Perovskite film formation

Abstract

An antisolvent treatment of as-deposited perovskite wet film. According to the method, it is possible to make high quality perovskite semiconductor via controlled intermediate phase formation. This novel treatment enables printing of large area perovskite semiconducting films for industrial scale perovskite semiconductor device production. The method comprises forming a perovskite film via depositing a perovskite precursor solution to a surface of a substrate to form a film having a first surface, and exposing the first surface to a flow of antisolvent.

Description

Perovskite film formationTechnical field

[0001] The present invention relates to a method of forming a perovskite film.Background Art

[0002] Perovskite semiconductors are a class of materials with exceptional properties for optoelectronic devices. Perovskites have the general formula ABXs, where A is a monovalent cation, such as methylammonium (MA, CHsNHs ) or formamidinium (FA, (NFF^CFF), B is a divalent cation such as lead and tin, and X are halide anions. This class of materials offers a promising solution to address the growing demand for more efficient and cost-effective solar cells, and novel solar cell applications where the existing heavy silicon devices are not feasible.Perovskite solar cells (PSCs) have attracted significant attention due to their high-power conversion efficiency (PCE) that approaches their fundamental limit.

[0003] Currently, less than 5% of the electric power worldwide is produced by solar cells while it is the lowest-cost option for most of the world, that can also help reduce the carbon footprint of electricity production. In comparison, the global power demand is expected to grow 3.3 times by 2050, and 60 % of that growth will be covered by solar cells. Perovskite solar cells would help meet the global power demand, while helping reduce the reliance on fossil fuels for electricity generation.

[0004] Currently, practical implementations of perovskite solar-cells have been limited, because there are no known methods of manufacturing perovskite solar cells on a large scale. Known methods are limited to small-scale manufacturing, which is limiting the large-scale implementation of perovskite solar cells.

[0005] A known method of thin film perovskite semiconductor production is printing. This is usually a three-stage process: first, printing of a liquid film, second, pre -crystallisation stage and third, full-crystallisation / drying.

[0006] In the first stage, a thin liquid film is formed. This is usually achieved via spin coating. Spin-coating comprises dripping a perovskite precursor solution on a spinning substrate, such that the centripetal force on the solution creates an even covering of the perovskite precursor solution on the substrate. This is an approach unsuitable for scaled up industrial production.

[0007] Other known methods for forming a liquid film (the first stage of manufacture) include slot-die coating, knife -coating, bar-coating, blade-coating, spray coating, gravure printing and inkjet printing. For each of these, the three-stage printing process holds valid. The deposition of the perovskite liquid film differs between them. In slot-die coating, the liquid is extruded from a printerhead onto the substrate to produce thin perovskite film on the substrate. In knife-, blade- and barcoating, the liquid is dropped onto the substrate and a blade / knife / bar motion spreads the liquid onto the substrate to form the film. In inkjet, the printer head extrudes droplets of the liquid and through the head’s motion the substrate is covered with a thin film.

[0008] In the second stage, pre-crystallisation is achieved by spin-coating an antisolvent dropped onto the perovskite film formed in the first stage. The antisolvent is used to facilitate the removal of the host solvent(s) and initiate crystallization of the perovskite film. Antisolvent-solvent interactions modulate the crystallisation dynamics. Timing of the antisolvent treatment may affect resultant film morphology, electronic quality, and photovoltaic performance. Therefore, treatment with antisolvent may help produce high-quality perovskite films and devices. According to the state of the art, it is considered that fast pre-crystallisation results in superior perovskite films. Known methods of pre-crystallisation are aimed at increasing the rate of perovskite crystallisation in the pre-crystallisation step. The use of spin -coating in this stage of perovskite production is unsuitable for scaled up industrial production.

[0009] Some known methods use an air knife or a static antisolvent bath to control the pre- crystallisation stage. Other known methods include proving an antisolvent with additives such as potassium thiocyanate to assist film formation. However, these methods do not produce perovskite films of high quality.

[0010] The third stage of full -crystallisation is achieved by film drying. Usually annealing via heating or photo-curing is used. When spin coating is used for the second stage, the third stage occurs after stopping the substrate spinning during spin-coating. During this step the crystals grow to the final crystal structure of the film.

[0011] Opto-electronic behaviour of perovskite solar-cells may be affected by the perovskite crystal structure. Large and / or dense crystals with a low defect concentration are preferable for perovskite opto-electronic devices. Defects, such as impurities, grain boundaries, vacancies, interstitial atoms, and others reduce the efficiency of a perovskite optoelectronic.

[0012] The known printing process is cumbersome and is limited to small scale production in a lab. It is particularly difficult to implement at large scale. Developing a perovskite thin -film manufacturing process which may result in high quality densely packed crystals is key to implementing perovskite opto-electronic devices at a large scale.

[0013] Known perovskite film deposition methods exhibit other problems such as haziness in the perovskite films.

[0014] CN106757342A discloses a method for growing perovskite single crystals by antisolvent diffusion, wherein a small container open containing a perovskite precursor solution is placed in alarge closed container containing the antisolvent. The pre -crystallisation step of the invention comprises the antisolvent diffusing to the surface of the perovskite precursor solution.

[0015] CN108520918A discloses a method for growing a perovskite fdm by vertically immersing a substrate in a solution of perovskite precursor. The solution may be stirred to replenish the precursor solution supply to the substrate surface. Anti-solvent is dripped into the precursor solution to force deposition of the perovskite precursor onto the substrate. The method disclosed in CN 108520918A is equivalent to a printing step in the aforementioned three-stage process, in that this method provides a way to deposit a fdm of perovskite onto a substrate. However, this method provides no way for controlling the crystallisation rate during the pre-crystallisation stage.

[0016] CN109638162A discloses preparation method for making perovskite fdms, the method comprising preparing a layer of lead iodide by spin coating, then spin-coating the surface of the lead iodide fdm with a caesium bromide solution and then annealing. This method is difficult to scale up to industrial production, due to the use of spin coating. This method also does not provide a way for controlling the crystallisation rate of the perovskite.

[0017] CN 112853486A discloses a method to preparing a two-dimensional perovskite single crystal in air. The method comprises the following steps: first, the precursor is dissolved in an organic solvent to prepare a precursor solution; second, the anti-solvent is added dropwise to the precursor solution; and third, the anti -solvent low-temperature heating method is used to quickly invert the temperature of the precursor solution to crystallize the two-dimensional perovskite. The method is aimed at producing a two-dimensional perovskite crystal by using high crystallisation rates.

[0018] CN 114517332A discloses a preparation method for erbium -doped two-dimensional perovskite single crystal. The method comprises mixing a perovskite precursor solution and a solution of erbium chloride to form a two-dimensional perovskite single crystal.Summary of Invention

[0019] According to the present invention there is provided a method of forming a perovskite film comprising: depositing a perovskite precursor solution to a surface of a substrate to form a film having a first surface, and exposing the first surface to a flow of antisolvent, wherein the flow rate of antisolvent occurs is in the range of equal to or greater than O.Olm / s and equal to or less than Im / s.

[0020] Desirably, the perovskite precursor solution comprises a perovskite precursor dissolved in a first solvent.

[0021] Desirably, the perovskite precursor has the formula ABXs, where A is a monovalent cation, B is a divalent cation, and X represents a single type of halide anion selected from a list containing iodide, bromide, or chloride.

[0022] Desirably, the perovskite precursor comprises one or more of: MAPbh. FAPbh. CsFAPbh.

[0023] Desirably, the first solvent and the antisolvent have a miscibility gap at a temperature at which the method is performed.

[0024] Desirably, the first solvent comprises one or more of: dimethylformamide, dimethyl sulfoxide, triethyl phosphate, y-valerolactone, Gamma-butyrolactone, Acetonitrile, N- N-methyl-2- pyrrolidone, Methyl acetate , tetrahydrofuran, 2-methoxyethanol, ethanol, 2-pyrrolidinone, 2- Methyltetrahydrofuran .

[0025] Desirably, the antisolvent comprises one or more of: diethyl ether, ethyl acetate, toluene, chlorobenzene, diphenyl ether, methylanisole, anisole, dichlorobenzene, dichloromethane, ethanol, methanol, isopropanol, butanol, chloroform, butyl acetate, trifluorotoluene, hexane, m-xylene, mesitylene, (R)-(+) -limonene, diethyl carbonate, dimethyl carbonate, or optionally, the antisolvent is selected from one of: diethyl ether, ethyl acetate, toluene, chlorobenzene, diphenyl ether, methylanisole, anisole, dichlorobenzene, dichloromethane, ethanol, methanol, isopropanol, butanol, chloroform, butyl acetate, trifluorotoluene, hexane, m-xylene, mesitylene, (R)-(+)-limonene, diethyl carbonate, dimethyl carbonate.

[0026] Desirably, the flow rate of antisolvent is provided parallel to the first surface.

[0027] Desirably, the exposing of the substrate to the flow of the antisolvent takes place for 1 to 200 s, preferably, 10 to 150 s.

[0028] Desirably, the substrate is annealed after the exposing the first surface to a flow of antisolvent.

[0029] Desirably, the substrate is an anode layer.

[0030] More desirably, the anode layer is optically transparent.

[0031] Desirably, the substrate has a stacked structure comprising the anode layer and an n-type semiconductor layer, and the perovskite precursor solution is deposited on the surface of the n-type semiconductor layer.

[0032] Desirably, a p-type semiconductor layer is deposited on the perovskite film, optionally, by spin-coating, slot-die coating, blade coating, spray coating or evaporation.

[0033] Desirably, a cathode layer is deposited on the p-type semiconductor layer.

[0034] Desirably, the substrate is a cathode layer.

[0035] Desirably, the cathode layer is optically transparent.

[0036] Desirably, the substrate has a stacked structure comprising the cathode layer and a p-type semiconductor layer, and the perovskite precursor solution is deposited on the surface of the p-type semiconductor layer.

[0037] Desirably, a n-type semiconductor layer is deposited on the perovskite film, optionally, by spin-coating, slot-die coating, blade coating, spray coating or vacuum evaporation.

[0038] Desirably, an anode layer is deposited on the n-type semiconductor layer.

[0039] Desirably, the p-type semiconductor layer is selected from one of: spiro-OMeTAD, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine, regioregular poly(3 -hexylthiophene-2,5 -diyl), carbon black, carbon-60, carbon allotropes, nickel oxide and self-assembled monolayers (SAMs) such as Me-4PACz ([4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic Acid).

[0040] Desirably, the cathode layer comprises one or more of: gold, carbon, allotropes of carbon, silver, copper.

[0041] Desirably, the anode layer comprises one or more of: indium tin oxide, fluorine-doped tin oxide, tin oxide, titanium, nickel, and carbon.

[0042] Desirably, the n-type semiconductor layer is selected from one of: tin oxide, zinc oxide, titanium oxide, carbon -60, [6,6] -phenyl-C61 -butyric acid methyl, and 2,9-Dimethyl-4,7-diphenyl- 1 , 10-phenanthroline .

[0043] Desirably, the flow of antisolvent over the first surface is generated by a peristaltic pump.

[0044] Desirably, the flow of antisolvent over the first surface is generated by the following steps: disposing substrate in a container, wherein the container holds the antisolvent; stirring the antisolvent; wherein optionally, the antisolvent is stirred via magnetic stirring or mechanical stirring.

[0045] According to the present invention, there is provided a method of manufacturing electronic devices, such as solar cells, comprising the method according to the present invention or one of its variants.

[0046] According to the present invention, there is provided an optoelectronic device, such as a solar cell, manufactured by the method according to the present invention or one of its variants.Brief description of the drawings

[0047] Exemplary embodiments of the invention are described below with reference to the accompanying Figures, in which:

[0048] Figure la is a schematic of an example of a setup for carrying out the method of the present invention. Figure lb shows a real example of a container of Figure la.

[0049] Figure 2a is a schematic of another example of a setup for carrying out the method of the present invention. Figure 2b show an example of the method illustrated in Figure 2a.

[0050] Figure 3 is a miscibility chart of some known solvents.

[0051] Figures 4a-4e are contour plots of photoluminescence spectra of M APbF (methylammonium lead halide) recorded as a function of time and the corresponding Scanning Electron Microscopy (SEM) images of the final annealed films, wherein the films were exposed tovarious antisolvent flows. Figure 4f shows extracted photoluminescence parameters, including peak intensity and peak position.

[0052] Figure 4f shows photoluminescence data of the film, based on the photoluminescence peak maximum. Figure 4f (a) is a graph showing the time evolution of the photoluminescence peak intensity of the films during exposure to the anti -solvent, Figure 4f (b) is graph showing the time evolution of the photoluminescence peak position of the films during exposure to the anti-solvent.

[0053] Figure 5a shows transmittance data of a film, based on the position of 480 nm wavelength peak in photoluminescence. In Figure 5a a main graph shows the time evolution of the transmittance of the films during exposure to the anti -solvent, and an inset graph, showing the time evolution of the transmittance of the films during an annealing stage.

[0054] Figure 5b shows transmittance data of a film, based on the position of 750 nm wavelength peak in photoluminescence. In Figure 5b a main graph shows the time evolution of the transmittance of the films during exposure to the anti -solvent, and an inset graph, showing the time evolution of the transmittance of the films during an annealing stage.

[0055] Figure 6a shows a comparison of the photoluminescence intensity and transmittance of the 480 nm light of a perovskite film in a stationary antisolvent bath and in a flow of antisolvent. The red colour represent transmittance while black colour photoluminescence.

[0056] Figure 6b shows photos of different antisolvent treated films.

[0057] Figure 7a shows a focused region of x-ray diffraction (XRD) spectra of MA Pbh films on glass slides. Figure 7b shows time -correlated single photon counting (TCSPC) fitted plots of MA Pbh films on glass slides.

[0058] Figures 8a-8d show the photovoltaic performance distribution with the white square box within the bars representing the mean value. Figure 8e shows a plot of current density against applied voltage of a perovskite film formed according to an embodiment of the present invention. Figure 8f shows a cross-section of device formed by the method according to an embodiment of the present invention.Detailed Description

[0059] The present inventors have developed a method of perovskite film manufacturing which can be scaled to industrial production which achieves perovskite films with improved optoelectronic properties.

[0060] The present inventors have devised a method for perovskite film manufacturing which enables superior control of the pre -crystallisation process. The present inventors have determined that, contrary to the expectations, fast pre-crystallisation doesn’t necessarily lead to perovskite films of higher quality (such as films with larger grain sizes with fewer defects). The presentinventors have determined that this method may also reduce haziness of perovskite films, which is a common problem with the deposition methods according to the prior art.

[0061] The spin-coating according to the prior art often includes three stages.

[0062] In the first stage, a perovskite precursor film is deposited on the substrate. The perovskite precursor solution comprises a perovskite precursor dissolved in a first solvent. The first solvent can be a single solvent or mixture of two or more solvents. Some of the first solvent may evaporate after deposition of the precursor solution on the substrate . The deposition may be achieved via spin-coating, or other methods such as blade coating, screen printing, spray coating, gravure printing and slot die coating. Slot die coating is a technique for forming extruded thin films which may be formed on a substrate.

[0063] In the second stage, pre-crystallisation is achieved by spin-coating an antisolvent dropped onto the perovskite film formed in the first stage. Antisolvents are fluids that do not dissolve the solute (e.g. the perovskite precursor) but are miscible with the first solvent of the solution, at the temperature that the process is carried out. Antisolvent are solvents that exhibit lower solubility for the perovskite precursors than the solvent of the perovskite precursor. The antisolvent may help extract the first solvent which is used to dissolve the precursor materials and the antisolvent may lower the precursor solubility, leading to local supersaturation, initiating the nucleation, precipitation and solidification of the dissolved materials into a film. Promoting nucleation with an antisolvent may increase uniformity of the film, which may result in a better morphology and electronic quality, and subsequently, an improved performance and stability.

[0064] In other words, the antisolvent is different to the first solvent, and the perovskite precursor does not dissolve in the antisolvent. The antisolvent preferably has a solubility mismatch with the first solvent at the temperature that the process is carried out. According to known techniques, anti -solvent is applied via spin coating to achieve a uniform film of antisolvent across the perovskite film surface so that crystallisation is uniform across the surface of the film. Known techniques also include immersion of the perovskite film printed in the first stage in the antisolvent bath.

[0065] The antisolvent is used to facilitate the removal of the host solvent(s) and initiate crystallization of the perovskite film. Antisolvent-solvent interactions modulate the crystallisation dynamics. Timing of the antisolvent treatment may affect resultant film morphology, electronic quality, and photovoltaic performance. Therefore, treatment with antisolvent may help produce high-quality perovskite films and devices.

[0066] The attempts that have been made to control the crystallisation rate during the pre- crystallisation stage, such as using an air knife or a static antisolvent bath, do not enable a high level of control over perovskite crystallisation behaviour during the pre-crystallisation stage.Furthermore, the known methods of controlling crystallisation behaviour are aimed at increasing the crystallisation rate during the pre-crystallisation stage. This does not allow high quality perovskite fdms to be achieved.

[0067] When an antisolvent is added to the perovskite precursor solution, it may locally reduce the solubility of the solution, thereby creating local supersaturation conditions. As a consequence, there may appear regions that are rich in perovskite precursor, which may aid the nucleation and crystallization of the perovskite.

[0068] In the third stage, the substrate and the perovskite fdm formed thereon are heated to anneal the perovskite film and remove the solvent (which may comprise the first solvent and / or the antisolvent) from the perovskite film. Additionally, or alternatively, air may be blown across the surface of the film or films may be photo / photo-thermal annealed to facilitate removal of the solvent from the perovskite film.

[0069] According to the present invention, there is provided a method of forming a perovskite film comprising depositing a perovskite precursor solution to a first surface of a substrate to form a film having a first surface, and exposing the first surface of the substrate to a flow of antisolvent. This method of the present invention may be implemented instead of known methods of carrying out the second stage of the perovskite formation. The method according to the present invention results in the application of an antisolvent in a controlled manner which improves the final perovskite film quality.

[0070] According to the invention, the substrate is not spun. It is kept stationary. Alternatively, the substrate can be moved in a direction opposite or lateral to the flow of antisolvent during antisolvent application. There is a relative linear motion of the substrate and anti-solvent. This technique may be referred to as dynamic antisolvent. This is the opposite configuration to what is known in the prior art, wherein the substrate is rotated during the spin-coating process that the antisolvent can be applied, or the alternative known method in which both the perovskite film and antisolvent bath are still.

[0071] Providing a flow of antisolvent over the surface of the perovskite precursor film leads to superior qualities of the perovskite films. This is contrary to what is expected, because providing a flow of antisolvent may result in lower crystallisation rates than known techniques such as spincoating. The present inventors have determined that the relatively low crystallisation rates achieved by dynamic antisolvent enables to have high control over the crystallisation rate, which results in superior films. The present inventors have discovered that this results in superior perovskite films particularly when forming single -halide perovskite films.

[0072] The perovskite precursor solution may be applied to a substrate surface by any suitable means, such as printing, to form a film of perovskite precursor solution on substrate. Theperovskite precursor film has a first surface, which is not in contact with the substrate and is parallel to the surface of the substrate.

[0073] A flow of antisolvent is applied to the first surface of the perovskite precursor film. The flow of antisolvent may be provided parallel to or substantially parallel to the first surface of the perovskite precursor film. This may be carried out by any suitable means. For example, the substrate may be placed in a container comprising the antisolvent, and the antisolvent may be stirred by any suitable means, such as magnetic stirring and / or mechanical stirring.

[0074] Alternatively, the present invention can also be used with continuous in-line processing methods. For example, the antisolvent may be arranged to form a laminar flow over the surface of a continuous perovskite precursor film. This embodiment may be suitable for large scale applications.

[0075] An example of a setup for carrying out the method of the present invention is shown schematically in Figure la. According to this example, a container 203 having an inlet 201 and an outlet 202 is provided. A substrate 100 having a perovskite precursor film formed thereon is placed in the container, such that the surface of the perovskite precursor film is exposed to the antisolvent flow. In figure la, the antisolvent flow is provided in direction x. A pump is provided to drive the antisolvent into inlet 201 and out of outlet 202, such that a flow of antisolvent 300 is provided over the surface of the perovskite precursor film (i.e. the first surface).

[0076] The flow of antisolvent may be laminar. This may further improve control over the crystallisation behaviour of the perovskite.

[0077] The flow of antisolvent may apply a pressure on the first surface, for example, the flow of antisolvent may exert a surface pressure of 2 kPa to 10 kPa on the first surface. For example, the antisolvent may be stirred (e.g. magnetically or mechanically) to create a vortex, and the substrate may be disposed within the antisolvent vortex so that the antisolvent flows over the first surface and exerts a pressure on the first surface. The antisolvent experiences centrifugal acceleration, which increases pressure radially outward from a central axis of a vortex (the axis around which the antisolvent revolves). The fluid may be in solid-body rotation (i.e., all fluid particles rotate at the same angular velocity co) meaning the surface pressure at distance r from the central axis may be defined by Formula 1 as:P(r) = Po+ 12 pa>2r2[Formula 1] where: P(r) is the pressure at distance r from the central axis, Po is the pressure at central axis (r=0), p is the fluid density, co is the angular velocity of the fluid (rad / s), r is the radial distance from the rotation axis. The substrate may be positioned in the antisolvent vortex such that the surfacepressure on the first surface is in the region of 2 kPa to 10 kPa. A surface pressure in this range may be provided by disposing the substrate in a vortex of diethyl ether, revolving at 500-1000 rpm. For example, when the method is carried out at atmospheric pressure, Po is Patm(Patm is the atmospheric pressure, 101,325 Pa), fluid density for diethyl ether is 730 kg / m3, and angular velocity may be in the range of 500 rpm-1000 rpm. According to Formula 1, the mean pressure difference (minus P0) ranges from 2.44 kPa (500 rpm) to 9.77 kPa (1000 rpm), with a middle point of 5.50 kPa from 750 rpm. Antisolvent surface pressure may be provided by providing the substrate in a vortex of antisolvent, although there are other possible ways of providing antisolvent surface pressure on the first surface of the perovskite precursor film. Providing the flow of antisolvent in a way that exerts pressure on the first surface may improve removal of the host solvent. The solvent pressure may aid the infiltration of the antisolvent, facilitating the solvent / antisolvent exchange (which may otherwise only happen at the interface of the first surface and antisolvent).

[0078] Optionally, the setup shown on Figure la may be adapted so that the substrate is moved in the opposite to the flow of antisolvent (in the direction opposite direction x in Figure la). This may be particularly advantageous for large scale industrial applications, in which large substrates may be processed.

[0079] Figure lb shows an example of container 203 having inlet 201 and outlet 202.

[0080] A further example of a setup for carrying out the method of the present invention is shown schematically Figure 2a. According to this example, the substrate 100 may be placed vertically in a container 204 containing antisolvent. Preferably, there may be enough antisolvent so that the entire substrate is immersed in the solvent. A flow of antisolvent 200 may be provided by a magnetic stirring means 205 which is disposed in container 204. Stirring of the antisolvent leads to a flow of antisolvent 300 over the surface of the substrate 100. The substrate may be placed so that its surface is exposed to a flow of substrate. It may not be preferable to place the substrate 100 perpendicular or substantially perpendicular to a flow of solvent. Horizontal placement may be preferable. This is because there may be no or limited flow of antisolvent over the first surface of the perovskite precursor film which is disposed on the substrate, when the substrate is disposed perpendicular or substantially perpendicular to a flow of solvent. Horizonal placement of the substrate enables large-scale applications of this method, e.g. in industry.

[0081] Figure 2b shows an example of the method illustrated in Figure 2a. Step 1 shows the printed film of perovskite precursor dipped in antisolvent prior to an antisolvent flow being applied. Step 2 shows the film after exposure to an antisolvent flow by the method shown schematically in Figure 2a.

[0082] The perovskite precursor comprises a perovskite dissolved in a solvent, the precursor having the formula ABX3. The A site may be occupied by one or more of MA, FA, Cs. The B site may be occupied by one or more of: Pb and Sn. The X site may be occupied by a halide, such as I, Br, Cl. When the X site is occupied by a single type of halogen atom, the perovskite may be referred to as a single -halide perovskite.

[0083] The perovskite precursor solution may comprise a perovskite precursor dissolved in a first solvent. The perovskite precursor may comprise methylammonium lead iodide (MAPbls), formamidinium lead iodide (FAPbh). caesium formamidium lead iodide (CsFAPbh) or one or more compounds that are precursors to one or more of MAPbls, FAPbh. or CsFAPbh. In an embodiment, the perovskite precursor may consist of one or more of methylammonium lead iodide (MAPbh), formamidinium lead iodide (FAPbh), caesium formamidium lead iodide (CsFAPbh), or one or more compounds that are precursors to one or more of MAPbh, FAPbh, or CsFAPbh. Optionally, incidental impurities may be present in the perovskite precursor.

[0084] MA is an abbreviation for methylammonium, FA is an abbreviation for formamidinium.

[0085] Alternatively, the precursor may comprise mixed cation compositions, such that site A in the perovskite composition formula ABX3 comprises a mix of different cations, such as MA and FA. The cation site may be doped with an element such as caesium. The perovskite precursor may consist of said mixed cation composition(s), and optionally, incidental impurities / additives, as well as the solvent they are dissolved in.

[0086] Optionally, X can comprise one or more halide atoms, such as chlorine, bromine or iodine. In a preferred embodiment, the perovskite precursor contains only one type of halide atom, so that a single-halide perovskite film is formed. A single-halide perovskite is a perovskite composition having the formula ABX3, wherein the X site is occupied by one type of halide. The present inventors found that lowering the pre-crystallisation rate is particularly effective for improving film quality of single-halide perovskites.

[0087] Optionally, the perovskite precursor may comprise compounds which react to form a perovskite. For example, a mix of ammonium iodide (MAI) and lead iodide (PbB) which react to form a MAPbh film. Optionally, the perovskite precursor may consist of compounds which react to form a perovskite and optionally, incidental impurities.

[0088] The first solvent and the antisolvent may have a miscibility gap at a temperature at which the method is performed. In other words, a phase diagram of the first solvent and the antisolvent may have a miscibility gap at a temperature at which the method is performed. If the first solvent and the antisolvent have a miscibility gap, the antisolvent may be particularly effective at removing the first solvent from the perovskite precursor film and initiating crystallisation of the perovskite film. A miscibility gap is a region in a phase diagram for a mixture of components where themixture exists as two or more phases - any region of composition of mixtures where the constituents are not completely miscible. If the first solvent and the antisolvent have a miscibility gap at a temperature at which the method is performed, then they do not form a solution and instead exist as two separate phases.

[0089] Preferably, the perovskite precursor is less soluble in the antisolvent than in the first solvent at the temperature that the method of forming the perovskite film is carried out at. For example, PbE solubility is below 0.1 molar in the antisolvent diethyl ether, while over a magnitude higher in first solvents like DMF and DMSO.

[0090] The pre-crystallisation method according to the present invention may be carried out at ambient temperature, such as 15-30 °C.

[0091] The first solvent may be any suitable solvent that the perovskite precursor can be dissolved in.

[0092] The first solvent may comprise a plurality of solvents.

[0093] The one or more solvents of the first solvent may be selected from one of more of: dimethylformamide (DMF), dimethyl sulfoxide, triethyl phosphate (TEP), y-valerolactone (GVL), Gamma-butyrolactone (GBL), Acetonitrile (ACN), Methyl acetate (MAAc), 2-methoxyethanol(2- ME), ethanol, 2-pyrrolidinone, 2-Methyltetrahydrofuran (2ME-THF). The first solvent may consist of one or more of: dimethylformamide (DMF), dimethyl sulfoxide (DMSO), triethyl phosphate (TEP), y-valerolactone (GVL), Gamma-butyrolactone (GBL), Acetonitrile (ACN), Methyl acetate (MAAc), 2-methoxyethanol(2-ME), ethanol, 2-pyrrolidinone, 2-Methyltetrahydrofuran (2ME- THF). These solvent abbreviations are used throughout the description and figures.

[0094] The first solvent may comprise or consist of one of more of: dimethylformamide (DMF), dimethyl sulfoxide, triethyl phosphate (TEP), y-valerolactone (GVL), Gamma-butyrolactone (GBL), Acetonitrile (ACN), Methyl acetate (MAAc), 2-methoxyethanol(2-ME), ethanol, 2- pyrrolidinone, 2-Methyltetrahydrofuran (2ME-THF). The first solvent may consist of one or more of: dimethylformamide (DMF), dimethyl sulfoxide (DMSO), triethyl phosphate (TEP), y- valerolactone (GVL), Gamma-butyrolactone (GBL), Acetonitrile (ACN), Methyl acetate (MAAc), 2-methoxyethanol(2-ME), ethanol, 2-pyrrolidinone, 2-Methyltetrahydrofuran (2ME-THF).

[0095] Optionally, incidental impurities may be present in the first solvent.

[0096] The antisolvent may be any suitable solvent which has a solubility mismatch with the first solvent, i.e. it may be a solvent which is immiscible in the first solvent and / or has limited miscibility in the first solvent.

[0097] The antisolvent may be selected from one or more of: diethyl ether (DE), ethyl acetate (EA), toluene (TOL), chlorobenzene (CB), anisole (ANI), diphenyl ether, methylanisole, dichlorobenzene, dichloromethane, ethanol, methanol, isopropanol, butanol, chloroform, butylacetate, trifluorotoluene, hexane, m-xylene, mesitylene, (R)-(+)-limonene, diethyl carbonate, dimethyl carbonate. These solvent abbreviations are used throughout the description and figures.

[0098] The antisolvent may consist or comprise one or more of: diethyl ether (DE), ethyl acetate (EA), toluene (TOL), chlorobenzene (CB), anisole (ANI), diphenyl ether, methylanisole, dichlorobenzene, dichloromethane, ethanol, methanol, isopropanol, butanol, chloroform, butyl acetate, trifluorotoluene, hexane, m-xylene, mesitylene, (R)-(+)-limonene, diethyl carbonate, dimethyl carbonate.

[0099] Optionally, incidental impurities / additives may be present in the antisolvent.

[0100] Different antisolvents may lead to different crystallisation rates.

[0101] An example of possible first solvent and antisolvent pairs is shown on Figure 3. Figure 3 is a miscibility chart prepared by Sigma Aldrich (Sigma Aldrich is a registered trade mark). Solvent pairs which are marked as immiscible may be used as the first solvent and antisolvent. These solvent pairs may be particularly preferable as they may control the rate of crystallisation and intermediate phases formation, and slow down the crystallisation of the perovskite solid. Solvent pairs which are marked as miscible may also form suitable solvent and antisolvent combinations to control the rate of crystallisation and intermediate phases formation, speed up the crystallisation of the perovskite solid. While certain solvent pairs are marked as miscible, they may have a solubility mismatch at a temperature at which the method is performed making them suitable for a perovskite film formation method according to the present invention.

[0102] The antisolvent flow rate may be in the range of equal to or greater than O.Olm / s and equal to or less than Im / s. If the flowrate of antisolvent is too high, the perovskite precursor may be swept past the substrate surface or may be swept off the substrate surface, which may prevent perovskite crystal nucleation. If the flowrate of antisolvent is too low, then the antisolvent may not be replenished quickly enough to promote nucleation of the perovskite on the substrate surface . A flow rate that is too low may lead to non-uniform film formation, which may be hazy. The antisolvent flow is provided parallel to or substantially parallel to the first surface. The antisolvent flow rate of O.Olm / s and equal to or less than Im / s is preferably applied to that the antisolvent flows parallel to or substantially parallel to the first surface. The antisolvent may continuously flow over the surface of perovskite precursor film. The flow rate may be equal to or greater than 0. Im / s and equal to or less than 0.9 m / s. The flow rate may be equal to or greater than 0.2 m / s and equal to or less than 0.8 m / s. The flow rate may be equal to or greater than 0.3 m / s and equal to or less than 0.7 m / s. When the flow rate falls any of these ranges, there is a good balance of perovskite crystal nucleation and antisolvent replenishment. The flow rate is the speed at which the antisolvent passes over the first surface.

[0103] To provide a flow of antisolvent to a perovskite precursor film, the substrate comprising the perovskite precursor film may be placed in a chamber (such as container 203 of Figure la) , wherein the antisolvent can flow into an inlet of the chamber (such as inlet 201 of Figure la) and out of an outlet of the chamber (such as outlet 202 of Figure la). Accordingly, a flow of antisolvent within the chamber may be achieved. According to this embodiment, a flow of antisolvent is applied parallel to the substrate surface, which has a perovskite precursor film formed thereon. A peristaltic pump may be provided to move the antisolvent within the chamber.

[0104] Alternatively, the substrate having the perovskite precursor film formed thereon may be immersed vertically, or substantially vertically, to an inner side of a container containing the antisolvent. The antisolvent may be stirred to create a vortex, such that antisolvent flows in a uniform manner across the surface of the container and the substrate. The antisolvent may be stirred by magnetic stirring and / or mechanical stirring. Magnetic stirring may comprise placing a magnetic stirrer bar in the antisolvent and rotating is by an applied magnetic field. This method may be particularly advantageous because film haziness may be reduced. This method may be referred to as “vortex flow”. Mechanical stirring may be particularly useful for large-scale industrial applications of this method.

[0105] The antisolvent flow rate may be selected depending on the first solvent of the perovskite precursor solution. For example, if the first solvent comprises a number of solvents, the flow rate may be selected so that the solvents of the first solvent are extracted at a different rate. For example, when the first solvent comprises DMF and DMSO and the antisolvent comprises DE, the antisolvent extracts DMF relatively quickly compared to DMSO. If DMF is extracted too quickly this can force formation of too many nuclei and subsequently very small crystals which may not be desirable. Therefore, the antisolvent flow rate may be selected so that the first solvent or component of the first solvent is extracted at an optimal rate and in a uniform manner to achieve optimal nuclei distribution for dense perovskite films. In another example, if the antisolvent is extracted slowly, crystallisation rate may be too slow which may lead to fewer crystals and pinholes.

[0106] The exposing of the substrate to the flow of the antisolvent takes place for 1 to 200 s, preferably, 10 to 150 s, more preferably 20 to 120 s, even more preferably 30 to 110 s, further even more preferably 40 to 100 s. The time may be selected depending on the thickness of the perovskite precursor film. The time may be selected based on the flow rate. According to the present invention, the flow of antisolvent may be applied at ambient temperature, such as 10-30°C, or preferably 15-25 °C, or even more preferably 17-23 °C.

[0107] The substrate may be annealed after the exposing the first surface to a flow of antisolvent. Accordingly, the third stage of perovskite film formation may be carried out. The temperature ofannealing may be in the range of 50-220°C, or preferably 60-180 °C, or more preferably 70 - 150°C, or even more preferably 80-140 °C, or further even more preferably 90-130 °C. The substrate may be annealed for 1 - 600 seconds, or preferably, for 5 - 300 seconds, or more preferably for 10 - 200 seconds, or even more preferably, 15 - 100 seconds, or further even more preferably for 20 - 60 seconds.

[0108] The method of the present invention may result in a perovskite fdm having a surface roughness (Ra) or 1-5 nm.

[0109] The method according to the present invention may be used to make part of a solar cell battery or a full solar cell.

[0110] For example, the substrate may be an anode layer (in a device structure referred to here as p-i-n). An anode layer of a solar cell is an electrode at which oxidation occurs during cell discharge.[oni] The anode layer may be optically transparent. This may be particularly suitable for solar cells, which may require a transparent electrode so that light can be transmitted to the optoelectronic layer (i.e. the perovskite layer).

[0112] The anode layer may have an n-type semiconductor disposed thereon, and the perovskite precursor solution may be deposited thereon. An n-type semiconductor layer in a solar cell may be referred to as an electron-transport layer, because it transports electrons to the anode when an electron is generated in the optoelectronic layer (i.e. the perovskite layer).

[0113] A p-type semiconductor layer may be deposited on the perovskite fdm by any suitable means, such as by spin-coating, slot-die coating, blade coating, spray coating or evaporation. The p-type semiconductor layer may be referred to as a hole -transport layer in a solar cell, because the p-type semiconductor layer transports holes to the anode when a hole is generated in the optoelectronic layer (i.e. the perovskite layer).

[0114] A cathode layer may be applied to the p-type semiconductor layer by any suitable means, such as vacuum evaporation.

[0115] A perovskite solar cell may be obtained by forming layers of a solar cell in the opposite order. In other words, the substrate may be a cathode layer (in a device structure referred to here as p-i-n). The cathode layer may be optically transparent.

[0116] The cathode layer may have a p-type semiconductor disposed thereon, and the perovskite precursor solution may be deposited thereon.

[0117] The p-type semiconductor layer may be deposited on the cathode layer by any suitable means, such as spin-coating, slot-die coating or blade -coating.

[0118] An n-type semiconductor layer may be deposited on the perovskite fdm, optionally, by spin-coating, slot-die coating, blade coating, spray coating or vacuum evaporation .

[0119] An anode layer may be deposited on the n-type semiconductor by any suitable means, such as vacuum evaporation.

[0120] The p-type semiconductor layer may be selected from one of: spiro-OMeTAD, poly[bis(4- phenyl)(2,4,6-trimethylphenyl)amine, regioregular poly(3-hexylthiophene-2,5-diyl), carbon-60 and nickel oxide. The p-type semiconductor layer may consist of one or more of: : spiro-OMeTAD, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine, regioregular poly(3 -hexylthiophene-2,5 -diyl), carbon-60 and nickel oxide and incidental impurities, as well as self-Assembled Monolayers (SAMs) such as Me-4PACz ([4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic Acid).

[0121] Spiro-OMeTAD is a known p-type semiconductor, which may be used as a hole -transport material in optoelectronic devices. Spiro-OMeTAD has the IUPAC name: N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxyphenyl)-9,9'-spirobi[9H-fluorene]-2,2',7,7'- tetramine.

[0122] Carbon-60 is a fullerene of carbon having the formula C60. Other fullerenes of carbon may be used.

[0123] The cathode layer may be selected from one of: gold, carbon, allotropes of carbon, silver, copper. Allotropes of carbon such as graphene may be used.

[0124] The anode layer may comprise one or more of: indium tin oxide (ITO), fluorine-doped tin oxide, tin oxide (SnO2), titanium, nickel, and carbon.

[0125] The n-type semiconductor may be selected from one of tin oxide, zinc oxide, titanium oxide, carbon-60, [6,6] -phenyl-C61 -butyric acid methyl, 2,9-Dimethyl-4,7-diphenyl-l,10- phenanthroline.

[0126] An example of a stacked structure that can be formed by a method according to the present invention is as follows: (ITO / SnO2 / MAPbl3 / Spiro-OMeTAD / Gold).

[0127] An example of perovskite fdms which were formed by the method according to the present invention using different antisolvents is discussed in detail in section Method 1. Method lincludes photoluminescence spectroscopy analysis of the fdms formed.

[0128] [Method i]

[0129] A perovskite precursor solution was made by dissolving of MAPbh in DMF:DMSO (9: 1.1 v:v) without additives to produce a IM solution of MAPbh.

[0130] The perovskite precursor fdm was printed on five substrates, each substrate having size 25x75mm substrate.

[0131] Each of the substrates was exposed to a flow of a different antisolvent. The antisolvents were: diethyl ether (DE), ethyl acetate (EA), toluene (Tol), chlorobenzene (CB) and anisole (Ani). These solvent abbreviations are used throughout the description and figures. These antisolventshave contrasting miscibility with the precursor solvent DMF / DMSO. These solvents have the added benefit of being cheap.

[0132] For the antisolvent step, the substrate having the perovskite precursor film formed thereon was transferred to chamber filled with liquid antisolvent, simulating bathing treatment in an industrialised roll-to-roll process. In-situ transmittance and photoluminescence spectroscopy (PL) was conducted using optical fibres and achieving minimal optical losses, which enabled analysis of the evolution of the perovskite crystallisation as a function of time for all antisolvents . Other techniques are available for analysing the evolution of perovskite crystallisation.

[0133] Photoluminescence spectroscopy results may be used to analyse the films formed by different antisolvents, so that the performance of different antisolvents can be compared.

[0134] Figures 4a-4e presents contour plots of the PL evolution during antisolvent treatment. The excitation wavelength was 405 nm, and the measurements were acquired at one-second intervals for 180 seconds immediately after placing the printed film in the antisolvent. The colour maps show that the fastest PL intensity rise was in EA and Ani, at 5 seconds and 9 seconds respectively, while the slowest PL rise was in DE. Both DE and Tol showed increasing fluorescence up to 180 sec but the DE-treated film had four times higher final intensity. This may suggest that the DE- treated film may lead to better quality crystals. The PL peak position was also tracked (Figure 4f). The DE PL peak indicates that the DE-treated perovskite film may exhibited the slowest evolution and smallest overall peak shift reaching only to 710 nm at 180 s. Figure 4f(a) is a graph showing the time evolution of the PL intensity of the films during exposure to the anti -solvent, Figure 4f(b) is graph showing the time evolution of the PL peak position of the films during exposure to the anti -solvent.

[0135] Perovskite nanocrystals and intermediates may exhibit quantum confinement effects, and the average crystallite size will affect the PL peak position from Figure 4f(a) and 4f(b). The procedure assumes insignificant changes in the nanocrystals / intermediate phases dielectric constant and the effective masses of electrons and holes for the duration of the antisolvent treatment.

[0136] The rate of nanocrystal growth observed between the antisolvents may be different. The results on Figures 4a-4f suggest that a DE-treated sample has the slowest growth rate while the EA- treated sample has the fastest growth rate. For EA, large nanocrystals of 6.5 nm are formed almost immediately upon contact with the solvent (calculated from the PL peak position), whereas for DE, the nanocrystals reach an average size of only up to 5.0 nm by 180 seconds (calculated from the PL peak position). For DE, there is a much larger distribution of crystals (65 nm) than for EA (50 nm), which correlates with the observation in SEM images in Figure 4a-4e. These results indicate that the nanocrystal growth rate and size distribution achieved during the antisolvent bath treatment stepmay affect the quality of the fully-formed perovskite film after the final heating stage (the third stage) is complete.

[0137] The films were then annealed to complete perovskite crystal, as described in the section Device Fabrication. During annealing, the substrates were heated to a temperature which facilitated perovskite crystal growth. This may be referred to as the “heating stage”.

[0138] The heating (third) stage of the perovskite film formation process was probed using transmittance from the samples upon heating.

[0139] Figure 5a shows transmittance data of a film, based on the position of 480 nm wavelength peak in photoluminescence. In Figure 5a a main graph shows the time evolution of the transmittance of the films during exposure to the anti -solvent, and an inset graph, showing the time evolution of the transmittance of the films during an annealing stage.

[0140] Figure 5b shows transmittance data of the film at 750 nm. Figure 2b shows a main graph, showing the time evolution of the transmittance of the films during exposure to the anti -solvent, and an inset graph, showing the time evolution of the transmittance of the films during an annealing stage.

[0141] Wavelengths of 480 nm and 750 nm were chosen in photoluminescence experiments to probe the higher bandgap intermediate phases and the fully formed perovskite crystals, respectively. The results agree with the PL data from Figure 4a-4f and further reveal that DE may be the slowest acting solvent during heating. In the results shown, DE may need up to double the amount of time to fully convert the films into perovskite crystals. The final films formed with DE had 27% transmittance at 750 nm suggesting near ideal film thickness for device.

[0142] DE antisolvent treatment may produce the best quality (pinhole -free) films, although with small crystal sizes, while EA and ANI antisolvents may produce films which are lower in quality than those treated with DE with pinholes and uneven distribution of crystal sizes possibly due to the loss of MAI and perovskite nuclei.

[0143] This may be linked to the fact that DE is the only solvent from the studied here with a poor miscibility to DMSO, one of the precursor solvents, while all others are miscible with both DMSO and DMF. The first stage of the DE treatment is DMF extraction from the precursor solution, which may initiate or accelerate the crystallisation of intermediate phases, such as PbE DMSO and MAI PbL DMSO. The intermediate precursor phases may retard the perovskite crystallisation process, which may be enforced with thermal annealing. This may be followed by slow DMSO extraction due to DMSO volatility, which may lead to the formation of higher quality nanocrystals of MA PbL compared to the other antisolvents, as observed by the high PL intensity. There is also the possibly for introducing a nonstoichiometric intermediate phases xMAI PbL xDMSO (x<l)26.In other antisolvents studied, perovskite crystallisation may be mostly complete within the antisolvent bath treatment which may negatively impact the final film quality.

[0144] Balancing the rate of DMF and DMSO extraction may be preferable, as it may lead to intermediate MAI PbL DMSO phases and improved control of the rate of crystal growth. Slowing down these rates may be beneficial and may enable successful crystallisation of perovskite films.

[0145] DE may be a particularly advantageous solvent due to its low cost, low toxicity and low environmental impact.

[0146] The method according to the present invention may reduce or eliminate haziness in the perovskite film, which may occur in perovskite films formed by a method according to the prior art.

[0147] Hazy films occur regularly in spin-coated and printed films. This is due to uneven flow of the antisolvent dispense rate, its amount, the spinning rate, and the difference in miscibility between the antisolvent and the host solvent in the precursor solution.

[0148] For antisolvent bath-treated large-area printed perovskite films, it is also common to see irregular stains forming on the film during the antisolvent bath. Bath-treated means that the antisolvent is stationary, and is not provided as a flow over the surface of the perovskite precursor film.

[0149] The present inventors have determined that stains may occur due to undesirably rapid nucleation, which may happen irregularly along the films, and that this may be driven by the extraction of DMF from the deposited precursor solution.

[0150] The present inventors have determined that providing a flow of anti-solvent, rather than a stationary bath, minimises haziness of the films and improve the uniformity of the large -area films. This may be due to the formation of intermediate phases. Figure 6a shows the photoluminescence intensity and transmittance of the 480 nm light of a perovskite film in a stationary antisolvent bath and in a flow of antisolvent. These results were collected during a DE antisolvent flow rate of 1 L / min across the surface of a perovskite film

[0151] From the PL intensity changes on Figure 6a, it may be concluded that the formation of intermediate phases and nanocrystals is faster under the flow of antisolvent, as the maximum PL was reached at 60 s. The transmission data reached a plateau at the same time. This is likely due to faster and more uniform DFM extraction as compared to the stationary bath. The maximum PL intensity for the flow treatment was 75% higher than that under the stationary antisolvent treatment, which indicates better quality crystals were formed under the antisolvent flow by 60 s. The decrease of PL after 60 s suggests that there exists an optimal time length of the antisolvent treatment.

[0152] After annealing, both samples have transmittance of around 27% at 750 nm. This suggests achieving a similar films thickness. The main difference lies in the surface quality of the film as judged by the near-infrared region of the transmittance spectra, where the flow-treated film showed 60-75 % transmittance compared to 50-60% for the stationary bath-treated film, indicating a much better film quality and a smoother perovskite -air interface for the flow -treated sample. This is a common phenomenon observed in semiconductors, whereby less scattering films with a smooth film-air surface (mirror like surface) exhibit high transmission at wavelengths longer than the optical bandgap.

[0153] The smaller roughness of the flow treated film is confirmed from the pictures in Figure 6b (compare DE vs. DE Flow films), where mirror like film is observed for DE Flow only.

[0154] Furthermore, we probed the PL FWHM, which showed that at 180 s, the flow process produces more uniform crystal size distribution (59 nm) than the stationary (67 nm). At the same time, the PL peak position at 180 s indicates larger intermediate phase crystallite sizes for the flow process (5.44 nm for flow vs. 4.64 nm for stationary).

[0155] To reduce or remove the haziness, the following procedures may be implemented in the method according to the present invention. First, the flow of antisolvent over the surface of the perovskite precursor film may be kept substantially parallel or parallel to the surface of the perovskite precursor film. Additionally, or alternatively, a higher flow rate may be provided. A faster and more uniform flow rate may reduce or eliminate haziness in the films.

[0156] The film labelled “DE Flow” in Figure 6b was made by immersing the substrate vertically in a container comprising DE, and creating a fast and uniform flow across the inner sidewall of the container by magnetic stirring. This film exhibited particularly low haziness.

[0157] The in-situ PL data in Figure 6a shows that the flow antisolvent treatment reaches a peak maximum at 60 s and drops afterwards. This may suggest that there may be an optimum time or range of times for which the antisolvent flow may be applied. Antisolvent flow duration times of 5, 10, 20, 40 and 60 s were tested and the quality of the final films was then analysed with X-ray powder diffraction (XRD), shown in Figure 7a.

[0158] All films exhibited a major peak at 14.2° corresponding to the (110) plane of the perovskite tetragonal structure, according to the PDF card [PDF 01-084-7607 (ICDD, 2018)]. The peak at 12.7° is assigned to the (001) plane of the PbE hexagonal structure [PDF 00-007-0235 (ICDD, 1957)]. A strong PbE peak can be identified for the 5 s treated film, while the 10 s treated film shows no peak of PbE. The PbE peak again emerges for 20 - 60 s treated films, although with a smaller intensity than the 5 s sample.

[0159] This suggests that 10 s treated film may result in a MA PbE film of high quality.

[0160] Further studies were carried out to investigate the quality of the films using Time- Correlated Single Photon Counting (TCSPC), which may indicates the trap state density of films.

[0161] Figure 7b shows the PL decays fitted with a single exponential function, and the trend agrees with the XRD results, where the 10 s treated sample had the longest lifetime of 164.23 ns confirming that particularly high crystal quality is obtained for 10 s treatment time. These results suggest that treating a MAPbL precursor film with 10 s of DE flow may lead to a perovskite film of particularly high quality.

[0162] [Device fabrication]

[0163] The method according to the present invention may be used to make devices, such as solar cells. A device may have a stacked structure, such as: ITO (75x25 mm) / SnO2 (Slot die coated) / MAPbl3 (Slot die coated) / Spiro-MeOTAD (Spin coated) / Au (Vacuum evaporated). An example of a cross-section of such as device is shown in Figure 8f.

[0164] The fabrication procedure used DE antisolvent and vortex flow for application of the antisolvent flow. Details of the device fabrication are presented in the section Experimental Details.

[0165] Different antisolvent bath duration times were tested, and Figure 8a-d presents the resulting device parameters. The highest current density (Jsc) and smallest variation is observed for 10 s and 20 s treated fdms. In terms of open -circuit voltage (Voc) distributions, the 5 s and 10 s treated devices (Figure 8b) produce the highest voltage of 1.05 V (of the measured devices). From Figure 8c, the forward scan of 5 s treated devices showed a low fdl factor (FF). In contrast, the rest showed a similar level of around 70%. Combining these results, Figure 8e shows the distribution of power conversion efficiency (PCE).

[0166] Devices having a film which has been treated for 10 s by the antisolvent flow may have particularly good optoelectronic properties. The PCE of the 20, 40 and 60 s treatment drops gradually with increasing time, yet it remained in the range of 15% to 16%.

[0167] Treating a perovskite precursor films for 10 s with DE may led to the formation of particularly well -distributed intermediate phases and may produce an excellent quality films. For samples treated for longer than 10 s, the PCE drops, which may be attributed to the decrease in the crystal quality as seen in TCSPC and the traces of PbL in the XRD. This may be due to a shift of the stoichiometry of PbL DMSO and MAI PbL DMSO intermediate phases to the former, which could be due to the loss of MAI or DMSO into the DE flow. Thus, although the miscibility between DE and DMSO is low, prolonged flow washing would still decompose the intermediate phases causing MAI loss, indicating that optimisation of the antisolvent treatment time should be an essential step in scale-up activities.

[0168] Performance of the device may be further enhanced when the substrate comprises spin- coated SnCh instead of slot die-coated SnC although both substrates may be used to achieve fdms of good quality.

[0169] A device formed by a method according to the present invention may have the following structure SnO? (Spin coated, 60 nm-thick) / 560 nm MAPbh (Slot die coated) / 190 nm Spiro- MeOTAD (Spin coated) / 100 nm Au (Vacuum evaporated). The PCE of this device 18.57 %. All steps of the, except for vacuum evaporation, may be performed in an ambient environment with humidity around 30-50 % and temperature around 23 °C. The device testing was also conducted in an ambient environment (no glove box or clean room use).

[0170] [Experimental details]

[0171] [Materials]

[0172] SnCE was purchased from Alfa Aesar (44592, 15% in H2O colloidal dispersion), Methylammonium iodide was purchased from Greatcell Solar, Dimethyl sulfoxide was from Fisher Scientific, Spiro-MeOTAD was purchased from Ossila. All other materials and solvents are purchased from Sigma Aldrich.

[0173] [Device fabrication]

[0174] ITO (8 ohm / square) substrates (25 x 75 mm) were cleaned in acetone and IPA under ultrasonication for 15 min, and plasma treated for another 15 min. SnO2 was mixed with water (HPLC grade) in 1:3 wt%, stirred for 30 min and filtered using a 0.45 pm PTFE filter before use. The parameter setting for slot die coating of the SnO2 layer was: 15 mm / s, dispense rate: 5 pL / s, and stage pre-heating at 50 °C. After the coating, the substrates were annealed on a hot plate set at 150 °C for 30 min. For spin-coated SnO2, the same solution was used with 3000 RPM for 30 s, followed by the same annealing process.

[0175] Perovskite layer: 1. 1 M perovskite precursor solution was prepared by dissolution of the MAI with PbE in DMF, DMSO mixed solvent (9: 1.1, volume ratio) at 50 °C stirring for one hour. The precursor solution was filtered with a 0.45 pm PTFE filter before use. The parameter setting for slot die coating was: 5 mm / s, dispense rate: 1 pL / s, and stage pre-heating at 40 °C. After printing, the film was transferred into a quartz chamber for in -situ analysis or a nylon beaker for device making. After the antisolvent bath, the films were annealed at 100°C for 10 mins on a hot plate covered with a glass petri dish. Note that 1 M precursor was used on pure microscope glass slides for in -situ analysis. For device making, SnCE-coatcd ITO was again treated with plasma cleaning for 15 mins before perovskite deposition.

[0176] After thermal annealing, perovskite films were quickly removed and cut into three pieces of 25x25 mm substrates. Spiro-MeOTAD was dissolved in anhydrous toluene in 60 mg / ml concentration with additives of 28.8 pL / ml of 4-tert-butylpyridine and 22 pL / ml of Li-bis(trifluoromethanesulfonyl)imide (Li-TFSI), which comes from a stock of 350 mg Li-TFSI in 1 ml anhydrous acetonitrile. The solution was stirred overnight at 60 °C and fdtered with a 0.22 pm fdter before use. This was spin coated at 3000 RPM for 30 s without further treatment. 100 nm gold was deposited via thermal evaporation. Chemical preparations were conducted in an N2-filled glove box, and all other experiments were conducted in ambient conditions with 20 °C temperature and 30 - 40% humidity.

[0177] [Characterisation]

[0178] In-situ photoluminescence was performed using a self-assembled optical holder stage with a Wasatch WP-VISNIR-R-50 spectrometer equipped with Avantes AvaLight-HPLED miniature high power 405 nm LED source. Relevant short-pass and long-pass fdters were needed to calibrate the final spectra. For the measurement, integration time was set at 500 ms with a 1 s interval of measurements.

[0179] In-situ transmittance was performed using a self-assembled U-shaped fibre holder for optical alignment. Avantes AvaSpec-ILS2048CL-EVO-RS-UA ultra-low light fibre optic UV / VIS / NIR spectrometer was used with AvaLight-DHC compact deuterium -halogen light source. For the measurement, integration time was set at 50 ms, 20 averages with 1 s interval of measurements.

[0180] After the printing, the as-printed film was submerged in the quartz chamber. PL and transmittance were carried out 180 times with 1 s intervals. Afterwards, the substrate was placed onto a customised hotplate at 100 °C. The hot plate was suspended in the air with a 4 mm diameter through-hole drilled in the middle, the same two optical fibres were moved across the hole, and the transmittance was recorded for 60 sec with 1 s intervals.

[0181] For in-situ flow analysis, a Kamoer lab WIFI UIP stepper motor peristaltic pump was used to provide the flow. The flow rate was kept at 1 L / min. Viton™ Fluoroelastomer tubing was used to transfer the solvents.

[0182] In-situ data was fitted using self-written algorithms run by Spyder 4.2.5 under Python 3.8.

[0183] Time-correlated single photon counting (TCSPC) was performed using Edinburgh Instruments FLS1000 photoluminescence spectrometer. The excitation wavelength was 369.80 nm, and the emission wavelength was 770 nm. The time range was set at one ps, the channel range was 1024, and the bandwidth was 20 nm. The data-acquiring time was 10 mins.

[0184] Scanning electron microscopy (FEI Inspect F) was used for observing perovskite morphology and device architecture. X-ray diffraction was performed using a Siemens D5005 diffractometer from 5° to 70° using Cu Ka radiation. Crystal peaks were identified using the International Centre for Diffraction Data (ICDD) database.

[0185] Slot die coater L2005A1-UK was purchased from Ossila. Spin coater WS650MA was from Laurell. For solar cell performance, a customised sample holder was used for making the electrical connection. A Keithley 2400 SMU was used as a data acquisition instrument with self-written LabVIEW controlling software. The one sun illumination was provided by an Oriel® Soli A™ class ABB solar simulator and calibrated using Newport 91160B calibrated reference cell meter. The active area was defined using a pre-cut aluminium mask with 0.2 cm2. The scan range was -0.1 V to 1.1 V, and the voltage increment was 0.01 V.

[0186] A perovskite film formed by the method of the present invention can be used in an optoelectronic device such as a solar cell, solar battery, laser, sensor, light emitting diode, display, catalysis and solar fuels.

[0187] A method of manufacturing solar cells, or solar batteries, or lasers, or sensors, or light emitting diodes, or displays, or catalysis and solar fuels may comprise the method according to any one of the preceding claims.

[0188] Where the term “comprising”, “comprise” or “comprises” is used, said term may substituted by “consisting of, “consist of or “consists of respectively, or by “consisting essentially of, “consist essentially of or “consists essentially of respectively. Reference to “comprising” certain features is interpreted as meaning that it includes those features, but that it does not exclude the presence of other features. On the other hand, the wording “consist of’ is used to mean that no further features are present apart from the ones following said wording. Any reference to a numerical range or single numerical value also includes values that are about that range or single value.

[0189] Having described the invention it will be appreciated that variations may be made on the above described embodiments which are not intended to be limiting. The invention is defined in the appended claims and their equivalents.

[0190] This application claims priority from GB2402190.9 filed on 16 February 2024, the contents of which are hereby incorporated by reference.

[0191] Embodiments of the present disclosure can be further described by the following clauses:1. A method of forming a perovskite film comprising: depositing a perovskite precursor solution to a surface of a substrate to form a film having a first surface, and exposing the first surface to a flow of antisolvent, wherein the flow rate of antisolvent occurs is in the range of equal to or greater than O.Olm / s and equal to or less than lm / s3.2. A method of forming a perovskite film comprising:depositing a perovskite precursor solution to a surface of a substrate to form a film having a first surface, and exposing the first surface to a flow of antisolvent, wherein the perovskite precursor has the formula ABX3, where A is a monovalent cation, B is a divalent cation, and X represents a single type of halide anion selected from a list containing iodide, bromide, or chloride.3. The method according to any of the preceding clauses, wherein the perovskite precursor solution comprises a perovskite precursor dissolved in a first solvent.4. The method according to any of the preceding clauses, wherein the perovskite precursor has the formula ABX3, where A is a monovalent cation, B is a divalent cation, and X represents a single type of halide anion selected from a list containing iodide, bromide, or chloride.5. The method according to any of the preceding clauses, wherein the perovskite precursor comprises one or more of: MAPbh. FAPbh. CsFAPbfi.6. The method according to any one of clauses 2 to 5, wherein the first solvent and the antisolvent have a miscibility gap at a temperature at which the method is performed.7. The method according to any one of clauses 2 to 6, wherein the first solvent comprises one or more of: dimethylformamide, dimethyl sulfoxide, triethyl phosphate, y-valerolactone, Gammabutyrolactone, Acetonitrile, N- N-methyl -2 -pyrrolidone, Methyl acetate, tetrahydrofuran, 2- methoxyethanol, ethanol, 2-pyrrolidinone, 2-Methyltetrahydrofuran.8. The method according to any of the preceding clauses, wherein the antisolvent is selected from one of: diethyl ether, ethyl acetate, toluene, chlorobenzene, diphenyl ether, methylanisole, anisole, dichlorobenzene, dichloromethane, ethanol, methanol, isopropanol, butanol, chloroform, butyl acetate, trifluorotoluene, hexane, m-xylene, mesitylene, (R)-(+)-limonene, diethyl carbonate, dimethyl carbonate.9. The method according to any of the preceding clauses wherein the flow rate of antisolvent is provided parallel to the first surface.10. The method according to any of the preceding clauses, wherein the exposing of the substrate to the flow of the antisolvent takes place for 1 to 200 s, preferably, 10 to 150 s.11. The method according to any of the preceding clauses, wherein the substrate is annealed after the exposing the first surface to a flow of antisolvent.12. The method according to any of the preceding clauses, wherein the substrate is an anode layer.13. The method according to clause 12, wherein the anode layer is optically transparent.14. The method according to clause 13, wherein the substrate has a stacked structure comprising the anode layer and an n-type semiconductor layer, and the perovskite precursor solution is deposited on the surface of the n-type semiconductor layer.15. The method according to any of the preceding clauses, wherein a p-type semiconductor layer is deposited on the perovskite film after the perovskite film is formed, optionally, by spincoating, slot-die coating, blade coating, spray coating or evaporation.16. The method according to clause 15, wherein a cathode layer is deposited on the p-type semiconductor layer.17. The method according to any one of clauses 1-11, wherein the substrate is a cathode layer.18. The method according to clause 17, wherein the cathode layer is optically transparent.19. The method according to clause 17 or clause 18, wherein the substrate has a stacked structure comprising the cathode layer and a p-type semiconductor layer, and the perovskite precursor solution is deposited on the surface of the p-type semiconductor layer.20. The method according to any of the preceding claims, wherein a n-type semiconductor layer is deposited on the perovskite film, optionally, by spin-coating, slot-die coating, blade coating, spray coating or vacuum evaporation.21. The method according to clause 20, wherein an anode layer is deposited on the n-type semiconductor layer.22. The method according to clause 15, 16 or 19-21, wherein the p-type semiconductor layer is selected from one of: spiro-OMeTAD, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine, regioregular poly(3-hexylthiophene-2,5-diyl), carbon black, carbon-60, carbon allotropes, nickel oxide and self-assembled monolayers (SAMs) such as Me-4PACz ([4-(3,6-Dimethyl-9H-carbazol- 9-yl)butyl]phosphonic Acid) .23. The method according to any one of clauses 16-22, wherein the cathode layer comprises one or more of: gold, carbon, allotropes of carbon, silver, copper.24. The method according to any one of clauses 12-16 or 21, wherein the anode layer comprises one or more of: indium tin oxide, fluorine -doped tin oxide, tin oxide, titanium, nickel, and carbon.25. The method according to any one of clauses 14-16 and 20-21, wherein the n-type semiconductor layer is selected from one of: tin oxide, zinc oxide, titanium oxide, carbon -60, [6,6] - phenyl-C61 -butyric acid methyl, and 2,9-Dimethyl-4,7-diphenyl-l,10-phenanthroline.26. The method according to any of the preceding clauses, wherein the flow of antisolvent over the first surface is generated by a peristaltic pump.27. The method according to any of clauses 1 to 26, wherein the flow of antisolvent over the first surface is generated by the following steps: disposing substrate in a container, wherein the container holds the antisolvent; stirring the antisolvent; wherein optionally, the antisolvent is stirred via magnetic stirring or mechanical stirring.28. A method of manufacturing electronic devices, such as solar cells, comprising the method according to any one of the preceding clauses.29. An optoelectronic device, such as a solar cell, manufactured by the method according to any one of the preceding clauses.

Claims

Claims1. A method of forming a perovskite film comprising: depositing a perovskite precursor solution to a surface of a substrate to form a film having a first surface, and exposing the first surface to a flow of antisolvent, wherein the flow rate of antisolvent occurs is in the range of equal to or greater than O.Olm / s and equal to or less than Im / s.

2. The method according to claim 1, wherein the perovskite precursor solution comprises a perovskite precursor dissolved in a first solvent.

3. The method according to claim 1 or claim 2, wherein the perovskite precursor has the formula A BX where A is a monovalent cation, B is a divalent cation, and X represents a single type of halide anion selected from a list containing iodide, bromide, or chloride.

4. The method according to any of the preceding claims, wherein the perovskite precursor comprises one or more of: MAPbh. FAPbh. CsFAPbh.

5. The method according to any one of claims 2 to 4, wherein the first solvent and the antisolvent have a miscibility gap at a temperature at which the method is performed.

6. The method according to any one of claims 2 to 5, wherein the first solvent comprises one or more of: dimethylformamide, dimethyl sulfoxide, triethyl phosphate, y-valerolactone, Gammabutyrolactone, Acetonitrile, N- N-methyl-2 -pyrrolidone, Methyl acetate, tetrahydrofuran, 2- methoxyethanol, ethanol, 2-pyrrolidinone, 2-Methyltetrahydrofuran.

7. The method according to any of the preceding claims, wherein the antisolvent comprises one or more of: diethyl ether, ethyl acetate, toluene, chlorobenzene, diphenyl ether, methylanisole, anisole, dichlorobenzene, dichloromethane, ethanol, methanol, isopropanol, butanol, chloroform, butyl acetate, trifluorotoluene, hexane, m-xylene, mesitylene, (R)-(+)-limonene, diethyl carbonate, dimethyl carbonate, or optionally, the antisolvent is selected from one or more of: diethyl ether, ethyl acetate, toluene, chlorobenzene, diphenyl ether, methylanisole, anisole, dichlorobenzene, dichloromethane, ethanol, methanol, isopropanol, butanol, chloroform, butyl acetate, trifluorotoluene, hexane, m- xylene, mesitylene, (R)-(+)-limonene, diethyl carbonate, dimethyl carbonate .

8. The method according to any of the preceding claims wherein the flow rate of antisolvent is provided parallel to the first surface.

9. The method according to any of the preceding claims, wherein the exposing of the substrate to the flow of the antisolvent takes place for 1 to 200 s, preferably, 10 to 150 s.

10. The method according to any of the preceding claims, wherein the substrate is annealed after the exposing the first surface to a flow of antisolvent.

11. The method according to any of the preceding claims, wherein the substrate is an anode layer.

12. The method according to claim 11, wherein the anode layer is optically transparent.

13. The method according to claim 11 or claim 12, wherein the substrate has a stacked structure comprising the anode layer and an n-type semiconductor layer, and the perovskite precursor solution is deposited on the surface of the n-type semiconductor layer.

14. The method according to any of the preceding claims, wherein a p-type semiconductor layer is deposited on the perovskite film, optionally, by spin-coating, slot-die coating, blade coating, spray coating or evaporation.

15. The method according to claim 14, wherein a cathode layer is deposited on the p-type semiconductor layer.

16. The method according to any one of claims 1-10, wherein the substrate is a cathode layer.

17. The method according to claim 16, wherein the cathode layer is optically transparent.

18. The method according to claim 16 or claim 17, wherein the substrate has a stacked structure comprising the cathode layer and a p-type semiconductor layer, and the perovskite precursor solution is deposited on the surface of the p-type semiconductor layer.

19. The method according to any one of claims 1-10 or 15-18, wherein a n-type semiconductor layer is deposited on the perovskite fdm, optionally, by spin-coating, slot-die coating, blade coating, spray coating or vacuum evaporation.

20. The method according to claim 19, wherein an anode layer is deposited on the n-type semiconductor layer.

21. The method according to claim 14, 15 or 18-20, wherein the p-type semiconductor layer is selected from one of: spiro-OMeTAD, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine, regioregular poly(3-hexylthiophene-2,5-diyl), carbon black, carbon-60, carbon allotropes, nickel oxide, and self-assembled monolayers (SAMs) such as Me-4PACz ([4-(3,6-Dimethyl-9H- carbazol-9-yl)butyl]phosphonic Acid) .

22. The method according to any one of claims 15-21, wherein the cathode layer comprises one or more of: gold, carbon, allotropes of carbon, silver, copper.

23. The method according to any one of claims 11-15 or 20, wherein the anode layer comprises one or more of: indium tin oxide, fluorine-doped tin oxide, tin oxide, titanium, nickel, and carbon.

24. The method according to any one of claims 13-15 and 19-20, wherein the n-type semiconductor layer is selected from one of: tin oxide, zinc oxide, titanium oxide, carbon -60, [6,6] - phenyl-C61 -butyric acid methyl, and 2,9-Dimethyl-4,7-diphenyl-l,10-phenanthroline.

25. The method according to any of the preceding claims, wherein the flow of antisolvent over the first surface is generated by a peristaltic pump.

26. The method according to any of claims 1 to 23, wherein the flow of antisolvent over the first surface is generated by the following steps: disposing substrate in a container, wherein the container holds the antisolvent; stirring the antisolvent; wherein optionally, the antisolvent is stirred via magnetic stirring or mechanical stirring.

27. A method of manufacturing electronic devices, such as solar cells, comprising the method according to any one of the preceding claims.

28. An optoelectronic device, such as a solar cell, manufactured by the method according to any one of the preceding claims.

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