Method of production of perovskite single crystal & uses thereof

Abstract

In various embodiments, the present invention provides methods of production of metal halide perovskite single crystals, metal halide perovskite single crystals, methods of production of electrodes on metal halide perovskite single crystals, methods of production of optoelectronic devices comprising metal halide perovskite single crystals, and optoelectronic devices comprising metal halide perovskite single crystals. Metal halide perovskite single crystals and optoelectronic devices comprising metal halide perovskite single crystals according to the invention may find particular utility in X-ray detectors, gamma-ray detectors, neutron detectors, proton detectors, or visible photodetectors.

Description

[0001] 008832545

[0002] 1

[0003] Method of production of perovskite single crystal & uses thereof

[0004] Field of the Invention

[0005] The present invention relates to methods of production of metal halide perovskite single crystals and particularly, although not exclusively, to methods of production of single crystals suitable for use in high- performance radiation detectors, such as X-ray and gamma-ray detectors. It also relates to methods for production of optoelectronic devices incorporating such single crystals and methods for production of electrodes on such single crystals.

[0006] Background

[0007] Metal halide perovskites have attracted tremendous scientific interest since they were first reported in 2009 as light harvesters for efficient solar cells. Perovskites are inexpensive photoactive semiconductors featuring large extinction coefficients, long charge carrier lifetimes, high photoluminescence quantum yields (PLQY) and tuneable bandgaps, which allow control of light absorption and emission on demand. These properties make them useful for a broad range of optoelectronic applications, such as solar cells, LEDs, lasers, and sensors.

[0008] In particular, cost effective single crystals of metal halide perovskites prepared via solution methods have found use in radiation detectors, such as X-ray and gamma-ray detectors. However, the quality of single crystals is paramount for high-performance of such detectors.

[0009] Various techniques for production of high-quality perovskite single crystals via solution methods are known in the art. For example, Zhao et al, “High-yield growth of FACsPbBrs single crystals with low defect density from mixed solvents for gamma-ray spectroscopy”, nature photonics (2023), https: / / doi.org / 10.1038 / s41566-023-01154-8 [9], proposes the growth of centimetre-sized formamidinium- caesium lead bromide (FACsPbBrs) single crystals at high yield from low-purity (98%) precursors using a solution method. Supplementary Figure 8 from this work shows that a fabricated y-ray detector utilising FACsPbBrs crystals with a thickness of about 1.5 mm produced according to methods disclosed therein and having an asymmetrical Au / Bi metal electrode configuration achieved ~10 nA / cm2current density at a reverse (-100V) bias voltage.

[0010] However, there is an ongoing desire to provide improved techniques for production of reproducible single crystals of metal halide perovskites in a facile manner.

[0011] Additionally, known processes for incorporating single crystals in optoelectronic devices can be complex and time consuming. There is also an ongoing desire to provide techniques for integration of single crystals of metal halide perovskites into optoelectronic devices in a facile manner.

[0012] The present invention has been devised in light of the above considerations. 008832545

[0013] 2

[0014] Summary of the Invention

[0015] In a first aspect, the present invention provides a method of production of a metal halide perovskite single crystal having a formula selected from:

[0016] AMX3

[0017] A'2MX4, or

[0018] A'2An-i MnXsn+i , wherein n > 1 ; the method including steps of: providing a metal halide perovskite solution comprising ions of A and / or A’, M and X in a solvent mixture, wherein:

[0019] A and / or A’ is an organic or inorganic cation;

[0020] M is a divalent cation selected from the group consisting of: Pb2+, Sn2+, Ge2+or combinations thereof;

[0021] X is a halide selected from the group consisting of: F; Ch, Br, I' or combinations thereof; and wherein the solvent mixture comprises dimethylformamide (DMF), gamma-Butyrolactone (GBL) and at least one of dimethyl sulfoxide (DMSO) and / V-methylpyrrolidone (NMP), the DMSO and / or NMP being present in the solvent mixture in amounts of 5 vol% or less, based on the total volume of the solvent mixture; and wherein the method further comprises a step of processing the metal halide perovskite solution to form the metal halide perovskite single crystal.

[0022] The present inventors have found that such methods may lead to production of metal halide perovskite single crystals of high quality. In particular, the inventors have found that small amounts of DMSO and / or NMP added to the solvent mixture can act as nucleation inhibiting agents. The present inventors theorise that this is a result of formation of an intermediate product in solution (M-DMSO-AX I M-DMSO-A’X or M- NMP-AX / M-NMP-A’X, depending on the nature of the nucleation inhibiting agent added) that decreases the total number of nucleation sites available, thereby allowing for better controlled formation of one bulk, high-quality single crystal in solution vs formation of a plurality of small crystals.

[0023] As noted above, A and / or A’ is an organic or inorganic cation. The notation A is used to represent a cation that occupies an intralayer site in the metal halide perovskite structure. The notation A’ is used to represent a cation that occupies an interlayer site in the metal halide perovskite structure.

[0024] The nature of A and / or A’ is not particularly limited, and it is contemplated that the present invention may find application across a broad range of possible organic and / or inorganic cations. However, in some preferred embodiments, A and / or A’ may be selected from the group consisting of: formamidinum (FA), methyammonium (MA), dimethylammonium (DMA), cesium (Cs), potassium (K), phenylethylammonium (PEA), butylammonium (BA), and combinations thereof.

[0025] Suitably, A may be selected from the group consisting of: formamidinum (FA), methyammonium (MA), dimethylammonium (DMA), cesium (Cs), potassium (K) and combinations thereof, although it is contemplated that other cations may also be suitable. 008832545

[0026] 3

[0027] A’ may be a bigger organic cation than A. Suitably, A’ may be selected from the group consisting of: phenylethylammonium (PEA), butylammonium (BA), and combinations thereof, although it is contemplated that other cations may also be suitable.

[0028] For example, to form 2D or quasi-2D perovskites, A’ may be selected from the group consisting of: linear alkylammonium cations, diammonium cations, cycloalkylammonium cations with saturated ring backbones, aromatic ammonium cations, aromatic diammonium species, heteroaryl / heteroalkyl ammonium cations, and combinations thereof.

[0029] Linear alkylammonium cations comprise alkyl chains of any length terminated with one -NH3+group, such as ethylammonium (C2), propylammonium (C3), butylammonium (C4), pentylammonium (C5), hexylammonium (C6), etc. Diammonium cations comprise alkyl chains of any length terminated with two - NH3+groups, such as ethylenediammonium, propanediammonium, butanediammonium, hexanediammonium, etc. Cycloalkylammonium cations with saturated ring backbones comprise, e.g., cyclopentylammonium, cyclohexylammonium, cycloheptylammonium. Aromatic ammonium cations comprise, e.g., phenylethylammonium, benzylammonium, naphthylmethylammonium. Aromatic diammonium species comprise, e.g., phenylenediammonium, xylylenediammonium.

[0030] Heteroaryl / heteroalkyl ammonium cations comprise, e.g., pyridinium or imidazolium derivatives, where consistent with perovskite formation.

[0031] The step of processing the metal halide perovskite solution to form the metal halide perovskite single crystal may include one or more of:

[0032] (i) heating the metal halide perovskite solution to a predetermined temperature sufficient to form the metal halide perovskite single crystal via inverse temperature crystallisation; ii) evaporating a proportion of the solvents present in the solvent mixture at ambient condition and / or via vacuum extraction of the solvent, to form the metal halide perovskite single crystal; and / or

[0033] (Hi) cooling the metal halide perovskite solution to form the metal halide perovskite single crystal.

[0034] The use of an inverse temperature crystallisation method may be particular preferred, due to the high controllability of such methods. The present inventors have found that employing inverse temperature crystallisation methods from mixed solvent solutions which include small amounts of DMSO and / or NMP, particularly high quality crystals can be formed. Where inverse temperature crystallisation is used to form the metal halide perovskite single crystal, the predetermined temperature may be in a range of from 30 °C to 90 °C, preferably in a range of from 30 °C to 70 °C, or in a range of from 35 °C to 65 °C, preferably about 40 °C. The predetermined temperature may correspond to the inverse temperature solubility for dissolved metal halide perovskite in the solvent mixture. The crystals may be allowed to grow for any suitable time period, e.g. some methods, the crystals may be allowed to grow over a time period of between 1 and 7 days. In other methods, shorter or longer timer periods may be used.

[0035] A processing step which involves cooling the metal halide perovskite solution to form the metal halide perovskite single crystal may find particular applicability in methods where the metal halide perovskite has 008832545

[0036] 4 a formula of A2MX4, or A'2An-iMnXsn+i, wherein n > 1. Such methods may be employed where the metal halide perovskite solution is already at an elevated temperature (e.g. a temperature above ambient temperature such as a temperature >25 °C) - e.g. where the metal halide perovskite solution is heated in order to encourage dissolution of one or more precursor compounds to form the solution.

[0037] The metal halide perovskite solution may be formed in any suitable manner known in the art. Suitably, it may be formed by dissolving one or more metal halide perovskite precursor compounds in the solvent mixture.

[0038] The one or more metal halide perovskite precursor compounds may comprise MX2 and AX and / or A’X. Equimolar amounts of MX2 and AX may be dissolved in the solvent mixture. Alternatively, the amounts of MX2 and AX and / or A’X dissolved in the solvent mixture may not be equimolar. Excess amounts of either MX2 and AX and / or A’X may be dissolved in the solvent mixture to form the metal halide perovskite solution

[0039] The identity of the metal halide perovskite may constitute a wide variety of compounds represented by combinations of the A and / or A’, M and X components identified above, although some compounds within this grouping may be preferred in view of their suitability for high-quality single crystal formation and / or their suitability for use in optoelectronic devices.

[0040] Preferably A comprises or consists of FA, MA or Cs. Preferably M is Pb2+. Preferably X is I; Br or Ch That is, the metal halide perovskite may be more preferably selected from the group consisting of: FAPbBrs, MAPbBrs, CsPbBrs, FAPbCE, MAPbCE, and CsPbBrs. Most preferably, the metal halide perovskite may be FAPbBrs. Where the metal halide perovskite is FAPbBrs, the one or more metal halide perovskite precursor compounds may comprise FABr and PbBr2.

[0041] As noted above, the present invention utilises small amounts of DMSO and / or NMP added to the solvent mixture, with the amount of DMSO and / or NMP present in the solvent mixture being 5 vol% or less, based on the total volume of the solvent mixture.

[0042] The DMSO may be present in the solvent mixture in amounts of 4 vol% or less, or from 1 vol% to 3 vol%, more preferably in amounts of around 2 vol%, based on the total volume of the solvent mixture.

[0043] The NMP may be present in the solvent mixture in amounts of in amounts of 4 vol% or less, 3 vol % or less, 2 vol % or less, or from 0.1 vol% to 2 vol%, more preferably in amounts of around 0.5 to 1 vol%, based on the total volume of the solvent mixture. In some cases the use of NMP may be preferred over the use of DMSO as smaller amounts of NMP may be necessary vs the amount of DMSO required to achieve similar effects.

[0044] Where the solvent mixture comprises both DMSO and NMP, the total amount of both of these components in solution may be in 4 vol% or less, e.g. from 1 vol% to 3 vol%, more preferably in amounts of around 2 vol%, based on the total volume of the solvent mixture.

[0045] It has been found that the use of DMSO and / or NMP in amounts as described above may lead to a good balance between formation of high-quality single crystals and process efficiency (in particular with respect 008832545

[0046] 5 to crystallisation time). If larger volumes of DMSO and / or NMP are used, the crystallisation time may be extended, with it taking a longer time to produce the desired single crystal. Additionally, a higher processing temperature may be required when the crystal is formed via an inverse temperature crystallisation method, which may increase the cost and reduce the efficiency of the process. In smaller volumes of DMSO and / or NMP are used, the resultant crystal quality may be lower: obvious defects may be seen.

[0047] The solvent mixture may conveniently comprise dimethylformamide (DMF) and gamma-Butyrolactone (GBL) in a ratio of from 1 :3 to 1 :1 , preferably in a ratio of about 1 :1. It has been found that such ratios may provide optimised solubility of the precursor compounds in the solvent mixture.

[0048] The method may further include dissolving guanidinium bromide (GuaBr) in the metal halide perovskite solution. It has been found that GuaBr may also act as a nucleation inhibitor, further slowing nucleation of the AMX3 crystals from solution during the step of processing the metal halide perovskite solution to form the metal halide perovskite single crystal. When GuaBr is added, it may be added in amounts of from 1 to 3 mol%, preferably 3 mol%, based on the amount of A + A’ + X in solution, e.g. based on the amount of AX where AX is used as a precursor for preparing the metal halide perovskite solution.

[0049] If the single crystal is simply removed from the perovskite solution and dried, residual solution on the surface can etch back the crystal, leading to a very rough morphology, and any remaining solution may rapidly recrystallise on the surface. Accordingly, the method may further include one or more washing and / or drying steps. The one or more washing and / or drying steps may comprise: removing the single crystal from the growth solution (e.g. using tweezers, such as PTFE tweezers); dipping the single crystal into oil (e.g. Kemet Lubricating Fluid, Type K, Code: 301124); rinsing the single crystal within the oil to remove residual perovskite solution from the surface; removing the single crystal from the oil, and removing and residual oil (e.g. by transferring the single crystal into an organic solvent (e.g. chlorobenzene, toluene, anisole, isopropanol, or another solvent capable of removing the oil without damaging the crystal) to remove the oil); drying the single crystal (e.g. in ambient air, or with a flow of nitrogen or compressed air to assist drying).

[0050] As noted above, methods according to the first aspect can result in formation of metal halide perovskite single crystals of higher quality than those known in the art. One suitable metric for assessing the quality of the metal halide perovskite single crystal is determination of the full width at half maximum (FWHM) of X-ray diffraction (XRD) rocking curve for the single crystal. Another suitable metric for assessing the quality of the metal halide perovskite single crystal is determination of the homogeneity of contrast in optical microscopy images of the crystal (e.g. in light reflection mode and / or polarized light reflection mode and / or light transmission mode). A further suitable metric for assessing the quality of the metal halide perovskite single crystal is determination of the uniformity of photoluminescence (one- and / or two- photon measurements) emission throughout the bulk of the crystal. 008832545

[0051] 6

[0052] In a second aspect, the present invention provides a metal halide perovskite single crystal having a formula selected from:

[0053] AMX3

[0054] A'2MX4, or

[0055] A'2An-i MnXsn+i , wherein n > 1 , wherein:

[0056] A and / or A’ is an organic or inorganic cation;

[0057] M is a divalent cation selected from the group consisting of Pb2+, Sn2+, Ge2+or combinations thereof;

[0058] X is a halide selected from the group consisting of F-, Cl; Br-, I- or combinations thereof.

[0059] Preferably the metal halide perovskite single crystal has a full width at half maximum (FWHM) of X-ray diffraction (XRD) rocking curve of 0.01 degrees or less. More preferably the metal halide perovskite single crystal has a full width at half maximum (FWHM) of X-ray diffraction (XRD) rocking curve of less than 0.005 degrees, e.g. in some embodiments, the full width at half maximum (FWHM) of X-ray diffraction (XRD) rocking curve may be 0.0047 or less.

[0060] Where the single crystal displays a FWHM of XRD rocking curve in the above ranges, this is an indication that the crystal has low defect density, and is of high quality, making it suitable for use in a wide range of applications.

[0061] The single crystal may have a surface roughness of 2 nm or less. The surface roughness may suitably be measured using a profilometer, e.g. by using a stylus profilometer which comprises a diamond-tip stylus that contacts the sample and maintains a constant stylus force as the sample stage moves the sample under the stylus tip to trace a profile. One example of a suitable profilometer than can be used for such measurement is the DektakXT ®. The resolution of this profilometer is around 0.5 nm.

[0062] The metal halide perovskite single crystal may be macroscopically large in size. That is, the metal halide perovskite single crystal may have at least one dimension that is 1 mm or more, 2 mm or more, 3mm or more, 5 mm or more, 1 cm or more, 5 cm or more or 10 cm or more. Preferably the single crystal has at least two dimensions of such size. That is, the single crystal may have at least one surface having an area of 1 mm x 1 mm or more, 2 mm x 2 mm or more, 3 mm x 3 mm or more, 5 mm x 5 mm or more, 1 cm x 1 cm or more, 2 cm x 2 cm or more, 5 cm x 5 cm or more, or 10 cm x 10 cm or more. In general, larger crystal may be preferable. A crystal having at least one surface having an area of 2 cm x 2 cm or more may be required in order to provide suitable coverage of a number of commercially available device substrates by the crystal. Accordingly, it may be preferred for the crystal to be of such size. Larger sizes may be preferred for industrial use.

[0063] Crystals according to the second aspect may be obtained from methods according to the first aspect. Other compositional preferences discussed above in relation to the first aspect apply also to the second aspect: e.g. A and / or A’ may suitably be selected from the group consisting of: formamidinum (FA), methyammonium (MA), cesium (Cs), phenylethylammonium (PEA), butylammonium (BA), and combinations thereof, although it is contemplated that other cations may also be suitable. 008832545

[0064] 7

[0065] The present inventors have found that methods according to the first aspect can be used to form freestanding metal halide perovskite single crystals, or can alternatively be used for direct growth of metal halide perovskite single crystals onto a substrate. Where the method comprises direct growth of metal halide perovskite single crystals onto a substrate, the method may form part of a method of production of an optoelectronic device.

[0066] Accordingly, in a third aspect, the present invention provides a method of production of an optoelectronic device, the method comprising: providing a metal halide perovskite solution comprising ions of A and / or A’, M and X in a solvent mixture, wherein

[0067] A and / or A’ is an organic or inorganic cation;

[0068] M is a divalent cation selected from the group consisting of Pb2+, Sn2+, Ge2+or combinations thereof;

[0069] X is a halide selected from the group consisting of F; Cl; Br, I' or combinations thereof; and wherein the solvent mixture comprises dimethylformamide (DMF), gamma-Butyrolactone (GBL) and at least one of dimethyl sulfoxide (DMSO) and / V-methylpyrrolidone (NMP), the DMSO and / or NMP being present in the solvent mixture in amounts of 5 vol% or less, based on the total volume of the solvent mixture; and wherein the method further comprises steps of: immersing a device substrate in the metal halide perovskite solution; and processing the metal halide perovskite solution to form the metal halide perovskite single crystal via monolithic growth upon the device substrate.

[0070] Optional features set out above in relation to the first aspect apply also to this aspect. For example, the compositional features of the metal halide perovskite solution in a solvent mixture apply also to this aspect. The metal halide perovskite single crystal may have a formula selected from AMX3, A2MX4, or A'2An-i MnXsn+i , wherein n > 1. Other compositional preferences discussed above in relation to the first aspect apply also to the second aspect: e.g. A and / or A may suitably be selected from the group consisting of: formamidinum (FA), methyammonium (MA), cesium (Os), phenylethylammonium (PEA), butylammonium (BA), and combinations thereof, although it is contemplated that other cations may also be suitable.

[0071] The step of processing the metal halide perovskite solution to form the metal halide perovskite single crystal via monolithic growth upon the substrate may include one or more of:

[0072] (i) heating the metal halide perovskite solution to a predetermined temperature sufficient to form the metal halide perovskite single crystal via inverse temperature crystallisation; ii) evaporating a proportion of the solvents present in the solvent mixture at ambient condition and / or via vacuum extraction of the solvent, to form the metal halide perovskite single crystal; and / or

[0073] (iii) cooling the metal halide perovskite solution to form the metal halide perovskite single crystal. 008832545

[0074] 8

[0075] As noted above in relation to the first aspect, the use of an inverse temperature crystallisation method may be particular preferred, and may take place at a predetermined temperature in a range of from 30 °C to 90 °C, preferably in a range of from 30 °C to 70 °C, or in a range of from 35 °C to 65 °C, preferably about 40 °C.

[0076] The nature of the device substrate is not particularly limited, and it is contemplated that such methods may find wide applicability for formation of metal halide perovskite crystals via monolithic growth upon a wide range of possible substrates. However, there may be specific advantages provided by selection of specific substrates. The device substrate may comprise or consist of a complementary metal-oxide- semiconductor (CMOS) device. The device substrate may comprise or consist of one or more integrated circuits. In other words, preferably the device substrate is not a plain silicon substrate. The device substrate may comprise an array of pixel electrodes, each electrode connected to a corresponding pixel circuit. Each pixel electrode may be formed from a suitable conductive material, e.g. metal, such as aluminium (Al) or gold (Au). The pixel electrodes may be separated by insulating regions formed of a suitable insulating material, e.g. a polymeric or resin material, e.g. polyimide.

[0077] In some examples the device substrate may comprise or consist of an application specific integrated circuit (ASIC). The ASIC may be a high energy X-ray imaging technology (HEXITEC) ASIC: the structure of such devices is discussed in detail in Jones et al “Spectroscopic X-ray imaging at MHz frame rates — the HEXITECMHZ ASIC” (2022)

[0017] , The HEXITEC ASIC is a sophisticated chip used in X-ray imaging. It comprises a plurality of aluminum pads that connect to a detector material, each aluminium pad defining a pixel configured for capturing X-ray photons. Beneath these pads, each pixel has electronics, including a preamplifier and a shaper, to amplify and refine the signal. This signal is then converted from analog to digital by an ADC. The chip's digital circuits process and organize the data, which is finally transmitted for image reconstruction. Supported by a silicon substrate and powered by a precise biasing circuit, the ASIC enables high-resolution X-ray imaging. In conventional detectors employing the HEXITEC ASIC, detector materials in a single crystal form (perovskite, CdTe, CZT or Si) are typically bonded to the aluminium pads using conventional methods such as silver-loaded epoxy, indium bump bonding, anisotropic conductive paste / tape, etc. These conventional methods are costly and time-consuming.

[0078] By providing methods in which the single crystal is grown monolithically directly onto a device substrate, the number of processing steps for formation of optoelectronic devices can be reduced. The need for a specific bonding step of bonding a detector material to a pixel electrode (e.g. using silver-loaded epoxy) can be eliminated. Additionally, a higher-quality interface may be provided between the single crystal and the device substrate, due to the lack of bonding material between the pixel electrode and the single crystal detector material.

[0079] The method may include performing one or more surface modification steps on the device substrate (e.g. CMOS device / ASIC) prior to immersing the device substrate in the metal halide perovskite solution.

[0080] The one or more surface modification steps may include forming a self-assembled monolayer (SAM) having high affinity with metal halide perovskites on the surface of the device substrate. 008832545

[0081] 9

[0082] By providing arrangements in which the device substrate (e.g. CMOS device I ASIC) is surface-modified to provide a self-assembled monolayer (SAM) having high affinity with metal halide perovskites on its surface, adhesion between the metal halide perovskite single crystal and the device substrate (e.g. ASIC) may be improved as compared with non-surface-modified device substrate (which commonly comprises surfaces which are formed from hydrophobic materials such as polyimide).

[0083] The self-assembled monolayer may comprise a plurality of anchoring molecules comprising a head group attached to the substrate and a tail group directly or indirectly attached to the head group.

[0084] The head group may be selected from the group consisting of: an amine group (-NH2); a carboxylic acid group (-COOH); a phosphonic acid group (-PO3H2); a thiol group (-SH); and an ethoxysilane group (-O-SI- (Si-OH)). The specific head group may be selected based on the nature of the device substrate to which the SAM is to be bonded. For example, where the device substrate comprises Al regions, the use of phosphonic acid groups may be preferred. Where the device substrate comprises Au regions, the use of thiol head groups may be preferred. Where the device substrate comprises polyimide regions, the use of amine groups may be preferred.

[0085] The tail group may be selected from the group consisting of: -NH2, -NH3+, -Cl, -Br, -I, N, -NO2, and -CF3. Preferably, the tail group comprises -NH2.

[0086] It has been found that provision of SAMs which include -NH2 tail groups are particularly preferred as they can provide for particularly good adhesion between the metal halide perovskite crystal and the SAM (and accordingly between the metal halide perovskite crystal and the device substrate).

[0087] The surface modification treatment which is required to form suitable SAMs may vary depending on the surface composition of the device substrate.

[0088] The method may include a surface treatment step of treating one or more surface regions of the device substrate with one or more diamine derivates, e.g. with ethylenediamine. Where the device substrate includes one or more surface regions which are formed of polyimide, the method may include a surface treatment step of treating the polyimide surface regions of the device substrate with one or more diamine derivates, e.g. with ethylenediamine.

[0089] For example, the surface treatment may include steps of immersing the device substrate in an aqueous solution of ethylenediamine, for a predetermined time period at a predetermined temperature. The aqueous solution of ethylenediamine may be a solution of 1 vol % to 50 vol% ethylenediamine in water, suitably about 10 vol% ethylenediamine in water. The predetermined time period may be from 1 minute to 10 hours, suitably about 1 hour. The predetermined temperature may be from 20 °C to 50 °C, suitably about 40 °C.

[0090] After treating the polyimide surface regions of the device substrate with one or more diamine derivates, the device substrate may be washed (e.g. with DI water and IPA)

[0091] The method may include a surface treatment step of treating one or more surface regions of the device substrate with an acid, e.g. with 2-aminoethylphosphonic acid. Where the device substrate includes one 008832545

[0092] 10 or more surface regions which are formed of aluminium, the method may include a surface treatment step of treating the aluminium surface regions of the device substrate with an acid, e.g. with 2- aminoethylphosphonic acid. Suitably, the surface treatment may include steps of immersing the device substrate in an ethanolic solution of 2-aminoethylphosphonic acid, for a predetermined time period at a predetermined temperature. The ethanolic solution of of 2-aminoethylphosphonic acid may comprise 0.1 wt% to 10 wt%, suitably 1 wt% of of 2-aminoethylphosphonic acid in ethanol. The predetermined time period may be from 1 hour to 24 hours, suitably about 12 hours (e.g. overnight). The predetermined temperature may be from 20 °C to 30 °C, suitably about 25 °C (e.g. at room temperature).

[0093] After the one or more surface treatment steps have been performed, the method may further include producing a metal halide perovskite single crystal on the device substrate, e.g. using methods in accordance with the first aspect. For example, after the one or more surface treatment steps have been performed, the device substrate may be positioned at the bottom of a glass vessel containing a metal halide perovskite solution for producing the single crystal.

[0094] After the one or more surface treatment steps have been performed, the method may further include steps of removing the device substrate having the metal halide perovskite single crystal formed upon it from the solution.

[0095] The device substrate + metal halide perovskite single crystal may be subject to one or more washing and / or drying steps after being removed from the solution. For example, the device substrate + metal halide perovskite single crystal may be subject to one or more washing and / or drying steps in accordance with the first aspect.

[0096] The method may further include a step of depositing one or more electrodes on the metal halide perovskite single crystal. The electrode(s) may be formed from any suitable material. In some arrangements, the electrode(s) may be formed from gold. The electrode(s) may be deposited on an opposing side of the metal halide perovskite single crystal to the side of the metal halide perovskite single crystal which is attached to the device substrate. Suitable methods for depositing one or more electrodes on the metal halide perovskite single crystal may include: thermal evaporation in a vacuum; E beam deposition; forming a carbon electrode by printing; forming a metal electrode by printing (e.g. printing of metal nanoparticles). The electrode(s) may be formed according to a method as set out in the fifth aspect of the present invention (see below).

[0097] The resulting optoelectronic device may be a radiation detector, e.g. an X-ray detector, a gamma-ray detector, a neutron detector, a proton detector, or a visible photodetector.

[0098] The method may further include a step of preconditioning the optoelectronic device. The preconditioning step may comprise subjecting the device to forward-reverse (F-R) bias conditioning, for example by subjecting the device to forward-bias conditioning and subsequently switching to reverse-bias conditioning. Forward-bias conditioning and / or reverse-bias conditioning may be performed by applying a predetermined bias voltage to the device. For example, forward-bias conditioning may be performed by applying a bias voltage of approximately -1000 V (absolute voltage approximately 1000 V), or another 008832545

[0099] 11 appropriate predetermined voltage. Reverse-bias conditioning may be performed by applying a bias voltage of approximately 1000 V (absolute voltage approximately 1000 V), or another appropriate predetermined voltage. Multiple conditioning cycles may be performed by repeatedly subjecting the device to forward-reverse (F-R) bias conditioning, e.g. subjecting the device to two or more cycles of forward-reverse (F-R) bias conditioning.

[0100] Methods according to the third aspect can result in formation of optoelectronic devices of higher quality than those known in the art. One suitable metric for assessing the quality of the optoelectronic device is determination of the dark current of the optoelectronic device, e.g. determination of a steady-state value of the dark current, a stability of the dark current during exposure to radiation (such as X-rays or gammarays), a dark current density, or a dark current drift. Another suitable metric for assessing the quality of the optoelectronic device is determination of the detection sensitivity of the device (e.g. detection sensitivity to radiation such as X-rays or gamma-rays). Further suitable metrics for assessing the quality of the optoelectronic device include determination of the charge mobility (hole mobility or electron mobility), the charge mobility-lifetime product (hole mobility-lifetime product or electron mobility-lifetime product), and the energy resolution of spectral detection (e.g. gamma-ray spectra).

[0101] It is further contemplated that the methods of surface treatment of a device substrate as described above in relation to the third aspect may find application outside of methods of formation of an optoelectronic device. For example, it is contemplated that the methods may be separately applied to a device substrate that is then sold as a surface-modified device substrate product.

[0102] Accordingly, in a fourth aspect, the present invention provides a method of surface modification of a device substrate including a step of forming a self-assembled monolayer (SAM) on a surface of the device substrate.

[0103] The device substrate may comprise or consist of a complementary metal-oxide-semiconductor (CMOS) device. The device substrate may comprise or consist of one or more integrated circuits. For example the device substrate may comprise or consist of an application specific integrated circuit (ASIC). The ASIC may be a high energy X-ray imaging technology (HEXITEC) ASIC. The device substrate may comprise an array of pixel electrodes, each electrode connected to a corresponding pixel circuit. Each pixel electrode may be formed from a suitable conductive material, e.g. a metal, such as aluminium (Al) or gold (Au). The pixel electrodes may be separated by insulating regions formed of a suitable insulating material, e.g. a polymeric or resin material, e.g. polyimide. The pixel electrode size may range from several tens of nanometers up to about 10 mm. The pixel size may be between 100 pm and 500 pm, with approximately 250 pm being a particularly suitable dimension.

[0104] Other features as set out above in relation to third aspect relating to the surface modification of the device substrate apply also to this aspect. That is, the SAM may have a high affinity with perovskite structures (AMX3, A2MX4, and / or A'2An-iMnX3n+i crystals). The SAM preferably comprises a plurality of anchoring molecules comprising a head group attached to the substrate and a tail group directly or indirectly attached to the head group. The tail group preferably comprises -NH2. The method may include a surface treatment step of treating one or more surface regions of the device substrate with one or more 008832545

[0105] 12 diamine derivates, e.g. with ethylenediamine. The method may include a surface treatment step of treating one or more surface regions of the device substrate with an acid, e.g. with 2- aminoethylphosphonic acid.

[0106] When integrating a single crystal of metal halide perovskite into an optoelectronic device such as an X- ray detector, in general it is desirable to form pixel electrodes on one side the single crystal for high resolution imaging. Plain electrodes can then be formed on the other side of the single crystal, for flipchip bonding to ASICs.

[0107] Standard microfabrication techniques such as photolithography are usually relied upon to form the pixel electrodes in many optoelectronic devices known in the art. However, such standard techniques typically involve aqueous developer solutions or organic lift-off solvents, such as / V-methylpyrrolidone (NMP), acetone, and dimethylformamide (DMF), which are incompatible with metal halide perovskites due to their high solubility and degradation in such environments. This sensitivity severely limits the applicability of standard lithographic methods for patterning fine electrodes on perovskite single crystals.

[0108] The fabrication of pixel electrodes with micron-scale pixel pitch on metal halide perovskite single crystals is particularly challenging. While shadow masks can be used to define large-area electrode structures, they offer poor resolution and are inadequate for achieving small pixel pitches below several hundred microns. In contrast, photolithography offers sub-micron resolution, making it essential for advanced integration, but only if the perovskite material can be protected during processing.

[0109] There is thus an ongoing desire to provide improved techniques for forming electrodes (in particular pixel electrodes with micron-scale pixel pitch) on metal halide perovskite single crystals in a facile manner.

[0110] Accordingly, in a fifth aspect, the present invention provides a method of production of electrodes on a metal halide perovskite single crystal, the method comprising steps of: forming a protective layer on a surface of the metal halide perovskite single crystal; and forming one or more electrodes on the protective layer.

[0111] The present inventors have found that the use of a conformal, chemically robust, and electronically compatible (i.e. charge-transporting) protective layer can provide shielding of a metal halide perovskite single crystal during photolithographic processing. Forming such a protective layer on the surface of the single crystal allows conventional photoresists and patterning techniques to be used without degrading the underlying crystal, facilitating easier production of electrodes (in particular pixel electrodes with micron-scale pixel pitch) on the crystal.

[0112] The composition of the protective layer is not particularly limited, and it is contemplated that the present invention may find application across a broad range of possible compositions of the protective layer. However, in some preferred embodiments, the protective layer may comprise, consist essentially of, or consist of a metal oxide and / or a metal chalcogenide. The metal oxide may be selected from the group consisting of SnC>2, TiCh, ZnO, NiO, AI2O3, V2O5, MoOs, SiC>2, ln2Os, HfC>2, ZrC>2, Ta2Os, Nb20s, Y2O3, WOx, and combinations thereof. The metal chalcogenide may be selected from the group consisting of M0S2, MoSe2, and combinations thereof. The present inventors have found that tin-based layers, e.g. 008832545

[0113] 13 layers comprising or consisting of SnC>2 or other tin oxides, may be particularly well-suited to providing the desirable properties of the protective layer.

[0114] The step of forming one or more electrodes on the protective layer may comprise forming an array of electrodes on the protective layer. For example, a 1D or 2D array of electrodes may be formed. The array may be a square array or a rectangular array, although it will be appreciated that the shape of the array is not particularly limited and that the array may be formed in any shape as desired for the particular applications of the device. Similarly, each electrode may be a square electrode, but the shape of each electrode is not particularly limited and other shapes (e.g. circular electrodes) may be used.

[0115] The array may correspond to an array of pixels. In this case, the electrodes may be referred to as pixel electrodes. Throughout the present disclosure, the terms “pixel electrodes” and “array of pixels” are used interchangeably.

[0116] The array of pixels may be formed with a pixel pitch of 10 mm or less, optionally 1 mm or less, optionally 500 pm or less, optionally approximately 250 pm. In typical applications of the array of pixels, a minimum pixel pitch on the scale of tens of nanometres is desired. Accordingly, the array of pixels may be formed with a pixel pitch of at least 1 nm, optionally at least 10 nm. The array of pixels may be formed with a pixel pitch of from 100 pm to 500 pm. The array of pixels may be formed with an inter-pixel edge separation of 100 pm or less, optionally approximately 50 pm. The inter-pixel edge separation corresponds to the gap between adjacent pixels. As described above, forming the array of pixels with micron-scale structure can provide high resolution detection when the metal halide perovskite single crystal is integrated into an optoelectronic device such as an X-ray detector or gamma-ray detector.

[0117] The one or more electrodes may be formed with a thickness of 10 pm or less, optionally approximately 100 nm. Nevertheless, it will be appreciated that larger electrode thicknesses, e.g. thicknesses at the millimetre scale may also be suitable, dependent on the capability of the electrode formation method (e.g. evaporation). The one or more electrodes may be formed from a suitable conductive material, e.g. a metal. Metals having a work function of about 2.8-4.7 eV are particularly suitable. The metal may be selected from the group consisting of Ti, Au, Al, Bi, Cr, In, Ag, Ca, Cu, Ga, Gain, Nd, Pb, Sn, Tb, V, Cd, La, Mg, TI, W, Zn, As, Hf, Mn.

[0118] The protective layer may be formed by atomic layer deposition. The present inventors have found that atomic layer deposition is a particularly convenient process for forming the protective layer without damaging the single crystal. The precursor and reactant for atomic layer deposition should be chosen in accordance with the desired composition of the protective layer. Each composition will have an optimal precursor. Where the protective layer is a tin-based layer, the step of forming the protective layer on the surface of the metal halide perovskite single crystal may suitably comprise depositing a tin oxide (e.g. SnO2) on the surface by atomic layer deposition. In this example, the atomic layer deposition may be performed with tetrakis(d imethylamino)tin( IV) (TDMASn) as a precursor and H2O as a reactant, although it is contemplated that other precursors may also be suitable. 008832545

[0119] 14

[0120] The protective layer may be formed with a thickness of 200 nm or less, optionally approximately 100 nm.

[0121] The protective layer may be formed with a thickness of 10 nm or more, e.g. with a thickness of from 10 nm to 100 nm.

[0122] The step of forming one or more electrodes on the protective layer may comprise forming the one or more electrodes using a photolithography process. As noted above, the presence of the protective layer permits standard photolithographic processes to be used without damaging the single crystal. This includes the use of aqueous developer solutions or organic lift-off solvents. As such, any suitable photolithography process may be used to form the one or more electrodes.

[0123] In some examples, forming the one or more electrodes using a photolithography process comprises: forming a patterned photoresist layer on the protective layer by photolithography; forming an electrically conductive layer on the patterned photoresist layer and exposed parts of the protective layer; removing the patterned photoresist layer, thereby forming the one or more electrodes from the electrically conductive layer.

[0124] The patterned photoresist layer is a layer of photoresist which is non-uniform, for example because only some of the photoresist has been developed. As a result, only a portion of the protective layer is covered by the patterned photoresist layer. The exposed parts of the protective layer are those parts of the protective layer that are not covered by the patterned photoresist layer.

[0125] After the patterned photoresist layer is removed, the electrically conductive layer remains on the protective layer only at the locations where the patterned photoresist layer was not present (i.e. at the parts of the protective layer that were exposed prior to formation of the electrically conductive layer). In other words, the parts of the electrically conductive layer that are formed on top of the patterned photoresist layer are removed when the patterned photoresist layer is removed. In this way, the pattern of the photoresist layer defines the pattern in which the one or more electrodes are formed.

[0126] Forming the patterned photoresist layer on the protective layer by photolithography may comprise: depositing a photoresist onto the protective layer to form a photoresist layer; exposing at least part of the photoresist layer to light via a photomask; developing the exposed photoresist, thereby forming the patterned photoresist layer.

[0127] As discussed above, the presence of the protective layer permits standard photolithographic processes to be used without damaging the single crystal. As such, any suitable photoresist and photomask may be used, and any suitable methods of depositing and developing the photoresist may be used. For example, either a positive photomask or a negative photomask may be used.

[0128] In some examples, the photoresist is deposited by spin-coating. The photoresist may be annealed prior to exposure. Any suitable light may be used in the exposure step, depending on the choice of photoresist. In some examples, UV light is used, e.g. light with a wavelength of approximately 400 nm.

[0129] In some examples, the exposed photoresist is developed in a water-based solution. The exposed photoresist may be rinsed after development, e.g. rinsed with pure water. 008832545

[0130] 15

[0131] Forming the electrically conductive layer on the patterned photoresist layer and exposed parts of the protective layer may comprise depositing the electrically conductive layer by evaporation, although it will be appreciated that the present invention is not limited to forming the electrically conductive layer by evaporation and any suitable process may be used.

[0132] Removing the patterned photoresist layer may comprise dissolving the patterned photoresist layer in one or more solvents. The one or more solvents may comprise organic solvents, for example N- methylpyrrolidone (NMP), acetone, and / or dimethylformamide (DMF). Multiple dissolving steps may be performed sequentially to ensure that all the patterned photoresist layer (and the parts of the electrically conductive layer formed on the patterned photoresist layer) is removed. Different dissolving steps may use different solvents.

[0133] The metal halide perovskite single crystal may be produced according to a method as set out in the first aspect of the invention. Optional features set out above in relation to the first aspect apply also to this aspect. The metal halide perovskite single crystal may be a metal halide perovskite single crystal according to the second aspect. Optional features set out above in relation to the second aspect apply also to this aspect.

[0134] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0135] Summary of the Figures

[0136] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0137] Figure 1 shows (a)207Pb NMR spectra and (b)1H NMR spectra of solutions of FAPbBrs in DMF:GBL (1:1 volume ratio), with addition of 0 vol% (black), 2 vol% (red), and 5 vol% (blue) DMSO.

[0138] Figure 2 shows images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of (a) 0 vol%, (b) 1 vol%, and (c) 2 vol% DMSO, respectively.

[0139] Figure 3 shows (a)17O NMR spectra of a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 2 vol% DMSO, before (red) and after (blue) crystal growth, and (b) FTIR spectra of the same solution, before (red) and after (blue) crystal growth, as well as neat solvent (green).

[0140] Figure 4 shows a schematic illustration of molecular interactions between DMSO and precursor ions.

[0141] Figure 5 shows conceptual diagrams of precursor environments without (left) and with (right) DMSO.

[0142] Figure 6 shows images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of (a) 0.5 vol%, and (b) 1 vol% NMP, respectively.

[0143] Figure 7 shows images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of 3 mol% GuaBr with respect to the amount of FABr in solution, and addition of (a) 0 vol%, (b) 1 vol%, (c) 2 vol%, and (d) 3 vol% DMSO, respectively. 008832545

[0144] 16

[0145] Figure 8 shows optical microscopy images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of 0 vol% (left column), 1 vol% (middle column), and 2 vol% (right column) DMSO, taken in light reflection mode (top row), polarized light reflection mode (middle row), and light transmission mode (bottom row). Scale bars in the optical images represent 0.5 mm.

[0146] Figure 9 shows optical microscopy images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of 0.5 vol% (left column), and 1 vol% (right column) NMP, taken in light reflection mode (top row), polarized light reflection mode (middle row), and light transmission mode (bottom row). Scale bars in the optical images represent 0.5 mm.

[0147] Figure 10 shows 3D two-photon photoluminescence (PL) mapping of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 : 1 volume ratio), with addition of (a) 0 vol%, (b) 1 vol%, (c) and 2 vol% DMSO, respectively.

[0148] Figure 11 shows two-photon excitation PL spectra of a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1:1 volume ratio), with addition of 2 vol% DMSO, recorded at different excitation depths (excitation depth increases from red plots to blue plots).

[0149] Figure 12 shows charge carrier diffusion dynamics measured via two-photon excitation PL for a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 2 vol% DMSO, at excitation depths of (a) 0 pm, (b) -4 pm, (c) -8 pm, (d), -12 pm, (e) -16 pm, and (f) -20 pm, respectively. In each case, the plots show: time evolution of the one-dimensional spatial distribution of PL profiles (left plot); selected one-dimensional spatial PL profiles at different times, with data plotted as solid lines and Gaussian fits plotted as dotted lines (middle plot); and time evolution of the squared broadening quantity, a2, extracted from the Gaussian fits (right plot).

[0150] Figure 13 shows XRD patterns of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of (a) 0 vol%, (b) 1 vol%, and (c) 2 vol% DMSO, respectively.

[0151] Figure 14 shows XRD patterns of a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 2% DMSO (red) compared against that of a commercially available MAPbBrs single crystal (black). A comparison against MAPbBrs was performed in view of the lack of commercial availability of FAPbBrs crystals.

[0152] Figure 15 shows XRD rocking curve measurements of a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 2% DMSO (red) compared against that of a commercially available MAPbBrs single crystal (black). The 2% DMSO FAPbBrs crystal shows narrower FWHM (0.0047 degree) than the commercial MAPbBrs (FWHM of 0.0173 degree).

[0153] Figure 16 shows XRD patterns of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of (a) 0.5 vol%, and (b) 1 vol% NMP, respectively.

[0154] Figure 17 shows XRD rocking curve measurements of FAPbBrs crystals grown from a solution of

[0155] FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 0.5 vol% (light blue) and 1 vol% (dark blue) NMP. 008832545

[0156] 17

[0157] Figure 18 shows an image of a commercially available HEXITEC device which can act as a device substrate in various methods according to the invention.

[0158] Figure 19 shows a schematic cross-sectional diagram of part of the HEXITEC device of Figure 6 showing the process of chemical surface modification to provide a SAM on the HEXITEC device surface.

[0159] Figure 20 shows various images of a FAPbBrs crystal monolithically grown on a surface-modified HEXITEC device by methods according to the present invention.

[0160] Figure 21 shows a (FAo.ssMAo.ioCso.osPbOo.ssBro.is ) single crystal grown directly on a surface-modified HEXITEC device by methods according to the present invention.

[0161] Figure 22 shows an image of the FAPbBrs crystal monolithically grown on a surface-modified HEXITEC device shown in Figure 20, after gold electrode evaporation under vacuum (106Pa) pressure.

[0162] Figure 23 shows the single pixel X-ray / gamma-ray detectors having a Bi-FAPbBrs-Au device structure, formed with the 2% DMSO FAPbBrs single crystal.

[0163] Figure 24 shows a schematic figure of one possible X-ray measurement set up for single pixel X-ray detectors.

[0164] Figure 25 shows the X-ray induced current of the X-ray detectors shown in Fig. 23 at different applied bias voltages.

[0165] Figure 26 shows a comparison of current density against bias voltage for (a) a device as disclosed in Zhao et al, “High-yield growth of FACsPbBrs single crystals with low defect density from mixed solvents for gamma-ray spectroscopy”, nature photonics (2023) [9], compared against (b) a device of Fig. 23, according to the present invention.

[0166] Figure 27 shows (a) a schematic of a Schottky-type X-ray detector architecture as used in the devices of Fig. 23, having a Bi-FAPbBrs-Au device structure formed with the 2% DMSO FAPbBrs single crystal, and dark current behaviour of such a detector under (b) conventional reverse-bias (Reverse-only) preconditioning, (c) forward-bias (Forward-only) preconditioning and (d) forward to reverse bias switching (Forward-Reverse).

[0167] Figure 28 shows behaviour of a Schottky-type X-ray detector as shown Fig. 23, having a Bi-FAPbBrs-Au device structure formed with the 2% DMSO FAPbBrs single crystal, in Energy-Integrating detector (EID) mode after forward-reverse (F-R) bias conditioning: (a) photocurrent and dark current across various electric fields for various X-ray dose rates, (b) dark current density as a function of electric field compared to known perovskite-based Schottky detectors disclosed in references [1] to

[0016] , (c) signal-to-noise ratio (SNR) versus X-ray dose rate across various electric fields, and (d) current stability during continuous X- ray exposure at high dose rate and high electric field.

[0168] Figure 29 shows behaviour of a detector as shown Fig. 23, having a Bi-FAPbBrs-Au device structure formed with the 2% DMSO FAPbBrs single crystal, operating in single-photon counting mode for a / y radiation after forward-reverse (F-R) bias conditioning: (a) normalised signals averaged from -300 pulses, recorded from a F-R bias preconditioned detector exposed to 5.5 MeV a-particles from a241Am 008832545

[0169] 18 source across various electric fields, (b) histograms of rise times recorded under various electric fields, (c) peak channel position of the a-particle signal as a function of applied voltage, (d) drift velocity as a function of electric field derived from rise time measurements, and (e) gamma-ray spectra from241Am (59.5 keV) recorded under various electric fields.

[0170] Figure 30 shows a schematic of a process for producing pixel electrodes on a 2 vol% DMSO-grown FAPbBrs single-crystal by methods according to the present invention, using atomic layer deposited (ALD) tin oxide (SnCh) as a protective layer, a water-based solvent for photoresist development, and N MP / Acetone based solvent for photoresist lift-off.

[0171] Figure 31 shows optical images of a FAPbBr3 / ALD-SnO2 / photoresist structure formed by methods according to the present invention, taken at the centre and corners of the structure after photoresist development.

[0172] Figure 32 shows optical images of Titanium (Ti) pixel electrodes deposited on a 2 vol% DMSO-grown FAPbBrs single crystal by methods according to the present invention, taken at the centre and corners of the structure.

[0173] Detailed Description of the Invention

[0174] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0175] The discussion below includes description of suitable methodologies for production of metal halide perovskite single crystals according to the present invention. It also includes discussion of methods of surface modification of a device substrate including a step of forming a self-assembled monolayer (SAM) on a surface of the device substrate, and methods for subsequently growing a metal halide perovskite single crystal on the surface-modified device substrate to form an optoelectronic device.

[0176] In addition, the discussion below includes description of suitable methodologies for formation of pixel electrodes on a metal halide perovskite single crystal according to the present invention.

[0177] Production and characterisation of single crystals having a FAPbBrs structure

[0178] A 1.4M saturated solution of FAPbBrs was prepared in a solvent mixture of DMF:GBL (1 :1 vol%) to form a precursor solution, and various samples of this precursor solution were modified by addition of nucleation inhibiting agents including NMP and / or DMSO, and GuaBr, in various amounts.

[0179] To elucidate the role of DMSO additives in modulating precursor interactions during crystal growth, solution-state NMR spectroscopy was performed on FAPbBrs precursor solutions (1 :1 vol% DMF:GBL) with various DMSO concentrations.

[0180] Fig. 1 shows (a)207Pb NMR spectra and (b)1H NMR spectra of precursor solutions containing 0 vol% (black), 2 vol% (red), and 5 vol% (blue) DMSO. The207Pb NMR spectra in Fig. 1(a) exhibit a clear 008832545

[0181] 19 downfield shift upon addition of DMSO, indicative of coordination between DMSO and Pb2+ions. This shift reflects increased electron density at the Pb nucleus, arising from electron donation by the lone pair on the oxygen atom in DMSO. The1H NMR spectra in Fig. 1(b) show peak broadening in the presence of DMSO, suggesting hindered rotational motion of FA+due to weak interactions with DMSO molecules.

[0182] To form the crystals, the samples of precursor solution were initially heated to 60 °C until one or more seed crystals were observed to appear. At this point, the temperature was decreased to around 40 °C to allow for crystal growth via the inverse temperature crystallisation (ITO) method. The crystals were allowed to grow for a time period of between 4 and 7 days. Experiments were also performed in which the samples of precursor solution were initially heated to 40 °C - in this case, the seed crystals were observed after 1 to 2 days.

[0183] Fig. 2 shows images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of (a) 0 vol%, (b) 1 vol%, and (c) 2 vol% DMSO, respectively. In Fig. 2(a), many small crystals can be seen: because of uncontrolled growth without addition of a nucleation inhibiting additive, many nucleation sites appear and many small crystals form in the beaker. With the use of DMSO additive, the number of nucleation sites significantly decreased - it is theorised that this is the result of DMSO interacting with Pb2+ions to form an intermediate product, Pb-DMSO-FABr. The effect of a reduced number of nucleation sites is seen in Fig. 2(b) and (c). With the optimal concentration of DMSO (2% in vol), only one bulk single crystal was formed in the solution in Fig. 2(c).

[0184] Fig. 3 shows (a)17O NMR spectra of a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 2 vol% DMSO, before (red) and after (blue) crystal growth, and (b) FTIR spectra of the same solution, before (red) and after (blue) crystal growth, as well as neat solvent (green) (i.e. DMF:GBL in 1 :1 volume ratio, with 2 vol% DMSO but without FAPbBrs).

[0185] The17O NMR spectra in Fig. 3(a) provide direct evidence of DMSO coordination through its oxygen atom. The inset panel shows that before crystal growth the DMSO17O NMR signals were very weak due to DMSO binding to Pb2+and FA+, leaving very few free DMSO molecules in the system. However, free DMSO molecules exhibited well-resolved17O resonances after crystal growth, due to the removal of Pb2+and FA+from the solution. The FTIR spectra in Fig. 3(b) reveal suppression of the S=O stretching vibration at 1062 cm-1in the presence of Pb2+and FA+(see inset panel), which recovered after crystal harvesting, corroborating the reversible coordination of DMSO during crystallisation.

[0186] Fig. 4 shows a schematic illustration of molecular interactions between DMSO and precursor ions, providing a suggestion of how DMSO forms coordination complexes with both Pb2+and FA+in solution. Fig. 5 shows conceptual diagrams of precursor environments without (left) and with (right) DMSO, providing a suggestion of how DMSO coordination slows nucleation and enables controlled crystallisation.

[0187] Based on the coordination chemistry discussed above and depicted in Figs. 4 and 5, the following crystallisation mechanism is proposed. In the absence of DMSO, loosely bound precursors readily aggregate, leading to rapid and uncontrolled nucleation. In contrast, DMSO-coordinated species impose a kinetic barrier to crystallisation, as dissociation of the coordination bonds is required prior to nucleation. 008832545

[0188] 20

[0189] Upon heating and solvent evaporation, gradual dissociation of Pb2+and FA+from the intermediate complexes allows slow release of precursors into the crystallisation, enabling controlled and oriented crystal growth.

[0190] As noted above, various nucleation inhibiting agents were investigated. Fig. 6 shows images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1:1 volume ratio), with addition of (a) 0.5 vol%, and (b) 1 vol% NMP, respectively. Similar to DMSO, with the use of NMP additive, the number of nucleation sites significantly decreased - it is theorised that this is the result of DMSO interacting with Pb2+ions to form an intermediate product, Pb-NMP-FABr. The effect of a reduced number of nucleation sites is seen in Fig. 6(a) and (b).

[0191] Fig. 7 shows images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of 3% GuaBr and addition of (a) 0 vol%, (b) 1 vol%, (c) 2 vol%, and (d) 3 vol% DMSO, respectively. Similarly to Fig. 2, it can be seen than when no DMSO was added, many small crystal formed, but with addition of amount of DMSO of 1 vol% and greater, the resulting crystals were large and high-quality.

[0192] As can be seen from Figs. 2, 6, and 7, the resulting crystals which were formed by methods including addition of a nucleation inhibiting agent (DMSO or NMP) in small amounts were macroscopically large single crystals, with perfect rectangular shapes. For crystals formed using DMSO as a nucleation inhibiting agent, the highest quality crystals were found to be those formed from solutions containing DMSO in amount of around 2 vol% based on the total volume of the solvent mixture, although crystals formed from solutions containing DMSO in amounts of from 1-3 vol% were also found to be of suitably high quality. For crystals formed using NMP as a nucleation inhibiting agent, the highest quality crystals were found to be those formed from solutions containing NMP in amount of around 0.5 to 1 vol% based on the total volume of the solvent mixture.

[0193] The quality of the resulting crystals was characterised by a variety of techniques.

[0194] Optical microscopy was performed on some of the resulting crystals formed using DMSO or NMP as a nucleation inhibiting agent; the results are shown in Figs. 8 and 9. Fig. 8 shows optical microscopy images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of 0 vol% (left column), 1 vol% (middle column), and 2 vol% (right column) DMSO. Fig. 9 shows optical microscopy images of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 : 1 volume ratio), with addition of 0.5 vol% (left column), and 1 vol% (right column) NMP. In each case, the optical microscopy images were taken in light reflection mode (top row), polarized light reflection mode (middle row), and light transmission mode (bottom row). Scale bars in the optical images represent 0.5 mm.

[0195] As shown in Fig. 8, crystals grown without DMSO displayed heterogeneous contrast in reflection, polarised reflection, and transmission modes, indicating grain boundaries and defects. In contrast, crystals grown with 1% DMSO showed moderate improvement, while those prepared with 2% DMSO exhibited uniform contrast across all imaging modes, suggesting superior crystallinity and fewer internal 008832545

[0196] 21 defects. Crystals grown with NMP (see Fig. 9) displayed homogeneous contrast in reflection, polarised reflection, and transmission modes, indicating good uniformity.

[0197] Some of the resulting crystals formed using DMSO as a nucleation inhibiting agent were examined using two-photon photoluminescence. Figure 10 shows 3D two-photon PL mapping of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio), with addition of (a) 0 vol%, (b) 1 vol%, (c) and 2 vol% DMSO, respectively. Figure 11 shows two-photon excitation PL spectra of a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1:1 volume ratio), with addition of 2 vol% DMSO, recorded at different excitation depths (excitation depth increases from red plots to blue plots).

[0198] Crystals grown without DMSO exhibited non-uniform PL intensity (see Fig. 10(a)), attributed to compositional inhomogeneity and a high density of trap states. In contrast, crystals grown with DMSO displayed uniform PL emission throughout the bulk (see Fig. 10(b)-(c)), further supporting the notion that DMSO additives improve crystal quality by reducing defect density and compositional fluctuation. The two-photon excitation PL spectra of Fig. 9 demonstrate that, in the bulk of the crystal, emission was red- shifted at larger excitation depths due to self-absorption of emission.

[0199] Two-photon excitation PL spectroscopy was also used to investigate the intrinsic charge transport properties of crystals grown using the amount of DMSO that led to the highest crystal quality (2 vol% DMSO). Fig. 12 shows charge carrier diffusion dynamics measured via two-photon excitation PL for a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1:1 volume ratio) with addition of 2 vol% DMSO, at excitation depths of (a) 0 pm, (b) -4 pm, (c) -8 pm, (d), -12 pm, (e) -16 pm, and (f) -20 pm, respectively.

[0200] In each case, the plots show: time evolution of the one-dimensional spatial distribution of PL profiles (left plot); selected one-dimensional spatial PL profiles at different times, with data plotted as solid lines and Gaussian fits plotted as dotted lines (middle plot); and time evolution of the squared broadening quantity, a2, extracted from the Gaussian fits (right plot). The plots represent bulk charge carrier diffusion dynamics using two-photon excitation, and they show that the carrier distribution broadens with time. The diffusion coefficient, D, can be extracted from the gradient of the data points in the right plots.

[0201] A diffusion coefficient, D, of 0.48 cm2 / s was obtained at the crystal surface (z = 0 pm, see Fig. 12(a)), with comparable values measured throughout the bulk up to z = -20 pm (see Fig. 12(f)), reflecting spatially uniform carrier diffusion characteristics. Applying the Einstein relation yields an estimated carrier mobility of 19.2 cm2 / V s , underscoring the high electronic quality of the crystal. This corresponds to an effective (ambipolar) mobility at zero field and is limited by whatever recombination was happening in the PL experiment.

[0202] The quality of the resulting crystals was also determined by X-ray diffraction (XRD) analysis, in particular, by rocking curve measurement analysis. Crystals that were formed in the presence of small amounts of nucleation inhibition agents (e.g. with small amounts of DMSO and / or NMP in solution) showed superior performance compared to commercial MAPbBrs. 008832545

[0203] 22

[0204] The XRD characterisation results of some of the resulting crystals grown in the presence of DMSO are shown in Figs. 13 to 15: Fig. 13 shows XRD patterns of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of (a) 0 vol%, (b) 1 vol%, and (c) 2 vol% DMSO, respectively, Fig. 14 shows XRD patterns of a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 2% DMSO (red) compared against that of a commercially available MAPbBrs single crystal (black), and Fig. 15 shows XRD rocking curve measurements of a FAPbBrs crystal grown from a solution of FAPbBrs in DMF:GBL (1:1 volume ratio) with addition of 2% DMSO (red) compared against that of a commercially available MAPbBrs single crystal (black).

[0205] Fig. 13 confirms the phase purity and the single crystal nature of the crystals across all samples, with peaks indicating the presence of 100, 200 and 300 planes seen.

[0206] Fig. 14 confirms the single crystal nature of the crystals, with peaks indicating the presence of 100 and 200 planes seen.

[0207] Fig. 15 confirms the high-quality of the single crystals, with the 2% DMSO FAPbBrs crystal showing narrower FWHM (0.0047 degrees) than the commercial MAPbBrs (which had a FWHM of 0.0173 degrees).

[0208] The XRD characterisation results of some of the resulting crystals grown in the presence of NMP are shown in Figs. 17 and 18: Fig. 17 shows XRD patterns of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of (a) 0.5 vol%, and (b) 1 vol% NMP, respectively, and Fig. 18 shows XRD rocking curve measurements of FAPbBrs crystals grown from a solution of FAPbBrs in DMF:GBL (1 :1 volume ratio) with addition of 0.5 vol% (light blue) and 1 vol% (dark blue) NMP.

[0209] Fig. 17 confirms the phase purity and the single crystal nature of the crystals across all samples, with peaks indicating the presence of 100, 200 and 300 planes seen.

[0210] Fig. 18 confirms the high-quality of the single crystals, with the 0.5 vol% NMP FAPbBrs crystal showing a narrow FWHM of 0.008 degrees and the 1.0 vol% NMP FAPbBrs crystal showing a narrow FWHM of 0.0049 degrees.

[0211] Surface roughness measurements were also performed using a DektakXT® profilometer, and it was also found that these crystals had crystal surfaces which were atomically flat with a roughness of around 2 nm or less.

[0212] Forming a self-assembled monolayer (SAM) on a surface of a device substrate

[0213] Following successful formation of high-quality FAPbBrs single crystals as noted above, it was contemplated that it would be desirable to grow these crystals directly onto a device such as a HEXITEC device (in other words, the HEXITEC device acting as a device substrate for deposition of the crystals).

[0214] Fig. 18 shows an image of a commercially available HEXITEC device. The circuits of the HEXITEC comprise silicon, however the exposed surface of the HEXITEC is formed of metal aluminium (Al) and polyimide. These layers are hydrophobic, and so monolithic single crystal growth with good adhesion to 008832545

[0215] 23 the HEXITEC surface is challenging as the crystals easily slip off the HEXITEC surface after the crystal growth.

[0216] It was theorised that suitable surface modification of the HEXITEC device may allow for improved adhesion between perovskite single crystals and the surface of the HEXITEC device, thereby allow for monolithic growth of the crystal directed on the device surface. In particular, it was theorised that surface modification to provide a self-assembled monolayer (SAM) having high affinity with metal halide perovskites on the surface of the device substrate would provide suitable adhesion between the single crystals and the surface of the HEXITEC device.

[0217] Fig. 19 shows a schematic cross-sectional diagram of part of the HEXITEC device of Figure 18 showing the process of chemical surface modification to provide a SAM on the HEXITEC device surface.

[0218] The surfaces of commercial HEXITEC ASICs were chemically modified with ethylenediamine and 2- aminoethylphosphonic acid to create a perovskite adhesive self-assembled monolayer. More specifically, the following process for surface modification was followed:

[0219] First, ethylenediamine was used to treat the polyimide surface regions of the HEXITEC device (the ‘device substrate’). The HEXITEC device was immersed in a solution of 10 vol% ethylenediamine in water, for 1 hour at 40 °C. After this time, the HEXITEC device was then removed from the solution, and washed with DI water and IPA. The concept of the using diamine derivatives is to break the imide groups in the polyimide and to turn it into functionalised amide in the form of self-assembled monolayer (SAM).

[0220] Second, 2-aminoethylphosphonic acid was used to treat the Al surface regions of the HEXITEC device. The HEXITEC device was immersed in a solution of 1 wt% 2-aminoethylphosphonic acid in EtOH, overnight at room temp (25 °C). After this time, the HEXITEC device was then removed from the solution.

[0221] The resulting SAM layer (shown schematically as the thin layer above the Al pad / Polyimide layer in Fig. 19) includes a plurality of anchoring molecules comprising a head group attached to the substrate and a tail group directly or indirectly attached to the head group, where the tail group of each molecule comprises -NH2. It was found that providing of a SAM layer having -NH2 tail groups allowed for suitable good adhesion of metal halide perovskite crystals (including crystals of the form AMX3) to the SAM layer (and, as a result, to the device substrate).

[0222] In order to test the effectiveness of the surface modification methods, a series of experiments were conducted in which metal halide perovskite crystals were grown directly on surface-modified device substrates.

[0223] Growth of AMX3 single crystals on surface-modified device substrates to form an optoelectronic device

[0224] Fig. 20 shows various images of a FAPbBrs crystal monolithically grown on a surface-modified HEXITEC device by methods according to the present invention. The modified HEXITEC was immersed in the 1 ,4M saturated solution of FAPbBrs prepared in a solvent mixture of DMF:GBL (1: 1 vol%) with additional 2 vol% DMSO. The precursor solution containing HEXITEC was heated to 60 °C until one seed crystal 008832545

[0225] 24 appeared. At this point, the temperature was decreased to around 40 °C to allow for crystal growth over time. The crystal was allowed to grow for a time period of 7 days.

[0226] Fig. 21 shows a (FAo.ssMAo.ioCso.osPbOo.ssBro.is ) single crystal grown directly on a surface-modified HEXITEC device by methods according to the present invention. A precursor solution containing 1.1 M of Pbh, 0.2M of PbBr2, 1.1 M of FAI, 0.13M of MABr and 0.065M of CsBr was prepared in GBL solvent. 0.8 vol% formic acid was added. Additionally 2 vol% DMSO was added. The precursor solution containing HEXITEC was heated to 90 °C until one seed crystal appeared. At this point, the temperature was decreased to 60 °C and then increased from 60 °C to 68 °C with the heating rate of 1 °C / hour. Once the temperature reached 68 °C, the temperature was held at 68 °C for one day to allow for crystal growth over time. The crystal was allowed to grow for a time period of 1 day.

[0227] Fig. 22 shows an image of the FAPbBrs crystal monolithically grown on a surface-modified HEXITEC device shown in Figure 20, after gold electrode evaporation under vacuum (10~6Pa) pressure. Monolithically grown FAPbBrs crystal on a surface-modified HEXITEC was placed inside the vacuum evaporation chamber and gold was thermally evaporated to form the electrode with the thickness of 100 nm.

[0228] Figs. 23(a) and (b) show images of single pixel X-ray / gamma-ray detectors made from the 2 vol% DMSO FAPbBrs single crystals. The detector structure is Bi(100nm)-FAPbBr3-Au(100nm). The bottom electrode is Bi and the top centre electrode is Au. AnAu-guard band electrode is also provided for low leakage current.

[0229] Fig. 24 shows a schematic figure of one possible X-ray measurement set up for single pixel X-ray detectors. X-ray source generated from the W-anode while the detector device is placed at the holder inside the chamber. The detector device is connected to the control system for data acquisition.

[0230] Fig. 25 shows the X-ray induced current response of the X-ray detectors of Fig. 23 at different applied bias voltage.

[0231] Fig. 26 shows a comparison of current density against bias voltage for (a) a device as disclosed in Zhao et al, “High-yield growth of FACsPbBrs single crystals with low defect density from mixed solvents for gamma-ray spectroscopy”, nature photonics (2023) [9], compared against (b) a device of Fig. 23, according to the present invention. At reverse (-100V) bias voltage, the detector by Zhao et al, shows ~10 nA / cm2current density, whilst the detector according to the present invention shows ~20 nA / cm2current density, demonstrating comparable performance.

[0232] Fig. 27 shows (a) a schematic of a Schottky-type X-ray detector architecture as used in the devices of Fig. 23, having a Bi-FAPbBrs-Au device structure formed with the 2% DMSO FAPbBrs single crystal, and dark current behaviour of such a detector under (b) conventional reverse-bias (Reverse-only) preconditioning, (c) forward-bias (Forward-only) preconditioning and (d) forward to reverse bias switching (Forward-Reverse).

[0233] As shown in Fig. 27(a), the detector structure comprises a FAPbBrs single crystal active layer sandwiched between a bismuth (Bi) bottom electrode and gold (Au) top electrode. The top electrode includes a 008832545

[0234] 25 guard-band design (central and guard electrodes) to avoid edge effects. This geometry ensures efficient charge collection while suppressing lateral leakage currents.

[0235] Under conventional operation, reverse-bias preconditioning leads to dark current stabilisation over time, primarily due to ionic redistribution. Although prolonged reverse biasing has been reported to reduce trap-related dark current by partially passivating defects, this process is slow and results in a moderate dark current floor. As shown in Fig. 27(b), standard reverse-only preconditioning at 1000 V results in stable dark current floor (-136 nA / cm2) after extended biasing for -3 hours.

[0236] An alternative approach for further dark current reduction was explored using forward bias preconditioning - see Fig. 27(c). Upon applying forward bias of -1000V, the absolute value of the dark current initially increases substantially, which could be interpreted as a sign of device degradation.

[0237] However, the present inventors hypothesise that this increase instead arises from charge injection across the reduced forward barrier, enhanced by field-induced ion migration and band bending. As ion migration reaches saturation and band bending equilibrates, the device attains a steady-state condition, and the dark current correspondingly stabilises. During this process, the injected carriers may be passivating traps, which is not addressed under reverse bias.

[0238] Following forward-only biasing, the device was switched to reverse bias (F-R biasing, see Fig. 27(d)). An initial transient in dark current was observed, likely due to re-equilibration of mobile ions, followed by a marked decrease in steady-state dark current from 136 nA / cm2down to 54 nA / cm2at 1000 V. This supports the hypothesis that forward bias facilitates trap passivation through charge injection.

[0239] The present inventors have found that a single F-R conditioning cycle is sufficient to achieve and maintain a low dark current during continuous operation under reverse bias for at least 21 hours, with potential for longer duration (see Fig. 28(d), discussed below).

[0240] Fig. 28 shows behaviour of a Schottky-type X-ray detector as shown Fig. 23, having a Bi-FAPbBrs-Au device structure formed with the 2% DMSO FAPbBrs single crystal, in Energy-Integrating detector (EID) mode after forward-reverse (F-R) bias conditioning: (a) photocurrent and dark current across various electric fields for various X-ray dose rates, (b) dark current density as a function of electric field compared to known perovskite-based Schottky detectors disclosed in references [1] to

[0016] , (c) signal-to-noise ratio (SNR) versus X-ray dose rate across various electric fields, and (d) current stability during continuous X- ray exposure at high dose rate and high electric field.

[0241] The detector was exposed to dose rates ranging from 498 to 7,858 nGy / s using a 60 kV X-ray source, and photocurrent was measured across various electric fields. The photocurrent vs time plots in Fig. 28(a) show that when X-ray illumination is turned on at each dose rate, the current rises accordingly above the dark level, and returns to baseline when X-rays are off. A linear increase in photocurrent with dose rate was observed, demonstrating stable and low-noise operation with effective dark current suppression, enabled by F-R bias preconditioning. 008832545

[0242] 26

[0243] Table 1 shows data extracted from Fig. 22, demonstrating that the detector sensitivity increased from

[0244] 7,341.1 to 8,847.4 C Gy-1cm-2as the electric field increased from 411 to 4115 V / cm with negligible dark current drift, confirming enhanced charge collection at higher fields.

[0245] Table 1

[0246] To benchmark dark current performance, dark current density values were compared against perovskite Schottky-type detectors disclosed in references [1] to

[0016] - see Fig. 28(b). The F-R conditioned devices according to the present invention consistently exhibited among the lowest dark current densities reported to date, demonstrating over an order of magnitude reduction in dark current density compared to known detectors.

[0247] The limit of detection (LoD) was extracted from the signal-to-noise ratio (SNR) versus dose rate plot shown in Fig. 28(c). The LoD is defined as the dose rate at which SNR = 3 using linear fitting. LoD values remained consistently low across all applied fields, varying only from 1.2 to 4.4 nGy / s (see Table 1 ). While the LoD at the highest field (4115 V / cm) shows a modest increase to 3.7 nGy / s, this variation is minimal with only 3.2 nGy / s across an order-of-magnitude electric field range (411 to 4115 V / cm). This indicates the robustness of the F-R bias protocol in suppressing baseline noise with very low dark current drift (see Table 1) and enabling low-dose detection.

[0248] To evaluate long-term reliability, the F-R preconditioned detector was subjected to continuous X-ray exposure at a high dose rate of 20.3 Gy / s and electric field of 4,115 V / cm. No performance degradation was observed over ~21 hours of operation with total accumulated X-ray dose of 1 .485 Gy, demonstrating excellent device stability (see Fig. 28(d)).

[0249] Resolving low-energy gamma-ray photons remains a major challenge for perovskite detectors as solution-grown FAPbBrs detectors have typically suffered from higher dark current and trap-mediated noise, precluding single-photon resolution. However, the low dark current and excellent performance 008832545

[0250] 27 enabled by the 2 vol% DMSO FAPbBrs detector according to the present invention is promising for spectroscopic radiation detection.

[0251] Fig. 29 shows behaviour of a detector as shown Fig. 23, having a Bi-FAPbBrs-Au device structure formed with the 2% DMSO FAPbBrs single crystal, operating in single-photon counting mode for a / y radiation after forward-reverse (F-R) bias conditioning: (a) normalised signals averaged from -300 pulses from exposure to 5.5 MeV a-particles from a241Am source across various electric fields, (b) histograms of rise times recorded under various electric fields, (c) peak channel position of the a-particle signal as a function of applied voltage, (d) drift velocity as a function of electric field derived from rise time measurements, and (e) gamma-ray spectra from241Am (59.5 keV) recorded under various electric fields.

[0252] The single-particle response was first evaluated using a-particles (5.5 MeV) from a241Am unsealed source. The measurement was carried out in air, and the distance between the source and the detector was around 0.5 cm. Averaged preamplifier signal pulses (from -300 individual pulses) are shown in Fig. 29(a), which shows increasingly faster rise times with higher electric fields.

[0253] Rise time was defined from 0% to 90% of the pulse, which corresponds to 80% of the signal amplitude. This value was then linearly corrected to estimate the full 0-100% rise time, ensuring consistency with time-of-flight-based charge transport analysis. Rise time histograms (see Fig. 29(b)) showed a fielddependent rise time shortening from 7.9 ± 1.1 ps to 1.0 ± 0.1 ps, with the fastest observed value being 0.75 ps. Rise time is directly linked to charge carrier mobility, where shorter rise times indicate faster charge extraction, which translates into higher mobility and more complete charge collection efficiency, both essential for spectroscopic performance.

[0254] Fig. 29(c) shows the peak channel number of the a-particle signal plotted against applied bias voltage from a multichannel analyser (Easy MCA 8k). The data was fitted using the single carrier Hecht equation, yielding a hole mobility-lifetime product of pi = 3.2 * 10-4cm2 / V. Additionally, electric field-dependent rise time analysis with a linear fit to drift velocity (see Fig. 29(d)) yielded a hole mobility of p = 31 .8 ± 2.7 cm2V-1s“1.

[0255] Critically, the 2 vol% DMSO FAPbBrs detector successfully resolved the 59.5 keV gamma-ray emission from a sealed241Am source, exhibiting distinct and stable photopeaks across a range of electric fields as shown in Fig. 29(e). Data extracted from Fig. 29(e) is shown in Table 2. 008832545

[0256] 28

[0257] Table 2

[0258] At lower fields (411 and 823 V / cm), the photopeak was only marginally distinguishable from the noise floor. Therefore, energy resolution (the ratio of FWHM to peak channel number, expressed here as a percentage) was quantified at higher electric fields of 1646, 2469, 3292, and 4115 V / cm, yielding energy resolution values of 23.1%, 20.5%, 19.1%, and 18.6%, respectively. The best energy resolution of 18.6% represents the narrowest reported to date for solution-grown FAPbBrs detectors.

[0259] The 59.5 keV gamma-ray emission was selected to characterise the 2 vol% DMSO FAPbBrs detector as it presents the most challenging requirement due to low signal-to-noise ratio. While higher-energy photons may yield even better resolution, resolving 59.5 keV already demonstrates the detector's capability under demanding conditions. This energy range is also highly relevant for practical applications such as medical imaging (e.g. , identification of individual elements via their K-edge for iodine and barium contrast agents), and X-ray fluorescence (XRF), all of which require precise photon energy discrimination below 100 keV.

[0260] Production of pixel electrodes on AMX3 single crystals

[0261] Following successful formation of high-quality FAPbBrs single crystals as noted above, it was contemplated that it would be desirable to produce pixel electrodes on the single crystals for higher- resolution X-ray / gamma-ray imaging (compared to, e.g., the single Au electrode detector shown schematically in Fig. 27(a)). In particular, it would be desirable to produce pixel electrodes with micron- scale pixel pitch.

[0262] To produce such pixel electrodes on high-quality FAPbBrs single crystals, the present inventors used photolithography in combination with an ALD-SnCh protective layer. The protective layer shields the FAPbBrs crystal so that standard photolithographic techniques involving aqueous developer solutions and / or organic lift-off solvents can be used without degrading the underlying crystal. Pixel electrodes can thus be fabricated on-demand using conventional photoresists and patterning techniques, and the pixel size can be precisely controlled through photomask design.

[0263] Using this approach, the present inventors successfully demonstrated pixel electrode arrays with 250 pm pixel pitch and 50 pm inter-pixel gap on single perovskite crystals, compatible with subsequent flip-chip bonding to ASICs. The resulting pixel electrode perovskite crystals can be integrated with commercially available ASICs or custom CMOS designs for high-resolution X-ray and gamma-ray imaging applications.

[0264] Fig. 30 shows a schematic of a process for producing pixel electrodes on a 2 vol% DMSO-grown FAPbBrs single-crystal by methods according to the present invention, using atomic layer deposited (ALD) tin oxide (SnO2) as a protective layer, a water-based solvent for photoresist development, and 008832545

[0265] 29

[0266] N MP / Acetone based solvent for photoresist lift-off. The process involved the following steps, illustrated in respective panels of Fig. 30.

[0267] 1 ) ALD-SnC>2 Deposition. A 100 nm thick layer of ALD-SnCh was deposited by atomic layer deposition on top of a 2 vol% DMSO-grown FAPbBrs single crystal. The precursor for deposition was tetrakis(dimethylamino)tin(IV) (TDMASn, EpiValence) and the reactant was H2O. The chamber temperature was 100 °C and the precursor bubbler temperature was 75 °C. The pulsing sequence was: 0.6 s TDMASn pulse, 30 s purge, 0.1 s H2O pulse, 30 s purge. The growth rate was 0.1 nm / cycle.

[0268] 2) Photoresist Deposition. A layer of photoresist (MICROPOSIT™ S1813™) was spin-coated onto the ALD-SnO2-protected FAPbBr3 single crystal (7.2 mm x 7.6 mm x 2 mm), then annealed at 100 °C for 3 minutes.

[0269] 3) Exposure. Using a mask aligner, a positive photomask was aligned on the crystal. UV light (400 nm) was applied for 30 seconds.

[0270] 4) Photoresist Development. The exposed photoresist was developed in a water-based solution (MICROPOSIT™ MF™-319) for 30 seconds, followed by rinsing with pure water.

[0271] 5) The unexposed photoresist remained intact, defining the pixel pattern.

[0272] 6) Metal Electrode Deposition. A 100 nm thick layer of titanium (Ti) was evaporated onto the developed structure. Ti directly contacted the exposed FAPbBr3 / ALD-SnO2 regions while also covering the remaining photoresist areas.

[0273] 7) Lift-off Process. The remaining photoresist was dissolved in NMP (1 min) and acetone (3 min), removing excess photoresist / Ti.

[0274] 8) After the lift-off process, only the Ti pixel electrodes were left on the ALD-SnC>2-coated FAPbBrs crystal.

[0275] Fig. 31 shows optical images of a FAPbBr3 / ALD-SnO2 / photoresist structure formed by methods according to the present invention, taken at the centre and corners of the structure after photoresist development. This shows the structure at step 5) of the process shown in Fig. 30. At the end of the process, Ti will remain in the areas where photoresist has been removed. That is, the negative space of the remaining photoresist layer defines the pixel pattern.

[0276] Fig. 32 shows optical images of Titanium (Ti) pixel electrodes deposited on a 2 vol% DMSO-grown FAPbBrs single crystal by methods according to the present invention, taken at the centre and corners of the structure. This shows the structure at step 8) of the process shown in Fig. 30. The Ti pixel electrodes have 250 pm pixel pitch, with 50 pm gaps between pixels (the gaps being the separation between the edges of adjacent pixels). A total of 400 pixels were deposited in a 20x20 array.

[0277] *** 008832545

[0278] 30

[0279] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0280] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0281] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0282] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0283] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0284] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0285] References

[0286] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0287] [1] J. Pang et al, “Reconfigurable perovskite X-ray detector for intelligent imaging”, Nature Communications 15, Article number: 1769 (2024), https: / / doi.org / 10.1038 / s41467-024-46184-0

[0288] [2] M. Han et al, “Suppression of Ionic and Electronic Conductivity by Multilayer Heterojunctions Passivation Toward Sensitive and Stable Perovskite X-Ray Detectors”, Adv. Funct. Mater. 33, 2303375 (2023), https: / / doi.org / 10.1002 / adfm.202303376 008832545

[0289] 31

[0290] [3] Y. Liu et al, “Ligand assisted growth of perovskite single crystals with low defect density”, Nature Communications 12, Article number: 1686 (2021), https: / / doi.org / 10.1038 / s41467-021-21934-6

[0291] [4] Y. Liu et al, “Triple-Cation and Mixed-Halide Perovskite Single Crystal for High-Performance X-ray Imaging”, Adv. Mater. 33, 2006010 (2021 ), https: / / doi.org / 10.1002 / adma.202006010

[0292] [5] Z. Ni et al, “Identification and Suppression of Point Defects in Bromide Perovskite Single Crystals Enabling Gamma-Ray Spectroscopy”, Adv. Mater. 36, 2406193 (2024), https: / / doi.Org / 10.1002 / adma.202406193

[0293] [6] L. Pan et al, “Ultrahigh-Flux X-ray Detection by a Solution-Grown Perovskite CsPbBr3 Single-Crystal Semiconductor Detector”, Adv. Mater. 35, 2211840 (2023), https: / / doi.org / 10.1002 / adma.202211840

[0294] [7] Y. He et al, “3D / 2D Perovskite Single Crystals Heterojunction for Suppressed Ions Migration in Hard X-Ray Detection”, Adv. Funct. Mater. 31, 2104880 (2021 ), https: / / doi.org / 10.1002 / adfm.202104880

[0295] [8] L. Zhao et al, “Surface-defect-passivation-enabled near-unity charge collection efficiency in bromide- based perovskite gamma-ray spectrum devices”, Nature Photonics 18, 250-257 (2024), https: / / doi.Org / 10.1038 / S41566-023-01356-0

[0296] [9] L. Zhao et al, “High-yield growth of FACsPbBr3 single crystals with low defect density from mixed solvents for gamma-ray spectroscopy”, Nature Photonics 17, 315-323 (2023), https: / / doi.Org / 10.1038 / S41566-023-01154-8

[0297]

[0010] M. C. De Siena et al, “Extreme y-Ray Radiation Tolerance of Spectrometer-Grade CsPbBr3 Perovskite Detectors”, Adv. Mater. 35, 2303244 (2023), https: / / doi.org / 10.1002 / adma.202303244

[0298]

[0011] L. Pan et al, “Perovskite CsPbBr3 Single-Crystal Detector Operating at 1010 Photons s-1 mm-2 for Ultra-High Flux X-ray Detection”, Adv. Optical Mater. 11 , 2202946 (2023), https: / / d0i.0rg / l 0.1002 / adom.202202946

[0299]

[0012] Y. He et al, “High spectral resolution of gamma-rays at room temperature by perovskite CsPbBr3 single crystals”, Nature Communications 9, Article number: 1609 (2018), https: / / doi.org / 10.1038 / s41467- 018-04073-3

[0300]

[0013] Y. He et al, “CsPbBr3 perovskite detectors with 1.4% energy resolution for high-energy y-rays”, Nature Photonics 15, 36-42 (2021 ), https: / / doi.org / 10.1038 / s41566-020-00727-1

[0301]

[0014] Y. He et al, “Sensitivity and Detection Limit of Spectroscopic-Grade Perovskite CsPbBr3 Crystal for Hard X-Ray Detection”, Adv. Funct. Mater. 32, 2112925 (2022), https: / / doi.org / 10.1002 / adfm.202112925 008832545

[0302] 32

[0303]

[0015] L. Pan et al, “Ion Migration Enhances the Performance of Perovskite CsPbBr3 y-Ray Detectors”, Adv. Mater. Technol. 10, 2401548 (2024), https: / / doi.org / 10.1002 / admt.202401548

[0016] Y. Feng et al, “Interface Energy-Level Reorganization for Efficient Perovskite y-Ray Detectors”,

[0304] Angew.Chem. Int. Ed. 63, e202412685 (2024), https: / / doi.org / 10.1002 / anie.202412685

[0305]

[0017] L. Jones et al, “Spectroscopic X-ray imaging at MHz frame rates — the HEXITECMHZ ASIC”, J INST 7 C10012 (2022), https: / / doi.org / 10.1088 / 1748-0221 / 17 / 10 / C10012

Claims

00883254533Claims:

1. A method of production of a metal halide perovskite single crystal having a formula selected from:AMX3A'2MX4, orA'2An-iMnX3n+i, wherein n > 1 ; the method including steps of: providing a metal halide perovskite solution comprising ions of A and / or A’, M and X in a solvent mixture; whereinA and / or A’ is an organic or inorganic cation;M is a divalent cation selected from the group consisting of: Pb2+, Sn2+, Ge2+or combinations thereof;X is a halide selected from the group consisting of: F-, CI-, Br, I- or combinations thereof; and wherein the solvent mixture comprises dimethylformamide (DMF), gamma-Butyrolactone (GBL) and at least one of dimethyl sulfoxide (DMSO) and / V-methylpyrrolidone (NMP), the DMSO and / or NMP being present in the solvent mixture in amounts of 5 vol% or less, based on the total volume of the solvent mixture; and wherein the method further comprises a step of processing the metal halide perovskite solution to form the metal halide perovskite single crystal.

2. The method according to claim 1 wherein the step of processing the metal halide perovskite solution to form the metal halide perovskite single crystal includes one or more of:(i) heating the metal halide perovskite solution to a predetermined temperature sufficient to form the metal halide perovskite single crystal via inverse temperature crystallisation; ii) evaporating a proportion of the solvents present in the solvent mixture at ambient condition and / or via vacuum extraction of the solvent, to form the metal halide perovskite single crystal; and / or(iii) cooling the metal halide perovskite solution to form the metal halide perovskite single crystal.

3. The method according to claim 1 or claim 2 wherein A and / or A’ are selected from the group consisting of: formamidinum (FA), methyammonium (MA), dimethylammonium (DMA), cesium (Cs), potassium (K), phenylethylammonium (PEA), butylammonium (BA), and combinations thereof.

4. The method according to any one of the preceding claims wherein the metal halide perovskite single crystal has an AMX3structure, and is selected from the group consisting of FAPbBr3, MAPbBr3, CsPbBr3, FAPbCE, MAPbCh, and CsPbBr35. The method according to any one of the preceding claims wherein the solvent mixture comprises: DMSO present in the solvent mixture in amounts of from 1 vol% to 3 vol%, optionally in amounts of around 2 vol%; and / orNMP present in the solvent mixture in amounts of from 0.1 vol% to 2 vol%, optionally in amounts of around 0.5 vol% to 1 vol%.008832545346. The method according to any one of the preceding claims wherein the solvent mixture comprises dimethylformamide (DMF) and gamma-Butyrolactone (GBL) in a ratio of from 1 :3 to 1 : 1 preferably in a ratio of about 1 :1.

7. The method according to any one of the preceding claims wherein the method further includes dissolving guanidinium bromide (GuaBr) in the metal halide perovskite solution.

8. The method according to claim 6 wherein the guanidinium bromide (GuaBr) is present in the metal halide perovskite solution in amounts of from 1 to 3 mol%, preferably 3 mol% based on the amount of A + A’ + X in solution.

9. A method of production of electrodes on a metal halide perovskite single crystal, the method comprising steps of: forming a protective layer on a surface of the metal halide perovskite single crystal; and forming one or more electrodes on the protective layer.

10. The method according to claim 9, wherein the protective layer comprises a metal oxide and / or a metal chalcogenide.

11. The method according to claim 10, wherein the metal oxide is selected from the group consisting of SnC>2, TiC>2, ZnO, NiO, AI2O3, V2O5, MoOs, SiC>2, ln2C>3, HfC>2, ZrC>2, Ta2Os, Nb20s, Y2O3, WOx, and combinations thereof.

12. The method according to claim 10 or claim 11 , wherein the metal chalcogenide is selected from the group consisting of M0S2, MoSe2, and combinations thereof.

13. The method according to any one of claims 9 to 12, wherein the step of forming one or more electrodes on the protective layer comprises forming an array of electrodes on the protective layer.

14. The method according to claim 13, wherein the array corresponds to an array of pixels.

15. The method according to claim 14, wherein the array of pixels is formed with a pixel pitch of 10 mm or less, optionally 1 mm or less, optionally 500 pm or less, optionally approximately 250 pm.

16. The method according to claim 14 or claim 15, wherein the array of pixels is formed with an interpixel edge separation of 100 pm or less, optionally approximately 50 pm.

17. The method according to any one of claims 9 to 16, wherein the one or more electrodes are formed with a thickness of 10 pm or less, optionally approximately 100 nm.0088325453518. The method according to any one of claims 9 to 17, wherein the one or more electrodes are formed from a metal, optionally a metal selected from the group consisting of Ti, Au, Al, Bi, Cr, In, Ag, Ca, Cu, Ga, Gain, Nd, Pb, Sn, Tb, V, Cd, La, Mg, TI, W, Zn, As, Hf, Mn.

19. The method according to any one of claims 9 to 18, wherein the protective layer is formed by atomic layer deposition.

20. The method according to any one of claims 9 to 19, wherein the protective layer comprises SnO2.

21. The method according to claim 20, wherein the step of forming the protective layer on the surface of the metal halide perovskite single crystal comprises depositing SnO2 on the surface by atomic layer deposition.

22. The method according to claim 21 , wherein the atomic layer deposition is performed with tetrakis(dimethylamino)tin(IV) (TDMASn) as a precursor and H2O as a reactant.

23. The method according to any one of claims 9 to 22, wherein the protective layer is formed with a thickness of 200 nm or less, optionally approximately 100 nm.

24. The method according to any one of claims 9 to 23, wherein the step of forming one or more electrodes on the protective layer comprises forming the one or more electrodes using a photolithography process.

25. The method according to claim 24, wherein forming the one or more electrodes using a photolithography process comprises: forming a patterned photoresist layer on the protective layer by photolithography; forming an electrically conductive layer on the patterned photoresist layer and exposed parts of the protective layer; removing the patterned photoresist layer, thereby forming the one or more electrodes from the electrically conductive layer.

26. The method according to claim 25, wherein forming the patterned photoresist layer on the protective layer by photolithography comprises: depositing a photoresist onto the protective layer to form a photoresist layer; exposing at least part of the photoresist layer to light via a photomask; developing the exposed photoresist, thereby forming the patterned photoresist layer.

27. The method according to claim 25 or 26, wherein forming the electrically conductive layer on the patterned photoresist layer and exposed parts of the protective layer comprises depositing the electrically conductive layer by evaporation.0088325453628. The method according to any one of claims 25 to 27, wherein removing the patterned photoresist layer comprises dissolving the patterned photoresist layer in one or more solvents, optionally wherein the one or more solvents comprise / V-methylpyrrolidone (NMP) and / or acetone.

29. The method according to any one of claims 9 to 28, wherein the metal halide perovskite single crystal is produced according to a method as set out in any one of claims 1 to 8.

30. A method of production of an optoelectronic device, the method comprising providing a metal halide perovskite solution comprising ions of A and / or A’, M and X in a solvent mixture, wherein:A and / or A’ is an organic or inorganic cation;M is a divalent cation selected from the group consisting of Pb2+, Sn2+, Ge2+or combinations thereof;X is a halide selected from the group consisting of F; Cl; Br, I' or combinations thereof; and wherein the solvent mixture comprises dimethylformamide (DMF), gamma-Butyrolactone (GBL) and at least one of dimethyl sulfoxide (DMSO) and / V-methylpyrrolidone (NMP), the DMSO and / or NMP being present in the solvent mixture in amounts of 5 vol% or less, based on the total volume of the solvent mixture; and wherein the method further comprises steps of: immersing a device substrate in the metal halide perovskite solution; and processing the metal halide perovskite solution to form a metal halide perovskite single crystal via monolithic growth upon the substrate.

31. The method according to claim 30 wherein A and / or A’ are selected from the group consisting of: formamidinum (FA), methyammonium (MA), dimethylammonium (DMA), cesium (Cs), potassium (K), phenylethylammonium (PEA), butylammonium (BA), and combinations thereof.

32. The method according to claim 30 or claim 31 wherein the metal halide perovskite single crystal has an AMX3 structure, and is selected from the group consisting of FAPbBrs, MAPbBrs, CsPbBrs, FAPbCI3, MAPbCI3, and CsPbBr333. The method according to any one of claims 30 to 32 wherein the step of processing the metal halide perovskite solution to form the metal halide perovskite single crystal via monolithic growth upon the substrate includes one or more of:(i) heating the metal halide perovskite solution to a predetermined temperature sufficient to form the metal halide perovskite single crystal via inverse temperature crystallisation; ii) evaporating a proportion of the solvents present in the solvent mixture at ambient condition and / or via vacuum extraction of the solvent, to form the metal halide perovskite single crystal and / or(iii) cooling the metal halide perovskite solution to form the metal halide perovskite single crystal.0088325453734. The method according to any one of claims 30 to 33 wherein the device substrate comprises a complementary metal-oxide-semiconductor (CMOS) device, optionally wherein the CMOS device comprises an application specific integrated circuit (ASIC), optionally a high energy X-ray imaging technology (HEXITEC) ASIC.

35. The method according to claim 34 wherein the method includes performing one or more surface modification steps on the CMOS device prior to immersing the CMOS device in the solvent.

36. The method according to claim 35 wherein the surface modification steps include forming a selfassembled monolayer (SAM) having high affinity with metal halide perovskites on the surface of the ASIC.

37. The method according to claim 36 wherein the self-assembled monolayer comprises a plurality of anchoring molecules comprising a head group attached to the substrate and a tail group directly or indirectly attached to the head group, wherein the tail group comprises -NH2.

38. The method according to any one of claims 30 to 37 wherein the method further comprises steps of: removing the device substrate + metal halide perovskite single crystal from the solution; and depositing one or more electrodes on the metal halide perovskite single crystal.

39. The method according to claim 38, wherein the one or more electrodes are formed according to a method as set out in any one of claims 9 to 29.

40. A metal halide perovskite single crystal having a formula selected from:AMX3A'2MX4, orA'2An-i MnXsn+i , wherein n > 1 , wherein:A and / or A’ is an organic or inorganic cation;M is a divalent cation selected from the group consisting of Pb2+, Sn2+, Ge2+or combinations thereof;X is a halide selected from the group consisting of F-, Ch, Br-, I- or combinations thereof; wherein the metal halide perovskite single crystal has a full width at half maximum (FWHM) of X- ray diffraction (XRD) rocking curve of less than 0.005 degrees.

41. The metal halide perovskite single crystal according to claim 40 wherein A and / or A’ are selected from the group consisting of: formamidinum (FA), methyammonium (MA), dimethylammonium (DMA), cesium (Cs), potassium (K), phenylethylammonium (PEA), butylammonium (BA), and combinations thereof.0088325453842. The metal halide perovskite single crystal according to claim 40 or claim 41 wherein the metal halide perovskite single crystal has an AMX3 structure, and is selected from the group consisting of FAPbBrs, MAPbBrs, CsPbBr3, FAPbCh, MAPbCh, and CsPbBr343. The metal halide perovskite single crystal according to any one of claims 40 to 42 wherein the metal halide perovskite single crystal has a surface roughness of 2 nm or less.

44. The metal halide perovskite single crystal according to any one of claims 40 to 43 wherein the metal halide perovskite single crystal has at least one dimension of 1 mm or greater.

45. The method according to any one of claims 9 to 29, wherein the metal halide perovskite single crystal is a metal halide perovskite single crystal according to any one of claims 40 to 44.

46. An optoelectronic device comprising a metal halide perovskite single crystal according to any one of claims 40 to 44, optionally wherein the optoelectronic device is an X-ray detector, a gamma-ray detector, a neutron detector, a proton detector, or a visible photodetector.

47. The optoelectronic device according to claim 46, wherein the device is formed according to a method as set out in any one of claims 30 to 39.

48. The optoelectronic device according to claim 46 or claim 47, wherein the device comprises one or more electrodes formed directly or indirectly on the metal halide perovskite single crystal so as to be in electrical contact with the metal halide perovskite single crystal.

49. The optoelectronic device according to claim 48, wherein the one or more electrodes are formed according to a method as set out in any one of claims 9 to 29.

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