Tin perovskites, methods for preparation and applications therefor

Abstract

A process for producing a layer of a tin perovskite comprising: producing a precursor layer of a tin perovskite precursor on a substrate from a precursor solution comprising tin perovskite precursors in a solvent; subjecting the precursor layer to aging under controlled time and temperature conditions to regulate tin perovskite crystal growth to produce an aged layer; and annealing the aged layer to produce the tin perovskite. Additionally contemplated is a tin halide perovskite of formula: (CsX1)w(PEAyFA(1-y)SnX2)(1-w) wherein X1 is a halide or pseudohalide, 0.03≤w≤0.07, 0<y<1.0, and X2 is a halide, or a combination of two halides. Also, a device comprising a layer of a tin halide perovskite, wherein the tin halide perovskite is a mixed cation tin halide perovskite, comprising between 3 and 7 mol% caesium, with respect to the total moles of caesium and tin, and a combination of two organic cations.

Description

Tin perovskites, methods for preparation and applications thereforFIELD

[0001] The present application relates to new methods for the production of tin perovskites, new perovskite materials and devices incorporating the perovskites. The perovskites have particular application in photovoltaic devices, such as solar cells. This application claims priority from Australian provisional patent application 2024903310, the entirety of which is incorporated herein by this cross-reference.BACKGROUND

[0002] Lead halide perovskites have garnered enormous interest from both academia and industry for next-generation low-cost thin-film photovoltaic (PV) technologies, however a major disadvantage of leadbased perovskites is their toxicity. Tin halide perovskites (THPs) have emerged as a potential alternative to lead halide perovskites, due to their non-toxic nature and bandgap suitability for solar spectra harvesting. However tin halide perovskites suffer from a number of challenges. Those challenges include the rapid oxidation from Sn2+to Sn4+, fast crystallization kinetics leading to disoriented growth and therefore poor film quality, and poor power conversion efficiencies (PCE). There is still a considerable performance gap between lead-halide perovskites and tin-halide perovskites in terms of the achievable PCEs. To date, the best reported PCE for a tin-based perovskite is 15.67%.

[0003] Researchers in the field have explored a variety of approaches to improve tin perovskites materials and their preparation to improve their performance in photovoltaic applications, in terms of efficiency (PCE) and stability. Some strategies rely on controlling the reagents and solvent combinations to improve controlled crystal growth. In one approach, a so-called “one-step” procedure is proposed for the preparation of tin perovskites. Sn l2-dimethylsulfoxide (DMSO) adducts are prepared with enhanced co-ordination, and when combined with other reagents the desired tin perovskite are produced with out-of-plane oriented crystal growth, yielding films with fewer pinholes and a more uniform orientation. (Jiang et al, One-Step Synthesis of Snl2.(DMSO)xAdducts for High-Performance Tin Perovskite Solar Cells, J. Am. Chem. Soc. 2021 , 143, 10970 - 10976 Other approaches have focused on improving defect passivation, working on the interface between the tin perovskite layer of the device and adjacent layers, and using different additives as crystallization templates to control the crystallization process.

[0004] While a range of approaches have been explored, there remains a need for improvements in the morphology and quality of tin perovskites, and improved methods for the production of tin perovskite films for use in photovoltaic applications. It is an object of the present application to provide improved processes for the production of tin perovskite films, and new tin halide perovskite materials, or to at least provide the public with a useful choice.SUMMARY

[0005] According to the present application, there is provided a process for the production of a layer of a tin perovskite, the process comprising:- producing a precursor layer of a tin perovskite precursor on a substrate from a precursor solution, the precursor solution comprising tin perovskite precursors in a solvent;- subjecting the precursor layer to aging under controlled time and temperature conditions to regulate tin perovskite crystal growth to produce an aged layer, and- annealing the aged layer to produce the tin perovskite.

[0006] The tin perovskite is preferably a tin halide perovskite, and the tin perovskite precursor is preferably a tin halide perovskite precursor.

[0007] In some embodiments, one or more of the following conditions apply:- the aging is performed at an aging temperature of between 55°C and -40°C and an aging time of between 1 and 240 minutes, with the proviso that when the aging temperature is between 20°C and 55 °C, the aging time is a minimum of 10 minutes;- the precursor layer of tin perovskite precursor is produced by applying said precursor solution to the substrate at a precursor solution temperature of less than 20°C;- the precursor layer of tin perovskite precursor is produced by applying said precursor solution on the substrate and then applying an antisolvent solution onto the precursor solution at an antisolvent temperature of less than 20°C; or- the tin perovskite is a mixed cation tin halide perovskite, comprising between 3 and 7% cesium, based on as a mol% with respect to the total moles of cesium and tin, and a combination of two organic cations.

[0008] According to one particular embodiment, there is provided a process for the production of a layer of a tin perovskite, the process comprising:- producing a layer of a perovskite precursor solution on a substrate, wherein the perovskite precursor solution comprises precursors of the tin perovskite in a solvent;- subjecting the layer of perovskite precursor solution to aging at an aging temperature of between 55°C and - 40°C for a time period of between 1 and 240 minutes to produce an aged layer, with the proviso that when the aging temperature is between 20°C and 55°C the time period is a minimum of 10 minutes, and- annealing the aged layer of tin perovskite.

[0009] Any tin perovskite of any desired formula may be produced in layer form with improved morphology making use of the process as outlined above. In some embodiments, the tin perovskite is a tin halide perovskite.

[0010] Independently of this, the applicant has developed a new tin halide perovskite composition, and new devices comprising the tin halide perovskite, with advantageous properties. In relation to the new tin halide perovskite and devices, while it is highly advantageous to make use of the above method for the production of a layer of the tin halide perovskite in such devices, alternative methods could be used, or a combination of techniques could be used (including the above-described process) for controlling the crystallization of the tin halide perovskite.

[0011] In a second aspect, there is provided a tin halide perovskite based on a composition of formula (I):(CsX1)w(PEAyFA(i-y)SnX23)(i-w) formula (I) wherein X1is a halide or pseudohalide,0.03 < w < 0.07,0 < y < 1.0, andX2is a halide, or a combination of two halides.

[0012] In a third aspect, the present application provides a device comprising a layer of a tin halide perovskite, wherein the tin halide perovskite is a mixed cation tin halide perovskite, comprising between 3 and 7% cesium, based on mol% with respect to the total moles of cesium and tin, and a combination of two organic cations. Preferably, the tin halide perovskite is based on a composition of formula (I) as defined above.

[0013] The device may be a photovoltaic device, a thermoelectric device, a sensor, a memory device, a light emitting device. In some embodiments the device is a photovoltaic device. The photovoltaic device may be a solar cell, a light emitting diode (LED), detector or photodetector.

[0014] The photovoltaic device may be a solar cell.

[0015] The present application further provides a tin perovskite produced by the process described above, and a device comprising the tin perovskite produced by the process.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Various examples and embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0017] Figure 1 is a schematic diagram illustrating the steps in the process for the preparation of a layer of tin perovskite in accordance with one embodiment, with each sequential step illustrated in parts (a) to (f).

[0018] Figure 2 is a schematic diagram of the layers of a photovoltaic device in the form of a solar cell of one embodiment, incorporating a layer of tin perovskite.

[0019] Figure 3 presents the data demonstrating colloidal stabilization and crystallization kinetics of tin halide perovskite films with and without Cs+incorporation, with the following parts: (a) Schematic illustration of colloidal stabilization of PEA-rich colloids in the precursor solutions, and crystal nucleation and growth processes of 2D-3D THPs with and without Cs+incorporation, (b) Colloidal cluster size distributions of THP precursor solutions with different Cs+ratios from 0-20 mol% characterized by dynamic light scattering.(c) In-situ observation of XRD patterns of CsO and Cs5 wet films upon post-annealing treatment starting from 20°C to 80°C (ramp rate: 1 °C min1) and staying at 80°C for 10 min. (d) XRD patterns (3-15°) of CsO and Cs5 wet films before annealing, (e) XRD patterns of polycrystalline CsO and Cs5 films after post-annealing treatment (insets are amplified diffraction peaks of 2D perovskites within 3-13°).

[0020] Figure 4 demonstrates tin halide perovskite phase distribution and crystal orientation, including: (a) 2D GIWAXS patterns of CsO and Cs5 THP films with selected incident angles of 0.05°, 0.16°, 0.20°, and 0.50°, where the missing out-of-plane wedges were omitted for clarity, (b) Azimuthal distribution of (110)3D plane of CsO and Cs5 THP films, with varying X-ray scattering depth defined by incident angles, (c) Azimuthal FWHM variations of (110)3D plane of CsO and Cs5 THP films with different incident angles. ToF- Sl MS depth profiles of (d) CsO and (e) Cs5 THP films.

[0021] Figure 5 shows the device features, performance and stability test results for a device of one embodiment, including: (a) Schematic illustration of the device structure and the corresponding cross- sectional SEM image of a tin halide perovskite solar cell produced in accordance with one embodiment, (b) J- I / curves and stabilized power outputs of exemplary devices containing the tin halide perovskite with CsO and Cs5. (c) EQE plots and integrated photocurrents of CsO and Cs5 devices, (d) Statistical PCE distributions of THPSCs with different Cs molar ratios (20 devices for each condition from different batches), (e) The comparison between certified PCE values from the previously reported THPSCs and this work, (f) The shelf stability of unencapsulated THPSCs under dark conditions in N2 at 35 ± 5°C for 6,000 hours, (g) The shelf stability of unencapsulated THPSCs under dark conditions in N2 at 35 ± 5°C for 5,000 hours, where the THPSCs tested were produced under “target” conditions and compared to those produced under “control” conditions, (h) The operational stability of unencapsulated THPSCs under continuous one-sun illumination at open-circuit status in N2 at 60 ± 5°C for 1 ,500, where the THPSCs tested were produced under “target” and “control” conditions, (i) J-V curves of exemplary devices containing the tin halide perovskite produced under “target” conditions and compared to those produced under “control” conditions, (j) EQE plots and integrated photocurrents of devices containing the tin halide perovskite produced under “target” conditions and compared to those produced under “control” conditions, (k) Statistical PCE distributions of THPSCs containing the tin halide perovskite produced under “target’ conditions and compared to those produced under “control” conditions (20 devices for each condition from different batches).

[0022] Figure 6 is a plot showing the bandgap for devices of embodiments of the invention containing tin halide perovskites with CsO (left-hand curve) and Cs5 (right-hand curve).

[0023] Figure 7 is plot demonstrating enhanced peak intensity, indicating enhanced crystallinity, for tin halide perovskites with different levels of Cs content (CsO to Cs10), and demonstrating that Cs5 produces the greatest crystallinity of the samples tested.

[0024] Figure 8 is a PL mapping comparing (a) a tin halide perovskite without Cs incorporation, against (b) the tin halide perovskite containing 5% Cs and made in accordance with the process of the present application.

[0025] Figure 9 presents SEM images of tin halide perovskites containing with different levels of Cs content (CsO to Cs10).DETAILED DESCRIPTION

[0026] The process, the tin perovskite (including tin halide perovskites) and devices will now be described in further detail with reference to a number of non-limiting embodiments of the invention, following discussion of the meaning of a number of terms used in the specification. Unless otherwise indicated, all scientific and technical terms used in the specification have the meaning as is commonly understood to persons skilled in the art of the invention.Definitions

[0027] The term “perovskite” refers to a material that belongs to a class of materials with a three- dimensional structure related to that of the mineral known as perovskite, that is, CaTiOs. This extends to those materials comprising one or more layers with the three-dimensional structure corresponding to that of the CaTiOs. Perovskites may be of the general formula ABX3, where A refers to one or more cations, B refers to a second cation (in this case, a divalent metal cation), and X refers to one or more anions.

[0028] “Tin perovskite”, also referred to as “tin-based perovskite”, is a perovskite containing tin - Sn2+. Using the general formula indicated above in which B is Sn, tin perovskites may be represented as ASnXs. A “tin halide perovskite” is a perovskite in which an anion component (X of ASnXs) of the perovskite is provided by one or more halide anions. The term “halide” refers to a halogen anion. The halogens are fluorine, chlorine, bromine, iodine and astatine. The halides are preferably selected from one or more of I-, Br , Ch and F-. The term “tin halide perovskite” encompasses tin perovskites containing one or more halides, and optionally a pseudohalide. The term “pseudohalide” refers to a polyatomic analogue of a halide, such as (SCN)-, (SeCN)-, (PCO)-, (OCI)-, (OBr)- and (01)-. The tin halide perovskite preferably comprises not more than two different halide species. The tin halide perovskite of some embodiments are free of pseudohalides. Preferably the halides of the tin halide perovskite are selected from one or both of iodide and bromide. In some embodiments, the halide is iodide, or a combination of iodide and bromide. Where a combination of iodide and bromide is present in the tin halide perovskite, the amount of bromide is preferably not more than 30mol% of the total halide content.

[0029] The cations of the tin perovskite, including the cations A of the tin perovskite of formula ABX3, may be selected from any suitable cations as are known in the art for perovskites. The tin halide perovskites of the present application are preferably mixed cation tin halide perovskites. “Mixed cation” refers to the presence of two or more cation species. The cations may be one or more of alkali metal cations and organic cations. In the present application, a combination of two organic cations is preferred. “Organic cation” refers to a cation comprising carbon. The cation may comprise further elements, for example, the cation may comprise hydrogen, nitrogen or oxygen. The organic cations may be among any of the organic cations known in the art for the production of perovskites, such as those used in the production of the lead halide perovskites of the prior art. These may be selected from formamidinium (“FA”), phenylethylammonium (“PEA”), methylammonium (MA), dimethylammonium (DMA), any of the alkyl butylammonium (BA),guanidinium (Gua) or dialkyl ammonium cations. “Alkali metal cation” refers to the positively charged group 1 elements. These are Li+, Na+, K+, Rb+, Cs+and Fr. The alkali metal cation is preferably caesium (Cs+).

[0030] The term “alkyl” refers to straight chain or branched chain saturated hydrocarbon group having 1 to 20 carbon atoms, and preferably 1 to 6 carbon atoms. Where appropriate, the alkyl group may have a specified number of carbon atoms, for example, Ci-6alkyl which includes alkyl groups having 1, 2, 3, 4, 5 or 6 carbon atoms in a linear or branched arrangement. Examples of suitable alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, s-butyl, amyl, 1 ,2-dimethylpropyl, 1 ,1 -dimethylpropyl, pentyl, i-pentyl, hexyl, 1 -methylpentyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1 ,2- dimethylbutyl, 1 ,3-dimethylbutyl, 1 ,2,2-trimethylpropyl, 1,1,2-trimethylpropyl, 2- methylbutyl, 3-methyl butyl, 4- methylbutyl, n-hexyl, 2-methylpentyl, 3-methylpentyl, 4- methylpentyl, 5-methylpentyl, 2-ethylbutyl and 3- ethylbutyl.

[0031] The term “precursor” in the context of a tin perovskite precursor refers to any chemical compounds used in the synthesis of the perovskite, such as the tin halide perovskite, and to any intermediate product(s) formed from those chemical compounds before complete formation of the tin perovskite. The precursors of a perovskite of the formula ABX3 are typically combinations of a cation and an anion. So, the precursors are typically AX and BX, where each of A, B and X are as defined previously (including references to sub-species as described herein, including A1, A2, A2', A2ii, X1, X2, X2' and X2ii). Thus, precursors may include A1X1, BX2, BX2', A2X2', A2iiX2iiand so forth. In embodiments of the invention wherein the perovskite is a tin perovskite of the general formula ASnXs, the tin perovskite precursors may include Snl, SnBr, SnCI, SnF, SnSCN, SnSeCN, SnPCO, SnOCI, SnOBR, SnOI, formamidinium iodide, formamidinium bromide, formamidinium chloride, formamidinium fluoride, formamidinium thiocyanate, formamidinium phosphaethynolate, formamidinium hypochlorite, formamidinium hypobromite, formamidinium hypoiodite, phenylethylammonium iodide, phenylethylammonium bromide, phenylethylammonium chloride, phenylethylammonium fluoride, phenylethylammonium thiocyanate, phenylethylammonium phosphaethynolate, phenylethylammonium hypochlorite, phenylethylammonium hypobromite, phenylethylammonium hypoiodite, Csl, CsBr, CsCI, CsF, CsSCN, CsSeCN, CsPCO, CsOCI, CsOBR, CsOI, Rbl, RbBr, RbCI, RbF, KI, KBr, KCI, KF, methylammonium iodide, methylammonium bromide, methylammonium chloride, methylammonium fluoride, dialkylammonium iodide, dialkylammonium bromide, dialkylammonium chloride, dialkylammonium fluoride, guanidinium iodide, guanidinium bromide, guanidinium chloride and guanidinium fluoride, In other embodiments of the invention wherein the tin perovskite is a tin halide perovskite, the tin halide perovskites may include Snl, SnBr, SnCI, SnF, formamidinium iodide, formamidinium bromide, formamidinium chloride, formamidinium fluoride, phenylethylammonium iodide, phenylethylammonium bromide, phenylethylammonium chloride, phenylethylammonium fluoride, Csl, CsBr, CsCI, CsF, Rbl, RbBr, RbCI, RbF, KI, KBr, KCI, KF, methylammonium iodide, methylammonium bromide, methylammonium chloride, methylammonium fluoride, dialkylammonium iodide, dialkylammonium bromide,dialkylammonium chloride, dialkylammonium fluoride, guanidinium iodide, guanidinium bromide, guanidinium chloride and guanidinium fluoride

[0032] The term “precursor solution” refers to a combination of one or more precursors in a solvent. In some embodiments, the term “solution” refers to a mixture in which the solid is dissolved in the liquid, however the general expression also extends to colloidal solutions where the solid is dispersed in the liquid. The precursor solution may in some instances refer to a solution of all perovskite precursor compounds in a solvent. This may be referred to as a “complete precursor solution”. For clarity, it will be noted that the precursor solution containing all perovskite precursors required for the formation of the target perovskite may develop nucleated crystals of the perovskite material, with crystallization progressing thereafter. There may be a reduced solvent content over time. For example, in the case of spin coating, by the end of the spin coating process, much (although not all) of the solvent will have been driven away to leave nucleated crystals and precursor in a reduced volume of solvent. Through the application of the precursor solution on the substrate, the tin perovskite precursor compounds - the reagents - may react or otherwise be rearranged into a different tin perovskite precursor form, and with continued nucleation and crystal growth this progresses towards formation of the target tin perovskite.

[0033] The term “solvent’ refers to any suitable solvent or combination of solvents for use in the relevant procedure. A solvent may be a polar solvent or a non-polar solvent. When referring to a solvent used for the precursor solution, the solvent is preferably a polar solvent or comprises at least one polar solvent. Examples of polar solvents include water, alcohol solvents (such as methanol, ethanol, n-propanol, isopropanol and n-butanol), ether solvents (such as dimethyl ether, diethyl ether and tetrahydrofuran), ester solvents (such as acetate ethyl), carboxylic acid solvents (such as formic acid and ethanoic acid), ketone solvents (such as acetone), amide solvents (such as dimethylformamide and diethylformamide), amine solvents (such as triethylamine), nitrile solvents ( such as acetonitrile), sulfoxide (dimethylsulfoxide) solvents and halogenated solvents (such as dichloromethane, chloroform and chlorobenzene). Two sub-class of polar solvents are the polar protic solvents, and the polar aprotic solvents. Examples of protic polar solvents include water, alcohol solvents and carboxylic acid solvents. Preferably, the solvent for the perovskite precursors is a polar solvent. More preferably, the solvent is a polar aprotic solvent. Examples of aprotic polar solvents include ketone solvents (such as acetone), amide solvents (such as dimethylformamide and diethylformamide), nitrile solvents (such as acetonitrile), sulfoxide solvents (dimethyl sulfoxide) and halogenated solvents (such as dichloromethane, chloroform and chlorobenzene). For example, the solvent may be one or more of dimethylformamide (DMF), dimethylsulfoxide (DMSO) and gamma-butyrolactone (GBL). Another class of solvents is the non-polar (or apolar) solvent class.

[0034] The term “antisolvent” refers to a solvent that is a poor solvent for the perovskite. The antisolvent may be used to cause supersaturation of the perovskite in the solvent to thereby drive nucleation and commence crystal growth of the perovskite. Suitable antisolvents include the non-polar solvents.Examples of non-polar solvents include alkanes (such as pentane and hexane), cycloalkanes (such as cyclopentane and cyclohexane) and arenes (such as benzene, toluene and xylene). Preferred antisolvents include chlorobenzene, anisole, diethyl ether, toluene, pentane, hexane and chloroform.

[0035] The term “substrate” refers to any structural component on which it is desired to produce a layer of a perovskite. For example, the substrate may comprise a component or layer of a device, such as a photovoltaic device. A substrate may be a rigid substrate or a flexible substrate. Examples of rigid substrates are glass, plastic or metal, optionally coated with coating layers. Flexible substrates may include polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), among others, which may optionally be coated with a coating layer.

[0036] The term “aging” refers to a process during which crystal growth is allowed to take place, following the production of the layer of precursor. Aging may be described as a pause or interval in processing, before a more active or extreme action is placed on the precursor layer. Aging is suitably conducted in an active temperature and time controlled environment. In the context of the present application, aging is conducted at controlled temperature conditions, and for a controlled time period, to regulate perovskite crystal growth. In preferred embodiments, the temperature is actively controlled to a temperature or temperature profile over a selected time period to enable the desired crystal morphology to develop. The active control may be by way of controlled temperature environment, as distinct from transient movement of the perovskite precursor layer through an unregulated temperature zone from one stage of processing (or production) to the next - such as from spin-coating to annealing.

[0037] The term “annealing” refers to a heat-treatment (or application of heat) to remove the residue organic solvent from the spin-coated perovskite film and promote the crystallization of perovskite crystal. The outcome may be to increase order in a disordered or less ordered perovskite crystal structure. Annealing is typically performed at elevated temperatures, such as at least 50°C, for example, at about 80°C.

[0038] The term “layer” refers to any structure which is substantially laminar in form (for instance extending substantially in two perpendicular directions, but limited in its extension in the third perpendicular direction). A layer may have a thickness which varies across the layer, although it is preferably approximately constant. The "thickness" of a layer refers to the average thickness of a layer. The thickness of layers may easily be measured, for instance by using microscopy, such as electron microscopy of a cross section of a film. The term “region” encompasses “layer” and also extends to non-laminar areas, zones or the like.

[0039] The term "transparent" refers to material or object allows visible light to pass through almost undisturbed so that objects behind can be seen. The term "semi- transparent" refers to material or object which has a transmission (alternatively and equivalently referred to as a transmittance) to visible light intermediate between a transparent material or object and an opaque material or object. Typically, a transparent material will have an average transmission for visible light (wavelength of from 370 to 740 nm) of between 90 and 100%, and semi-transparent materials will have an average transmission for visible light offrom 10 to 90%. Semi-transparent materials more typically have an average transmission for visible light of from 40% to 90%.

[0040] The term "electrode" refers to a conductive material or object through which electric current enters or leaves an object, substance, or region. The term "negative electrode" refers to an electrode through which electrons leave a material or object (i.e. an electron collecting electrode). A negative electrode is typically referred to as an "anode". The term "positive electrode" refers to an electrode through which holes leave a material or object (i.e. a hole collecting electrode). A positive electrode is typically referred to as a "cathode". Within a photovoltaic device, electrons flow from the positive electrode / cathode to the negative electrode / anode, whilst holes flow from the negative electrode / anode to the positive electrode / cathode. The term “electrode” encompasses transparent conducting oxide (“TCO”) layers and metal film layers of a device.

[0041] The term "charge transporter" refers to a region, layer or material through which a charge carrier (i.e. a particle carrying an electric charge), is free to move. In photovoltaic devices electrons act as mobile negative charge carriers and holes act as mobile positive charges. The term "electron transporter" therefore refers to a region, layer or material through which electrons can easily flow and that will typically reflect holes (a hole being the absence of an electron that is regarded as a mobile carrier of positive charge in a semiconductor). Conversely, the term "hole transporter" refers to a region, layer or material through which holes can easily flow and that will typically reflect electrons.

[0042] The term “passivation layer” refers to a layer of a passivation material on surface (such as a semi-conductor surface) to chemically and / or electrically stabilize that surface. The stabilization may comprise protection of that the surface from oxidation or another undesirable chemical reaction of the surface. The passivation layer may additionally or alternatively address surface defects. The passivation material may be an inorganic passivation material or an organic passivation material. Examples of suitable passivation materials for forming the passivation layer include guanidinium bromide (GuaBr), phenylethylammonium bromide (PEABr), phenylethylammonium iodide (PEAI), butylammonium iodide (BAI), butylammonium bromide (BABr), ethylammonium iodide (EAI) and ethylammonium bromide (EABr).

[0043] The article “a” and “an” are used to refer to one or more of the objects described. Thus, for example, “a cation” refers to one or more cations.

[0044] The term “comprising” and corresponding terms such as “comprises” is used in the inclusive sense, to refer to the presence of the named integers, with the possible inclusion of any other additional integers. The term “consisting of” is used in the exclusive sense to refer to the presence of the named integers only, to the exclusion of any other unnamed integers. The term "consisting essentially of" refers to a composition comprising the components of which it consists essentially as well as other components, provided that the other components do not materially affect the essential characteristics of the composition. Typically, a composition consisting essentially of certain components will comprise greater than or equal to 95 wt% of those components or greater than or equal to 99 wt% of those components.Process for the production of tin perovskite

[0045] The process of the present application involves the production of a layer of a tin perovskite, the method generally comprising:- producing a precursor layer of a tin perovskite precursor on a substrate;- subjecting the precursor layer to aging under controlled time and temperature conditions to regulate tin perovskite crystal growth to produce an aged layer, and- annealing the aged layer to produce the tin perovskite.

[0046] The applicants have investigated techniques to regulate the crystallization kinetics of the tin perovskite during fabrication. It was found that a key step leading to a significantly improved crystallization outcome involved the adoption of an aging process designed to slow solvent evaporation, therefore promoting the nucleation and growth of a high-quality tin-based perovskite thin film. The form of “regulation” of the tin perovskite crystal growth is suitably a “slowing” of the tin perovskite crystallization. The “slowing” of the crystallization of the tin perovskite refers to slower crystallization as compared to the uncontrolled, high rate of crystallization of tin perovskites.

[0047] The conditions for the aging step are suitably an aging temperature of between 55°C and - 40°C and an aging time of between 1 and 240 minutes, with the proviso that when the aging temperature is between 20°C and 55°C, the aging time is a minimum of 10 minutes.

[0048] The aging step retards the fast crystallization of tin-based perovskites, enabling high-quality thin films with low defect densities and thus improved photovoltaic performance. The slower crystallization mitigates against microstructural defects and improves device stability against external stimuli.

[0049] During the spin-coating process, two-dimensional (2D) perovskite can form at the air-solvent interface, subsequently serving as a template to direct the growth of three-dimensional (3D) perovskite from the top to the bottom. Due to the highly oriented nature of the 2D perovskite, the subsequent growth of 3D perovskite based on this template also exhibits high orientation, which enhances charge transfer efficiency.

[0050] Aging of spin-coated tin-based perovskite films (whether at room temperature or otherwise) has not previously been considered in the field of tin perovskite thin film production, for a number of reasons. One challenge is that the as-deposited film contains undercoordinated Sn2+, which is vulnerable to oxidation to Sn4+during aging (Sn2+d Sn4++ Sn°). It is conventional therefore to immediately subject the precursor layer to annealing without delay to complete the crystallisation and stabilisation of the tin perovskite. Other strategies have instead been utilised with the objective of slowing down crystallisation, such as using a phosphonic acid in the perovskite precursor solution which is expelled during crystallisation and through this mechanism slows down crystallisation. However, that approach has led to larger grain size production which has an adverse impact. Other adjustments to the precursor solution (such as the one-step approach discussed above) have also been used. It has been found in the present work that aging the perovskite film promotes slow crystallization as the solvent evaporates slowly. By maintaining the temperature to below55°C (and preferably significantly lower), the low temperature minimises or prevents the oxidation of Sn2+due to the exothermic nature of this reaction. Additional controls may also be put in place to minimise or avoid oxidation of the Sn2+such as conducting the aging in an inert gas atmosphere.

[0051] The aging temperature may be controlled to be at a constant temperature, or the temperature may be moderated across the aging process, provided that the aging temperature is within the defined temperature range for a minimum of 50%, preferably 60%, 70%, 80% or 90% of the aging time period. Preferably the aging is conducted within the temperature range specified for at least 95% of the aging time period or substantially all of the aging time period.

[0052] The aging temperature is preferably controlled to be not more than 55°C, preferably not more than 50°C, 45 °C, 40°C, 35°C, 30°C, 25°C, 20°C, 15°C, 10°C, 5°C, 0°C, -5°C, -10°C, -15°C or -20°C. The temperature may be as low as -40°C, however lower temperatures may also be suitable. Any maximum temperature and minimum can be combined to form a range. Temperature ranges of particular benefit are less than 20°C to -40°C, 15°C or less to -40°C, less than 15°C to -40°C, 10°C or less to -40°C, 5°C to - 40°C, 0°C to -40°C and -5°C to -40°C.

[0053] The duration of aging may be between 1 and 240 minutes. Generally, a balance between aging time and temperature is desired. A shorter duration may contain costs because beyond a certain time period (for a given aging temperature), further aging time may not further improve the crystallization process and final perovskite morphology. The time period may therefore be a minimum of 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 40 minutes or 50 minutes. The aging time is suitably not longer than 240 minutes, 210 minutes, 180 minutes, 150 minutes or 120 minutes. Any minimum and maximum aging time period may be combined to form a range. Examples of suitable ranges include 10-180 minutes, 10-120 minutes, 20-120 minutes and 20 - 70 minutes.

[0054] The temperature conditions during the aging step may be controlled by any suitable technique. The present applicant has developed a cooling plate on which the substrate is located, which was temperature-controlled and thus created a region of temperature control in close proximity to the cooling plate. Through contact with the cooling plate, via the substrate, the precursor layer was controlled to the desired temperature conditions during aging. However, it will be understood that any other means of temperature control could be used, such as a controlled temperature gas chamber environment, refrigeration control or otherwise.Precursor layer temperature control

[0055] The step of producing a precursor layer of tin perovskite precursor on the substrate may be by way of any form of solution phase deposition. Examples include spin coating (also known as centrifugal coating), slot-die coating, wide slot nozzle coating, screen printing, inkjet printing, doctor blading (scraper coating), spray coating or roll-to-roll coating. Roll-to-roll (R2R) coating is an industrial process used to apply coatings to a flexible substrate as it is continuously fed from one roller to another. This method is highlyefficient and is used in various industries, including electronics, packaging, and solar cells. Spin coating and R2R coating are particularly suitable methods of application of the precursor solution.

[0056] In each technique for producing or applying the precursor layer on the substrate, a precursor solution comprising tin perovskite precursors in a solvent is applied to the substrate. The applicant has found that controlling the temperature of the precursor solution to be less than 20°C allows the production of a tin perovskite with improved properties. That is, the low temperature precursor layer production conditions retard the crystallization kinetics of the tin-based perovskite layer, independently of the aging process. The temperature is preferably not more than 20°C, 18°C, 15°C, 10°C, 5°C, 0°C, -5°C, -10°C, -15°C or -20°C. The temperature may be as low as -40°C, however lower temperatures may also be suitable. A temperature range for the precursor solution at the time of application to the substrate may be based on any maximum temperature indicated above with a minimum of -40°C. Examples include 15°C - -40°C, 10°C - -40°C, 5°C - -40°C and 0°C - -40°C. If an aging temperature of 10°C or less is used, or an aging time of at least 10 minutes or 20 minutes is used, then the temperature of application of the precursor solution may be higher than 20°C. The temperature may be, for instance, between 55°C and -40°C.

[0057] In some embodiments, the precursor layer of tin perovskite precursor is produced by applying said precursor solution on the substrate and then applying an antisolvent solution onto the precursor solution at an antisolvent temperature of less than 20°C. The application of an antisolvent is conventional in spin coating processes. The temperature of the antisolvent is preferably not more than 20°C, 18°C, 15°C, 10°C, 5°C, 0°C, -5°C, -10°C, -15°C or -20°C. The temperature may be as low as -40°C, however lower temperatures may also be suitable.

[0058] The antisolvent may be any solvent that is a poor solvent for the perovskite. The antisolvent causes supersaturation of the perovskite in the solvent (i.e. the solvent of the perovskite precursor solution) to thereby drive nucleation and commence crystal growth of the perovskite. The antisolvents may be a nonpolar solvent. Examples include chlorobenzene, anisole, diethyl ether, toluene, pentane, hexane and chloroform, among others.

[0059] In spin coating processes, one or both of the precursor solution and the antisolvent may be controlled to be within one of the controlled temperature ranges indicated above. Preferably, both the precursor solution and the antisolvent are controlled to be 20°C or less, preferably they are each controlled to be not more than 18°C, 15°C, 10°C, 5°C, 0°C, -5°C or -10°C. The temperature may be as low as -40°C, - 30°C , -20°C or -15°C.

[0060] Following aging, the aged layer is annealed to produce the tin perovskite. The annealing may be performed under any suitable annealing conditions. For example, the annealing temperature may be at least about 60°C, 70°C, 80°C or higher. The time of annealing may be at least 5 minutes. The annealing time is preferably not more than 120 minutes. Any minimum and maximum time period may be combined toform a range for the annealing time. The time may be for instance, between 10 and 100 minutes, between 10 and 80 minutes, between 5 minutes and 80 minutes, between 30 and 80 minutes or otherwise.

[0061] The process for the production of the layer of perovskite in accordance with one embodiment of the invention involves the combination of (a) controlled-temperature spin-coating as described above, (b) controlled-temperature aging as described above, and (c) annealing. A preferred process comprising all three steps is illustrated schematically in Figure 1 . As shown in Figure 1(a), in a first step, a perovskite precursor solution (1), which is cooled to a solution temperature of -10°C, is dropped onto a substrate (2), which is spun at high speed about a central axis as indicated by the arrow, to be spread evenly across the surface of the substrate. In a second step shown in Figure 1 (b), an antisolvent (3), also cooled to an antisolvent temperature of -10°C, is dropped onto the layer of perovskite precursor (4) which remains on the substrate surface following step 1 (a). In Figure 1 (c), spinning continues as indicated by the arrow. As shown in Figure 1 (d), at the end of spin coating, the substrate (1) with the layer of perovskite precursor (4) is aged on a cooling plate (5) at a temperature of -20°C for 50 minutes. Thereafter, as shown in Figure 1 (e), the substrate (1) bearing the layer of now aged perovskite (6) is subjected to annealing on a hot plate (7) set to a temperature of 80°C for about 10 minutes. The final product shown in Figure 1 (f) is a substrate (1) containing a layer of the crystalline perovskite (8). As will be appreciated, the process may be conducted with different temperature settings for one or more of the perovskite precursor solution, the antisolvent, and the aging step in the process shown schematically in Figure 1 . Where the temperature is at ambient or above for either or both of the perovskite precursor solution and the antisolvent, the solution or antisolvent may be heated rather than cooled. If the aging is conducted at ambient temperature, a cooling plate is not required, and if the aging temperature is above ambient, a heating plate or another heating source may be used.Perovskite Formulation

[0062] As noted above, any tin perovskite of any desired formula may be produced in layer form with improved morphology making use of the process as outlined above.

[0063] In some embodiments, the tin perovskite is a tin halide perovskite. In some embodiments, the tin halide perovskite is a mixed cation tin halide perovskite. In some embodiments, the tin halide perovskite is a mixed cation tin halide perovskite, comprising one or more organic cations and up to 10 mol% cesium, based on the total moles of cesium and tin. In some embodiments, the tin halide perovskite is a mixed cation tin halide perovskite, comprising between 3 and 7% cesium, based on as a mol% with respect to the total moles of cesium and tin, and a combination of two organic cations.

[0064] The tin halide perovskite may be based on a composition of formula (I) as outlined above. The expression “based on” means that the tin halide perovskite has the composition as defined by the given formula, or is derived from a precursor composition of that formula, with possible modifications arising through crystallization and / or any other treatments applied, such as annealing. It is noted that the precise formula of a perovskite can be difficult to articulate precisely. It is conventional however to make use of such formulae toarticulate the foundation (or precursor) formulation on which the perovskite is based, and to refer to this as the chemical formula for the perovskite itself.

[0065] The tin halide perovskite may be based on a composition of formula (Ila):(A1X1)W(A2BX23)(I.W) Formula (II) wherein A1is an alkali metal cation,A2represents one or more organic cations,B is to a divalent metal cation,X1represents a halide or pseudohalide,X2represents one or more halides, and0 < w < 0.2.

[0066] The tin halide perovskite may be based on a composition of formula (II):(A1X1)W(A2BX23)(I-W) Formula (Ila) wherein A1is an alkali metal cation,A2represents one or more organic cations,B is to a divalent metal cation, each of X1and X2independently represents one or more halides, and0 < w < 0.2.

[0067] In some embodiments, the perovskite is based on a composition of formula (III):(A1X1)W((A2i)y(A2ii)(i-y)BX23)(i-w) Formula (III) wherein A1is an alkali metal cation,A2' is a first organic cation,A2iiis a second organic cation,B is a divalent metal cation, each of X1and X2independently represent one or more halides or pseudohalides (preferably halides),0 < w < 0.2, and0 < y < 1.

[0068] In some embodiments, the perovskite is based on a composition of formula (IV):(A1X1)W((A2i)y(A2ii)(i.y)B((X2i)z(X2ii)(3-z))(i-w)Formula (IV) wherein A1is an alkali metal cation,A2' is a first organic cation,A2iiis a second organic cation,B is a divalent metal cation,X1, X2' and X2iieach independently represent a halide, with the proviso that X2' and X2iiare different,0 < w < 0.2,0 < y < 1 , and0 < z < 3.

[0069] In the above formulae (II) to (IV), preferably 0 < w < 0.1 , more preferably 0.01 < w < 0.1 , 0.01 < w < 0.07, and most preferably 0.03 < w < 0.07. In some embodiments w is about 0.05.

[0070] In the above formulae (II) to (IV), A1is preferably Cs.

[0071] The organic cations of the tin perovskite of some embodiments may be selected from any organic cations known for use in the production of perovskites. As examples, reference is made to formamidinium (FA), phenylethylammonium (PEA), methylammonium (MA), dimethylammonium (DMA), alkyl ammonium, dialkyl ammonium, butylammonium (BA) and guanidinium (Gua). In the above formulae (II) to(IV), each A2, A2' and A2iimay be selected from formamidinium, phenylethylammonium, methylammonium, dimethylammonium, alkyl ammonium, dialkyl ammonium, butylammonium and guanidinium. In the case of A2, where A2represents two organic cations, each A2is independently selected from the above organic cations. In some embodiments, A2represents PEA and FA.

[0072] In the above formulae (II) to (IV), each X1, X2, X2' and X2iiis preferably selected from |-, Br, Cl- and F-. X1is preferably iodide. X2is preferably iodide or a combination of iodide and bromide. X2' is preferably bromide and X2iiis iodide.

[0073] In the above formulae (II) to (IV), preferably 0 < y < 0.3. Preferably 0 < y < 0.2, and more preferably 0.05 < y < 0.2. For example, y may be about 0.15.

[0074] In the above formulae (II) to (IV), preferably 0 < z < 0.5, more preferably 0 < z < 0.3, more preferably 0.005 < z < 0.3, more preferably 0.01 < z < 0.2. In some embodiments, z is about 1 .5.

[0075] In some embodiments, the present application provides a tin halide perovskite of formula (I): (CsX1)w(PEAyFA(i-y)SnX23)(i-w) Formula (I) wherein X1is a halide or pseudohalide, 0.03 < w < 0.07, 0 < y < 1.0, and X2is a halide, or a combination of two halides.

[0076] In formula (I), X1is preferably a halide, and more preferably iodide, and X2is selected from iodide, bromide, or a combination of iodide and bromide. In formula (I), the preferred values for y are as outlined above for formulae (II) to (IV).

[0077] In some embodiments, the present application provides a tin halide perovskite is of formula(V):(CsX1)w(PEAy FA(i-y)Sn((X2i)z(X2ii)(3-z)))(i-W) Formula (VI) wherein X1, X2' and X2iieach independently represent a halide, with the proviso that X2' and X2iiare different, 0.03 < w < 0.07, 0 < y < 1.0, and0 < z < 3.

[0078] In formula (V), each X1, X2' and X2iiis preferably selected from k, Br, Cl- and F-. X1is preferably iodide. X2' is preferably bromide and X2iiis iodide. In formula (V), the preferred values for w, y and z are as outlined above for formula (II) to (IV).

[0079] The band gap of the perovskite material can be tuned by adjusting the relative content of elements that form the perovskite. For example, by adjusting the bromide content in a perovskite comprising iodide and bromide as the halides between 0 and 3, the bandgap of the perovskite of some embodiments can be modified between 1 .35 (or 1 .40) eV and 2.3 eV. In some embodiments, the band gap is controlled to be between 1 .4 and 2.0 eV, preferably between 1 .6 and 1 .9 eV. In alternative embodiments, the band gap is controlled to be between 1 .35 and 1 .48 eV.

[0080] To produce these perovskites from a precursor solution, suitable tin halide perovskite precursors are selected and combined in a suitable solvent, and thereafter the precursor solution is applied to the selected substrate.

[0081] In general terms, suitable tin halide perovskite precursors for combining in solution may comprise combinations of AX and SnX, in the ratios required to achieve the desired perovskite formulation (A and X having the definitions as set out previously - including sub-species as described herein, including A1, A2, A2', A2ii, X1, X2, X2iand X2ii). Thus, precursors may include A1X1, SnX22, Sn(X2ii)2, A2X2iand A2"X2ii. In some embodiments, the precursors comprise CsX1(e.g. Csl), SnX22, PEAX2' and FAX2". In some embodiments, the precursors comprise Csl, Snl2, PEABr and FAI. These tin halide perovskite precursors are included in a solution with a suitable solvent, such as a polar aprotic solvent, or combination of such solvents. An example of a precursor A1X1for producing a tin halide perovskite with a pseudohalide component as the X1component is Cs(SCN).

[0082] In some embodiments, the solvent for the perovskite precursors is a polar aprotic solvent. Examples include dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), dichloromethane, tetrahydrofuran (THF), ethyl acetate, acetone, acetonitrile, hexamethylphosphoric triamide (HMPT), gammabutyrolactone (GBL), N-methyl-2-pyrrol idone (NMP) and combinations of two or more thereof. However, it is possible for other solvents to be selected from the solvent classes described previously.

[0083] The final concentration of the tin halide perovskite precursors in the precursor solution is usually about 0.8 M to 1 .4 M.

[0084] The precursor solution may be provided by way of two separate precursor solutions, each comprising a subset of the tin halide perovskite precursors. In this instance, the precursor solutions may be applied sequentially or in combination on the substrate.

[0085] The precursor solution may be applied on any suitable substrate for the intended purpose. Details of preferred substrates are discussed below with reference to features of the device.Device Features and Production

[0086] In the following, the complete process for the preparation of a layer of tin perovskite as a component of a photovoltaic device (specifically, a solar cell) is described. However, it will be understood that the steps in the process may be applied more generally in the production of tin perovskite layers in devices for use in other applications.

[0087] The general devices that may incorporate the tin perovskite of the present application may in broadest terms comprise a pair of electrodes and a layer of tin perovskite. The device suitably comprises a first electrode, a hole transport layer, a tin perovskite layer, an electron transport layer and a second electrode. The tin perovskite may be the tin perovskite as produced by the process described above, and / or the tin perovskite may be of the formula I, II, III, IV or V defined previously. The tin perovskite in some embodiments is a mixed cation tin halide perovskite, comprising between 3 and 7% cesium, based on mol% with respect to the total moles of cesium and tin, and a combination of two organic cations.

[0088] In some embodiments, there is provided a photovoltaic device comprising a layer of tin perovskite as produced by the process described above, and / or the tin halide perovskite of formula I defined previously. In some embodiments, the tin halide perovskite comprises between 3 and 7% cesium, based on mol% with respect to the total moles of cesium and tin, and a combination of two organic cations.

[0089] The photovoltaic device may comprise the following:I. a first electrode layer;II. a hole transport layer (p-type semiconductor);III. optionally, a passivation layer;IV. a tin perovskite layer;V. an electron transport layer (n-type semiconductor);V. optionally, a hole blocking layer;VI. a second electrode layer.

[0090] In some embodiments, the layers of the photovoltaic device are in the order as outlined above. This arrangement is of the inverted, p-i-n structure. Depositing of the layers is typically in the order from I. to VI. In other embodiments, the photovoltaic device is of the regular n-i-p structure. In such embodiments, the hole transport layer and the electron transport layers are reversed.

[0091] The first electrode layer may comprise a first electrode material. The first electrode material may be any suitable electrically conductive material. Suitable conductive materials include metals, transparent conductive oxides, graphite and conductive polymers. Examples of metals that can form the first material of electrode include silver, gold, copper, aluminum, platinum, palladium or tungsten. The first electrode material usually comprises or consists essentially of a transparent conductive oxide. The transparent conductive oxide may be selected from fluorine doped tin oxide (FTO), tin and indium oxide (ITO) or aluminum and zinc oxide (AZO). The first electrode layer preferably comprises ITO. This may be on a rigid support, such as a glass support.

[0092] The next layer in a device of p-i-n structure may be a hole transport layer. This may comprise an inorganic or organic hole transport material (p-type semiconductor material). Typically, the material is an organic hole transport material, such as a polymeric hole transport material, or a selfassembled monolayer material layer. Examples of suitable materials include dimethoxy carboazol phosphinic acids such as MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid), spiro-OMeTAD (2,2', 7,7'-tetrakis-(N, N-di-p-methoxyphenylamine) 9,9'-spirobifluorene), P3HT (poly (3-hexylthiophene)), PCPDTBT (poly [2,1 ,3-benzothiadiazol-4,7-diyl [4,4-bis (2-ethylhexyl)-4H-cyclopenta [2,1-b: 3,4J] dithiophene-2, 6-diyl]]), PVK (poly (N-vinylcarbazole)), polyTPD (poly(N, N'-bis-4-butylphenyl-N, N'- bisphenyl)benzidine), benzodithiophene derivatives, poly(3,4-ethylenedioxythiophene) (PEDOT), polystyrene sulfonate (PSS), and blends or combinations thereof. The hole transport material may comprise carbon nanotubes or other conductive carbon nanomaterials.

[0093] In some embodiments, the device may comprise more than one hole transport layer. One such layer may comprise a self-assembled monolayer of a hole transport material. A preferred hole transport layer is MeO-2PACz. The other may comprise a conductive polymer layer, such as PEDOT:PSS. The PEDOT:PSS layer may be in contact with the first electrode, and the MeO-2PACz layer may follow the PEDOT:PSS layer.

[0094] Each hole transport layer may have a thickness of between 10 nm to 500 nm. For example, the thickness can be from 50 nm to 200 nm.

[0095] The device optionally, although preferably, comprises a passivation layer between the hole transport layer and the perovskite layer. The applicant has found that GuaBr is a particularly suitable passivation layer material for this purpose. However, other passivation layer materials may be used, including but not limited to PEABr, PEAI, BAI, BABr, EAI, EABr.

[0096] The tin perovskite layer forms the next layer of the device. The preceding layer, or combination of layers (e.g. I, II and optional layer III), may be referred to as the substrate during the production of the layer of tin perovskite. The perovskite layer and its formation has been described previously. The thickness of this layer may be between 10 and 1000nm. For example, the thickness may be between 10 and 500nm, or between 10 and 300nm. The thickness may be between 50 and 500nm, or between 50 and 300nm.

[0097] The next layer in a device of p-i-n structure is an electron transport layer. This may comprise an organic or inorganic electron transport material (p-type semiconductor material). Examples of suitable organic electron transport materials include fullerene, ICBA (indene-C60-bisadd uct), PCBM (phenyl- C61 -butyric acid methyl ester) and Ceo. Examples of inorganic electron transport materials include metal oxides, metal sulfides, metal selenides, metal telluriums, perovskites, amorphous silicon, semiconductors of type n of group IV, semiconductors of type n of the group lll-V, n-type semiconductors of group I l-VI, n-type semiconductors of group l-VII, n-type semiconductors of group IV-VI, n-type semiconductors of group V-VIand Type II semiconductors of group I l-V, any of which may be doped or undoped. Tin dioxide is one notable example.

[0098] The device may further comprise a hole blocking layer, in contact with the second electrode. Suitable hole blocking layer materials include bathocuproine (BCP) and zirconium(IV) acetyl aceton ate (ZrAcac)4.

[0099] The device comprises a second electrode layer. The second electrode material may be any suitable electrically conductive material. Suitable conductive materials include metals, transparent conductive oxides, graphite and conductive polymers. Preferred second electrode materials are silver and gold, preferably silver.

[0100] In some embodiments, the device comprises the layers as illustrated in Figure 2, including: an indium-tin oxide glass layer (9) - constituting a first electrode layer; a PEDOT:PSS layer (10)- constituting a first hole transport layer; a MeO-2PACz layer (11) - constituting a second hole transport layer; a GuaBr layer (12) - constituting a passivation layer; a tin perovskite layer (8); an ICBA layer (13) - constituting an electron transport layer; a BCP layer (14) - constituting a hole blocking layer; and a silver electrode layer (15) - constituting the second electrode layer.

[0101] In some embodiments, the solar cell has a power conversion efficiency (PCE) of at least 15%, or at least 16%. The PCE may even be above 16.5%, or above 17.0%.

[0102] The device may be in the form of a tandem cell. That is, the device may be a tandem perovskite-silicon solar cell. The device may be a 2-terminal or 4-terminal form of tandem solar cell. In such devices, the perovskite cell components may be arranged as a top layer on a silicon bottom cell, or otherwise. When the device is in the form of a tandem cell, an even higher PCE can be achieved.

[0103] In construction of the device, a layered approach is typically followed, commencing from the preparation or supply of a first electrode component, formation of a following layer or layers, production of the layer of tin perovskite as described herein, followed by the formation of additional layers and incorporating any other device components.

[0104] Following from the above description of the device layers, the substrate preferably comprises a first electrode layer, one of a hole transport layer and an electron transport layer and optionally a passivation layer. Preferably, the substrate comprises a first electrode layer, one or more hole transport layers and a passivation layer. In the example of Figure 2, the combination of layers (9) to (12) constitute the substrate (1).Examples

[0105] A number of experiments were conducted to test and identify suitable conditions for the production of tin perovskite layers in a test device, being a solar cell. The examples are illustrative and should not be read as limiting on the scope of the invention described more generally herein.General Procedure

[0106] The following general procedure and was followed for the production of solar cells containing a tin halide perovskite layer with varied characteristics.

[0107] Materials

[0108] ICBA was purchased from 1 -Material; [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonicAcid (Meo-2Pacz) was purchased from TCI, Poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) (Clevios™ P VP Al 4083) was purchased from xi’an Yuri Solar Co., Ltd, tin iodide (99.999%), tin powder, tin fluoride (99.999%), lead thiocyanate (99.999%), caesium iodide (Csl), ammonium thiocyanate (NH4SCN), bathocuproine (BCP) (99%) N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), isopropyl alcohol (IPA), chlorobenzene, and toluene were purchased from Sigma-Aldrich. Guanidinium bromide (GuaBr), formamidinium iodide (FAI), phenethylammonium Bromide (PEABr) were purchased from Greatcell Solar Materials.

[0109] Equipment[001 10] A conventional glove box and spin coater was used for producing a spin coated layer of the tin halide perovskite on the substrate. For aging, a cooling plate was built for controlled cooling of the perovskite precursor layer following spin-coating. The cooling plate comprised a steel plate measuring 100mm by 100mm with an embedded 12706 Thermoelectric Cooler Peltier module and three cooling fans integrated at the rear of the steel plate for heat exhaust. The cooling plate was located in the glove box for controlling the crystal growth during aging. For examples conducted with an aging temperature above ambient, a heating plate was used in place of the cooling plate. The heating plate was a commercial stirring hot plate of Thermo Scientific Cimarec HPS. The aging temperature indicated in the tables presented below refers to the set temperature for the cooling or heating plate.[001 11] Tin halide perovskite precursor solution preparation[001 12] Synthesis of Snfe: 0.8 M - 1 .4 M of h is fully dissolved in 0.75 ml of Dimethyl sulfoxide (DMSO) first, then, 3 ml of N,N-Dimethylformamide (DMF) is added into the solution. Excessive Sn powder is added into the solution and stirred overnight to make Sn I2 solution in mixed solvent (DMF:DMSO=4:1).[001 13] Tin Halide Perovskite Precursor preparation: FAI, PEABr, SnF2, NH4SCN with a molar ratio of 0.85: 0.15: 0.08: 0.05 were dissolved in prepared Snl2 solution (DMF: DMSO = 4:1 V / V). This preliminary tin halide perovskite precursor solution is used to form tin halide perovskites with 85% formamidinium and 15% phenylethylammonium as the A cation component, and for producing the CsO (or Cs 0%) tin halide perovskite of the test work. To produce a tin halide perovskite comprising a mixed organiccation (85% FA and 15% PEA), and with up to 10% Cs cation, the preliminary tin halide perovskite precursor solution was combined with a solution of Csl in the ration required to achieve the target Cs content. The Csl solution was prepared by dissolving Csl in DMSO to prepare a stock solution of Csl with concentration of 1 ,5M. The Csl solution was then added to the preliminary tin halide perovskite precursor solution at different ratios (x: 1 -x) to form compositions of the formula CsIxtPEAo.isFAo.ssSnhssBro isJi-x- The test work presented further below demonstrated the relative efficiencies of solar cell devices with Csl ratio from 0% to 10%. For the test work in Example 1 , the perovskite composition was based on x = 0.05 (i.e. 5% Csl) in this formula. During the step of forming a layer of the tin halide perovskite precursor described below, this precursor solution is dropped on the substrate by spin coating, followed by antisolvent.[001 14] Final composition: CsIoostPEAo isFAossSn^ssBro isJo ss[001 15] Device fabrication:[001 16] (i) T ransparent conducting oxide (ITO glass) layer: Patterned IT 0 glass sheets were cleaned with detergent solution, acetone and isopropanol for 15 min sequentially. The glass is then cleaned with UV-Ozone before use.[001 17] (ii) Hole transport layers: Poly (3, 4-ethylened ioxyth iophene) poly (styrenesulfonate)(PEDOT:PSS) (Clevios P) was filtered using 0.45 pm PTFE syringe filters and spin-coated onto patterned ITO substrates at 6000 rpm. The substrates were annealed at 140 °C for 20 min in ambient condition. Meo- 2PACz (0.15 mg / mL in IPA) is loaded onto the substrate for 10 s before spin-coating at 6000 rpm for 30 s. The substrate is then annealed at 100 °C for 10 min, followed by transferring into a N2-filled glovebox for further deposition.[001 18] (ill) Passivation layer: GuaBr (2 mg / mL in IPA) is spin-coated onto the substrate at 4000 rpm for 15 s, then the substrate is annealed at 100 °C for 10 min.[001 19] (iv) Perovskite layer: Before the deposition of perovskite film, the antisolvent, toluene, and perovskite precursor solutions were firstly cooled to the temperature indicated in the given test procedure (and if not otherwise indicated, cooling was to a temperature of -10 °C). Cooling of the solutions took place inside a N2 filled glovebox. If a temperature above ambient was used in the given test procedure, in place of cooling the relevant solution(s) on a cooling plate in the N2 filled glovebox, the cooling plate was substituted with a heating plate and the solution(s) heated under the same conditions. The temperatures for the perovskite precursor solution and antisolvent indicated in the test procedures and tables below refer to the temperatures set on the cooling plate or the heating plate used to cool or heat the relevant solution or antisolvent. To deposit the perovskite layer, 80 pL of perovskite precursor was spun onto substrate layer spinning at 5000 rpm for 30 seconds. About 10 seconds after the commencement of the spinning cycle at 5000 rpm, 500 pL of toluene was quickly dropped over a 1-2 second time period onto the spinning substrate. The use of the antisolvent such as toluene serves to drive nucleation and commence crystal growth.

[0120] Following spin coating, the film is aged for the time and at the temperature indicated in each aging experiment (other than in Example 1 where no aging was performed). The range of interest was 55°C to -40°C, and a time of 1 minute to 240 minutes. Where conditions are not indicated, the aging temperature was -10°C and the aging time was 50 minutes. Aging was conducted in dark conditions. The thickness of the active layer was 300nm. The aging temperature indicated in the experiments and tables presented below refers to the set temperature for the cooling plate or heating plate. Accordingly, there may be some degree of temperature variation (typically + / - 5°C, and potentially up to + / - 10°C) between the temperature of the plate and the film temperature during the course of the aging.

[0121] The film was then annealed at 80°C for 50 minutes, unless otherwise indicated.

[0122] (v) Electron transport layer: ICBA (18 mg / mL in chlorobenzene) is spin coated onto the perovskite layer at 1000 rpm for 30s, then annealed at 80 °C for 30 minutes.

[0123] (vi) Hole blocking layer: 6 nm of BCP was deposited with a thermal evaporation system.

[0124] (vii) Current collector: The deposition of the BCP layer was followed by the sequential deposition of a 80nm thick layer of Ag onto the substrate with the thermal evaporation system.Example 1

[0125] In this example, tests were performed to study the impact of controlling the precursor solution and antisolvent temperature on the production of the perovskite film. The measure of the perovskite film quality was the power conversion efficiency of the solar cell containing the tin halide perovskite film layer. In other tests conducted concurrently (and set out below) it was found that the best tin halide perovskite was that containing 5% cesium, and the mixed organic cation (in this case 85% FA and 15% PEA), so this was the selected tin halide perovskite used in this example.

[0126] The general test procedure outlined above was followed to produce solar cells with a tin perovskite layer (Csloo5(PEAo i sFAossSn IgssBro isJoss), with control of the precursor solution and the antisolvent temperature during spin coating at each of four different temperatures: (a) 40°C, (b) 20°C, (c) 0°C and (d) -10°C. In this example, no aging was performed, and the perovskite layers were annealed immediately following deposition. The performance of the solar cells so produced was tested and the results are set out in Table 1 below, where it is noted that Vocrefers to open-circuit voltage, Jscrefers to short-circuit current density (measured in mA cm2), FF refers to the fill factor, and PCE refers to the power conversion efficiency:Table 1 :

[0127] The results demonstrate that lowering the temperature of the precursor solution and the antisolvent yields a significant improvement in the morphology of the film produced, as reflected by the PCE results. There is a marked improvement for temperatures of 20°C and less, and further improvement at 0°C or less. This procedural step on its own has a positive impact on the perovskite film produced, but this impact is further improved through the utilization of the aging process of Example 2 below.Example 2

[0128] In this example, tests were performed to study the impact of aging the layer of perovskite precursor solution following spin-coating and prior to annealing, at different temperatures. Again, the measure of the perovskite film quality was the power conversion efficiency of the solar cell containing the tin halide perovskite film layer, when produced with different aging temperature conditions.

[0129] The general test procedure outlined above was followed to produce solar cells with a tin perovskite layer (Csloo5(PEAo i sFAossSn IgssBro isJoss), with the precursor solution and antisolvent temperatures set at -10°C, and aging conducted for a fixed 10 minute period at each of four different temperatures: (a) 40°C, (b) 20°C, (c) 0°C and (d) -10°C. Temperature of aging was controlled by controlling the temperature of the cooling plate (or heating plate) on which the substrate coated with the layer of perovskite precursor was positioned during the aging process. The performance of the solar cells so produced was tested and the results are set out in Table 2 below:Table 2:particularly that the lower the aging temperature, the better the morphology of the film so produced. There is a notable improvement for temperatures of 20°C and less, and a step-up improvement at 0°C and then again at -10°C. Based on this trend, it is anticipated that further temperature reduction during aging to as low as -40°C will result in further improvement in PCE outcome. In Example 3, further test work was conducted to explore the impact that different aging times have on the perovskite layer.Example 3

[0131] In this example, tests were performed to study the impact that the aging duration has on the perovskite layer produced. Again, the measure of the perovskite film quality was the power conversionefficiency of the solar cell containing the tin halide perovskite film layer, when produced with different aging duration conditions.

[0132] The general test procedure outlined above was followed to produce solar cells with a tin perovskite layer (Csloo5(PEAo i sFAossSn k.ssBro.isJo.ss), with the precursor solution and antisolvent temperatures set at -10°C during spin-coating, and aging conducted at a temperature of -10°C, for each of eight different time periods, being 10, 30, 50, 70, 90, 110, 130 and 150 minutes. The aging temperature was controlled at -10°C through controlling the temperature of the cooling plate to -10°C. In later test work, the 50 minute aging test was repeated with the same conditions. The performance of the solar cells so produced was tested and the results are set out in Table 3 below (with the later repeated 50 minute aging test presented as example 3(c’)):Table 3:

[0133] The PCE results demonstrate a peak performance is achieved when aging for about 50 minutes, when the aging temperature is -10°C (cooling plate temperature). While there is a drop in the quality of the perovskite film layer when aging at this temperature is continued for a longer time period, the results remain strong for continued aging up to 1 10 minutes, and only drop below that achieved for 30 minutes of aging when aging continues to 130 minutes and beyond. It will be noted that at different aging temperatures, the duration of aging may be shorter or longer, as can be determined by following the procedure outlined herein. It is anticipated that further reduction in the aging temperature will further improve the morphology of the perovskite film layer. It is particularly notable that even at the conditions studied so far, the highest recorded PCE to date was achieved for a tin halide perovskite solar cell, at 17.30%.Example 4

[0134] In this example, having demonstrated the efficacy of aging at controlled temperatures and times (with peak results achieved at -10°C and 50 minutes), further tests were performed to study the impactof precursor solution cooling and antisolvent cooling on the perovskite film, where the film aging conditions are kept constant.

[0135] The general test procedure outlined above was followed to produce solar cells. In each of the test runs of Example 4, aging was conducted at -10°C for 50 minutes. In example 4(a), the precursor and antisolvent were cooled to -10°C for use in the spin-coating step, in example 4(b) only the perovskite precursor solution was cooled to -10°C (and the antisolvent was dropped at room temperature), and in example 4(c), the perovskite precursor solution was spin-coated onto the substate at room temperature, followed by antisolvent dropping at -10°C. The example 4(a) results correspond to example 3(c) above, and this example was repeated - with the repeated experimental results now indicated as example 4(a’). The solar cell performance results are set out in Table 4 below:Table 4:crystallisation control comes from the aging conditions - with reduced aging times having a major impact, and the time of aging being capable of control to optimise crystallisation. However, controlling the temperature of one or both (and preferably both) of the perovskite precursor solution and the antisolvent, in addition to aging control, produces optimal results.

[0137] It will be noted that not all available techniques for applying a layer of a perovskite precursor solution to a substrate involve the use of an antisolvent. Other techniques for application of the perovskite precursor solution may be used, such as slot dye coating, inkjet printing, doctor blading and spray coating. In such instances, controlled temperature aging may provide the best form of crystallisation control for the production of a high quality tin halide perovskite.Example 5

[0138] The long-term stability of the fabricated tin halide perovskite solar cell produced in accordance with Example 4(a) above (identical to Example 3(c)) was examined under dark and continuous light illumination inside the N2 filled glovebox. The device was unencapsulated. The results of the long-term stability test are shown in Figure 5(f). The device exhibited excellent shelf stability retaining 93% of the initial PCE after storing in N2 under dark conditions for 6,000 hours. This was also compared to the equivalent device containing no caesium cation - the CsO results are also shown in Figure 5(f) - lower line. The comparative test work leading to the selection of a 5% caesium cation content is set out in Examples 6 below. A device produced with the tin halide perovskite of example 4(a’) was also produced and was shown to retain95% of the initial PCE after storing in N2 under dark conditions for 5,000 hours, as shown in Figure 5(g). In this figure, the device with the tin halide perovskite produced in accordance with example 4(a’) - the “target” - was compared to a “control” which was based on the equivalent device produced with the same Cs5 tin halide perovskite formulation, but with the precursor solution and antisolvent dropped at room temperature, and no aging. Figure 5(h) shows the operational stability of the same device (example 4(a’)) under continuous one-sun illumination at open-circuit status in N2 at 60 ± 5°C for 1 ,500 hours, with the “target” and “control” referring to the devices produced under the same conditions as indicated for Figure 5(g). Example 6

[0139] In this example, tests were conducted to develop an optimal tin halide perovskite (THP) formulation with the potential to yield two dimensional (2D)-th ree dimensional (3D) heterostructures throughout for additional control on crystallization of the perovskite and development of a high-quality THP thin film. A challenge with development of 2D structure in the film is the delayed maturation of the colloidal clusters in the precursor solution, leading to slower nucleation of the 2D THP, compared to the 3D counterpart, resulting in phase segregation and compromised photovoltaic performance and device durability.

[0140] In addition to the crystallization control steps studied in Examples 1-4, test work was conducted on a formulation for the THP that would be conducive to the development of 2D-3D heterostructure with homogeneous microstructure. This required study of the colloidal chemistry and crystallization engineering of mixed-dimensional perovskites. In this example, THPSCs were produced using the general procedure outlined above, but with variations in the amount of the Csl stock solution in the blend, to yield the target Cs+ratios (0-20 mol% based on the total moles of Cs and Sn - also denoted as CsO to Cs20.) Solar cells of the structure indicated in the general procedure were prepared. In tests that required an active area of the solar cell to be defined, this was done using a metal shadow mask of 0.0576 cm2.

[0141] Two different organic cations were selected for the A-site of the THP. Formidinium (FA) cation was selected as the primary organic cation for the A-site of the THP. Combined with this, a minor amount of the bulkier phenylethylammonium (PEA) cations was selected to induce the formation of 2D perovskite frameworks that can template the oriented growth of 3D FASnh. The small FA+cation (-253 pm) acts as a hard Lewis acid to form strong hydrogen bonding with iodostannate complexes of [SnX6]nm‘, constituting aggregated FA-based colloidal clusters in the THP precursor solutions. Conversely, the soft and bulky PEA+cations (-552 pm) loosely coordinate with iodostannate complexes, resulting in more isolated PEA-rich clusters that must undergo further agglomeration and coalescence periods to reach the critical nucleation radius of 2D THPs. During the crystallization of such phase-segregated 2D-3D THP heterostructures, the inadequacy of 2D perovskite crystal templates at the air-solution interface may lead to disoriented 3D THP grains at the shallow surface region with abundant trap states (e.g. Sn2+and F vacancies) and pronounced pinholes, while the subsequently bottom-enriched 2D THP segments are expected to inhibit the interfacial charge transport, both of which contribute to the inferior PV performance and device stability ofthe resultant 2D-3D THPSCs. In this regard, establishing homogeneous 2D-3D THP heterostructures through crystallization regulation is crucial for propelling the ongoing advancement of efficient and durable THPSCs. Colloidal stability and nucleation kinetics

[0142] To achieve the desired homogeneous 2D-3D heterostructures, a Cs-containing triple-cation precursor engineering strategy was developed to modulate the colloidal dynamics and nucleation kinetics of the 2D-3D THPs. Given the dispersive nature of colloidal THP precursor solutions, the dissociated A cations (e.g. FA+, PEA+, Cs+) are electrostatically attracted by the negatively charged iodostannate complexes of [SnX6]nm- to form an electric double-layer (EDL) configuration as FA-based or PEA-rich colloids (Fig. 3a). Based on the DLVO (Derjaguin, Landau, Verwey and Overbeek) theory, the stability of these colloids is mainly determined by total repulsive potential (H / totai) in the dispersing medium by integrating the Van der Waals attraction (l / IA) and the electrical double-layer repulsion (l / l / r) between two identical colloidal particles. In this scenario, the aggregation barrier (AH / *) is primarily dependant on the colloidal particle radius (R), where bulky colloids possess a high aggregation barrier that makes them difficult to form large clusters. As the inherently large size of the PEA+cation that expands the stern and diffuse layers in the EDLs configuration (RPEA > RFA), the bulkier PEA-rich colloids possess a higher aggregation barrier than FA colloids (A 1 / I / *PEA > A 1 / I / *FA). Consequently, the more isolated PEA colloids are of smaller cluster size (FPEA < that causes sluggish nucleation kinetics of 2D phases, while coagulated FA-based clusters favor fast nucleation of 3D THPs. Upon Cs+incorporation in the precursor solution, smaller Cs+cations can form strong ionic bonds with the inner iodostannate adducts, replacing PEA+and FA+cations in the EDLs and causing the volume to shrink for both colloids. Considering the vast size discrepancy between Cs+and PEA+cations, PEA-rich colloids are expected to undergo more significant colloidal contraction upon Cs+diffusion, while this phenomenon is less pronounced in their FA counterparts.

[0143] To verify this hypothesis, 0.8 M precursor solutions of PEA2Sn I4, FASnh, and CsSnh were prepared for dynamic light scattering (DLS) measurements. It was found that their average cluster size variance agreed with the expected trend of CsSnh > FASnh > PEA2Snk Incorporating 5 mol% Cs into the precursor solution of PEA2Snl4 was revealed to enlarge its average cluster size by 75%, while the FASnh clusters were expanded by only 40%. To examine the effect of Cs+cations on tuning the colloidal aggregation of the 2D-3D THP system, triple-cation (Cs-FA-PEA) precursor solutions with different Cs+ratios ranging from 1 to 20 mol% were prepared for DLS measurements. The samples were given the codes CsO, Cs1 , Cs3, Cs7, Cs10, Cs15 and Cs20, collectively referred to as “Csx”, with the number “x” indicating the mol% of Cs in each sample. The value of “x” corresponds to the “x” in the formula (Csl)x / ioo(PEAo i5FAo 85Sn k.ssBro.isJi-x / ioo. As shown in their DLS analysis of Fig. 3b, the average cluster size of 2D-3D THP precursor solutions experienced a vigorous growth from 369 nm to 934 nm as x is increased from CsO to Cs5, while the less significant increase rate from Cs7 to Cs20 can be attributed to the saturated Cs cations in the EDL structure.In this context, the CsO and Cs5 precursor solutions were selected for verifying the conceptual design as follows.

[0144] In-situ X-ray diffraction (XRD) was employed to elucidate the nucleation and crystal growth kinetics of CsO and Cs5 THP films after spin-coating (wet film at 0 min) and upon post-annealing treatment from 20°C to 80°C (ramp rate: 1 °C min1) in N2 (Figure 3c). While the CsO wet film only exhibited sharp (110) and (220) peaks assigned to 3D FASnh, the Cs5 wet film displayed both 3D perovskite peaks and the premature formation of 2D perovskite (n = 2, n refers to the layer number of corner-sharing octahedra), indicating the presence of 2D perovskite nuclei at the shallow surface region of the bulk film immediately after spin-coating (Figure 3d). Without Cs-assisted nucleation, 2D perovskites (n = 1 and 2) appeared progressively in the CsO film only upon thermal annealing from 20°C to 30°C at 10 min. The uneven 2D phase distribution with different n values can be attributed to the delayed nucleation of 2D perovskites during the top-down crystallization upon solvent evaporation, along with the gradient depletion of FA+cations and supersaturated PEA+cations at the substrate / perovskite interface. In contrast, the steadily increasing XRD intensity at 4.2°, 8.2°, and 12.4° (002, 004, and 006 planes) indicates the homogeneous growth of 2D perovskite (n = 2) domains across the Cs5 film, as illustrated in the distinguishing nucleation and growth kinetics of CsO and Cs5 films in Fig. 1 a. Benefiting from the templated growth effect of 2D perovskites, the Cs5 film exhibited improved crystallinity with suppressed structural defects, as indicated by the more pronounced (1 10) and (220) peaks of 3D perovskites with a reduced full width at the half maximum (FWHM) in the fully crystallized Cs5 film (Figure 3e). Notably, the dominant XRD peaks of both the 2D and 3D perovskites right shift to higher angles with increased Cs ratios, indicating a reduction in the lattice spacing and the successful substitution of FA+cations for Cs+in both 2D (n = 2) and 3D perovskite crystal phases. With this context, the excessive Cs+incorporation can cause a higher order of lattice mismatch for disoriented growth of the perovskite crystals, which explains the reduced crystallinity in the Cs10 film.2D phase distribution and crystal orientation

[0145] Grazing-incident wide-angle X-ray scattering (GIWAXS) was used to resolve the final microstructure of perovskite films with and without Cs+incorporation. Varying the incident angle enables different X-ray penetration and scattering depths, allowing semi-quantitative depth profiles of the CsO and Cs5 films to be developed. In the control CsO sample, the absence of 2D perovskite diffraction spots at small grazing angles (i.e. within 3-35 nm of the top surface region) and their gradual appearance with increased penetration depth (-90-330 nm) confirm that 2D perovskite scattering domains tend to reside preferentially deep within the bulk film toward the buried perovskite / substrate interface (Figure 4a). In stark contrast, pronounced 2D perovskite Bragg spots are observed throughout the Cs5 film sample at all grazing angles, emerging from the top surface to the deep within the bulk film, revealing the formation of a well-distributed 2D-3D perovskite heterostructure. An azimuthal analysis of the (110) scattering plane at different X-ray scattering depths indicates the presence of 2D perovskite segments at the shallow surface can induce animproved out-of-plane orientation of 3D perovskite crystals, as evidenced by narrower azimuthal FWHMs across the Cs5 film (Figure 4b-c). Consistent with the previous XRD result, the excessive incorporation of Cs+cations induces a lattice mismatch effect and increased disorientation of 3D perovskite grains, reflected through the Debye-Scherrer rings in the 2D GIWAXS patterns of Cs10 film with much higher azimuthal FWHMs. Benefiting from enhanced crystal orientation and crystallinity, the Cs5 film shows a compact surface morphology with negligible pinholes (Figure 9).

[0146] The uniform distribution of 2D THP segments and Cs+cations across the bulk polycrystalline films is further confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and in-depth X-ray photoelectron spectroscopy (XPS), respectively. As the component variations of CsO and Cs5 films indicated by ToF-SIMS results (Figure 4d-e), the PEA+signals are found to peak near the substrate in the CsO film (aggregated 2D perovskites at the bottom perovskite / substrate interface), while the Cs5 sample exhibits a more even distribution of PEA+throughout the film. The constructed 2D-3D perovskite heterostructure leads to a more homogeneous energy landscape across the bulk film facilitating charge carrier transport. Besides, the uniform distribution of Cs+cations across the bulk THP film is verified by the depth XPS profile of Cs5 film, which agrees with lattice substitution of FA+cations for Cs+in both 2D and 3D THP domains corroborated by XRD peak shift.Photovoltaic performance and device stability

[0147] Solar cell devices were fabricated using the THP, the solar cells having a p-i-n architecture of IT 0 / PEDOT:PSS / THP / ICBA / BCP / Ag, where the perovskite film (“absorber”) thickness is -300 nm (Figure 5a). By measuring the current density-voltage (J- 1 / ) curves under simulated one-sun AM1 .5 illumination, the optimised Cs5 device delivered an impressive PCE of 17.13% with an open-circuit voltage ( Voc) of 0.99 V, a short-circuit current density (Jsc) of 23.07 mA cm2, and a fill factor (FF) of 74.83%, whereas the best CsO device only achieved a PCE of 13.49% (Figure 5b). Consistently, the Cs5 device exhibited a higher stabilized PCE of 17.04% under steady-state maximum power output tracking, which is higher than that (13.31 %) of the CsO device. (Note that additional test results discussed below achieved even higher PCE for the tin halide perovskite solar cell). As shown in the external quantum efficiency (EQE) plots of Figure 5c, the Cs5 device presented much improved photocurrent response across the visible and near infrared region, yielding an integrated of 22.34 mA erm2that is comparable with the value extracted from the J-V curve. In the statistic PV performance distribution from THPSCs with different Cs ratios, the Cs5 devices achieved the highest average PCE of 16.57 ± 0.34% in 20 devices from different batches (Figure 5d), further confirming the optimal Cs molar ratio of 5% and the high producibility of the target devices.

[0148] Subsequently, the long-term stability of the fabricated THPSCs with and without Cs+incorporation was examined under dark and continuous light illumination, and the results are shown in Figure 5f. The unencapsulated Cs5 device exhibited excellent shelf stability retaining 93% of the initial PCE after storing in N2 under dark conditions for 6,000 hours, while the CsO device experienced a fast performancedecay after 2,000 hours (Figure 5f). Such improved shelf stability in the Cs5 devices can be attributed to the presence of 2D perovskite segments across the bulk THP film that provide steric hindrance for the oxygen and moisture permeation from the atmosphere. The operational stability of unencapsulated CsO and Cs5 devices was compared under continuous LED light illumination (100 mW cm2equivalent to one-sun intensity) at 60 ± 5°C. The target Cs5 device maintained over 92% of the original PCE after 1 ,500 hours.

[0149] Additional test devices were fabricated in two groups: (A) Cs5 tin halide perovskite formulation produced in accordance with the optimal conditions of example 4(a’) - denoted the “target” - and (B) the same Cs5 tin halide perovskite formulation, but produced using the sub-optimal conditions of room temperature precursor solution and antisolvent, and without aging - denoted the “control”. As above, the test solar cells were constructed with a p-i-n architecture of ITO / PEDOT:PSS / THP / ICBA / BCP / Ag, where the perovskite film thickness is -300 nm (same as Figure 5a). By measuring the current density-voltage (J-V) curves under simulated one-sun AM1.5 illumination, the optimised “target” device delivered an impressive PCE of 17.30% with an open-circuit voltage (Voc) of 1 .016 V, a short-circuit current density ( JSc) of 22.95 mA erm2, and a fill factor (FF) of 74.19%, whereas the best control device only achieved a PCE of 15.12% (Figure 5i). As shown in the external quantum efficiency (EQE) plots of Figure 5j, the target device presented much improved photocurrent response across the visible and near infrared region, yielding an integrated Jscof 22.68 mA erm2that is comparable with the value extracted from the J-V curve. In the statistic PV performance distribution, the target devices achieved higher average PCE of 16.24 ± 0.60% in 20 devices from different batches compared to control devices (Figure 5k), further confirming the optimal temperature control range and the high producibility of the target devices. The achieved highest PCE of the target device, of 17.30%, is the highest value among all reported PCE for THPSCs to date.

[0150] The “target” and “control” devices that were the subject of the tests presented in Figures 5i, 5j and 5k were also subjected to long-term stability tests under dark and continuous light illumination. The device produced under the “target” conditions of example 4(a’) retained 95% of the initial PCE after storing in N2 under dark conditions for 5000 hours (Figure 5g), which compared favourably with the “control” produced using room temperature precursor solution and antisolvent, and without aging. That test device (“target”) also demonstrated an improved retention of the initial PCE (93%) under continuous LED light illumination (100 mW erm2equivalent to one-sun intensity), at 60 ± 5°C, compared to the control (Figure 5h).Properties of tin halide perovskites and devices containing the tin halide perovskites

[0151] Tests were conducted to determine the impact on the bandgap for tin halide perovskite photovoltaic devices containing 5% Cs, and 0% Cs in the tin halide perovskite layer. As shown in Figure 6, including 5% Cs in the tin halide perovskite reduced the bandgap from 1 ,43eV to 1 .41 eV. Further variations in the bandgap can be achieved by changing the relative amount of Br to I, with an increased Br content increasing the bandgap.

[0152] Crystallinity of the tin halide perovskites produced in accordance with the process was also studied. Figure 7 presents a plot demonstrating enhanced peak intensity, indicating enhanced crystallinity, for tin halide perovskites with different levels of Cs content (CsO to Cs10). The data presented demonstrates that Cs5 produces the greatest crystallinity of the samples tested.

[0153] PL mapping was also conducted comparing (a) a tin halide perovskite without Cs incorporation, against (b) the tin halide perovskite containing 5% Cs and made in accordance with the process of the present application. The results presented in Figure 8 demonstrate an enhanced light response at 500-750 for the tin halide perovskite containing Cs, due to better crystallinity and more even distribution of 2D perovskite in the bulk film.

[0154] These examples are provided to demonstrate the background science behind the invention and to demonstrate the construction and performance of one form of device that may be prepared making use of the present invention. It will be understood that many variations may be made to these examples when putting the invention to practice in different applications, without departing form the spirit and scope of the invention.

Claims

CLAIMSWhat is claimed is:1 . A process for the production of a layer of a tin perovskite, the process comprising:- producing a precursor layer of a tin perovskite precursor on a substrate from a precursor solution, the precursor solution comprising tin perovskite precursors in a solvent;- subjecting the precursor layer to aging under controlled time and temperature conditions to regulate tin perovskite crystal growth to produce an aged layer, and- annealing the aged layer to produce the tin perovskite.

2. The process of claim 1 , wherein the tin perovskite is a tin halide perovskite.

3. The process of claim 1 or claim 2, wherein one or more of the following conditions apply:- the aging is performed at an aging temperature of between -40°C and 55°C and an aging time of between 1 and 240 minutes, with the proviso that when the aging temperature is between 20°C and 55 °C, the aging time is a minimum of 10 minutes;- the precursor layer of tin perovskite precursor is produced by applying said precursor solution to the substrate at a precursor solution temperature of less than 20°C;- the precursor layer of tin perovskite precursor is produced by applying said precursor solution on the substrate and then applying an antisolvent solution onto the precursor solution at an antisolvent temperature of less than 20°C; or- the tin perovskite is a mixed cation tin halide perovskite, comprising between 3 and 7% cesium, based on as a mol% with respect to the total moles of cesium and tin, and a combination of two organic cations.

4. The process of claim 1 or claim 2, wherein the aging is performed at an aging temperature of between -40°C and 55 °C and an aging time of between 1 and 240 minutes, with the proviso that when the aging temperature is between 20°C and 55°C, the aging time is a minimum of 10 minutes.

5. The process of claim 4, wherein the aging temperature is selected from one of the following ranges: 20°C to -40°C, 15°C or less to -40°C, less than 15°C to -40°C, 10°C or less to -40°C, 5°C to -40°C, 0°C to -40°C or -5°C to -40°C.

6. The process of claim 4 or claim 5, wherein the aging time is between one of the following ranges: 10 - 180 minutes, 10 - 120 minutes, 20 - 120 minutes and 20 - 70 minutes.

7. The process of any one of claims 4 to 6, wherein the precursor layer of tin perovskite precursor is produced by applying said precursor solution to the substrate at a precursor solution temperature of less than 20 degrees °C, or 10°C or less, or 0°C or less, or -10°C or less.

8. The process of claim 7, wherein the precursor layer of tin perovskite precursor is produced by applying said precursor solution on the substrate and then applying an antisolvent solution onto the precursor solution at an antisolvent temperature of less than 20°C, or less than 10°C or less, or 0°C or less, or -10°C or less.

9. The process of any one of claims 1 to 8, wherein the tin perovskite is selected from one of the following classes: (i) a mixed cation tin halide perovskite, (ii) a mixed cation tin halide perovskite, comprising one or more organic cations and up to 10 mol% cesium, based on the total moles of cesium and tin, or (iii) a mixed cation tin halide perovskite, comprising between 3 and 7% cesium, based on as a mol% with respect to the total moles of cesium and tin, and a combination of two organic cations.

10. The process of claim 9, wherein the or each organic cation of the tin halide perovskite is selected from the group consisting of: formamidinium (“FA”), phenylethylammonium (“PEA”), methylammonium (MA), dimethylammonium (DMA), alkyl butylammonium (BA), guanidinium (Gua) and dialkyl ammonium cations.11 . The process of any one of claims 1 to 10, wherein the substrate comprises a first electrode layer, either a hole transport layer or an electron transport layer, and optionally a passivation layer.

12. The process of any one of claims 1 to 1 1 , wherein the process further comprises producing a first electrode layer, a hole transport layer, an electron transport layer and a second electrode layer, to thereby produce a photovoltaic device.

13. A tin halide perovskite of formula (I):(CsX1)w(PEAyFA(i-y)SnX23)(i-w) formula (I) wherein X1is a halide or pseudohalide,0.03 < w < 0.07,0 < y < 1.0, andX2is a halide, or a combination of two halides.

14. The tin halide perovskite of claim 13, wherein the tin halide perovskite is produced by the process of any one of claims 1 to 12.

15. A tin perovskite produced by the process of any one of claims 1 to 12.

16. A device comprising a layer of a tin halide perovskite, wherein the tin halide perovskite is a mixed cation tin halide perovskite, comprising between 3 and 7% cesium, based on mol% with respect to the total moles of cesium and tin, and a combination of two organic cations.

17. The device of claim 16, wherein the device is a photovoltaic device, or the device is a solar cell.

18. The device of claim 16 or claim 17, wherein the layer of tin halide perovskite is produced by the process of any one of claims 1 to 12.

19. The device of any one of claims 16 to 18, with a band gap of between 1 .6 and 1 .9 eV.

Images