Perovskite photodetectors with metallocene-based compounds

Abstract

A perovskite photodetector (100) comprising a perovskite layer (110), an electron transporting layer (106) and a hole blocking layer (108) disposed between the electron transporting layer and the perovskite layer. The hole blocking layer comprises a metallocene substituted with a substituent having an O, S, N or P atom.

Description

[0001] PEROVSKITE PHOTODETECTORS WITH METALLOCENE-BASED COMPOUNDS

[0002] This invention relates to materials for perovskite photodetectors with metallocene-based compounds.

[0003] BACKGROUND

[0004] Commercial photodetectors use silicon-based semiconductor materials and other inorganic semiconductor materials, such as gallium phosphide and lead sulfide. Due to their complex and expensive manufacturing process under vacuum conditions, further commercial development is limited, and new candidates are required.

[0005] Polymer and quantum dot photodetectors have emerged due to their low-cost solution fabrication process and broadly detected spectrum, covering from visible to near-infrared range. However, their low light absorption capacity and poor electric performance limit practical applications. Perovskite materials have become promising candidates due to their excellent electric characteristics (such as high carrier mobility and long carrier diffusion length), good optical characteristics (such as light absorption capacity and tuneable band gap) and their simple and low-cost synthesis process under non-vacuum conditions. However, perovskite photodetectors suffer from relatively large dark currents, especially when using Pb-Sn perovskites, which are detrimental for the sensitivity to low light intensity.

[0006] There is a growing demand for high resolution and rapid response photodetectors with long-term stability. These photodetectors are crucial in hyperspectral Light Detection and Ranging (LiDAR) systems, imaging, and optical communications. Lead-halide perovskite (LHP) photodetectors have potential as broadband photodetectors due to their optoelectronic properties, such as compositional tuneability, high charge carrier mobility, extended charge carrier lifetimes, long diffusion lengths, and low-temperature solution processability. However, several challenges remain in devices with LHPs, primarily due to their inherent material instability. For example, their sensitivity to moisture and oxygen promotes the deprotonation of organic cations, leading to the decomposition of the absorber layer. Moreover, various environmental stressors, including electric fields and light exposure, can lead to a higher density of ionic defects and greater ion migration. This often results in significant hysteresis and device degradation, particularly under reverse bias, as holes can tunnel to the perovskite layer from the silver (Ag)electrode and interact with mobile iodine (I2) interstitials. The resultant I2 are unstable and mobile, meaning that they are prone to escape the perovskite layer. Additionally, it has been reported that I2 can traverse Ceo, typically used in the electron transport layer, and oxidize the Ag contact, causing shunting and substantial recombination losses within the device.

[0007] Efforts to address these issues and improve perovskite photodetectors (PPDs) include material-driven strategies, where compositional or additive engineering mitigates bulk and surface defects of the perovskite layer, and interface engineering, which improves energy level alignment between neighbouring layers or raises energy barriers to promote charge transfer and suppress undesired charge injection. Beyond these approaches, using a thin buffer layer has proven crucial in preventing the injection of charge carriers from the adjacent electrodes into the perovskite layer in the absence of light. Any deficiencies in the performance of the blocking layer increases the leakage current and thus the dark current density ( / D). A range of materials, including bathocuproine (BCP), bathophenanthroline (BPhen), atomic layer deposition-processed tin oxide (ALD-SnCh), and lithium fluoride (LiF), have been reported to effectively suppress shunting paths and simultaneously enhance reverse bias stability. While charge-blocking layers are beneficial in reducing JDin PPDs, their design requires meticulous consideration to avoid substantial energy level mismatches or chemical instability issues that could impede fast charge extraction, limiting the response speed and specific detectivity (£>*).

[0008] Mitigating performance degradation and improving the stability of PPDs is critical to realising their commercial viability.

[0009] There remains a need for improved perovskite photodetectors and, in particular perovskite photodetectors with improved stability and performance, specifically high response speeds, reduced noise spectral density, improved specific detectivity, and improved linear dynamic range.

[0010] WO2023203134 discloses a perovskite-based photovoltaic cell, which includes an interface layer comprising a metallocene substituent with a substituent having an O, S, N or P group, for example ferrocene substituted with a thienyl-carboxylate group. It is believed that the electron-rich substituent, e.g. the thienyl group, may form a coordinate bond with Pb of the perovskite and / or that the lone electron pairs of the O, S, N or P group are capable of binding to uncoordinated metal defects,e.g. Pb defects, at the perovskite surface and / or that the flexibility of metallocenes around the metal-aromatic bond may ameliorate stresses between the electron transport layer and the perovskite layer. Thus, the use of a metallocene interface layer boosts the electron transfer at the perovskite / ETL interface. However, there is no disclosure of perovskite photodetectors, let alone perovskite photodetectors that comprise a hole blocking layer comprising a metallocene substituent.

[0011] SUMMARY

[0012] The inventors have surprisingly found that a layer of a metallocene-based compound between the perovskite layer and the electron transport layer (ETL) can successfully eliminate hole back-injection. Without wishing to be bound by theory, this is believed to be due to the interfacial energy offset between the valence band of the perovskite and the energetic levels of the metallocene derivatives.

[0013] The inventors have achieved a perovskite photodetector with a stable JDover 400 continuous light / dark on / off cycles under reverse bias, indicating the versatility of metallocene-based compounds in perovskite photodetectors and improved perovskite photodetector performance while maintaining both efficiency and stability under reverse bias.

[0014] The present invention provides ultrasensitive (dark current values in the order of nA cm-2at -0.5V) and fast broadband perovskite photodetectors approaching 10 MHz. This enables speeds high enough to support video applications.

[0015] The perovskite photodetectors have ultra-low noise current approaching the sensitivity of the instrument used and achieved the highest specific detectivity (1013Jones) reported to date.

[0016] In a first aspect, the invention provides a perovskite photodetector comprising: a first electrode;

[0017] a second electrode;

[0018] a perovskite layer and an electron transport layer disposed between the first and second electrodes; and

[0019] a hole blocking layer disposed between the perovskite layer and the electron transport layer and in direct contact with the perovskite layer, the hole block layer comprising a metallocene substituted with at least one substituent R1comprising at least one of an O, S, N or P atom.The metallocene may be a compound of formula (I):

[0020] [Metallocene]p

[0021] (I)

[0022] wherein:

[0023] Metallocene is a metallocene group comprising a metal bound to two aromatic or heteroaromatic groups Ar1;

[0024] p is at least 1; and

[0025] at least one Metallocene is substituted with at least one substituent R1.

[0026] The compound of formula (I) may have formula (la):

[0027]

[0028] wherein:

[0029] M is a metal ion;

[0030] Ar1in each occurrence is a monocyclic or polycyclic aromatic or heteroaromatic group;

[0031] M and the two Ar1groups form the Metallocene;

[0032] at least one Ar1is substituted with at least one R1;

[0033] R2is a group for satisfying the valency of M;

[0034] q is 0 or a positive integer; and

[0035] R3in each occurrence is independently H or a substituent.

[0036] The metallocene may be ferrocene.

[0037] R1may be a group of formula (II):

[0038] -A-B

[0039] (II)

[0040] wherein A is a divalent group comprising O, S, N or P; and B is H, C1-12 alkyl, optionally substituted aryl or optionally substituted heteroaryl.

[0041] A may be selected from formula (III) or formula (IV)-(R5)f-Z-(R5)g- (III)

[0042] -(R50)r(IV)

[0043] wherein:

[0044] R5in each occurrence is independently a hydrocarbon group;

[0045] f and g are each independently 0 or 1;

[0046] R5is a Cl-4 alkylene group, preferably ethylene; and

[0047] Z is O, S, COO, C( = S)O, C(=O)S, CONR4, CSNR4, OC( = O)O, OC(=O)NR4, OC( = O)PR4' NR4, PR4, -OP(=O)(OR4)-O-, -NR4-P( = O)(NR42)-NR4-, wherein R4is H, optionally substituted C1-12 alkyl or optionally substituted phenyl.

[0048] The bond between the metallocene and R1may be a carbon-oxygen bond in which a C atom of the metallocene is bound to an O atom of R1.

[0049] A may be -C(=O)-O-; -O-C(=O)- or -C(=O)-.

[0050] B may be selected from optionally substituted phenyl and an optionally substituted 5-membered heteroaryl comprising one or more ring atoms selected from O, S and N.

[0051] B may be optionally substituted thiophene or optionally substituted furan or optionally substituted phenyl.

[0052] The hole blocking layer may have a thickness between 0.5 and 5 nm. The hole blocking layer may have a thickness of about 0.8 nm.

[0053] In a second aspect the invention provides a device comprising the perovskite photodetector. The device comprising the perovskite photodetector may be a camera, medical imaging system, consumer electronic device, environmental sensor, a communication network device or a wearable device, for instance wherein the device is a wearable sensor, CCT camera, night-vision device, or UV-Vis-NIR spectrometer.

[0054] In a third aspect the invention provides the use of a substituted-metallocene as described herein, in a perovskite photodetector for blocking hole transfer from the cathode to the perovskite layer, reducing ion migration across the perovskite interface, promoting the reduction of generated I2, or improving stability of theperovskite photodetector, wherein the substituted-metallocene is as described herein.

[0055] In a fourth aspect the invention provides a method of blocking hole transfer from the cathode to the perovskite layer, reducing ion migration across the perovskite interface, promoting the reduction of generated I2, or improving stability in a perovskite photodetector, comprising disposing a hole blocking layer between the perovskite layer and the electron transport layer in the perovskite photodetector, wherein the hole block layer is in direct contact with the perovskite layer and the hole blocking layer comprises a metallocene as described herein.

[0056] LIST OF FIGURES

[0057] Figure 1 provides a schematic illustration of a perovskite photodetector; Figure 2 provides a schematic illustration of a perovskite photodetector cell according to one of the examples, i.e. comprising a ferrocene hole blocking layer;

[0058] Figure 3 shows JV curves for a perovskite photodetector with and without a ferrocene-based compound under dark and light (AM 1.5G illumination) conditions;

[0059] Figure 4 shows the specific detectivity plots of perovskite photodetectors; Figure 5 shows the cut-off frequency plots of perovskite photodetectors.

[0060] DETAILED DESCRIPTION

[0061] With reference to Figure 1, an 'inverted' perovskite photodetector 100 comprising a p-n or p-i-n junction is described (where holes are collected at a transparent electrode).

[0062] transparent substrate 102 is provided. This forms the base or support for the perovskite photodetector 100. Incident light 116 enters the perovskite photodetector 100 through the transparent substrate 102. Substrate 102 may be formed of glass, or any other suitable transparent material.

[0063] The perovskite photodetector 100 comprises a perovskite layer 110. In use, the perovskite layer 110 absorbs light incident on the perovskite photodetector 100. The term 'light-absorbing' in relation to the perovskite(s) (and by extension the layer 110 comprising said one or more perovskites) refers to its role in absorbing light so as to act as a light absorbing material which allows to convert the light 116 into electrical energy. A perovskite type compound exhibits strong absorption withrespect to light 116 incident on the perovskite photodetector 100, and the bandgap of a perovskite semiconductor can be tuned to a desired band gap energy Eg.

[0064] Incident light 116 passes through the substrate layer 102 into the active layer 110, whereupon at least a portion of the incident light 116 is absorbed by exciting an electron across a semiconductor band gap so as to enable electrical generation. In particular, the electron is excited from a valence band of the semiconductor, across the bandgap, to a conduction band. The excited electron sits in the conduction band, and a corresponding hole (a vacancy or absence of an electron, rather than a physical particle in and of itself) remains in the valence band of the semiconductor.

[0065] An asymmetry within the perovskite layer 110 acts to separate the excited electron away from the hole, moving the charge carriers (holes and electrons) away from the point of electron promotion for collection and current generation. In the examples described herein, this asymmetry is provided by a junction within the perovskite layer 110 (such as a p-n or p-i-n junction). It will therefore be understood that the perovskite layer can include any suitable semiconductor junction. However, the asymmetry within the perovskite layer may be provided in any other suitable manner.

[0066] In some examples, the perovskite layer 110 can include one or more heterojunctions. Heterojunctions can be formed within the perovskite layer 110 by way of two different, undoped, perovskite materials. Thus, the perovskites referred to herein may both be undoped semiconductors. Alternatively, the perovskite(s) may be doped with p-type or n-type dopants to form a junction. In other words, they may be doped (throughout and / or at the surface) with at least one dopant material of greater valency than the bulk material (to provide n-type doping) and / or may be doped with at least one dopant material of lower valency than the bulk material (to give p-type doping). N-type doping will tend to increase the n-type character of the semiconductor material, while p-type doping will tend to reduce the degree of the natural n-type state (e.g. due to defects). Such doping may be made with any suitable material including F, Sb, N, Ge, Si, C, In, InO and / or Al. Suitable dopants and doping levels will be evident to those of skill in the art.

[0067] In some examples, light-absorbing perovskite layer 110 comprises one or more metal halide perovskites. In some examples, the light-absorbing layer maycomprise two different metal halide perovskites configured to form a semiconductor heterojunction within layer 110. Any perovskite(s) capable of performing the desired light-absorbing and charge separation functions may be used in lightabsorbing layer 110.

[0068] An electron transport layer (ETL) 106 is provided. The ETL (or n-type charge extraction layer) comprises an electron transport material. Any electron transport material known to the skilled person may be used. The ETL may comprise or consist of an organic electron transport material, an inorganic electron transport material or mixtures thereof. Example electron transport materials include organic materials such as fullerenes, metal oxides such as TiO?, ZnO, SnO?, SiO?, or ZrO?.

[0069] Fullerenes are preferred. Fullerenes may be selected from any known fullerene including Ceo fullerene and C70 fullerene, each of which is optionally substituted with one or more substituents. Exemplary substituents include C1-12 alkyl wherein one or more non-adjacent C atoms of the C1-12 alkyl may be replaced with O, S, CO or COO and optionally substituted phenyl, and wherein two substituents may be linked to form a monocyclic or polycyclic ring. Exemplary fullerenes include Ceo, PCBM and ICBA.

[0070] Electron transport materials may encourage a flow of electrons from the n-type perovskite, away from the junction within layer 110, while blocking the movement of holes. In this way, electrons accumulate at a first electrical conductor 104. In use, the first electrical conductor 104 is negatively charged due to the accumulation of electrons. When the perovskite photodetector cell is connected to an external load, the electrons leave the solar cell 100 via the first electrical conductor 104.

[0071] The first conductor 104 may be formed of any suitable conducting material, such as Ag, Au, Cu, etc.

[0072] A hole transport layer (HTL) 112 comprising or consisting of one or more hole transport materials can also be provided within perovskite photodetector 100. The HTL may be located proximate to the transparent substrate (holes are collected at the electrode proximate the substrate). Any hole-transport material known to the skilled person may be used.Example hole transport materials include organic hole-transport materials, inorganic hole-transport materials or combinations thereof. Organic hole-transport materials may be polymeric or non-polymeric. Exemplary polymeric hole-transport materials include polythiophenes, for example poly(3-hexylthiophene) (P3HT); poly(arylamines) for example PTTA; and doped PEDOT, for example PEDOT:PSS. Exemplary non-polymeric organic hole-transport materials are compounds containing one or more arylamine groups, for example spiro-OMeTAD and selfassemble monolayers, for example those based on phosphoric acid group-containing carbazole-based small organic molecules such as (2-(9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz), (2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl)phosphonic acid (MeO-2PACz), or (4-(3,6-dimethyl-9H-carbazol-9-yl)butyl)phosphonic acid (Me-4PACz). Exemplary inorganic hole-transport materials include copper-based materials (e.g. CuOx, CuSCN, Cui, etc.), nickel-based materials (e.g. NiOx), two-dimensional layered materials such as chalcogens (e.g. M0S2, WS2, etc.).

[0073] Hole transport materials may encourage a flow of holes away from the junction within active layer 110, while blocking the movement of electrons. In this way, holes accumulate at a second electrical conductor 114. In use, the second electrical conductor 114 is positively charged due to the accumulation of holes.

[0074] The second conductor 114 may be any transparent conducting material. In some examples, the second conductor 114 is a transparent conducting film (TCF). In some examples, the TCF is a transparent conducting oxide (TCO) layer. In some examples, the TCO layer comprises indium-tin oxide (ITO), fluorine-doped tin oxide (FTO) or doped zinc oxide.

[0075] A hole blocking layer (HBL) 108 is provided between the electron transport layer 106 and the active layer 110. The HBL 108 improves the extraction of electrons, increasing the light detection of the perovskite photodetector, and improve the stability of the perovskite photodetector 100.

[0076] The HBL 108 interfaces directly with the active layer 110. In other words, the HBL 108 is in direct contact with the active layer 110. The HBL 108 can be deposited directly on the active layer 110, or may be otherwise formed.One or more additional layers (not shown) may be provided within the perovskite photodetector 100. For example, optional additional hole blocking layers may be provided between the ETL 106 and the contact 104. Similarly, one more optional electron blocking layers may be provided between the HTL 112 and the contact 114 and / or between the perovskite layer 110 and the HTL 112. Any other layers may be provided within perovskite photodetector cell 100, as appropriate.

[0077] The present inventors have surprisingly found that the presence of such a metallocene in hole blocking layers (HBL) in perovskite photodetectors (PPDs) enhances the stability and performance of PPDs, specifically providing PPDs with high response speeds, reduced noise spectral density, improved specific detectivity, improved linear dynamic range and improved device stability

[0078] The metallocene preferably is a compound of formula (I):

[0079] [Metallocene]p

[0080] (I)

[0081] wherein:

[0082] Metallocene is a metallocene group comprising a metal bound to two aromatic or heteroaromatic groups Ar1;

[0083] p is at least 1, optionally 1, 2 or 3; and

[0084] at least one Metallocene is substituted with at least one substituent R1wherein R1is a group comprising an O, S, N or P atom.

[0085] Optionally, the compound of formula (I) has formula (la):

[0086]

[0087] wherein:

[0088] M is a metal ion;

[0089] Ar1in each occurrence is a monocyclic or polycyclic aromatic or heteroaromatic group;

[0090] M and the two Ar1groups form the Metallocene;at least one Ar1is substituted with at least one R1wherein R1is a group comprising an O, S, N or P atom;

[0091] R2is a group for satisfying the valency of M;

[0092] q is 0 or a positive integer, preferably 0 or 2;

[0093] R3in each occurrence is independently H or a substituent; and

[0094] p is at least 1.

[0095] Exemplary Ar1groups include, without limitation, C4-C8 aromatic groups, i.e., cyclobutadiene, cyclopentadienyl, benzene, cycloheptatrienyl or cyclooctatetraene; and Cs heteroaromatic groups, e.g., pyrrole, each of which may be unfused or fused to one or more further rings, preferably one or more benzene rings. Exemplary fused groups Ar1include benzocyclopentadienyl and fluorenyl.

[0096] Metallocene preferably comprises a metal M bound to two cyclopentadienyl groups Ar1. Ar1may consist of the cyclopentadienyl group or the cyclopentadienyl may be fused to one or more further rings, preferably one or more aromatic rings, e.g. one or more benzene rings as in benzocyclopentadienyl or fluorenyl.

[0097] M may be Fe2+, Co2+, Cr2+, Ni2+or V2+, preferably Fe2+. For each of these compounds, q is 0.

[0098] M may be Zr or Ti. For each of these compounds, q is 2 and R2may be any suitable group capable of bonding to Zr orTi, for example methyl, ammonia, dialkylamines, phosphines, CO or halogen, e.g. Cl, such as in metallocene dihalides.

[0099] The two Ar1groups of the or each Metallocene may be linked - other than through M - by a divalent group, for example a C1-6 alkylene or a group of formula Si(R3)? wherein R3in each occurrence is independently a C1-12 hydrocarbyl group, e.g. C1-12 alkyl or phenyl. It will therefore be understood that compounds of formula (I) include ansa-metallocenes.

[0100] In a preferred embodiment, M and Ar1form ferrocene, i.e. M is Fe; each Ar1is cyclopentadienyl; and y is 0.

[0101] Preferably, R1is the only substituent of the Ar1groups.

[0102] Preferably, p is 1, 2 or 3, more preferably 1.Compounds of formula (la) may be selected from formulae (lb), (Ic) or (Id):

[0103]

[0104] wherein tl is 0, 1 or 2, preferably 0 or 1; t2 is 0 or 1, preferably 1; and at least one of tl and / or t2 is at least 1.

[0105] In some embodiments, R1is a group of formula (II):

[0106] -A-B

[0107] (II)

[0108] wherein A is a divalent group comprising O, S, N or P; and B is H, C1-12 alkyl, optionally substituted aryl or optionally substituted heteroaryl.

[0109] Group A may comprise any group capable of binding to Pb. Exemplary groups A include, without limitation, ethers, thioethers, amines, phosphines, phosphoryl ethers, carbonates, carbamates, carboxylates, amides, thioamides, phosphonamides, thiocarboxylates, aminocarboxylates, and phosphocarboxylates. R1 may comprise only one group A. R1may comprise two or more groups A.

[0110] Exemplary groups A include groups of formulae (III) and (IV):

[0111] -(R5)f-Z-(R5)g- (III)

[0112] -(R5O)r(IV)

[0113] wherein:

[0114] R5in each occurrence is independently a hydrocarbon group;f and g are each independently 0 or 1;

[0115] R5is a Ci-4 alkylene group, preferably ethylene;

[0116] j is 1-10; and

[0117] Z is O, S, COO, C(=S)O, C(=O)S, CONR4, CSNR4, OC(=O)O, OC(=O)NR4, OC( = O)PR4' NR4, PR4, -OP(=O)(OR4)-O-, or -NR4-P(=O)(NR42)-NR4-, wherein R4is H, optionally substituted C1-12 alkyl or optionally substituted phenyl.

[0118] Hydrocarbon groups R5are preferably selected from C1-6 alkylene; optionally substituted phenylene; and C1-6 alkylene-phenylene.

[0119] phenylene group of an R5group may be unsubstituted or substituted with one or more substituents selected from C1-6 alkyl.

[0120] In the case where R5is C1-6 alkylene-phenylene, the group Z may be bound to either the alkylene or the phenylene group.

[0121] A particularly preferred group A is -O-C(=O)-, which may be linked to Metallocene through the O atom or the C atom(-C(=O)-O-), preferably through the O atom. A particularly preferred group A is also -C(=O)-.

[0122] B is preferably an optionally substituted aryl or heteroaryl, more preferably phenyl or a 5-membered heteroaromatic comprising one or more of N, S and O ring atoms, for example furan, thiophene, pyrrole, imidazoles and oxazole. Thiophene and furan are particularly preferred.

[0123] Optional substituents of an optionally substituted alkyl or alkylene group as described anywhere herein include F, Cl, OR4and NR42wherein R4is a C1-6 alkyl.

[0124] Optional substituents of any optionally substituted aromatic or heteroaromatic group as described anywhere herein, including but not limited to substituents R3of formula (la), include F, Cl, CN, NO2, C1-6 alkyl wherein one or more H atoms may be replaced with F, OR4and NR42wherein R4is a C1-6 alkyl.

[0125] Perovskites

[0126] The perovskites may be any material with the CatBXs crystal structure (perovskite structure, commonly referred to as the "ABX3" structure), where Cat and B are cations and X is an anion. B is preferably Pb or Sn.The perovskite is suitably a perovskite of formula CatPbXs or CatSnXs wherein Cat is a metal cation, an organic cation or a combination thereof and X is selected from at least one of I, Br and Cl.

[0127] Exemplary groups Cat include alkali metal cations, preferably Cs; ammonium cations, for example methylammonium; and amidinium ions, for example formamidinium.

[0128] Preferably, X includes two of I, Br and Cl.

[0129] Preferably, Cat comprises both a metal cation and an organic cation.

[0130] Preferably, Cat comprises two different organic cations.

[0131] Examples of perovskites suitable for use as a light-absorbing layer include: ammonium trihalogen plumbates such as CHsNHsPbls, CHsNHsPbCh, CHsNHsPbFs and CHsNHsPbBn; mixed-halide ammonium trihalogen plumbate perovskites with general formula CH3NH3Pb[Hali]3-x[Hal2]x wherein [Hali] and [Hah] are independently selected from among F, Cl, Br and I with the proviso that [Hall] and [Hal2] are non-identical and wherein 0 < x < 3, preferably wherein x is an integer (e.g. 1, 2 or 3, preferably 1 or 2); CsSnXs perovskites wherein X is selected from among F, Cl, Br and I, preferably I; organometal trihalide perovskites with the general formula (RNH3)BX3 where R is CH3, CnH2n or CnH2n+i, n is an integer in the range 2 < n < 10, preferably 2 < n < 5, e.g. n=2, n=3, or n=4, most preferably n=2 or n=3, X is a halogen (F, I, Br or Cl), preferably I, Br or Cl, and B is Pb or Sn; and combinations thereof. In some examples, a perovskite composition of Csx FA MAi-j^i-xPb IzBri-z^, where x= (0~0.95 ), y= ( 0~l ), z= ( 0~l ) is used, where MA and FA denote methylammonium and formamidinium, respectively.

[0132] Photodetector formation

[0133] Photodetectors as described herein may be formed by any method known to the skilled person. Preferably, the photoactive layer is formed by depositing a solution comprising the photoactive material. Suitable solvents for deposition of the photoactive material include polar solvents such as DMF and DMSO. Preferably, the electron transport layer is formed by depositing a solution comprising the material(polymer, oxides, etc). Suitable solvents for deposition of the ETL material include polar solvents such as methanol, DMF and DMSO.

[0134] Solutions may be deposited by any method known to the skilled person, for example spin-coating, dip-coating, slot-die coating, doctor blade coating and bar coating.

[0135] EXAMPLES

[0136] As described herein, the chemicals used include the following:

[0137] • The solvents, including dimethylformamide (DMF), dimethyl sulfoxide (DMSO), isopropanol (IPA) and chlorobenzene (CB) were purchased from J&K (China) and used as received.

[0138] • Perovskite precursors, Caesium iodide (CsI, 99.99 %, Sigma Aldrich), formamidinium iodide (FAI, >99.9 %, GreatCell Solar), methylammonium chloride (MACI, 99.0 %, Sigma-Aldrich), and methylammonium iodide (MAI, >99.9%, GreatCell Solar).

[0139] • [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) (98.0 %, TCI).

[0140] • bathocuproine (BCP) (>99.5%, Lumtec).

[0141] • Lead iodide (Pbl2), and lead bromide (PbBr2) purchased from TCI (Japan).

[0142] • C60, poly[bis(4 -phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) (Mn 6,000- 15,000), methylammonium chloride (MACI) and bathocuproine (BCP, purity of 99.9%) purchased from Xi'an Polymer Light Technology Corporation (China).

[0143] • High purity silver was purchased from commercial sources.

[0144] • Glass substrates patterned with ITO (15 sq-1) were received from Yingkou Shangsheng Business Co., Ltd. (China).

[0145] General procedure for the synthesis of l,l'-bis[(2-thienylcarbonyl)oxy]ferrocene (FCTC2), l',l"'-bis[(2-thienylcarbonyl)oxy]-l,l"-biferrocene (FC2TC2), l',l""'-bis[(2-thienylcarbonyl)oxy]-l,l":l'",l""-terferrocene (FC3TC2), l,l'-bis[(2-furanylcarbonyl)oxy]ferrocene, l,l'-bis[(lH-2-pyrrolylcarbonyl)oxy]ferrocene and l,l'-bis[(2-phenylcarbonyl)oxy]ferrocene

[0146] Fcl2, FC2I2 or FC3I2 (1 equiv), CU2O (1.5 equiv) and respective carboxylic acid (3 equiv) were refluxed for 24 h in CH3CN. The rection mixture was diluted withCH2CI2, filtered and the filtrate washed with saturated aqueous NaHCOs until the aqueous phase became colourless. The organic phase was dried over Na2SO4, filtered and concentrated in vacuo. The crude residue was purified by flash chromatography (n-hexane / CH2CI2) and the resulting product recrystallized from CH2CI2 and n-hexane to afford the desired compound.

[0147] l,l'-Bis[(2-thienylcarbonyl)oxy]ferrocene (FCTC2) - as reported in Z. Li, B. Li, X. Wu, S. A. Sheppard, S. Zhang, D. Gao, N. J. Long and Z. Zhu, Science, 2022, 376, 416-420

[0148]

[0149] FCTC2was prepared according to the general procedure using Fcl2(158 mg, 0.36 mmol), 2-thiophenecarboxylic acid (139 mg, 1.08 mmol), CU2O (77 mg, 0.54 mmol) and CH3CN (15 mL) to afford orange crystals (108 mg, 68%).XH NMR (400 MHz, CDCI3): 6 7.78 (d, 2H,3JHH = 3.8 Hz, H-5'), 7.56 (d,3JHH = 4.9 Hz, 2 x H-3'), 7.06 (dd,3JHH = 5.0 Hz,4JHH = 3.8 Hz, 2 x H-4'), 4.68 (t,3JHH = 2.0 Hz, 2 x H-2 and H-5), 4.10 (t, 4H,3JHH = 2.0 Hz, 2 x H-3 and H-4);13C{1H} NMR (100 MHz, CDCh): 6 160.34 (2 x C, 2 x C=O), 134.24 (2 x CH, 2 x C-5'), 133.3 (2 x C, 2 x C-2'), 133.22 (2 x CH, 2 x C-3'), 127.99 (2 x CH, 2 x C-4'), 116.58 (2 x C, 2 x C-l), 64.82 (4 x CH, 2 x C-3 and C-4), 62.22 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 437.9672 [M]+(mCaic 437.9683); Calcd for C2oHi4Fe04S2: C 54.81, H 3.22. Found: C 54.62, H 3.29.

[0150] l',l'"-Bis[(2-thienylcarbonyl)oxy]-l,l"-biferrocene (FC2TC2) - as reported in B. Li, D. Gao, S. A. Sheppard, W. D. J. Tremlett, Q. Liu, Z. Li, A. J. P.

[0151] White, R. K. Brown, X. Sun, J. Gong, S. Li, S. Zhang, X. Wu, D. Zhao, C. Zhang, Y. Wang, X. C. Zeng, Z. Zhu and N. J. Long, J. Am. Chem. Soc., 2024, 146, 13391-13398

[0152]

[0153] Fc2Tc2was prepared according to the general procedure using FC2I2 (947 mg, 1.52 mmol), 2-thiophenecarboxylic acid (585 mg, 4.57 mmol), CU2O (327 mg, 2.28 mmol) and CH3CN (80 mL) to afford orange crystals (652 mg, 69%). Rf 0.32 (CH2CI2 / n-hexane 1:1);XH NMR (400 MHz, CDCh): 6 7.76 (dd, 2H,3J = 3.8,4J = 1.2 Hz, 2 x H-5"), 7.59 (dd, 2H,3J = 4.9,4J = 1.3 Hz, 2 x H-3"), 7.11 (dd, 2H,3J = 5.0,3J = 3.7 Hz, 2 x H-4"), 4.44 (pseudo-t, 4H, J = 1.8 Hz, 2 x H-3 and H-4), 4.39 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-3' and H-4'), 4.16 (pseudo-t, 4H, J = 1.8 Hz, 2 x H-2 and H-5), 3.81 (pseudo-t, 4H, J = 1.9 Hz, 2 x H-2' and H-5');13C{1H} NMR (100 MHz, CDCh): 6 160.4 (2 x C, 2 x C=O), 134.2 (2 x CH, 2 x C-5"), 133.4 (2 x C, 2 x C-2"), 133.0 (2 x CH, 2 x C-3"), 127.9 (2 x CH, 2 x C-4"), 116.3 (2 x C, 2 x 1'), 84.4 (2 x C, 2 x C-l), 69.2 (4 x CH, 2 x C-2 and C-5), 67.7 (4 x CH, 2 x C-3 and C-4), 64.5 (4 x CH, 2 x C-2' and C-5'), 62.0 (4 x CH, 2 x C-3’ and C-4’); HRMS (ESI+): m / z 621.9655 [M]+(mCaic 621.9658); Calcd for C3oH22Fe204S2: C 57.90, H 3.56. Found: C 57.68, H 3.49.

[0154] l,,l"",-Bis[(2-thienylcarbonyl)oxy]-l,l":l"',l""-terferrocene (Fc3Tc2) -as reported in B. Li, D. Gao, S. A. Sheppard, W. D. J. Tremlett, Q. Liu, Z. Li, A. J. P. White, R. K. Brown, X. Sun, J. Gong, S. Li, S. Zhang, X. Wu, D. Zhao, C. Zhang, Y. Wang, X. C. Zeng, Z. Zhu and N. J. Long, J. Am. Chem. Soc., 2024, 146, 13391-13398

[0155]

[0156] FC3TC2was prepared according to the general procedure using Fc3I2(718 mg, 0.89 mmol), 2-thiophenecarboxylic acid (343 mg, 2.67 mmol), Cu2O (191 mg, 1.34 mmol) and CH3CN (100 mL) to afford red crystals (0.36 mg, 50%). Rf 0.42 (CH2CI2 / n-hexane 3:2);XH NMR (400 MHz, CDCh): 6 7.73 (dd, 2H,3J = 3.7 Hz,4J = 1.3 Hz, 2 x H-5'"), 7.58 (dd, 2H,3J = 5.0 Hz,4J = 1.3 Hz, 2 x H-3'"), 7.10 (dd, 2H,3J = 5.0,3J = 3.7 Hz, 2 x H-4'"), 4.33 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-3" and H-4"), 4.30 (pseudo-t, 4H, J = 1.8 Hz, 2 x H-3 and H-4 or 2 x H-3' and H-4'), 4.21 (pseudo-t, 4H, J = 1.9 Hz, 2 x H-2 and H-5 or 2 x H-2’ and H-5’), 4.06 (pseudo-t, 4H, J = 1.9 Hz, 2 x H-3 and H-4 or 2 x H-3’ and H-4’), 3.89 (pseudo-t, 4H, J = 1.8 Hz, 2 x H-2 and H-5 or 2 x H-2’ and H-5’), 3.77 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-2" and H-5");13C{1H} NMR (100 MHz, CDCh): 6 160.4 (2 x C, 2 x C=O), 134.2 (2 x CH, 2 x C-5'"), 133.5 (2 x C, 2 x C-2'"), 133.0 (2 x CH, 2 x C-3'"), 127.9 (2 xCH, 2 x C-4'"), 116.2 (2 x C, 2 x 1"), 85.1 (2 x C, 2 x C-l or 2 x C-l'), 83.3 (2 x C, 2 x C-l or 2 x C-l'), 69.0 (4 x CH, 2 x C-2 and C-5 or 2 x C-2' and C-5'), 69.0 (4 x CH, 2 x C-2 and C-5 or 2 x C-2' and C-5'), 67.8 (4 x CH, 2 x C-3 and C-4 or 2 x C-3' and C-4'), 67.5 (4 x CH, 2 x C-3 and C-4 or 2 x C-3' and C-4'), 64.5 (4 x CH, 2 x C-2" and C-5"), 61.9 (4 x CH, 2 x C-3" and C-4"); HRMS (ESI+) : m / z 805.9632 [M]+(mCaic 805.9634); Calcd for C4oH3oFe304S2: C 59.58, H 3.75. Found: C 59.28, H 3.49.

[0157] l,l'-Bis[(2-furanylcarbonyl)oxy]ferrocene

[0158]

[0159] l,l -Bis[(2-furanylcarbonyl)oxy]ferrocene was prepared according to the general procedure using Fcl2(311 mg, 0.71 mmol), 2-furoic acid (239 mg, 2.13 mmol), Cu2O (152 mg, 1.07 mmol) and CH3CN (30 mL). The crude residue was purified by flash chromatography (CH2CI2 / n-hexane 1:1 CH2CI2 neat) and recrystallised from CH2CI2 / n-hexane to afford orange-yellow crystals (205 mg, 71%). Rf 0.43 (CH2CI2 / n-hexane 4:1);XH NMR (400 MHz, CDCh): 6 7.56 (s, 2H, 2 x H-5'), 7.19 (d, 2H,3JHH = 3.5 Hz, 2 x H-3') 6.49-6.48 (m, 2H, 2 x H-4'), 4.69 (s, 4H, 2 x H-2 and H-5), 4.09 (s, 4H, 2 x H-3 and H-4);13C{1H} NMR (100 MHz, CDCh): 6 156.7 (2 x C, 2 x C=O), 146.9 (2 x CH, 2 x C-5'), 144.2 (2 x C, 2 x C-2'), 118.9 (2 x CH, 2 x C-3'), 116.3 (2 x C, 2 x C-l), 112.1 (2 x CH, 2 x C-4'), 64.8 (4 x CH, 2 x C-3 and C-4), 62.2 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 406.0144 [M]+(mCaic 406.0140); Calcd for C20Hi4FeO5: C 59.14, H 3.47. Found: C 59.40, H 3.47.

[0160] l,l'-Bis[(lH-2-pyrrolylcarbonyl)oxy]ferrocene

[0161]

[0162] l,r-Bis[(lH-2-pyrrolylcarbonyl)oxy]ferrocene was prepared according to the general procedure using Fcl2(113 mg, 0.26 mmol), 2-furoic acid (86 mg, 0.77 mmol), Cu2O (55 mg, 0.39 mmol) and CH3CN (10 mL). The crude residue was purified by flash chromatography (CH2CI2neat) and recrystallised from acetone / n-hexane to afford yellow crystals (29 mg, 26%). Rf 0.14 (CH2CI2 neat);XH NMR (400 MHz, (CD3)2CO): 6 11.03 (br s, 2H, 2 x NH), 7.11 (dd, 2H,3JHH = 2.5 Hz,4JHH = 1.5 Hz, 2 x H-5'), 6.91 (dd, 2H,3JHH = 3.8 Hz,4JHH = 1.5 Hz, 2 x H-3'), 6.23 (dd, 2H,3JHH = 3.8 Hz,4JHH = 2.5 Hz, 2 x H-4'), 4.60 (t, 4H,3JHH = 2.0 Hz, 2 x H-2 and H-5), 4.08 (t, 4H,3JHH = 2.0 Hz, 2 x H-3 and H-4);13C{1H} NMR (100 MHz, (CD3)2CO): 6 159.5 (2 x C, 2 x C=O), 125.2 (2 x CH, 2 x C-5'), 125.1 (2 x C, 2 x C-2'), 117.3 (2 x C, 2 x C-l), 117.1 (2 x CH, 2 x C-3'), 110.9 (2 x CH, 2 x C-4'), 65.2 (4 x CH, 2 x C-3 and C-4), 63.0 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 404.0458 [M]+(mcaic 404.0459). Calcd for C2oHi6FeN204: C 59.43, H 3.99, N 6.93. Found: C 59.64, H 4.03, N 6.80.

[0163] l,l'-Bis[(2-phenylcarbonyl)oxy]ferrocene (FcPhC2 or WT009)

[0164]

[0165] l,l'-Bis[(2-phenylcarbonyl)oxy]ferrocene was prepared according to the general procedure using Fcl2(108 mg, 0.25 mmol), 2-furoic acid (90 mg, 0.74 mmol), Cu2O (53 mg, 0.37 mmol) and CH3CN (10 mL). The crude residue was purified by flash chromatography (CH2CI2 / n-hexane 1:1 CH2CI2neat) and recrystallised from CH2CI2 / n-hexane to afford yellow-orange crystals (95 mg, 89%). Rf 0.44 (CH2CI2 / n-hexane 1:1);XH NMR (400 MHz, CDCI3): 6 8.02 (d, 4H,3J = 7.1 Hz, 2 x H-2' and H-6'), 7.54 (t, 2H,3J = 7 A Hz, 2 x H-4'), 7.36 (t, 4H,3J = 7.8 Hz, 2 x H-3' and H- 5'), 4.70 (t, 4H,3J = 2.0 Hz, 2 x H-2 and H-5), 4.12 (t, 4H,3J = 2.0 Hz, 2 x H-3 and H-4);13C{1H} NMR (100 MHz, CDCI3): 6 165.0 (2 x C, 2 x C=O), 133.4 (2 x CH, 2 x C-4'), 130.0 (4 x CH, 2 x C-2' and C-6'), 129.7 (2 x C, 2 x C-l'), 128.53 (4 x CH, 2 x C-3' and C-5'), 116.6 (2 x C, 2 x C-l), 64.8 (4 x CH, 2 x C-3 and C-4), 62.3 (4 x CH, 2 x C-2 and C-5); MS (ESI+): m / z 426.0564 [M]+(mcaic 426.0554); Calcd for C24Hi8FeO4: C 67.63, H 4.26. Found: C 67.51, H 4.10.

[0166] General procedure for the synthesis of l,l'-bis(2-thienylcarbonyl)ferrocene, l,l'-bis(2-furanylcarbonyl)ferrocene and 1,1'-bis(2-phenylcarbonyl)ferrocene

[0167] Ferrocene (1 equiv) was added to a stirred solution of AICI3(5 equiv) and respective acid chloride (5 equiv) in CH2CI2and stirred rt for 18 h. The reaction mixture wascooled to 0 °C and ice-cooled water added. The mixture was separated, and the aqueous phase extracted with CH2CI2 (x3). The combined organic layers were washed with satd. aq. NaHCOs, water and brine, dried over Na2SO4, filtered and concentrated in vacuo. The crude residue was purified by flash chromatography (n-hexane / EtOAc) and the resulting product recrystallized to afford the desired compound.

[0168] l,l'-Bis(2-thienylcarbonyl)ferrocene- as reported in A. A. O. Sarhan, M. S. Ibrahim, M. M. Kamal, K. Mitobe and T. Izumi, Monatshefte fur Chemie -Chemical Monthly, 2009, 140, 315-323

[0169]

[0170] l,l'-Bis(2-thienylcarbonyl)ferrocene was prepared according to the general procedure using AICH (4.44 mg, 33.3 mmol), benzoyl chloride (3.56 mL, 33.3 mmol), ferrocene (1.24 g, 6.67 mmol) and CH2CI2 (50 mL). The crude residue was purified by flash chromatography (CH2CI2 neat) and the resulting product recrystallized from CH2CI2 / n-hexane to afford l,l'-bis(2-thienylcarbonyl)ferrocene (1.49 g, 55%) as red-pink fluffy needles. Rf O.lO (CH2CI2 neat);XH NMR (400 MHz, CDCI3): 6 7.84 (d, 2H,3JHH = 3.8 Hz, 2 x H-3'), 7.63 (d, 2H,3JHH = 5.0 Hz, 2 x H-5'), 7.14-7.12 (m, 2H, 2 x H-4'), 5.06 (s, 4H, 2 x H-3 and H-4), 4.60 (s, 4H, 2 x H-2 and H-5);13C{1H} NMR (100 MHz, CDCh): 6 188.4 (2 x C, 2 x C=O), 143.9 (2 x C, 2 x C-l'), 132.7 (4 x CH, 2 x C-5'), 132.1 (4 x CH, 2 x C-3'), 128.0 (2 x CH, 2 x C-4'), 80.5 (2 x C, 2 x C-l), 74.8 (4 x CH, 2 x C-2 and C-5), 72.8 (4 x CH, 2 x C-3 and C-4); MS (ESI+): m / z 406.9849 [M + H]+(mCaic 406.9863); Calcd for C20Hi4FeO2S2: C 59.12, H 3.47. Found: C 58.80, H 3.09.

[0171] l,l'-Bis(2-furanylcarbonyl)ferrocene (F4) - as reported in A. A. O. Sarhan, M. S. Ibrahim, M. M. Kamal, K. Mitobe and T. Izumi, Monatshefte fur Chemie - Chemical Monthly, 2009, 140, 315-323

[0172]

[0173] l,l'-Bis(2-furanylcarbonyl)ferrocene was prepared according to the general procedure using AlCh (10.8 g, 81.1 mmol), 2-furoyl chloride (8.0 mL, 81.1 mmol), ferrocene (3.02 g, 16.2 mmol) and CH2CI2 (120 mL). The crude residue was purified by flash chromatography (n-hexane / EtOAc 4:1 1:1) and the resulting product recrystallized from CH2CI2 and n-hexane to afford l,l'-bis(2-furanylcarbonyl)ferrocene (3.52 g, 58%) as red-orange crystals. Rf 0.37 (n-hexane / EtOAc 1:1);XH NMR (400 MHz, CDCh): 6 7.55 (s, 2H, 2 x H-5'), 7.27 (d, 2H,3JHH = 3.6 Hz, 2 x H-3') 6.54 (s, 2H, 2 x H-4'), 5.17 (s, 4H, 2 x H-2 and H-5), 4.57 (s, 4H, 2 x H-3 and H-4);13C{1H} NMR (100 MHz, CDCh): 6 153.6 (2 x C, 2 x C=O), 145.8 (2 x C, 2 x C-2'), 145.8 (2 x CH, 2 x C-5'), 117.3 (2 x CH, 2 x C-3'), 112.3 (2 x CH, 2 x C-4'), 79.3 (2 x C, 2 x C-l), 74.4 (4 x CH, 2 x C-3 and C-4), 72.6 (4 x CH, 2 x C-2 and C-5); HRMS (ESI+): m / z 375.0320 [M + H]+(mCaic 375.0320); Calcd for C20Hi4FeO2: C 64.20, H 3.77. Found: C 64.27, H 3.50.

[0174] l,l'-Bis(2-phenylcarbonyl)ferrocene (CB606)- as reported in A. A. O.

[0175] Sarhan, M. S. Ibrahim, M. M. Kamal, K. Mitobe and T. Izumi, Monatshefte fur Chemie - Chemical Monthly, 2009, 140, 315-323

[0176]

[0177] l,l'-Bis(2-phenylcarbonyl)ferrocene was prepared according to the general procedure using AlCh (1.79 g, 13.4 mmol), benzoyl chloride (1.56 mL, 13.4 mmol), ferrocene (502 mg, 2.69 mmol) and CH2CI2(20 mL). The crude residue was purified by flash chromatography (n-hexane / EtOAc 9:1 1:1) and the resulting product recrystallized from n-hexane to afford l,l'-bis(2-phenylcarbonyl)ferrocene (745 mg, 70%) as red fluffy needles. Rf 0.63 (n-hexane / EtOAc 1:1);XH NMR (400 MHz, CDCh): 6 7.78 (d, 4H,3JHH = 7.0 Hz, 2 x H-2' and H-6'), 7.55 (t, 2H,3JHH = 7.4 Hz, 2 x H-4'), 7.43 (t, 4H,3JHH = 7.6 Hz, 2 x H-3' and H-5'), 4.92 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-3 and H-4), 4.58 (pseudo-t, 4H, J = 2.0 Hz, 2 x H-2 and H-5);13C{1H} NMR (100 MHz, CDCI3): 6 198.1 (2 x C, 2 x C=O), 139.3 (2 x C, 2 x C-1'), 132.1 (2 x CH, 2 x C-4'), 128.5 (4 x CH, 2 x C-3' and C-5'), 128.3 (4 x CH, 2 x C-2' and C-6'), 79.6 (2 x C, 2 x C-l), 74.8 (4 x CH, 2 x C-2 and C-5), 73.3 (4 x CH, 2 x C-3 and C-4); HRMS (ESI+) : m / z 395.0717 [M + H]+(mCaic 395.0734); Calcd for C24Hi8FeO2: C 73.12, H 4.60. Found: C 72.92, H 4.46.

[0178] Perovskite Photodetector Preparation

[0179] Perovskite photodetectors were fabricated in an inverted p-i-n structure on glass substrates patterned with indium tin oxide (ITO). The substrates were sequentially cleaned with acetone and 2-propanol (IPA) by ultrasonication for 10 min in each solvent. The substrates were then dried with N2and treated with UV-ozone for 15 min.

[0180] The hole transporting layer (HTL), perovskite, and Fc-derivative layers were spin-coated in a N2-filled glove box.

[0181] For the control device, an HTL of MeO-2PACz dissolved in ethanol (1 mM) was spun on ITO at 3000 rpm for 30 s and dried at 100°C for 10 min.

[0182] The perovskite layers were synthesized with varying compositions:

[0183] (1) For 1.2 M Cso.o5(FA5 / 6MAi / 6)o.95Pb(Io.85Bro.i5)3, the perovskite precursor solution with an 1% excess of Pbl2was prepared by mixing Pbl2, PbBr2, MAI, FAI, and CsI in a DMSO:DMF (4:l / v:v) mixed solvent. 10 mol% MACI was then added to the precursor solution and stirred at 60 °C for 1 h. The perovskite solution was deposited in a two-step process, first at 1000 rpm for 1 s, followed by at 5000 rpm for 27 s. 150 mL of chlorobenzene (CB) was dripped during the second spincoating. The film was then annealed at 100°C for 30 min.

[0184] (2) For 1.5 M MAPbls (MAPI), the perovskite solution was prepared by dissolving MAI and Pbl2in a solvent mixture of DMSO:DMF (l:9 / v:v), which was then heated at 60°C for 60 min. The pre-heated perovskite solution was spin-coated at 4000 rpm for 20 s, during which 0.4 mL of diethyl ether was dripped onto the film at 6s after the start. Subsequently, the perovskite film was annealed at 100°C for 30 min.It was found that if the modified Fc interlayer was too thin it failed to completely cover the entire perovskite layer. If the modified Fc interlayer was excessively thick the movement of charge carriers was hindered. 0.3 mg / mL of the Fc derivatives were used to enable direct tunnelling of charge carriers and prevent undesired charge injection from the contacts.

[0185] For the devices treated with Fc-derivatives, 0.3 mg / mL of the Fc derivatives were dissolved in CB, and dynamically spin-coated onto the perovskite layer at 4000 rpm for 40 s.

[0186] The samples were then dried at 100°C for 4 min.

[0187] About 40 nm of Ceo, 10 nm of BCP, and 100 nm of Ag were thermally evaporated as the ETL, HBL, and top electrode, respectively, to finish the stack of the device. The device had a pixel area of 0.045 cm2,

[0188] A summary of the p-i-n PPD's configuration is given in Fig. 2, where the device was built on a patterned indium tin oxide (ITO) substrate. The mixed-cation perovskite of Cso.o5(FA5 / 6MAi / 6)o.95Pb(Io.85Bro.i5)3 treated with MACI (CsFAMA, where MA is methylammonium and FA is formamidinium) was sandwiched between the HTL of [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) and the ETL of Ceo. A bathocuproine (BCP) buffer layer was sandwiched between the ETL and Ag electrode. A thin layer of either l,l -bis[(2-phenylcarbonyl)oxy]ferrocene (WT009) or l,l -bis(2-phenylcarbonyl)ferrocene (CB606) was introduced between the perovskite layer and the ETL.

[0189] PPDs with the two Fc-derivatives, l,l -bis[(2-phenylcarbonyl)oxy]ferrocene (WT009) or l,l -bis(2-phenylcarbonyl)ferrocene (CB606), were evaluated

[0190] Experimental parameters and measurements

[0191] • J-V measurements were conducted with a Keithley 4200 Source- Measurement unit (scan rate of 25 mV s-1), and an Oriel Instruments Solar Simulator in the range of -0.5 to +1.3 V. The measurements were carried out in ambient air and under AM 1.5G light of 100 mW cm-2, calibrating its intensity using a Si reference cell.• Noise power spectral density was measured by a Keithley 4200 Source Measurement unit and a digital oscilloscope (Siglent, SDS6054A) coupled with a high-speed current amplifier (Femto, DHPCA-100) to provide a variable gain.

[0192] • NMR spectra were recorded at ambient temperature on a Bruker Avance 400 MHz spectrometer and calibrated to the residual solvent peaks of CDCh at 7.26 and 77.2 ppm forXH and13C{1H}, respectively. Peak assignment was aided through the use of COSY, edited HSQC and HMBC experiments.

[0193] • Cyclic voltammetry analyses were conducted on a Gamry 600TM potentiostat to record the redox potentials associated with each ferrocene (Fc) moiety. The samples were analysed in a cell comprising of glassy carbon working, platinum counter and silver pseudo reference electrode. The samples (2 mM) were dissolved in an electrolyte solution of tetrabutylammonium hexafluorophosphate (0.1 M) in anhydrous and degassed CH2CI2, and analysed at scan rates of 50, 100, 250, 500 and 1000 mVs-1. Each sample was referenced to a ferrocene / ferrocenium (Fc / Fc+) internal reference and all scans corrected for solution resistance with values obtained from potentiostatic electrochemical impedance spectroscopy (EIS) measurements. UV-Vis analyses were conducted on an Agilent Technologies Cary 60 UV-Vis Spectrophotometer. All samples (5-1000 pM) were analysed at room temperature using a quartz cell with a path length of 1 cm in CH2CI2.

[0194] • High resolution mass spectrometry (HRMS) data were recorded by direct injection on a Waters LCT time-of-flight (ToF) Mass Spectrometer using electrospray ionization (ESI).

[0195] • Elemental analyses were analysed using a ThermoFlash 2000 Analyzer.

[0196] Perovskite characterizations

[0197] For the UV-Vis spectroscopy measurements, quartz / perovskite, quartz / perovskite / Fc derivative, and quartz / Fc derivative samples were prepared, and the absorption spectra were measured by using an Agilent Cary 60 UV-Vis spectrophotometer. Kelvin probe coupled with ambient photoemission spectroscopy (KP-APS) was performed on ITO / perovskite, ITO / perovskite / Fc derivative, ITO / MeO-2PACz and ITO / Fc derivative samples with a KP Technology SKP5050 Scanning Kelvin Prove and an APS02 Air Photoemission System. Time-resolved PL measurements were carried out using a Horiba Delta Flex (PPD-900, Horiba Scientific) operated at repetition rate of 1 MHz with low-fluence irradiation. Theglass / perovskite / Fc derivative samples were excited using a 467 nm continuous wave (CW) laser diode with a pulse duration of <200 ps (NanoLED). ToF-SIMS measurements were conducted on ITO / MeO-2PACz / perovskite / Fc-derivative samples, sputtering from the Fc layer into the perovskite film. The samples were depth profiled using an IONTOF ToF-SIMS V instrument. A 25keV Bi+3 ion beam in HCBM was used to analyse the sample, over a 150 x 150 pm2area with 128 x 128 pules. The samples were sputtered using a 10 keV Arisooo+GCIB over an area of 400x400 pm2. Final sputter crater depths were measured using the Zygo NexView optical interferometer. Both Top-view SEM and device cross-sectional images were taken on a field SEM (FE-SEM) Gemini 1 Zeiss Sigma 300. The images were acquired by secondary electron detector (SE2) under an accelerating voltage of 2.00 kV while keeping a low working distance. Spectroscopic ellipsometry measurements were performed on Si / Fc derivative films using a Variable Angle Spectroscopic Ellipsometer (in the spectral range 192 - 1690 nm) to extract accurate optical constants.

[0198] The thickness of the ferrocene layers was less than 1 nm, based on additional ellipsometry measurements. Optical and morphological investigations showed a negligible impact of the ferrocene layers on the optical properties and crystallinity of the perovskite layer.

[0199] The electrical properties of the Fc-derivative interlayers were investigated using air photoemission spectroscopy (APS). The optical bandgaps (Efl) were obtained from UV-Vis absorption spectra. The corresponding energies of CBM (LUMO) were determined by subtracting VBM (HOMO) from the Eg. The schematic energy level alignment of the entire device stack showed that the Fc derivatives have shallower HOMO levels than the adjacent layers, resulting in an interface energy offset (A4>H0M0) between the perovskite and Fc layers.

[0200] Current Density-Voltage ( / - V) Characteristics

[0201] To study the impact of the functionalised Fc-derived layers on mitigating parasitic charge injection, the current density-voltage ( / - V) characteristics were measured under light (AM 1.5 G simulated sunlight illumination) and dark conditions. The JDwas recorded at -0.5 V to facilitate charge extraction while reducing potential contributions from mobile ions which have been observed at higher voltages. The current density-voltage results are shown in Figure 3.On average, the JDof the reference was 2.4 x IO-8A cm-2, and the presence of the injection barrier created by the CB606 or WT009 interlayers reduced the value to 2.3 x IO-9A cm-2and 2.4 x IO-9A cm-2, respectively. Thus, the presence of the Fc-derived layers significantly improved JD. WT009 demonstrated this effect across different perovskite compositions.

[0202] The reduction in JDis mainly due to the A4>H0M0at the perovskite / Fc-derivative interface, which effectively restricts the flow of holes from the Ceo into the active layer. Under reverse bias, this effect would be intensified as the potential barrier steepens. Similar trends were observed in the noise current, random fluctuation in electrical signals that restricts the detection of the smallest optical signals, which suggests the prevalence of shot noise in our perovskite system.

[0203] Specific detectivity

[0204] Specific detectivity (£>*), a numerical metric that quantifies the sensitivity of a device at a certain wavelength, is the ability to discern subtle changes in light intensity accurately. D* was determined as follows:

[0205]

[0206] where A is the active area of the PD, A is the electrical bandwidth, and R is the responsivity. Here, the R, closely related to external quantum efficiency (EQE), is a measure of wavelength-dependent electrical current generated in response to incident optical power. The specific detectivity plots are shown in Figure 4.

[0207] All devices exhibited peak responses at 740 nm. The reference exhibited a peak D' value of 3.2 x 1011Jones (cm-Hz1 / 2W-1). The incorporation of the Fc-derivative layer increased the peak D* value to 1.9 x 1013Jones for CB606, and 1.9 x 1013Jones for WT009, which is similar to that of commercially available Si-based photodiodes.

[0208] The enhanced D* of the devices containing the Fc-derivative layer stemmed from their lower noise levels, potentially expanding the operational range by allowing the device to detect weaker light intensities.Linear Dynamic Range

[0209] Linear dynamic range (LDR) is the range within which photocurrent is linearly proportional to input light intensities. This was determined by extrapolating photocurrent density (Jph) at a given incident optical power as follows.

[0210] LDR = 20 - \og^ (2)

[0211] Jmaxand Jminare the upper and lower limits of Jphthat fall into the linear region, where the photocurrent changes in proportion to incident light intensity.

[0212] The WT009 devices demonstrated an extended LDR of 117 dB relative to the reference device, which only showed linear dependence on intensity over 113 dB. This was primarily due to the lower signal detection threshold in the WT009 device resulting in a significantly reduced Jmin, which aligns with its lower Snand JD.

[0213] Cut-Off Frequency

[0214] The cut-off frequency ( / _3 de) is defined as the frequency at which the response of a device drops by -3 dB from its initial signal amplitude. The cut-off frequency was measured under different illumination wavelengths and Figure 5 depicts the cut-off frequency curves obtained under 530 nm illumination, with each point representing the optical signal intensity at the corresponding frequency relative to that measured at 500 Hz. The CB606- and WT009-based devices both showed a significant increase in their response toward the high-frequency end of the spectrum before the cut-off. These increases were more pronounced when reverse bias was applied, which also extended the bandwidth limits beyond 1 MHz. The higher photoresponse at higher frequencies was observed over a range of visible illumination wavelengths. However, the reference exhibited a rather constant f_3dBof 0.9 MHz regardless of the bias applied.

[0215] The bias dependency in the devices containing the Fc-derivative layer are believed to be caused by charge carrier accumulation under stronger electric fields.

[0216] Summary

[0217] The above results are summarised in Table 1.Table 1

[0218]

[0219] The two Fc-derivatives, l,l'-bis[(2-phenylcarbonyl)oxy]ferrocene (WT009) or 1,1 - bis(2-phenylcarbonyl)ferrocene (CB606), were evaluated as materials for a supplementary hole-blocking layer (HBL), besides the conventional bathocuproine (BCP) buffer layer in PPDs. It was surprisingly found that these materials created a secondary energetic barrier that effectively blocks the parasitic hole injection from the Ag contact to the perovskite layer. This substantially reduced JDand noise spectral density and improved specific detectivity and linear dynamic range.

[0220] Although the energetic barrier between the conduction band minimum (CBM) of the perovskite and the lowest unoccupied molecular orbital (LUMO) of the Fc derivatives impedes charge transfer under short circuit conditions, this is ameliorated under reverse bias conditions, resulting in PPDs with a high response speed of 150 ns and 1.3 MHz at -0.5 V in the optical and electrical responses.

[0221] The Fc-based compounds can effectively reduce ion migration across the perovskite interface by promoting the reduction of generated h, which ultimately enhanced reverse bias device stability under in-operando conditions. Ion migration under reverse bias and illumination conditions has been shown to form localised clusters of defects and trigger chemical reactions or ion build-up at the interfaces between the perovskite and adjacent transport layers, which are ultimately associated with substantial increases in JD, inefficient charge collection, and device failure over time. Although Ceo and BCP can function as hole injection barriers, they are not sufficient to entirely prevent these adverse consequences of ion migration. The improved stability stemmed from the

[0222]

[0223] of Fc-derived compounds, which effectively block holes introduced from the metal Ag, thus preventing the oxidation of T ions. Furthermore, the Fc-derived compounds have the potential to inhibit the loss of T ions from the perovskite layer through the reduction of I2 formed on the perovskite surface. Fc-derived compounds undergo complete oxidation when an equivalent of I2 is added. This was indicated by the broadening and disappearanceof cyclopentadienyl (Cp) and phenyl peaks in theXH NMR spectrum due to the formation of the paramagnetic Fe3+complex. The redox potential of FcPhc? (114 mV vs Fc / Fc+) is lower than that of I2 (193 mV vs Fc / Fc+) confirming the ability of I2 to oxidize FcPhc2. Similar observations with other Fc and related derivatives on perovskite devices have also suggested the suppression of I2 migration through a catalytic redox process, with the formation of a Fcl-type complex and subsequent re-reduction through the oxidation of Pb defects.

[0224] While the present invention has been described in connection with specific forms and embodiments, it will be appreciated that various modifications can be made without departing from the spirit or scope of the invention.

[0225] This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) grant number EP / X52556X / 1.

Claims

CLAIMS1. A perovskite photodetector comprising:a first electrode;a second electrode;a perovskite layer and an electron transport layer disposed between the first and second electrodes; anda hole blocking layer disposed between the perovskite layer and the electron transport layer and in direct contact with the perovskite layer, the hole block layer comprising a metallocene substituted with at least one substituent R1comprising at least one of an O, S, N or P atom.

2. The perovskite photodetector according to claim 1 wherein the metallocene substituted with at least one substituent R1comprising at least one of an O, S, N or P atom is a compound of formula (I):[Metallocene]p(I)wherein:Metallocene is a metallocene group comprising a metal bound to two aromatic or heteroaromatic groups Ar1;p is at least 1; andat least one Metallocene is substituted with at least one substituent R1.

3. The perovskite photodetector according to claim 2 wherein the compound of formula (I) has formula (la):wherein:M is a metal ion;Ar1in each occurrence is a monocyclic or polycyclic aromatic or heteroaromatic group;M and the two Ar1groups form the Metallocene;at least one Ar1is substituted with at least one R1;R2is a group for satisfying the valency of M;q is 0 or a positive integer; andR3in each occurrence is independently H or a substituent.

4. The perovskite photodetector according to any one of the preceding claims wherein the metallocene is ferrocene.

5. The perovskite photodetector according to any one of the preceding claims wherein R1is a group of formula (II) :-A-B(II)wherein A is a divalent group comprising O, S, N or P; and B is H, C1-12 alkyl, optionally substituted aryl or optionally substituted heteroaryl.

6. The perovskite photodetector according to claim 5 wherein A is selected from formula (III) or formula (IV):-(R5)f-Z-(R5)g- (III)-(R6O)j- (IV)wherein:R5in each occurrence is independently a hydrocarbon group;f and g are each independently 0 or 1;R5is a Cl-4 alkylene group, preferably ethylene;j is 1-10; andZ is O, S, C(=O), COO, C( = S)O, C(=O)S, CONR4, CSNR4, OC(=O)O, OC( = O)NR4, OC(=O)PR4' NR4, PR4, -OP(=O)(OR4)-O-, -NR4-P(=O)(NR42)- NR4-, wherein R4is H, optionally substituted C1-12 alkyl or optionally substituted phenyl.

7. The perovskite photodetector according to claim 5 or 6 wherein A is -C(=O)-O-; -O-C(=O)- or -C(=O)-.

8. The perovskite photodetector according to any one of claims 5-7 wherein B is selected from optionally substituted phenyl and an optionally substituted 5-membered heteroaryl comprising one or more ring atoms selected from O, S and N.

9. The perovskite photodetector according to any one of claims 5-8 wherein B is optionally substituted thiophene or optionally substituted furan or optionally substituted phenyl.

10. The perovskite photodetector according to any one of the preceding claims wherein the thickness of the hole blocking layer is between 0.5 and 5 nm, or about 0.8 nm.

11. A device comprising the perovskite photodetector according to any one of claims 1 to 10.

12. The device according to claim 11, wherein the device is a camera, medical imaging system, consumer electronic device, environmental sensor, a communication network device or a wearable device, for instance wherein the device is a wearable sensor, CCT camera, night-vision device, or UV-Vis-NIR. spectrometer.

13. Use of a substituted-metallocene in a perovskite photodetector for blocking hole transfer from the cathode to the perovskite layer, reducing ion migration across the perovskite interface, promoting the reduction of generated I2, or improving stability of the perovskite photodetector, wherein the substituted-metallocene is the metallocene as defined in any one of claims 1 to 9.

14. A method of blocking hole transfer from the cathode to the perovskite layer , reducing ion migration across the perovskite interface, promoting the reduction of generated I2, or improving stability in a perovskite photodetector, comprising disposing a hole blocking layer between the perovskite layer and the electron transport layer in the perovskite photodetector, wherein the hole block layer is in direct contact with the perovskite layer and the hole blocking layer comprises a metallocene as defined in any one of claims 1 to 9.

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