Perovskite material layer

Abstract

A layer of perovskite material for use in an energy harvesting device, the layer having a thickness of less than 60 µm and the perovskite material comprising crystalline material of formula (LC)2(SC)n-1MnX3n+1 with a polar crystallographic point group, wherein each LC is a cation of formula A-B-C, wherein A is selected from ethyl, ethenyl, ethynyl, C1- 4 alkoxymethyl, C1-4 alkylthio, aryl, and heteroaryl, each of which is optionally substituted one or more times with fluoro and wherein the aryl and heteroaryl are optionally substituted one or more times with a substituent selected from C1-4 alkyl, C2-4 alkenyl, C2-4 alkynyl, C1-4 fluoroalkyl, C2-4 fluoroalkenyl, C2-4 fluoroalkynyl, C1-4 alkoxy, hydroxy, formyl, carboxyl and heteroaryl, B is absent or is selected from C1-6 alkylene, C2-6 alkenylene and C2-6 alkynylene, each of which is optionally substituted with fluoro, and C is –N(R1)3+ or – CR1N(R1)2N(R1)2+, wherein each R1 is independently selected from H and C1-6 alkyl, each SC is a cation selected from Cs+, N(R2)4+ and R2CN(R2)2N(R2)2 +, wherein each R2 is independently selected from H and methyl, each M is selected from Pb2+, Sn2+ and Ge2+, each X is halide, n is an integer selected from 1 to 6, and the material comprising photovoltaic and ferroelectric or piezoelectric properties.

Description

[0001] PEROVSKITE MATERIAL LAYER

[0002] FIELD

[0003] The present disclosure relates to a layer of perovskite material for use in an energy harvesting device, and to a method of forming the layer of perovskite material. The present disclosure also relates to an energy harvesting device comprising the layer of perovskite material, a method of forming an energy harvesting device and the use of a layer of perovskite material in an energy harvesting device.

[0004] BACKGROUND

[0005] Ferroelectric materials have a permanent polarisation, which can be electrically switched or reversed and has hysteresis properties. These materials may find application in a wide range of technologies, including but not limited to memory devices, such as random access memory, actuators, sensors, energy harvesting device and / or energy storage. Ferroelectric materials can exist in both bulk and thin film forms. For example, in thin film form, ferroelectric materials may have a potential to realize next-generation devices in energy and electronic sectors. Such devices may allow for ultra-high density data storage, low power consumption, ultrafast performance, and / or a lightweight. Thin films of ferroelectric material may facilitate the integration of such devices with emerging microelectronic technologies. Ferroelectric thin films may additionally be considered as key components in nanoscale devices, such as logic gates, memory devices, neuromorphic computing, field effect transistors, e.g. negative capacitance field effect transistors, and in photovoltaic applications.

[0006] Known ferroelectric materials that are employed in a number of applications include oxide perovskite-based ferroelectric materials, such as PbZri-xTixOs, BaTiCh, BiFeOs, LiNbOs, and binary oxide materials, such as HfC>2. However, these materials have an inherent brittleness and contain expensive and rare-earth metals, such as Zr, Nb, Li, Bi, or Hf. These rare-earth metals require complex, high temperature and energy- intensive processing and supply of these rare-earth metals may be limited. Processes for forming thin films including these rare-earth metals include epitaxial processes, such as molecular beam epitaxy, and pulsed laser deposition. This may limit the application of such thin films in smart technologies, such as the Internet of Things (loT) and / or wearable devices. loT and wearable devices require compact, flexible, highly scalable and ultralight-weight energy harvesting devices and / or energy storage devices, e.g. to be compatible with distributed sensing, e.g., in smart buildings, precision agriculture,

[0007] 55667482-1 smart cities and / or complete digitisation of manufacturing, and continuous monitoring and acquisition of data.

[0008] Organic ferroelectric materials, such as PVDF and its co-polymerwith TrFE, have properties, such as a lightweight, mechanical flexibility, biocompatibility and easy processability. The low melting point, e.g. about 177 °C, low spontaneous polarisation, and high coercive field, e.g. about 50 MV / mm may limit their application in loT and / or wearable devices. The inherent challenge of obtaining thin ferroelectric PVDF films, e.g. with a thickness of less than one micrometre, limits the integration of this material to micro-nanoscale electronic applications. As such, there is a need for materials with ferroelectric properties, which are as robust as oxide-based materials and easily fabricable, but also provide a high flexibility, and scalability.

[0009] This background serves only to set a scene to allow a person skilled in the art to better appreciate the following description. Therefore, none of the above discussion should necessarily be taken as an acknowledgement that that discussion is part of the state of the art or is common general knowledge. One or more aspects / embodiments of the disclosure may or may not address one or more of the background issues.

[0010] SUMMARY

[0011] According to a first aspect of the present disclosure there is provided a layer of perovskite material for use in an energy harvesting device. The layer has a thickness of less than 60 pm. The perovskite material comprises crystalline material of formula (LC)2(SC)n-i MnXsn+i with a polar crystallographic point group. Each LC is a cation of formula A-B-C. A is selected from ethyl, ethenyl, ethynyl, Ci-4alkoxymethyl, Ci-4al kylthio, aryl, and heteroaryl, each of which is optionally substituted one or more times with fluoro. The aryl and heteroaryl are optionally substituted one or more times with a substituent selected from Ci-4alkyl, C^alkenyl, C^alkynyl, C luoroalkyl, C^fluoroalkenyl, C2- 4fluoroalkynyl, Ci-4alkoxy, hydroxy, formyl, carboxyl and heteroaryl. B is absent or is selected from Ci-6alkylene, C^alkenylene and C^alkynylene, each of which is optionally substituted with fluoro. C is -N(R1)s+or -CR1N(R1)2N(R1)2+, wherein each R1is independently selected from H and Ci-ealkyl. Each SC is a cation selected from Cs+, N(R2)4+and R2CN(R2)2N(R2)2+, wherein each R2is independently selected from H and methyl. Each M is selected from Pb2+, Sn2+and Ge2+. Each X is halide, n is an integer selected from 1 to 6. The perovskite material comprises photovoltaic and ferroelectric or piezoelectric properties. n may be an integer selected from 3 to 6.

[0012] 55667482-1 With increasing n, a bandgap of the perovskite material may decrease. This may make the material more suitable for photovoltaic applications. With increasing n, a phase purity of the perovskite material may decrease. For example, the perovskite material may comprise an increased mix of phases with different values for n. With decreasing n, a bandgap of the perovskite material may increase and / or ferroelectric properties of the material may increase. The inventor has found that when n is selected from 3 to 6, the perovskite material may comprise improved photovoltaic and ferroelectric properties (or improved photovoltaic and piezoelectric properties).

[0013] Each X may be selected from chloride, iodide or a combination of two or more halides. When X is iodide, a bandgap of the perovskite material may be smaller compared to when X is chloride. This may make the layer of perovskite material suitable for use in one or more photovoltaic applications, such in an energy harvesting device that comprises a photovoltaic device. The layer of perovskite material may additionally comprise ferroelectric or at least piezoelectric properties, as will be described herein. When X is chloride, a bandgap of the perovskite material may be larger compared to when X is iodide. This may result in improved ferroelectric properties of the layer of perovskite material. For example, a leakage current of layer of perovskite material may be reduced. The layer of perovskite material may additionally comprise photovoltaic properties. It will be appreciated that, for example, a selection of X may allow for one or more properties of the layer of perovskite material to be tuned or improved.

[0014] The polar crystallographic point group may be an orthorhombic crystal system with a polar space group of Cmc2i.

[0015] A may be selected from ethyl, ethenyl, ethynyl, trifluoromethyl, and phenyl. The phenyl may optionally be substituted one or more times with a substituent selected from Ci-4alkyl and fluoro.

[0016] The Ci-6alkylene, C^alkenylene and C^alkynylene may be linear.

[0017] B may be selected from Ci-4alkylene, C^alkenylene and C^alkynylene. Each of Ci-4alkylene, C^alkenylene and C^alkynylene may optionally be substituted with fluoro.

[0018] Each R1may be independently selected from H and methyl.

[0019] C may be -N(H)s+.

[0020] Each LC may be butylammonium or phenethylammonium.

[0021] Each R2may be independently selected from H and methyl.

[0022] Each SC may be methylammonium or formamidinium.

[0023] Each M may be Pb2+.

[0024] The layer of perovskite material may have a thickness of 50 nm to 50 pm.

[0025] 55667482-1 The layer of perovskite material may have a thickness of less than 5 pm. A layer of material having a thickness between about 10 A and about 3 pm may also be referred to as a thin film. The layer of perovskite material may have a thickness between about 150 nm and 3 pm. The layer may have a thickness between about 250 nm and 2.5 pm. The use of a thin layer of perovskite material in an energy harvesting device may result in a lightweight and / or portable energy harvesting device, allow for decreased amounts of material to be used and / or printability of the perovskite material. This may make the energy harvesting device suitable, e.g. for loT applications, where lightweight, compact and / or portable devices are favoured over bulky and heavy energy sources.

[0026] According to a second aspect of the present disclosure there is provided an energy harvesting device comprising a first electrode, a second electrode, and a layer of perovskite material according to the first aspect, the layer of perovskite material being arranged between the first and second electrodes.

[0027] The device may comprise a ferroelectric device and / or a piezoelectric device.

[0028] The device may further comprise a barrier layer. The barrier layer may be arranged between at least one of the first and second electrodes and the layer of perovskite material. The barrier layer may comprises a metal material or a polymer material. The metal material may comprise Cr, C^Os, and / or Barium. The polymer material may comprise phenyl-Cei butyric acid methyl ester, Bathocuproine, polymeric C60, styrene ethylene butylene styrene or polymethyl methacrylate.

[0029] The barrier layer may be configured to reduce a diffusion of halide atoms from the layer of perovskite material to the at least one of the first and second electrodes and / or reduce a reaction between the halide atoms and the conducting material of the at least one the first and second electrodes. For example, halide atoms diffusing from the layer of perovskite material may react with a metal material of the at least one of the first and second electrodes, e.g. to form irreversible metal-halide compounds. Additionally or alternatively, metal atoms may diffuse from the at least one of the first and second electrodes into the layer of perovskite material, thereby acting as device short-circuiting paths and / or recombination centres. By arranging the barrier layer between the layer of perovskite material and the at least one of the first and second electrodes, the formation of the device short-circuiting paths and / or recombination centres may be reduced or prevented.

[0030] The device may comprise a photovoltaic device. The device may further comprise an electron transport layer. The electron transport layer may be arranged between at least one of the first and second electrodes and the layer of perovskite material. The

[0031] 55667482-1 device may further comprise a hole transport layer. The hole transport layer may be arranged between at least one other of the first and second electrodes and the layer of perovskite material. At least one of the first and second electrodes may be configured to be transparent to at least a portion of light to be absorbed by the layer of perovskite material.

[0032] According to a third aspect of the present disclosure there is provided a method of forming a layer perovskite material. The method comprises depositing a layer of precursor perovskite material on a support layer using a deposition process, and annealing the deposited layer of precursor perovskite material at a temperature of about 25 °C to about 125 °C to form a layer of perovskite material according to the first aspect.

[0033] The deposition process may comprise a blade coating process. This process may allow for a scalable formation of the layer of perovskite material. The blade coating process may use a blade speed between about 1 mm / s and about 30 mm / s. For example, blade coating process may use a blade speed of about 15 mm / s.

[0034] The deposition process may comprise a spin-coating process.

[0035] The method may comprise annealing the deposited layer of precursor perovskite material at a pressure, e.g. between about 0.01 bar and about 1 bar. This may allow for removal of one or more solvents, e.g. complete removal of the one or more solvents, aid the formation of a uniform layer and / or increase a crystallisation rate.

[0036] The method may allow for a facilitated formation of the layer of perovskite material, e.g. at a low temperature, such as a temperature between about 25 °C to about 125 °C.

[0037] For the avoidance of doubt, the term “obtainable” includes materials obtained by the method of the third aspect, as well as materials not obtained by the method of the third aspect but that are identical to those obtained by the method of the third aspect. In some embodiments, the layer of perovskite material having a thickness of less than 60 pm is obtained by the method according to the third aspect.

[0038] According to a fourth aspect of the present disclosure there is provided a layer of perovskite material having a thickness of less than 60 pm obtainable by the method according to the third aspect.

[0039] According to a fifth aspect of the present disclosure there is provided an energy harvesting device comprising the layer of perovskite material according to the fourth aspect.

[0040] 55667482-1 According to a sixth aspect of the present disclosure there is provided a use of the layer of perovskite material according to the first or fourth aspect in an energy harvesting device.

[0041] According to a seventh aspect of the present disclosure there is a method of forming an energy harvesting device. The method comprises forming or providing a first electrode, forming a layer of perovskite material, the layer of perovskite material being formed using the method according to the third aspect, and forming or providing a second electrode, wherein the layer of perovskite material is arranged between the first and second electrodes.

[0042] The method may comprise forming a barrier layer between at least one of the first and second electrodes and the layer of perovskite material.

[0043] The above summary is intended to be merely exemplary and non-limiting. The disclosure includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. It should be understood that features defined above in accordance with any aspect of the present disclosure or below relating to any specific embodiment of the disclosure may be utilised, either alone or in combination with any other defined feature, in any other aspect or embodiment or to form a further aspect or embodiment of the disclosure.

[0044] BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1A depicts an exemplary crystal structure of a layer of perovskite material according to an embodiment of the present disclosure;

[0046] Figure 1 B depicts a flow chart in overview of a method of forming a layer of perovskite material for use in an energy harvesting device;

[0047] Figure 2 schematically depicts an exemplary deposition process that may be used in the method of Figure 1 B;

[0048] Figure 3 depicts absorbance spectra that have been measured for exemplary layers of perovskite material;

[0049] Figure 4 depicts an X-ray diffraction pattern that has been measured for an exemplary layer of perovskite material;

[0050] Figure 5 depicts an X-ray diffraction pattern that has been measured for another exemplary layer of perovskite material;

[0051] Figures 6A and 6B depict scanning electron images of exemplary layers of perovskite material;

[0052] 55667482-1 Figure 7 depicts Raman spectra that have been measured for exemplary layers of perovskite material;

[0053] Figure 8 depicts graphs obtained by differential scanning calorimetry measurements for exemplary layers of perovskite material;

[0054] Figure 9 schematically depicts an exemplary energy harvesting device according to an embodiment of the present disclosure;

[0055] Figure 10 schematically depicts another exemplary energy harvesting device;

[0056] Figure 11A schematically depicts another exemplary energy harvesting device comprising a n-i-p structure;

[0057] Figure 11 B schematically depicts another exemplary energy harvesting device comprising a p-i-n structure;

[0058] Figure 12 depicts a flow chart in overview of a method of forming an energy harvesting device according to an embodiment of the present disclosure;

[0059] Figure 13 depicts a flow chart in overview of a method of forming another energy harvesting device;

[0060] Figure 14 depicts a graph of measured polarisations of an exemplary energy harvesting device in dependence of a voltage having different frequencies;

[0061] Figure 15 depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0062] Figure 16 depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0063] Figure 17 depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage f;

[0064] Figure 18 depicts a graph of measured leakage currents of an exemplary energy harvesting device in dependence of a voltage;

[0065] Figure 19 depicts a graph of a measured pyroelectric current and a measured temperature gradient of an exemplary energy harvesting device in dependence of a time;

[0066] Figure 20 depicts a graph of a pyroelectric current and a temperature gradient ^ of an exemplary energy harvesting device in dependence of a time;

[0067] Figure 21A depicts a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric force microscope;

[0068] 55667482-1 Figure 21 B depicts a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a voltage applied to a tip of the piezoelectric force microscope;

[0069] Figure 22A depicts a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope;

[0070] Figure 22B depicts a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope;

[0071] Figure 23A depicts a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time, the open-circuit voltage having been measured for different forces applied to the device;

[0072] Figure 23B depicts a graph of a measured short-circuit current of an energy harvesting device in dependence on time, the short-circuit current having been measured for different forces applied to the device;

[0073] Figure 24A depicts a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time, the open-circuit voltage having been measured for different forces applied to the device;

[0074] Figure 24B depicts a graph of a measured short-circuit current of an energy harvesting device in dependence on time, the short-circuit current having been measured for different forces applied to the device;

[0075] Figure 25A depicts a graph of a measured external quantum efficiency of an energy harvesting device in dependence on a wavelength;

[0076] Figure 25B depicts a graph of measured current densities of a number of energy harvesting devices in dependency on voltages applied to each device;

[0077] Figure 25C depicts a distribution of open-circuit voltages under 1 Sun illumination determined for a number of energy harvesting devices;

[0078] Figure 25D depicts a graph of measured current densities of a number of energy harvesting devices in dependency on voltages applied to each device;

[0079] Figure 25E depicts a distribution of open-circuit voltages determined for a number of energy harvesting devices under an illumination of 1000 lux;

[0080] Figure 25F depicts a distribution of power conversion efficiencies determined for a number of energy harvesting devices under 1 Sun illumination;

[0081] Figure 25G depicts a distribution of power conversion efficiencies determined for a number of energy harvesting devices under an illumination of 1000 lux;

[0082] 55667482-1 Figure 26 depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0083] Figure 27A depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0084] Figure 27B depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0085] Figures 28A to 28C each depict a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0086] Figure 29A depicts a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope;

[0087] Figure 29B depicts a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to the tip of the piezoelectric microscope;

[0088] Figure 30A depicts a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope;

[0089] Figure 30B depicts a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to the tip of the piezoelectric microscope;

[0090] Figure 31 A depicts a graph of a measured short-circuit current of an energy harvesting device in dependence on time, the short-circuit current having been measured for different forces applied to the device;

[0091] Figure 31 B depicts a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time, the open-circuit voltage having been measured for different forces applied to the device;

[0092] Figure 32A depicts a graph of a measured short-circuit current of an energy harvesting device in dependence on time, the short-circuit current having been measured for different forces applied to the device;

[0093] Figure 32B depicts a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time, the open-circuit voltage having been measured for different forces applied to the device;

[0094] Figure 33A depicts a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time, the open-circuit voltage having been measured for different forces applied to the device;

[0095] 55667482-1 Figure 33B depicts a graph of a measured short-circuit current of an energy harvesting device in dependence on time, the short-circuit current having been measured for different forces applied to the device;

[0096] Figure 34A depicts a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time, the open-circuit voltage having been measured for different forces applied to the device;

[0097] Figure 34B depicts a graph of a measured short-circuit current of an energy harvesting device in dependence on time, the short-circuit current having been measured for different forces applied to the device;

[0098] Figure 35A depicts a graph of a distribution of short-circuit current densities that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0099] Figure 35B depicts a graph of a distribution of open-circuit voltages that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0100] Figure 35C depicts a graph of a distribution of power conversion efficiencies that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0101] Figure 35D depicts a graph of a distribution of fill factors that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0102] Figure 36A depicts a graph of a distribution of short-circuit current densities that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0103] Figure 36B depicts a graph of a distribution of open-circuit voltages that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0104] Figure 36C depicts a graph of a distribution of power conversion efficiencies that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0105] Figure 36D depicts a graph of a distribution of fill factors that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0106] Figure 37A depicts a graph of a distribution of short-circuit current densities that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0107] Figure 37B depicts a graph of a distribution of open-circuit voltages that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0108] 55667482-1 Figure 37C depicts a graph of a distribution of power conversion efficiencies that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0109] Figure 37D depicts a graph of a distribution of fill factors that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0110] Figure 37E depicts a graph of steady state power conversion efficiencies that have been determined for a number of energy harvesting devices under 1 Sun illumination;

[0111] Figure 38A depicts a graph of a distribution of short-circuit current densities that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0112] Figure 38B depicts a graph of a distribution of open-circuit voltages that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0113] Figure 38C depicts a graph of a distribution of power conversion efficiencies that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0114] Figure 38D depicts a graph of a distribution of fill factors that have been determined for a number of energy harvesting devices under 1000 lux illumination;

[0115] Figure 38E depicts a graph of a steady state power conversion efficiency that has been determined for an energy harvesting device under 1000 lux illumination;

[0116] Figures 39A and 39B each depict an absorbance spectrum another exemplary layer of perovskite material;

[0117] Figure 40A depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0118] Figure 40B depicts a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage;

[0119] Figure 41A depicts a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time, the open-circuit voltages having been measured for a force of 20N applied to each device;

[0120] Figure 41 B depicts a graph of measured short-circuit currents of a number of energy harvesting devices in dependence on time, the short-circuit currents having been measured for a force of 20N applied to each device;

[0121] Figure 42A depicts a graph of a measured power of a number of energy harvesting devices in dependence on a resistance of a load resistor, the powers having been measured for a force of 20N applied to each device;

[0122] 55667482-1 Figure 42B depicts a graph of a measured power of a number of energy harvesting devices in dependence on a resistance of a load resistor, the powers having been measured for a force of 20N applied to each device;

[0123] Figure 43A shows a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time, the open-circuit voltages having been measured for a force applied to each device;

[0124] Figure 43B depicts a graph of measured open-circuit voltages of an energy harvesting device in dependence on time, the open-circuit voltages having been measured for a force applied to the device with no illumination, under illumination with visible light and under illumination with ultraviolet light;

[0125] Figure 43C depicts a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time the open-circuit voltages having been measured for a force applied to each device and under illumination with infrared light;

[0126] Figure 44 depicts absorbance spectra of exemplary layers of perovskite material;

[0127] Figure 45A depicts a graph of a measured voltage and a measured current of an energy harvesting device in dependence on a resistance of a load resistor;

[0128] Figure 45B depicts a graph of a measured voltage and a measured current of an energy harvesting device in dependence on a resistance of a load resistor;

[0129] Figure 46A depicts a graph of measured open-circuit voltages of an energy harvesting device in dependence on time, the open-circuit voltages having been measured for a force of 10 N applied to the device;

[0130] Figure 46B depicts a graph of measured open-circuit voltages of an energy harvesting device in dependence on time, the open-circuit voltages having been measured for a force of 10 N applied to the device;

[0131] Figure 47A depicts a graph of measured open-circuit voltages of an energy harvesting device in dependence on time, the open-circuit voltages having been measured for different forces applied to the device;

[0132] Figure 47B depicts a graph of measured open-circuit voltages of an energy harvesting device in dependence on time, the open-circuit voltages having been measured for different forces applied to the device;

[0133] Figure 48A depicts a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time, the open-circuit voltages having been measured for a force applied to each device;

[0134] 55667482-1 Figure 48B depicts a graph of measured open-circuit voltages of an energy harvesting devices in dependence on time, the open-circuit voltages having been measured for a force applied to the device under illumination with infrared light;

[0135] Figure 48C depicts a graph of measured open-circuit voltages of an energy harvesting devices in dependence on time, the open-circuit voltages having been measured for a force applied to the device under illumination with ultraviolet light;

[0136] Figure 48D depicts a graph of measured open-circuit voltages of an energy harvesting devices in dependence on time, the open-circuit voltages having been measured for a force applied to the device under illumination with visible light;

[0137] Figure 49 depicts a graph of measured current densities in dependency on voltages applied to an energy harvesting device;

[0138] Figure 50A depicts a graph of measured current densities in dependency on voltages applied to an energy harvesting device;

[0139] Figure 50B depicts a graph of measured current densities in dependency on voltages applied to an energy harvesting device;

[0140] Figure 51A depicts a graph of measured current densities in dependency on voltages applied to an energy harvesting device;

[0141] Figure 51 B depicts a graph of measured current densities in dependency on voltages applied to an energy harvesting device;

[0142] Figure 52A depicts a graph of determined piezoelectric coefficients in dependency on a force applied to a number of energy harvesting devices;

[0143] Figure 52B depicts a graph of determined piezoelectric coefficients in dependency on a force applied to an energy harvesting device;

[0144] Figure 52C depicts a graph of determined piezoelectric coefficients in dependency on a force applied to a number of energy harvesting devices;

[0145] Figure 53A depicts a graph of measured open-circuit voltages of an energy harvesting device in dependence on time;

[0146] Figure 53B depicts a graph of measured current of an energy harvesting device in dependence on time; and

[0147] Figure 54 depicts absorbance spectra of exemplary layers of perovskite material.

[0148] DETAILED DESCRIPTION OF DRAWINGS

[0149] In the discussion that follows, reference is made to a number of terms, which are to be understood to have the meanings provided below, unless a context indicates to the contrary. The nomenclature used herein for defining compounds, in particular the

[0150] 55667482-1 compounds described herein, is intended to be in accordance with the rules of the International Union of Pure and Applied Chemistry (IUPAC) for chemical compounds, specifically the “IUPAC Compendium of Chemical Terminology (Gold Book)” (see A. D. Jenkins et al., Pure & Appl. Chem., 68, 2287-2311 (1996)). For the avoidance of doubt, if an IUPAC rule is contrary to a definition provided herein, the definition herein is to prevail.

[0151] The term “alkyl” is well known in the art and defines univalent groups derived from alkanes by removal of a hydrogen atom from any carbon atom, wherein the term “alkane” is intended to define acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, wherein n is an integer s 1. Ci-ealkyl may refer but is not limited to methyl, ethyl, n-propyl, / so-propyl, n-butyl, sec-butyl, / so-butyl and terf-butyl, n-pentyl, neo-pentyl, and n-hexyl. Ci-4alkyl refers to any one selected from the group consisting of methyl, ethyl, n-propyl, / so-propyl, n-butyl, sec-butyl, / so-butyl and terf-butyl.

[0152] The term “fluoroalkyl” is therefore understood to refer to univalent groups derived from alkyl groups wherein one or more hydrogen atoms have been replaced with a fluorine atom. For example, Ci-4fluoroalkyl may refer but is not limited to fluoromethyl, difluoromethyl, trifluoromethyl, trifluoroethyl, perfluoroethyl, and hexafluoroisopropyl.

[0153] The term “alkylene” is well known in the art and defines divalent groups derived from alkanes by removal of two hydrogen atoms from any one or two carbon atoms. Ci- ealkylene may refer but is not limited to methylene, ethylene, n-propylene, / so-propylene, n-butylene, sec-butylene, / so-butylene and terf-butylene, n-pentylene, neo-pentylene, and n-hexylene. Ci-4alkylene refers to any one selected from the group consisting of methylene, ethylene, n-propylene, / so-propylene, n-butylene, sec-butylene, / so-butylene and terf-butylene.

[0154] The term “alkenyl” defines univalent groups derived from alkenes by removal of a hydrogen atom from any carbon atom, wherein the term “alkene” is intended to define acyclic branched or unbranched hydrocarbons having one carbon-carbon double bond and the general formula CnH2n, where n is an integer > 2. C^alkenyl refers to any one selected from the group consisting of ethenyl, prop-1 -enyl, prop-2-enyl, 1-methyl- ethenyl, but-1-enyl, but-2-enyl, 1-methyl-prop-1-enyl, 1-methyl-prop-2-enyl, 2-methyl- prop-1-enyl, and 2-methyl-prop-2-enyl.

[0155] The term “fluoroalkenyl” is therefore understood to refer to univalent groups derived from alkenyl groups wherein one or more hydrogen atoms have been replaced with a fluorine atom. For example, C^fluoroalkenyl may refer but is not limited to fluoroethenyl, difluoroethenyl, trifluoroethenyl, 1-fluoroprop-1-enyl, 3-trifluoroprop-1-

[0156] 55667482-1 enyl, perfluoroprop-1 -enyl, 1-difluoroprop-2-enyl, perfluoroprop-2-enyl, 2- trifluoromethylprop-1-enyl, perfluorobut-1-enyl, and perfluorobut-2-enyl.

[0157] The term “alkenylene” is well known in the art and defines divalent groups derived from alkenes by removal of two hydrogen atoms from any one or two carbon atoms. C2- salkenylene may refer but is not limited to ethenylene, prop-1 -enylene, prop-2-enylene,

[0158] 1-methyl-ethenylene, but-1 -enylene, but-2-enylene, 1-methyl-prop-1 -enylene, 1-methyl- prop-2-enylene, 2-methyl-prop-1 -enylene, and 2-methyl-prop-2-enylene, pent-1 - enylene, pent-2-enylene, hex-1 -enylene, hex-2-enylene, and hex-3-enylene. C^alkenyl refers to any one selected from the group consisting of ethenylene, prop-1 -enylene, prop-

[0159] 2-enylene, 1-methyl-ethenylene, but-1 -enylene, but-2-enylene, 1-methyl-prop-1- enylene, 1-methyl-prop-2-enylene, 2-methyl-prop-1 -enylene, and 2-methyl-prop-2- enylene.

[0160] The term “alkynyl” defines univalent groups derived from alkynes by removal of a hydrogen atom from any carbon atom, wherein the term “alkyne” is intended to define acyclic branched or unbranched hydrocarbons having one carbon-carbon triple bond and the general formula CnH2n-2, where n is an integer > 2. C^alkynyl refers to any one selected from the group consisting of ethynyl, 1-propynyl, 2-propynyl, 1-butynyl, 2- butynyl, 3-butynyl, and 1-methyl-prop-2-ynyl.

[0161] The term “fluoroalkynyl” is therefore understood to refer to univalent groups derived from alkynyl groups wherein one or more hydrogen atoms have been replaced with a fluorine atom. For example, C^fluoroalkynyl may refer but is not limited to fluoroethynyl, 3-trifluoro-1-propynyl, 1-difluoro-2-propynyl, perfluoro-2-propynyl, 4- trifluoro-1-butynyl, perfluoro-1-butynyl, 4-trifluoro-2-butynyl, perfluoro-2-butynyl, and 1- trifluoromethyl-prop-2-ynyl.

[0162] The term “alkynylene” is well known in the art and defines divalent groups derived from alkynes by removal of two hydrogen atoms from any one or two carbon atoms. C2- ealkynylene may refer but is not limited to ethynylene, 1-propynylene, 2-propynylene, 1- butynylene, 2-butynylene, 1-methyl-but-1-ynylene, 1 -pentynylene, 2-pentynylene, 1- hexynylene, 2-hexynylene, and 3-hexynylene. C^alkynyl refers to any one selected from the group consisting of ethynylene, 1-propynylene, 2-propynylene, 1-butynylene, 2- butynylene, 3-butynylene, and 1-methyl-prop-2-ynylene.

[0163] The term “hydroxy” is well known in the art and defines the univalent group derived from water by removal of one hydrogen atom. Hydroxy groups are often depicted as -OH.

[0164] 55667482-1 The term “alkoxy” defines univalent groups derived from alcohols by removal of a hydrogen atom from an -OH group, wherein the term “alcohol” is intended to define groups derived from alkanes by the replacement of a hydrogen atom with a hydroxy group. C2-4alkoxy refers to any one selected from the group consisting of methoxy, ethoxy, n-propoxy, / so-propoxy, n-butoxy, sec-butoxy, / so-butoxy and tert-butoxy.

[0165] The term “alkoxymethyl” is therefore understood to refer to univalent groups derived from methyl groups wherein one hydrogen atom is replaced with an alkoxy group. An alkoxymethyl group may be depicted as alkyl-O-CH2- For example, Ci- 4alkoxymethyl refers to any one selected from the group consisting of methoxymethyl, ethoxymethyl, n-propoxymethyl, / so-propoxymethyl, n-butoxymethyl, sec-butoxymethyl, / so-butoxymethyl and terf-butoxymethyl.

[0166] The term “alkylthio” defines univalent groups derived from alkylthiols by removal of the hydrogen atom of the thio group. The term “alkylthiol” refers to alkanes wherein one hydrogen atom has been replaced with a thio group, wherein thio refers to -SH. Ci- 4alkylthio refers to any one selected from the group consisting of methylthio, ethylthio, n- propylthio, / so-propylthio, n-butylthio, sec-butylthio, / so-butylthio and terf-butylthio.

[0167] The term "aryl" may refer to a single or fused ring system comprising one or more aromatic rings. The term may refer to a mono- or polycyclic aromatic hydrocarbon system having 6 to 14 carbon ring atoms, in some cases 6 to 10 carbon ring atoms. Where the aryl is a fused ring system, at least one of the rings is aromatic and the other ring(s) may be aromatic or aliphatic. Representative examples of suitable aryl groups include, but are not limited to, phenyl, biphenyl, naphthyl, pyrenyl, perylenyl, 1 -naphthyl, 2- naphthyl and anthracenyl. For example, an aryl may be selected from phenyl and naphthyl, such as phenyl. As used herein, “substituted aryl” refers to an aryl group as defined herein which comprises one or more substituents on the aromatic ring. When an aryl group is substituted, any hydrogen atom(s) may be replaced with the substituent(s), providing valencies are satisfied.

[0168] The term “heteroaryl” may refer to a single or fused ring system comprising one or more aromatic rings, wherein the one or more aromatic rings comprise one or more O, N and / or S atoms. The term may refer to a mono- or polycyclic heteroaromatic system having 5 to 14 ring atoms, in some cases 5 to 10 ring atoms. Where the heteroaryl is a fused ring system, at least one of the rings is aromatic and the other ring(s) may be aromatic or aliphatic. Representative examples of heteroaryl groups may include, but are not limited to, pyrrolyl, furanyl, thiophenyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl, thiazolyl, pyridinyl, pyrimidinyl, pyridazinyl, pyrazinyl, indolyl, benzofuranyl,

[0169] 55667482-1 benzothiazolyl, benzimidazolyl, indazolyl, benzoxazolyl, benzisoxazolyl, benzodioxanyl, benzothiadiazolyl, naphthyridinyl etc. For example, a heteroaryl may be selected from thiophenyl and pyridinyl. As used herein, “substituted heteroaryl” refers to a heteroaryl group as defined herein which comprises one or more substituents on the heteroaromatic ring. When a heteroaryl is substituted, any hydrogen atom(s) may be replaced with the substituent(s), providing valencies are satisfied.

[0170] The term “formyl” is well known in the art and defines the univalent group derived from formaldehyde by removal of one hydrogen atom. Formyl groups are often depicted as -C(O)H.

[0171] The term “carboxyl” is also well known in the art and defines the univalent group derived from formic acid by removal of the hydrogen atom from the carbon atom. Carboxyl groups are often depicted as -C(O)OH, or -CO2H.

[0172] The term “halide” refers to fluoride, chloride, bromide, or iodide. Typically, halide refers to chloride, bromide, or iodide.

[0173] The term “about” herein, when qualifying a number or value, is used to refer to values that lie within ± 5% of the value specified. For example, annealing a deposited layer of precursor perovskite material at a temperature of about 25 °C to about 125 °C is understood to include annealing at a temperature from 23.75 °C to 128.75 °C.

[0174] Certain embodiments provide a layer of perovskite material for use in an energy harvesting device, the layer having a thickness of less than 60 pm and the material comprises crystalline material of formula (LC)2(SC)n-iMnX3n+i with a polar crystallographic point group wherein: each LC is a large spacer cation of formula A-B-C, wherein: A is selected from ethyl, ethenyl, ethynyl, Ci-4alkoxymethyl, Ci-4alkylthio, aryl, and heteroaryl, each of which is optionally substituted one or more times with fluoro and wherein the aryl and heteroaryl are optionally substituted one or more times with a substituent selected from Ci-4alkyl, C^alkenyl, C^alkynyl, Ci-4fluoroalkyl, C^fluoroalkenyl, C^fluoroalkynyl, Ci-4alkoxy, hydroxy, formyl, carboxyl and heteroaryl; B is absent or is selected from Ci- ealkylene, C^alkenylene and C^alkynylene, each of which is optionally substituted with fluoro; and C is -N(R1)s+or -CR1N(R1)2N(R1)2+, wherein each R1is independently selected from H and Ci-ealkyl; each SC is a small cation selected from Cs+, N(R2)4+and R2CN(R2)2N(R2)2+, wherein each R2is independently selected from H and methyl; each M is selected from Pb2+, Sn2+and Ge2+; each X is halide; and n is an integer selected from 1 to 6 and the material comprises or is a photovoltaic and ferroelectric material or a photovoltaic and piezoelectric material. For example, the material may comprise photovoltaic and ferroelectric properties or photovoltaic and piezoelectric properties. A

[0175] 55667482-1 ferroelectric material is also a piezoelectric material. However, not all piezoelectric materials may also be ferroelectric materials. Additionally, a ferroelectric material is a pyroelectric material.

[0176] A ferroelectric material may be understood as a material having an electric polarisation that can be reversed upon exposure to an electric field. The electric polarisation is due to an asymmetric distribution of ions in a crystal lattice of the ferroelectric material.

[0177] A photovoltaic material may be understood as a material having an ability to convert light absorbed by the material into a plurality of charge carriers, such as electrons and holes, using the photovoltaic effect.

[0178] A pyroelectric material may be understood as a material having an ability to generate a voltage, when the material is heated or cooled.

[0179] The co-existence of photovoltaic and ferroelectric or photovoltaics and piezoelectric properties of the layer of perovskite material may allow for use of the layer of perovskite material in an energy harvesting device, such as an energy harvesting device of an loT and / or wearable device.

[0180] As described above, the layer of perovskite material has a thickness of less than 60 pm. For example, the layer of perovskite material may have a thickness of 50 nm to 50 pm. A layer of material having a thickness between about 5 pm and about 100 pm may also be referred to a thick film. As such, in embodiments where the layer of perovskite material has a thickness between about 5 pm and about 50 pm, the layer of perovskite material is a thick film of perovskite material.

[0181] In some embodiments, the layer of perovskite material may have a thickness of less than 5 pm. A layer of material having a thickness between about 10 A and about 3 pm may also be referred to as a thin film. As such, in embodiments where the layer of perovskite material has a thickness between about 10 A and about 3 pm, the layer of perovskite material is a thin film of perovskite material. In some embodiments, the layer has a thickness between about 150 nm and 3 pm. In some embodiments, the layer has a thickness between about 250 nm and 2.7 pm. The use of a thin layer of perovskite material in an energy harvesting device may result in a lightweight and / or portable energy harvesting device, allow for decreased amounts of material to be used and / or printability of the perovskite material. This may make the energy harvesting device suitable, e.g. for loT applications, where lightweight, compact and / or portable devices are favoured over bulky and heavy energy sources.

[0182] 55667482-1 The thickness may be measured using any suitable technique known in the art, for example using a profilometer such as a Dektak 150 Stylus profilometer. It will be appreciated that the terms “layer” and “film” may be interchangeably used.

[0183] As described above, n is an integer selected from 1 to 6. In some embodiments, n is an integer selected from 2 to 6 or 1 to 5. Typically, n is selected from 3 to 6. With increasing n, a bandgap of the perovskite material may decrease. This may make the material more suitable for photovoltaic application. With increasing n, a phase purity of the perovskite material may decrease. For example, the perovskite material may comprise an increased mix of phases with different values for n. With decreasing n, a bandgap of the perovskite material may increase and / or ferroelectric properties of the material may increase. The inventor has found that when n is selected from 3 to 6, the perovskite material may comprise improved photovoltaic and ferroelectric properties (or improved photovoltaic and piezoelectric properties).

[0184] In some embodiments, each X is chloride, bromide, iodide, or a combination of two or more halides. Typically, each X is chloride, iodide, or a combination of two or more halides. More typically, X is iodide or chloride. When X is iodide, a bandgap of the perovskite material may be smaller compared to when X is chloride. This may make the layer of perovskite material suitable for use in an energy harvesting device that comprises a photovoltaic device. The layer of perovskite material may additionally comprise ferroelectric or at least piezoelectric properties, as will be described herein. When X is chloride, a bandgap of the perovskite material may be larger compared to when X is iodide. This may result in improved ferroelectric properties of the layer of perovskite material. For example, a leakage current of layer of perovskite material may be reduced. The layer of perovskite material may additionally comprise photovoltaic properties. It will be appreciated that, for example, a selection of X may allow for one or more properties of the layer of perovskite material to be tuned or improved.

[0185] The term “polar crystallographic point group” refers to point groups wherein there is more than one point that every symmetry operation leaves unmoved, thus creating a polar direction. Polar crystallographic point groups are Ci , C2, Cs, C2v, C3, Csv, C4, C4v, Ce, Cev. In some embodiments, the polar crystallographic point group is an orthorhombic crystal system, for example one with a space group selected from Cmc2i , Pmm2, Pmc2i , Pcc2, Pma2, Pca2i, Pnc2, Pmn2i, Pba2, Pna2i, Pnn2, Cmm2, Ccc2, mm2, f em2, Kma2, Kea2, Fmm2, Fdd2, \mm2, \ba2 and \ma2 (each denoted by its international short symbol). Typically, the polar crystallographic point group is Cmc2i.

[0186] 55667482-1 The cation of formula A-B-C may also be referred to as large cation ion or large spacer cation. Examples of large spacer cations known in the art for use in perovskite materials include those falling within the A-B-C structure defined herein and described by Zou et al., in Mater. Chem. Front, 2024, 8, 82-103 and / or by Sirbu et al., in Adv. Energy Mater. 2021 , 2003877. For example, the large spacer cation may be an alkylammonium, such as butylammonium, or a tetraalkylammonium, such as tetrabutylammonium or trimethylhexylammonium. Alternatively, the larger spacer cation may comprise an aromatic entity, for example it may be a phenylammonium or a phenylalkylammonium, where the phenyl is optionally substituted with halo, methyl, hydroxy, carboxyl or ethenyl, or the large spacer cation may be a thiophenylammonium, thiophenylalkylammonium, pyridylammonium or pyridylalkylammonium, where the thiophenyl and pyridyl are optionally substituted with halo, methyl, hydroxy, carboxyl or ethenyl.

[0187] In some embodiments, A is selected from ethyl, ethenyl, ethynyl, trifluoromethyl, and phenyl, wherein the phenyl is optionally substituted one or more times with a substituent selected from Ci-4alkyl and fluoro. In some embodiments, A is ethyl or phenyl. Typically, A is ethyl.

[0188] In some embodiments, the Ci-6alkylene, C^alkenylene and C^alkynylene are linear. That is to say, there are no branched groups. Examples of linear Ci-ealkylene groups include methylene, ethylene, n-propylene, n-butylene, n-pentylene, and n- hexylene; examples of linear C^alkenylene groups include ethenylene, prop-1 -enylene, prop-2-enylene, but-1 -enylene, but-2-enylene, pent-1 -enylene, pent-2-enylene, hex-1 - enylene, hex-2-enylene, and hex-3-enylene; and examples of linear C^alkynylene groups include ethynylene, 1-propynylene, 2-propynylene, 1-butynylene, 2-butynylene, 1 -pentynylene, 2-pentynylene, 1 -hexynylene, 2-hexynylene, and 3-hexynylene.

[0189] In some embodiments, B is selected from Ci-4alkylene, C^alkenylene and C2- 4alkynylene, each of which is optionally substituted with fluoro. Typically, B is C2alkylene, which may be referred to as ethylene.

[0190] In some embodiments, each R1is independently selected from H and methyl. Typically, each R1is H.

[0191] In some embodiments, C is -N(H)s+. In some embodiments, each LC is butylammonium (BA) or phenethylammonium (PEA). Typically, each LC is butylammonium, or ‘BA’. Wherein one or more LC is butylammonium, it may be depicted as:

[0192] 55667482-1 Wherein one or more LC is phenethylammonium, it may be depicted as:

[0193] In some embodiments, each R2is independently selected from H and methyl.

[0194] As described above, each SC is a cation selected from Cs+, N(R2)4+and R2CN(R2)2N(R2)2+. This cation may also be referred to as a small cation or a small spacer cation. In some embodiments, each SC is N(R2)4+. Typically where each SC is N(R2)4+, one R2is methyl, and three R2are H. That is to say, in some embodiments, each SC is methylammonium, or ‘MA’. Where an SC is methylammonium, it may be depicted as:

[0195] H3CNH3+

[0196] In some embodiments, each SC is R2CN(R2)2N(R2)2+. Typically, where each SC is R2CN(R2)2N(R2)2+, each R2is H. That is to say, each SC is formamidinium, or ‘FA’. Where an SC is formamidinium, it may be depicted as:

[0197] Although the above description describes that each M is selected from Pb2+, Sn2+and Ge2+, it will be appreciated that each M may be a Group IV dication. In some embodiments, each M is Pb2+.

[0198] In some embodiments, the layer of perovskite material comprises crystalline material of the formula (BA)2(MA)n-iPbnX3n+i, wherein n is an integer selected from 1 to 6, and X is Br or I. In some embodiments, X is Br and n is 1 or 2. For example, the layer of perovskite material may comprise crystalline material of the formula BA2PbBr4, or BA2MAPb2Br?. In some embodiments, X is I and n is 2 or 4. For example, the layer may comprise crystalline material of the formula BA2MAPb2l?, or BA2MAsPb4li3.

[0199] In some embodiments, the layer of perovskite material comprises crystalline material of the formula (PEA)2(MA)n-iPbnX3n+i, wherein n is an integer selected from 1 to 6, and X is Br. In some embodiments, X is Br and n is 1 , 2, 3 or 4. For example, the layer of perovskite material may comprise crystalline material of the formula (PEA)2PbBr4, (PEA)2MAPb2Br7, (PEA)2MA2Pb3Brio, or (PEA)2MA3Pb4Bri4. When each LC is phenethylammonium (PEA), the large cation may be considered to be bulkier than a butylammonium (BA) cation. This may make it difficult to orient or arrange phenethylammonium cations between at least two neighbouring inorganic sub-lattices or Pb-X sub-lattices. The arrangement of the large cations will be described in more detail

[0200] 55667482-1 in relation to Figure 1A. The inventor has found, e.g. unexpectantly found, that the layer of perovskite material comprising crystalline material of the formula (PEA)2(MA)n- iPbnX3n+i, as described herein, has at least improved ferroelectric properties.

[0201] In some embodiments, the layer of perovskite material comprises crystalline material of the formula (BA)2(FA)n-iPbnX3n+i, wherein n is an integer selected from 1 to 6, and X is Br. In some embodiments, X is Br and n is 2, 3, 4 or 5. For example, the layer of perovskite material may comprise crystalline material of the formula (BA)2(FA)Pb2Br?, (BA)2(FA)2Pb3Brio, (BA)2(FA)3Pb4Bri3, or (BA)2(FA)4Pb5Bri6.

[0202] Figure 1 A shows an exemplary crystal structure of a layer of perovskite material according to an embodiment of the present disclosure. As described above, the perovskite material comprises crystalline material of formula (LC)2(SC)n-iMnX3n+i. In this example, each M is Pb2+, each X is Br and n is equal to 1 . The perovskite material comprises a layered structure, such as a Ruddlesden-Pooper (R-P) layered structure. It will be appreciated that any of the perovskite materials described herein may comprise the layered structure.

[0203] The large and small cations, which are labelled “LC” and “SC”, respectively, in Figure 1 A, function as spacer layers, which interact through van der Waal’s forces. The crystal structure comprises a plurality of PbXe units, such as [PbBre]4' units or [PbBre]4' octahedral units. The PbXe units define a sub-lattice 1 , such as an inorganic sub-lattice or Pb-X sub-lattice. In the embodiment shown in Figure 1A, each sub-lattice 1 comprises four [PbBre]4' units. However, it will be understood that each sub-lattice is not limited to comprising four [PbBre]4' units.

[0204] The large cations LC define a spacer layer between at least two neighbouring sub-lattices 1. For example, the large cations LC may space the two neighbouring sublattices 1 from each other in a first direction, which is indicated as the x-direction in Figure 1A.

[0205] Each small cation SC is arranged between at least two neighbouring PbXe units. For example, each small cation SC may space the two neighbouring PbXe units from each other in a second direction, which is indicated as the z-direction in Figure 1A. The first and second directions are perpendicular, e.g. substantially perpendicular to each other.

[0206] At least one end of each of the large and small cations LC, SC is hydrogen bonded to a PbXe unit. The layered structure of the perovskite material may be less rigid compared to other layered perovskite material, e.g. the Dion-Jacobson layered perovskite material. This in turn may result in increased ferroelectric properties of the

[0207] 55667482-1 perovskite material described herein. The ferroelectric properties of the perovskite material may be due to tilting of each PbXe unit, e.g. due to a presence of stereochemically active lone pair of electrons in the Pb2+, a dynamic molecular motion of the large cations LC, which are coulombically bonded to halogens in each PbXe unit, and a reorientation of the small cations SC. The PbXe units are distributed in an y-z plane, as indicated in Figure 1A, and the large cations LC are distributed along an out-of-plane direction, e.g. the x-direction indicated in Figure 1A. The corner-sharing PbXe units are tilted due to the stereochemically active lone pair of electrons in the Pb2+cation. This distortion of the PbXe units can lead to a deviation of equatorial Br-Pb-Br bond angles relative to their standard value of 90°, bridging of Br-Pb-Br bond angles, e.g. relative to their standard value of 180°, and changes to the Pb-Br bond lengths. This distortion of the PbXe units shifts the centre of the negative charge. The large cations LC and the PbXe units interact interacts via the coulombic and hydrogen bonding interaction. The large LC cations have dynamic freedom for molecular motion and the orientation of the dipole moment of these cations favours the shift of a positively centre of charge. In the example of n=2, where the small cation SC is also present, their methyl groups may get fully ordered in the layered structure, e.g. due to the N-H— -Br hydrogen bonding. This coupled motion or shift of the positive and negative centre of charge may lead to the polarisation in the exemplary perovskite materials described herein.

[0208] The layer of perovskite material may be characterisable by one or more parameters. For example, the layer of perovskite material may have a specific: absorption spectrum, phase transition temperature, Raman spectrum and / or X-ray diffraction (XRD) spectrum. The determination and / or measurement of these parameters are described further below.

[0209] The absorption spectrum may be measured over a range of 300 to 800 nm using a spectrometer, such as a Cary 300 Bio Spectrometer.

[0210] The phase transition temperature may be measured by differential scanning calorimetry (DSC), e.g. using a NETZSCH DSC 204 F1 calorimeter, where the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. When a solid sample begins to phase transition, more or less heat is required to increase the temperature of the sample at the same rate as the reference, owing to the absorption or release of heat by the sample as it undergoes an endothermic or exothermic transitioning process. Phase transitions are depicted as a peak with either positive or negative heat flow in the DSC thermogram. For the avoidance of doubt, all phase transition temperatures referred to herein were

[0211] 55667482-1 obtained by DSC. To assess the phase transition temperature, the temperature should be varied from below to above the phase transition temperature of the sample. A temperature range of 0 °C to 500 °C or 450 °C may be used, at a ramp rate of, for example, 5 K / min.

[0212] The Raman spectra may be measured over the range of 0 to 500 cm-1using a spectrometer, such as a Renishaw Re 04 Raman spectrometer and microscope.

[0213] The layer of perovskite material may have a particular structure characterisable by a particular X-ray diffraction pattern. For the avoidance of doubt, all X-ray diffraction peaks referred to herein were obtained using a Cu Ka1 source, with an output wavelength of about 1.54 A (e.g. 1.54060 A). Any suitable instrument may be used to obtain an X-ray diffractogram, including, for example, a Bruker D8 Discover (EIGER2R- 500K 2d detector) instrument using Cu Ka1 (A = 1.54060 A). Data may be collected in a range of 5° to 60° for 20, optionally with a step size of 0.02° and optionally with a time step of 1 s, cumulative time per step (2d detector) of 777 s. For the avoidance of doubt, an X-ray diffraction pattern comprises a peak at a specific 20 value when the relative intensity in respect of the largest peak (which has a relative intensity of 100%) is at least 0.5 or 1 %.

[0214] In some embodiments, the layer of perovskite material absorbs light having a wavelength between about 300 and about 750 nm, such as between about 350 nm and about 700 nm or between about 380 nm and about 670 nm. In particular embodiments, the layer of perovskite material has an absorption spectrum comprising an absorption maximum at a wavelength ranging from about 300 to about 750 nm, such as about 350 nm to about 700 nm. Where the layer of perovskite material comprises BA2PbBr4, the layer of perovskite material may have an absorption maximum at a wavelength of about 395 to about 405 nm, such as about 400 nm. Where the layer of perovskite material comprises BA2MAPb2Br?, the layer of perovskite material may have an absorption maximum at a wavelength of about 405 nm to about 480 nm, such as about 450 nm. Where the layer of perovskite material comprises BA2MAPb2l?, the layer of perovskite material may have an absorption maximum at a wavelength of about 550 nm to about 600 nm, such as 590 nm. Where the layer of perovskite material comprises BA2MA3Pb4li3, the layer of perovskite material may have an absorption maximum at a wavelength of about 600 nm to about 670 nm, such as 650 nm. Where the layer of perovskite material comprises (BA)2FAPb2Br?, the layer of perovskite material may have an absorption maximum at a wavelength of about 430 nm to about 470 nm, such as about 453 nm. Where the layer of perovskite material comprises (BA)2(FA)2Pb3Br , the

[0215] 55667482-1 layer of perovskite material may have an absorption maximum at a wavelength of about 430 nm to about 470 nm, such as 458 nm. Where the layer of perovskite material comprises BA2PbBr4, the layer of perovskite material may have an absorption maximum at a wavelength of about 360 nm to about 450 nm, such as 415 nm. Where the layer of perovskite material comprises BA2FAPb2Br?, the layer of perovskite material may have an absorption maximum at a wavelength of about 410 nm to about 510 nm, such as 460 nm. Where the layer of perovskite material comprises BA2FA2PbsBrio, the layer of perovskite material may have an absorption maximum at a wavelength of about 410 nm to about 510 nm, such as 462 nm. Where the layer of perovskite material comprises BA2FAsPb4Bri3, the layer of perovskite material may have an absorption maximum at a wavelength of about 430 nm to about 520 nm, such as 474 nm. Where the layer of perovskite material comprises BA2FA4PbsBri6, the layer of perovskite material may have an absorption maximum at a wavelength of about 460 nm to about 560 nm, such as 511 nm. Figures 3, 39A, 39B, 44 and 54 show exemplary absorption spectra, which have been measured for the layer of perovskite material. These absorption spectra will be described below in more detail.

[0216] In some embodiments, the layer of perovskite material comprises crystalline perovskite material, such as material of the formula BA2MAPb2Br?, with an X-ray diffraction (XRD) pattern comprising two or more, such as five or more or 8 or more, peaks at a 20 ± 0.2 selected from any 4.7, 9.1 , 13.7, 15.4, 18.2, 27.5, 30.6, 32.2, and 41.8, as measured by XRD using a Cu Ka1 source. In some embodiments, the XRD pattern comprises peaks at a 20 ± 0.2 of 13.7 and 27.5. These peaks may have a relative intensity of about 100% and 97%, respectively. In some embodiments, the XRD pattern comprises peaks at a 20 ± 0.2 of 13.7, 27.5, and 30.6. These peaks may have a relative intensity of about 100%, 97%, and 93% respectively. In some embodiments, the XRD pattern comprises peaks at a 20 ± 0.2 of 9.1 , 13.7, 27.5, 30.6, and 32.2. These peaks may have a relative intensity of about 58%, 100%, 97%, 93%, and 64% respectively. In some embodiments, the XRD pattern comprises peaks at a 20 ± 0.2 of 4.7, 9.1 , 13.7,

[0217] 18.2, 27.5, 30.6, and 32.2. These peaks may have a relative intensity of about 47%, 58%, 100%, 45%, 97%, 93%, and 64% respectively.

[0218] In some embodiments, the layer comprises crystalline perovskite material, such as material of the formula BA2PbBr4, with an XRD pattern comprising two or more, such as four or more, or 6 or more, peaks at a 2© ± 0.2 selected from any 6.7, 13.2, 19.7,

[0219] 26.3, 32.9, and 39.6, as measured by XRD using a Cu Ka1 source. In some embodiments, the XRD pattern comprises peaks at a 2© ± 0.2 of 6.7 and 26.3. These

[0220] 55667482-1 peaks may have a relative intensity of about 100% and 96%, respectively. In some embodiments, the XRD pattern comprises peaks at a 20 ± 0.2 of 6.7, 19.7, 26.3, and 32.9. These peaks may have a relative intensity of about 100%, 35%, 96%, and 78%, respectively. In some embodiments, the XRD pattern comprises peaks at a 20 ± 0.2 of 6.7, 13.2, 19.7, 26.3, 32.9, and 39.6. These peaks may have a relative intensity of about 100%, 29%, 35%, 96%, 78%, and 23%, respectively. Figures 4 and 5 show exemplary XRD patterns, which have been measured for the layer of perovskite material. These XRD patterns will be described below in more detail.

[0221] In some embodiments, the layer of perovskite material has or is characterisable by a Raman spectrum comprising one or more peaks and / or shoulder peaks at about 45 to 55, about 75 to 85, about 115 to about 125 and about 135 to about 145 cm-1, such as at about 50, 80, 120 and / or 140 cm-1, as measured by a Raman spectrometer. Where the layer of perovskite material comprises material of the formula BA2PbBr4, the layer may have a Raman spectrum comprising one or more peaks at about 50, 80, and / or 120 cm-1. Where the layer of perovskite material comprises material of the formula BA2MAPb2Br?, the layer of perovskite material may have a Raman spectrum comprising a peak at about 50 cm-1, and one or more shoulder peaks at about 80 and / or 140 cm-1. Figure 7 shows exemplary Raman spectra, which have been measured for the layer of perovskite material. These Raman spectra will be described below in more detail.

[0222] In some embodiments, the layer of perovskite material undergoes a phase transition at temperatures at about or more than 200 K, 250 K, 300 K or 350 K, such as about or more than 300 K or 350 K. Differential scanning calorimetry (DSC) may be used to measure such phase transitions. For example, upon heating the layer, the layer of perovskite material may have a differential scanning calorimetry (DSC) curve comprising a peak, sometimes referred to as an endothermic peak, and / or upon cooling the layer of perovskite material, the DSC curve may comprise a trough, sometimes referred to as an exothermic peak, as measured by a differential scanning calorimeter. Where the layer of perovskite material comprises crystalline material of the formula BA2PbBr4, the DSC curve may comprise an endothermic peak at about or more than 350 K, such as at about 380 to 390 K, e.g. at about 386 K, and / or an exothermic peak at about or more than 350 K, such as at about 375 K to 385 K, e.g. at about 382 K. Where the layer of perovskite material comprises crystalline material of the formula BA2MAPb2Br?, the DSC curve may comprise an endothermic peak at about or more than 340 K, such as at about 345 K to about 355 K, e.g. at about 350 K, and / or an exothermic

[0223] 55667482-1 peak at about or more than 340 K, such as at about 340 K to about 350 K, e.g. at about 346 K.

[0224] In some embodiments, the layer of perovskite material has an enthalpy of transition of from about 9 to about 11 J g-1, or from about 4 to about 6 J g-1, as measured by DSC. Where the layer of perovskite material comprises crystalline material of the formula BA2PbBr4, the layer of perovskite material may have an enthalpy of transition of from about 9 to about 11 J g-1. Where the layer of perovskite material comprises crystalline material of the formula BA2MAPb2Br?, the layer of perovskite material may have an enthalpy of transition of from about 4 to about 6 J g-1.

[0225] In some embodiments, the DSC curves are obtained from samples under a nitrogen environment, and / or from samples held in an aluminium crucible. Figure 8 shows exemplary DSC curves, which have been measured for the layer of perovskite material. These absorption spectra will be described below in more detail.

[0226] Figure 1 B shows a flow chart in overview of a method of forming a layer of perovskite material for use in an energy harvesting device. At stage 2, the method comprises depositing a layer of precursor perovskite material on a support layer, e.g. using a deposition process. In some embodiments, the support layer comprises a substrate layer or substrate. In other embodiments, the support layer comprises a layer of an energy harvesting device, such as an electrode, an electrode transport layer, a hole transport layer, a barrier layer or another layer of the device.

[0227] The precursor perovskite material may comprise a precursor solution or suspension. The precursor solution or suspension comprises one or more perovskite precursors mixed in at least one solvent. For example, the perovskite precursors may be mixed with the solvent and stirred for a duration of time, such as about an hour at room temperature.

[0228] The precursor perovskite material may comprise a salt of LC, such as a halide salt, e.g. a bromide or iodide salt, and may or may not include a salt of SC. The precursor perovskite material may also comprise a salt of M, such as a halide salt, e.g. a bromide or iodide salt. Each X may be provided in the precursor perovskite material as the counterion of an LC, M, and / or (when present) an SC salt. The perovskite precursors may be mixed in any suitable solvent, such as one or more selected from dimethyl sulfoxide, N, N-dimethylformamide, dihydrolevoglucosenone, N-butyl-2-pyrrolidinone, N- octyl-2-pyrrolidone, N-cyclohexyl-3-pyrrolidone, N-benzyl-2-pyrrolidone, ethyl acetate, dichloromethane, tetra hydrofuran, 2-methyl-tetrahydrofuran, 1 ,3, -dioxolane, propylene carbonate, dimethyl carbonate, N-formyl morpholine and acetonitrile.

[0229] 55667482-1 For example, in embodiments where the perovskite material comprises BA2MAPb2Br?, the perovskite precursors comprise n-butylammonium bromide (BABr), methylammonium bromide (MABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 154.05 mg of BABr, 56 mg of MABr, 367.01 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N- Dimethylformamide.

[0230] In embodiments where the perovskite material comprises BA2PbBr4, the perovskite precursors comprise n-butylammonium bromide (BABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N- Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 308.1 mg of BABr, 367.01 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N- Dimethylformamide.

[0231] In embodiments where the perovskite material comprises BA2MAPb2l?, the perovskite precursors comprises n-butylammonium iodide (BAI), methylammonium iodide (MAI) and lead iodide (PI). The solvent comprises anhydrous Dimethyl sulfoxide and N, N-Dimethylformamide. The precursor solution comprises a ratio of BAI: MAI: PI of 2:1 :2 with 3 % of anhydrous Dimethyl sulfoxide and 97 % of N, N-Dimethylformamide.

[0232] In embodiments where the perovskite material comprises BA2MAsPb4li3, the perovskite precursors comprises n-butylammonium iodide (BAI), methylammonium iodide (MAI) and lead iodide (PI) The solvent comprises anhydrous Dimethyl sulfoxide and N, N-Dimethylformamide. The precursor solution comprises a ratio of BAI:MAI:PI of 2:3:4 with 3 % of anhydrous Dimethyl sulfoxide and 97 % of N, N-Dimethylformamide.

[0233] In embodiments where the perovskite material comprises (PEA)2PbBr4, the perovskite precursors comprise Phenethylammonium bromide (PEABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 202.9 mg of PEABr, 367.01 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N-Dimethylformamide.

[0234] In embodiments where the perovskite material comprises (PEA)2MAPb2Br?, the perovskite precursors comprise Phenethylammonium bromide (PEABr), methylammonium bromide (MABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 202.9 mg of PEABr, 111.97 mg of MABr,

[0235] 55667482-1 367.01 mg of PbBr2, 30|JL of anhydrous Dimethyl sulfoxide and 970 pL of N, N- Dimethylformamide.

[0236] In embodiments where the perovskite material comprises (PEA)2MA2PbsBrio or (PEA)2MAsPb4Bri4, the perovskite precursors comprise Phenethylammonium bromide (PEABr), methylammonium bromide (MABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 101.45 mg of PEABr, 55.99 mg of MABr, 183.51 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N- Dimethylformamide.

[0237] In embodiments where the perovskite material comprises (BA)2FAPb2Br?, the perovskite precursors comprise n-butylammonium bromide (BABr), formamidinium bromide (FABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 308.1 mg of BABr, 124.97 mg of FABr, 734.02 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N-Dimethylformamide.

[0238] In embodiments where the perovskite material comprises (BA)2(FA)2PbsBrio, the perovskite precursors comprise n-butylammonium bromide (BABr), formamidinium bromide (FABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 308.1 mg of BABr, 249.94 mg of FABr, 1101.3 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N-Dimethylformamide.

[0239] In embodiments where the perovskite material comprises (BA)2(FA)sPb4Bri3, the perovskite precursors comprise n-butylammonium bromide (BABr), formamidinium bromide (FABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 308.1 mg of BABr, 374.91 mg of FABr, 1468.04 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N-Dimethylformamide.

[0240] In embodiments where the perovskite material comprises (BA)2(FA)4Pb5Bri6, the perovskite precursors comprise n-butylammonium bromide (BABr), formamidinium bromide (FABr) and lead bromide (PbBr2). The solvent comprises anhydrous dimethyl sulfoxide (DMSO) and N, N-Dimethylformamide (DMF). For example, 1 mL precursor solution comprises 154 mg of BABr, 249.94 mg of FABr, 917.5 mg of PbBr2, 30pL of anhydrous Dimethyl sulfoxide and 970 pL of N, N-Dimethylformamide.

[0241] The method may comprise preparing the precursor solution or suspension in a nitrogen environment, such as nitrogen filled compartment or box.

[0242] 55667482-1 At stage 4, the method comprises annealing the deposited layer of precursor perovskite material at a temperature of about 25 °C to about 125 °C. The deposited layer of precursor perovskite material may be annealed at one or more temperatures, such as 25 °C, 50 °C and / or 100 °C. For example, in embodiments where the perovskite material comprises BA2MAPb2Br?or BA2PbBr4, the temperature may be about 25 °C, 50 °C and / or 100 °C. In embodiments where the perovskite material comprises BA2MAPb2l? or BA2MA3Pb4li3, the deposited layer of precursor perovskite material may be annealed at one or more temperatures, such as 25 °C, 50 °C and / or 100 °C. In embodiments where the perovskite material comprises (PEA)2PbBr4, (PEA)2MAPb2Br?, (PEA)2MA2Pb3Br , or (PEA)2MA3Pb4Bri4, the deposited layer of precursor perovskite material may be annealed at one or more temperatures, such as 50 °C. It will be appreciated that in other embodiments, the deposited layer of precursor perovskite material may be annealed at one or more different temperatures, such as 25 °C and / or 100 °C.

[0243] In embodiments where the perovskite material comprises (BA)2(FA)Pb2Br?, (BA)2(FA)2Pb3Brio, (BA)2(FA)3Pb4Bri3 or (BA)2(FA)4Pb5Bri6the deposited layer of precursor perovskite material may be annealed at one or more temperatures, such as about 25 °C (which may also be referred to as room temperature). It will be appreciated that in other embodiments, the deposited layer of precursor perovskite material may be annealed at one or more different temperatures, such as 50 °C and / or 100 °C.

[0244] The deposited layer of perovskite precursor perovskite material may be annealed for a time duration, e.g. between 2 minutes and 15 minutes, such as between 3 minutes and 10 minutes. The deposited layer of perovskite precursor perovskite material may be annealed at one or more temperatures. At each temperature, the deposited layer of perovskite precursor perovskite material may be annealed for a duration of time. For example, in some embodiments, the deposited layer of precursor perovskite material may be annealed at a first temperature for a first duration, at a second temperature for a second duration, and / or at a third temperature for a third duration. In some embodiments, the first temperature is about 25 °C and the first duration is less than 10 minutes. The second temperature is about 50 °C and the second duration is less than 5 minutes, such as about 3 minutes. The third temperature is about 100 °C and the third duration is less than 5 minutes, such as about 3 minutes. It will be appreciated that in other embodiments, the first, second and / or third temperature and / or the first, second and / or third duration may be different from those described above.

[0245] The deposited layer of precursor perovskite material may be annealed at a pressure between about 0.01 bar and about 1 bar, such as about 0.1 bar. This may allow

[0246] 55667482-1 for removal of the solvents, e.g. complete removal of the solvents, aid the formation of a uniform layer or film and / or increase a crystallisation rate.

[0247] The deposited layer of perovskite precursor perovskite material may be annealed at one or more pressures. At each pressure, the deposited layer of perovskite precursor perovskite material may be annealed for a duration of time. For example, in some embodiments, for the first duration, the deposited layer of precursor perovskite material is annealed at a pressure of 0.1 bar. The deposited layer of precursor perovskite material is annealed at the pressure of 0.1 bar for a part of the second and third durations, such as 1 minute. For the remainder of the second and third durations, the deposited layer of precursor perovskite material is annealed at atmospheric pressure, e.g. about 1 bar. For the remainder of the second and third durations, the deposited layer of precursor perovskite material may be annealed in the nitrogen environment mentioned above. It will be appreciated that in other embodiments, the pressure and / or the first, second and / or third duration may be different from those described above.

[0248] The method described above may allow for a facilitated formation of the layer of perovskite material, e.g. at a low temperature, such as a temperature between about 25 °C to about 125 °C.

[0249] Figure 2 schematically illustrates an exemplary deposition process that may be used in the method shown in Figure 1 B, e.g. for depositing the layer of precursor perovskite material on the support layer. In this embodiment, the deposition process comprises a blade coating process. This process may allow for a scalable formation of the layer of perovskite material.

[0250] The blade coating process comprises depositing the precursor perovskite material 6 on the support layer 8. The blade coating process comprises moving or passing a blade 10 over the support layer 8, e.g. to spread the precursor perovskite material evenly across a surface of the support layer 8. A speed of the blade 10 may be between about 1 mm / s and 30 mm / s. For example, a volume of 10 pL of precursor perovskite material may be deposited on the support layer 8 with a speed of the blade 10 of about 15 mm / s. The blade coating process may be performed in an ambient air environment. This may facilitate a manufacture of layer of perovskite material.

[0251] It will be appreciated that the deposition process described herein is not limited to the blade coating process. For example, in other embodiments, the deposition process may comprise a spin coating process. Spin coating may be understood as a process comprising the use of centrifugal force to spread a solution or suspension, such as the

[0252] 55667482-1 precursor solution or suspension, evenly across a surface, for example, for depositing the layer of precursor perovskite material.

[0253] In embodiments, where BA2MAPb2l? or BA2MAsPb4li3 is formed, the precursor perovskite material may be deposited on the support layer using the spin coating process. For example, a volume of about 100 pL of precursor perovskite material may be statically deposited on the support layer and the spin coating process may be started. The support layer may be rotated at a speed between 500 rpm and 1500 rpm, such as about 1000 rpm, to spread the precursor perovskite material on the support layer. An acceleration or ramp rate used in the spin coating process may be between 800 rpm / s and 1500 rpm / s, such as about 1000 rpm / s. A duration of the spin coating process may be between 5 seconds and 50 seconds, such as about 30 seconds.

[0254] Figure 3 shows absorbance spectra of exemplary layers of perovskite material. In the example shown in Figure 3, the absorbance spectra were measured for BA2MAPb2Br?, which is labelled as n=2 in Figure 3, and BA2PbBr4, which is labelled as n=1 in Figure 3. The absorbance spectra may also be referred to as US-Vis spectra. In this example, each layer of perovskite material was deposited on a substrate comprising glass.

[0255] Each absorbance spectrum has an absorption edge, which is indicated by the dashed line in Figure 3. The absorption edge for BA2PbBr4was measured at about 400 nm, which results in a bandgap of about 2.97 eV. The absorption edge for BA2MAPb2Br? was measured at about 450 nm, which results in a bandgap of about 2.76 eV. This may allow for use of the layers of perovskite materials in indoor and / or outdoor photovoltaic applications. The absorbance spectra shown in Figure 3 may be indicative of the layers of perovskite material comprising photovoltaic properties. No defect related emission peaks can be observed in the absorbance spectra shown in Figure 3. This may be indicative of a high quality of the layers of BA2MAPb2Br? and BA2PbBr4.

[0256] Figure 4 shows an X-ray diffraction pattern that has been measured for an exemplary layer of perovskite material. In the example shown in Figure 4, the X-ray diffraction pattern has been measured for BA2MAPb2Br?. A full width at half maximum (FWHM) of 0.06° for (600) hkl planes was observed. By comparing the X-ray diffraction pattern shown in Figure 4 with that of epitaxial perovskite oxide layers, it can be determined that the layer of BA2MAPb2Br? crystallises in an orthorhombic system with a polar space group of Cmc2i, e.g. when the deposited precursor perovskite material is annealed at about 50 °C, as described above.

[0257] 55667482-1 Figure 5 shows an X-ray diffraction pattern that has been measured for another exemplary layer of perovskite material. In the example shown in Figure 5, the X-ray diffraction pattern has been measured for a layer of BA2PbBr4. By comparing the X-ray diffraction spectrum shown in Figure 5 with that of the corresponding single crystal, which is described in Lin Li et al. “Two-Dimensional Hybrid Perovskite-Type Ferroelectric for Highly Polarization-Sensitive Shortwave Photodetection,’’ J. Am. Chem. Soc. 2019, 141 , 6, 2623-2629, it can be determined that the layer of BA2PbBr4 crystallises in an orthorhombic system with a polar space group of Cmc2i, e.g. when the deposited precursor perovskite material is annealed at about 50 °C, as described above. In the examples, shown in Figures 4 and 5, the layers of perovskite material were deposited on respective layers of aluminium, which were formed on respective glass substrates.

[0258] Figures 6A and 6B show scanning electron images of exemplary layers of perovskite material. The scanning electron images were obtained using a scanning electron microscope, such as a HITACHI S-4800 scanning electron microscopy instrument. In the example shown in Figure 6A, the scanning electron image was obtained for a layer of perovskite material comprising BA2PbBr4. In the example shown in Figure 6B, the scanning electron image was obtained for a layer of perovskite material comprising BA2MAPb2Br?. In the examples shown in Figures 6A and 6B, the layers of perovskite material comprising BA2MAPb2Br? and BA2PbBr4 were formed on the same substrates as those described in relation to Figures 4 and 5. The deposited precursor perovskite material was annealed at about 50 °C for each of BA2MAPb2Br? and BA2PbBr4, as described above. Figures 6A and 6B show large domains of both BA2MAPb2Br? and BA2PbBr4 layers. This may support the layered structure of these perovskite materials. The layered structure of the layer of perovskite material comprising BA2MAPb2Br? is more compact relative to the layered structure of the layer of perovskite material comprising BA2PbBr4. Domain features can be better seen in the scanning electron image of the layer of perovskite material comprising BA2MAPb2Br? compared to the scanning electron image of the layer of perovskite material comprising BA2PbBr4. The scanning electron image of the layer of perovskite material comprising BA2PbBr4 shows that the domains are larger in size than the domains shown in the scanning electron image of the layer of perovskite material comprising BA2MAPb2Br?.

[0259] Figure 7 shows Raman spectra that have been measured for exemplary layers of perovskite material. In the example shown in Figure 7, the Raman spectra have been measured for a layer of perovskite material comprising BA2PbBr4, which is indicated by

[0260] 55667482-1 the label “n1” in Figure 7, and a layer of perovskite material comprising BA2MAPb2Br?, which is indicated by the label “n2” in Figure 7.

[0261] The Raman spectra may be used to probe structural dynamics of the small and large cations’ orientational, conformational and rotational motion and how the associated motion may influence the Pb-X bonding and the PbXe units. The Raman spectra show a number of peaks below 200 cm-1. The peaks may also be referred to as modes. These modes can be associated with vibrations or translations of the inorganic sub-lattice, such as Pb-X vibrations. These modes can also be associated with translational and rotational motion of the small cation coupled to the Pb-X sub-lattice. Large cation LC related vibrations may occur at wavenumbers greater than 300 cm-1.

[0262] Figure 7 shows a comparison of the Raman spectra measured for layers of perovskite material comprising BA2MAPb2Br?and BA2PbBr4, which have been formed by annealing the respective layers of precursor perovskite material at a temperature of about 50 °C. The sharp mode at about 50 cm-1has been previously been observed for 3-dimensional MAPbBrs. The mode at 80 cm-1may be associated with Pb-X bending vibrations in the layered structure of the layers of perovskite material. The mode at about 50 cm-1, which is present in the Raman spectra for both layers, may be due to strong local polar fluctuations in the Pb-X unit, which can be anharmonic. The Raman peaks at about 80 cm-1for the layer of perovskite material comprising BA2PbBr4 appears to be sharper and more prominent compared to the corresponding peak for the layer of perovskite material comprising BA2MAPb2Br?. This may be due to better-defined vibrational states for a plurality of [PbX4]2' units, which are present in BA2PbBr4. A weak bond between the large cation LC chains and [PbX4]2' unit, the absence of hydrogen bonding between the small cation and the [PbX4]2unit, and a stronger lateral bonds between the corner shared [PbX4]2' units may enhance vibrational modes of Pb-Br bonds for the layer of perovskite material comprising BA2PbBr4. This may lead to the sharper Raman peak at ~ 80 cm-1for this layer of perovskite material. The shoulder peak at 120 cm-1and broad feature at 143 cm-1may be due to Pb-X stretching, e.g. out of plane stretching, coupled with movement of the large cation. The 120 cm-1shoulder peak is not clearly visible in the Raman spectra for the layer of perovskite material comprising BA2MAPb2Br?, which was formed at 50 °C, but may be observed in a Raman spectrum (not shown) measured for a layer of perovskite material comprising BA2MAPb2Br? layer, which may be formed by annealing the precursor perovskite materials at a temperature of about 25 °C. In such Raman spectrum, a broadening of this peak can be observed. This may be an indication that these structures are highly dynamic and may be due to a

[0263] 55667482-1 reorientation of the small cations. This reorientation may produce a dynamic disorder distributed across the sub-lattice, or distribution of dynamic equilibrium positions or strain induced deformation of the Pb-X sub-lattice. This in turn may result in the perovskite material having ferroelectric properties. The 120 cm-1shoulder peak may also be related to the coupled polar anharmonic vibrations or dynamic disorder involving multiple polar vibrational modes.

[0264] Figure 8 shows graphs obtained by differential scanning calorimetry (DSC) measurements for exemplary layers of perovskite material. In the example shown in Figure 8, the DSC measurements have been performed on a powder of perovskite material comprising BA2PbBr4, which is indicated by the label “n=1” in Figure 8, and a powder of perovskite material comprising BA2MAPb2Br?, which is indicated by the label “n=2” in Figure 8. Each powder was formed by depositing a layer of perovskite material on a respective substrate, which in this embodiment comprises a glass substrate. Subsequent to the deposition, each layer of perovskite material was scraped off the substrate to form the respective powder. DSC measurements may be performed to understand the thermodynamic structural phase transition behaviour of the layers of perovskite material comprising BA2MAPb2Br? and BA2PbBr4. The DSC measurements were performed in a cooling cycle and a heating cycle. DSC measures the energy absorbed or released during the heating cycle or the cooling cycle. It may be provide information regarding a phase change, crystallisation and / or melting of the layers of perovskite material comprising BA2MAPb2Br? and BA2PbBr4. In this example, DSC measurements were carried out using NETZSCH DSC 204 F1 equipment with the DSC 204F1 t-sensor / E in a nitrogen environment.

[0265] In Figure 8, a reversible phase transition can be observed at around 350 K for the BA2MAPb2Br? layer, when cooling and heating this layer through a temperature range of -20 °C to 105 °C. Above a temperature of about 350K, a crystal structure of the layer of perovskite material comprising BA2MAPb2Br? may transform to a centrosymmetric structure with a space group of Cmca. Such a crystal structure may exhibit reduced or no piezoelectric and ferroelectric properties. As such, in some embodiments, the precursor perovskite material may be annealed at a temperature below 355 K (about 82 °C) to form a BA2MAPb2Br? layer comprising piezoelectric and / or ferroelectric properties. However, it will be appreciated that in some embodiments, the annealing temperature may be above 82 °C, such as about 100 °C, as described herein. As will be described below, in some embodiments, at an annealing temperature above 82 °C, the layer of perovskite material may still exhibit piezoelectric and / or ferroelectric properties.

[0266] 55667482-1 In Figure 8, a reversible phase transition can be observed at around 385 K for the layer of perovskite material comprising BA2PbBr4, when cooling and heating this layer through a temperature range of -20 °C to 105 °C. The higher phase transition temperature measured for BA2PbBr4 compared to that measured for BA2MAPb2Br? may be due to a high binding energy in its layered structure. As shown in Figure 8, the enthalpy of transition is found to be higher (9.5 and 10.6 J / g) for the layer of perovskite material comprising BA2PbBr4 compared to the layer of perovskite material comprising BA2MAPb2Br? (5.84 and 4.98 J / g). This may be correlated to a higher formation energy of lower n-phases in the BA2PbBr4 layer.

[0267] The measurements described above in relation to Figures 3 to 8 were performed on a layer of perovskite material comprising BA2PbBr4 having a thickness of about 1 pm and a layer of perovskite material comprising BA2MAPb2Br? having a thickness of about 360 nm.

[0268] Figure 9 schematically shows an exemplary energy harvesting device 12 according to an embodiment of the present disclosure. The device 12 comprises a first electrode 14. The device 12 comprises a second electrode 16. The first and second electrodes 14, 16 each comprise a conducting material. In some embodiments, the conducting material comprises a metal material, such as aluminium, gold, nickel, another metal material or a combination of metal materials. In other embodiments, the conducting material comprises a transparent conducting oxide, such as indium tin oxide, fluorinedoped tin oxide (FTO), carbon, aluminium-doped zinc oxide (AZO) or another transparent conducting oxide. The first and second electrode may comprise the same conducting material or a different conducting material.

[0269] The device 12 comprises a layer of perovskite material 18. The layer of perovskite material 18 may comprise any of the features of the layer of perovskite material described above. The layer of perovskite material 18 is arranged between the first and second electrode 14, 16.

[0270] The device 12 also comprises a substrate layer 20. The substrate layer may comprise a glass substrate, a polymer substrate, or another substrate. The polymer substrate may comprise a Polyethylene terephthalate (PET) substrate or another polymer substrate. The polymer substrate may allow for the formation of a flexible energy harvesting device. For example, when the device is used as a piezoelectric device, the polymer substrate may prevent or reduce damage to the device.

[0271] In this embodiments, the first electrode 14 is arranged between the substrate layer 20 and the layer of perovskite material 18. However, it will be appreciated that in

[0272] 55667482-1 other embodiments, the second electrode may be arranged between the substrate layer and the layer of perovskite material.

[0273] As described above, the layer of perovskite material 18 has at least piezoelectric and / or ferroelectric properties. The device 12 can be a ferroelectric device or a piezoelectric device.

[0274] In embodiments where the device 12 is a piezoelectric device, a mechanical force on the layer of perovskite material 18 can generate a plurality of charge carriers. The charge carriers may diffuse to the first and second electrodes 14, 16 for extraction as electrical energy.

[0275] In embodiments where the device 12 is a ferroelectric device, a mechanical force on the layer of perovskite material 18 and / or a temperature change of the layer of perovskite material 18 can generate a plurality of charge carriers. The charge carriers may diffuse to the first and second electrodes for extraction as electrical energy. This is because a ferroelectric material has the ability to maintain a polarisation, the orientation of which can be electrically switched. The ferroelectric material has piezoelectric properties and pyroelectric properties. As such, applying a mechanical force to and / or changing a temperature of the ferroelectric material can change a magnitude of the polarisation the ferroelectric material, which can generate a plurality of charge carriers. The charge carriers may diffuse to the first and second electrodes 14, 16 for extraction as electrical energy.

[0276] Figure 10 schematically shows another exemplary energy harvesting device. The device shown in Figure 10 may have any of the features of the device 12 shown in Figure 9. In this embodiment, the device 12 comprises a barrier layer 22. The barrier layer 22 is arranged between the layer of perovskite material 18 and the second electrode 16. The barrier layer 22 may be configured to reduce a diffusion of halide atoms from the layer of perovskite material 18 to the second electrode 16 and / or reduce a reaction between the halide atoms and the conducting material of the second electrode 16. For example, halide atoms diffusing from the layer of perovskite material 18 may react with a metal material of the second electrode 16 to form irreversible metal-halide compounds. Additionally or alternatively, metal atoms may diffuse from the second electrode 16 into the layer of perovskite material 18, thereby acting as device short-circuiting paths and / or recombination centres. However, by arranging the barrier layer 22 between the layer of perovskite material 18 and the second electrode 16, the formation of the device shortcircuiting paths and / or recombination centres may be reduced or prevented.

[0277] 55667482-1 In some embodiment, the barrier layer 22 comprise a metal material, such as chromium, barium, chromium oxide, preferably chromium trioxide (C^Os), another metal material or a combination thereof.

[0278] In some embodiment, the barrier layer 22 comprises a polymer material. The polymer material may comprise PCBM (phenyl-Cei butyric acid methyl ester, or PCeiBM), PMMA (polymethyl methacrylate), SEBS (styrene ethylene butylene styrene), SEBS-g-MA (polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene-graft- maleic anhydride), PET (polyethylene terephthalate), PDMS (polydimethylsiloxane), PEN (polyethylene naphthalate), PS (polystyrene), parylene, PVDF (polyvinylidene difluoride), and any suitable co-polymers. Often, the polymer material comprises PCBM, PDMS, PET, SEBS, and SEBS-g-MA. In embodiments where the layer of perovskite material 18 comprises BA2PbBr4, or BA2MAPb2Br?, the polymer material may comprise PCBM, PDMS, or PET. In embodiments where the layer of perovskite material 18 BA2MAPb2l?, or BA2MAsPb4li3, the polymer material may comprise PDMS, PET, SEBS, or SEBS-g-MA. In embodiments where the device 12 comprises a piezoelectric device, the polymer material may comprise PET, PDMS, SEBS, or SEBS-g-MA.

[0279] Although the barrier layer is shown in Figure 10 as being arranged between the layer of perovskite material 18 and the second electrode 16, it will be appreciated that in other embodiments, the barrier layer may be arranged between the layer of perovskite material and the first electrode.

[0280] Figure 11A schematically shows another exemplary energy harvesting device. In this embodiment, the device 12 is provided in the form of a photovoltaic device. The photovoltaic device may also be referred to as solar cell.

[0281] The device 12 comprises an electron transport layer 24. In this embodiment, the electron transport layer 24 is arranged between the first electrode 14 and the layer of perovskite material 18. The electron transport layer 24 may be configured to facilitate and / or increase a transport of electrons generated in the layer of perovskite material 18 to the first electrode 14 and / or to reduce a recombination of the charge carriers. The electron transport layer 24 may comprise an oxide material, such as tin oxide (SnC>2), titanium dioxide (TiC>2), zinc oxide (ZnO) or the like.

[0282] The device 12 comprises a hole transport layer 26. In this embodiment, the hole transport layer 26 is arranged between the layer of perovskite material 18 and the second electrode 16. The hole transport layer 26 may be configured to facilitate the extraction and / or transport of holes generated in the layer of perovskite material 18 to the second electrode 16 and / or to prevent the transport of electrons generated in the layer of

[0283] 55667482-1 perovskite material 18 to the second electrode 26. The hole transport layer 26 may comprise an organic compound, such as Spiro-MeOTAD (CsiHesl^UOs) or the like, or a polymer material, such as Poly(3-hexylthiophene) (P3HT), Poly[bis(4-phenyl)(2,4,6- trimethylphenyl)amine (PTAA), Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (PolyTPD) or the like. The device 12 shown in Figure 11A comprises a n-i-p structure.

[0284] Figure 11 B schematically shows another exemplary energy harvesting device. In this embodiment, the device 12 is provided in the form of another exemplary photovoltaic device.

[0285] The device 12 comprises an electron transport layer 24. In this embodiment, the electron transport layer 24 is arranged between the second electrode 16 and the layer of perovskite material 18. The electron transport layer 24 may be configured to facilitate and / or increase a transport of electrons generated in the layer of perovskite material 18 to the second electrode 16 and / or to reduce a recombination of the charge carriers. The electron transport layer 24 may comprise a polymer material, such as PCBM or any other suitable polymer material. The electron transport layer 24 may comprise a hole blocking layer. The hole blocking layer may comprise an organic compound, such as bathocuproine (BCP) or any other suitable material or organic compound. Additionally or alternatively, the electron transport layer 24 may comprise an oxide material, such as tin oxide (SnC>2), titanium dioxide (TiC>2), zinc oxide (ZnO) or the like.

[0286] The device 12 comprises a hole transport layer 26. In this embodiment, the hole transport layer 26 is arranged between the layer of perovskite material 18 and the first electrode 14. The hole transport layer 26 may be configured to facilitate the extraction and / or transport of holes generated in the layer of perovskite material 18 to the first electrode 14 and / or to prevent the transport of electrons generated in the layer of perovskite material 18 to the first electrode 14. The hole transport layer 26 may comprise a hole transport polymer material, such as Poly(N,N'-bis-4-butylphenyl-N,N'- bisphenyl)benzidine and Poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt- 2,7-(9,9-dioctylfluorene)] (Poly TPD / PFN), Poly(3-hexylthiophene) (P3HT) or another suitable polymer material, organic compound or the like. In some embodiments, the hole transport layer may comprise (2-(9H-carbazol-9-yl)ethyl)phosphonic acid, another example of a hole transport polymer material. This polymer may self-assemble to form a self-assembled monolayer (SAM). The device 12 shown in Figure 11 B comprises a p- i-n structure.

[0287] Figure 12 shows a flow chart in overview of a method of forming an energy harvesting device according to an embodiment of the present disclosure.

[0288] 55667482-1 At stage 30, the method comprises forming or providing a first electrode. The first electrode may comprise any of the features of the first electrode 14 described above. The first electrode 14 may be formed on a substrate layer. The substrate layer may comprise any of the features of the substrate layer 20 described above.

[0289] At stage 32, the method comprises forming a layer of perovskite material. The layer of perovskite material may comprise any of the features of the layer of perovskite material 18 described above. The layer of perovskite material 18 is formed according to the method described in relation to Figure 1 B.

[0290] At stage 34, the method comprises forming or providing a second electrode. The second electrode may comprise any of the features of the second electrode 16. The layer of perovskite material 18 is formed between the first and second electrodes 14, 16.

[0291] The method of forming the device 12 will be described in the following in more details in relation to exemplary embodiments of the device 12.

[0292] In a first exemplary embodiment, the device 12 comprises the ferroelectric device mentioned above.

[0293] In some embodiments, the layer of perovskite material 18 comprises BA2MAn- iPbnBr3n+i with n=1 or n=2. When n=1 , the layer of perovskite material 18 comprises BA2PbBr4. When n=2, the layer of perovskite material 18 comprises BA2MAPb2Br?. A thickness of the layer of perovskite material may be about 1 .2 pm.

[0294] In some embodiments, the layer of perovskite material 18 comprises (PEA^MAn- iPbnX3n+i with n=1 , n=2, n=3 and n=4. When n=1 , the layer of perovskite material 18 comprises (PEA)2PbBr4. When n=2, the layer of perovskite material comprises (PEA)2MAPb2Br?. When n=3, the layer of perovskite material 18 comprises (PEA)2MA2Pb3Brio. When n=4, the layer of perovskite material 18 comprises (PEA)2MA3Pb4Bri4. A thickness of the layer of perovskite material may be between about 900 nm and about 2 pm. in some embodiment, the layer of perovskite material 18 comprises (BA)2(FA)n- iPbnBr3n+i with n=2, n=3, n=4 and n=5. When n=2, the layer of perovskite material 18 comprises (BA)2(FA)Pb2Br?. When n=3, the layer of perovskite material 18 comprises (BA)2(FA)2Pb3Brio. When n=4, the layer of perovskite material 18 comprises

[0295] (BA)2(FA)3Pb4Bri3. When n=5, the layer of perovskite material 18 comprises

[0296] (BA)2(FA)4PbsBri6. A thickness of the layer of perovskite material may be between about 2.5 pm.

[0297] At stage 30, the first electrode 14 is formed on the substrate layer 20. The substrate layer 20 comprises a glass substrate. Prior to forming the first electrode 14,

[0298] 55667482-1 the substrate layer 20 is cleaned using an ultrasonic cleaning process and / or a UV-ozone treatment process. For example, ultrasonic cleaning process may be sequentially performed in a number of cleaning liquids. For example, the number of cleaning liquids may comprise sodium dodecyl sulphate (SDS), deionized water, acetone, and isopropyl alcohol. The substrate layer 20 may be exposed to UV-ozone for a selected duration. This may reduce or remove one or more surface impurities from a surface of the substrate layer 20. In this embodiment, a duration of the UV-ozone treatment may be selected to be around 15 minutes.

[0299] The first electrode 14 is formed on the substrate layer 20 using a deposition process, such as a thermal evaporation process or another deposition process. The conductive material may be deposited on the substrate layer 20 at a pressure of about 2x1 O'6mbar. A thickness of the first electrode may be between about 50 nm and 200 nm, such as about 100 nm. In this embodiment, the conductive material comprises aluminium. However, it will be appreciated that in other embodiments another conductive material may be used. The first electrode 14 may be plasma ashed prior to depositing the layer of perovskite material thereon. This may increase a wettability of the precursor perovskite material on the first electrode 14. This in turn may lead to an increased or complete coverage of the first electrode 14 by the layer of perovskite material, which may prevent short-circuiting of the device 12.

[0300] At stage 32, the layer of perovskite material 18 is formed on the first electrode 14 as described above in relation to Figure 1 B. In this embodiment, the deposition process comprises the blade coating process described above. In this embodiment, a thickness of the layer of perovskite material 18 is about 1 .2 pm.

[0301] At stage 34, the second electrode 16 is formed on the layer of perovskite material 18. The second electrode 16 is formed on the layer of perovskite material 18 using a deposition process, such as a thermal evaporation process or another deposition process. The conductive material may be deposited on the layer of perovskite material 18 at a pressure of about 4x1 O'6mbar. This pressure may also be referred to as a working pressure. Prior to the deposition of the conductive material, a pressure in an evaporation chamber may be about 2x1 O'6mbar. This pressure may also be referred to as a base pressure. A thickness of the second electrode 16 may be between about 50 nm and 200 nm, such as about 100 nm. In this embodiment, the conductive material comprises aluminium. However, it will be appreciated that in other embodiments another conductive material may be used. The above stages may be used to form the device 12 shown in Figure 9.

[0302] 55667482-1 In a second exemplary embodiment, the method may comprise any of the steps described in relation to the first exemplary embodiment. Additionally, the method comprises forming a barrier layer, such as the barrier layer 22, between at least one of the first and second electrode 14, 16 and the layer of perovskite material 18, e.g. to form the device 12 shown in Figure 10. For example, the barrier layer 22 may be formed between the second electrode 16 and the layer of perovskite material 18. The barrier layer 22 may be formed on the layer of perovskite material 18 using a deposition process, such as a thermal evaporation process or another deposition process.

[0303] In some embodiments, the barrier layer comprises a metal material, such as chromium. The metal material may be deposited on the layer of perovskite material 18 at a pressure of about 4x1 O'6mbar. This pressure may also be referred to as a working pressure. Prior to deposition of the metal material, a pressure in an evaporation chamber may be about 2x1 O'6mbar. This pressure may also be referred to as a base pressure. A thickness of the barrier layer 22 may be between about 10 nm and 50 nm, such as 20 nm. Oxygen may be present during the deposition of the metal material. In this embodiment, the presence of oxygen may result in a barrier layer comprising chromium and / or Cr2C>3. It will be appreciated that in other embodiments, the barrier layer may comprise another metal material.

[0304] In some embodiments, the barrier comprises a polymer material, such as PCBM. The method may comprise forming a barrier layer solution or suspension. For example, the PCBM solution or suspension may be prepared by mixing PCBM into a solvent, such as chlorobenzene (CB). This solution or suspension may be stirred for a duration of time, such as 4 hour to 5 hours, at a temperature of about 60 °C. The method may comprise filtering the barrier layer solution or suspension, e.g. using a filter, such as a 0.2 pm filter, prior to depositing the polymer material on the layer of perovskite material 18. It will be appreciated that in other embodiments, the barrier layer may comprise another polymer material.

[0305] In a third exemplary embodiment, the device 12 comprises the ferroelectric device mentioned above in relation to the first or second exemplary embodiment. In this embodiment, the layer of perovskite material 18 comprises BA2MAn-iPbnl3n+i with n=2 or n=4. When n=2, the layer of perovskite material 18 comprises BA2MAPb2l7. When n=4, the layer of perovskite material 18 comprises BA2MA3Pb4li3.

[0306] In this embodiment, stages 30 and 34 are the same as described in relation to the first exemplary embodiment.

[0307] 55667482-1 At stage 32, the layer of perovskite material 18 is formed on the first electrode 14 as described above in relation to Figure 1 B. In this embodiment, the deposition process comprises the spin coating described above. A thickness of the layer of perovskite material 18 is between about 300 nm and 500 nm.

[0308] In a fourth exemplary embodiment, the device 12 comprises the piezoelectric device mentioned above.

[0309] In some embodiments, the layer of perovskite material 18 comprises BA2MAn- iPbnBr3n+i with n=1 or n=2. When n=1 , the layer of perovskite material 18 comprises BA2PbBr4. A thickness of the layer of perovskite material is about 350 nm. When n=2, the layer of perovskite material 18 comprises BA2MAPb2Br?. A thickness of the layer of perovskite material is about 1.2 pm.

[0310] In some embodiments, the layer of perovskite material 18 comprises (PEA^MAn- iPbnX3n+i with n=1 , n=2, n=3 and n=4. When n=1 , the layer of perovskite material 18 comprises (PEA)2PbBr4. When n=2, the layer of perovskite material comprises (PEA)2MAPb2Br?. When n=3, the layer of perovskite material 18 comprises (PEA)2MA2Pb3Brio. When n=4, the layer of perovskite material 18 comprises (PEA)2MA3Pb4Bri4. A thickness of the layer of perovskite material may be between about 900 nm and about 2 pm.

[0311] In some embodiment, the layer of perovskite material 18 comprises (BA)2(FA)n- iPbnBr3n+i with n=2, n=3 and n=4. When n=2, the layer of perovskite material 18 comprises (BA)2(FA)Pb2Br?. When n=3, the layer of perovskite material 18 comprises (BA)2(FA)2Pb3Brio. When n=4, the layer of perovskite material 18 comprises

[0312] (BA)2(FA)3Pb4Bri3. When n=5, the layer of perovskite material 18 comprises

[0313] (BA)2(FA)4PbsBri6. A thickness of the layer of perovskite material may be between about 2.5 pm.

[0314] At stage 30, the first electrode 14 is provided. In this embodiment, the first electrode is patterned or pre-patterned on the substrate layer 20. The conductive material of the first electrode 14 comprise a transparent conductive oxide, such as indium tin oxide. The substrate layer 20 comprises a polymer substrate layer, such as a PET substrate. The polymer substrate may be a flexible substrate layer. However, it will be appreciated that in other embodiments, the first electrode may comprise another transparent conductive oxide material and / or the substrate may comprise another polymer material, a glass material or another substrate.

[0315] Prior to stage 32, the substrate layer 20 and the first electrode 14 provided thereon are cleaned using an ultrasonic cleaning process and / or a UV-ozone treatment

[0316] 55667482-1 process. For example, ultrasonic cleaning process may be sequentially performed in a number of cleaning liquids. For example, the number of cleaning liquids may comprise sodium, deionized water, acetone, and isopropyl alcohol. The substrate layer 20 the first electrode 14 provided thereon may be exposed to UV-ozone for a selected duration. This may reduce or remove one or more surface impurities, such as organic contaminants, from a surface of the substrate layer 20 and the first electrode 14 and / or increase a wettability of the substrate layer 20 and the first electrode 14. In this embodiment, a duration of the UV-ozone treatment is selected to be around 15 minutes. However, it will be appreciated that in other embodiments, another duration of the UV-ozone treatment may be used.

[0317] At stage 32, the layer of perovskite material 18 is formed on the first electrode 14 as described above in relation to Figure 1 B. In this embodiment, the deposition process comprises the blade coating process described above.

[0318] At stage 34, the second electrode 16 is provided. In this embodiment, the second electrode is patterned or pre-patterned on a further substrate layer. The conductive material of the second electrode 16 comprise a transparent conductive oxide, such as indium tin oxide. The further substrate comprises a polymer substrate layer, such as a PET substrate. However, it will be appreciated that in other embodiments, the second electrode may comprise another transparent conductive oxide material and / or the further substrate comprises another polymer material, a glass material or another substrate. It will be appreciated that in other embodiments, the further substrate may not be present.

[0319] The method comprises cleaning the further substrate layer and the second electrode 16. The further substrate layer and the second electrode 16 provided thereon may be cleaned in the same manner as the substrate layer 20 and the first electrode provided thereon.

[0320] The method may comprise forming the barrier layer 22. In this embodiment, the barrier layer 22 comprises a polymer material, such as an elastomeric material. In this embodiment, the polymer material comprises polydimethylsiloxane (PDMS). By forming the barrier layer 22 from the elastomeric material, damage to the layer of perovskite material 18, e.g. due to a mechanical force applied to the device 12, may be reduced or prevented. In other words, the barrier layer 22 may act as a cushion layer.

[0321] In this embodiment, the barrier layer 22 may be formed by forming a barrier layer material solution. The barrier layer material solution may be formed by mixing a polymer material base, such as a PDMS base, with a curing agent or curing solution. A ratio of the polymer material base to the curing agent or curing solution may be 10: 1. The barrier

[0322] 55667482-1 layer material solution may be placed in a vacuum environment for a time duration, such as 30 minutes. However, it will be appreciated that in other embodiments, the barrier layer may comprise another polymer material and / or may be differently formed.

[0323] The barrier layer material is deposited on the further substrate layer and second electrode 16 using a deposition process, such as a spin coating process. For example, the barrier layer material is deposited by spin coating at 4000 rpm for 60 seconds. An acceleration or ramp rate used in the spin coating process may be between 1000 rpm / s and 3000 rpm / s, such as about 2000 rpm / s.

[0324] Subsequent to the deposition of the barrier layer material, the method comprises annealing the barrier layer material, e.g. to form the barrier layer 22. In this embodiment, the barrier layer material is annealed for a duration of time, such as 1 minute. The barrier layer material may be annealed at a temperature between 25°C and 60°C, such as 25°C, 50°C or any other suitable temperature.

[0325] The method may comprise arranging the barrier layer 22 on the layer of perovskite material 18 to form the device 12. The barrier layer 22 is arranged on the layer of perovskite material 18 such that the barrier layer 22 is arranged between the layer of perovskite material 18 and the second electrode 16.

[0326] The method may comprise annealing the device 12 at a temperature of 50 °C for a duration of time, such as 5 minutes. This may allow for removal of the solvents, e.g. complete removal of the solvents, aid the formation of uniform films and / or increase a crystallisation rate. Alternatively or additionally, the method may comprise annealing the device 12 at a temperature of about 25 °C for a duration of time, such as about 8 hours.

[0327] The method may comprise connecting metal contacts to the first and second electrodes 14, 16, respectively. The metal contacts may be provided in the form of metal wires or other suitable contacts. In this embodiment, the metal contacts comprise copper wires, which are connected to the first and second electrodes 14, 16 using a conductive paste, such as silver paste, or another conductive adhesive.

[0328] The device 12 may be covered by a polymer material, e.g. to protect the device 12 from a surrounding environment. For example, the device 12 may be covered by the polymer material to protect the device 12 from moisture, ambient humidity and / or the like. The polymer material may comprise Kapton, such as a Kapton film or Kapton tape, or another polymer material, such as another polymer or plastic film or the like.

[0329] In a fifth exemplary embodiment, the device 12 comprises the piezoelectric device mentioned above. In this embodiment, the layer of perovskite material 18 comprises BA2MAn-iPbnl3n+i with n=2 or n=4. When n=2, the layer of perovskite material

[0330] 55667482-1 18 comprises BA2MAPb2l?. A thickness of the layer of perovskite material 18 is about 350 nm. When n=4, the layer of perovskite material 18 comprises BA2MA3Pb4li3. A thickness of the layer of perovskite material 18 is about 1 .2 pm.

[0331] In this embodiment, stage 30 is the same as stage 30 of the third exemplary embodiment. The substrate layer 20 may be attached to a rigid substrate, such as a glass substrate to aid the cleaning and deposition process.

[0332] At stage 32, the layer of perovskite material 18 is formed on the first electrode 14 as described above in relation to Figure 1 B. In this embodiment, the deposition process comprises the spin coating described above.

[0333] At stage 34, the second electrode 16 is provided. In this embodiment, the second electrode is patterned or pre-patterned on a further substrate. The conductive material of the second electrode 16 comprise a transparent conductive oxide, such as indium tin oxide. The further substrate comprises a polymer substrate, such as a PET substrate. However, it will be appreciated that in other embodiments, the second electrode may comprise another transparent conductive oxide material and / or the further substrate comprises another polymer material, a glass material or another substrate. It will be appreciated that in some embodiments, the further substrate may not be present.

[0334] The method comprises cleaning the further substrate and the second electrode 16. The further substrate and the second electrode 16 provided thereon may be cleaned in the same manner as the substrate layer 20 and the first electrode provided thereon.

[0335] The method may comprise forming the barrier layer 22. In this embodiment, the barrier layer 22 comprises a polymer material, such as Polystyrene-b / oc -poly(ethylene- ra n-butylene)- bl ock-po\y sty rene-graff-maleic anhydride (SEBS-g-MA). SEBS is of hydrophobic nature, which may improve a stability of the device 12. Maleic anhydride is of hydrophilic nature, which may result in an improved bond with the layer of perovskite material 18, which has a hydrophobic nature. This may allow for the formation of a firmly packed device with resistance against common environmental effects. SEBS-g-MA with a 10 wt.% (0.1 g / 1 mL ratio) is mixed with a solvent, such as a toluene anhydrous 99.8 % solvent, and stirred for an hour at a temperature of about 80°C. The dissolved solution may be aged for a day, before depositing it on the second electrode 16.

[0336] It will be appreciated that in other embodiments, the barrier layer may comprise another polymer material. The barrier layer 22 is formed on the further substrate layer and second electrode 16 using a deposition process, such as a spin coating process. For example, the barrier layer 22 is deposited by spin coating at 1000 rpm for 10 seconds. An acceleration or ramp rate used in the spin coating process may be between

[0337] 55667482-1 800 rpm / s and 1500 rpm / s, such as about 1000 rpm / s. Subsequent to the deposition of the barrier layer 22, the method comprises annealing the barrier layer 22. In this embodiment, the barrier layer 22 is annealed at a temperature of 50 °C for a duration of time, such as 1 minute.

[0338] The method may comprise arranging the barrier layer 22 on the layer of perovskite material 18 to form the device 12. The barrier layer 22 is arranged on the layer of perovskite material 18 such that the barrier layer 22 is arranged between the layer of perovskite material 18 and the second electrode 16.

[0339] The method may comprise annealing the device 12 at a temperature of 80 °C for a duration of time, such as 2 to 3 hours. This may allow the barrier layer 22 to completely dry out and to be attached to the layer of perovskite material 18. During this step, a load, such as a 5 kg load, may be applied to the device 12 to firmly attach the barrier layer 22 to the layer of perovskite material 18.

[0340] The method may comprise connecting metal contacts to the first and second electrodes 14, 16, respectively. The metal contacts may be provided in the form of metal wires or other suitable contacts. In this embodiment, the metal contacts comprise copper wires, which are connected to the first and second electrodes 14, 16 using a conductive paste, such as silver paste, or another conductive adhesive.

[0341] The device 12 may be covered by a polymer material, e.g. to protect the device 12 from a surrounding environment. For example, the device 12 may be covered by the polymer material to protect the device 12 from moisture, ambient humidity and / or the like. The polymer material may comprise Kapton, such as a Kapton film or Kapton tape, or another polymer material, such as another polymer or plastic film or the like.

[0342] Figure 13 shows a flow chart in overview of a method of forming an energy harvesting device according to an embodiment of the present disclosure.

[0343] In a first exemplary embodiment, the device 12 comprises the exemplary photovoltaic device shown in Figure 11 A. In this embodiment, the layer of perovskite material 18 comprises BA2MAn-iPbnBr3n+i with n=1 or n=2. When n=1 , the layer of perovskite material 18 comprises BA2PbBr4. When n=2, the layer of perovskite material 18 comprises BA2MAPb2Br7. However, it will be described that in other embodiments, the layer of perovskite material may comprise BA2MAPb2l7, BA2MAsPb4li3, (PEA)2PbBr4, (PEA)2MAPb2Br7, (PEA)2MA2Pb3Brio, or (PEA)2MA3Pb4Bri4, (BA)2(FA)Pb2Br7, (BA)2(FA)2Pb3Brio, (BA)2(FA)3Pb4Bri3, or (BA)2(FA)4Pb5Bri6or another perovskite material.

[0344] 55667482-1 At stage 40, the first electrode 14 is provided. In this embodiment, the first electrode is patterned or pre-patterned on the substrate layer 20. The conductive material of the first electrode 14 comprise a transparent conductive oxide, such as indium tin oxide. The substrate layer 20 comprises a glass substrate. However, it will be appreciated that in other embodiments, the first electrode may comprise another transparent conductive oxide material and / or the substrate may comprise a polymer material, or another substrate. The method comprises cleaning the substrate layer 20 and the first electrode 14 as described above.

[0345] At stage 42, the method comprises forming an electron transport layer, such as the electron transport layer 24 described above. The electron transport layer is formed on the substrate layer 20 and the first electrode 14 using a deposition process, such as a spin coating process. For example, a solution or suspension comprising an electron transport layer precursor material may be spin coated on the substrate layer 20 and first electrode 14. In this embodiment, the electron transport layer 24 comprises tin oxide (SnC>2). The method comprises spin coating a volume of a tin oxide solution or suspension, such as about 100 pL, at 3000 rpm for 30 seconds on the substrate layer 20 and the first electrode 14. The method comprises annealing the electron transport layer precursor material at a temperature of about 150 °C for a duration of time. In this embodiment, the duration of time comprises 30 minutes. It will be appreciated that in other embodiment, the electron transport layer may be formed from a different material and / or using a different deposition process.

[0346] At stage 44, the layer of perovskite material 18 is formed on the electron transport layer 24 as described above in relation to Figure 1 B. In this embodiment, the deposition process comprises the blade coating process described above.

[0347] At stage 46, the method comprises forming a hole transport layer, such as the hole transport layer 26. The hole transport layer is formed on the layer perovskite material 18 using a deposition process, such as a spin coating process. For example, a solution or suspension comprising a hole transport layer precursor material may be deposited, e.g. spin coated, on the layer of perovskite material 18. In this embodiment, the hole transport layer 26 comprises Spiro-MeOTAD.

[0348] For the preparation of a Spiro-MeOTAD solution or suspension, Spiro-MeOTAD (72.3 mg) was dissolved in chlorobenzene (1 mL), mixed with a 4-tertbutylpyridine (t-BP, 28.8 pL), lithium bis(trifluoromethanesulfonyl)imide solution (LiTFSI, 17.5 pL, which may be prepared by dissolving 52 mg LiTFSI in 100 pL acetonitrile) and tris(2-(1 / 7-pyrazol-1- yl)-4-terf-butylpyridine)cobalt(ll I) tri[bis(trifluoromethane)sulfonimide] (FK209 Co(lll)

[0349] 55667482-1 TFSI salt, 29 pL, which may be prepared by dissolving 30 mg FK209 Co(lll) TFSI salt in 100 pL acetonitrile).

[0350] The method comprises spin coating a volume of the Spiro-MeOTAD solution or suspension, such as about 55 pL, at 4000 rpm for 30 seconds on the layer of perovskite material 18. The method comprises annealing the hole transport layer precursor material at a temperature of about 150 °C for a duration of time. In this embodiment, the duration of time comprises 30 minutes. It will be appreciated that in other embodiment, the hole transport layer may be formed from a different material and / or using a different deposition process. The method may comprise doping the hole transport layer 26 with a dopant material. The dopant material may comprise oxygen or another suitable dopant. In this embodiment, the substrate layer 20, first electrode 14, electron transport layer 24, layer of perovskite material 18 and the hole transport layer 26 were stored in an oxygen environment for a duration of time, such as about 12 hours, to dope the hole transport layer 26 with oxygen.

[0351] At stage 48, the method comprises forming a second electrode, such as the second electrode 16. The second electrode 16 is formed on the hole transport layer 26 using a deposition process, such as a thermal evaporation process. The conductive material may be deposited on the hole transport layer 26 at a pressure of about 3x1 O'6mbar. In this embodiment, the conductive material comprises gold. However, it will be appreciated that in other embodiments, the second electrode may comprise another conductive material. A thickness of the second electrode 16 may be between about 40 nm and 100 nm, such as about 60 nm.

[0352] In a second exemplary embodiment, the device 12 comprises the exemplary photovoltaic device shown in Figure 11 B. In this embodiment, the layer of perovskite material 18 comprises BA2MAn-iPbnl3n+i with n=2 or n=4. When n=2, the layer of perovskite material 18 comprises BA2MAPb2l7. When n=4, the layer of perovskite material 18 comprises BA2MA3Pb4li3. However, it will be described that in other embodiments, the layer of perovskite material may comprise BA2PbBr4, BA2MAPb2Br7, BA2MAPb2l7, BA2MA3Pb4li3, (PEA)2PbBr4, (PEA)2MAPb2Br7, (PEA)2MA2Pb3Brio, or (PEA)2MA3Pb4Bri4, (BA)2(FA)Pb2Br7, (BA)2(FA)2Pb3Brio, (BA)2(FA)3Pb4Bri3, or (BA)2(FA)4PbsBri6 or another perovskite material.

[0353] At stage 40, the first electrode 14 is provided. In this embodiment, the first electrode comprises any of the features of the first electrode described in relation to the first exemplary embodiment.

[0354] 55667482-1 At stage 42, the method comprises forming a hole transport layer, such as the hole transport layer 26 described above. The hole transport layer 26 is formed on the substrate layer 20 and the first electrode 14 using a deposition process, such as a spin coating process. For example, a solution or suspension comprising a hole transport layer precursor material may be spin coated on the substrate layer 20 and the first electrode 14.

[0355] In this embodiment, the hole transport layer 26 comprises Poly TPD / PFN. For example, 1.4 mg / mL Poly-TPD may be mixed with 1 mL chlorobenzene and stirred for a duration of time, such as about 4 hours, at room temperature. Poly-TPD may be spin coated on the substrate layer 20 and the first electrode 14 at 6000 rpm for 30 seconds.

[0356] 1 mg / mL PFN-P1 may be mixed with 995 pL methanol and stirred for a duration of time, such as about 2 hours, at about 60 °C. Subsequently, 5 pL aceticacid may be added and stirred for another duration of time, such as 2 hours, at about 60 °C. 100 pL of PFN-P1 may be spin coated on the Poly-TPD at 3000 rpm for 30 s. It will be appreciated that in other embodiment, the hole transport layer may be formed from a different material and / or using a different deposition process.

[0357] At stage 44, the layer of perovskite material 18 is formed on the hole transport layer 26. In this embodiment, the deposition process comprises a spin coating process. For example, a layer of the perovskite precursor material may be spin coated on the hole transport layer 26 at 1000 rpm for 30 seconds. The deposited layer of precursor perovskite material may be annealed. In embodiments where the layer of perovskite material comprises BA2MA3Pb4li3, the deposited layer of precursor perovskite material may be annealed at room temperature and a pressure between about 0.01 bar and about 1 bar, such as about 0.1 bar. In embodiments where the layer of perovskite material comprises BA2MAPb2l?, the deposited layer of precursor perovskite material may be annealed at a temperature of about 100°C. The deposited layer of precursor perovskite material may be annealed at a pressure of between about 0.01 bar and about 1 bar, such as about 0.1 bar. The layer of perovskite material comprising BA2MA3Pb4li3 may have a thickness of about 1.6 pm. The layer of perovskite material comprising BA2MAPb2l? may have a thickness of about 700 nm.

[0358] At stage 46, the method comprises forming an electron transport layer, such as the electron transport layer 24. The electron transport layer 24 may comprise a polymer material, such as PCBM. The method may comprise forming an electron transport layer solution or suspension. For example, the PCBM solution or suspension may be prepared by mixing PCBM into a solvent, such as chlorobenzene (CB). This solution or suspension

[0359] 55667482-1 may be stirred for a duration of time, such as 4 hour to 5 hours at a temperature of about 60 °C. The method may comprise filtering the electron transport layer solution or suspension, e.g. using a filter, such as a 0.2 pm filter, prior to depositing the polymer material on the layer of perovskite material 18. The layer of PCBM may have a thickness of about 40nm.

[0360] In this embodiment, the electron transport layer 24 comprises a hole blocking layer. The hole blocking layer may comprise an organic compound, such as bathocuproine (BCP) or any other suitable material or organic compound. A BCP solution or suspension may be prepared by mixing BCP into a solvent, such as ethanol. This solution or suspension may be spin coated on the layer of PCBM, for example at 3000 rpm for 50s, to form a layer of BCP. The layer of BCP have a thickness of about 5 nm. It will be appreciated that in other embodiment the layer of BCP may have a thickness of more or less than 5 nm.

[0361] At stage 48, the method comprises forming a second electrode, such as the second electrode 16. The second electrode 16 is formed on the electron transport layer 24. The second electrode 16 may be formed as described in relation to the first exemplary embodiment, as described above in relation to Figure 13. The second electrode may have a thickness between about 40 nm and 150 nm, such as about 100 nm.

[0362] It will be appreciated that the electron transport layer 24 may be formed between at least one of the first and second electrodes 14, 16 and the layer of perovskite material 18. The hole transport layer 26 may be formed between at least one other of the first and second electrodes 14, 16 and the layer of perovskite material 18.

[0363] Figure 14 shows a graph of measured polarisations of an exemplary energy harvesting device in dependence of a voltage having different frequencies. Each of the curves shown on Figure 14 may also be referred to a P-E loop or hysteresis curve. Each curve has been measured for the device 12 shown in Figure 9, which was formed according to the method described in the exemplary first embodiment. For example, the curves were measured by applying a voltage at the first and second electrode 14, 16. The voltage was varied between -1 V to 1 Volt and the polarisation of the layer of perovskite material 18 was measured in dependence of the varying voltage at the first and second electrodes 14. The P-E loops shown in Figure 14 were measured at frequencies of the voltage ranging from 1 Hz to 200 Hz. In this embodiment, the layer of perovskite material comprises BA2MAPb2Br?. The polarisation curves shown in Figure 14 indicate that the device 12 behaves like a resistor and not a ferroelectric device. This

[0364] 55667482-1 may be due to a diffusion of halide atoms from the layer of perovskite material 18 to the second electrode 16 and / or a reaction between the halide atoms and conducting material of the second electrode 16, which may cause the device 12 to short-circuit, as described above.

[0365] Figure 15 shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -20 V and 20 V and a frequency of the voltage was 50 Hz. An electric field between the first and second electrodes 14, 16 is proportional to a voltage (or potential difference) between the first and second electrodes 14, 16. The polarisation and current have been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises chromium and / or C^Ch and the layer of perovskite material comprises BA2MAPb2Br?. In this example, a current-voltage response is similar to that of a ferroelectric material. This may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. A saturation polarisation of about 0.3 pC / cm2and a coercive field (Ec) of about 100 kV / cm were determined. The saturation polarisation may be understood as the polarisation that does not increase further when the voltage increases. The coercive field may be understood as the strength of the electric field at which the polarisation is equal to zero. It may also be understood as a measure of the ability of a ferroelectric material to withstand an external electric field without becoming depolarised.

[0366] Figure 16 shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -200 V and 200 V and a frequency of the voltage was 50 Hz. The graph has been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PCBM and the layer of perovskite material comprises BA2PbBr4. In this example, a saturation polarisation of about 7.5 pC / cm2and a coercive field of about 2.1 MV / cm were determined.

[0367] Figure 17 shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -200 V and 200 V and a frequency of the voltage was 50 Hz. The graph has been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PCBM and the layer

[0368] 55667482-1 of perovskite material comprises BA2MAPb2Br?. In this example, a saturation polarisation of about 4 pC / cm2and a coercive field of about 0.3 MV / cm were determined. The higher saturation polarisation value of the layer of perovskite material comprising BA2PbBr4 compared to the layer of perovskite material comprising BA2MAPb2Br? may be related to an open two-dimensional confined environment for the organic spacer cation SC, when n=1. In this case, a single layer of corner-sharing BXe octahedra is connected to the two molecules of butylammonium via weak hydrogen bonding, which may allow for an increased freedom for molecular vibration.

[0369] The higher coercive field value that has been determined for BA2PbBr4 may be due to a reduced thickness of this layer of perovskite material 18 relative to the layer of perovskite material 18 that comprises BA2MAPb2Br?.

[0370] The saturation polarisation and coercive field values determined from the measured curves shown in Figures 15 to 17 and may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. It can be seen from Figures 16 and 17 that the saturation polarisation and the coercive field for the devices comprising the barrier layer that comprises PCBM are increased relative to the device comprising the barrier layer that comprises chromium and / or C^Os. This may be due to the barrier layer that comprises PCBM providing an improved reduction of short-circuiting paths and / or recombination centres in the layer of perovskite material 18 relative to the barrier layer that comprises chromium and / or C^Os. The barrier layer comprising PCBM may also lead to a passivation of defects in the layer of perovskite material 18. A yield of the devices 12 comprising the barrier layer 22 that comprises PCBM may be considered as being improved to 80% to 100 %.

[0371] Figure 18 shows a graph of measured currents of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -4 V and 4 V. The upper two curves shown in Figure 18 have been measured for the device 12 shown in Figure 9, which was formed according to the method described in the first exemplary embodiment in relation to Figure 12. The lower two curves shown in Figure 18 have been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PCBM and the layer of perovskite material comprises BA2MAPb2Br?. It can be seen from Figure 18 that the provision of the barrier layer 22 that comprises PCBM results in a reduced current, which is also referred to as a leakage current. This reduced leakage current may be due to the reduction of short-circuiting paths and / or recombination

[0372] 55667482-1 centres in the layer of perovskite material 18. Additionally, the inventor has found that the barrier layer 22 that comprises PCBM allows for deposition of the conductive material of the second electrode 16 with an improved uniformity. This may be due to PCBM filling valleys that may be present in the layer of perovskite material 18. The improved uniformity of the second electrode 16 may cause the increased yield of the devices 12 comprising the barrier layer 22 that comprises PCBM.

[0373] Figure 19 shows a graph of a measured pyroelectric current and a measured temperature gradient of an exemplary energy harvesting device in dependence of a time. The curves shown in Figure 19 have been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PCBM and the layer of perovskite material comprises BA2PbBr4.

[0374] Figure 20 shows a graph of a pyroelectric current and a temperature gradient ^ of an exemplary energy harvesting device in dependence of a time. The curves shown in Figure 20 have been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PCBM and the layer of perovskite material comprises BA2MAPb2Br?.

[0375] The curves shown in Figures 19 and 20 have been obtained by performing pyroelectricity measurements on the devices 12, respectively. In this example, pyroelectricity may be understood as the ability to generate a temporary voltage, when the layer of perovskite material 18 is heated or cooled. In this example, the temperature was varied between about 45°C and about 55°C. The resulting pyroelectric current is I = pA ^ , where p is the pyroelectric coefficient, A is the area of the device 12. The pyroelectric current is proportional and in phase with the temperature gradient A small shift in a maximum of the temperature gradient and the pyroelectric current I can be seen in Figures 19 and 20. This shift may be due to a time lag between a heat propagation to the layer of perovskite material 18 and the heat generated at the irradiation source. The pyroelectric coefficient for the device 12 comprising the layer of perovskite material 18 that comprises BA2PbBr4 was determined as about 0.45 pC / m2°C. The pyroelectric coefficient for the device 12 comprising the layer of perovskite material 18 that comprises BA2MAPb2Br? was determined as about 0.43 pC / m2°C. These pyroelectric coefficients are indicative of the exemplary perovskite materials 18 having ferroelectric properties.

[0376] 55667482-1 Figure 21A shows a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric force microscope. Figure 21 B shows a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric force microscope. The curves shown in Figures 21a and 21 B have been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PCBM and the layer of perovskite material comprises BA2PbBr4.

[0377] Figure 22A shows a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope. Figure 22B shows a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope. The graphs have been measured for the device 12 shown in Figure 10, which was formed according to the method described in the second exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PCBM and the layer of perovskite material comprises BA2MAPb2Br?.

[0378] Piezoelectric force microscopy (PFM) is based on the piezoelectric effect. A voltage across the layer of perovskite material 18 may cause a mechanical deformation of the layer of perovskite material 18, which may be in the form of an extension or contraction. This mechanical extension of the layer of perovskite material 18 can be measured in PFM. PFM can also be used to study ferroelectric domains. For example, the voltage can switch regions of ferroelectric domains. In PFM, the voltage is applied between a tip of the piezoelectric force microscope and a surface of the device 12 to generate the piezoelectric field in the layer of perovskite material 18. The tip is connected to the cantilever of the piezoelectric force microscope. The applied voltage may cause the layer of perovskite material 18 to contract or expand, which results in a deflection of the cantilever. Both the amplitude and phase of the deflection of the cantilever can be monitored. The amplitude signal may provide information on a signal strength, which in turn may be indicative of a piezoelectric tensor of the layer of perovskite material 18. The phase may provide information on the direction of polarization. In this example, a vertical piezoelectric force mode was used and the voltage was varied between -10 V and 10 V. Figures 21A and 22A each show that the respective amplitude curve represents an amplitude loop. Figures 21 B and 22B each show that the respective phase represents a phase loop. The amplitude and phase loops are indicative of ferroelectric domain

[0379] 55667482-1 switching, which in turn is indicative of the layers of perovskite material 18 having ferroelectric properties.

[0380] In Figures 21 B and 22B, a 180°C phase shift can be observed, which may be indicative of the layers of perovskite material 18 retaining spontaneous polarization.

[0381] An effective piezoelectric coefficient d33effwas determined as 18 ± 2 pm / V for the layer of perovskite material 18 comprising BA2MAPb2Br? and as 12±2 pm / for the layer of perovskite material 18 comprising BA2PbBr4. The effective piezoelectric coefficients d33effwere determined based on amplitude curves shown in Figures 21A and 22A, respectively, and a sensitivity of the tip, which is 58 nmA / for the layer of perovskite material 18 comprising BA2MAPb2Br? and 56.8 nmA / for the layer of perovskite material 18 comprising BA2PbBr4. The determined effective piezoelectric coefficient d33effvalue is higher for the layer of perovskite material 18 comprising BA2MAPb2Br?. This may be due to a thickness of this layer being larger than a thickness the layer of perovskite material 18 comprising BA2PbBr4. In this example, the layer of perovskite material 18 comprising BA2PbBr4 has a thickness of about 0.6 pm. The layer of perovskite material 18 comprising BA2MAPb2Br? has a thickness of about 1.2 pm. The noise in Figures 21 A and 21 B may be due to a higher coercive field of the layer of perovskite material 18 comprising BA2PbBr4 relative to a coercive field of the layer of perovskite material 18 comprising BA2MAPb2Br?.

[0382] The piezoelectric coefficients d33for the exemplary layers of perovskite material 18 were additionally determined using the Berlincourt method. In this method, the piezoelectric coefficient may be determined based on a voltage that is generated, when a mechanical force is applied to the device 12, e.g. the layer of perovskite material 18. In this example, the piezoelectric coefficients have been measured for the same devices 12 as those mentioned above in relation to Figures 21A to 22B. The piezoelectric coefficient d33for the layer of perovskite material 18 comprising BA2PbBr4 was determined as about 3.5 pC / N to 4 pC / N. The piezoelectric coefficient d33for the layer of perovskite material 18 comprising BA2MAPb2Br?was determined as about 2 pC / N to 2.5 pC / N. The higher d33coefficient that has been determined for the layer of perovskite material 18 comprising BA2PbBr4 is in line with the higher saturation polarisation that has been determined for this layer in the P-E measurements described above.

[0383] Figure 23A shows a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time. The open-circuit voltage has been measured for different forces applied to the device. Figure 23B shows a graph of a measured short- circuit current of an energy harvesting device in dependence on time. The short-circuit

[0384] 55667482-1 current has been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS and the layer of perovskite material comprises BA2PbBr4.

[0385] Figure 24A shows a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time. The open-circuit voltage has been measured for different forces applied to the device. Figure 24B shows a graph of a measured short- circuit current of an energy harvesting device in dependence on time. The short-circuit current have been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS and the layer of perovskite material comprises BA2MAPb2Br?.

[0386] For the measurements of the open-circuit voltage of the devices 12, a force of 1 N, 6N, 10N and 20N was applied to each device 12, e.g. using a linear motor arrangement at a frequency of 1 Hz. An active area of each device 12 was 2.3 cm2.

[0387] The open-circuit voltage of the devices 12 may be measured by connecting the first and second electrodes 14, 16 of each device 12 to a device for measuring an electric potential, such as a voltmeter. The voltmeter is configured such that a current passing though this arrangement is zero, e.g. substantially zero, when no force is applied to each device 12. When the force is applied, a voltage may be generated over the layer of perovskite material 18. This voltage is measured by the voltmeter. For a force of 10N, the open-circuit voltage was determined as about 8V for both the device 12 comprising the layer of perovskite material 18 that comprises BA2MAPb2Br? and the device 12 comprising the layer of perovskite material 18 that comprises BA2PbBr4. The open-circuit voltage may be considered as the peak-to-peak voltage in the graphs shown in Figures 23A and 24A.

[0388] The short-circuit current of each device 12 may be measured by connecting the first and second electrodes 14, 16 of each device 12 to a device for measuring an electric current, such as an ammeter. The ammeter is configured such that a potential difference between the first and second electrodes 14, 16 of each device is zero, e.g. substantially zero, when no force is applied to each device 12. For a force of 10N, the short-circuit current was determined as about 5 pA for both the device 12 comprising the layer of perovskite material 18 that comprises BA2MAPb2Br? and the device 12 comprising the

[0389] 55667482-1 layer of perovskite material 18 that comprises BA2PbBr4. The short-circuit current may be considered as the peak-to-peak current in the graphs shown in Figures 23B and 24B. In this example, the layer of perovskite material 18 comprising BA2PbBr4 has a thickness of about 350 nm. The layer of perovskite material 18 comprising BA2MAPb2Br? has a thickness of about 1 pm. It can be seen from Figures 23A to 24B, that the open-circuit voltage values and short-circuit current values are similar for the two devices 12. The determined values for the open-circuit voltage and short-circuit current are indicative of the layers of perovskite material 18 having piezoelectric properties.

[0390] Figure 25A shows a graph of a measured external quantum efficiency (EQE) of an energy harvesting device in dependence on a wavelength. In this example, the device comprises a photovoltaic device, such as the device 12 shown in Figure 11 A. In this example, the layer of perovskite material 18 comprises BA2MAPb2Br?. The external quantum efficiency can be understood as a ratio of charge carriers collected by the device 12 to the number of photons incident on the device 12. From Figure 25A, it can be seen that the external quantum efficiency is the highest in a wavelength range of about 300 nm to 470 nm. For example, the device 12 mainly absorbs photons having a wavelength in this range. This is also in agreement with the absorption spectra shown in Figure 3.

[0391] Figure 25B shows a graph of measured current densities in dependency on voltages applied to a number of energy harvesting devices. Each device comprises a photovoltaic device, such as the device 12 shown in Figure 11 A. In this example, the layer of perovskite material 18 comprises BA2MAPb2Br?. The current densities were measured under 1 sun illumination (AM 1.5G, 100 mW / cm2) of each device 12. The current densities were measured in a number of Forward Scans, which are labelled as “FW” in Figure 25B. During a Forward Scan, the voltage across each device 12 was increased from zero to the open-circuit voltage. The current densities were also measured in a number of Reverse Scans, which are labelled as “RV” (RS) in Figure 25B. During a Reverse Scan, the voltage across each device 12 was decreased from the open-circuit voltage to zero. The current densities were measured for a device 12 with a layer of perovskite material that has been annealed at room temperature, which is labelled as “RT” in Figure 25B, a device 12 with a layer of perovskite material that has been annealed at about 55°C, a device 12 with a layer of perovskite material that has been annealed at about 80°C and a device 12 with a layer of perovskite material that has been annealed at about 100°C.

[0392] 55667482-1 From the measured current density curves shown in Figure 25B, short-circuit current densities between about 0.2 mA / cm2and 0.6 mA / cm2were determined. An average short-circuit density was determined as about 0.4 mA / cm2. Each short-circuit current density was determined for a voltage across each device 12 being equal to zero. A short-circuit current density may be considered as a maximum current density available from a device 12.

[0393] Figure 25C shows a distribution of open-circuit voltages determined for a number of energy harvesting devices. Each device comprises a photovoltaic device, such as the device 12 shown in Figure 11A. In this example, the layer of perovskite material 18 comprises BA2MAPb2Br?. The open-circuit voltages were determined for a device 12 with a layer of perovskite material that has been annealed at room temperature, which is labelled as “25 (RT)” in Figure 25C, a device 12 with a layer of perovskite material that has been annealed at about 50°C, a device 12 with a layer of perovskite material that has been annealed at about 55°C and a device 12 with a layer of perovskite material that has been annealed at about 100°C. From Figure 25C, an average open-circuit voltage of about 1.2 V was determined for the devices 12. Each open-circuit voltage was determined for a current density equal to zero. An open-circuit voltage may be considered as a maximum voltage available from a device 12 and / or as a measure of recombination of charge carriers in the device 12.

[0394] From the measured current densities and the applied voltages, an average power conversion efficiency of about 0.08% was determined for the devices 12. The power conversion efficiency may be understood as a ratio of electrical power to optical power.

[0395] Figure 25D shows a graph of measured current densities in dependency on voltages applied to a number of energy harvesting devices. Each device comprises a photovoltaic device, such as the device 12 shown in Figure 11 A. In this example, the layer of perovskite material 18 comprises BA2MAPb2Br?. The current densities were measured for the same devices as those mentioned in relation to Figures 25B. The current densities were measured under indoor illumination of each device 12 by a white light emitting diode using an illuminance of 1000 lux. The current densities were measured in a number of Forward Scans, which are labelled as “FW” in Figure 25D. The current densities were also measured in a number of Reverse Scans, which are labelled as “RV” (RS) in Figure 25D.

[0396] From the measured current density curves shown in Figure 25D, short-circuit current densities between about 1 pA / cm2and about 3 pA / cm2were determined. An average short-circuit density was determined as about 2 pA / cm2.

[0397] 55667482-1 Figure 25E shows a distribution of open-circuit voltages determined for a number of energy harvesting devices. The open-circuit voltages were determined for the same devices as those mentioned in relation to Figures 25B and 25C. From Figure 25E, an average open-circuit voltage of about 0.7 V was determined for the devices 12.

[0398] From the measured current densities and the applied voltage, an average power conversion efficiency of about 0.16% was determined for the devices 12 under indoor illumination.

[0399] Figure 25F shows a distribution of power conversion efficiencies (PCE) determined for a number of energy harvesting devices. The power conversion efficiencies shown in Figure 25F were determined under 1 Sun illumination of each device 12. The power conversion efficiencies were determined for the same devices as those mentioned in relation to Figure 25C. Power conversion efficiencies between about 0.04% and about 0.11% were determined. An average power conversion efficiency of about 0.08% was determined for the devices 12.

[0400] Figure 25G shows a distribution of power conversion efficiencies (PCE) determined for a number of energy harvesting devices. The power conversion efficiencies shown in Figure 25F were determined under an illumination of 1000 lux of each device 12. The power conversion efficiencies were determined for the same devices as those mentioned in relation to Figures 25B and 25D. Power conversion efficiencies between about 0.04% and about 0.18% were determined. An average power conversion efficiency was determined as about 0.16%. It can be seen that the power conversion efficiencies determined for the devices 12 under an illumination of 1000 lux is increased relative to the power conversion efficiencies determined for the devices 12 under 1 Sun illumination. This may be due to a bandgap of the perovskite material of the devices 12 being more suitable for indoor illumination than to 1 Sun illumination. A variation of the determined power conversion efficiencies shown in Figures 25F and 25G may be due to one or more variations in morphology and / or crystallinity of the perovskite material, e.g. due to the different annealing temperatures.

[0401] The determined values for the open-circuit voltage, short-circuit current and power conversion efficiency are indicative of the layers of perovskite material 18 of the devices 12 having photovoltaic properties. As such, the layers of perovskite material described herein may be considered as being photovoltaic and ferroelectric or piezoelectric. This may allow for use of one or more of these layers in an energy harvesting device.

[0402] 55667482-1 Figure 26 shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -1.5 V and 1.5 V and a frequency of the voltage was 10 Hz. The graph has been measured for the device 12 shown in Figure 9, which was formed according to the method described in the third exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MAPb2l?, which is labelled as “n=2” in Figure 26. In this example, the layer of precursor perovskite materials was annealed at a temperature of about 100 °C. In this example, a currentvoltage response is similar to that of a ferroelectric material. This may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. A saturation polarisation of about 4 pC / cm2and a coercive field (Ec) of about 5 kV / cm were determined.

[0403] Figure 27A shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -4 V and 4 V and a frequency of the voltage was 100 Hz. The voltage applied to the device 12 may correspond to an electric field of about 25 kV. The graph has been measured for the device 12 shown in Figure 9, which was formed according to the method described in the third exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MA3Pb4li3, which is labelled as “n=4” in Figure 27A. In this example, the layer of precursor perovskite materials was annealed at a temperature of about 100 °C. In this example, a currentvoltage response is similar to that of a ferroelectric material. This may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. A saturation polarisation of about 23 pC / cm2and a coercive field (Ec) of about 12 kV / cm were determined.

[0404] Figure 27B shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -1.5 V and 1.5 V and a frequency of the voltage was 10 Hz. The voltage applied to the device 12 may correspond to an electric field of about 10 kV. In this example, the polarisation and current were measured for a lower voltage and a lower electric field than those shown in Figure 27A. The graph has been measured for the device 12 shown in Figure 9, which was formed according to the method described in the third exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MA3Pb4li3, which is also labelled as “n=4” in Figure 27B. In this example, the layer of precursor perovskite materials was annealed at a

[0405] 55667482-1 temperature of about 100 °C. In this example, a current-voltage response is similar to that of a ferroelectric material. This may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. A saturation polarisation of about 15 pC / cm2and a coercive field (Ec) of about 3 kV / cm were determined.

[0406] Each of Figures 28A to 28C shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. The graphs have been measured for the device 12 shown in Figure 9, which was formed according to the method described in the third exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MA3Pb4li3. In this example, the layer of precursor perovskite material was annealed at a temperature of about 25 °C. This temperature may also be referred to as room temperature.

[0407] To measure the polarisation shown in Figure 28A, the voltage was varied between -1.5 V and 1.5 V and a frequency of the voltage was 10 Hz. The polarisation was measured for an electric field of 5 kV and 10kV. A saturation polarisation of about 3 pC / cm2and about 1 pC / cm was determined. The coercive field (Ec) was determined as 3 kV / cm for an electric field of 5 kV and 10 kV.

[0408] To measure the current shown in Figure 28B, the voltage was varied between - 1.5 V and 1.5 V and a frequency of the voltage was 10 Hz. The current was measured for an electric field of 5 kV and 10kV.

[0409] To measure the polarisation shown in Figure 28C, the voltage was varied between -4 V and 4 V and a frequency of the voltage was 10 Hz. The voltage applied to the device 12 may correspond to an electric field of about 25 kV. A saturation polarisation of about 6 pC / cm2was determined.

[0410] From the graphs shown in Figure 28C, it can be seen that a current-voltage response is similar to that of a ferroelectric material. This may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. Additionally, the polarisationvoltage responses shown in Figures 28A to 28C are indicative of the layer of perovskite material 18 comprising ferroelectric properties. The graphs shown in Figures 28A to 28C indicate that it is possible to form a layer of perovskite material 18 having ferroelectric properties at room temperature. This may facilitate a manufacture of the layer of perovskite material 18.

[0411] Figure 29A shows a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope. Figure 29B shows a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to the tip of the

[0412] 55667482-1 piezoelectric microscope. The graphs have been measured for the device 12 shown in Figure 9, which was formed according to the method described in the third exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MA3Pb4li3.

[0413] In this example, the voltage applied to the tip was varied between -10 V and 10 V. Figures 29A shows that the measured amplitude curve represents an amplitude loop. Figures 29B shows that the measured phase represents a phase loop. The amplitude and phase loops are indicative of ferroelectric domain switching, which in turn is indicative of the layers of perovskite material 18 having ferroelectric properties.

[0414] In Figures 29A and 29B, a 180°C phase shift can be observed, which is indicative of the layers of perovskite material 18 retaining spontaneous polarization.

[0415] Figure 30A shows a graph of a measured amplitude of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to a tip of the piezoelectric microscope. Figure 30B shows a graph of a measured phase of a cantilever of a piezoelectric force microscope in dependence on a voltage applied to the tip of the piezoelectric microscope. The graphs have been measured for the device 12 shown in Figure 9, which was formed according to the method described in the third exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MAPb2l?.

[0416] In this example, the voltage applied to the tip was varied between -10 V and 10 V. Figure 30A shows that the measured amplitude curve represents an amplitude loop. Figure 30B shows that the measured phase represents a phase loop. The amplitude and phase loops are indicative of ferroelectric domain switching, which in turn is indicative of the layers of perovskite material 18 having ferroelectric properties.

[0417] In Figures 30A and 30B, a 180°C phase shift can be observed, is indicative of the layers of perovskite material 18 retaining spontaneous polarization.

[0418] An effective piezoelectric coefficient d33effwas determined as 1.96 pC / N, for the layer of perovskite material 18 comprising BA2MAPb2l? and as 142 pC / N for the layer of perovskite material 18 comprising BA2MA3Pb4li3. The effective piezoelectric coefficients d33effwere determined based on the amplitude curves shown in Figures 29B and 29B, respectively, and a sensitivity of the tip, which is 10.4 nmA / for the layer of perovskite material 18 comprising BA2MAPb2l? and 10.2 nmA / for the layer of perovskite material 18 comprising BA2MA3Pb4li3.

[0419] Figure 31A shows a graph of a measured short-circuit current of an energy harvesting device in dependence on time. The short-circuit current has been measured

[0420] 55667482-1 for different forces applied to the device. Figure 31 B shows a graph of a measured opencircuit voltage of an energy harvesting device in dependence on time. The open-circuit voltage has been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fifth exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MAsPb4li3.

[0421] Figure 32A shows a graph of a measured short-circuit current of an energy harvesting device in dependence on time. The short-circuit current has been measured for different forces applied to the device.

[0422] Figure 32B shows a graph of a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time. The open-circuit voltage has been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fifth exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MAPb2l?.

[0423] For the measurements of the open-circuit voltage of the device 12, a force was applied to the device 12 by finger tapping, hand tapping and foot tapping on the device. An active area of each device 12 was 2.5 cm2. A short-circuit current of between about 1 pA and 3 pA and an open-circuit voltage of about between 1 V and 4 V were measured ,the force was applied to the device 12 finger tapping, hand tapping and foot tapping. The determined values for the open-circuit voltage and short-circuit current are indicative of the layers of perovskite material 18 having piezoelectric properties.

[0424] Figure 33A shows a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time. The open-circuit voltage has been measured for different forces applied to the device. Figure 33B shows a graph of a measured short- circuit current of an energy harvesting device in dependence on time. The short-circuit current has been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fifth exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MA3Pb4li3. In this example, the force was varied between 1 N and 40 N.

[0425] Figure 34A shows a graph of a measured open-circuit voltage of an energy harvesting device in dependence on time. The open-circuit voltage has been measured for different forces applied to the device. Figure 34B shows a graph of a measured short-

[0426] 55667482-1 circuit current of an energy harvesting device in dependence on time. The short-circuit current has been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fifth exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises BA2MAPb2l7.

[0427] For the measurements of the open-circuit voltage and the short-circuit current of the device 12, a force of ranging from 1 N to 24 N was applied to the devices 12, e.g. using a linear motor arrangement at a frequency of 1 Hz. An active area of each device 12 was 2.5 cm2.

[0428] The open-circuit voltages and the short-circuit currents of the devices 12 were measured as described above. For a force of 40 N, the open-circuit voltage was determined as about 2V for the device 12 comprising the layer of perovskite material 18 that comprises BA2MA3Pb4li3. For a force of 20 N, the open-circuit voltage was determined as about 4V for the device 12 comprising the layer of perovskite material 18 that comprises BA2MAPb2l?. The open-circuit voltage may be considered as the peak- to-peak voltage in the graphs shown in Figures 33A and 34A. The short-circuit current of the devices 12 were measured as described above.

[0429] For a force of 40 N, the short-circuit current was determined as about 2 pA for the device 12 comprising the layer of perovskite material 18 that comprises BA2MA3Pb4li3. For a force of 20 N, the short-circuit current was determined as about 4 pA for the device 12 comprising the layer of perovskite material 18 that comprises BA2MAPb2l?. The determined values for the open-circuit voltage and short-circuit current are indicative of the layers of perovskite material 18 having piezoelectric properties.

[0430] Figure 35A shows a graph of a distribution of short-circuit current densities (Jsc) that have been determined for a number of energy harvesting devices. In this example, each device comprises a photovoltaic device, such as the device 12 shown in Figure 11 B. In this example, the layer of perovskite material 18 comprises BA2MAPb2l?. The hole transport layer of one of the devices 12 comprises (2-(9H-carbazol-9- yl)ethyl)phosphonic acid. This device 12 is labelled as “I7 2PACz” in Figure 35 A. The hole transport layer of another one of the devices 12 comprises Poly TPD / PFN. This device 12 is labelled as “I7 Organic” in Figure 35A. The short-circuit current densities were determined for 1 sun illumination (AM 1.5G, 100 mW / cm2) of each device 12. The short-circuit current densities were determined based on a number of Forward Scans, which are labelled as “FW” in Figure 35A, and a number of Reverse Scans, which are

[0431] 55667482-1 labelled as “RV” (RS) in Figure 35A. An average short-circuit density of about 0.5 mA / cm2was determined for the device 12 labelled as “I72PACz”. An average short-circuit density of about 0.3 mA / cm2was determined for the device 12 labelled as “I7 Organic”. This short-circuit density is lower than that determined for the device 12 labelled as “I7 2PACz”.

[0432] Figure 35B shows a graph of a distribution of open-circuit voltages (Voc) that have been determined for a number of energy harvesting devices. The distribution of open-circuit voltages shown in Figure 35B was determined for the same devices as the distribution of short-circuit current densities shown in Figure 35A. An average opencircuit voltage of about 0.9 V was determined for the device 12 labelled as “I7 2PACz”. An average open-circuit voltage of about 0.8 V was determined for the device 12 labelled as “I7 Organic”. This open-circuit voltage is lower than that determined for the device 12 labelled as “I7 2PACz”.

[0433] Figure 35C shows a graph of a distribution of power conversion efficiencies (PCE) that have been determined for a number of energy harvesting devices. The distribution of power conversion efficiencies shown in Figure 35C was determined for the same devices as the distribution of short-circuit current densities shown in Figure 35A. An average power conversion efficiency of about 0.15 % was determined for the device 12 labelled as “I7 2PACz”. An average power conversion efficiency of about 0.1 % was determined for the device 12 labelled as “I7 Organic”. This power conversion efficiency is lower than that determined for the device 12 labelled as “I72PACz”.

[0434] Figure 35D shows a graph of a distribution of fill factors that have been determined for a number of energy harvesting devices. The distribution of fill factors shown in Figure 35D was determined for the same devices as the distribution of short-circuit current densities shown in Figure 35A. An average fill factor of about 40 % was determined for the device 12 labelled as “I7 2PACz”. An average fill factor of about 30 % was determined for the device 12 labelled as “I7 Organic”. This fill factor is lower than that determined for the device 12 labelled as “I7 2PACz”.

[0435] Figure 36A shows a graph of a distribution of short-circuit current densities (Jsc) that have been determined for a number of energy harvesting devices. The distribution of short-circuit densities shown in Figure 36A was determined for the same devices as the distribution of short-circuit current densities shown in Figure 35A. The short-circuit current densities were determined for an illumination of 1000 lux of each device 12. The short-circuit current densities were determined based on a number of Forward Scans, which are labelled as “FW” in Figure 36A, and a number of Reverse Scans, which are

[0436] 55667482-1 labelled as “RV” (RS) in Figure 36A. An average short-circuit density of about 0.045 mA / cm2was determined for the device 12 labelled as “I7 2PACz”. An average short- circuit density of about 0.045 mA / cm2was determined for the device 12 labelled as “I7 Organic”. Figure 36B shows a graph of a distribution of open-circuit voltages (Voc) that have been determined for a number of energy harvesting devices. The distribution of open-circuit voltages shown in Figure 36B was determined for the same devices as the distribution of short-circuit current densities shown in Figure 35A. An average opencircuit voltage of about 0.6 V was determined for the device 12 labelled as “I7 2PACz”. An average open-circuit voltage of about 0.8 V was determined for the device 12 labelled as “I7 Organic”. This open-circuit voltage is higher than that determined for the device 12 labelled as “l7 2PACz”.

[0437] Figure 36C shows a graph of a distribution of power conversion efficiencies (PCE) that have been determined for a number of energy harvesting devices. The distribution of power conversion efficiencies shown in Figure 36C was determined for the same devices as the distribution of short-circuit current densities shown in Figure 35A. An average power conversion efficiency of about 6 % was determined for the device 12 labelled as “I7 2PACz”. An average power conversion efficiency of about 10 % was determined for the device 12 labelled as “I7 Organic”. The higher power conversion efficiency for the device 12 labelled “I7 Organic” may be due to an improved quality, e.g. a decrease in a number of defects, at an interface between the hole transport layer 26 and the layer of perovskite material 18.

[0438] Figure 36D shows a graph of a distribution of fill factors that have been determined for a number of energy harvesting devices. The distribution of fill factors shown in Figure 36D was determined for the same devices as the distribution of short- circuit current densities shown in Figure 35A. An average fill factor of about 80 % was determined for the device 12 labelled as “I72PACz”. An average fill factor of about 70 % was determined for the device 12 labelled as “I7 Organic”. This fill factor is lower than that determined for the device 12 labelled as “I72PACz”.

[0439] Referring to Figures 35A to 36D, the power conversion efficiencies and fill factors of the devices 12 determined under indoor illumination are higher than the power conversion efficiencies and fill factors of the devices 12 determined under 1 Sun illumination. This may be due to a bandgap of the perovskite material of the devices 12 being more suitable for indoor illumination than to 1 Sun illumination.

[0440] Figure 37A shows a graph of a distribution of short-circuit current densities (Jsc) that have been determined for a number of energy harvesting devices. In this example,

[0441] 55667482-1 each device comprises a photovoltaic device, such as the device 12 shown in Figure 11 B. In this example, the layer of perovskite material 18 comprises BA2MA3Pb4li3. The hole transport layer of one of the devices 12 comprises (2-(9H-carbazol-9- yl)ethyl)phosphonic acid. This device 12 is labelled as “l13 2PACz” in Figure 37A. The hole transport layer of another one of the devices 12 comprises Poly TPD / PFN. This device 12 is labelled as “113 Organic” in Figure 37A. The short-circuit current densities were determined for 1 sun illumination (AM 1.5G, 100 mW / cm2) of each device 12. The short-circuit current densities were determined based on a number of Forward Scans, which are labelled as “FW” in Figure 37A, and a number of Reverse Scans, which are labelled as “RV” (RS) in Figure 37A. An average short-circuit density of about 1 .5 mA / cm2was determined for the device 12 labelled as “113 2PACz”. An average short-circuit density of about 1.5mA / cm2was determined for the device 12 labelled as “113 Organic”. In this example, the average short-circuit densities determined for the devices are about the same.

[0442] Figure 37B shows a graph of a distribution of open-circuit voltages (Voc) that have been determined for a number of energy harvesting devices. The distribution of open-circuit voltages shown in Figure 37B was determined for the same devices as the distribution of short-circuit current densities shown in Figure 37A. The open-circuit voltages were determined for 1 sun illumination (AM 1.5G, 100 mW / cm2) of each device 12. An average open-circuit voltage of about 0.8 V was determined for the device 12 labelled as “113 2PACz”. An average open-circuit voltage of about 1.1 V was determined for the device 12 labelled as “113 Organic”. This open-circuit voltage is higher than that determined for the device 12 labelled as “I72PACz”.

[0443] Figure 37C shows a graph of a distribution of power conversion efficiencies (PCE) that have been determined for a number of energy harvesting devices. The distribution of power conversion efficiencies shown in Figure 37C was determined for the same devices as the distribution of short-circuit current densities shown in Figure 37A. An average power conversion efficiency of about 0.3 % was determined for the device 12 labelled as “113 2PACz”. An average power conversion efficiency of about 0.5 % was determined for the device 12 labelled as “113 Organic”. The higher power conversion efficiency for the device 12 labelled “113 Organic” may be due to an improved quality, e.g. a decrease in a number of defects, at an interface between the hole transport layer 26 and the layer of perovskite material 18.

[0444] Figure 37D shows a graph of a distribution of fill factors (FF) that have been determined for a number of energy harvesting devices. The distribution of fill factors

[0445] 55667482-1 shown in Figure 37D was determined for the same devices as the distribution of short- circuit current densities shown in Figure 37A. An average fill factor of about 30 % was determined for the device 12 labelled as “113 2PACz”. An average fill factor of about 30 % was determined for the device 12 labelled as “113 Organic”.

[0446] Figure 37E shows a graph of steady state power conversion efficiencies that have been determined for a number of energy harvesting devices. The power conversion efficiencies shown in Figure 37E were determined for the same devices as the distribution of short-circuit current densities shown in Figure 37A. A steady state power conversion efficiency of about 0.17 % was determined for the device 12 labelled as “113 2PACz”. A steady state power conversion efficiency of about 0.4 % was determined for the device 12 labelled as “113 Organic”. This steady state power conversion efficiency is higher than that determined for the device 12 labelled as “I72PACz”.

[0447] Figure 38A shows a graph of a distribution of short-circuit current densities (Jsc) that have been determined for a number of energy harvesting devices. In this example, each device comprises a photovoltaic device, such as the device 12 shown in Figure 11 B. In this example, the layer of perovskite material 18 comprises BA2MA3Pb4li3. The hole transport layer of one of the devices 12 comprises (2-(9H-carbazol-9- yl)ethyl)phosphonic acid. This device 12 is labelled as “l13 2PACz” in Figure 38A. The hole transport layer of another one of the devices 12 comprises Poly TPD / PFN. This device 12 is labelled as “113 Organic” in Figure 38A. The short-circuit current densities were determined for an illumination of 1000 lux of the devices 12. The short-circuit current densities were determined based on a number of Forward Scans, which are labelled as “FW” in Figure 38A, and a number of Reverse Scans, which are labelled as “RV” (RS) in Figure 38A. An average short-circuit current density of about 0.03 mA / cm2was determined for the device 12 labelled as “113 2PACz”. An average short-circuit density of about 0.06 mA / cm2was determined for the device 12 labelled as “113 Organic”. This short-circuit current density is higher than that determined for the device 12 labelled as “I7 2PACz”.

[0448] Figure 38B shows a graph of a distribution of open-circuit voltages (Voc) that have been determined for a number of energy harvesting devices. The distribution of open-circuit voltages shown in Figure 38B was determined for the same devices as the distribution of short-circuit current densities shown in Figure 38A. An average opencircuit voltage of about 0.5 V was determined for the device 12 labelled as “113 2PACz”. An average open-circuit voltage of about 1.2 V was determined for the device 12 labelled

[0449] 55667482-1 as “113 Organic”. This open-circuit voltage is higher than that determined for the device 12 labelled as “l7 2PACz”.

[0450] Figure 38C shows a graph of a distribution of power conversion efficiencies (PCE) that have been determined for a number of energy harvesting devices. The distribution of power conversion efficiencies shown in Figure 38C was determined for the same devices as the distribution of short-circuit current densities shown in Figure 38A. An average power conversion efficiency of about 2 % was determined for the device 12 labelled as “113 2PACz”. An average power conversion efficiency of about 14 % was determined for the device 12 labelled as “113 Organic”. The higher power conversion efficiency for the device 12 labelled “113 Organic” may be due to an improved quality, e.g. a decrease in a number of defects, at an interface between the hole transport layer 26 and the layer of perovskite material 18.

[0451] Figure 38D shows a graph of a distribution of fill factors (FF) that have been determined for a number of energy harvesting devices. The distribution of fill factors shown in Figure 38D was determined for the same devices as the distribution of short- circuit current densities shown in Figure 38A. An average fill factor of about 60 % was determined for the device 12 labelled as “113 2PACz”. An average fill factor of about 60 % was determined for the device 12 labelled as “113 Organic”. In this example, the average fill factors determined for both devices are about the same.

[0452] Figure 38E shows a graph of a steady state power conversion efficiency that has been determined for an energy harvesting device. The steady state power conversion efficiency shown in Figure 38E was determined for the device labelled “113 Organic” in Figure 38A under indoor illumination. The steady state power conversion efficiency was determined as about 12 %.

[0453] Referring to Figures 37A to 38E, the power conversion efficiencies and fill factors of the devices 12 determined under indoor illumination are higher than the power conversion efficiencies and fill factors of the devices 12 determined under 1 Sun illumination. This may be due to a bandgap of the perovskite material of the devices 12 being more suitable for indoor illumination than to 1 Sun illumination.

[0454] The determined values for the open-circuit voltage, short-circuit current, fill factor and power conversion efficiency are indicative of the layers of perovskite material 18 of the devices 12 having photovoltaic properties. As such, the layers of perovskite material described herein may be considered as comprising photovoltaic and ferroelectric or piezoelectric properties. This may allow for use of one or more of these layers in an energy harvesting device.

[0455] 55667482-1 Figures 39A and 39B each show an absorbance spectrum of another exemplary layer of perovskite material. In the example shown in Figure 39A, the absorbance spectrum was measured for a layer of perovskite material comprising BA2MAPb2l?. In the example shown in Figure 39B, the absorbance spectrum was measured for layer of perovskite material BA2MA3Pb4li3.

[0456] Each absorbance spectrum has a main absorption edge, which is indicated by the dashed line in Figures 39A and 39B. This absorption edge for BA2MAPb2l? was measured at about 590 nm, which results in a bandgap of about 2.10 eV. The absorption edge for BA2MA3Pb4li3 was measured at about 650 nm, which results in a bandgap of about 1 .90 eV. This may make these layers of perovskite material suitable for indoor and / or outdoor photovoltaic applications. The absorbance spectra shown in Figures 39A and 39B may be indicative of the layers of perovskite material comprising photovoltaic properties.

[0457] In addition to this prominent absorption peak, there are small excitonic peaks in the longer wavelength regions. These excitonic peaks show the presence of higher ‘n’ phases in the 2D halide perovskites. For example, for BA2MAPb2l?, the absorption edge near 630 nm, which is indicated in Figure 39A by the dotted line, may be due to the presence of a n=3 phase. The additional peak around 660 nm may be due to the present of a n=4 phase. For BA2MA3Pb4li3, the excitonic peak near 680 nm may be due to the presence of a n=5 phase.

[0458] Figure 40A shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -40 V and 40 V and a frequency of the voltage was 40 Hz. The polarisation and current have been measured for the device 12 shown in Figure 9, which was formed according to the method described in the first exemplary embodiment in relation to Figure 12.

[0459] In this example, the layer of perovskite material 19 comprises (PEA)2PbBr4. In this example, a current-voltage response is similar to that of a ferroelectric material. This may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. A saturation polarisation of about 1 .3 pC / cm2and a coercive field (Ec) of about 200 kV / cm were determined.

[0460] Figure 40B shows a graph of a measured polarisation and a measured current of an exemplary energy harvesting device in dependence of a voltage. In this example, the voltage was varied between -65 V and 65 V and a frequency of the voltage was 40 Hz. The polarisation and current have been measured for the device 12 shown in Figure 10,

[0461] 55667482-1 which was formed according to the method described in the second exemplary embodiment in relation to Figure 12.

[0462] In this example, the layer of perovskite material 19 comprises (PEA)2MAPb2Br? and the barrier layer 22 comprises PCBM.

[0463] In this example, a current-voltage response is similar to that of a ferroelectric material. This may be indicative of the layer of perovskite material 18 acting as a ferroelectric material. A saturation polarisation of about 0.7 pC / cm2and a coercive field (Ec) of about 66 kV / cm were determined.

[0464] Figure 41 A shows a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time. The open-circuit voltages have been measured for a force of 20N applied to each device.

[0465] Figure 41 B shows a graph of measured short-circuit currents of a number of energy harvesting devices in dependence on time. The short-circuit currents have been measured for a force of 20N applied to each device. In this example, each device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 of each device comprises (PEA)2MAn-iPbnX3n+i, wherein in a first device, n=1 , in a second device, n=2, in a third device, n=3 and in a fourth device, n=4. The open-circuit voltages and the short-circuit currents were measured as described above.

[0466] Referring to Figure 41A, the measured open-circuit voltages increase with increasing n. For example, the open-circuit voltage was measured as about 4V for the first device, where n=1. The open-circuit voltage was measured as about 6V for the fourth device, where n=4.

[0467] Referring to Figure 41 B, the measured short-circuit currents increase with increasing n. For example, the short-circuit current was measured as about 1 pA for the first device, where n=1. The short-circuit current was measured as about 2 pA for the fourth device, where n=4.

[0468] The determined values for the open-circuit voltages and short-circuit currents are indicative of the layers of perovskite material 18 having piezoelectric properties.

[0469] Figure 42A shows a graph of a measured power of a number of energy harvesting devices in dependence on a resistance of a load resistor. The power has been measured for a force of 20N applied to each device.

[0470] 55667482-1 Figure 42B shows a graph of a measured power of a number of energy harvesting devices in dependence on a resistance of a load resistor. The power has been measured for a force of 20N applied to each device. The force was applied using a linear accelerometer. Each device was connected to a rectifier circuit and the load resistor. The power was obtained from measurements of output currents and voltages for varying resistances of the load resistor.

[0471] In these examples, each device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. The graph shown in Figure 42B was measured for devices comprising a further barrier layer. The further barrier layer is arranged between the layer of perovskite material 18 and the barrier layer 22, which are shown in Figure 10. In this example, the further barrier layer comprises a polymer material, such as PCBM. However, it will be appreciated that in other examples, the further barrier layer may comprise another material, such as another polymer material, metal material or any other suitable material.

[0472] The layer of perovskite material 18 of each device comprises (PEA^MAn- iPbnX3n+i, wherein in a first device, n=1 , in a second device, n=2, in a third device, n=3 and in a fourth device, n=4. It can be seen from Figures 42A and 42B, that the measured power has a maximum across the load resistance of around 3x105Ohm. The maximum power increases with n increasing from 1 to 4. This may be indicative of an increase of the piezoelectric properties of the layers of perovskite material with increasing n.

[0473] In can be seen from Figure 42B that the maximum powers measured for the devices comprising the further barrier layer are increased compared to the maximum powers measured for the devices without the further barrier layer. This may be due to the further barrier layer aiding a reduction in a diffusion of halide atoms from the layer of perovskite material 18 to the second electrode 16. This in turn may reduce a formation of short-circuiting paths and / or recombination centres in the devices.

[0474] Figure 43A shows a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time. The open-circuit voltages have been measured for a force applied to each device.

[0475] In this example, each device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 of each device 12 comprises (PEA)2MAn-i PbnXsn+i , wherein in a first device, n=1 , in a second device, n=2, in a third

[0476] 55667482-1 device, n=3 and in a fourth device, n=4. For the measurements of the open-circuit voltage of each device 12, a force was applied to each device 12 by finger tapping on the device. An active area of each device 12 was 2.5 cm2. The measured open-circuit voltages are similar for the different devices. For example, the open-circuit voltage was measured as about 6V for each device 12.

[0477] Figure 43B shows a graph of measured open-circuit voltages of an energy harvesting device in dependence on time. The open-circuit voltages have been measured for a force applied to the device. The open-circuit voltages have been measured with no illumination of the device and under illumination of the device with visible light, which is labelled as “Vis Light” in Figure 43B, and with ultraviolet light, which is labelled as “UV Light” in Figure 43B. A wavelength of the ultraviolet light is in the range of about 350 nm to 390 nm. The visible light has a spectrum of a cool white LED.

[0478] In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 comprises (PEA)2MAn-iPbnX3n+i, wherein n=4. For the measurements of the open-circuit voltages of device 12, a force was applied to the device 12 by finger tapping on the device. An active area of the device 12 was 2.5 cm2.

[0479] When the device 12 is not illuminated, an open-circuit voltage was measured as about 8V. It can be seen from Figure 43B that the measured open-circuit voltage is increased under illumination of the device 12. For example, when the device 12 is illuminated with visible light, an increased open-circuit voltage of about 9V was measured. For example, when the device 12 is illuminated with ultraviolet light, an opencircuit voltage of about 10V was measured. This open-circuit voltage is increased relative to that measured under illumination of the device 12 with visible light. The illumination of the device 12 may aid a polarisation or orientation of the molecules of the layer of perovskite material. This in turn may enhance the ferroelectric and piezoelectric properties of the layer of perovskite material.

[0480] Figure 43C shows a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time. The open-circuit voltages have been measured for a force applied to each device. The open-circuit voltages have been measured under illumination of the devices with infrared light, such as near infrared light. A wavelength of the infrared light is about 1100 nm. The devices 12 are the same as those described in relation to Figure 43A. For the measurement of the open-circuit voltage of each device 12, a force was applied to the device 12 by finger tapping on each device. The measured open-circuit voltages are similar for the different devices. For the first device where n=1 , the open-circuit voltage was measured as about 13V. For the

[0481] 55667482-1 second to fourth devices (n=2, n=3 and n=4), the open circuit voltage was measured as about 15V. In this example, the measured open-circuit voltages are higher than those measured in the examples shown in Figures 43A and 43B. This may be due to the dipole moment of the LC cations. For example, the infrared light may enable rotational motion of the LC cations and / or an improved alignment of the LC cations. This in turn may lead to an enhanced polarisation of the layer of perovskite material, which may result in the increased open-circuit voltages of the devices under illumination with the infrared light.

[0482] Figure 44 shows absorbance spectra of exemplary layers of perovskite material. In the example shown in Figure 44, the absorbance spectra were measured for (BA)2FAPb2Br?, which is labelled as n=2 in Figure 44, and (BA)2(FA)2PbsBrio, which is labelled as n=3 in Figure 44. The absorbance spectra may also be referred to as US-Vis spectra.

[0483] Each absorbance spectrum has a main absorption edge, which is indicated by the dashed lines in Figure 44 and another shoulder peak around 540 nm which could be related to the presence of higher n phases. The absorption edge for (BA)2FAPb2Br? and (BA)2(FA)2PbsBrio was measured at about 453 and 458 nm, respectively, which results in a bandgap of about 2.73 and 2.70 eV. This may allow use of the layers of perovskite materials in indoor and / or outdoor photovoltaic applications. The absorbance spectra shown in Figure 44 may be indicative of the layers of perovskite material comprising photovoltaic properties.

[0484] Figure 45A shows a graph of a measured voltage and a measured current of an energy harvesting device in dependence on a resistance of a load resistor. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 comprises (BA)2(FA)2PbsBrio.

[0485] Figure 45B shows a graph of a measured voltage and a measured current of an energy harvesting device in dependence on a resistance of a load resistor. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 comprises (BA)2(FA)Pb2Br?.

[0486] A force of 20N was applied to each device using a linear accelerometer. Each device was connected to a rectifier circuit and the load resistor. The currents and voltages were measured for varying resistances of the load resistor. In Figures 45A and

[0487] 55667482-1 45B, a power for the different resistances of the load resistor may be determined by multiplying a measured voltage with a measured current for a given resistance. A maximum power of around 0.8 pWwas measured for the device the layer of perovskite material that comprises (BA)2(FA)2PbsBrio and a maximum power of around 1.12 pW measured for the device comprising the layer of perovskite material that comprises (BA)2(FA)Pb2Br7.

[0488] Figure 46A shows a graph of measured open-circuit voltages of an energy harvesting device in dependence on time. The open-circuit voltages have been measured for a force of 10 N applied to the device. The open-circuit voltages were measured after application of a direct current voltage VDC of 0V, 5V, 10V and 20V to the device. The direct current voltage VDC is applied to the device to cause poling in the layer of perovskite material. Poling may be understood as forcing dipoles in the layer of perovskite material to orient themselves in a prescribed direction. The force was applied using a linear accelerometer. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 comprises (BA)2(FA)2PbsBrio. It can be seen from Figure 46A that the open-circuit voltage increases with increasing direct current voltage VDC. For example, when the direct current voltage VDC is 0V, the open-circuit voltage was measured as about 5 V (peak to peak). When the direct current voltage VDC is increased to 20V, the open-circuit voltage was measured as about 8V (peak to peak).

[0489] Figure 46B shows a graph of measured open-circuit voltages of an energy harvesting device in dependence on time. The open-circuit voltages have been measured for a force of 10 N applied to the device. Different direct current voltage VDC were applied to the device to cause poling in the layer of perovskite material, as described in relation to Figure 46A. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 comprises (BA)2FAPb2Br?. It can be seen from Figure 46B that the open-circuit voltage increases with increasing direct current voltage VDC. For example, when the direct current voltage VDC is 0V, the open-circuit voltage was measured as about 5 V (peak to peak). When the direct current voltage VDC is increased to 20V, the open-circuit voltage

[0490] 55667482-1 was measured as about 8V (peak to peak). The measurements shown in Figure 46A and 46B are indicative of the layers of perovskite material having piezoelectric properties.

[0491] Figure 47A shows a graph of measured open-circuit voltages of an energy harvesting device in dependence on time. The open-circuit voltages have been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises (BA)2FAPb2Br?. The barrier layer 22 comprises PDMS.

[0492] For the measurements of the open-circuit voltage of the device 12, a force was applied to the device 12 by arm folding, finger tapping and foot pressing on the device. For example, arm folding may bend the device, thereby applying a bending force to the device. An active area of the device 12 was 2.5 cm2. An open-circuit voltage of about 2V to 6 V peak to peak was measured.

[0493] Figure 47B shows a graph of measured open-circuit voltages of an energy harvesting device in dependence on time. The open-circuit voltages have been measured for different forces applied to the device. In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the layer of perovskite material comprises (BA)2(FA)2PbsBrio. The barrier layer 22 comprises PDMS.

[0494] For the measurements of the open-circuit voltage of the device 12, a force was applied to the device 12 by arm folding, finger tapping and foot pressing on the device. An active area of the device 12 was 2.5 cm2. An open-circuit voltage of about 2 V to about 6 V peak to peak was measured of the LC cations and / or an improved alignment of the LC cations. This in turn may lead to an enhanced polarisation of the layer of perovskite material, which may result in the increased open-circuit voltages under illumination with the infrared light.

[0495] Figure 48A shows a graph of measured open-circuit voltages of a number of energy harvesting devices in dependence on time. The open-circuit voltages have been measured for a force applied to each device.

[0496] In this example, each device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The layer of perovskite material 18 of each device comprises

[0497] 55667482-1 (BA)2(FA)n-iPbnBr3n+i, wherein in a first device, n=2, in a second device, n=3, in a third device, n=4 and in a fourth device, n=5. For the measurements of the open-circuit voltage of each device 12, a force was applied to the device 12 by finger tapping on the device. An active area of the device 12 was 2.5 cm2. The measured open-circuit voltages are similar for the different devices. For example, the open-circuit voltage was measured as about 6V for each device 12.

[0498] Figure 48B shows a graph of measured open-circuit voltages of an energy harvesting devices in dependence on time. The open-circuit voltages have been measured for a force applied to the device under illumination with infrared light, which is labelled as “”IR Light” in Figure 48B. The infrared light comprises the same wavelength as that described in relation to Figure 43C. The devices are the same as those mentioned in relation to Figure 48A. The force was applied to each device 12 by finger tapping.

[0499] It can be seen from Figure 48B that the measured open-circuit voltage increased under illumination of the devices 12 with infrared light relative to the open-circuit voltages shown in Figure 48A. For example, when the devices 12 are illuminated with infrared light, an increased open-circuit voltage of about 10V was measured.

[0500] Figure 48C shows a graph of measured open-circuit voltages of an energy harvesting devices in dependence on time. The open-circuit voltages have been measured for a force applied to the device under illumination with ultraviolet light, which is labelled as “UV Light” in Figure 48C. The ultraviolet light comprises the same wavelength as that described in relation to Figure 43B. The devices are the same as those mentioned in relation to Figure 48A.

[0501] It can be seen from Figure 48C that the measured open-circuit voltages are increased under illumination of the devices 12 with ultraviolet light relative to the opencircuit voltages shown in Figure 48A. For example, when the devices 12 are illuminated with ultraviolet light, an increased open-circuit voltage of about 8V was measured. The open-circuit voltages measured under illumination of the devices 12 with ultraviolet light are lower relative to the open-circuit voltages shown in Figure 48B. The illumination of the device 12 may aid a polarisation or orientation of the molecules of the layer of perovskite material. This in turn may enhance the ferroelectric and piezoelectric properties of the layer of perovskite material. The increased open-circuit voltages measured under illumination of the devices with infrared light may be due to the dipole moment of the LC cations. For example, the infrared light may enable rotational motion of the LC cations and / or an improved alignment of the LC cations. This in turn may lead

[0502] 55667482-1 to an enhanced polarisation of the layer of perovskite material, which may result in the increased open-circuit voltages under illumination with the infrared light.

[0503] Figure 48D shows a graph of measured open-circuit voltages of an energy harvesting devices in dependence on time. The open-circuit voltages have been measured for a force applied to the device under illumination with visible light. The visible light comprises the same spectrum as that described in relation to Figure 43B. The devices are the same as those mentioned in relation to Figure 48A.

[0504] It can be seen from Figure 48D that the measured open-circuit voltages are increased under illumination of the devices 12 with visible light relative to the open-circuit voltages shown in Figure 48A. For example, when the devices 12 are illuminated with visible light, an increased open-circuit voltage of about 7.5 V was measured. The opencircuit voltage measured under illumination of each device 12 with visible light are lower relative to the open-circuit voltages shown in Figure 48B. As described above, this may be due to the dipole moment of the LC cations. For example, the infrared light may enable rotational motion of the LC cations and / or an improved alignment of the LC cations. This in turn may lead to an enhanced polarisation of the layer of perovskite material, which may result in the increased open-circuit voltages under illumination with the infrared light.

[0505] Figure 49 shows a graph of measured current densities in dependency on voltages applied to an energy harvesting device. The device comprises a photovoltaic device, such as the device 12 shown in Figure 11 A. In this example, the layer of perovskite material 18 comprises (PEA)2 MAsPb4 I , the hole transport layer 26 comprises Poly(3-hexylthiophene) (P3HT), the electron transport layer 24 comprises tin oxide (SnC>2), the second electrode 16 comprises gold, the first electrode 14 comprise indium tin oxide (ITO) and the substrate layer 20 comprises glass. A thickness of the layer of perovskite material 18 is between about 600nm and about 1000nm. It will be appreciated that in other embodiment one or more different materials may be used for one or more layers of the device.

[0506] The current densities were measured under 1 Sun illumination (AM 1.5G, 100 mW / cm2) of the device 12. The current densities were measured in a number of Forward Scans, which are labelled as “FW” in Figure 49. During a Forward Scan, the voltage across the device 12 was increased from zero to the open-circuit voltage. The current densities were also measured in a number of Reverse Scans, which are labelled as “RV” in Figure 49. During a Reverse Scan, the voltage across the device 12 was decreased from the open-circuit voltage to zero. The current densities were measured for a device 12 with a layer of perovskite material that has been annealed at room temperature for 5

[0507] 55667482-1 minutes, followed by thermal annealing at 100°C for 10 minutes. The layer of perovskite material was prepared at room temperature using the blade coating method described herein.

[0508] From the measured current density curves shown in Figure 49, a short-circuit current density of about -8.9 mA / cm2, an open-circuit voltage of about 0.5 V, a power conversion efficient of about 1.4 %, a fill factor of about 30%, a maximum power of about -1 .4 W, a shunt resistance of about 64.6 Ohm*cm2, and a series resistance of about 33.6 Ohm*cm2was determined in the Forward Scans.

[0509] From the measured current density curves shown in Figure 49, a short-circuit current density of about -9.1 mA / cm2, an open-circuit voltage of about 0.7 V, a power conversion efficient of about 2.1 %, a fill factor of about 35%, a maximum power of about -2.1 W, a shunt resistance of about 128.7 Ohm*cm2, and a series resistance of about

[0510] 27.6 Ohm*cm2was determined in the Reverse Scans.

[0511] Figure 50A shows a graph of measured current densities in dependency on voltages applied to an energy harvesting device. The device comprises a photovoltaic device, such as the device 12 described in relation to Figure 49.

[0512] The current densities were measured under indoor illumination of the device 12 by a white (5000 K) light emitting diode using an illuminance of 200 lux. As described above, the current densities were measured in a number of Forward Scans, which are labelled as “FW” in Figure 50A. The current densities were also measured in a number of Reverse Scans, which are labelled as “RV” in Figure 50A.

[0513] From the measured current density curves shown in Figure 50A, a short-circuit current density of about -0.03 mA / cm2, an open-circuit voltage of about 0.3 V, a power conversion efficient of about 6.7 %, a fill factor of about 46%, a shunt resistance of about

[0514] 13047.6 Ohm*cm2, and a series resistance of about 2410.8 Ohm*cm2was determined in the Forward Scans.

[0515] From the measured current density curves shown in Figure 50A, a short-circuit current density of about -0.03 mA / cm2, an open-circuit voltage of about 0.3 V, a power conversion efficient of about 7.4 %, a fill factor of about 47%, a shunt resistance of about 42543.2 Ohm*cm2, and a series resistance of about 1522.3 Ohm*cm2was determined in the Reverse Scans.

[0516] Figure 50B shows a graph of measured current densities in dependency on voltages applied to an energy harvesting device. The device comprises a photovoltaic device, such as the device 12 described in relation to Figure 49.

[0517] 55667482-1 The current densities were measured under indoor illumination of the device 12 by a white (5000 K) light emitting diode using an illuminance of 1000 lux. As described above, the current densities were measured in a number of Forward Scans, which are labelled as “FW” in Figure 50B. The current densities were also measured in a number of Reverse Scans, which are labelled as “RV” in Figure 50B.

[0518] From the measured current density curves shown in Figure 50B, a short-circuit current density of about -0.08 mA / cm2, an open-circuit voltage of about 0.3 V, a power conversion efficient of about 2.7%, a fill factor of about 36%, a maximum power of about -0.01 W, a shunt resistance of about 4685.6 Ohm*cm2, and a series resistance of about 1408.7 Ohm*cm2was determined in the Forward Scans.

[0519] From the measured current density curves shown in Figure 50B, a short-circuit current density of about -0.09 mA / cm2, an open-circuit voltage of about 0.4 V, a power conversion efficient of about 5 %, a fill factor of about 50%, a maximum power of about -0.02, a shunt resistance of about 14124.5 Ohm*cm2, and a series resistance of about 721 .8 Ohm*cm2was determined in the Reverse Scans.

[0520] Figure 51 A shows a graph of measured current densities in dependency on voltages applied to an energy harvesting device. The device comprises a photovoltaic device, such as the device 12 described in relation to Figure 49. However, in this example, the layer of perovskite material 18 comprises (PEA)2 MA2Pbs ho.

[0521] The current densities were measured under 1 Sun illumination (AM 1.5G, 100 mW / cm2) of the device 12. The current densities were measured in a number of Forward Scans, which are labelled as “FW” in Figure 51A. The current densities were also measured in a number of Reverse Scans, which are labelled as “RV” in Figure 49. The current densities were measured for a device 12 with a layer of perovskite material that has been vacuum annealed at 100°C for 10 minutes. The layer of perovskite material was prepared at room temperature using the blade coating method described herein.

[0522] From the measured current density curves shown in Figure 51A, a short-circuit current density of about -10 mA / cm2, an open-circuit voltage of about 0.7 V, a power conversion efficient of about 1.9 %, a fill factor of about 26%, a maximum power of about -1 .9 W, a shunt resistance of about 45 Ohm*cm2, and a series resistance of about 61.5 Ohm*cm2was determined in the Forward Scans.

[0523] From the measured current density curves shown in Figure 51A, a short-circuit current density of about -8.5 mA / cm2, an open-circuit voltage of about 0.9 V, a power conversion efficient of about 3.2 %, a fill factor of about 42%, a maximum power of about

[0524] 55667482-1 -3.2 W, a shunt resistance of about 50699.6 Ohm*cm2, and a series resistance of about 45.4 Ohm*cm2was determined in the Reverse Scans.

[0525] Figure 51 B shows a graph of measured current densities in dependency on voltages applied to an energy harvesting device. The device comprises a photovoltaic device, such as the device 12 described in relation to Figure 51A.

[0526] The current densities were measured under indoor illumination of the device 12 by a white (2700 K - warm light) light emitting diode using an illuminance of 1000 lux. The current densities were measured in a number of Forward Scans, which are labelled as “FW” in Figure 51 B. The current densities were also measured in a number of Reverse Scans, which are labelled as “RV” in Figure 51 B.

[0527] From the measured current density curves shown in Figure 51 B, a short-circuit current density of about -0.1 mA / cm2, an open-circuit voltage of about 0.6 V, a power conversion efficient of about 6.3 %, a fill factor of about 32%, a maximum power of about -0.02 W, a shunt resistance of about 8414.5 Ohm*cm2, and a series resistance of about 3896.7 Ohm*cm2was determined in the Forward Scans.

[0528] From the measured current density curves shown in Figure 51 B, a short-circuit current density of about -0.09 mA / cm2, an open-circuit voltage of about 0.8 V, a power conversion efficient of about 12%, a fill factor of about 51%, a maximum power of -0.04 W, a shunt resistance of about 37740.1 Ohm*cm2, and a series resistance of about 1522.3 Ohm*cm2was determined in the Reverse Scans.

[0529] At least the above determined values for the open-circuit voltage, short-circuit current and power conversion efficiency are indicative of the layers of perovskite material 18 comprising (PEA)2 MA2Pbs ho and (PEA)2 MAsPb4 I , respectively, of the devices 12 having photovoltaic properties. As such, the layers of perovskite material described herein may be considered as being photovoltaic and ferroelectric or piezoelectric. This may allow for use of one or more of these layers in an energy harvesting device.

[0530] Figure 52A shows a graph of determined piezoelectric coefficients in dependency on a force applied to a number of energy harvesting devices. A static pre-load force was applied to each device by clamping each device. The status pre-load force was varied from 1 N to 10 N. A dynamic force of about 0.1 N or 0.25N was also applied. The piezoelectric properties of the layer of perovskite material 18 were measured using the Berlincourt method described above.

[0531] In this example, each device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22

[0532] 55667482-1 comprises PCBM. The first and second electrodes 14, 16 each comprise aluminium. A thickness of the first electrode 14 is about 50 nm. A thickness of the second electrode 16 is about 100 nm. The substrate layer 20 comprises glass. In the example shown in Figure 52A, the layer of perovskite material 18 comprises (PEA)2 MAsPb4 I . It will be appreciated that in other embodiments, the device may be provided without the barrier layer and / or one or more layers of the device may comprise a different thickness and / or material.

[0533] In Figure 52A, each device 12 is labelled as a “pixel.” Each substrate layer has an area of 2 cm times 2 cm and includes 8 devices 12. Each device has an area of about 4 mm times 2.5 mm.

[0534] From the graph shown in Figure 52, a maximum piezoelectric coefficient (d33) of about 1.7 pC / N was determined for some pixels.

[0535] Figure 52B shows a graph of determined piezoelectric coefficients in dependency on a force applied to an energy harvesting device. In the example shown in Figure 52B, the device 12 is the same as those described in relation to Figure 52A. However, the layer of perovskite material 18 comprises (PEA)2 MA2Pbs ho. The piezoelectric coefficient was determined in the same manner as that described in relation to Figure 52A. A piezoelectric coefficient (d33) of about 2.25 pC / N was determined in this example. The determined piezoelectric coefficient is higher than that determined in the example described in relation to Figure 52A.

[0536] Figure 52C shows a graph of determined piezoelectric coefficients in dependency on a force applied to a number of energy harvesting devices. In this example, each device is the same as that described in relation to Figure 52B. In this example, a positive piezoelectric coefficient (d33) of about 6 pC / N was determined for some devices, e.g. pixels, while some other devices, e.g. pixels, showed a negative piezoelectric coefficient (d33) ranging from about -0.15 to 6 pC / N, as shown in Figure 52C.

[0537] Figure 53A shows a graph of measured open-circuit voltages of an energy harvesting device in dependence on time. The open-circuit voltage has been measured for a force (of about 20 N) applied to the device under different illumination sources. The open-circuit voltage may also be referred to as a piezo voltage.

[0538] In this example, the device comprises a piezoelectric device, such as the device 12 shown in Figure 10, which was formed according to the method described in the fourth exemplary embodiment in relation to Figure 12. In this example, the barrier layer 22 comprises PDMS. The first and second electrodes 14, 16 each comprises indium tin oxide (ITO). The substrate layer 20 and the further substrate layer each comprise a

[0539] 55667482-1 polymer material, such as a Polyethylene terephthalate (PET). The layer of perovskite material 18 of the device 12 comprises (BA)2(FA)2PbnBrio. For the measurements of the open-circuit voltage of the device 12, a force of 20 N was applied to the device 12 by a linear accelerator. An active area of the device 12 was 2.5 cm2. The measured opencircuit voltage has been measured while no illumination was present, which is labelled as “w / o light” in Figure 53A, and while the device 12 was illuminated with visible light, ultraviolet light, which is labelled as “UV” in Figure 53A, and infrared light, which is labelled as “IR” in Figure 53A. The visible light, ultraviolet light and the infrared light comprise any of the features of the visible light, ultraviolet light and the infrared light described above.

[0540] As can be seen in Figure 53A, the open-circuit voltage is reduced under illumination of the device 12 with ultraviolet and visible light, while the open-circuit voltage is increased under illumination of the device 12 with infrared light. The open-circuit voltage is decreased by a factor of approximately 0.7 times under illumination of the device 12 with visible light, but this value is around 0.6 times under illumination of the device with ultraviolet light. The open-circuit voltage can be considered as a surface phenomenon. As such, photo-generated charge carriers may affect the open-circuit voltage neutralization (screening), under illumination of the device with ultraviolet and visible light. This effect may be more apparent under illumination of the device with ultraviolet light due to more photo-induced light carriers being generated than with visible light, which may resist the open-circuit voltage even more.

[0541] Figure 53B shows a graph of measured current of an energy harvesting device in dependence on time. The current has been measured for a force applied to the device. In this example, the force applied to the device is the same as that described in relation to Figure 53A. The current may be a short-circuit current. The current may also be referred to as a piezo-generated current or piezo current.

[0542] In this example, the device is the same as the device 12 described in relation to Figure 53A. For the measurements of the current, a force of 20 N was applied to the device 12 by a linear accelerator on the device. The measured current has been measured while no illumination was present, which is labelled as “w / o light” in Figure 53B and while the device was illuminated with visible light, ultraviolet light, which is labelled as “UV” in Figure 53B, and infrared light, which is labelled as “IR” in Figure 53B. The visible light, ultraviolet light and the infrared light comprises any of the features of the visible light, ultraviolet light and the infrared light described above.

[0543] 55667482-1 From Figure 53B, it can be seen that the current increases, when the device 12 is illuminated with ultraviolet and visible light sources. The piezo-generated current signal may be weaker, but the light-induced additional electron-hole pair generation may increase a conductivity of the layer or perovskite material 18. This may improve a mobility of the charge carriers, which may result in an increase of the current.

[0544] As can be seen in Figure 53A and 53B, the open-circuit voltage and current vary depending on the wavelength of the light, which is used to illuminate the device 12. The current enhancement is highest for infrared illumination, which indicates the pyroelectric properties of the films, whereas the open-circuit voltage is enhanced for illumination of the device 12 with visible and ultraviolet light. These pyroelectric properties of these materials can be further used for harvesting the periodic thermal variations in the ambient or surroundings.

[0545] Bandgap excitation may be possible, when the device 12 is illuminated with ultraviolet and visible light, whereas the illumination of the device 12 with infrared light can cause trapped charges to be released or activate the pyroelectric properties. However, visible light-generated photo carriers in the layer of perovskite material 18 may be relatively lower than when the device 12 is illuminated with ultraviolet light. This may be because most of the visible spectrum is outside of the bandgap of the material of the layer of perovskite material 18. Therefore, the enhancement of the current, when the device 12 is illuminated with visible light, is 1.6 times, whereas, when the device 12 is illuminated with ultraviolet light, the enhancement of the current increases to 1.7 times. In addition, a rapid rotational disordering of the inorganic sublattice may also be possible for excitation above the bandgap of the perovskite material.

[0546] In contrast, the open-circuit voltage and current increases 1.3 times and 1.2 times, respectively, when the device 12 is illuminated with infrared light. However, this photo energy is below the band gap of the material of the layer of perovskite material 18, so no band-to-band absorption may occur. Instead, the infrared light may generate additional charges due to the pyroelectric (infrared light on / off cycles) effect, which complements the piezoelectric signal. As a result, the open-circuit voltage and current is increased. It has been previously shown that, under infrared light excitation, the rotational dynamics of organic spacer cations can re-orient their dipole moment and the resulting lattice coupling can induce distortions and changes in the surrounding halide octahedra.

[0547] Figure 54 shows absorbance spectra of exemplary layers of perovskite material. In the example shown in Figure 54, the absorbance spectra were measured for BA2PbBr4, which is labelled as n=1 in Figure 54, BA2FAPb2Br?, which is labelled as n=2

[0548] 55667482-1 in Figure 54, BA2FA2Pb3Br , which is labelled as n=3 in Figure 54, BA2FAsPb4Bri3, which is labelled as n=4 in Figure 54, and BA2FA4PbsBri6, which is labelled as n=5 in Figure 54. The absorbance spectra may also be referred to as US-Vis spectra. In this example, each layer of perovskite material was deposited on a substrate comprising glass.

[0549] Each absorbance spectrum has an absorption edge, which is indicated by the dashed line in Figure 54. The absorption edge wavelength corresponding to each n value is about 415 nm (n= 1 ), 460 nm (n=2), 462 nm (n=3), 474 nm (n=4) and 511 nm ( n=5). The wavelengths correspond to bandgaps of about 2.99 eV (n= 1 ), 2.69 eV (n=2), 2.68 eV (n=3), 2.62 eV (n=4) and 2.43 eV. This may allow for use of the layers of perovskite materials in indoor and / or outdoor photovoltaic applications. The absorbance spectra shown in Figure 54 may be indicative of the layers of perovskite material comprising photovoltaic properties. No defect related emission peaks can be observed in the absorbance spectra shown in Figure 54. This may be indicative of a high quality of the layers of perovskite material.

[0550] It will be understood that references to a plurality of features may be interchangeably used with references to singular forms of those features, such as for example “at least one” and / or “each”. Singular forms of a feature, such as for example “at least one” or “each,” may be used interchangeably.

[0551] Although the disclosure has been described in terms of embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

[0552] 55667482-1

Claims

CLAIMS1 . A layer of perovskite material for use in an energy harvesting device, the layer having a thickness of less than 60 pm and the perovskite material comprising crystalline material of formula (LC)2(SC)n-iMnX3n+i with a polar crystallographic point group; wherein: each LC is a cation of formula A-B-C, wherein:A is selected from ethyl, ethenyl, ethynyl, Ci-4alkoxymethyl, Ci-4alkylthio, aryl, and heteroaryl, each of which is optionally substituted one or more times with fluoro and wherein the aryl and heteroaryl are optionally substituted one or more times with a substituent selected from Ci-4alkyl, C^alkenyl, C^alkynyl, Ci-4fluoroalkyl, C2- 4fluoroalkenyl, C^fluoroalkynyl, Ci-4alkoxy, hydroxy, formyl, carboxyl and heteroaryl;B is absent or is selected from Ci-ealkylene, C^alkenylene and C^alkynylene, each of which is optionally substituted with fluoro; andC is -N(R1)3+or -CR1N(R1)2N(R1)2+, wherein each R1is independently selected from H and Ci-ealkyl; each SC is a cation selected from Cs+, N(R2)4+and R2CN(R2)2N(R2)2+, wherein each R2is independently selected from H and methyl; each M is selected from Pb2+, Sn2+and Ge2+; each X is halide; n is an integer selected from 1 to 6; and the perovskite material comprising photovoltaic and ferroelectric or piezoelectric properties.

2. The layer of claim 1 , wherein n is an integer selected from 3 to 6.

3. The layer of claim 1 or claim 2, wherein each X is selected from chloride, iodide or a combination of two or more halides.

4. The layer of any one preceding claim, wherein the polar crystallographic point group is an orthorhombic crystal system with a polar space group of Cmc2i.

5. The layer of any one preceding claim, wherein A is selected from ethyl, ethenyl, ethynyl, trifluoromethyl, and phenyl, wherein the phenyl is optionally substituted one or more times with a substituent selected from Ci-4alkyl and fluoro.55667482-16. The layer of any one preceding claim, wherein the Ci-6alkylene, C^alkenylene and C^alkynylene are linear.

7. The layer of any one preceding claim, wherein B is selected from Ci-4alkylene, C^alkenylene and C^alkynylene, each of which is optionally substituted with fluoro.

8. The layer of any one preceding claim, wherein each R1is independently selected from H and methyl.

9. The layer of any one preceding claim, wherein C is -N(H)s+.

10. The layer of any one preceding claim, wherein each LC is butylammonium or phenethylammonium.

11. The layer of any one preceding claim, wherein each R2is independently selected from H and methyl.

12. The layer of any one preceding claim, wherein each SC is methylammonium or formamidinium.

13. The layer of any one preceding claim, wherein each M is Pb2+.

14. An energy harvesting device comprising: a first electrode; a second electrode; and a layer of perovskite material according to any preceding claim, the layer of perovskite material being arranged between the first and second electrodes.

15. The device of claim 14, wherein the device comprises a ferroelectric device and / or a piezoelectric device.

16. The device of claim 15, wherein the device further comprises a barrier layer arranged between at least one of the first and second electrodes and the layer of perovskite material.55667482-117. The device of claim 16, wherein the barrier layer comprises a metal material or a polymer material.

18. The device of claim 17, wherein the metal material comprises Cr, C^Os, and / or Barium and / or the polymer material comprises phenyl-Cei butyric acid methyl ester, Bathocuproine, polymeric C60, styrene ethylene butylene styrene or polymethyl methacrylate.

19. The device of claim 14, wherein the device comprises a photovoltaic device, the device further comprising: an electron transport layer arranged between at least one of the first and second electrodes and the layer of perovskite material; and a hole transport layer arranged between at least one other of the first and second electrodes and the layer of perovskite material, wherein at least one of the first and second electrodes is configured to be transparent to at least a portion of light to be absorbed by the layer of perovskite material.

20. A method of forming a layer perovskite material, the method comprising: depositing a layer of precursor perovskite material on a support layer using a deposition process; and annealing the deposited layer of precursor perovskite material at a temperature of about 25 °C to about 125 °C to form a layer of perovskite material according to any one of claims 1 to 13.

21. The method of claim 20, wherein the deposition process comprises a blade coating process or a spin-coating process, wherein optionally the blade coating process uses a blade speed between about 1 mm / s and about 30 mm / s.

22. The method of claim 20 or 21 , wherein the method comprises annealing the deposited layer of precursor perovskite material at a pressure between about 0.01 bar and about 1 bar.+23. A layer of perovskite material having a thickness of less than 60 pm obtainable by the method of any one of claims 20 to 22.55667482-124. An energy harvesting device comprising the layer of perovskite material of claim 23.

25. Use of the layer of perovskite material of any one of claims 1 to 13 or claim 23 in an energy harvesting device.55667482-1

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