Passivatied a / m / x material

Abstract

The invention provides a photovoltaic device which comprises a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]c wherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound. The invention also provides the use of the above-defined passivated A / M / X material as a sensitizer in a photovoltaic device. The present invention also relates to processes for producing a passivated A / M / X material and then (a) producing a photovoltaic device comprising the passivated A / M / X material or (b) using the passivated A / M / X material as a sensitizer in a photovoltaic device.

Description

[0001] PASSIVATION FIELD OF THE INVENTION The present invention relates to a photovoltaic device which comprises a passivated A / M / X material. The present invention also relates to use of a passivated A / M / X material as a sensitizer in a photovoltaic device. The present invention also relates to processes for producing a passivated A / M / X material and then (a) producing a photovoltaic device comprising the passivated A / M / X material or (b) using the passivated A / M / X material as a sensitizer in a photovoltaic device. BACKGROUND TO THE INVENTION Hybrid organic-inorganic metal halide perovskites are recognized as one of the most promising emerging semiconducting materials for optoelectronic applications due to their excellent properties including tuneable bandgaps, high absorption coefficients and long carrier diffusion lengths. See Saliba, M et al. Perovskite Solar Cells: From the Atomic Level to Film Quality and Device Performance; De Wolf, S. et al. Organometallic halide perovskites: Sharp optical absorption edge and its relation to photovoltaic performance; and Lim, J. et al. Elucidating the long-range charge carrier mobility in metal halide perovskite thin films. Although metal-halide perovskites are considered “defect-tolerant”, they still require careful growth and “defect-passivation-strategies” in order to reach the highest efficiency and exhibit improved long-term operational stability in optoelectronic devices. Non-radiative defect-mediated charge-carrier recombination can be inhibited by modifying perovskites with molecular compounds, which bind to or interact with different crystalline sites on the perovskite surfaces (see Noel, N. K. et al. Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic–Inorganic Lead Halide Perovskites; and Snaith, H. J. et al. Photovoltaic device comprising a metal halide perovskite and a passivating agent). This “molecular-passivation” strategy has been adopted broadly, crucial in delivering contemporary record light-to-electrical power conversion efficiencies (PCE) in triiodide perovskites (see Jiang, Q. et al. Surface passivation of perovskite film for efficient solar cells; and Park, J. et al. Controlled growth of perovskite layers with volatile alkylammonium chlorides). Perovskites comprising various compositions of “mixed cations and anions”, that see great promise in delivering phase-stable single-junction and tandem multi-junction solar cells, do not benefit from most molecular passivation strategies and, instead, see severe photovoltage deficits when integrated into solar cells. Photoluminescence quantum yield (PLQY) is commonly employed to assess the quality of perovskite light absorbers. Higher PLQY values indicate lower nonradiative recombination rates. When focusing on passivating isolated perovskite thin-films or nanocrystals, a broad range of approaches can be adopted. These include employing small molecular Lewis acids or bases and nanocrystal growth ligands, such as trioctylphosphine oxide and oleic acid. The latter can lead to internal luminescence efficiency approaching unity in both nanocrystals and polycrystalline thin films (see deQuilettes, D. W. et al. Photoluminescence Lifetimes Exceeding 8 μs and Quantum Yields Exceeding 30% in Hybrid Perovskite Thin Films by Ligand Passivation). However, the larger organic molecules generally required for stable organic passivation ligands also results in the inhibition of charge carrier transport in the films and have yet to be demonstrated in high-performance solar cells. As an alternative to molecular passivation, the growth of thin “shells” of metal oxide or metal hydroxide around perovskite nanocrystals has proven to be effective in both increasing luminescence efficiency and material stability (see Guggisberg, D. et al. Colloidal CsPbX3 Nanocrystals with Thin Metal Oxide Gel Coatings). But similarly, the thicker “insulating” metal (hydr)oxide coatings inhibit charge transport, and some of the best passivation methods that exhibit outstanding PLQY have yet to be proven compatible with integrating into devices to achieve high efficiency or photovoltaic (PV) parameters close to the radiative limit. Following the metal (hydr)oxide approach applied to polycrystalline thin films, a recent study showed that passivating the top surface of perovskite with (3-aminopropyl)trimethoxysilane (APTMS) could dramatically reduce nonradiative recombination, resulting in one of the highest PLQYs demonstrated to date for mixed-cation lead mixed-anion thin-film perovskites (see Jariwala, S. et al. Reducing Surface Recombination Velocity of Methylammonium-Free Mixed-Cation Mixed-Halide Perovskites via Surface Passivation; and Pothoof, J. et al. Surface Passivation Suppresses Local Ion Motion in Halide Perovskites). In the same study, no functional solar cells were reported (Jariwala, S. et al.) whilst, in the follow-up, the improvement in the open-circuit voltage (VOC) was far below what is suggested by the PLQY data (Shi, Y. et al. (3-Aminopropyl)trimethoxysilane Surface Passivation Improves Perovskite Solar Cell Performance by Reducing Surface Recombination Velocity). There remains a significant need to develop passivation strategies that result in efficient and stable photovoltaic devices. SUMMARY OF THE INVENTION The present invention stems from the finding that utilising specific passivating agents to produce a passivated A / M / X material can reduce the defect density of the A / M / X material, whilst also significantly enhancing long-term operational stability when the passivated A / M / X material is used in a photovoltaic device. Devices formed utilising the passivated A / M / X materials deliver, inter alia, high voltage, highly efficient perovskite solar cells with a broad range of compositions and band gaps. Furthermore, the materials and solar cells including the passivated A / M / X material are extremely stable under elevated temperature and simulated sun light. Accordingly, in a first aspect, the invention provides a photovoltaic device which comprises a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound. In a second aspect, the invention provides use of a passivated A / M / X material as a sensitizer in a photovoltaic device, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound. In a third aspect, the invention provides a process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (i) treating said crystalline A / M / X material with said passivating agent; or (ii) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: producing a photovoltaic device comprising the passivated A / M / X material. In a fourth aspect, the invention provides a process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (i) treating said crystalline A / M / X material with said passivating agent; or (ii) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: using the passivated A / M / X material as a sensitizer in a photovoltaic device. BRIEF DESCRIPTION OF THE FIGURES Fig. 1 shows silane molecules and their fabrication and optoelectronic properties. a) Structures of amino-silane (AS) molecules, (3-aminopropyl)trimethoxysilane (APTMS), trimethoxy(propyl)silane (PTMS), trimethoxy[3-(methylamino)propyl]silane (MAPTMS), (N, N-Dimethylaminopropyl)trimethoxysilane (DMAPTMS), [3-(2- aminoethylamino)propyl]trimethoxysilane (AEAPTMS) and 3-[2-(2- aminoethylamino)ethylamino]propyltrimethoxysilane [(AE)2APTMS]. b) Schematic of positive-intrinsic-negative solar cell architecture, where ETL is electron transport layer, HTL is hole transport layer, TE is top electrode, BE is bottom electrode, and TCO is transparent conductive oxide. c) Illustration of the vapour-based deposition adopted for passivation molecules: i) loading an AS molecule chosen from Fig. 1a in a petri dish; ii) heating the passivation molecule containing petri dish at 100 for a short period of time (i.e., a few minutes) until most of the molecules are vapourised; iii) placing an as-grown perovskite film sample inside the petri dish for tens of seconds to a couple of minutes. d) Photos of perovskite films made on TCO glass substrates and treated with the molecules shown in Fig. 1a and without any treatment (Ref, i.e., reference). e), f) Photoluminescence quantum yield (PLQY) e) and THz mobility (f) of I90Br10perovskite films treated with molecules shown in Fig. 1a and without any treatments (i.e., ref). Fig. 2 shows PLQY for I60Br40with amino-silane molecules and without treatment. Fig. 3 shows PLQY for the perovskite compositions of I90Br10, I77Br23and I60Br40with and without the AEAPTMS treatment. Fig. 4 shows a) PLQY and b) corresponding calculated quasi-Fermi level splitting (QFLS) for I60Br40with and without the AEAPTMS treatment processed on perovskite only, on top of the HTL / perovskite stack, or in-between perovskite and ETL (i.e., perovskite / ETL). All of the treatments are processed on the top surface of perovskite. Fig. 5 shows PLQY measured for I60Br40with and without the AEAPTMS treatment and processed for different treatment durations. Fig. 6 shows a) optical density, b) reflection, and c) transmission of I90Br10 treated with and without amino-silane molecules. Fig. 7 shows THz conductivity measurements conducted for two sets of ref I90Br10. Fig. 8 shows THz conductivity measurements conducted for I90Br10when treated with different amino-silane molecules. Fig. 9 shows SEM measurements conducted for I90Br10with and without amino-silane molecule treatments. Fig. 10 shows 2D perovskite phases formed on I90Br10when treated with the MAPTMS molecule. The fitting lattice parameter is based on one-layer Ruddlesden-Popper phases with Pbca space group. The fitting data shows these two peaks are both from the (001) family face with high orientation based on (grazing-incidence wide-angle X-ray scattering) GIWAXS data. Fig. 11 shows optimised structures of the silane-surface interactions. a) Side views of the simulated structures using DFT (density functional theory) and ab-initio molecular dynamics (AIMD) techniques of the interactions of the amino-silanes PTMS, APTMS, MAPTMS and AEAPTMS with the Pb / I terminated (001) FAPbI3surface. PbI6octahedra are highlighted to show that the major structural relaxation is found mostly in the topmost layer. b) Changes in the charge density profile around all the species involved in the silane-surface binding; the prominent charge accumulation (yellow) and depletion (blue) regions show N-Pb and O-Pb interactions. c) Binding energies of amino-silanes on the pristine surface and on the iodide Frenkel defect surface. d) Schematic diagram showing the perovskite surface with undercoordinated Pb cations adjacent to an iodide vacancy, and the binding of an AEAPTMS molecule via N-Pb and O-Pb bonds. Here, FA+cations are omitted for clarity. Fig. 12 shows current density and voltage characterisations conducted for I90Br10with various amino-silane surface treatments: a) PTMS, b) APTMS, c) MAPTMS, d) DMAPTMS, e) AEAPTMS, and f) (AE)2APTMS. Fig. 13 shows perovskite solar cell characterisation. a), b), c) Current density and voltage characteristics of the representative AEAPTMS-treated perovskite solar cells with an aperture size of 0.25 cm2using an absorber composition of I90Br10(a) I77Br23(b) I60Br40(c), where FB-SC indicates the scan direction from the forward bias (FB) to short circuit (SC) condition, and vice versa for SC-FB. Insets of a)-c) summarise critical PV performance parameters. d) Current density and voltage characteristics of the representative large-area 1- cm2AEAPTMS-treated cells based on the compositions of I90Br10, I77Br23, and I60Br40. e) corresponding maximum power point tracking (MPPT) efficiencies (η) for 120 seconds are shown. Fig. 14 shows EQE spectra (solid line) with photocurrent integrated over the AM1.5 (100 mW cm-2) solar spectrum and the first derivative of the corresponding EQE (dashed line, leading to an extracted bandgap value of 1.60 eV) for I90Br10treated with a) the AEAPTMS molecule and b) without the treatment. c) Corresponding ηMPPTfor AEAPTMS- based and ref cells measured over a time period of 120 seconds. Fig. 15 shows EQE spectra (solid line) with photocurrent integrated over the AM1.5 (100 mW cm-2) solar spectrum and the first derivative of the corresponding EQE (dashed line, leading to an extracted bandgap value of 1.67 eV) for I77Br23treated with a) the AEAPTMS molecule and b) without the treatment. c) Corresponding ηMPPTfor AEAPTMS- based and ref cells measured over a time period of 120 seconds. Fig. 16 shows EQE spectra (solid line) with photocurrent integrated over the AM1.5 (100 mW cm-2) solar spectrum and the first derivative of the corresponding EQE (dashed line, leading to an extracted bandgap value of 1.77 eV) for I60Br40treated with a) the AEAPTMS molecule and b) without the treatment. c) Corresponding ηMPPTfor AEAPTMS- based and ref cells measured over a time period of 120 seconds. Fig. 17 shows statistical results of VOCobtained from SC-FB and FB-SC scans based on 0.25 cm2solar cells using the perovskite compositions of I90Br10(1.60 eV, 26 cells), I77Br23(1.67 eV, 15 cells) and I60Br40(1.77 eV, 18 cells) treated with the AEAPTMS molecule or without the treatment (i.e. “ref” in Fig. 17). Fig. 18 shows statistical results of JSCobtained from SC-FB and FB-SC scans based on 0.25cm2solar cells using the perovskite compositions of I90Br10(1.60 eV, 26 cells), I77Br23(1.67 eV, 15 cells) and I60Br40(1.77 eV, 18 cells) treated with the AEAPTMS molecule or without the treatment (i.e. “ref” in Fig. 18). Fig. 19 shows statistical results of FF obtained from SC-FB and FB-SC scans based on 0.25cm2solar cells using the perovskite compositions of I90Br10(1.60 eV, 26 cells), I77Br23(1.67 eV, 15 cells) and I60Br40(1.77 eV, 18 cells) treated with the AEAPTMS molecule or without the treatment (i.e. “ref” in Fig. 19). Fig. 20 shows statistical results of PCE obtained from SC-FB and FB-SC scans based on 0.25cm2solar cells using the perovskite compositions of I90Br10(1.60 eV, 26 cells), I77Br23(1.67 eV, 15 cells) and I60Br40(1.77 eV, 18 cells) treated with the AEAPTMS molecule or without the treatment (i.e. “ref” in Fig. 20). Fig. 21 shows EQE spectra (solid line) with photocurrent integrated over the AM1.5 (100 mW cm-2) solar spectrum and the first derivative of the corresponding EQE (dashed line), leading to an extracted bandgap value of a) 1.60, b) 1.67, and c) 1.77 eV for 1-cm2cells treated with and without the AEAPTMS molecule. Fig. 22 shows operational stability and ageing characterisation. a) Evolution of average maximum power point (MPP) (with the corresponding standard deviations in a band plot) of encapsulated AEAPTMS-treated and reference (ref) I90Br10cells (including six AEAPTMS-based and five ref cells) aged under the open-circuit condition and full-spectrum simulated sunlight at 85℃ in ambient air with relative humidity in the laboratory 50~60%. MPP was tracked for 120 seconds for each cell at different ageing stages. The datasets including some solar cells with distinct degradation pathways, i.e., light-soaking and burn-in, are explicitly indicated. b) Evolution of normalised MPP for individual cells used and recorded for (a). For the champion cell, the time taken to reach 95% of its initial MPP (T95,Champ) is approximately 1,600 hours. c), d) PL-derived QFLS maps of the full-area 0.25-cm2 AEAPTMS-treated (c) and ref I90Br10(d) cells before ageing (0 h) and after ageing under the condition, as described in (a), for 600 h. e), f) Luminescence-derived charge collection quality (Qcol) maps for AEAPTMS-treated (e) and ref (f) cells calculated corresponding to (c) and (d), respectively. g), h) Statistical results of QFLS (g) and Qcol(h) shown in a violin plot, where the median, interquartile range and 1.5× interquartile range are indicated as a dot, a box, and a line, respectively, for all the cells recorded in (b). Each QFLS and Qcolmap covers an area of 5.38 mm × 4.67 mm. Fig. 23 shows evolution of average VOC(with the corresponding standard deviations in a band plot) of encapsulated AEAPTMS-treated and ref I90Br10 cells (including six AEAPTMS-based and five ref cells) aged under the open-circuit condition and full-spectrum simulated sunlight at 85℃ in ambient air with relative humidity in the laboratory 50~60%. Fig. 24 evolution of average JSC(with the corresponding standard deviations in a band plot) of encapsulated AEAPTMS-treated and ref I90Br10cells (including six AEAPTMS- based and five ref cells) aged under the open-circuit condition and full-spectrum simulated sunlight at 85℃ in ambient air with relative humidity in the laboratory 50~60%. Fig. 25 shows evolution of average FF (with the corresponding standard deviations in a band plot) of encapsulated AEAPTMS-treated and ref I90Br10cells (including six AEAPTMS-based and five ref cells) aged under the open-circuit condition and full-spectrum simulated sunlight at 85℃ in ambient air with relative humidity in the laboratory 50~60%. Fig. 26 shows evolution of EQE spectra before and after aged under the open-circuit condition and full-spectrum simulated sunlight at 85℃ in ambient air with relative humidity in the laboratory 50~60% with photocurrent integrated over the AM1.5 (100 mW cm-2) solar spectrum and I90Br10cells treated with (a) and without (b) the AEAPTMS molecule. Fig. 27 shows PL-derived QFLS maps of the full-area 0.25-cm2a) AEAPTMS-treated and b) ref I90Br10(b) cells before ageing (0 h) and after ageing for various periods of time. Each image covers an area of 5.38 mm × 4.67 mm. Fig. 28 shows Qcolmaps of the full-area 0.25-cm2a) AEAPTMS-treated and b) ref I90Br10cells before ageing (0 h) and after ageing for various periods of time. Each image covers an area of 5.38 mm × 4.67 mm. Fig. 29 shows current density and voltage characteristics of a set of p-i-n perovskite solar cells with the APTMS treatment and without the treatment (i.e. reference). Fig. 30 shows a) current density and voltage characterisation and b) corresponding ηMPPTfor a time period of 120 seconds conducted for 0.25cm2I90Br10solar cells using evapourated C60and BCP and treated with and without the AEAPTMS molecule. EQE spectra (solid line) with photocurrent integrated over the AM1.5 (100 mW cm-2) solar spectrum and the first derivative of the corresponding EQE (dashed line, leading to an extracted bandgap value of 1.67 eV) shown in c) and d) for cells with and without the AEAPTMS treatment, respectively. Fig. 31 shows 3D crystallographic characterisation of perovskite films. XRD series for the ref and amino-silane molecule treated I90Br10perovskite films, prepared on ITO glass substrates. Fig. 32 shows electronic density of states of AEAPTMS-treated perovskite. Electronic density of states of the perovskite surface with an iodide Frenkel defect before and after its interaction with the AEAPTMS molecule. A small increase in the band gap from 1.03 eV to 1.15 eV due to AEAPTMS-surface binding is found. Such broadening of the surface states signifies surface passivation effects. Trap like states below the valence band edge disappear upon interaction with AEAPTMS, again indicating the effective passivator role of AEAPTMS in eliminating these trap states and iodide vacancy defects through strong surface binding. Fig. 33 shows PCE-bandgap and VOC-bandgap literature data comparisons. Data are presented with different symbols according to their active pixel areas: squares for pixel areas <0.1 cm2; circles for pixel areas between 0.1 and 0.3 cm2; triangles for pixel areas equal to 1.0 cm². Dashed lines indicate the percentages of the detailed balance limits (e.g., 90%, 80%, etc.). Our representative 0.25-cm2and 1-cm2cells with bandgaps of 1.6 eV, 1.67 eV, and 1.77 eV are marked as red (left hand box), green (middle box), and blue (right hand box), respectively, and are circled by dashed boxes. All the data were extracted from the reverse scans from the operation of their respective cells (in a p-i-n configuration), as reported in Jacobsson, T. J. et al. An open-access database and analysis tool for perovskite solar cells based on the FAIR data principles. Fig. 34 shows characteristics of AEAPTMS-treated I60Br40cell. a) Photo of the solar cell design used in this study. b) Corresponding current density and voltage characteristics of 1-cm2AEAPTMS-treated I60Br40cell shown in (a). c), d) Corresponding EQEELobtained (c) under the operation of a voltage sweep between 0 and 4 V and (d) recorded for 600 seconds obtained by injecting the same cell (b) at the current density of 17 mA cm-2. Insets of c) and d) show the corresponding photo of the cell’s illumination at the current density of 17 mA cm-2and ηMPPTfor 120 seconds, respectively. Fig. 35 shows operational stability of I90Br10solar cells. a) Evolution of average PCE (with the corresponding standard deviations in a band plot) of encapsulated AEAPTMS-treated and ref I90Br10cells (including six AEAPTMS-based and five ref cells) aged under the open- circuit condition and full-spectrum simulated sunlight at 85℃ in ambient air with relative humidity in the laboratory 50~60%. The datasets including some solar cells with distinct degradation pathways, i.e., light-soaking and burn-in, are explicitly indicated. b) Evolution of normalised PCE for individual cells used and recorded for (a). For the champion cell, the time taken to reach 95% of its initial MPP (T95, Champ) is approximately 1,440 hours. Fig.36 shows operational stability and ageing characterisation. a) Evolution of average maximum power point (MPP) (with the corresponding standard deviations in a band plot) of encapsulated AEAPTMS-treated and reference (ref) I90Br10cells (including six AEAPTMS- based and five ref cells) aged under the open-circuit condition and full-spectrum simulated sunlight at 85℃ in ambient air with relative humidity in the laboratory 50~60%. MPP was tracked for 120 seconds for each cell at different ageing stages. The datasets including some solar cells with distinct degradation pathways, i.e., light-soaking and burn-in, are explicitly indicated. b) Evolution of normalised MPP for individual cells used and recorded for (a). For the champion cell, the time taken to reach 95% of its initial MPP (T95, Champ) is approximately 1,600 hours. DETAILED DESCRIPTION OF THE INVENTION The term “crystalline” as used herein indicates a crystalline compound, which is a compound having an extended 3D crystal structure. A crystalline compound is typically in the form of crystals or, in the case of a polycrystalline compound, crystallites (i.e. a plurality of crystals having particle sizes of less than or equal to 10 μm). The crystals together often form a layer. The crystals of a crystalline material may be of any size. Where the crystals have one or more dimensions in the range of from 1 nm up to 1000 nm, they may be described as nanocrystals. The term “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 term “crystalline A / M / X material”, as used herein, refers to a material with a crystal structure which comprises one or more A ions, one or more M ions, and one or more X ions. A ions and M ions are cations. X ions are anions. A / M / X materials typically do not comprise any further types of ions. The term “perovskite”, as used herein, refers to a material with a three-dimensional crystal structure related to that of CaTiO3or a material comprising a layer of material, which layer has a structure related to that of CaTiO3. The structure of CaTiO3can be represented by the formula AMX3, wherein A and M are cations of different sizes and X is an anion. In the unit cell, the A cations are at (0,0,0), the M cations are at (1 / 2, 1 / 2, 1 / 2) and the X anions are at (1 / 2, 1 / 2, 0). The A cation is usually larger than the M cation. The skilled person will appreciate that when A, M and X are varied, the different ion sizes may cause the structure of the perovskite material to distort away from the structure adopted by CaTiO3to a lower- symmetry distorted structure. The symmetry will also be lower if the material comprises a layer that has a structure related to that of CaTiO3. Materials comprising a layer of perovskite material are well known. For instance, the structure of materials adopting the K2NiF4-type structure comprises a layer of perovskite material. The skilled person will appreciate that a perovskite material can be represented by the formula [A][M][X]3, wherein [A] is at least one cation, [M] is at least one cation and [X] is at least one anion. When the perovskite comprises more than one A cation, the different A cations may distribute over the A sites in an ordered or disordered way. When the perovskite comprises more than one M cation, the different M cations may distribute over the B sites in an ordered or disordered way. When the perovskite comprises more than one X anion, the different X anions may distribute over the X sites in an ordered or disordered way. The symmetry of a perovskite comprising more than one A cation, more than one M cation or more than one X cation, will be lower than that of CaTiO3. For layered perovskites the stoichiometry can change between the A, M and X ions. As an example, the [A]2[M][X]4structure can be adopted if the A cation has too large an ionic radius to fit within the 3D perovskite structure. The term “perovskite” also includes A / M / X materials adopting a Ruddleson-Popper phase. Ruddleson- Popper phase refers to a perovskite with a mixture of layered and 3D components. Such perovskites can adopt the crystal structure, An-1A’2MnX3n+1, where A and A’ are different cations and n is an integer from 1 to 8, or from 2 to 6. The term “mixed 2D and 3D” perovskite is used to refer to a perovskite film within which there exists both regions, or domains, of AMX3and An-1A’2MnX3n+1perovskite phases. The term “metal halide perovskite”, as used herein, refers to a perovskite, the formula of which contains at least one metal cation and at least one halide anion. The term “mixed halide perovskite” as used herein refers to a perovskite which contains at least two types of halide anion. The term “mixed cation perovskite” as used herein refers to a perovskite which contains at least two types of A cation. The term “mixed metal perovskite” as used herein refers to a perovskite which contains at least two types of M cation. The term “organic-inorganic metal halide perovskite”, as used herein, refers to a metal halide perovskite, the formula of which contains at least one organic cation. The terms “disposing on” or “disposed on”, as used herein, refer to the making available or placing of one component on another component. The first component may be made available or placed directly on the second component, or there may be a third component which intervenes between the first and second component. For instance, if a first layer is disposed on a second layer, this includes the case where there is an intervening third layer between the first and second layers. Typically, “disposing on” refers to the direct placement of one component on another. Similarly, the terms “disposing between” or “disposed between”, as used herein, refer to the making available or placing of one (first) component between two other (second and third) components. The first component may be made available or placed directly between the second and third components, or there may be a further component which intervenes between the first and second components and / or between the first and third components. For instance, if a first layer is disposed between a second layer and a third layer, this includes the case where there is an intervening fourth layer between the first and second layers and an intervening fifth layer between the first and third layers. Often, however, “disposing between” refers to the direct placement or making available of one (first) component between two other (second and third) components. The term “layer”, as used herein, 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 over the extent of the layer. Typically, a layer has approximately constant thickness. The “thickness” of a layer, as used herein, 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, or by surface profilometry for instance using a stylus profilometer. The term “band gap”, as used herein, refers to the energy difference between the top of the valence band and the bottom of the conduction band in a material. The skilled person of course is readily able to measure the band gap of a semiconductor (including that of a perovskite) by using well-known procedures which do not require undue experimentation. For instance, the band gap of a semiconductor can be estimated by constructing a photovoltaic diode or solar cell from the semiconductor and determining the photovoltaic action spectrum. Alternatively, the band gap can be estimated by measuring the light absorption spectra either via transmission spectrophotometry or by photo thermal deflection spectroscopy. The band gap can be determined by making a Tauc plot, as described in Tauc, J., Grigorovici, R. & Vancu, a. Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi 15, 627–637 (1966) where the square of the product of absorption coefficient times photon energy is plotted on the Y-axis against photon energy on the x-axis with the straight line intercept of the absorption edge with the x-axis giving the optical band gap of the semiconductor. Alternatively, the optical band gap may be estimated by taking the onset of the incident photon-to-electron conversion efficiency, as described in Barkhouse et al. Device characteristics of a 10.1% hydrazineprocessed Cu2ZnSn(Se,S)4 solar cell. The term “semiconductor” or “semiconducting material”, as used herein, refers to a material with electrical conductivity intermediate in magnitude between that of a conductor and a dielectric. A semiconductor may be a negative (n)-type semiconductor, a positive (p)- type semiconductor or an intrinsic (i) semiconductor. A semiconductor may have a band gap of from 0.5 to 3.5 eV, for instance from 0.5 to 2.5 eV or from 1.0 to 2.0 eV (when measured at 300 K). The term “n-type region”, as used herein, refers to a region of one or more electron- transporting (i.e. n-type) materials. Similarly, the terms “n-type layer” refers to a layer of an electron-transporting (i.e. an n-type) material. An electron-transporting (i.e. an n-type) material could be a single electron-transporting compound or elemental material, or a mixture of two or more electron-transporting compounds or elemental materials. An electron- transporting compound or elemental material may be undoped or doped with one or more dopant elements. The term “p-type region”, as used herein, refers to a region of one or more hole- transporting (i.e. p-type) materials. Similarly, the term “p-type layer” refers to a layer of a hole-transporting (i.e. a p-type) material. A hole-transporting (i.e. a p-type) material could be a single hole-transporting compound or elemental material, or a mixture of two or more hole- transporting compounds or elemental materials. A hole-transporting compound or elemental material may be undoped or doped with one or more dopant elements. The term “electrode material”, as used herein, refers to any material suitable for use in an electrode. An electrode material will have a high electrical conductivity. The term “electrode” as used herein indicates a region or layer consisting of, or consisting essentially of, an electrode material. The term “passivation”, as used herein, refers to the inhibition of non-radiative defect- mediate charge-carrier recombination in an A / M / X material. In particular, as used herein, passivation is performed by modifying an A / M / X material with molecular compounds, which bind to or interact with different sites of the surface of the A / M / X material. The sites that the molecular compounds interact with on the surface of the A / M / X material may be crystal lattice sites exposed at the surface, or defect sites such as vacancies or interstitials and long range crystal defects such as step dislocations, add-atoms etc. The term “passivating agent”, as used herein, refers to a molecular compound used to passivate an A / M / X material. As used herein, an “alkyl group” is a substituted or unsubstituted, linear or branched chain saturated radical, it is often a substituted or an unsubstituted linear chain saturated radical, more often an unsubstituted linear chain saturated radical. A C1-C20alkyl group is an unsubstituted or substituted, straight or branched chain saturated hydrocarbon radical, having from 1 to 20 carbon atoms. Typically, it is C1-C10alkyl, for example methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl, or C1-C6alkyl, for example methyl, ethyl, propyl, butyl, pentyl or hexyl, or C1-C4alkyl, for example methyl, ethyl, i-propyl, n-propyl, t- butyl, s-butyl or n-butyl. When an alkyl group is substituted it typically bears one or more substituents selected from substituted or unsubstituted C1-C20alkyl, substituted or unsubstituted aryl (as defined herein), cyano, amino, C1-C10alkylamino, di(C1-C10)alkylamino, arylamino, diarylamino, arylalkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, C1-C20alkoxy, aryloxy, haloalkyl, sulfonic acid, sulfhydryl (i.e. thiol, -SH), C1-C10alkylthio, arylthio, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid and phosphonate ester. Examples of substituted alkyl groups include haloalkyl, hydroxyalkyl, aminoalkyl, alkoxyalkyl and alkaryl groups. The term alkaryl, as used herein, pertains to a C1-C20alkyl group in which at least one hydrogen atom has been replaced with an aryl group. Examples of such groups include, but are not limited to, benzyl (phenylmethyl, PhCH2-), benzhydryl (Ph2CH-), trityl (triphenylmethyl, Ph3C-), phenethyl (phenylethyl, Ph-CH2CH2-), styryl (Ph-CH=CH-), cinnamyl (Ph-CH=CH-CH2-). As discussed hereinbelow, an alkyl group, for instance a C1-20alkyl group, may optionally be interrupted by a secondary, tertiary or quaternary amine moiety as defined hereinbelow. That is to say, a secondary, tertiary or quaternary amine moiety may interrupt the carbon chain of the alkyl group, such that the nitrogen atom of the secondary, tertiary or quaternary amine moiety is bonded to two different carbon atoms within the carbon chain of the alkyl group. An alkyl group may be interrupted more than once, by a plurality of secondary, tertiary or quaternary amine moieties, each of which will interrupt the alkyl group at a different point along the carbon chain (i.e. between a different pair of carbon atoms in the carbon chain). This is particularly the case when the alkyl group is the organic group bonded to a silicon atom of the silane moiety of the organic compound of the passivating agent described herein, for instance the organic group R in the organic compound of formula (I) herein. Additionally or alternatively, such an alkyl group, for instance a C1-20alkyl group, may be substituted by one or more primary, secondary, tertiary or quaternary amine moieties as defined hereinbelow. Alternatively, an alkyl group may not be interrupted by a secondary, tertiary or quaternary amine moiety. In other words, the carbon chain of the alkyl group may be uninterrupted. This is usually the case when the alkyl group is not the organic group bonded to the silicon atom of the silane moiety of organic compound of the passivating agent described herein, for instance when the alkyl group is not the organic group R in the organic compound of formula (I) herein. Typically, a substituted alkyl group carries 1, 2 or 3 substituents, for instance 1 or 2. As used herein, an “aryl group” is a substituted or unsubstituted, monocyclic or bicyclic aromatic group which typically contains from 6 to 14 carbon atoms, preferably from 6 to 10 carbon atoms in the ring portion. Examples include phenyl, naphthyl, indenyl and indanyl groups. An aryl group is unsubstituted or substituted. When an aryl group as defined above is substituted it typically bears one or more substituents selected from C1-C6alkyl which is unsubstituted (to form an aralkyl group), aryl which is unsubstituted, cyano, amino, C1-C10alkylamino, di(C1-C10)alkylamino, arylamino, diarylamino, arylalkylamino, amido, acylamido, hydroxy, halo, carboxy, ester, acyl, acyloxy, C1-C20alkoxy, aryloxy, haloalkyl, sulfhydryl (i.e. thiol, -SH), C1-10alkylthio, arylthio, sulfonic acid, phosphoric acid, phosphate ester, phosphonic acid and phosphonate ester and sulfonyl. Typically it carries 0, 1, 2 or 3 substituents. A substituted aryl group may be substituted in two positions with a single C1-C6alkylene group, or with a bidentate group represented by the formula -X-(C1-C6)alkylene, or -X-(C1-C6)alkylene-X-, wherein X is selected from O, S and NR, and wherein R is H, aryl or C1-C6alkyl. Thus a substituted aryl group may be an aryl group fused with a cycloalkyl group or with a heterocyclyl group. The ring atoms of an aryl group may include one or more heteroatoms (as in a heteroaryl group). Such an aryl group (a heteroaryl group) is a substituted or unsubstituted mono- or bicyclic heteroaromatic group which typically contains from 6 to 10 atoms in the ring portion including one or more heteroatoms. It is generally a 5- or 6-membered ring, containing at least one heteroatom selected from O, S, N, P, Se and Si. It may contain, for example, 1, 2 or 3 heteroatoms. Examples of heteroaryl groups include thiophenyl, pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, furanyl, thienyl, pyrazolidinyl, pyrrolyl, oxazolyl, oxadiazolyl, isoxazolyl, thiadiazolyl, thiazolyl, isothiazolyl, imidazolyl, pyrazolyl, quinolyl and isoquinolyl. A heteroaryl group may be unsubstituted or substituted, for instance, as specified above for aryl. Typically it carries 0, 1, 2 or 3 substituents. The term “cycloalkyl”, as used herein, refers to a saturated or partially unsaturated cyclic hydrocarbon radical. A cycloalkyl group may be a C3-10cycloalkyl group, a C3-8cycloalkyl group or a C3-6cycloalkyl group. Examples of a C3-8cycloalkyl group include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclohexenyl, cyclohex-1,3-dienyl, cycloheptyl and cyclooctyl. Examples of a C3-6cycloalkyl group include cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl. The term “alkenyl”, as used herein, refers to a linear or branched chain hydrocarbon radical comprising one or more double bonds. An alkenyl group may be a C2-20alkenyl group, a C2-14alkenyl group, a C2-10alkenyl group, a C2-6alkenyl group or a C2-4alkenyl group. Examples of a C2-10alkenyl group are ethenyl (vinyl), propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl or decenyl. Examples of C2-6alkenyl groups are ethenyl, propenyl, butenyl, pentenyl or hexenyl. Examples of C2-4alkenyl groups are ethenyl, i- propenyl, n-propenyl, s-butenyl or n-butenyl. Alkenyl groups typically comprise one or two double bonds. The term “alkynyl”, as used herein, refers to a linear or branched chain hydrocarbon radical comprising one or more triple bonds. An alkynyl group may be a C2-20alkynyl group, a C2-14 alkynyl group, a C2-10 alkynyl group, a C2-6 alkynyl group or a C2-4 alkynyl group. Examples of a C2-10alkynyl group are ethynyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, octynyl, nonynyl or decynyl. Examples of C1-6alkynyl groups are ethynyl, propynyl, butynyl, pentynyl or hexynyl. Alkynyl groups typically comprise one or two triple bonds. Unless otherwise specified, the term “substituted”, as used herein in the context of substituted organic groups, refers to an organic group which bears one or more substituents selected from C1-10alkyl, aryl (as defined herein), cyano, amino, nitro, C1-10alkylamino, di(C1-10)alkylamino, arylamino, diarylamino, aryl(C1-10)alkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester, acyl, acyloxy, C1-10alkoxy, aryloxy, halo(C1-10)alkyl, sulfonic acid, thiol, C1-10alkylthio, arylthio, sulfonyl, phosphoric acid, phosphate ester, phosphonic acid and phosphonate ester. Examples of substituted alkyl groups include haloalkyl, perhaloalkyl, hydroxyalkyl, aminoalkyl, alkoxyalkyl and alkaryl groups. When a group is substituted, it may bear 1, 2 or 3 substituents. For instance, a substituted group may have 1 or 2 substituents. The term “halide” as used herein indicates the singly charged anion of an element in group VIII of the periodic table. “Halide” includes fluoride, chloride, bromide and iodide. The term “halo” as used herein indicates a halogen atom. Exemplary halo species include fluoro, chloro, bromo and iodo species. As used herein the term hydroxyl represents a group of formula: -OH As used herein the term thiol represents a group of formula: -SH As used herein the term acyl represents a group of formula: -C(=O)R, wherein R is an acyl substituent, for example, a substituted or unsubstituted C1-20alkyl group, or a substituted or unsubstituted aryl group. Examples of acyl groups include, but are not limited to, -C(=O)CH3(acetyl), -C(=O)CH2CH3(propionyl), -C(=O)C(CH3)3(t-butyryl), and -C(=O)Ph (benzoyl, phenone). As used herein the term acyloxy (or reverse ester) represents a group of formula: -OC(=O)R, wherein R is an acyloxy substituent, for example, substituted or unsubstituted C1-20alkyl group, or a substituted or unsubstituted aryl group, typically a C1-6alkyl group. Examples of acyloxy groups include, but are not limited to, -OC(=O)CH3(acetoxy), -OC(=O)CH2CH3, -OC(=O)C(CH3)3, -OC(=O)Ph, and -OC(=O)CH2Ph. As used herein the term ester (or carboxylate, carboxylic acid ester or oxycarbonyl) represents a group of formula: -C(=O)OR, wherein R is an ester substituent, for example, a substituted or unsubstituted C1-20alkyl group, or a substituted or unsubstituted aryl group (typically a phenyl group). Examples of ester groups include, but are not limited to, -C(=O)OCH3, -C(=O)OCH2CH3, -C(=O)OC(CH3)3, and -C(=O)OPh. As used herein, the terms “carboxy”, “carboxyl” and “carboxylic acid” each represent a group of the formula: -C(=O)OH, or -COOH. As would be understood by the skilled person, a carboxylic acid group can exist in protonated and deprotonated forms (for example, -C(=O)OH and -C(=O)O-), and in salt forms (for example, -C(=O)O-X+, wherein X+is a monovalent cation). A C1-10alkylthio group is a said C1-10alkyl group, preferably a C1-6alkyl group, attached to a sulfur atom. An arylthio group is an aryl group, preferably a phenyl group, attached to a sulfur atom. An example of an arylthio group is -SPh. A C1-10alkoxy group is a said substituted or unsubstituted C1-10alkyl group attached to an oxygen atom. A C1-6alkoxy group is a said substituted or unsubstituted C1-6alkyl group attached to an oxygen atom. A C1-4alkoxy group is a substituted or unsubstituted C1-4alkyl group attached to an oxygen atom. Said C1-20, C1-10, C1-6and C1-4alkyl groups are optionally interrupted as defined herein. Examples of C1-4alkoxy groups include, -OMe (methoxy), -OEt (ethoxy), -O(nPr) (n-propoxy), -O(iPr) (isopropoxy), -O(nBu) (n-butoxy), - O(sBu) (sec-butoxy), -O(iBu) (isobutoxy), and -O(tBu) (tert-butoxy). Further examples of C1-20alkoxy groups are -O(Adamantyl), -O-CH2-Adamantyl and -O-CH2-CH2-Adamantyl. An aryloxy group is a substituted or unsubstituted aryl group, as defined herein, attached to an oxygen atom. An example of an aryloxy group is -OPh (phenoxy). As used herein, an amino group is a radical of formula –NR2, wherein each R is a substituent. R is usually selected from hydrogen, alkyl, alkenyl, cycloalkyl, or aryl, wherein each of alkyl, alkenyl, cycloalkyl and aryl are as defined herein. Typically, each R is selected from hydrogen, C1-10alkyl, C2-10alkenyl, and C3-10cycloalkyl. Preferably, each R is selected from hydrogen, C1-6alkyl, C2-6alkenyl, and C3-6cycloalkyl. More preferably, each R is selected from hydrogen and C1-6 alkyl. Most preferably, each R is hydrogen. A typical amino group is an alkylamino group, which is a radical of formula –NR2wherein at least one R is an alkyl group as defined herein. A C1-6alkylamino group is an alkylamino group wherein at least one R is an C1-6alkyl group. As used herein, an imino group is a radical of formula R2C=N- or –C(R)=NR, wherein each R is a substituent. That is, an imino group is a radical comprising a C=N moiety, having the radical moiety either at the N atom or attached to the C atom of said C=N bond. R is as defined herein: that is, R is usually selected from hydrogen, alkyl, alkenyl, cycloalkyl, or aryl, wherein each of alkyl, alkenyl, cycloalkyl and aryl are as defined herein. Typically, each R is selected from hydrogen, C1-10alkyl, C2-10alkenyl, and C3-10cycloalkyl. Preferably, each R is selected from hydrogen, C1-6alkyl, C2-6alkenyl, and C3-6cycloalkyl. More preferably, each R is selected from hydrogen and C1-6alkyl. A typical imino group is an alkylimino group, which is a radical of formula R2C=N- or –C(R)=NR wherein at least one R is an alkyl group as defined herein. A C1-6alkylimino group is an alkylimino group wherein the R substituents comprise from 1 to 6 carbon atoms. The term “ether” as used herein indicates an oxygen atom substituted with two alkyl radicals as defined herein. The alkyl radicals may be optionally substituted, and may be the same or different. As used herein, the term “ammonium” indicates an organic cation comprising a quaternary nitrogen. An ammonium cation is a cation of formula R1R2R3R4N+. R1, R2, R3, and R4are substituents. Each of R1, R2, R3, and R4are typically independently selected from hydrogen, or from optionally substituted alkyl, alkenyl, aryl, cycloalkyl, cycloalkenyl and amino; the optional substituent is preferably an amino or imino substituent. Usually, each of R1, R2, R3, and R4are independently selected from hydrogen, and optionally substituted C1-10alkyl, C2-10alkenyl, C3-10cycloalkyl, C3-10cycloalkenyl, C6-12aryl and C1-6amino; where present, the optional substituent is preferably an amino group; particularly preferably C1-6amino. Preferably, each of R1, R2, R3, and R4are independently selected from hydrogen, and unsubstituted C1-10alkyl, C2-10alkenyl, C3-10cycloalkyl, C3-10cycloalkenyl, C6-12aryl and C1-6amino. In a particularly preferred embodiment, R1, R2, R3, and R4are independently selected from hydrogen, C1-10alkyl, and C2-10alkenyl and C1-6amino. Further preferably, R1, R2, R3, and R4are independently selected from hydrogen, C1-6 alkyl, C2-6 alkenyl and C1-6 amino. As used herein, the term “iminium” indicates an organic cation of formula (R1R2C=NR3R4)+, wherein R1, R2, R3, and R4are as defined in relation to the ammonium cation. Thus, in a particularly preferred embodiment, of the iminium cation, R1, R2, R3, and R4are independently selected from hydrogen, C1-10alkyl, C2-10alkenyl and C1-6amino. In a further preferable embodiment of the iminium cation, R1, R2, R3, and R4are independently selected from hydrogen, C1-6alkyl, C2-6alkenyl and C1-6amino. Often, the iminium cation is formamidinium, i.e. R1is NH2and R2, R3and R4are all H. The term “sensitizer”, as used herein, refers to a material which is capable of performing photoinduced charge generation, photoemission or electro emission. Often, the sensitizer is also capable of transporting charge (holes or electrons). For instance, when the sensitizer is said A / M / X material the sensitizer is generally also capable of transporting charge. The invention provides a photovoltaic device which comprises a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound. The term “silane moiety”, as used herein, refers to a moiety which contains a silicon atom bonded to four substituents, Rsil. Each of the four Rsilsubstituents in a silane moiety may be independently selected from inorganic groups, for instance H, OH and halo, and organic groups, for instance alkyl, aryl, alkoxy, acyl, ester and acyloxy groups, each of which may be unsubstituted or substituted. Sometimes, one or more of the four Rsilsubstituents comprises another silane moiety. For instance, an Rsilsubstituent may itself be a group of formula -OSiRsil3. Often, each of the four Rsilsubstituents of a silane moiety is independently either H or an organic group, and more typically, each Rsilis an organic group. For instance, each of the four Rsilsubstituents of a silane moiety may be independently selected from alkoxy groups and alkyl groups, each of which may be unsubstituted or substituted. In the present invention, the Rsilsubstituents in the silane moiety may be groups which, together with the silicon atom to which they are bonded, constitute the organic compound which comprises a silane moiety that is employed as a passivating agent. Any primary, secondary, tertiary or quaternary amine moiety that is present in the organic compound may be part of an Rsilsubstituent of the silane moiety. An “amine moiety”, as used herein, means a moiety in which a nitrogen atom is bonded to at least one group other than hydrogen, and which is a primary amine, a secondary amine a tertiary amine or a quaternary amine, as defined hereinbelow. Each group other than hydrogen in the amine moiety is typically an organic group, for instance an alkyl group or an aryl group which may itself be unsubstituted or substituted. A “primary amine”, as used herein, means a moiety in which nitrogen is bonded to one group other than hydrogen and to two hydrogen atoms, or to three hydrogens if the primary amine moiety is protonated. Thus, a primary amine moiety has the formula RNH2or RNH3+, where R is the group other than hydrogen. The group other than hydrogen is typically an organic group, for instance an alkyl group or an aryl group which may itself be unsubstituted or substituted. A “secondary amine”, as used herein, means a moiety in which nitrogen is bonded to two groups other than hydrogen and to one hydrogen atom, or to two hydrogens if the secondary amine moiety is protonated. Thus, a secondary amine moiety has the formula R2NH or R2NH2+, where the R groups are the groups other than hydrogen. The groups other than hydrogen are typically organic groups, for instance alkyl or aryl groups which may themselves be unsubstituted or substituted. The two groups other than hydrogen may optionally be bonded to each other, to form a ring together with the nitrogen of the secondary amine moiety (but typically, this is not the case). A “tertiary amine”, as used herein, means a moiety in which nitrogen is bonded to three groups other than hydrogen and to no hydrogen atoms, or to one hydrogen if the tertiary amine moiety is protonated. Thus, a tertiary amine moiety has the formula R3N or R3NH+, where the R groups are the groups other than hydrogen. The groups other than hydrogen are typically organic groups, for instance alkyl or aryl groups which may themselves be unsubstituted or substituted. Any one of the three groups other than hydrogen may optionally be bonded to another one of the groups other than hydrogen, to form a ring together with the nitrogen of the tertiary amine moiety. A “quaternary amine”, as used herein, means a cationic moiety in which nitrogen is bonded to four groups other than hydrogen. Thus, a quaternary amine moiety has the formula R4N+, where the R groups are the groups other than hydrogen. The groups other than hydrogen are typically organic groups, for instance alkyl or aryl groups which may themselves be unsubstituted or substituted. Any one of the four groups other than hydrogen may optionally be bonded to another one of the groups other than hydrogen, to form a ring together with the nitrogen of the tertiary amine moiety. In the present invention, the group or groups other than hydrogen, in the primary, secondary, tertiary or quaternary amine moiety, may be groups which, together with the nitrogen atom to which they are bonded and any hydrogen atoms which are bonded to said nitrogen atom, constitute the organic compound which comprises a silane moiety. The silane moiety may be part of a said group other than hydrogen. As mentioned above, the passivating agent of the invention comprises an organic compound. Typically, the organic compound comprises a silane moiety and an amine moiety, wherein the amine moiety is a secondary amine, a tertiary amine or a quaternary amine. The organic compound may comprise a silane moiety and a plurality of amine moieties, wherein at least one of the amine moieties is a secondary amine, a tertiary amine or a quaternary amine. The organic compound may comprise a silane moiety and from two to five amine moieties, for example two, three, four or five amine moieties, wherein at least one of the amine moieties is a secondary amine, a tertiary amine or a quaternary amine. Typically, when the organic compound comprises a plurality of amine moieties and at least one of the amine moieties is a secondary amine, a tertiary amine or a quaternary amine, it is also the case that at least one of the amine moieties is a primary amine. Often, when the organic compound comprises a plurality of amine moieties, at least one of the amine moieties is a primary amine, and each of the other amine moieties is independently selected from a secondary amine, a tertiary amine and a quaternary amine. Preferably, one of the amine moieties is a primary amine, and each of the other amine moieties is a secondary amine. Typically, when the organic compound comprises an amine moiety, the organic compound comprises an organic group bonded to a silicon atom of the silane moiety, wherein each of said amine moieties is present in said organic group. In this embodiment, typically, said organic group is a C1-20alkyl group, wherein each amine moiety which is a secondary, tertiary or quaternary amine may either interrupt the carbon chain of the C1-20alkyl group or be present as a substituent on the C1-20alkyl group, and wherein any amine moiety which is a primary amine is present as a substituent on the C1-20alkyl group. This C1-20alkyl group may for instance be a C1-10alkyl group. The C1-20alkyl group (or C1-10alkyl group) may be otherwise unsubstituted or substituted. In other words, apart from any amine moiety that is present as a substituent on the alkyl group, the alkyl group may be unsubstituted (“otherwise unsubstituted”) or substituted (“otherwise substituted”). For instance, the organic group (typically said C1-20or C1-10alkyl group) may be otherwise substituted by one or more groups selected from thiol, C1-10alkylthio, arylthio, carboxy, pyridyl and bipyridyl. Thus, in addition to any amine moiety that is present as a substituent on the alkyl group, the alkyl group may be substituted by one or more groups selected from thiol, C1-10alkylthio, arylthio, carboxy, pyridyl and bipyridyl. The organic compound may for instance be according to formula (I) (I) wherein: R is said organic group; and E1, E2and E3are independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted aryloxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryl, unsubstituted or substituted acyl, unsubstituted or substituted ester, unsubstituted or substituted acyloxy, hydroxyl and halo. Typically, in the compound of formula (I), the group R is a C1-20alkyl group, wherein each amine moiety which is a secondary, tertiary or quaternary amine may either interrupt the carbon chain of the C1-20alkyl group or be present as a substituent on the C1-20alkyl group, and wherein any amine moiety which is a primary amine is present as a substituent on the C1-20alkyl group, and wherein the C1-20alkyl group is otherwise unsubstituted or substituted (for instance, as discussed above it be otherwise substituted by one or more groups selected from thiol, C1-10alkylthio, arylthio, carboxy, pyridyl and bipyridyl). Often, in the compound of formula (I), the group R is a C1-10alkyl group, wherein each amine moiety which is a secondary, tertiary or quaternary amine may either interrupt the carbon chain of the C1-10alkyl group or be present as a substituent on the C1-10alkyl group, and wherein any amine moiety which is a primary amine is present as a substituent on the C1-10alkyl group, and wherein the C1-10 alkyl group is otherwise unsubstituted or substituted (for instance, it may be otherwise substituted by one or more groups selected from thiol, C1-10alkylthio, arylthio, carboxy, pyridyl and bipyridyl). Typically, E1, E2and E3are independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryloxy, and unsubstituted or substituted aryl. The groups E1, E2and E3are often unsubstituted or substituted C1-10alkoxy, for instance unsubstituted C1-10alkoxy. The groups E1, E2and E3may for instance be unsubstituted or substituted C1-6alkoxy, or unsubstituted or substituted C1-4alkoxy, and typically these alkoxy groups are unsubstituted. E1, E2and E3may for instance be independently selected from ethoxy and methoxy. For example E1, E2and E3may each be methoxy. Typically, the organic compound has the formula (II) wherein: E1, E2and E3are independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted aryloxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryl, unsubstituted or substituted acyl, unsubstituted or substituted ester, unsubstituted or substituted acyloxy, hydroxyl and halo; q is 0 or an integer from 1 to 5; each m is independently an integer from 1 to 5; each n is independently 0 or 1; p is 0 or an integer from 1 to 5; G is NR12 or CR13; each R1is independently H or unsubstituted or substituted C1-6alkyl (and is typically H or unsubstituted C1-6alkyl, for instance H, methyl or ethyl); wherein when each n is 0 and G is NR12, or when q is 0 and G is NR12, then at least one R1group is unsubstituted or substituted C1-6alkyl. Typically, in the compound of formula (II), E1, E2and E3are independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryloxy, and unsubstituted or substituted aryl. The groups E1, E2and E3are often unsubstituted or substituted C1-10alkoxy, for instance unsubstituted C1-10alkoxy. The groups E1, E2and E3may for instance be unsubstituted or substituted C1-6alkoxy, or unsubstituted or substituted C1-4alkoxy, and typically these alkoxy groups are unsubstituted. E1, E2and E3may for instance be independently selected from ethoxy and methoxy. Typically, at least one of E1, E2and E3is methoxy. Preferably, at least two of E1, E2and E3are methoxy. More preferably, E1, E2and E3are all methoxy. Typically, in the compound of formula (II), when G is NR12 and q is not 0, then p is not 0. Typically, in the compound of formula (II), G is NR12. Typically, in the compound of formula (II), each R1is independently H or methyl. Preferably, each R1is H. Typically, in the compound of formula (II), q is 0, 1 or 2. Typically, q is 1 or 2. Alternatively, q is 0. For example, q can be 0. Alternatively, q can be 1. Alternatively, q can be 2. Often, in the compound of formula (II), q is an integer from 1 to 5, each m is independently 1, 2, or 3 and each n is 1. Preferably, q is 1 or 2, each m is 2 and each n is 1. Typically, in the compound of formula (II), p is an integer from 1 to 5. For example, p is often 1. However, p is also typically 2. In other typical instances, p is 3. However, p may for instance be 4. In other cases, p may be 5. Typically, however, p is 1, 2, 3, or 4. Preferably, p is 1, 2, or 3. In the present invention, the organic compound is preferably selected from: (i) PTMS ; and (v) (AE)2APTMS More preferably, the organic compound is selected from (ii) MAPTMS, (iii) DMAPTMS, (iv) AEAPTMS, and (v) (AE)2APTMS. Often, the organic compound is selected from AEAPTMS and (AE)2APTMS. The organic compound is typically AEAPTMS. Often, however, the organic compound is (AE)2APTMS. The passivating agent may comprise a dimer or a polymer of the organic compound. The dimer of the organic compound may comprise (i) a first monomer unit, wherein the first monomer unit is an organic compound as defined above provided that the silicon atom of the silane moiety of the first monomer unit is substituted with a linking atom or group, and (ii) a second monomer unit, wherein the second monomer unit is an organic compound as defined above provided that the silicon atom of the silane moiety of the second monomer unit is substituted with said linking atom or group, wherein the linking atom or group is covalently bonded to the silicon atom of the silane moiety of the first monomer unit and to the silicon atom of the silane moiety of the second monomer unit, to link the first monomer unit to the second monomer unit. The first monomer unit may be an organic compound of formula (I) or formula (II) as defined above provided that one of E1, E2and E3of the first monomer unit is said linking atom or group, and the second monomer unit may be an organic compound of formula (I) or formula (II) as defined above provided that one of E1, E2and E3of the second monomer unit is said linking atom or group. The dimer may have formula (Ia): wherein L is said linking atom or group, and each R, E1and E2is independently as defined above. (Each R is an organic group, as defined above, wherein each of said amine moieties is present in said organic group.) The dimer may for instance have formula (IIa): wherein L is said linking atom or group, and each G, p, n, m, q, E1and E2is independently as defined above. In the dimer of formula (Ia) or formula (IIa), the linking atom or group is typically a linking atom. Often, L is a linking atom, such as a divalent atom, such as an oxygen atom, a sulfur atom, or a selenium atom. Typically, when L is a linking atom, the linking atom is an oxygen atom. L may be a linking group, such as an unsubstituted or substituted alkylene group, or an unsubstituted or substituted arylene group, or a group -O-Q-, -O-Q-O-, -Q-Q- or -Q-Q-Q- wherein O is an oxygen atom and each Q is an unsubstituted or substituted alkylene group or an unsubstituted or substituted arylene group. Typically, the or each alkylene group is an unsubstituted or substituted C1-C6alkylene group, more typically an unsubstituted C1-C6alkylene group, for instance a butylene, propylene or ethylene group. As the skilled person would understand, an arylene group is a bivalent aryl group, wherein the aryl group is as defined above. The arylene group may for instance be unsubstituted or substituted phenylene, for instance unsubstituted phenylene. The polymer of the organic compound may comprise a monomer unit which is an organic compound as defined above, provided that the silicon atom of the silane moiety is substituted with at least one linking atom or group, wherein the or each linking atom or group is covalently bonded to said silicon atom and is also covalently bonded to the silicon atom of the silane moiety of another monomer unit in the polymer. The monomer unit is typically an organic compound of formula (I) or formula (II) as defined above, provided that at least one of E1, E2and E3is a said linking atom or group, typically wherein two of E1, E2and E3are both a said linking atom or group. Each linking atom or group may be as further defined above for the dimer of formula (Ia) and formula (IIa). Often, each linking atom or group is a linking atom, for instance an oxygen atom. Typically, the polymer of the organic compound comprises a monomer unit of formula (Ib): wherein L is a said linking atom or group, and R and E1are as defined above. (R is an organic group, as defined above, wherein each of said amine moieties is present in said organic group.) Typically, the polymer comprises at least three monomer units of formula (Ib), for instance from 3 to 1000, or from 3 to 100, for example from 3 to 50, or from 3 to 20, for instance from 3 to 10, or from 3 to 5, monomer units of formula (Ib). Often, for instance, the polymer of the organic compound comprises a monomer unit of formula (IIb): wherein L is a said linking atom or group, and G, p, n, m, q and E1are as defined above. Typically, the polymer comprises at least three monomer units of formula (IIb), for instance from 3 to 1000, or from 3 to 100, for example from 3 to 50, or from 3 to 20, for instance from 3 to 10, or from 3 to 5, monomer units of formula (IIb). Typically, the polymer comprises a structure of formula (Ic): wherein each L is a said linking atom or group, and each R and E1is independently as defined above, and z is an integer of at least 3. (Each R is an organic group, as defined above, wherein each of said amine moieties is present in said organic group.) The integer z may for instance be from 3 to 1000, or from 3 to 100, for example from 3 to 50, or from 3 to 20, for instance from 3 to 10, or from 3 to 5. Often, for instance, the polymer comprises a structure of formula (IIc): wherein each L is a said linking atom or group, and each G, p, n, m, q and E1is independently as defined as defined above, and z is an integer of at least 3. The integer z may for instance be from 3 to 1000, from 3 to 100, or for example from 3 to 50, or from 3 to 20, for instance from 3 to 10, or from 3 to 5. In the present invention, the passivating agent may be disposed on a surface of the crystalline A / M / X material. The photovoltaic device may comprise a layer comprising the passivated A / M / X material. The layer comprising the passivated A / M / X material may comprise (i) a layer of the crystalline A / M / X material, and (ii) the passivating agent. The passivating agent may be disposed on a surface of the layer of the crystalline A / M / X material. The passivating agent may penetrate into the layer of the crystalline A / M / X material up to a certain depth beyond a surface of the layer of the crystalline A / M / X material. The passivating agent may for instance be disposed on a surface of the layer of the crystalline A / M / X material and penetrate into the layer of the crystalline A / M / X material up to a certain depth below said surface. Said depth may for instance be up to about 140 nm, or up to about 120 nm, or up to about 100 nm, or up to about 80 nm. Said depth may for instance be from about 10 nm to about 140 nm, for instance from about 20 nm to about 120 nm, or from about 30 nm to about 100 nm, or from about 40 nm to about 90 nm. Typically, the depth is about 70 nm. The layer comprising the passivated A / M / X material may comprise (i) a layer of the crystalline A / M / X material, and (ii) a layer of the passivating agent disposed on a surface of the layer of the crystalline A / M / X material. The layer comprising the passivated A / M / X material may consist essentially of the passivated A / M / X material. The layer comprising the passivated A / M / X material may consist of the passivated A / M / X material. The layer comprising the passivated A / M / X material may comprise a layer of the crystalline A / M / X material, and said passivating agent penetrating into a surface of the layer of the crystalline A / M / X material. The passivating agent may penetrate into the surface of the layer of the crystalline A / M / X material. It may penetrate into the surface of the layer of the crystalline A / M / X material up to a certain distance beyond the surface. The passivating agent may for instance penetrate up to about 140 nm into the surface of the layer of the crystalline A / M / X material (meaning up to about 140 nm beyond the surface), or up to about 120 nm, or up to about 100 nm, or up to about 80 nm. Typically, the passivating agent penetrates up to about 70 nm into the surface of the layer of the crystalline A / M / X material. The passivating agent may for instance penetrate from about 10 nm to about 140 nm into the surface of the layer of the crystalline A / M / X material, for instance from about 20 nm to about 120 nm, or from about 30 nm to about 100 nm, or from about 40 nm to about 90 nm into the surface, for instance about 70 nm into the surface. The layer comprising the passivated A / M / X material may comprise particles of the passivated A / M / X material. Each particle of the passivated A / M / X material may comprise (a) a particle of the crystalline A / M / X material and (b) said passivating agent disposed on a surface of the crystalline A / M / X material. The layer comprising the passivated A / M / X material may have a thickness of from about 10 nm to about 100 μm, or from about 10 nm to about 10 μm, or from about 100 nm to about 1000 nm. When the photovoltaic device comprises a layer comprising the passivated A / M / X material, the device may comprise a photoactive region, wherein the photoactive region comprises an n-type region comprising at least one n-type layer, a p-type region comprising at least one p-type layer, and, disposed between the n-type region and the p-type region: said layer comprising the passivated A / M / X material. Typically, the photoactive region is disposed between a first electrode and a second electrode. The first electrode may comprise a metal (for instance silver, gold, aluminium, copper, molybdenum or tungsten), an organic conducting material such as PEDOT:PSS, or a transparent conducting oxide (for instance fluorine doped tin oxide (FTO), aluminium doped zinc oxide (AZO), zinc doped indium oxide (IZO) or tin-doped indium oxide (ITO)). Typically, however, the first electrode is a transparent electrode. Thus, the first electrode typically comprises a transparent conducting oxide, preferably FTO, ITO, IZO or AZO. Typically, the first electrode is tin-doped indium oxide (ITO). The thickness of the layer of a first electrode is typically from 10 nm to 1000 nm, more typically from 40 to 400nm. The second electrode may be as defined above for the first electrode, for instance, the second electrode may comprise a metal (for instance silver, gold, aluminium or tungsten), an organic conducting material such as PEDOT:PSS, or a transparent conducting oxide (for instance fluorine doped tin oxide (FTO), aluminium doped zinc oxide (AZO), zinc doped indium oxide (IZO) or tin-doped indium oxide (ITO)). Typically, however, the second electrode comprises, or consists essentially of, a metal for instance an elemental metal (in which case the first electrode is typically a transparent electrode, as discussed above). Examples of metals which the second electrode material may comprise, or consist essentially of, include silver, gold, copper, molybdenum, aluminium, platinum, palladium, or tungsten. The second electrode may be disposed by vacuum evaporation. The thickness of the layer of a second electrode material is typically from about 10 to about 1000 nm, preferably from about 50 nm to about 150 nm. The second electrode may optionally include a further layer comprising a metal (for instance Cr) or a metal oxide (for instance Cr2O3), typically a layer comprising Cr, or a mixture of chromium and chromium (III) oxide (Cr / Cr2O3). The thickness of the further layer is typically between 1 nm to 10 nm. Typically, the second electrode comprises a layer of gold and a layer of chromium. Optionally, the thickness of the gold layer may be from about 50 nm to about 150 nm. Optionally, the thickness of the layer of chromium may be from about 1 nm to about 10 nm. Examples of electron transporting (n-type) materials are known to the skilled person. A suitable n-type material may be an organic or inorganic material. A suitable inorganic n- type material may be selected from a metal oxide, a metal sulphide, a metal selenide, a metal telluride, a perovskite, amorphous Si, an n-type group IV semiconductor, an n-type group III- V semiconductor, an n-type group II-VI semiconductor, an n-type group I-VII semiconductor, an n-type group IV-VI semiconductor, an n-type group V-VI semiconductor, and an n-type group II-V semiconductor, any of which may be doped or undoped. More typically, the n-type material is selected from a metal oxide, a metal sulphide, a metal selenide, and a metal telluride. Thus, the n-type layer may comprise an inorganic material selected from oxide of titanium, tin, zinc, niobium, tantalum, tungsten, indium, gallium, neodymium, palladium, or cadmium, or an oxide of a mixture of two or more of said metals. For instance, the n-type layer may comprise TiO2, SnO2, ZnO, Nb2O5, Ta2O5, WO3, W2O5, In2O3, Ga2O3, Nd2O3, PbO, or CdO. Other suitable n-type materials that may be employed include sulphides of cadmium, tin, copper, or zinc, including sulphides of a mixture of two or more of said metals. For instance, the sulphide may be FeS2, CdS, ZnS, SnS, BiS, SbS, or Cu2ZnSnS4. The n-type layer may for instance comprise a selenide of cadmium, zinc, indium, or gallium or a selenide of a mixture of two or more of said metals; or a telluride of cadmium, zinc, cadmium or tin, or a telluride of a mixture of two or more of said metals. For instance, the selenide may be Cu(In,Ga)Se2. Typically, the telluride is a telluride of cadmium, zinc, cadmium or tin. For instance, the telluride may be CdTe. The n-type layer may for instance comprise an inorganic material selected from oxide of titanium (e.g. TiO2), tin (e.g. SnO2), zinc (e.g. ZnO), niobium, tantalum, tungsten, indium, gallium, neodymium, palladium, cadmium, or an oxide of a mixture of two or more of said metals; a sulphide of cadmium, tin, copper, zinc or a sulphide of a mixture of two or more of said metals; a selenide of cadmium, zinc, indium, gallium or a selenide of a mixture of two or more of said metals; or a telluride of cadmium, zinc, cadmium or tin, or a telluride of a mixture of two or more of said metals. Examples of other semiconductors that may be suitable n-type materials, for instance if they are n-doped, include group IV elemental or compound semiconductors; amorphous Si; group III-V semiconductors (e.g. gallium arsenide); group II-VI semiconductors (e.g. cadmium selenide); group I-VII semiconductors (e.g. cuprous chloride); group IV-VI semiconductors (e.g. lead selenide); group V-VI semiconductors (e.g. bismuth telluride); and group II-V semiconductors (e.g. cadmium arsenide). Other n-type materials may also be employed, including organic and polymeric electron-transporting materials, and electrolytes. Suitable examples include, but are not limited to a fullerene or a fullerene derivative (for instance C60, C70, phenyl-C61-butyric acid methyl ester (PCBM, also referred to herein as PC61BM), PC71BM (i.e. phenyl C71butyric acid methyl ester), bis[C60]BM (i.e. bis-C60butyric acid methyl ester), and 1′,1′′,4′,4′′- Tetrahydro-di[1,4]methanonaphthaleno[1,2:2′,3′,56,60:2′′,3′′][5,6]fullerene-C60(ICBA)), an organic electron transporting material comprising perylene or a derivative thereof, poly{[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50- (2,20-bithiophene)} (P(NDI2OD-T2)) or bathocuproine (BCP). Typically, the n-type material is phenyl-C61-butyric acid methyl ester (PCBM) or C60. Typically, when PCBM it used it is deposited in the solution phase. Typically, when C60is used, it is deposited in the vapour phase by vacuum evaporation. Typically, BCP is used as an additional n-type material in the n-type layer. Thus, typically, PCBM is used in combination with BCP, or C60is used in combination with BCP. Examples of hole transporting (p-type) materials are known to the skilled person. The p-type material may be a single p-type compound or elemental material, or a mixture of two or more p-type compounds or elemental materials, which may be undoped or doped with one or more dopant elements. The p-type material may comprise an inorganic or an organic p-type material. For instance, the p-type material may be an organic p-type material. Suitable p-type materials may be selected from polymeric or molecular hole transporters. The p-type material may for instance comprise spiro-OMeTAD (2,2’,7,7’- tetrakis-(N,N-di-p-methoxyphenylamine)9,9’-spirobifluorene)), P3HT (poly(3- hexylthiophene)), PCPDTBT (Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)- 4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl]]), PVK (poly(N-vinylcarbazole)), HTM- TFSI (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide), spiro-OMETAD+-bis(trifluoromethanesulfonyl)imide- (spiro(TFSI)2), tBP (tert-butylpyridine), m-MTDATA (4,4′,4′′- tris(methylphenylphenylamino)triphenylamine), MeOTPD (N,N,N′,N′-tetrakis(4- methoxyphenyl)-benzidine), BP2T (5,5′-di(biphenyl-4-yl)-2,2′-bithiophene), Di-NPB (N,N′- Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine), α-NPB (N,N’-di(naphthalen-1- yl)-N,N’-diphenyl-benzidine), TNATA (4,4',4"-tris-(N-(naphthylen-2-yl)-N- phenylamine)triphenylamine), BPAPF (9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H- fluorene), spiro-NPB (N2,N7-Di-1-naphthalenyl-N2,N7-diphenyl-9,9'-spirobi[9H-fluorene]- 2,7-diamine), 4P-TPD (4,4-bis-(N,N-diphenylamino)-tetraphenyl), polyTPD (i.e. Poly[N,N’- bis(4-butylphenyl)-N,N’-bisphenylbenzidine]), Me-4PACz ([4-(3,6-dimethyl-9H-carbazol-9- yl)butyl]phosphonic acid), PTAA (i.e. poly(triaryl amine), also known as poly[bis(4- phenyl)(2,4,6-trimethylphenyl)amine]) or PEDOT:PSS. The p-type material may comprise carbon nanotubes. Usually, the p-type material is selected from spiro-OMeTAD, P3HT, PCPDTBT, polyTPD, Me-4PACz, spiro(TFSI)2and PVK. In one embodiment, the p-type material is Me-4PACz. Suitable p-type materials also include molecular hole transporters, polymeric hole transporters and copolymer hole transporters. The p-type material may for instance be a molecular hole transporting material, a polymer or copolymer comprising one or more of the following moieties: thiophenyl, phenelenyl, dithiazolyl, benzothiazolyl, diketopyrrolopyrrolyl, ethoxydithiophenyl, amino, triphenyl amino, carbozolyl, ethylene dioxythiophenyl, dioxythiophenyl, or fluorenyl. The p-type material may be doped, for instance with tertbutyl pyridine and LiTFSI. The p-type material may be doped to increase the hole-density. The p-type material may for instance be doped with NOBF4(Nitrosonium tetrafluoroborate), or 1,3,4,5,7,8-hexafluoro- tetracyanonaphthoquinodimethane (F6TCNNQ), to increase the hole-density. The hole-transporting material (p-type material) may be a solid state inorganic hole transporting material. For instance, the p-type layer may comprise an inorganic hole transporter comprising an oxide of nickel (e.g. NiO), vanadium, copper or molybdenum; CuI, CuBr, CuSCN, Cu2O, CuO or CIS; a perovskite; amorphous Si; a p-type group IV semiconductor, a p-type group III-V semiconductor, a p-type group II-VI semiconductor, a p- type group I-VII semiconductor, a p-type group IV-VI semiconductor, a p-type group V-VI semiconductor, and a p-type group II-V semiconductor, which inorganic material may be doped or undoped. The p-type layer may be a compact layer of said inorganic hole transporter. The p-type material may be an inorganic p-type material, for instance a material comprising an oxide of nickel, vanadium, copper or molybdenum; CuI, CuBr, CuSCN, Cu2O, CuO or CIS; amorphous Si; a p-type group IV semiconductor, a p-type group III-V semiconductor, a p-type group II-VI semiconductor, a p-type group I-VII semiconductor, a p- type group IV-VI semiconductor, a p-type group V-VI semiconductor, and a p-type group II- V semiconductor, which inorganic material may be doped or undoped. The p-type material may for instance comprise an inorganic hole transporter selected from CuI, CuBr, CuSCN, Cu2O, CuO and CIS. Often, the layer of a hole transporting (p-type) material is a solid state inorganic hole transporting material comprising an oxide of nickel, vanadium, copper or molybdenum. The solid state inorganic hole transporting material is typically present as a compact layer. The solid state inorganic hole transporting material may for instance comprise nickel oxide. For instance, the optoelectronic device may comprise a compact layer of nickel oxide. The layer comprising a crystalline A / M / X material may for instance be disposed directly on the layer of a hole transporting (p-type) material, for instance the layer comprising solid state inorganic hole transporting material comprising nickel oxide, preferably a compact layer of nickel oxide. Typically, however, the p-type region comprises Me-4PACz. Often, the p-type region comprises Me-4PACz, and the n-type region comprises phenyl-C61-butyric acid methyl ester (PCBM) or C60. Often, the n-type region further comprises BCP. Thus, the n-type region may comprise PCBM and BCP, or C60and BCP. The device of the present invention comprises a passivated A / M / X material, which comprises a passivating agent and a crystalline A / M / X material. The crystalline A / M / X material is typically a semiconductor. The crystalline A / M / X material comprises a compound of formula: [A]a[M]b[X]c, wherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18. a is often a number from 1 to 4, b is often a number from 1 to 3, and c is often a number from 1 to 8. Each of a, b and c may or may not be an integer. For instance, a, b or c may not be an integer where the compound adopts a structure having vacancies such that the crystal lattice is not completely filled. It is simple to provide very good control over the stoichiometry of A / M / X materials during their production and thus easy to form structures where a, b or c is not an integer (for instance a structure having vacancies in one or more of the A, M or X sites) as well as structures in which a, b and c are all integers. Accordingly, in some embodiments, one or more of a, b and c is a non-integer value. For example, one of a, b and c may be a non-integer value. In one embodiment, a is a non-integer value. In another embodiment, b is a non-integer value. In yet another embodiment, c is a non-integer value. In other embodiments, each of a, b and c are integer values. Thus, in some embodiments, a is an integer from 1 to 6; b is an integer from 1 to 6; and c is an integer from 1 to 18. a is often an integer from 1 to 4, b is often an integer from 1 to 3, and c is often an integer from 1 to 8. In the compound of formula [A]a[M]b[X]c, generally: [A] comprises one or more A cations, which A cations may for instance be selected from alkali metal cations or organic monocations; [M] comprises one or more M cations which are metal or metalloid cations selected from Pd4+, W4+, Re4+, Os4+, Ir4+, Pt4+, Sn4+, Pb4+, Ge4+, Te4+, Bi3+, Sb3+, Ca2+, Sr2+, Cd2+, Cu2+, Ni2+, Mn2+, Fe2+, Co2+, Pd2+, Ge2+, Sn2+, Pb2+, Yb2+and Eu2+, preferably Sn2+, Pb2+, Cu2+, Ge2+, and Ni2+; particularly preferably Pb2+and Sn2+; [X] comprises one or more X anions selected from halide anions (e.g. Cl–, Br–, and I-), O2–, S2–, Se2–, and Te2–; a is a number from 1 to 4; b is a number from 1 to 3; and c is a number from 1 to 8. Preferably the compound of formula [A]a[M]b[X]ccomprises a perovskite. The compound of formula [A]a[M]b[X]coften comprises a metal halide perovskite, for instance an organic-inorganic metal halide perovskite. [M] cations [M] comprises one or more M cations which are metal or metalloid cations. [M] may comprise two or more different M cations. [M] may comprise one or more monocations, one or more dications, one or more trications or one or more tetracations. Typically, the one or more M cations are selected from Ca2+, Sr2+, Cd2+, Cu2+, Ni2+, Mn2+, Fe2+, Co2+, Pd2+, Ge2+, Sn2+, Pb2+, Yb2+, Eu2+, Bi3+, Sb3+, Pd4+, W4+, Re4+, Os4+, Ir4+, Pt4+, Sn4+, Pb4+, Ge4+or Te4+. Preferably, the one or more M cations are selected from Cu2+, Pb2+, Ge2+or Sn2+. Typically, [M] comprises one or more metal or metalloid dications. For instance, each M cation may be selected from Ca2+, Sr2+, Cd2+, Cu2+, Ni2+, Mn2+, Fe2+, Co2+, Pd2+, Ge2+, Sn2+, Pb2+, Yb2+and Eu2+, preferably Sn2+, Pb2+, Cu2+, Ge2+, and Ni2+; preferably Sn2+and Pb2+. In some embodiments, [M] comprises two different M cations, typically where said cations are Sn2+and Pb2+. [A] cations and [X] anions In general, said one or more A cations are monocations. [A] typically comprises one or more A cations which may be organic and / or inorganic monocations. For instance, [A] may comprise at least two A cations which may be organic and / or inorganic monocations. Thus, the compound of formula [A]a[M]b[X]cmay be a mixed cation perovskite. [A] may comprise at least one A cation which is an organic cation and at least one A cation which is an inorganic cation. [A] may comprise at least two A cations which are both organic cations. [A] may comprise at least two A cations which are both inorganic cations. Where an A species is an inorganic monocation, A is typically an alkali metal monocation (that is, a monocation of a metal found in Group 1 of the periodic table), for instance Li+, Na+, K+, Rb+, Cs+, for example Cs+or Rb+, for instance Cs+. Typically, [A] comprises at least one organic monocation. Where an A species is an organic monocation, A is typically an ammonium cation, for instance methylammonium, or an iminium cation, for instance formamidinium. Each A cation may be selected from: an alkali metal cation, for instance Li+, Na+, K+, Rb+, Cs+; a cation of the formula [R1R2R3R4N]+, wherein each of R1, R2, R3, R4is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12aryl, and at least one of R1, R2, R3and R4is not hydrogen; a cation of the formula [R5R6N=CH-NR7R8]+, wherein each of R5, R6, R7and R8is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12 aryl; and C1-10 alkylamammonium, C2-10 alkenylammonium, C1-10alkyliminium, C3-10cycloalkylammonium and C3-10cycloalkyliminium, each of which is unsubstituted or substituted with one or more substituents selected from amino, C1-6alkylamino, imino, C1-6alkylimino, C1-6alkyl, C2-6alkenyl, C3-6cycloalkyl and C6-12aryl. Preferably, each A cation is selected from Cs+, Rb+, methylammonium [(CH3NH3)+], ethylammonium [(CH3CH2NH3)+], propylammonium [(CH3CH2CH2NH3)+]. Butylammonium [(CH3CH2CH2CH2NH3)+], pentylammonium [(CH3CH2CH2CH2CH2NH3)+], hexylammonium [(CH3CH2CH2CH2CH2CH2NH3)+], heptylammonium [(CH3CH2CH2CH2CH2CH2CH2NH3)+], octylammonium [(CH3CH2CH2CH2CH2CH2CH2CH2NH3)+], tetramethylammonium [(N(CH3)4)+], formamidinium [(H2N-C(H)=NH2)+], 1-aminoethan-1-iminium [(H2N– C(CH3)=NH2)+] and guanidinium [(H2N-C(NH2)=NH2)+]. [A] usually comprises one, two or three A monocations. [A] may comprises a single cation selected from methylammonium [(CH3NH3)+], ethylammonium [(CH3CH2NH3)+], propylammonium [(CH3CH2CH2NH3)+], dimethylammonium [(CH3)2NH+], tetramethylammonium [(N(CH3)4)+], formamidinium [(H2N–C(H)=NH2)+], 1-aminoethan-1- iminium [(H2N–C(CH3)=NH2)+], guanidinium [(H2N-C(NH2)=NH2)+], Cs+and Rb+. For instance [A] may comprise a single cation that is methylammonium [(CH3NH3)+]. Alternatively, [A] may comprise two cations selected from this group, for instance Cs+and formamidinium [(H2N-C(H)=NH2)+], or for instance Cs+and Rb+, or for instance methylammonium [(CH3NH3)+] and formamidinium [(H2N-C(H)=NH2)+]. [X] comprises one or more X anions. Typically, [X] comprises one or more halide anions, i.e. an anion selected from F-, Br-, Cl- and I-. Typically, each X anion is a halide. [X] typically comprises one, two or three X anions and these are generally selected from Br-, Cl- and I-. X may comprise two more different X anions. Typically, [X] comprises two or more different halide anions. [X] may for instance consist of two X anions, such as Cl and Br, or Br and I, or Cl and I. Therefore, the compound of formula [A]a[M]b[X]coften comprises a mixed halide perovskite. When [A] comprises one or more organic cations, the compound of formula [A]a[M]b[X]cmay be an organic-inorganic metal halide perovskite. Typically, said one or more A cations are monocations, said one or more M cations are dications, and said one or more X anions are one or more halide anions. Often, [A] comprises at least two different A cations as described herein and [X] comprises at least two different X anions as described herein. In some embodiments, [A] comprises at least three different A cations as described herein and [X] comprises at least two different X anions as described herein. Compound of formula [A]a[M]b[X]c- further detail Typically, a = 1, b = 1 and c = 3. Thus, the compound of formula [A]a[M]b[X]cmay be a compound of formula [A][M][X]3, wherein [A], [M] and [X] are as described herein. Typically, the crystalline A / M / X material comprises: a perovskite of formula (I): [A][M][X]3(I) wherein: [A] comprises one or more A cations which are monocations; [M] comprises one or more M cations which are metal or metalloid dications; and [X] comprises one or more anions which are halide anions. In some embodiments, the perovskite of formula (I) comprises a single A cation, a single M cation and a single X cation. i.e., the perovskite is a perovskite of the formula (IA): AMX3(IA) wherein A, M and X are as defined above. In a preferred embodiment, A is selected from (CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N–C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+; M is Pb2+or Sn2+and X is selected from Br-, Cl- and I-. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IA) selected from APbI3, APbBr3, APbCl3, ASnI3, ASnBr3and ASnCl3, wherein A is a cation as described herein. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IA) selected from CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3, CH3NH3SnI3, CH3NH3SnBr3, CH3NH3SnCl3, CsPbI3, CsPbBr3, CsPbCl3, CsSnI3, CsSnBr3, CsSnCl3, (H2N–C(H)=NH2)PbI3, (H2N–C(H)=NH2)PbBr3, (H2N– C(H)=NH2)PbCl3, (H2N–C(H)=NH2)SnI3, (H2N–C(H)=NH2)SnBr3and (H2N– C(H)=NH2)SnCl3, in particular CH3NH3PbI3or CH3NH3PbBr3, preferably CH3NH3PbI3. In one embodiment, the perovskite is a perovskite of the formula (IB): [AIxAII1-x]MX3(IB) wherein AIand AIIare as defined above with respect to A, wherein M and X are as defined above and wherein x is greater than 0 and less than 1. In a preferred embodiment, AIand AIIare each selected from (CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N–C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+; M is Pb2+or Sn2+and X is selected from Br-, Cl- and I-. AIand AIImay for instance be (H2N–C(H)=NH2)+and Cs+respectively, or they may be (CH3NH3)+and (H2N-C(H)=NH2)+respectively. Alternatively, they may be Cs+and Rb+respectively. Preferably, AIand AIIare (H2N– C(H)=NH2)+and Cs+respectively. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IB) selected from (CsxRb1-x)PbBr3, (CsxRb1-x)PbCl3, (CsxRb1-x)PbI3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]PbCl3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]PbBr3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]PbI3, [(CH3NH3)xCs1-x]PbCl3, [(CH3NH3)xCs1-x]PbBr3, [(CH3NH3)xCs1-x]PbI3, [(H2N–C(H)=NH2)xCs1-x]PbCl3, [(H2N–C(H)=NH2)xCs1-x]PbBr3, [(H2N–C(H)=NH2)xCs1-x]PbI3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]SnCl3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]SnBr3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]SnI3, [(CH3NH3)xCs1-x]SnCl3, [(CH3NH3)xCs1-x]SnBr3, [(CH3NH3)xCs1-x]SnI3, [(H2N–C(H)=NH2)xCs1-x]SnCl3, [(H2N–C(H)=NH2)xCs1-x]SnBr3, and[(H2N–C(H)=NH2)xCs1-x]SnI3,where x is greater than 0 and less than 1, for instance x may be from 0.01 to 0.99 or from 0.05 to 0.95 or 0.1 to 0.9. In one embodiment, the perovskite is a perovskite compound of the formula (IC): AM[XIyXII1-y]3(IC) wherein A and M are as defined above, wherein XIand XIIare as defined above in relation to X and wherein y is greater than 0 and less than 1. In a preferred embodiment, A is selected from (CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N– C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+; M is Pb2+or Sn2+; and XIand XIIare each selected from Br-, Cl- and I-. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IC) selected from APb[BryI1-y]3, APb[BryCl1-y]3, APb[IyCl1-y]3, ASn[BryI1-y]3, ASn[BryCl1-y]3, ASn[IyCl1-y]3, where y is greater than 0 and less than 1, and wherein A is a cation as described herein. y may be from 0.01 to 0.99. For instance, y may be from 0.05 to 0.95 or 0.1 to 0.9. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IC) selected from CH3NH3Pb[BryI1-y]3, CH3NH3Pb[BryCl1-y]3, CH3NH3Pb[IyCl1-y]3, CH3NH3Sn[BryI1-y]3, CH3NH3Sn[BryCl1-y]3, CH3NH3Sn[IyCl1-y]3, CsPb[BryI1-y]3, CsPb[BryCl1-y]3, CsPb[IyCl1-y]3, CsSn[BryI1-y]3, CsSn[BryCl1-y]3, CsSn[IyCl1-y]3, (H2N–C(H)=NH2)Pb[BryI1-y]3, (H2N–C(H)=NH2)Pb[BryCl1-y]3, (H2N– C(H)=NH2)Pb[IyCl1-y]3, (H2N–C(H)=NH2)Sn[BryI1-y]3, (H2N–C(H)=NH2)Sn[BryCl1-y]3, and (H2N–C(H)=NH2)Sn[IyCl1-y]3, where y is greater than 0 and less than 1, for instance y may be from 0.01 to 0.99 or from 0.05 to 0.95 or 0.1 to 0.9. In a preferred embodiment, the perovskite is a perovskite of the formula (ID): [AIxAII1-x]M[XIyXII1-y]3(ID) wherein AIand AIIare as defined above with respect to A, M is as defined above, XIand XIIare as defined above in relation to X and wherein x and y are both greater than 0 and less than 1. In a preferred embodiment, AIand AIIare each selected from ((CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N–C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+, preferably AIand AIIare (H2N–C(H)=NH2)+and Cs+respectively; M is Pb2+or Sn2+;and XIand XIIare each selected from Br-, Cl- and I-. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (ID) selected from (CsxRb1-x)Pb(BryCl1-y)3, (CsxRb1-x)Pb(BryI1-y)3, and (CsxRb1-x)Pb(ClyI1-y)3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]Pb[BryI1-y]3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]Pb[BryCl1-y]3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]Pb[IyCl1-y]3, [(CH3NH3)xCs1-x]Pb[BryI1-y]3, [(CH3NH3)xCs1-x]Pb[BryCl1-y]3, [(CH3NH3)xCs1-x]Pb[IyCl1-y]3, [(H2N–C(H)=NH2)xCs1-x]Pb[BryI1-y]3, [(H2N–C(H)=NH2)xCs1-x]Pb[BryCl1-y]3, [(H2N– C(H)=NH2)xCs1-x]Pb[IyCl1-y]3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]Sn[BryI1-y]3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]Sn[BryCl1-y]3, [(CH3NH3)x(H2N–C(H)=NH2)1-x]Sn[IyCl1-y]3, [(CH3NH3)xCs1-x]Sn[BryI1-y]3, [(CH3NH3)xCs1-x]Sn[BryCl1-y]3, [(CH3NH3)xCs1-x]Sn[IyCl1-y]3, [(H2N–C(H)=NH2)xCs1-x]Sn[BryI1-y]3, [(H2N–C(H)=NH2)xCs1-x]Sn[BryCl1-y]3, and [(H2N– C(H)=NH2)xCs1-x]Sn[IyCl1-y]3, where x and y are both greater than 0 and less than 1, for instance x and y may both be from 0.01 to 0.99 or from 0.05 to 0.95 or 0.1 to 0.9. In one embodiment, the perovskite is a perovskite of the formula (IE): A[MIzMII1-z]X3(IE) wherein MIand MIIare as defined above with respect to M, A and X are as defined above, and wherein z is greater than 0 and less than 1. In a preferred embodiment, A is selected from (CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N– C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+; MIis Pb2+and MIIis Sn2+; and X is selected from Br-, Cl- and I-. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IE) selected from CH3NH3[PbzSn1-z]Cl3, CH3NH3[PbzSn1-z]Br3, CH3NH3[PbzSn1-z]I3, Cs[PbzSn1-z]Cl3, Cs[PbzSn1-z]Br3, Cs[PbzSn1-z]I3, (H2N– C(H)=NH2)[PbzSn1-z]Cl3, (H2N–C(H)=NH2)[PbzSn1-z]Br3, and (H2N–C(H)=NH2)[PbzSn1-z]I3, where z is greater than 0 and less than 1, for instance z may be from 0.01 to 0.99 or from 0.05 to 0.95 or 0.1 to 0.9. In one embodiment, the perovskite is a perovskite of the formula (IF): [AIxAII1-x][MIzMII1-z]X3 (IF) wherein AIand AIIare as defined above with respect to A, MIand MIIare as defined above with respect to M, and X is as defined above and wherein x and z are both greater than 0 and less than 1. In a preferred embodiment, AIand AIIare each selected from (CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N–C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+; MIis Pb2+and MIIis Sn2+; and X is selected from Br-, Cl- and I-. AIand AIImay for instance be (H2N–C(H)=NH2)+and Cs+respectively, or they may be (CH3NH3)+and (H2N-C(H)=NH2)+respectively. Alternatively, they may be Cs+and Rb+respectively. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IF) selected from [(CH3NH3)x(H2N–C(H)=NH2)1-x][PbzSn1-z]Cl3, [(CH3NH3)x(H2N–C(H)=NH2)1-x][PbzSn1-z]Br3, [(CH3NH3)x(H2N–C(H)=NH2)1-x][PbzSn1-z]I3, [(CH3NH3)xCs1-x][PbzSn1-z]Cl3, [(CH3NH3)xCs1-x][PbzSn1-z]Br3, [(CH3NH3)xCs1-x][PbzSn1-z]I3, [(H2N–C(H)=NH2)xCs1-x][PbzSn1-z]Cl3, [(H2N– C(H)=NH2)xCs1-x][PbzSn1-z]Br3, [(H2N–C(H)=NH2)xCs1-x][PbzSn1-z]I3, where x and z are both greater than 0 and less than 1, for instance x and z may each be from 0.01 to 0.99 or from 0.05 to 0.95 or 0.1 to 0.9. In one embodiment, the perovskite is a perovskite compound of the formula (IG): A[MIzMII1-z][XIyXII1-y]3(IG) wherein A is as defined above, MIand MIIare as defined above with respect to M, and wherein XIand XIIare as defined above in relation to X and wherein y and z are both greater than 0 and less than 1. In a preferred embodiment, A is selected from (CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N–C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+; MIis Pb2+and MIIis Sn2+; and XIand XIIare each selected from Br-, Cl- and I-. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IG) selected from A[PbzSn1-z][BryI1-y]3, A[PbzSn1-z][BryCl1-y]3, A[PbzSn1-z][IyCl1-y]3, where y and z are both greater than 0 and less than 1, and wherein A is a cation as described herein. y and z may each be from 0.01 to 0.99. For instance, y and z may each be from 0.05 to 0.95 or 0.1 to 0.9. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IG) selected from CH3NH3[PbzSn1-z][BryI1-y]3, CH3NH3[PbzSn1-z][BryCl1-y]3, CH3NH3[PbzSn1-z][IyCl1-y]3, Cs[PbzSn1-z][BryI1-y]3, Cs[PbzSn1-z][BryCl1-y]3, Cs[PbzSn1-z][IyCl1-y]3, (H2N–C(H)=NH2)[PbzSn1-z][BryI1-y]3, (H2N– C(H)=NH2)[PbzSn1-z][BryCl1-y]3, and (H2N–C(H)=NH2)[PbzSn1-z][IyCl1-y]3, where y and z are both greater than 0 and less than 1, for instance y and z may each be from 0.01 to 0.99 or from 0.05 to 0.95 or 0.1 to 0.9. In one preferred embodiment, the perovskite is a perovskite of the formula (IH): [AIxAII1-x][MIzMII1-z][XIyXII1-y]3 (IH) wherein AIand AIIare as defined above with respect to A, MIand MIIare as defined above with respect to M, XIand XIIare as defined above in relation to X and wherein x, y and z are each greater than 0 and less than 1. In a preferred embodiment, AIand AIIare each selected from ((CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (H2N– C(H)=NH2)+, (H2N–C(CH3)=NH2)+, (H2N–C(NH2)=NH2)+, Cs+and Rb+; MIis Pb2+and MIIis Sn2+; and XIand XIIare each selected from Br-, Cl- and I-. For instance, the crystalline A / M / X material may comprise, or consist essentially of, a perovskite compound of formula (IH) selected from [(CH3NH3)x(H2N–C(H)=NH2)1-x][PbzSn1-z][BryI1-y]3, [(CH3NH3)x(H2N–C(H)=NH2)1-x][PbzSn1-z][BryCl1-y]3, (CH3NH3)x(H2N–C(H)=NH2)1-x][PbzSn1-z][IyCl1-y]3, [(CH3NH3)xCs1-x][PbzSn1-z][BryI1-y]3, [(CH3NH3)xCs1-x][PbzSn1-z][BryCl1-y]3, [(CH3NH3)xCs1-x][PbzSn1-z][IyCl1-y]3, [(H2N– C(H)=NH2)xCs1-x][PbzSn1-z][BryI1-y]3, [(H2N–C(H)=NH2)xCs1-x][PbzSn1-z][BryCl1-y]3, and [(H2N–C(H)=NH2)xCs1-x][PbzSn1-z][IyCl1-y]3, where x, y and z are each greater than 0 and less than 1, for instance x, y and z may each be from 0.01 to 0.99 or from 0.05 to 0.95 or 0.1 to 0.9. In one embodiment, a = 2, b = 1 and c = 4. In that embodiment, the crystalline A / M / X material comprises a compound (a “2D layered perovskite”) of formula (II): [A]2[M][X]4(II) wherein: [A] comprises one or more A cations which are monocations; [M] comprises one or more M cations which are metal or metalloid dications; and [X] comprises one or more X anions which are halide anions. In this embodiment, the A and M cations, and the X anions, are as defined above. In another embodiment, a = 2, b = 1 and c = 6. In that embodiment, the crystalline A / M / X material may in that case comprise a hexahalometallate of formula (III): [A]2[M][X]6(III) wherein: [A] comprises one or more A cations which are monocations; [M] comprises one or more M cations which are metal or metalloid tetracations; and [X] comprises one or more X anions which are halide anions. The hexahalometallate of formula (III) may in a preferred embodiment be a mixed monocation hexahalometallate. In a mixed monocation hexahalometallate, [A] comprises at least two A cations which are monocations; [M] comprises at least one M cation which is a metal or metalloid tetracation (and typically [M] comprises a single M cation which is a metal or metalloid tetracation); and [X] comprises at least one X anion which is a halide anion (and typically [X] comprises a single halide anion or two types of halide anion). In a mixed metal hexahalometallate, [A] comprises at least one monocation (and typically [A] is a single monocation or two types of monocation); [M] comprises at least two metal or metalloid tetracations (for instance Ge4+and Sn4+); and [X] comprises at least one halide anion (and typically [X] is a single halide anion or two types of halide anion). In a mixed halide hexahalometallate, [A] comprises at least one monocation (and typically [A] is a single monocation or two types of monocation); [M] comprises at least one metal or metalloid tetracation (and typically [M] is a single metal tetra cation); and [X] comprises at least two halide anions, for instance Br–and Cl- or Br- and I-. [A] may comprise at least one A monocation selected from any suitable monocations, such as those described herein for a perovskite. In the case of a hexahalometallate, each A cation is typically selected from Li+, Na+, K+, Rb+, Cs+, NH4+and monovalent organic cations. Monovalent organic cations are singly positively charged organic cations, which may, for instance, have a molecular weight of no greater than 500 g / mol. For instance, [A] may be a single A cation which is selected from Li+, Na+, K+, Rb+, Cs+, NH4+and monovalent organic cations. [A] preferably comprises at least one A cation which is a monocation selected from Rb+, Cs+, NH4+and monovalent organic cations. For instance, [A] may be a single inorganic A monocation selected from Li+, Na+, K+, Rb+, Cs+and NH4+. In another embodiment, [A] may be at least one monovalent organic A cation. For instance, [A] may be a single monovalent organic A cation. In one embodiment, [A] is (CH3NH3)+. In another embodiment, [A] is (H2N–C(H)=NH2)+. Preferably, [A] comprises two or more types of A cation. [A] may be a single A monocation, or indeed two A monocations, each of which is independently selected from K+, Rb+, Cs+, NH4+, (CH3NH3)+, (CH3CH2NH3)+, (CH3CH2CH2NH3)+, (N(CH3)4)+, (N(CH2CH3)4)+, (N(CH2CH2CH3)4)+, (H2N–C(H)=NH2)+and (H2N–C(CH3)=NH2)+. [M] may comprise one or more M cations which are selected from suitable metal or metalloid tetracations. Metals include elements of groups 3 to 12 of the Periodic Table of the Elements and Ga, In, Tl, Sn, Pb, Bi and Po. Metalloids include Si, Ge, As, Sb, and Te. For instance, [M] may comprise at least one M cation which is a metal or metalloid tetracation selected from Ti4+, V4+, Mn4+, Fe4+, Co4+, Zr4+, Nb4+, Mo4+, Ru4+, Rh4+, Pd4+, Hf4+, Ta4+, W4+, Re4+, Os4+, Ir4+, Pt4+, Sn4+, Pb4+, Po4+, Si4+, Ge4+, and Te4+. Typically, [M] comprises at least one metal or metalloid tetracation selected from Pd4+, W4+, Re4+, Os4+, Ir4+, Pt4+, Sn4+, Pb4+, Ge4+, and Te4+. For instance, [M] may be a single metal or metalloid tetracation selected from Pd4+, W4+, Re4+, Os4+, Ir4+, Pt4+, Sn4+, Pb4+, Ge4+, and Te4+. Typically, [M] comprises at least one M cation which is a metal or metalloid tetracation selected from Sn4+, Te4+, Ge4+and Re4+. In one embodiment [M] comprises at least one M cation which is a metal or metalloid tetracation selected from Pb4+, Sn4+, Te4+, Ge4+and Re4+. For instance, [M] may comprise an M cation which is at least one metal or metalloid tetracation selected from Pb4+, Sn4+, Te4+and Ge4+. Preferably, [M] comprises at least one metal or metalloid tetracation selected from Sn4+, Te4+, and Ge4+. As discussed above, the hexahalometallate compound may be a mixed-metal or a single-metal hexahalometallate. Preferably, the hexahalometallate compound is a single-metal hexahalometallate compound. More preferably, [M] is a single metal or metalloid tetracation selected from Sn4+, Te4+, and Ge4+. For instance, [M] may be a single metal or metalloid tetracation which is Te4+. For instance, [M] may be a single metal or metalloid tetracation which is Ge4+. Most preferably, [M] is a single metal or metalloid tetracation which is Sn4+. [X] may comprise at least one X anion which is a halide anion. [X] therefore comprises at least one halide anion selected from F–, Cl–, Br–and I–. Typically, [X] comprises at least one halide anion selected from Cl–, Br–and I–. The hexahalometallate compound may be a mixed-halide hexahalometallate or a single-halide hexahalometallate. If the hexahalometallate is mixed, [X] comprises two, three or four halide anions selected from F–, Cl–, Br–and I–. Typically, in a mixed-halide compound, [X] comprises two halide anions selected from F–, Cl–, Br–and I–. In some embodiments, [A] is a single monocation and [M] is a single metal or metalloid tetracation. Thus, the crystalline A / M / X material may, for instance, comprise a hexahalometallate compound of formula (IIIA) A2M[X]6 (IIIA) wherein: A is a monocation; M is a metal or metalloid tetracation; and [X] is at least one halide anion. [X] may be one, two or three halide anions selected from F–, Cl–, Br–and I–, and preferably selected from Cl–, Br–and I–. In formula (IIIA), [X] is preferably one or two halide anions selected from Cl–, Br–and I–. The crystalline A / M / X material may, for instance, comprise, or consist essentially of, a hexahalometallate compound of formula (IIIB) A2MX6-yX′y(IIIB) wherein: A is a monocation (i.e. the second cation); M is a metal or metalloid tetracation (i.e. the first cation); X and X′ are each independently a (different) halide anion (i.e. two second anions); and y is from 0 to 6. When y is 0 or 6, the hexahalometallate compound is a single-halide compound. When y is from 0.01 to 5.99 the compound is a mixed-halide hexahalometallate compound. When the compound is a mixed-halide compound, y may be from 0.05 to 5.95. For instance, y may be from 1.00 to 5.00. The hexahalometallate compound may, for instance, be A2SnF6-yCly, A2SnF6-yBry, A2SnF6-yIy, A2SnCl6-yBry, A2SnCl6-yIy, A2SnBr6-yIy, A2TeF6-yCly, A2TeF6-yBry, A2TeF6-yIy, A2TeCl6-yBry, A2TeCl6-yIy, A2TeBr6-yIy, A2GeF6-yCly, A2GeF6-yBry, A2GeF6-yIy, A2GeCl6-yBry, A2GeCl6-yIy, A2GeBr6-yIy, A2ReF6-yCly, A2ReF6-yBry, A2ReF6-yIy, A2ReCl6-yBry, A2ReCl6-yIyor A2ReBr6-yIy, wherein: A is K+, Rb+, Cs+, (R1NH3)+, (NR24)+, or (H2N–C(R1)=NH2)+, wherein R1is H, a substituted or unsubstituted C1-20alkyl group or a substituted or unsubstituted aryl group, and R2is a substituted or unsubstituted C1-10alkyl group; and y is from 0 to 6. Optionally, y is from 0.01 to 5.99. If the hexahalometallate compound is a mixed-halide compound, y is typically from 1.00 to 5.00. A may be as defined above. For instance, A may be Cs+, NH4+, (CH3NH3)+, (CH3CH2NH3)+, (N(CH3)4)+, (N(CH2CH3)4)+, (H2N–C(H)=NH2)+or (H2N–C(CH3)=NH2)+, for instance Cs+, NH4+, or (CH3NH3)+. The hexahalometallate compound may typically be A2SnF6-yCly, A2SnF6-yBry, A2SnF6-yIy, A2SnCl6-yBry, A2SnCl6-yIy, or A2SnBr6-yIy, wherein: A is K+, Rb+, Cs+, (R1NH3)+, (NR24)+, or (H2N–C(R1)=NH2)+, or A is as defined herein, wherein R1is H, a substituted or unsubstituted C1-20alkyl group or a substituted or unsubstituted aryl group, or R2is a substituted or unsubstituted C1-10 alkyl group; and y is from 0 to 6. In another embodiment, the hexahalometallate compound is A2GeF6-yCly, A2GeF6-yBry, A2GeF6-yIy, A2GeCl6-yBry, A2GeCl6-yIy, or A2GeBr6-yIy, wherein: A is K+, Rb+, Cs+, (R1NH3)+, (NR24)+, or (H2N–C(R1)=NH2)+, or A is as defined herein, wherein R1is H, a substituted or unsubstituted C1-20alkyl group or a substituted or unsubstituted aryl group, and R2is a substituted or unsubstituted C1-10alkyl group; and y is from 0 to 6. The hexahalometallate compound may, for instance, be A2TeF6-yCly, A2TeF6-yBry, A2TeF6-yIy, A2TeCl6-yBry, A2TeCl6-yIy, or A2TeBr6-yIy, wherein: A is K+, Rb+, Cs+, (R1NH3)+, (NR24)+, or (H2N–C(R1)=NH2)+, or A is as defined herein, wherein R1is H, a substituted or unsubstituted C1-20alkyl group or a substituted or unsubstituted aryl group, and R2is a substituted or unsubstituted C1-10 alkyl group; and y is from 0 to 6 or y is as defined herein. Often, y will be from 1.50 to 2.50. For instance, y may be from 1.80 to 2.20. This may occur if the compound is produced using two equivalents of AX′ and one equivalent of MX4, as discussed below. In some embodiments, all of the ions are single anions or cations. Thus, the crystalline A / M / X material may comprise, or consist essentially of, a hexahalometallate compound of formula (IIIC) A2MX6(IIIC) wherein: A is a monocation; M is a metal or metalloid tetracation; and X is a halide anion. A, M and X may be as defined herein. The hexahalometallate compound may be A2SnF6, A2SnCl6, A2SnBr6, A2SnI6, A2TeF6, A2TeCl6, A2TeBr6, A2TeI6, A2GeF6, A2GeCl6, A2GeBr6, A2GeI6, A2ReF6, A2ReCl6, A2ReBr6or A2ReI6, wherein: A is K+, Rb+, Cs+, (R1NH3)+, (NR24)+, or (H2N–C(R1)=NH2)+, wherein R1is H, a substituted or unsubstituted C1-20alkyl group or a substituted or unsubstituted aryl group, and R2is a substituted or unsubstituted C1-10alkyl group. A may be as defined herein. Preferably, the hexahalometallate compound is Cs2SnI6, Cs2SnBr6, Cs2SnBr6-yIy, Cs2SnCl6-yIy, Cs2SnCl6-yBry, (CH3NH3)2SnI6, (CH3NH3)2SnBr6, (CH3NH3)2SnBr6-yIy, (CH3NH3)2SnCl6-yIy, (CH3NH3)2SnCl6-yBry, (H2N–C(H)=NH2)2SnI6, (H2N– C(H)=NH2)2SnBr6, (H2N–C(H)=NH2)2SnBr6-yIy, (H2N–C(H)=NH2)2SnCl6-yIyor (H2N– C(H)=NH2)2SnCl6-yBrywherein y is from 0.01 to 5.99. For example, the hexahalometallate compound may be (CH3NH3)2SnI6, (CH3NH3)2SnBr6, (CH3NH3)2SnCl6, (H2N– C(H)=NH2)2SnI6, (H2N–C(H)=NH2)2SnBr6or (H2N–C(H)=NH2)2SnCl6. The hexahalometallate compound may be Cs2SnI6, Cs2SnBr6, Cs2SnCl6-yBry, (CH3NH3)2SnI6, (CH3NH3)2SnBr6, or (H2N–C(H)=NH2)2SnI6. The crystalline A / M / X material may comprise a bismuth or antimony halogenometallate. For instance, the crystalline A / M / X material may comprise a halogenometallate compound comprising: (i) one or more monocations ([A]) or one or more dications ([B]); (ii) one or more metal or metalloid trications ([M]); and (iii) one or more halide anions ([X]). The compound may be a compound of formula BBiX5, B2BiX7or B3BiX9where B is (H3NCH2NH3)2+, (H3N(CH2)2NH3)2+, (H3N(CH2)3NH3)2+, (H3N(CH2)4NH3)2+, (H3N(CH2)5NH3)2+, (H3N(CH2)6NH3)2+, (H3N(CH2)7NH3)2+, (H3N(CH2)8NH3)2+or (H3N–C6H4–NH3)2+and X is I–, Br–or Cl–, preferably I–. In yet further embodiments, the crystalline A / M / X materials may be double perovskites. Such compounds are defined in WO 2017 / 037448, the entire contents of which is incorporated herein by reference. Typically, the compound is a double perovskite compound of formula (IV): [A]2[B+][B3+][X]6(IV); wherein: [A] comprises one or more A cations which are monocations, as defined herein; [B+] and [B3+] are equivalent to [M] where M comprises one or more M cations which are monocations and one or more M cations which are trications; and [X] comprises one or more X anions which are halide anions. The one or more M cations which are monocations comprised in [B+] are typically selected from metal and metalloid monocations. Preferably, the one or more M cations which are monocations are selected from Li+, Na+, K+, Rb+, Cs+, Cu+, Ag+, Au+and Hg+. More preferably, the one or more M cations which are monocations are selected from Cu+, Ag+and Au+. Most preferably, the one or more M cations which are monocations are selected from Ag+and Au+. For instance, [B+] may be one monocation which is Ag+or [B+] may be one monocation which is Au+. The one or more M cations which are trications comprised in [B3+] are typically selected from metal and metalloid trications. Preferably, the one or more M cations which are trications are selected from Bi3+, Sb3+, Cr3+, Fe3+, Co3+, Ga3+, As3+, Ru3+, Rh3+, In3+, Ir3+and Au3+. More preferably, the one or more M cations which are trications are selected from Bi3+and Sb3+. For instance, [B3+] may be one trication which is Bi3+or [B3+] may be one trication which is Sb3+. Bismuth has relatively low toxicity compared with heavy metals such as lead. In some embodiments, the one or more M cations which are monocations (in [B+]) are selected from Cu+, Ag+and Au+and the one or more M cations which are trications (in [B3+]) are selected from Bi3+and Sb3+. An exemplary double perovskite is Cs2BiAgBr6. Typically, where the compound is a double perovskite it is a compound of formula (IVa): A2B+B3+[X]6(IVa); wherein: the A cation is as defined herein; B+is an M cation which is a monocation as defined herein; B3+is an M cation which is a trication as defined herein; and [X] comprises one or more X anions which are halide anions, for instance two or more halide anions, preferably a single halide anion. In yet another embodiment, the compound may be a layered double perovskite compound of formula (V): [A]4[B+][B3+][X]8(V); wherein: [A], [B+], [B3+] and [X] are as defined above. In some embodiments, the layered double perovskite compound is a double perovskite compound of formula (Va): A4B+B3+[X]8(Va); wherein: the A cation is as defined herein; B+is an M cation which is a monocation as defined herein; B3+is an M cation which is a trication as defined herein; and [X] comprises one or more X anions which are halide anions, for instance two or more halide anions, preferably a single halide anion or two kinds of halide anion. In yet another embodiment, the compound may be a compound of formula (VI): [A]4[M][X]6(VI); wherein: [A], [M] and [X] are as defined above (in relation to, for instance, compounds of formula (I) or (II)). However, preferably the compound is not a compound of formula (VI). Where the compound is a compound of formula (VI), the compound may preferably be a compound of formula (VIA) [AIAII]4[M][X]6(VIA); that is, a compound wherein [A] comprises two types of A monocation. In other preferred embodiments, the compound of formula (VI) may be a compound of formula (VIB): [A]4[M][XIXII]6 (VIB); that is, a compound of formula (VI) wherein [X] comprises two types of X anion. In yet other preferred embodiments, the compound of formula (VI) may be a compound of formula (VIC): [AIAII]4[M][XIXII]6(VIC); that is, a compound of formula (VI) wherein [A] comprises two types of A monocation and [X] comprises two types of X anion. In formulae (VIa), (VIb) and (VIc), each of : [A], [M] and [X] are as defined above (in relation to, for instance, compounds of formula (I) or (II)). In another embodiment, a = 1, b = 1 and c = 4. In that embodiment, the crystalline A / M / X material may in that case comprise a compound of formula (VII): [A][M][X]4(VII) wherein: [A] comprises one or more A cations which are monocations; [M] comprises one or more M cations which are metal or metalloid trications; and [X] comprises one or more X anions which are halide anions. The A monocations and M trications are as defined herein. An exemplary compound of formula (VII) is AgBiI4. It should be understood that the invention also encompasses processes for producing variants of the above-described structures (I), (II), (III), (IV), (V), (VI) and (VII) where one or more of the relevant a, b and c values are non-integer values. Preferably, the compound of formula [A]a[M]b[X]cis a compound of formula [A][M][X]3, a compound of formula [A]4[M][X]6or a compound of formula [A]2[M][X]6. For example, in preferred embodiments the compound of formula [A]a[M]b[X]cis a compound of formula (I), for instance a compound of formula (IA), (IB), (IC), (ID), (IE), (IF), (IG), (IH), (IIIA), or a compound of formula (IIIB),(IIIC), (VIA), (VIB), or (VIC). Generally, the compound of formula [A]a[M]b[X]cis a compound of formula (I), for instance a compound of formula (IA), (IB), (IC), (ID), (IE), (IF), (IG) or (IH). In some embodiments, the compound of formula [A]a[M]b[X]cis a compound wherein [A] comprises two or more different A cations. For examples, [A] may contain two types of cation or three types of A cation. In some embodiments, the compound of formula [A]a[M]b[X]cis a compound wherein [X] comprises two or more different X anions. For example, [X] may contain two types of anion, e.g. halide anions. In some embodiments, the compound of formula [A]a[M]b[X]cis a compound wherein [M] comprises two or more different M cations. For example, [X] may contain two types of anion, e.g. Sn2+and Pb2+. In one aspect of each of these embodiments, the compound of formula [A]a[M]b[X]cis a compound wherein [A] comprises two or more different A cations and wherein [X] comprises two or more different X anions. For example, [A] may contain two types of A cation and [X] may contain two types of X anion (e.g. two types of halide anion). [A] may contain three types of A cation and [X] may contain two types of X anion (e.g. two types of halide anion). In one aspect of each of these embodiments, the compound of formula [A]a[M]b[X]cis a compound wherein [A] comprises two or more different A cations and wherein [M] comprises two or more different M cations. For example, [A] may contain two types of A cation and [M] may contain two types of M cation (e.g. Sn2+and Pb2+). In one aspect of each of these embodiments, the compound of formula [A]a[M]b[X]cis a compound wherein [X] comprises two or more different X anions and wherein [M] comprises two or more different M cations. For example, [X] may contain two types of X anion (e.g. two types of halide anion) and [M] may contain two types of M cation (e.g. Sn2+and Pb2+). In one aspect of each of these embodiments, the compound of formula [A]a[M]b[X]cis a compound wherein [A] comprises two or more different A cations and wherein [X] comprises two or more different X anions and wherein [M] comprises two or more different M cations. For example, [A] may contain two types of A cation, [X] may contain two types of X anion (e.g. two types of halide anion) and [M] may contain two types of M cation (e.g. Sn2+and Pb2+). Typically, the compound of formula [A]a[M]b[X]c is a compound of formula [A][M][X]3, wherein [A], [M] and [X] are as defined above. Typically, each A cation is selected from: an alkali metal cation; a cation of the formula [R1R2R3R4N]+, wherein each of R1, R2, R3, R4is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12aryl, and at least one of R1, R2, R3and R4is not hydrogen; a cation of the formula [R5R6N=CH- NR7R8]+, wherein each of R5, R6, R7and R8is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12aryl; and C1-10alkylamammonium, C2-10alkenylammonium, C1-10alkyliminium, C3-10cycloalkylammonium and C3-10cycloalkyliminium, each of which is unsubstituted or substituted with one or more substituents selected from amino, C1-6alkylamino, imino, C1-6alkylimino, C1-6alkyl, C2-6alkenyl, C3-6cycloalkyl and C6-12aryl. Typically, each A cation is selected from Cs+, Rb+, formamidinium, guanidinium, methylammonium, ethylammonium, propylammonium. butylammonium, pentylammonium, hexylammonium, septylammonium and octylammonium, and is preferably selected from Cs+, Rb+, formamidinium, guanidinium, methylammonium, and ethylammonium. Typically, [X] comprises two or more different X anions. Typically, the two or more different X anions are two or more different halide anions. Preferably, [X] comprises Br and I. Typically, [A] comprises formamidinium. Often, [A] further comprises Cs+. Alternatively, [A] may consist essentially of formamidinium. Alternatively, [A] may consist of formamidinium. Typically, [A] does not comprise methylammonium. Alternatively, [A] consists of methylammonium and at least one A cation other than methylammonium, provided that the molar fraction of methylammonium in [A] is less than 15% of [A]. Typically, [M] comprises an M cation selected from Ca2+, Sr2+, Cd2+, Cu2+, Ni2+, Mn2+, Fe2+, Co2+, Pd2+, Ge2+, Sn2+, Pb2+, Yb2+and Eu2+. Typically, [M] comprises an M cation selected from Sn2+, Pb2+, Cu2+, Ge2+, and Ni2+; preferably selected from Sn2+and Pb2+. Preferably wherein [M] comprises Pb2+. Typically, the compound of formula [A]a[M]b[X]cis a compound of formula [Csx(H2N–C(H)=NH2)1-x]Pb[IyBr1-y]3 wherein x is greater than 0 and less than 1, and y is greater than 0 and less than 1. Typically, x is from about 0.05 to about 0.5, for example from about 0.08 to about 0.3, or from about 0.1 to about 0.2. For example, x may be 0.13, 0.15 or 0.17. Typically, y is from about 0.3 to about 0.99, for example from about 0.4 to about 0.95, or from about 0.50 to about 0.92. For example, y may be 0.9, or 0.6, or 0.77. The compound of formula [A]a[M]b[X]cmay for instance comprise Cs0.13FA0.87Pb(I0.9Br0.1)3, Cs0.15FA0.85Pb(I0.6Br0.4)3, or Cs0.17FA0.83Pb(I0.77Br0.23)3, where FA is the formamidinium cation, (H2N–C(H)=NH2)+. The photovoltaic device may for instance comprise an n-type region comprising at least one n-type layer; a p-type region comprising at least one p-type layer; and, disposed between the n-type region and the p-type region: a layer comprising said passivated A / M / X material. The photovoltaic device may for instance comprise a photoactive region, wherein the photoactive region comprises the n-type region, the p-type region, and, disposed between the n-type region and the p-type region: said layer comprising the passivated A / M / X material. The layer comprising said passivated A / M / X material may consist essentially of, or consist of, the passivated A / M / X material. The layer comprising said passivated A / M / X material may for instance comprise (or consist essentially of, or consist of) a thin film of the passivated A / M / X material. As the skilled person would understand, an n-type layer is a layer of an electron-transporting (i.e. an n-type) material, and a p-type layer is a layer of a hole-transporting (i.e. a p-type) material. The wide range of n-type and p-type materials are known to the skilled person, for instance as described in WO 2020 / 012195, the entire contents of which are incorporated herein by reference. The n-type layer may for instance comprise titanium dioxide, phenyl-C61-butyric acid methyl ester (PCBM) or C60, and the p- type layer may for instance comprise spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-p- methoxyphenylamine)9,9’-spirobifluorene) or Me-4PACz. The n-type layer may comprise BCP and either C60or phenyl-C61-butyric acid methyl ester (PCBM). The photovoltaic device may further comprise a first electrode and a second electrode. The first electrode may be in contact with the n-type region. The second electrode may be in contact with the p-type region. Typically, the photoactive region is disposed between the first electrode and the second electrode. The first and second electrodes may comprise any suitable electrically conductive material. The first electrode typically comprises a transparent conducting oxide, for instance FTO, ITO, or AZO, preferably ITO. The second electrode typically comprises one or more metals, for instance one or more metals selected from silver, gold, copper, aluminium, platinum, palladium, chromium or tungsten. Typically, the second electrode comprises gold and chromium. Each electrode may form a single layer or may be patterned. The photovoltaic device may be a solar cell. The photovoltaic device may be a single-junction photovoltaic device, for instance a single-junction solar cell. Alternatively, the photovoltaic device may be a tandem-junction or a multi-junction photovoltaic device, for instance a tandem-junction or a multi-junction solar cell. Accordingly, the photovoltaic device may comprise a first electrode, a second electrode, and, disposed between the first and second electrodes: a photoactive region as defined above (i.e. comprising said n-type region, said p-type region, and, disposed between the n-type region and the p-type region: said layer comprising the passivated A / M / X material); and at least one other photoactive region. The other photoactive region or regions may (each independently) be the same as or different from the photoactive region defined hereinbefore. The at least one other photoactive region may be at least one other photoactive region from photoactive regions used in conventional and known optoelectronic and photovoltaic devices. For instance, the at least one other photoactive region may comprise, or be, a photoactive region from a crystalline silicon photovoltaic cell or a photoactive region from a conventional thin film gallium arsenide, CIGS, CIS or CZTSSe photovoltaic device. The at least one other photoactive region may comprise, or be, a photoactive region which comprises a layer of a semiconductor. The semiconductor may for instance comprise silicon, for instance crystalline silicon. The semiconductor may for instance comprise a metal sulphide semiconductor or a metal selenide semiconductor, for instance copper zinc tin sulphide, copper zinc tin selenide, copper zinc tin selenide sulphide, copper indium gallium selenide, copper indium gallium diselenide or copper indium selenide. Use of passivated A / M / X materials The present invention also provides the use of a passivated A / M / X material as a sensitizer in a photovoltaic device, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound. The passivated A / M / X material may be a passivated A / M / X material as further defined anywhere herein. Processes for producing passivated A / M / X materials The present invention also provides processes for producing a passivated A / M / X material, wherein the process further comprises: producing a photovoltaic device comprising the passivated A / M / X material or using the passivated A / M / X material as a sensitizer in a photovoltaic device. Accordingly, the present invention provides a process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (i) treating said crystalline A / M / X material with said passivating agent; or (ii) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: producing a photovoltaic device comprising the passivated A / M / X material. The present invention also provides a process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (i) treating said crystalline A / M / X material with said passivating agent; or (ii) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: using the passivated A / M / X material as a sensitizer in a photovoltaic device. The passivated A / M / X material that is produced in these processes may be a passivated A / M / X material as defined anywhere herein. In the processes described herein, the passivated A / M / X material may be in the form of a layer. The process may comprise treating a layer comprising said crystalline A / M / X material with said passivating agent, wherein the layer comprising said crystalline A / M / X material is disposed on a substrate. The layer comprising said crystalline A / M / X material may in the first place be produced by any suitable process for producing a layer (e.g. a thin film) of a crystalline A / M / X material on a substrate. Processes for producing a layer of a crystalline A / M / X material comprising an [A]a[M]b[X]ccompound on a substrate are known in the art, and are described in, for instance, WO 2020 / 109787, WO 2020 / 012193 and WO 2020 / 012195. Such processes typically comprise exposing the substrate to the one or more A cations, the one or more M cations, and the one or more X anions of the [A]a[M]b[X]ccompound to be produced. Generally, the cations and anions are present in relative amounts that correspond to the stoichiometry of the same cations and anions in the [A]a[M]b[X]ccompound to be produced. The one or more A cations, the one or more M cations, and the one or more X anions may be present in one or more different phases selected from vapour phases and solution phases. For instance, such processes often comprise disposing a film- forming solution on the substrate, wherein the film-forming solution comprises a solvent, the one or more A cations, the one or more M cations, and the one or more X anions. Alternatively a plurality of solutions may be employed that together comprise the cations and anions in question. The disposing here may be via spin-coating or any other suitable solution deposition method. This is typically followed by thermal annealing, for instance at a temperature of from 50 to 150 ℃, e.g. at about 100 ℃. Alternatively, such processes typically comprise exposing the substrate to: a vapour phase which comprises, or a plurality of vapour phases which together comprise, the one or more A cations, the one or more M cations, and the one or more X anions. This may be followed by thermal annealing, for instance at a temperature of from 50 to 150 ℃, e.g. at about 100 ℃. As mentioned above the processes of the invention may comprise (i) treating said crystalline A / M / X material (which may optionally be in the form of a layer) with said passivating agent. The step of treating said crystalline A / M / X material with said passivating agent may comprise vapourising said passivating agent to produce the passivating agent in the vapour phase and treating said crystalline A / M / X material with said passivating agent in the vapour phase. Vapourising said passivating agent may be achieved by heating the passivating agent, for instance at a temperature of from 50 to 150 ℃, e.g. at about 100 ℃. The crystalline A / M / X material may be exposed to the resulting vapour, for instance for a period of time which may for instance be from 5 seconds to 5 minutes, for instance from 10 seconds to 2 minutes. Alternatively, the step of treating said crystalline A / M / X material with said passivating agent may comprise preparing a solution of said passivating agent, and treating the crystalline A / M / X material with said solution of the passivating agent. The treating here may be via spin-coating or any other suitable solution deposition method. Alternatively, the process may comprise (ii) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent. For instance, the process may comprise producing, on a substrate, a layer comprising said crystalline A / M / X material in the presence of said passivating agent, said producing comprising exposing the substrate to said one or more A cations, said one or more M cations, said one or more X anions and said passivating agent. The one or more A cations, the one or more M cations, the one or more X anions, and said passivating agent may be present in one or more different phases selected from vapour phases and solution phases. For instance, the one or more A cations, the one or more M cations, the one or more X anions, said passivating agent, and a solvent, may be present in a film-forming solution and the process may comprise disposing the film-forming solution on the substrate. Alternatively a plurality of solutions may be employed that together comprise the cations and anions in question and said passivating agent, and the process may comprise disposing each of said solutions on the substrate. The disposing here may be via spin-coating or any other suitable solution deposition method. This may be followed by thermal annealing, for instance at a temperature of from 50 to 150 ℃, e.g. at about 100 ℃. Alternatively, the process may comprise exposing the substrate to: a vapour phase which comprises, or a plurality of vapour phases which together comprise, the one or more A cations, the one or more M cations, the one or more X anions and said passivating agent. This may be followed by thermal annealing, for instance at a temperature of from 50 to 150 ℃, e.g. at about 100 ℃. The passivating agent employed in the processes of the present invention comprises, or consists essentially of, or consists of, an organic compound which comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine. The organic compound may be as further defined anywhere herein. Such organic compounds, which are employed herein as passivating agents, are commercially available and / or may be synthesised by the skilled person using routine methods. Dimers and polymers of such organic compounds as defined above may also be readily produced by routine methods. Such dimers and polymers (e.g. of formula Ia, IIa, Ic or IIc) may be produced in advance of the abovementioned process for producing the passivated A / M / X material, or they may be formed in situ, from the corresponding monomeric organic compounds (e.g. of formula I or II), during the process for producing the passivated A / M / X material. In the processes of the invention, the abovementioned substrate may comprise a first charge-transporting region comprising at least one layer of a first charge-transporting material. The substrate may further comprise a first electrode. The processes of the invention may further comprise disposing, on the layer comprising said crystalline A / M / X material: a second charge-transporting region comprising at least one layer of a second charge-transporting material. The process may further comprise disposing on the second charge-transporting region: a second electrode. Typically, the first charge-transporting region is a hole-transporting region and the first charge-transporting material is a hole-transporting material, and the second charge-transporting region is an electron-transporting region and the second charge-transporting material is an electron- transporting material. Alternatively, the first charge-transporting region may be an electron- transporting region and the first charge-transporting material is an electron-transporting material and the second charge-transporting region is a hole-transporting region and the second charge-transporting material is a hole-transporting material. The hole transporting (p-type) and the electron transporting (n-type) materials may be as further defined above in relation to the device of the invention, as may the first and second electrodes. Significant advantages are realised for devices, such as photovoltaic devices, that comprise passivated A / M / X materials as described herein. These advantages include (for example) improved device efficiency and stability. The invention will be described further in the Examples which follow. EXAMPLES 1. Methods Precursor material preparation Lead iodide (PbI2, 99.99%), lead bromide (PbBr2, >98.0%) and [4-(3,6-Dimethyl-9H- carbazol-9-yl)butyl]phosphonic acid (Me-4PACz, >99.0%) were purchased from TCI. Formamidinium iodide (FAI, >99.99%) was purchased from Dyenamo. [6,6]-phenyl-C61- butyric acid methyl ester (PC61BM, >99.5%) was purchased from Solenne BV. Bathocuproine (BCP, 98%) and caesium iodide (CsI, 99.9%) were purchased from Alfa Aesar. Unless stated otherwise, all other materials and solvents were purchased from Sigma-Aldrich. All the materials were used as received without further purification. To form the mixed-cation lead mixed-anion perovskite precursor solutions, CsI, FAI, PbI2, and PbBr2were prepared in the way corresponding to the exact stoichiometry for the hybrid perovskite composition [i.e. CsxFA1-xPb(I1-yBry)3; (i) for 1.6 eV cells, x = 0.13 and y = 0.10; (ii) for 1.67 eV cells, x = 0.17 and y = 0.23; (ii) for 1.77 eV cells, x = 0.15 and y = 0.4] in a mixed organic solvent system comprising N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) at the volume ratio of DMF:DMSO = 4:1. The perovskite precursor concentrations were 1.41 M and 1.39 M for 1.6 eV cells and 1.67 / 1.77 eV cells, respectively. The perovskite precursor solutions were stirred overnight in a nitrogen-filled glovebox and used without any further treatment. Film deposition and solar cell fabrication Tin-doped indium oxide (ITO) coated glass were cleaned in a series of ultrasonic cleaning baths using various solutions and solvents in the following sequence: 1) deionized water with 2% v / v solution of Decon 90 cleaning detergent; 2) deionized water; 3) acetone and 4) isopropanol (each step for ~10 minutes). After ultrasonic cleaning, substrates were dried with compressed nitrogen and then treated with UV-Ozone for 20 minutes before use. After the substrate cleaning procedure, Me-4PACz (0.5 mg mL-1in ethanol) was deposited by dispensing the as-prepared organic solution onto a spinning substrate at 3,000 rpm for 20 seconds, followed by thermal annealing at 130 ℃ for 5 min in ambient air. The deposition of the perovskite layers was carried out using a spin coater in a nitrogen-filled glovebox with the following processing parameters: starting at 1,000 rpm for 5 seconds (ramping time of 5 seconds from stationary status) and then 5,000 rpm (ramping time of 5 seconds from 1,000 rpm) for 30 seconds. Before the end of the spinning process, a solvent-quenching method was used by dropping anisole of 200 μL onto the spinning substrates at 35 seconds after the start of the spin-coating process. The thermal annealing process (100 ℃ for 50 minutes) was then carried out for the formation of the perovskite layer. For PC61BM-based cells, the as-prepared PC61BM solution (20 mg mL-1in a mixed solvent of chlorobenzene, CB, and 1,2- dichlorobenzene, o-DCB, at the volume ratio of CB : o-DCB = 3:1) was dynamically spun onto the perovskite layers at a speed of 2,000 rpm for 20 seconds. The samples were then annealed at 100 ℃ for 5 minutes. After cooling down to room temperature, the as-prepared BCP solution (0.5 mg mL-1in isopropanol) was dynamically spun onto the PC61BM layer at a speed of 4000 rpm for 20 seconds, followed by a brief thermal annealing process at 100 ℃ for ~1 min. Both PC61BM and BCP layers were processed inside the nitrogen-filled glovebox. For C60-based devices, the evaporation chamber was pumped down to a base pressure between 8×10–7and 2×10–6mbar for both the 20-nm C60and 2-nm BCP depositions. The walls of the chamber were maintained at 17 ℃ and the rotating substrate at 20 ℃ through two separate chillers. The solar cells were completed by thermal evaporation of Cr (3.5 nm) and Au electrodes (100 nm) through shadow masks under high vacuum (5×10–6mbar) using a thermal evaporator (Nano 36, Kurt J. Lesker) placed in the ambient environment. Solar cell characterisation and stability test Current-voltage and maximum power point measurements were measured using two Keithley 2400 series source metres in the ambient environment under both simulated sunlight (AM 1.5 irradiance generated by a Wavelabs SINUS-220 simulator) and in the dark. The active area of the solar cell was masked with a black-anodised metal aperture to either 0.25 or 1 cm2, within a light-tight holder. The current-voltage characteristics were taken from a “reverse” scan (i.e., from “forward bias” to “short circuit”) followed by a forward scan (i.e., from short circuit to forward bias) at a scan rate of 245 mV s-1. Subsequently, active maximum power point tracking measurements using a gradient descent algorithm were performed for 120 seconds to obtain the maximum power point tracked efficiency. The intensity of the solar simulator was set periodically such that the short-circuit current density from a KG3-filtered Si reference photodiode (Fraunhofer ISE) matched its 1-sun certified value. A local measurement of the intensity before each batch of solar cell measurements were performed, was made by integrating the spectrum obtained from the solar simulator’s internal spectrometer. By taking the ratio of this internal intensity measurement to one obtained at the time of calibration we determined the equivalent irradiance at the time of measurement. The perovskite solar cells were first covered by a thin layer of CYTOP™ (CTL-809M, AGC Chemicals), which was dried under vacuum, followed by an encapsulation process using a cover glass (LT-Cover, Lumtec) and UV adhesive (LT-U001, Lumtec) in a nitrogen-filled glovebox. All the encapsulated devices were aged using an Atlas SUNTEST CPS+ (1,500 W air-cooled Xenon lamp) light-soaking chamber under simulated full-spectrum AM1.5 sunlight with 77 mW cm-2irradiance and without applying any UV filters. All the aging tests performed in this work were conducted in open-circuit conditions. To carry out current-voltage characterisations, the samples were taken out from the chamber and tested at different ageing times, following the measurement protocol as described herein. The ageing chamber for storing the encapsulated samples was air-cooled with the temperature controlled at 85 ℃ (measured using a black standard temperature control unit). During the ageing period, the relative humidity in the laboratory was monitored in the range of 50~60%. Photoluminescence quantum yield Photoluminescence quantum yield (PLQY) measurements were conducted using the de Mello method (de Mello, J. C. et al. An improved experimental determination of external photoluminescence quantum efficiency). A 532 nm CW laser diode (ThorLabs DJ532-10), coupled into an optical fibre and into an integrating sphere, was used to photoexcite the samples. The intensity of the laser was adjusted to provide a 1 sun equivalent photon flux, equal to ~56 mW cm-2for this excitation wavelength and the bandgap of the absorbing material studied in an earlier article (Kirchartz, T. et al. Photoluminescence-Based Characterization of Halide Perovskites for Photovoltaics). A second optical fibre was used from the output of the integrating sphere to QEPro spectrometer, purchased from OceanOptics. The system was calibrated by using a calibrated halogen lamp with specified spectral irradiance (HL-3P-INT- CAL, OceanInsight), which was shone into to integrating sphere. A spectral correction factor was established to match the spectral output of the detector to the calibrated spectral irradiance of the lamp. External quantum efficiency The external quantum efficiency (EQE) of the devices was measured using a Fourier transform photocurrent spectroscopy system based on a Bruker Vertex 80v Fourier transform interferometer. The solar cells were masked with a metal aperture such that the whole active area was illuminated by a tungsten halogen lamp. To determine the EQE, the photocurrent spectrum of the device under test was divided by that of a calibrated Si reference cell (Newport) of a known EQE. The acquisition time for each photocurrent spectrum was ~60 s and was conducted in ambient conditions. To determine the equivalent short-circuit current density under 1 sun irradiance from the EQE measurements, the overlap integral of the AM1.5 photon flux (φAM1.5) spectrum with the EQE was calculated. Explicitly, this is given by ^^^^ୡ = ^ ^^^^^.ହ(^^)^^^^^^(^^) d^^^where q is the elementary charge and λ is the wavelength. Terahertz photoconductivity The photoinduced, time-resolved conductivity was measured using the optical-pump terahertz (THz)-probe technique as described in Ulatowski, A. et al. Revealing Ultrafast Charge-Carrier Thermalization in Tin-Iodide Perovskites through Novel Pump–Push–Probe Terahertz Spectroscopy. ACS Photonics 8, 2509-2518 (2021). The thin films were photoexcited with laser pulses from a Ti:Sapphire laser system (35-fs pulse duration, 800-nm central wavelength, 5-kHz repetition rate; a Spectra Physics MaiTai-Ascend-Spitfire regenerative amplifier). The frequency was then doubled to 400 nm using a BBO crystal. The excitation fluence was controlled with a neutral density filter wheel. The time delay between the optical pump pulse and the probing THz pulse was adjusted with optical delay stages. THz radiation was generated using a spintronic emitter (2 nm tungsten, 1.8 nm Co40Fe40B20, 2 nm platinum on quartz substrates). The transmitted THz radiation was measured using electro- optic sampling in a 1-mm thick ZnTe (110) crystal with an 800 nm-wavelength gate beam originating from the same laser system mentioned above. The polarisation of the gate beam was measured using a quarter-wave plate, Wollaston prism, and a balanced photodiode detector. The relative change of THz transmission through the photoexcited and un-excited thin film, ∆T, was measured by selectively blocking half of the pump pulses and half of THz pulses using two optical choppers and recording the resulting four different transmission signals (THz-on–pump-off, THz-off–pump-off, THz- on–pump-on, THz-off–pump-on) with a custom-made, FPGA-based data acquisition board. The photo-induced conductivity σ of a thin film can be expressed as: where d is the thickness of the thin film, nsubis the refractive index of the quartz substrate (nsub= 2.12), ∆T is the difference between the THz radiation transmitted through the photoexcited sample and the sample in the dark, and T is the electric field strength of the THz radiation transmitted through the sample in the dark. ^0and c are the permittivity of free space and speed of light, respectively. The charge-carrier sum mobility, µ, was calculated from the onset of THz conductivity at t = 0, using the relation between photoinduced conductivity and photoexcited charge-carrierpair density n,^^(^^) = ^^^^(^^)^^where e is the electron charge. The number density of photoexcited charge carriers n (t =0) was estimated from the absorption spectra, assuming that all absorbed photons were converted to electron-hole pairs. This assumption is reasonable for lead-halide perovskites at room temperature given their low exciton binding energy. Photoluminescence imaging The photoluminescence (PL) images taken for this work employed the same setup as Dasgupta, A. et al. Visualizing Macroscopic Inhomogeneities in Perovskite Solar Cells. The sample is excited optically with a 440-nm LED light, and electronically contacted by a source meter (Keithley 2636). The excitation intensity was controlled by controlling the current draw of the LED. The intensity of the LED which corresponds to an equivalent 1 sun illumination was determined by measuring the current at the short-circuit condition and varying the intensity till this current matched that measured on the solar simulator. The spatially resolved photoluminescence maps were obtained using a scientific CMOS camera sensor (Andor Zyla 4.2, Oxford Instruments). A long pass filter was used to stop the excitation light from entering the camera, so that only the PL was incident on the sensor. X-ray diffraction X-ray diffraction (XRD) patterns were measured at room temperature using a PANalytical X'Pert PRO diffractometer equipped with a Cu line focus X-ray tube run at 45 kV with a tube current of 40 mA (Cu Kα radiation, λ = 1.506 Å). In a Bragg-Brentano geometry setup, the 1D detector collected data from each sample, with a typical collection time of 60 minutes per sample. The alignment was calibrated using ITO, avoiding any shifts in peaks due to the sample tilting. The lattice parameters are calculated using the TOPAS software. Grazing-incidence wide-angle X-ray scattering The grazing-incidence wide-angle X-ray scattering (GIWAXS) data were recorded with a Rigaku SmartLab diffractometer, equipped with a HyPix-3000 hybrid pixel-array 2D detector and a rotating 3 kW Cu Kα source (operating at 40 kV, 45 mA). X-ray photons with energy of 8.048 keV and CBO-f optics were incident on an aligned sample surface, held at grazing incidence angles of 0.5° and 1.5°. The samples were mounted on a 2D-XRD attachment stage with a beam stop for the direct beam, which was positioned 65 mm away from the detector. Detector images were integrated and resampled into Q-space and were combined using scripts based on the PyFAI and pygix libraries. These libraries were also used for azimuthally integrated 1D profiles (Ashiotis, G. et al. The fast azimuthal integration Python library: pyFAI). For variable-angle GIWAXS measurements (αi = 0.5°−1.5°), samples were measured using a different 2D-XRD aperture slit configuration with parallel-beam optics, a 0.5° in-plane parallel-slit collimator, a 0.1 mm incident slit, and a 10 mm length-limiting slit at a single detector position, before processing as described herein. Scanning electron microscopy Scanning electron microscopy (SEM) samples were prepared following the perovskite solar cell fabrication process till the perovskite layer and treated with different amino-silane molecules afterward. Top-down SEM images were taken on an FEI Quanta 600 FEG at an acceleration voltage of 3 kV. The SEM chamber was pumped down to high vacuum of < 2 × 10-4mbar. Time-of-flight secondary ion mass spectrometry Perovskite samples for time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were prepared following the same procedure used for solar cells. Measurements were conducted using a TOF.SIMS 5 instrument (IONTOF GmbH), which was equipped with a bismuth primary ion source and an O2+sputter source to probe the positive atomic and / or fragment ions. The 3D ToF-SIMS data were acquired over an area of 100 × 100 µm using a 30 keV Bi+primary ion beam. This was followed by a sputtering process, with each cycle lasting for 1.5 seconds. During each sputtering cycle, a 350 × 350 µm area of the sample was bombarded with 1 keV O2+ion beams in an interlaced mode. The sample holder, containing the samples, was sealed in a container filled with argon gas and was transferred into the instrument immediately before testing. Electronic structure calculation and modelling First-principles simulations were performed within the framework of density functional theory (DFT) as implemented in Vienna Ab-initio Simulation Package (VASP) (see Kresse, G. et al. Ab initio molecular dynamics for liquid metals; Kresse, G et al. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set; and Kresse, G. et al. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane- wave basis set) using a projector augmented-wave (PAW) basis (see Blöchl, P. E. Projector augmented-wave method). The generalized gradient approximation (GGA) exchange- correlation functional of Perdew Burke-Ernzerhof (PBE) modified for solids (PBEsol) was used for all the bulk and surface structures (see Perdew, J. P. et al. Restoring the Density- Gradient Expansion for Exchange in Solids and Surfaces). VASP-recommended PAW pseudopotentials were used for all the atomic species (C:2s22p2, N:2s22p3, H:1s1, Pb:5d106s26p2, I:5s25p5, O:2s22p4, and Si:3s23p2). Grimme’s DFT-D3 dispersion correction was considered due to the presence of organic molecules (Grimme, S. et al. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu). A plane wave energy cut-off of 500 eV and a Brillouin zone (BZ) sampling using Γ-centred k-mesh are done for all the calculations. A k-point grid of 4 × 4 × 4 (4 × 4 × 1 for surface slabs) was used to calculate finer properties such as the density of states (DOS), charge density difference, surface energies, and binding energies. Gaussian smearing of 0.1 is used for all the structure optimizations and 0.01 for calculating finer properties. The cell volume, shape, and atomic positions for the bulk structure and only the atomic positions for the surface slabs were fully relaxed using the conjugate gradient algorithm until the force on each atom converge below 0.01 eV Å-1. Equivalently, an energy convergence criterion of 10-6eV was used for the electronic degrees of freedom. 2. Passivation with silane moieties It is shown herein that a vapour-based amino-silane passivation strategy reduces photovoltage deficits in solar cells comprising mixed-cation, mixed-anion perovskites to 100~120 mV. It is demonstrated that primary-amino saline passivation disrupts the perovskite crystallinity and destroys long-range order and charge transport, whereas amino-silane molecules incorporating secondary or tertiary amines offer both passivation and sustain the long-range conduction in the perovskite absorbers. Ab initio molecular dynamics simulations reveal atomic-scale details of strong passivator binding of the amino-silane molecules on the defective perovskite surface. Since surface Pb ions adjacent to iodide vacancies are severely undercoordinated, the amino-silane adsorption through both N-Pb and O-Pb bonds act by increasing the Pb coordination and eliminate surface vacancy defects, thereby promoting enhanced surface passivation. Importantly, it is shown that passivated and encapsulated perovskite solar cells can maintain 95 per cent of their initial efficiency for more than 1,500 hours when subjected to simulated full-spectrum simulated sunlight at 85 ℃ under an open- circuit condition in ambient air. It is thought that both the silane units and the amine units can interact with the perovskite surface and each other, and hence influence electronic passivation of the surface. Herein, the impact of APTMS vapour phase passivation is investigated in mixed-cation lead mixed-anion perovskite thin-films and in positive-intrinsic-negative (p-i-n) perovskite solar cells (as schematically shown in Fig. 1b) (see Jariwala, S. et al. Reducing Surface Recombination Velocity of Methylammonium-Free Mixed-Cation Mixed-Halide Perovskites via Surface Passivation and Pothoof, J. et al. Surface Passivation Suppresses Local Ion Motion in Halide Perovskites). For the perovskite absorber, the “methylammonium-free” composition, Cs0.13FA0.87Pb(I0.9Br0.1)3(FA = formamidinium) was employed.Consistent with the work in Jariwala, S. et al. Reducing Surface Recombination Velocity of Methylammonium-Free Mixed- Cation Mixed-Halide Perovskites via Surface Passivation, a substantial increase in PLQY was measured, from ~ 6% to 24%, for perovskite films before and after APTMS passivation. An illustration of the vapour-phase passivation treatment is shown in Fig. 1c. For integration into p-i-n PV devices [4-(3,6-dimethyl-9H-carbazol-9- yl)butyl]phosphonic acid (Me-4PACz) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) were employed, as the hole transport and electron transport layers, respectively. The current density-voltage (J-V) characteristics and other corresponding PV performance parameters obtained from the optimised APTMS-treated PV cells and the reference PV cells are shown in Fig. 29. It is shown that the PV performance of APTMS-treated cells is relatively poor and significantly worse than reference untreated cells, with significant hysteresis in the J- V characteristics. Without being bound by theory, it is postulated that a too thick a layer of APTMS has formed, which is inhibiting electron extraction. To understand if the structure of the silane molecule can have an impact upon either passivation or functionality in a PV cell, perovskite films were prepared and treated with APTMS and other amino-silane molecules with a range of deviations from APTMS (Fig. 1a), including trimethoxy(propyl)silane (PTMS), trimethoxy[3-(methylamino)propyl]silane (MAPTMS), (N, N-Dimethylaminopropyl)trimethoxysilane (DMAPTMS), [3-(2- aminoethylamino)propyl]trimethoxysilane (AEAPTMS) and 3-[2-(2- aminoethylamino)ethylamino]propyltrimethoxysilane [(AE)2APTMS] (Fig. 1d). Perovskite films with the compositions of Cs0.13FA0.87Pb(I0.9Br0.1)3and Cs0.15FA0.85Pb(I0.6Br0.4)3, hereafter abbreviated as I90Br10and I60Br40, respectively, were examined. Amino-silane (AS) molecules were deposited by vapour-phase by: i) loading an AS molecule chosen from Fig. 1a in a petri dish; ii) heating the passivation molecule containing petri dish at 100 for a short period of time (i.e., a few minutes) till all the molecules vapourised; iii) placing an as-grown perovskite film sample inside the petri dish for tens of seconds to a couple of minutes. All the amino-silane molecules (except for PTMS, which has no amino functionality) improve the PLQY for I90Br10as compared to the reference (as shown in Fig. 1e), whilst for I60Br40, apart from PTMS and DMAPTMS, enhanced PLQY was observed after treatment (Fig. 2). In particular, for the wider band gap I60Br40 perovskite (~1.77 eV), an increase in PLQY from ~ 1.5 to 3% was observed for the APTMS treatment. However, this value increases to over 7% and to over 20% for the AEAPTMS and (AE)2APTMS treatments respectively. A series of PLQY measurements was conducted on a variety of perovskite films and “half- stacks”, which were composed of either indium oxide tin oxide (ITO) glass substrate / hole transport layer (HTL, including Me-4PACz and aluminium oxide nanoparticles) / perovskite, or glass substrate / aluminium oxide nanoparticles / perovskite / electron transport layer (ETL, containing PC61BM and bathocuproine, BCP) using AEAPTMS. Significant improvements were observed for the amino-silane molecules that contain both primary and secondary amine groups [i.e., AEAPTMS and (AE)2APTMS)]. For example, AEAPTMS treated I90Br10, I60Br40and Cs0.17FA0.83Pb(I0.77Br0.23)3, hereafter abbreviated as I77Br23, see a large increase in PLQY (Fig. 3), with the PLQY for the glass / I60Br40 / ETL stack increasing from 0.07% for the control, to 4.11% for the AEAPTMS treated films (Fig. 4a). Meanwhile, in Fig. 4b, corresponding calculated quasi-Fermi level splitting (QFLS) values are shown for further comparison. This indicates that the passivation specifically inhibits non-radiative recombination at the perovskite / ETL interface. To probe the impact of the passivation treatments further, UV-visible absorption spectroscopy and non-contact transient terahertz photoconductivity measurements were performed on I90Br10samples, since we observed the largest enhancement in PLQY for this perovskite composition when using APTMS. In Fig. 6, UV-visible absorption spectra of perovskite films with and without surface passivation are shown. Surprisingly, for the perovskite films passivated with APTMS a considerable reduction in the optical density was observed, less than half over the entire absorption range, indicating a bulk change to the perovskite thin film following passivation. All the other passivated films exhibit a small reduction in optical absorption, in comparison to the reference films. Optical pump terahertz (THz) probe spectroscopy revealed the change in conductivity of a sample following photoexcitation and was used to determine the “short-range” charge carrier mobility. This technique revealed surprising differences in samples (Fig. 1f and Figs. 7 and 8). For the reference films, the sum of mobility is approximately 60 cm2 / Vs, which is a high value for lead-halide perovskites. For all passivation treatments, apart from PTMS, the mobility (Fig.1f) is reduced. For the APTMS-treated I90Br10samples, this reduction in mobility is precipitous, with the sum of mobilities values dropping to 1.5 cm2V-1s-1. This indicates that there is a significant disruption to the crystalline order throughout the APTMS-treated perovskite films on a length-scale short enough to inhibit the THz-derived charge carrier mobility. In contrast, the reduction in charge carrier mobility is much less severe for the other passivation molecules investigated; for instance, we determine a sum of mobilities of 46 cm2 / Vs for AEAPTMS-treated films. With such a severe disruption to crystalline order for the APTMS-treated films, inferred from both the drop in THz mobility and the optical absorption strength, it was expected that it may be possible to discern changes in the crystallinity of the thin films as determined by electron microscopy or X-ray diffraction (XRD). First, the influence of different amino-silane molecules on the film formation quality was investigated by examining the surface morphology of the samples by scanning electron microscopy (SEM, Fig. 9). Characteristic polycrystalline morphology was observed in all perovskite films with an SEM grain diameter of approximately 150~200 nm. However, for both the APTMS and MAPTMS-treated I90Br10perovskite films, significant fractions of the films appeared to be ill-defined and “cloudy”. This could be due to a complete change to the morphology within these regions or a thick capping layer of an amorphous material being present. For the AEAPTMS and (AE)2APTMS treated films, the polycrystalline structures were discernible over the entire surface area, yet there is more “haziness” to the images, suggestive of a surface coating with an insulating material. Fig.31 shows XRD 2θ plots for the series of treated films, which once again reveal striking differences. XRD traces of PTMS, DMAPTMS, and AEAPTMS treated samples did not differ significantly from those of untreated samples, and all exhibited the characteristic XRD pattern of the polycrystalline perovskite thin-films without any additional diffraction peaks or significant change in scattering intensity. For the (AE)2APTMS-treated sample, a slight decrease in peak intensities was observed, with the full width at half maximum (FWHM) of the first perovskite peak broadening, whilst this perovskite characteristic peak of AEAPTMS- treated sample shows a narrowing FWHM value, in comparison with the untreated films (see Table 1). However, for the APTMS-treated I90Br10sample, all the perovskite scattering peaks are reduced in intensity by over an order of magnitude. In addition, an extra peak at ~12 degrees was observed in the MAPTMS-treated sample, which can be ascribed to PbI2. 2θ (°) Ref PTMS APTMS MAPTMS DMAPTMS AEAPTMS (AE)2APTMS13.891 0.113 0.111 0.271 0.115 0.123 0.103 0.21719.709 0.084 0.080 0.214 0.081 0.098 0.079 0.16124.236 0.076 0.075 0.179 0.087 0.085 0.084 0.14428.086 0.075 0.073 0.289 0.057 0.050 0.048 0.13831.501 0.078 0.074 0.239 0.065 0.081 0.074 0.13334.607 0.082 0.083 0.232 0.084 0.098 0.077 0.12440.187 0.082 0.075 0.273 0.082 0.081 0.077 0.11543.953 0.089 0.068 0.225 0.082 0.093 0.078 0.137Table 1. Full width at half maximum values for perovskite characteristic peaks. From the THz mobility measurements, the SEM images, XRD patterns and UV-vis absorption traces, a clear picture has emerged. The APTMS treatment has a strong influence on the crystallinity throughout the perovskite films, introducing short-range scattering defects or disrupting the crystalline domains up to such an extent that the charge carriers undergo short- range scattering and have severely limited charge carrier mobility. Notably, even for perovskite nanocrystals with a crystal diameter of 7 nm, the charge carrier mobility is only reduced by a factor of three, in comparison to the “bulk mobility” in a polycrystalline film. Here, the reduction in mobility by a factor of 40 indicates that the break-up of the crystalline order, or introduction of charge-scattering sites, is on a much shorter length scale than 7 nm, and hence intra-grain, rather than simply at the polycrystalline grain boundaries. In contrast, applying AEAPTMS and (AE)2APTMS molecules appear to leave the crystalline order and charge carrier mobility largely unaffected. To explore how APTMS, MAPTMS, and AEAPTMS impact perovskites in more detail, grazing-incidence wide-angle X-ray scattering (GIWAXS) was employed to study their effects on the orientation and crystallinity of treated I90Br10perovskite films. GIWAXS at a lower incidence angle (0.5°) was employed, which is relatively close to the critical angle. This approach provided information about the surface region, which extends a few tens of nanometres deep into the sample. In the AEAPTMS-treated perovskite sample, the brighter ring colour observed suggested higher crystallinity near the surface compared to the reference sample. Additionally, after the treatment, a pair of broad rings appear in the low Q space (i.e., below 0.5 Å⁻¹). This observation suggested that the organic passivation molecules are located in the upper part of the perovskite layer, likely not extending deeper than the top 70 nm. At the higher incidence angle (1.5°), information about the deeper regions within the perovskite samples was obtained. Continuous Debye-Scherrer diffraction rings at Qxy=1.0, 1.4, 1.7, 2.0 Å-1were observed in all the samples, corresponding to (100), (110), (111) and (200) diffraction peaks respectively in 1D XRD patterns. This observation indicates that the isotropic orientation of the bulk perovskite is maintained with or without the passivation treatment. On the other hand, for the APTMS-treated perovskite sample, there are no clear Debye- Scherrer diffracting rings observed in the lower incidence angle GIWAXS pattern, consistent with the APTMS molecules causing “disintegration” of the perovskite structure. As seen in 1D in-plane and out-of-plane integrations from 2D GIWAXS, all perovskite-specific rings had vanished, and a broad isotropic ring appears around Qxy= 0.4 Å-1. Coupled with the knowledge that primary amines can strongly interact with and “solvate” metal halide perovskites, these observations indicated that the perovskite films undergo a solvation process during the APTMS treatment, reforming into a much less crystalline material with much shorter-range crystalline domains, which are barely discernible via X-ray diffraction. This APTMS-triggered solvation process results in a visual change to the films during treatment. The peculiarity is that from the UV-vis absorption, at least half the absorption strength remained, and the photoluminescence remained at the expected energy of the 3D perovskite and is extremely strong. This indicates that the 3D perovskite is still present but in the form of well-passivated nanoscale domains. For the MAPTMS sample, apart from the perovskite rings, a highly oriented 2D phase appeared by fitting one-layer Ruddlesden-Popper (RP) phases (Fig. 10). From in-plane and out-of-plane 1D line cuts, it is evident that the orientation of most of the 2D phase perovskite is along the out-of-plane direction. Sharp diffraction peaks emerged at the scattering vector Qxy= 0.43, 0.72, and 0.87 Å-1along the z-axis direction, which we indexed as the (002), (102), and (004) planes of the RP phase perovskite with a layer spacing of approximately 14.2 Å (Fig. 10). Lastly, to substantiate the claim regarding the depth of penetration of the treatment using APTMS, MAPTMS, and AEAPTMS, the films were further examined using time-of-flight secondary ion mass spectrometry (ToF-SIMS), looking at the elemental depth profile. Whilst APTMS and MAPTMS both exhibit varying degrees of Si signal penetration throughout the films, The Si signal for films treated with AEAPTMS molecules remained on the surface of the perovskite layer. This observation held even when the treatment duration was tripled. To gain further insights into the structures and interactions of the amino-silane molecules at the perovskite surfaces, density functional theory (DFT) and ab-initio molecular dynamics (AIMD) methods were used to investigate four types of silane molecules covering different possible amine positions: primary (APTMS), secondary (MAPTMS), both primary and secondary (AEAPTMS), as well in the absence of an amino group (PTMS). Their interactions on the pristine perovskite surface and on surfaces with an iodide vacancy defect were examined. It is well known that iodide vacancy defects play key detrimental roles affecting operational efficiency and stability of perovskite solar cells that include facilitating ion migration to surfaces and non-radiative photocarrier recombination. To investigate key adsorption and structural trends for a range of amino-silanes on FA-based, MA-free perovskite triiodides, the (001) PbI2terminated pristine FAPbI3surface was examined, since it is one of the most stable and studied halide perovskite surfaces. Fig.11 shows the optimised structures and calculated binding energies of the amino-silane molecules on the pristine surface and on the iodide vacancy defect surface. Three important features emerge. First, favourable binding energies show strong bonding affinity between the molecules and the surface with the highest and lowest binding energies found for AEAPTMS and PTMS respectively; interestingly, only for AEAPTMS does the surface binding energy increase significantly from the pristine surface to the iodide vacancy defect surface. Second, the strongest adsorption for AEAPTMS suggests the role of larger amino-silane molecules having both primary and secondary amine groups in providing effective surface defect passivation. As the surface Pb ions adjacent to an iodide vacancy are severely undercoordinated (with four-fold coordination versus octahedral in the bulk), the strong passivator adsorption of AEAPTMS acts by increasing the coordination around these Pb ions and eliminating the surface vacancy defects. Third, since there has been speculation around the precise atomistic interactions of the amino-silane molecules at the perovskite surfaces, the simulations reveal that the terminal amines in AEAPTMS bind to the surface Pb atoms with an N-Pb bond length of 2.51 Å, which is significantly shorter than the Pb-I bond length of 3.15 Å. The schematic Fig. 11d shows that the AEAPTMS molecule bridges the two Pb ions adjacent to the iodide vacancy via N-Pb bonds. O-Pb bonding with an average bond length of 2.8 Å is also shown. Changes in the charge density profile around all the species (Fig. 11b) show silane-surface charge transfer around the surface Pb cations, again indicating O-Pb and N-Pb chemical bonds. Such multidentate interactions increase the binding or anchoring strength on the surface, which enhances the effective passivation. Overall, the ab initio simulation results are in good agreement with the experimental findings as well as providing new structural and mechanistic insights at the atomic level. 3. Solar cells A series of solar cells were fabricated and measured to screen a range of amino-silane molecules (Fig. 12). AEAPTMS molecule results in the most significant improvement in PV performance parameters for the I90Br10-based perovskite. Cells treated with (AE)2APTMS also exhibited enhanced PV performance, however in certain cases, (AE)2APTMS-treated cells were found to show lower JSCand FF, hence a lower PCE as compared to AEAPTMS-treated cells, whilst requiring a very narrow processing window (<10 seconds, for the vapour treatment process). Representative J-V characteristics of reference and AEAPTMS-treated cells using I90Br10, I77Br23, and I60Br40are shown in Fig. 13a, 13b, and 13c, respectively. These cells, with an aperture of 0.25 cm2, exhibited the corresponding PV parameters of VOC= 1.22, 1.26, and 1.35 V, JSC= 22.3, 19.5, and 17.2 mA cm-2, FF = 83.4, 84.0, and 80.9%, and PCE = 22.6, 20.6, and 18.7%, respectively, for the scanning direction from the forward-bias to short-circuit (FB- SC) conditions. Figs. 14, 15, and 16 show the corresponding external quantum efficiency (EQE) and 120-second maximum power point tracking (MPPT) with MPP efficiency of 22.4, 20.7, and 18.6% for I90Br10, I77Br23, and I60Br40-based cells respectively. The EQE analysis revealed that the integrated JSCexhibit a negligible variation (<1%) compared to the JSC measured from the corresponding cells under simulated sunlight, as shown in Fig. 13a-3c. The EQE-derived PV bandgaps are 1.60, 1.67, and 1.77 eV for I90Br10, I77Br23, and I60Br40, respectively, with the statistical results for VOC, JSC, FF, and PCE shown in Figs. 17, 18, 19, and 20, respectively. Of particular note is that VOCcan reach as high as 1.28 and 1.38 V for 1.67 and 1.77- eV perovskites (i.e., I77Br23and I60Br40), respectively. Such high photovoltages from mixed- cation, mixed-anion perovskite cells are crucial for the development of next-generation perovskite-based tandem cells. Multi-junction perovskite-on-silicon and all-perovskite cells usually employ thermally evapourated C60 as an ETL. To demonstrate the compatibility of the passivation treatment disclosed herein with this ETL, I77Br23with an evapourated C60and BCP was fabricated. This resulted in an enhancement of VOCfrom 1.21 (control) to 1.26 V (passivated), confirming the suitability of the approach discussed herein for use in tandem cells. 1-cm2cells were also fabricated, and the corresponding J-V characteristics, MPPT, and EQE are shown in Fig. 13d, Fig. 13e, and Supplementary Fig. 21, respectively. The obtained PV parameter enhancement (Table 2) is similar to that seen in 0.25-cm2small-area cells with a minimum performance deficit when enlarging the cell area. The 1-cm2cell results suggest that the vapour-passivation approach induces a highly uniform passivation effect. For the different band gap absorbers, the detailed balance VOClimit was estimated and compared to the values obtained in the devices described herein. AEAPTMS can universally boost VOC to beyond 90% of theoretically achievable photovoltages. These results demonstrate photovoltages obtained from perovskite PV cells treated with AEAPTMS is approaching what is attainable from those made of single-crystal epitaxially grown III-V semiconductors.

[0002] Table 2. Corresponding PV performance parameters obtained for 1-cm2AEAPTMS- treated solar cells shown in Fig. 13d using I90Br10(i.e., 1.60 eV), I77Br23(i.e., 1.67 eV), and I60Br40(i.e., 1.77 eV) as the light absorbers. Next, how AEAPTMS impacts the solar cells long-term operational stability was investigated. Encapsulated reference and AEAPTMS-treated I90Br10cells were aged by exposing them to full-spectrum simulated sunlight at 85℃ under the open-circuit conditions in ambient air with relative humidity in the laboratory 50~60%, resembling an ageing condition close to the ISOS-L-3 protocol (ISOS: International Summit on Organic Photovoltaic Stability). The devices were taken out of the aging chamber, allowed to cool to room temperature and characterised under 1-sun simulated sun light at various time intervals. The average maximum power point (MPP) tracked efficiency and JV derived PCE of the cells, along with their standard deviations, are plotted in Fig.22a, whilst the evolution of the corresponding PV performance parameters: VOC, JSC, and FF are shown in Figs. 23, 24, and 25, respectively. The evolution of the MPP efficiency and PCE data of each individual cell recorded is plotted in Fig. 22b. The AEAPTMS-treated cells maintained high operation levels during the ageing process, and it took approximately 1,600 and 1,440 hours for the champion cell to decrease to 95% (T95, Champ) of its initial MPP (Fig. 22 a and b, Fig. 36) and PCE (Fig. 35), representing outstanding stability for mixed-cation mixed-halide perovskite PV cells. In comparison, this represents an order of magnitude enhancement upon previous best reported stability under the same aging conditions (a champion “T80” lifetime (time to reach 80% of peak performance) of 430 hours). It was observed that even when a cover glass slip was used for encapsulation with epoxy resin edge sealing to obtain the best stability a “capping layer” was required to coat the completed devices, for which CYTOP™ – an amorphous fluoropolymer – was used. In the absence of an on-cell capping layer, cell areas underwent discolouration. Since the cells were encapsulated in an N2filled glove box, this was assumed to be due to the loss of iodine during photoinduced degradation, which then left voids in the film and reacted with the metallic electrodes. When comparing the external quantum efficiency (EQE) spectra measured from aged cells (Fig. 26), no significant change in the EQE onset energy was observed, suggesting little compositional change during aging. However, the reference cells suffered from more severe panchromatic deterioration at in EQE than the AEAPTMS-treated cells, consistent with the more severe drop in JSC. Notably, the dips in the EQE spectra appeared to result from “sporadic” internal-shunting of the aged devices during measurements, which tended to rectify themselves. Given the observed variations in cell performance during ageing (Fig. 22a and 22b), PL imaging of cells was conducted to elucidate how the AEAPTMS was enhancing the long- term stability. PL-derived QFLS images of the representative full-area 0.25-cm2AEAPTMS- based and reference cells at different ageing stages are presented in Fig. 22c, 22d, and Fig. 27. Intriguingly, even the reference cell maintained relatively high and stable QFLS maps, comparable to its initial state, for up to 300 hours. This stability is evidenced by only 2.8% relative decay in the QFLS median (from 1.160 eV to 1.128 eV, Fig. 22g). However, after 600 hours of aging, clear regions of lower and higher QFLS appeared in the maps (Fig. 22d 22g, and 27), indicating that the cells degrade heterogeneously on the 100 µm to mm length- scale. QFLS of the complete device, should closely correlate with VOC. However, as show in Fig. 23, as the reference cell ages, the difference between QFLS and VOCgets progressively larger, reaching ~380 mV by the end of the aging process. This indicated that either a significant energetic shift (i.e., in the vacuum level) has occurred at either (i) one or both charge selective contacts, or (ii) the charge selectivity of one of the contacts has reduced. For the reference cell, in addition to loss in VOC, there is also a significant loss in the other PV performance parameters (Figs. 23-25). As such, the PL imaging suggested that the perovskite absorber can remain relatively intact when subjected to rigorous 85°C light soaking ageing conditions, whilst other parts of the cell may have undergone severe deterioration. The substantial decay observed in the FF indicated increase resistance to charge extraction. The AEAPTMS-treated cells exhibited even less deterioration in QFLS and remain homogeneous throughout the aging. Specifically, whilst the QFLS median was initially 1.227 eV at 0 hours, this value only dropped from 1.187 eV at 300 hours to 1.167 eV at 1,200 hours (Fig. 22c, 22g, and 27). Whilst the QFLS map offers valuable insights, it does not appear to directly correlate with the sequence of cell degradation. The luminescence-derived charge collection quality (Qcol) was therefore assessed. Qcolshould qualitatively scale with JSC. In Figures 22c and 22d, Qcolmaps are presented for the fresh and 600-hour aged devices and in Fig. 28 the complete dataset of Qcolthroughout the ageing process is shown. Consistent with the macroscopic device JV measurements, the reference cell showed a significant reduction in Qcolafter aging (see Fig. 22f). The median dropped from 0.963 in its initial state to 0.420 after ageing for 600 hours (see Fig. 22h), indicating a decay of over 56%. Notably, regions of lower Qcolcorrelated with the same regions of lower QFLS. Interestingly, even for the reference cells there remained quite large regions of high Qcolafter aging, indicating that the degradation is originating from specifically defective regions, rather than being homogeneous across the device. In contrast, the AEAPTMS-treated cells show an even higher initial median Qcol(0.985) which only dropped to 0.914 and remains relatively homogeneous after 1,200 hours of aging. Although the most stable devices exhibited an approximately linear decay during our stress testing, for both reference and AEAPTMS treated devices it was observed that some cells experienced positive light-soaking (i.e., performance gain) or burn- in (i.e., early decay) during ageing (Fig. 22b). This Example demonstrates that amino-silane molecules, with varying primary, secondary, and tertiary amine functional groups, significantly impact the PLQY, crystalline order, and charge carrier mobility of halide perovskites. AEAPTMS emerges as an exceptional surface passivator, establishing a strong bonding affinity with the perovskite surface. Notably, AEAPTMS treatment substantially elevates the PLQY of perovskites with bandgaps ranging from 1.6 to 1.8 eV, propelling their radiative efficiencies closer to theoretical limits. Moreover, AEAPTMS-treated perovskite solar cells exhibit state-of-the-art long-term stability under strenuous elevated temperature light soaking aging conditions, highlighting the potential of the present invention in advancing durable and efficient perovskite solar technologies.

[0003] Embodiments of the invention are described in the following numbered paragraphs. 1. A photovoltaic device which comprises a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound. 2. A photovoltaic device according to embodiment 1, wherein the organic compound comprises a silane moiety and an amine moiety, wherein the amine moiety is a secondary amine, a tertiary amine or a quaternary amine. 3. A photovoltaic device according to embodiment 1 or embodiment 2, wherein the organic compound comprises a silane moiety and a plurality of amine moieties, wherein at least one of the amine moieties is a secondary amine, a tertiary amine or a quaternary amine. 4. A photovoltaic device according to embodiment 1 or embodiment 2, wherein the organic compound comprises a silane moiety and from two to five amine moieties, wherein at least one of the amine moieties is a secondary amine, a tertiary amine or a quaternary amine. 5. A photovoltaic device according to embodiment 3 or embodiment 4, wherein at least one of the amine moieties is a primary amine. 6. A photovoltaic device according to embodiment 3 or embodiment 4 wherein one of the amine moieties is a primary amine, and each of the other amine moieties is independently selected from a secondary amine, a tertiary amine and a quaternary amine, preferably wherein one of the amine moieties is a primary amine, and each of the other amine moieties is a secondary amine. 7. A photovoltaic device according to any one of embodiments 2 to 6 wherein the organic compound comprises an organic group bonded to a silicon atom of the silane moiety, wherein each of said amine moieties is present in said organic group. 8. A photovoltaic device according to embodiment 7 wherein said organic group is a C1-20alkyl group, wherein each amine moiety which is a secondary, tertiary or quaternary amine may either interrupt the carbon chain of the C1-20 alkyl group or be present as a substituent on the C1-20alkyl group, and wherein any amine moiety which is a primary amine is present as a substituent on the C1-20alkyl group, and wherein the C1-20alkyl group is otherwise unsubstituted or substituted. 9. A photovoltaic device according to embodiment 7 or embodiment 8 wherein the organic group is substituted by one or more groups selected from thiol, C1-10alkylthio, arylthio, carboxy, pyridyl and bipyridyl. 10. A photovoltaic device according to any one of embodiments 7 to 9 wherein the organic compound has the formula (I) wherein: R is said organic group; and E1, E2and E3are independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted aryloxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryl, unsubstituted or substituted acyl, unsubstituted or substituted ester, unsubstituted or substituted acyloxy, hydroxyl and halo. 11. A photovoltaic device according to embodiment 10 wherein R is a C1-20alkyl group as defined in embodiment 8, and E1, E2and E3are unsubstituted or substituted C1-10alkoxy, preferably unsubstituted C1-10alkoxy, more preferably methoxy. 12. A photovoltaic device according to any one of the preceding embodiments wherein the organic compound has the formula (II) wherein: E1, E2and E3are as defined in embodiment 10, and are preferably independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryloxy, and unsubstituted or substituted aryl; q is 0 or an integer from 1 to 5; each m is independently an integer from 1 to 5; each n is independently 0 or 1; p is 0 or an integer from 1 to 5; G is NR12 or CR13; each R1is independently H or C1-6alkyl; wherein when each n is 0 and G is NR12, or when q is 0 and G is NR12, then at least one R1group is C1-6alkyl, and preferably when G is NR12 and q is not 0, then p is not 0. 13. A photovoltaic device according to embodiment 12 wherein E1, E2and E3are unsubstituted or substituted C1-10alkoxy, preferably unsubstituted C1-10alkoxy. 14. A photovoltaic device according to embodiment 12 or embodiment 13 wherein at least one of E1, E2and E3is methoxy, preferably wherein at least two of E1, E2and E3are methoxy, more preferably wherein E1, E2and E3are all methoxy. 15. A photovoltaic device according to any one of embodiments 12 to 14 wherein G is NR12. 16. A photovoltaic device according to any one of embodiments 12 to 15 wherein each R1is independently H or methyl, more preferably wherein each R1is H. 17. A photovoltaic device according to any one of embodiments 12 to 16, wherein q is 0, 1 or 2 preferably wherein q is 1 or 2. 18. A photovoltaic device according to any one of embodiments 12 to 17, wherein q is 0. 19. A photovoltaic device according to any one of embodiments 12 to 17, wherein q is an integer from 1 to 5, each m is independently 1, 2, or 3 and each n is 1, preferably wherein q is 1 or 2, each m is 2 and each n is 1. 20. A photovoltaic device according to any one of embodiments 12 to 19, wherein p is an integer from 1 to 5, preferably wherein p is 1, 2 or 3. 21. A photovoltaic device according to any one of embodiments 1 to 14 wherein the organic compound is selected from: (i) PTMS ; (ii) MAPTMS ; ; (iv) AEAPTMS , optionally wherein the organic compound is selected from (ii) MAPTMS, (iii) DMAPTMS, (iv) AEAPTMS, and (v) (AE)2APTMS. 22. A photovoltaic device according to any one of embodiments 1 to 14 and 21 wherein the organic compound is AEAPTMS. 23. A photovoltaic device according to any one of embodiments 1 to 14 and 21 wherein the organic compound is (AE)2APTMS. 24. A photovoltaic device according to any one of the preceding embodiments wherein said dimer of the organic compound comprises (i) a first monomer unit, wherein the first monomer unit is an organic compound as defined in any one of the preceding embodiments provided that the silicon atom of the silane moiety of the first monomer unit is substituted with a linking atom or group, and (ii) a second monomer unit, wherein the second monomer unit is an organic compound as defined in any one of the preceding embodiments provided that the silicon atom of the silane moiety of the second monomer unit is substituted with said linking atom or group, wherein the linking atom or group is covalently bonded to the silicon atom of the silane moiety of the first monomer unit and to the silicon atom of the silane moiety of the second monomer unit, to link the first monomer unit to the second monomer unit. 25. A photovoltaic device according to embodiment 24 wherein the first monomer unit is an organic compound as defined in any one of embodiments 10 to 20 provided that one of E1, E2and E3of the first monomer unit is said linking atom or group, and the second monomer unit is an organic compound as defined in any one of embodiments 10 to 20 provided that one of E1, E2and E3of the second monomer unit is said linking atom or group. 26. A photovoltaic device according to embodiment 24 or embodiment 25 wherein the dimer has formula (Ia) or formula (IIa): wherein L is said linking atom or group, and each R, E1and E2is independently as defined in embodiment 10 or embodiment 11; wherein L is said linking atom or group, and each G, p, n, m, q, E1and E2is independently as defined in any one of embodiments 12 to 20. 27. A photovoltaic device according to any one of embodiments 24 to 26 wherein the linking atom or group is a linking atom, wherein the linking atom is an oxygen atom. 28. A photovoltaic device according to any one of embodiments 1 to 23 wherein said polymer of the organic compound comprises a monomer unit which is an organic compound as defined in any one of embodiments 1 to 23, provided that the silicon atom of the silane moiety is substituted with at least one linking atom or group, wherein the or each linking atom or group is covalently bonded to said silicon atom and is also covalently bonded to the silicon atom of the silane moiety of another monomer unit in the polymer. 29. A photovoltaic device according to embodiment 28 wherein the monomer unit is an organic compound as defined in any one of embodiments 10 to 20 provided that at least one of E1, E2and E3is a said linking atom or group, typically wherein two of E1, E2and E3are a said linking atom or group. 30. A photovoltaic device according to embodiment 28 or embodiment 29 wherein each linking atom or group is a linking atom, wherein the linking atom is an oxygen atom. 31. A photovoltaic device according to any one of embodiments 28 to 30 wherein the polymer of the organic compound comprises a monomer unit of formula (Ib) or formula (IIb): wherein L is a said linking atom or group, and R and E1are as defined in embodiment 10 or embodiment 11; wherein L is a said linking atom or group, and G, p, n, m, q and E1are as defined in any one of embodiments 12 to 20, preferably wherein the polymer comprises at least three monomer units of formula (Ib) or formula (IIb). 32. A photovoltaic device according to any one of embodiments 28 to 31 wherein the polymer comprises a structure of formula (Ic) or formula (IIc): wherein each L is a said linking atom or group, and each R and E1is independently as defined in embodiment 10 or embodiment 11, and z is an integer of at least 3; wherein each L is a said linking atom or group, and each G, p, n, m, q and E1is independently as defined in any one of embodiments 12 to 20, and z is an integer of at least 3. 33. A photovoltaic device according to any one of the preceding embodiments wherein the passivating agent is disposed on a surface of the crystalline A / M / X material. 34. A photovoltaic device according to any one of the preceding embodiments which comprises a layer comprising the passivated A / M / X material. 35. A photovoltaic device according to embodiment 34 wherein the layer comprising the passivated A / M / X material comprises (i) a layer of the crystalline A / M / X material, and (ii) a layer of the passivating agent disposed on a surface of the layer of the crystalline A / M / X material. 36. A photovoltaic device according to embodiment 34 or embodiment 35 wherein the layer comprising the passivated A / M / X material consists of the passivated A / M / X material. 37. A photovoltaic device according to embodiment 34 wherein the layer comprising the passivated A / M / X material comprises particles of the passivated A / M / X material, optionally wherein the particles are nanoparticles, and optionally wherein each of the particles comprises a particle of the crystalline A / M / X material and said passivating agent disposed on a surface of the crystalline A / M / X material, and optionally wherein the layer further comprises a matrix material and said particles are dispersed in the matrix material. 38. A photovoltaic device according to any one of embodiments 34 to 37 which comprises a photoactive region, wherein the photoactive region comprises an n-type region comprising at least one n-type layer, a p-type region comprising at least one p-type layer, and, disposed between the n-type region and the p-type region: said layer comprising the passivated A / M / X material, optionally wherein the photoactive region is disposed between a first electrode and a second electrode. 39. A photovoltaic device according to any one of embodiments 34 to 38 wherein the layer comprising the passivated A / M / X material has a thickness of from about 10 nm to about 100 μm, optionally from about 10 nm to about 10 μm, preferably from about 100 nm to about 1000 nm. 40. A photovoltaic device according to any one of embodiments 1 to 33 wherein the passivated A / M / X material comprises particles of the passivated A / M / X material, optionally wherein the particles are nanoparticles, and optionally wherein each of the particles comprises a particle of the crystalline A / M / X material and said passivating agent disposed on a surface of the crystalline A / M / X material, and optionally wherein the particles are dispersed within a matrix material. 41. A photovoltaic device according to any one of the preceding embodiments, wherein the compound of formula [A]a[M]b[X]cis a compound of formula [A][M][X]3, wherein [A], [M] and [X] are as defined in embodiment 1. 42. A photovoltaic device according to any one of the preceding embodiments, wherein said one or more X anions are one or more halide anions, and preferably wherein said one or more A cations are monocations, said one or more M cations are dications. 43. A photovoltaic device according to any one of the preceding embodiments, wherein said one or more A cations comprise at least one organic cation. 44. A photovoltaic device according to any one of the preceding embodiments, wherein [A] comprises two or more different A cations. 45. A photovoltaic device according to any one of the preceding embodiments, wherein each A cation is selected from: an alkali metal cation; a cation of the formula [R1R2R3R4N]+, wherein each of R1, R2, R3, R4is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12aryl, and at least one of R1, R2, R3and R4is not hydrogen; a cation of the formula [R5R6N=CH-NR7R8]+, wherein each of R5, R6, R7 and R8 is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12aryl; and C1-10alkylamammonium, C2-10alkenylammonium, C1-10alkyliminium, C3-10cycloalkylammonium and C3-10cycloalkyliminium, each of which is unsubstituted or substituted with one or more substituents selected from amino, C1-6alkylamino, imino, C1-6alkylimino, C1-6alkyl, C2-6alkenyl, C3-6cycloalkyl and C6-12aryl; preferably wherein each A cation is selected from Cs+, Rb+, formamidinium, guanidinium, methylammonium, and ethylammonium. 46. A photovoltaic device according to any one of the preceding embodiments, wherein [X] comprises two or more different X anions, preferably wherein the two or more different X anions are two or more different halide anions, optionally wherein [X] comprises Br and I. 47. A photovoltaic device according to any one of the preceding embodiments, wherein [A] comprises formamidinium, optionally wherein [A] further comprises Cs+. 48. A photovoltaic device according to any one of the preceding embodiments, wherein [A] does not comprise methylammonium, or [A] consists of methylammonium and at least one A cation other than methylammonium, provided that the molar fraction of methylammonium in [A] is less than 15 % of [A]. 49. A photovoltaic device according to any one of the preceding embodiments, wherein [M] comprises an M cation selected from Ca2+, Sr2+, Cd2+, Cu2+, Ni2+, Mn2+, Fe2+, Co2+, Pd2+, Ge2+, Sn2+, Pb2+, Yb2+and Eu2+, optionally selected from Sn2+, Pb2+, Cu2+, Ge2+, and Ni2+; preferably selected from Sn2+and Pb2+, preferably wherein [M] comprises Pb2+. 50. A photovoltaic device according to any one of the preceding embodiments, wherein the compound of formula [A]a[M]b[X]cis a compound of formula [Csx(H2N–C(H)=NH2)1-x]Pb[IyBr1-y]3wherein x is greater than 0 and less than 1, and y is greater than 0 and less than 1. 51. A photovoltaic device according to any one of the preceding embodiments, wherein the photovoltaic device is a solar cell. 52. A photovoltaic device according to any one of the preceding embodiments, wherein the photovoltaic device is a tandem-junction or a multi-junction photovoltaic device. 53. Use of a passivated A / M / X material as a sensitizer in a photovoltaic device, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]c wherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound. 54. A process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (i) treating said crystalline A / M / X material with said passivating agent; or (ii) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: producing a photovoltaic device comprising the passivated A / M / X material. 55. A process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (i) treating said crystalline A / M / X material with said passivating agent; or (ii) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: using the passivated A / M / X material as a sensitizer in a photovoltaic device. 56. A process according to embodiment 54 or embodiment 55 wherein the passivated A / M / X material is in the form of a layer and the process comprises (i) treating a layer comprising said crystalline A / M / X material with said passivating agent, wherein the layer comprising said crystalline A / M / X material is disposed on a substrate; or (ii) producing, on a substrate, a layer comprising said crystalline A / M / X material in the presence of said passivating agent, said producing comprising exposing the substrate to said one or more A cations, said one or more M cations, said one or more X anions and said passivating agent, optionally wherein said one or more A cations, said one or more M cations, said one or more X anions and said passivating agent are present in one or more different phases selected from vapour phases and solution phases. 57. A process according to embodiment 56 wherein the substrate comprises a first charge- transporting region comprising at least one layer of a first charge-transporting material, and optionally a first electrode, optionally wherein the process further comprises disposing, on the layer comprising said crystalline A / M / X material: a second charge-transporting region comprising at least one layer of a second charge-transporting material and, optionally, a second electrode, preferably wherein the first charge-transporting region is a hole-transporting region and the first charge-transporting material is a hole-transporting material, and the second charge-transporting region is an electron-transporting region and the second charge- transporting material is an electron-transporting material, or the first charge-transporting region is an electron-transporting region and the first charge-transporting material is an electron-transporting material and the second charge-transporting region is a hole- transporting region and the second charge-transporting material is a hole-transporting material. 58. A process according to any one of embodiments 54 to 57, wherein (i) treating said crystalline A / M / X material with said passivating agent comprises vapourising said passivating agent to produce the passivating agent in the vapour phase and treating said crystalline A / M / X material with said passivating agent in the vapour phase. 59. A process according to any one of embodiments 54 to 57, wherein (i) treating said crystalline A / M / X material with said passivating agent comprises preparing a solution of said passivating agent, and treating said crystalline A / M / X material with said solution of the passivating agent.

Claims

CLAIMS 1. A photovoltaic device which comprises a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound.

2. A photovoltaic device according to claim 1 wherein the organic compound comprises a silane moiety and an amine moiety, wherein the amine moiety is a secondary amine, a tertiary amine or a quaternary amine.

3. A photovoltaic device according to claim 1 or claim 2 wherein the organic compound comprises a silane moiety and a plurality of amine moieties, wherein at least one of the amine moieties is a secondary amine, a tertiary amine or a quaternary amine, and optionally wherein at least one of the amine moieties is a primary amine.

4. A photovoltaic device according to claim 2 or claim 3 wherein the organic compound comprises an organic group bonded to a silicon atom of the silane moiety, wherein each of said amine moieties is present in said organic group.

5. A photovoltaic device according to claim 4 wherein said organic group is a C1-20alkyl group, wherein each amine moiety which is a secondary, tertiary or quaternary amine may either interrupt the carbon chain of the C1-20alkyl group or be present as a substituent on the C1-20alkyl group, and wherein any amine moiety which is a primary amine is present as a substituent on the C1-20alkyl group, and wherein the C1-20alkyl group is otherwise unsubstituted or substituted.

6. A photovoltaic device according to claim 4 or claim 5 wherein the organic compound has the formula (I)wherein: R is said organic group; and E1, E2and E3are independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted aryloxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryl, unsubstituted or substituted acyl, unsubstituted or substituted ester, unsubstituted or substituted acyloxy, hydroxyl and halo.

7. A photovoltaic device according to claim 6 wherein R is a C1-20alkyl group as defined in claim 5, and E1, E2and E3are unsubstituted or substituted C1-10alkoxy, preferably unsubstituted C1-10alkoxy, more preferably methoxy.

8. A photovoltaic device according to any one of the preceding claims wherein the organic compound has the formula (II)wherein: E1, E2and E3are as defined in claim 6, and are preferably independently selected from H, unsubstituted or substituted C1-10alkoxy, unsubstituted or substituted C1-10alkyl, unsubstituted or substituted aryloxy, and unsubstituted or substituted aryl; q is 0 or an integer from 1 to 5; each m is independently an integer from 1 to 5; each n is independently 0 or 1; p is 0 or an integer from 1 to 5; G is NR12 or CR13; each R1is independently H or C1-6alkyl; wherein when each n is 0 and G is NR12, or when q is 0 and G is NR12, then at least one R1group is C1-6alkyl, and preferably when G is NR12 and q is not 0, then p is not 0.

9. A photovoltaic device according to any one of the preceding claims wherein the organic compound is selected from:(i) PTMS; (ii) MAPTMS ;; (iv) AEAPTMS, optionally wherein the organic compound is selected from (ii) MAPTMS, (iii) DMAPTMS, (iv) AEAPTMS, and (v) (AE)2APTMS.

10. A photovoltaic device according to any one of the preceding claims wherein said dimer of the organic compound comprises (i) a first monomer unit, wherein the firstmonomer unit is an organic compound as defined in any one of the preceding claims provided that the silicon atom of the silane moiety of the first monomer unit is substituted with a linking atom or group, and (ii) a second monomer unit, wherein the second monomer unit is an organic compound as defined in any one of the preceding claims provided that the silicon atom of the silane moiety of the second monomer unit is substituted with said linking atom or group, wherein the linking atom or group is covalently bonded to the silicon atom of the silane moiety of the first monomer unit and to the silicon atom of the silane moiety of the second monomer unit, to link the first monomer unit to the second monomer unit.

11. A photovoltaic device according to claim 10 wherein the dimer has formula (Ia) or formula (IIa):wherein L is said linking atom or group, and each R, E1and E2is independently as defined in claim 6 or claim 7;wherein L is said linking atom or group, and each G, p, n, m, q, E1and E2is independently as defined in claim 8.

12. A photovoltaic device according to any one of claims 1 to 9 wherein said polymer of the organic compound comprises a monomer unit which is an organic compound as defined in any one of claims 1 to 9, provided that the silicon atom of the silane moiety is substituted with at least one linking atom or group, wherein the or each linking atom or group is covalently bonded to said silicon atom and is also covalently bonded to the silicon atom of the silane moiety of another monomer unit in the polymer.

13. A photovoltaic device according to claim 12 wherein the polymer of the organic compound comprises a monomer unit of formula (Ib) or formula (IIb):wherein L is a said linking atom or group, and R and E1are as defined in claim 6 or claim 7;wherein L is a said linking atom or group, and G, p, n, m, q and E1are as defined in claim 8, preferably wherein the polymer comprises at least three monomer units of formula (Ib) or formula (IIb).

14. A photovoltaic device according to any one of claim 12 or claim 13 wherein the polymer comprises a structure of formula (Ic) or formula (IIc):wherein each L is a said linking atom or group, and each R and E1is independently as defined in claim 6 or claim 7, and z is an integer of at least 3;wherein each L is a said linking atom or group, and each G, p, n, m, q and E1is independently as defined claim 8, and z is an integer of at least 3.

15. A photovoltaic device according to any one of the preceding claims, wherein each A cation is selected from: an alkali metal cation; a cation of the formula [R1R2R3R4N]+, wherein each of R1, R2, R3, R4is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12 aryl, and at least one of R1, R2, R3 and R4 is not hydrogen; a cation of the formula [R5R6N=CH-NR7R8]+, wherein each of R5, R6, R7and R8is independently selected from hydrogen, unsubstituted or substituted C1-20alkyl, and unsubstituted or substituted C6-12aryl; and C1-10alkylamammonium, C2-10alkenylammonium, C1-10alkyliminium, C3-10cycloalkylammonium and C3-10cycloalkyliminium, each of which is unsubstituted or substituted with one or more substituents selected from amino, C1-6alkylamino, imino, C1-6alkylimino, C1-6alkyl, C2-6alkenyl, C3-6cycloalkyl and C6-12aryl; preferably wherein each A cation is selected from Cs+, Rb+, formamidinium, guanidinium, methylammonium, and ethylammonium.

16. A photovoltaic device according to any one of the preceding claims, wherein [X] comprises two or more different X anions, preferably wherein the two or more different X anions are two or more different halide anions, optionally wherein [X] comprises Br and I.

17. A photovoltaic device according to any one of the preceding claims, wherein [A] comprises formamidinium, optionally wherein [A] further comprises Cs+.

18. A photovoltaic device according to any one of the preceding claims, wherein [M] comprises an M cation selected from Ca2+, Sr2+, Cd2+, Cu2+, Ni2+, Mn2+, Fe2+, Co2+, Pd2+, Ge2+, Sn2+, Pb2+, Yb2+and Eu2+, optionally selected from Sn2+, Pb2+, Cu2+, Ge2+, and Ni2+; preferably selected from Sn2+and Pb2+, preferably wherein [M] comprises Pb2+.

19. A photovoltaic device according to any one of the preceding claims, wherein the compound of formula [A]a[M]b[X]cis a compound of formula [Csx(H2N–C(H)=NH2)1-x]Pb[IyBr1-y]3wherein x is greater than 0 and less than 1, and y is greater than 0 and less than 1.

20. A photovoltaic device according to any one of the preceding claims, wherein the photovoltaic device is a solar cell.

21. A photovoltaic device according to any one of the preceding claims, wherein the photovoltaic device is a tandem-junction or a multi-junction photovoltaic device.

22. Use of a passivated A / M / X material as a sensitizer in a photovoltaic device, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, andwherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound.

23. A process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine, or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (iii) treating said crystalline A / M / X material with said passivating agent; or(iv) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: producing a photovoltaic device comprising the passivated A / M / X material.

24. A process for producing a passivated A / M / X material, wherein the passivated A / M / X material comprises a crystalline A / M / X material and a passivating agent, wherein the crystalline A / M / X material comprises a compound of formula [A]a[M]b[X]cwherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18, and wherein the passivating agent comprises an organic compound, wherein the organic compound comprises a silane moiety and (a) does not comprise an amine moiety, or (b) comprises an amine moiety which is a secondary amine, a tertiary amine or a quaternary amine,or the passivating agent comprises a dimer or a polymer of the organic compound, wherein the process comprises: (iii) treating said crystalline A / M / X material with said passivating agent; or (iv) producing said crystalline A / M / X material from said one or more A cations, said one or more M cations and said one or more X anions, in the presence of said passivating agent; and thereby producing the passivated A / M / X material, and wherein the process further comprises: using the passivated A / M / X material as a sensitizer in a photovoltaic device.

25. A process according to claim 23 or claim 24 wherein the passivated A / M / X material is in the form of a layer and the process comprises (i) treating a layer comprising said crystalline A / M / X material with said passivating agent, wherein the layer comprising said crystalline A / M / X material is disposed on a substrate; or (ii) producing, on a substrate, a layer comprising said crystalline A / M / X material in the presence of said passivating agent, said producing comprising exposing the substrate to said one or more A cations, said one or more M cations, said one or more X anions and said passivating agent, optionally wherein said one or more A cations, said one or more M cations, said one or more X anions and said passivating agent are present in one or more different phases selected from vapour phases and solution phases.

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