PROCESS OF FORMING A PHOTOACTIVE LAYER OF A PEROVSKITE PHOTOACTIVE DEVICE

Abstract

A process of forming a photoactive layer of a planar perovskite photoactive device comprising: applying at least one layer of a first precursor solution to a substrate to form a first precursor coating on at least one surface of the substrate, the first precursor solution comprising MX.sub.2 and AX dissolved in a first coating solvent, wherein the molar ratio of MX.sub.2:AX=1:n with 0<n<1; and applying a second precursor solution to the first precursor coating to convert the first precursor coating to a perovskite layer AMX.sub.3, the second precursor solution comprising AX dissolved in a second coating solvent, the first precursor solution reacting with the second precursor solution to form a perovskite layer AMX.sub.3 on the substrate, wherein A comprises an ammonium group or other nitrogen containing organic cation, M is selected from Pb, Sn, Ge, Ca, Sr, Cd, Cu, Ni, Mn, Co, Zn, Fe, Mg, Ba, Si, Ti, Bi, or In, X is selected from at least one of F, Cl, Br or I.

Claims

1. A process of forming a photoactive layer of a planar perovskite photoactive device comprising: applying at least one layer of a first precursor solution to a substrate to form a first precursor coating on at least one surface of the substrate, the first precursor solution comprising MX.sub.2 and AX dissolved in a first coating solvent, wherein the molar ratio of MX.sub.2:AX=1:n with 0<n<1; and applying a second precursor solution to the first precursor coating to convert the first precursor coating to a perovskite layer AMX.sub.3, the second precursor solution comprising AX dissolved in a second coating solvent, the first precursor solution reacting with the second precursor solution to form a perovskite layer AMX.sub.3 on the substrate, thereby forming a photoactive layer of a planar perovskite photoactive device, wherein A comprises an ammonium group or other nitrogen containing organic cation, M is selected from Pb, Sn, Ge, Ca, Sr, Cd, Cu, Ni, Mn, Co, Zn, Fe, Mg, Ba, Si, Ti, Bi, or In, X is selected from at least one of F, Cl, Br or I.

2. The process according to claim 1, wherein the molar ratio of MX.sub.2:AX=1:n with 0<n≦0.5.

3-5. (canceled)

6. The process according to claim 1, wherein the process further involves the step of: drying the first precursor coating prior to applying the second precursor solution.

7-8. (canceled)

9. The process according to claim 1, wherein the process further involves the step of: subjecting the substrate with first precursor coating to a solvent vapour soaking process before applying the second precursor solution to the first precursor coating.

10. The process according to claim 9, wherein the substrate is subjected to the solvent vapour soaking process immediately after the first precursor solution is applied to the substrate.

11. The process according to claim 1, wherein MX.sub.2 and AX are soluble in the first coating solvent, AX is soluble in the second coating solvent, and MX.sub.2 has a low to zero solubility in the second coating solvent.

12. The process according to claim 1, wherein the first coating solvent comprises DMF, DMSO, γ-butyrolactone, acetone, acetyl acetone, ethyl acetoacetate, NMP, DMAC, THF or combinations thereof.

13. The process according to claim 1, wherein the second coating solvent is at least one of including isopropanol, n-butanol, isobutanol, ethanol, methanol, acetic acid, ethylene glycol, propylene glycol, glycerol, allyl alcohol, propagyl alcohol, inositol or combinations thereof.

14. The process according to claim 1, wherein A in AX comprises an organic cation having the formula (R.sub.1R.sub.2R.sub.3R.sub.4N), wherein: R.sub.1 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl; R.sub.2 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl; R.sub.3 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl; and R.sub.4 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl.

15. The process according to claim 1, wherein A in AX comprises an organic cation having the formula (R.sub.5R.sub.6N═CH—NR.sub.7R.sub.8), and wherein: R.sub.5 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl; R.sub.6 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl; R.sub.7 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl; and R.sub.8 is hydrogen, unsubstituted or substituted C1-C20 alkyl, or unsubstituted or substituted aryl.

16. The process according to claim 14, wherein AX is selected from the group consisting of CH.sub.3NH.sub.3X and HC(NH.sub.2).sub.2X, and wherein X is selected from at least one of F, Cl, Br or I.

17. The process according to claim 1, wherein the perovskite layer comprises an organo-metal halide perovskite.

18. The process according to claim 17, wherein the perovskite layer comprises at least one of CH.sub.3NH.sub.3MX.sub.3 or HC(NH.sub.2).sub.2MX.sub.3, in which, M is selected from Pb, Sn, Tl, Bi, or In; and X is selected from at least one of F, Cl, Br or I.

19-20. (canceled)

21. The process according to claim 1, wherein the second precursor solution comprises from 5 to 75 wt % AX.

22. (canceled)

23. The process according to claim 1, wherein the substrate comprises at least one of a polymer, metal, ceramic or glass.

24. The process according to claim 1, wherein the substrate includes one or more layers or coatings selected from at least one of: at least one coating of a transparent conductive oxide (TCO); at least one hole transporting layer comprising an organic or inorganic semiconductor; or at least one electron transporting layer comprising an organic or inorganic conductor.

25. (canceled)

26. The process according to claim 1, wherein the first precursor coating has a dry layer thickness from 100 nm to 600 nm.

27. The process according to claim 1, wherein at least one of the first perovskite precursor solution or the second precursor solution further comprises a perovskite crystallisation retardant comprising a polymer additive which is soluble in the respective first coating solvent or second coating solvent.

28. The process according to claim 27, wherein the polymer additive is selected from the group consisting of poly vinyl alcohol, poly vinyl acetate (PVAc), ABS, poly amides, poly acrylics, poly imide, poly acrylonitrile, poly butyl methacrylate, poly butadiene, poly carboxy methyl cellulose, poly ethers, poly ethylene acrylates, poly glycols, poly isocyanates, poly methacrylates, poly vinyl butyral, poly vinyl fluoride, poly vinyl methyl ethers, poly amines, polyethylene oxide, polyethylene glycol Poly(2-ethyl-2-oxazoline) and combinations thereof.

29. (canceled)

30. The process according to claim 1, wherein AX from the first precursor solution is incorporated into the final perovskite layer AMX.sub.3.

31-35. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

[0117] The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

[0118] FIG. 1 provides an illustration of the layers comprising a solar cell incorporating a photoactive layer according to the present invention in (A) a conventional solar cell structure; and (B) an inverted solar cell structure; (C) a tandem solar cell structure.

[0119] FIG. 2 provides a schematic illustration of slot die coating with a gas-quenching process for the fabrication of pinhole-free first precursor solution (MX.sub.2 plus AX additive) layer showing (A) Overall printer schematic; and (B) Slot die printing head schematic.

[0120] FIG. 3 provides six optical microscope images of PbI.sub.2 and a series of PbI.sub.2 plus different % composition CH.sub.3NH.sub.3I films prepared by slot die coating and dried naturally at room temperature.

[0121] FIG. 4 is a plot of absorbance change during dipping process for conversion of PbI.sub.2 with and without CH.sub.3NH.sub.3I additive to perovskite.

[0122] FIG. 5 provides output curves of four different perovskite solar cells with a device configuration of ITO/ZnO/perovskite (CH.sub.3NH.sub.3PbI.sub.3)/doped P3HT/Ag fabricated by sequential deposition process.

[0123] FIG. 6 provides a plot of power conversion efficiency versus molar ratio of CH.sub.2NH.sub.3I added to PbI.sub.2 showing the effect of MAI additive on performance of perovskite devices fabricated by same soaking-free procedure.

[0124] FIG. 7 shows a current density-voltage (J-V) curves of perovskite devices fabricated by same soaking-free process using conventional pure PbI.sub.2 and CH.sub.3NH.sub.3I and polymer added PbI.sub.2 precursors.

[0125] FIG. 8 shows the relative conversion rates of intermediates of sequentially deposited perovskite layer monitored by absorbance of the film during dipping conversion process in CH.sub.3NH.sub.3I in 2-propanol.

[0126] FIG. 9a shows a photograph of a roll-to-roll conversion process of CH.sub.3NH.sub.3I added intermediate according to one embodiment of the present invention.

[0127] FIG. 9b shows a photograph of an intermediate and perovskite during the same roll-to-roll process shown in FIG. 9a.

[0128] FIG. 10 provides a current density-voltage (J-V) curve of roll-to-roll printed perovskite device fabricated by one embodiment of the present invention

DETAILED DESCRIPTION

[0129] Photovoltaic cells, particularly thin film and flexible solar cells are formed as a multilayer coating on a substrate. As shown in FIG. 1, this multilayer coating structure can be arranged on the substrate in at least two different arrangements termed in the art as (A) a conventional structure, or (B) as an inverted structure (FIG. 1(B)).

[0130] As shown in FIG. 1(A), a conventional structure is formed on a substrate having the following layers successively layered on a surface thereof: a transparent conductor (such as a transparent conductive oxide (TCO), conducting polymer or thin metal) with or without conducting grids, followed by a hole transporting layer; followed by the photoactive layer; followed by an electron transporting layer, and followed by a conductor layer (typically a metal).

[0131] As shown in FIG. 1(B), an inverted structure is formed on a substrate having the following layers successively layered on a surface thereof: a transparent conductor (such as a transparent conductive oxide (TCO), conducting polymer or thin metal) with or without conducting grids, followed by an electron transporting layer; followed by the photoactive layer; followed by a hole transporting layer, and followed by a conductor layer (typically a metal).

[0132] It should be appreciated that the hole transporting layer or electron transporting layers could be omitted in some embodiments of the above conventional and inverted structures. These layers can therefore be optional in certain embodiments.

[0133] As shown in FIG. 1(C), a tandem structure is formed on a substrate using two stacked solar cell structures, i.e. a top cell and a bottom cell of the conventional or inverted structure. The stacked structure includes two different solar cell of the same or different configurations. The example provided comprises the following layers successively layered thereon: Transparent conductor layer (TCO, conducting polymer or thin metal) with or without collecting grids; followed by a Top cell—a Perovskite solar cell either type A (FIG. 1A) or type B (FIG. 1B); followed by a Transparent conductor layer (TCO, conducting polymer or thin metal) with or without conducting grids; followed by a Bottom cell (perovskite, organic, inorganic or silicon solar cell); followed by a Metal (or conductor) layer. A substrate such as glass, plastic, metal or ceramic could also be used but should be understood to be optional. Tandem and multilayer/junction structures will be discussed in more detail below.

[0134] Each layer can be formed by one of a number of coating techniques know in the art including casting, doctor blading, screen printing, inkjet printing, pad printing, knife coating, meniscus coating, slot die coating, gravure coating, reverse gravure coating, kiss coating, micro-roll coating, curtain coating, slide coating, spray coating, flexographic printing, offset printing, rotatory screen printing, or dip coating. It should be appreciated that a person skilled in the art would be able to adopt a suitable technique to apply each layer based on techniques known in the art.

[0135] The various layers can comprise a number of suitable components currently known in the art. Examples include: [0136] Suitable transparent conductive oxides (TCO) include tin doped indium oxide (ITO), fluoride-doped tin oxide (FTO), doped zinc oxide such as aluminium doped zinc oxide (AZO), or indium doped cadmium-oxide; [0137] Suitable hole transporting layers include a transparent conducting polymer such as at least one of Poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate mixture (PEDOT:PSS), poly(4,4-dioctylcyclopentadithiophene); doped P3HT (Poly(3-hexylthiophene-2,5-diyl)) or the like; [0138] Suitable electron transporting layers include zinc oxide, titanium dioxide, tungsten trioxide or the like; [0139] Suitable conductor layers comprise aluminium, silver, magnesium, copper, gold or suitable alloys thereof or the like; and [0140] Suitable substrates include metals, polymers, ceramics, or glasses.

[0141] In a perovskite type photoactive device, such as a photovoltaic cell, the photoactive layer comprises an organic-inorganic perovskite-structured semiconductor.

[0142] The skilled person will appreciate that a perovskite material can be represented by the formula [A][M][X].sub.3, wherein [A] is at least one cation, [M] is at least one cation and [X] is at least one anion. When the perovskite comprise more than one A cation, the different A cations may distributed over the A sites in an ordered or disordered way. When the perovskite comprises more than one M cation, the different M cations may distributed over the M sites in an ordered or disordered way. When the perovskite comprise more than one X anion, the different X anions may be distributed over the X sites in an ordered or disordered way. The symmetry of a perovskite comprising more than one A cation, more than one M cation or more than one X cation, will be lower than that of CaTiO.sub.3. A perovskite is a crystalline compound. Thus, the layer of the perovskite semiconductor without open porosity typically consists essentially of crystallites of the perovskite.

[0143] As mentioned in the preceding paragraph, the term “perovskite”, as used herein, refers to (a) a material with a three-dimensional crystal structure related to that of CaTiO.sub.3 or (b) a material comprising a layer of material, wherein the layer has a structure related to that of CaTiO.sub.3. Although both of these categories of perovskite may be used in the devices according to the invention, it is preferable in some circumstances to use a perovskite of the first category, (a), i.e. a perovskite having a three-dimensional (3D) crystal structure. Such perovskites typically comprise a 3D network of perovskite unit cells without any separation between layers. Perovskites of the second category, (b), on the other hand, include perovskites having a two-dimensional (2D) layered structure. Perovskites having a 2D layered structure may comprise layers of perovskite unit cells that are separated by (intercalated) molecules; an example of such a 2D layered perovskite is [2-(I-cyclohexenyl)ethylammonium]2PbBr.sub.4. 2D layered perovskites tend to have high exciton binding energies, which favours the generation of bound electron-hole pairs (excitons), rather than free charge carriers, under photoexcitation. The bound electron-hole pairs may not be sufficiently mobile to reach the p-type or n-type contact where they can then transfer (ionise) and generate free charge. Consequently, in order to generate free charge, the exciton binding energy has to be overcome, which represents an energetic cost to the charge generation process and results in a lower voltage in a photovoltaic cell and a lower efficiency. In contrast, perovskites having a 3D crystal structure tend to have much lower exciton binding energies (on the order of thermal energy) and can therefore generate free carriers directly following photoexcitation. Accordingly, the perovskite semiconductor employed in the devices and processes of the present invention is preferably a perovskite of the first category, (a), i.e. a perovskite which has a three-dimensional crystal structure. This is particularly preferable when the optoelectronic device is a photovoltaic device.

[0144] In order to provide highly efficient photovoltaic devices, the absorption of the absorber/photoactive region should ideally be maximised so as to generate an optimal amount of current. Consequently, when using a perovskite as the absorber in a solar cell, the thickness of the perovskite layer should ideally be in the order of from 300 to 600 nm, in order to absorb most of the sun light across the visible spectrum. In particular, in a solar cell the perovskite layer should generally be thicker than the absorption depth (which is defined as the thickness of film required to absorb 90% of the incident light of a given wavelength, which for the perovskite materials of interest is typically above 100 nm if significant light absorption is required across the whole visible spectrum (400 to 800 nm)), as the use of a photoactive layer in photovoltaic devices with a thickness of less than 100 nm can be detrimental to the performance of the device.

[0145] In contrast, electroluminescent (light-emitting) devices do not need to absorb light and are therefore not constrained by the absorption depth. Moreover, in practice the p-type and n-type contacts of electroluminescent devices are typically chosen such that once an electron or hole is injected on one side of the device it will not flow out of the other (i.e. they are selected so as to only inject or collect a single carrier), irrespective of the thickness of the photoactive layer. In essence, the charge carriers are blocked from transferring out of the photoactive region and will thereby be available to recombine and generate photons, and can therefore make use of a photoactive region that is significantly thinner.

[0146] As used herein, the term “thickness” refers to the average thickness of a component of an optoelectronic device. Typically, therefore, when the optoelectronic device is a photovoltaic device, the thickness of the perovskite layer is greater than 100 nm. The thickness of the perovskite layer in the photovoltaic device may for instance be from 100 nm to 100 μm, or for instance from 100 nm to 700 nm. The thickness of the perovskite layer in the photovoltaic device may for instance be from 200 nm to 100 μm, or for instance from 200 nm to 700 nm.

[0147] The present invention provides a process of forming a photoactive layer of a perovskite type photoactive device. There are currently two methods streams of forming perovskite layers on a substrate.

[0148] One stream is using conventional blend solution of MX.sub.2 and AX in either a stoichiometric ratio (MX.sub.2:AX=1:1) or non-stoichiometric ratios (MX.sub.2:AX=1:n, n>1). AX is typically volatile so that excess amount can be removed by thermal annealing process. Various methods have been developed to fabricate defect free perovskite films

[0149] The second stream is a two-step process, called sequential deposition process. MX.sub.2 is easier to form defect free film compared to the AMX.sub.3. Therefore, the process consists of coating of MX.sub.2 followed by conversion process to AMX.sub.3. In many cases, a precursor solution comprising AX is applied by dipping, slot die coating or other process to the first MX.sub.2 coating for this conversion. Unfortunately, the conversion speed of dense MX.sub.2 films can be very slow. Therefore, additional soaking processes can be used to create sub-micro-size cracks which allows AX to penetrate into MX.sub.2 as for example is taught in Hwang et al, Toward Large Scale Roll-to-Roll Production of Fully Printed Perovskite Solar Cells DOI: 10.1002/adma.201404598 the contents of which are to be understood to be incorporated into this specification by this reference. The gas quenched and soaked PbI.sub.2 show significantly faster reaction speeds and darker appearance after completion of the conversion as compared to conventionally formed MX.sub.2 films which are converted by dipping or slot die processes.

[0150] A further process, the three step process is taught in Zhao Y. et al., “Three-step sequential solution deposition of PbI.sub.2-free CH.sub.3NH.sub.3PbI.sub.3 perovskite”, Journal of Materials Chemistry A, 3, pages 9086-9091, 26 Nov. 2014 the contents of which are to be understood to be incorporated into this specification by this reference. Zhao Y. et al. teaches a three-step sequential solution process for complete conversion of precursor to CH.sub.3NH.sub.3PbI.sub.3 without PbI.sub.2 residue. In this three-step method, a thermally unstable stoichiometric PbI.sub.2.CH.sub.3NH.sub.3Cl precursor film is first deposited on the mesoporous TiO.sub.2 substrate, followed by thermal decomposition to form PbI.sub.2, which is finally converted into CH.sub.3NH.sub.3PbI.sub.3 by dipping in a regular isopropanol solution of CH.sub.3NH.sub.3I at room temperature. In comparison to the two-step approach using similar processing conditions, the three-step method enables the formation of the PbI.sub.2 film through the thermal decomposition of the PbI.sub.2.CH.sub.3NH.sub.3Cl precursor film. This facilitates a rapid conversion of PbI.sub.2 to CH.sub.3NH.sub.3PbI.sub.3 without any traceable residue PbI.sub.2 in the final conversion step, leading to an improved device performance. It should be understood that this process used CH.sub.3NH.sub.3Cl to produce porous PbI.sub.2 for full conversion to CH.sub.3NH.sub.3PbI.sub.3 without PbI.sub.2 residue. This process does not improve processability nor retard perovskite crystallization during perovskite formation. The main focus of this three step process is to produce porous PbI.sub.2 film for accelerated conversion in a physical route (in contrast to the chemical route of the present invention).

[0151] Due to the volatile nature of AX, equivalent or excess amount of AX compared to MX.sub.2 have been widely used when forming a perovskite layer using a perovskite precursor solution (single step) or precursor solutions (sequential deposition process, including the prior two step and three step processes discussed above).

[0152] Furthermore, in Zhao the thermally unstable stoichiometric PbI.sub.2.CH.sub.3NH.sub.3Cl precursor film is first deposited on a high temperature sintered meso-porous TiO.sub.2 structure. This is not a planar perovskite as provided by the present invention. In addition, in Zhao, a porous PbI.sub.2 layer is created by eliminating Methylammonium chloride (MACl—CH.sub.3NH.sub.3Cl) via a heating process. MACl instead of Methylammonium iodide (MAI) because MACl is more volatile and easy to remove. MAI is then added to produce the requisite perovskite. In the present invention, compound AX from the first precursor solution (for example MAI) is included in step 1 to improve processability and is incorporated into the final perovskite layer AMX.sub.3.

[0153] The process of the present invention comprises the sequential coating or deposition sequential deposition of forming a perovskite layer. In the inventive process, a first coating of a first perovskite precursor solution comprising a metal halide (MX.sub.2) component mixed with an AX additive in a first coating solvent is applied (in which the molar ratio of MX.sub.2:AX=1:n with 0<n<1), then a second coating of a second precursor solution comprising an ammonium halide or other organic halide reactant (AX) in a second coating solvent which forms to the selected perovskite precursor is applied. Once the layer of AX is applied to the initial MX.sub.2 coating, MX.sub.2 and AX react to form AMX.sub.3. Importantly, the amount of AX additive in the first precursor solution is less than the stoichiometric required for the reaction with MX.sub.2 to form the perovskite crystals to AMX.sub.3, i.e. a molar ratio of MX.sub.2:AX=1:n with 0<n<1. However, it should be appreciate that the exact molar ratio is dependent on the nature of the components MX.sub.2 and AX and can vary accordingly.

[0154] Thus, despite conventional wisdom of using equivalent or excess amount of AX compared to MX.sub.2 as taught in all previous methods including Zhao Y. et al., the inventors have surprisingly discovered that the addition of a less than stoichiometric amount (i.e. greater than 0 but less than 1) of AX to MX.sub.2 in the first precursor solution applied to the substrate interferes with the crystallization of the perovskite layer in the sequential deposition process. Whilst a number of additives have been previously mixed with MX.sub.2 in an attempt to control crystallisation, the use of AX in the first precursor solution advantageously does not contaminate the perovskite composition, as this additive is a reactant added in the second step.

[0155] No prior process has used a precursor solution having a lower than 1 molar ratio of AX in MX.sub.2. In this regard, no previous research or process has discovered the advantages of the present invention that the addition of a less than stoichiometric amount (i.e. greater than 0 but less than 1) of AX to MX.sub.2 in the first precursor solution applied to the substrate interferes with the crystallization of the perovskite layer in the sequential deposition process.

[0156] Any number of coating application techniques can be used to apply the first precursor solution and the second precursor solution to the substrate. The applied coatings of the first precursor solution and the second precursor solution can be applied by any number of suitable application techniques as discussed previously. In exemplary embodiments and as discussed in the following examples, the first precursor solution is applied using a slot die technique. The second precursor solution is the applied using either a dipping process or another technique, for example slot die coating. In some embodiments, the first precursor solution and the second precursor solution is applied using a slot die technique. An example of one type of slot die coating process and device is shown in FIG. 2. This is described in more detail below. It should be appreciated that other slot die coating devices and process are equally applicable for applying a coating of the first precursor solution and/or second precursor solution to a substrate, and that the present invention should not be limited to the illustrated device.

[0157] As demonstrated in some of the following examples, the process of the present invention can further include the step of drying the applied coating of the first precursor solution and/or the second precursor solution. The drying step can comprise any number of drying process including air dry, convective drying, ambient drying, heat treatment, annealing or the like at a temperature suitable for the perovskite layer to crystallise. In some embodiments, each layer is air dried. In other embodiments, convective or forced drying techniques are used. In some embodiments, heat can be applied to encourage evaporation of the respective first coating solvent or second coating solvent.

[0158] In some embodiments, as will be demonstrated in some of the following examples, a gas-quenching technique is used to rapidly dry the respective coating layer of first or second precursor solution. Gas-quenching comprising the rapid cooling and drying of the applied coating or the first and/or second precursor solution through the application of a drying gas, such as nitrogen, argon or other inert gas. Slot die coating followed by gas quenching has been found to provide a dense and uniform film which can be converted to perovskite film following application of the second precursor solution.

[0159] As mentioned previously, FIG. 2 provides an example of one type of slot die printing device which can be used to coat the substrate with the first precursor solution and/or the second precursor solution in accordance with the process of the present invention. The set up shown in FIG. 2 can also be used for the gas quenching process described above. In this set up, a modified 3D printer based slot-die coater 100 is used for coating purposes, for example as is taught in D. Vak, et al “3D Printer Based Slot-Die Coater as a Lab-to-Fab Translation Tool for Solution-Processed Solar Cells”, Adv. Energy Mater. 2014, DOI: 10.1002/aenm.201401539, the disclosure of which should be understood to be incorporated into this specification by this reference. The 3D printer 100 (FIG. 2(A)) comprises a hot plastic extrusion 3D printer (Felix 2.0) which is adapted to actuate a syringe of precursor solution to feed this solution through a slot die printing head 101 (FIG. 2(B)) including a slot nozzle 105. The slot printing head 101 includes a stepper motor (a generic NEMA 17 stepper motor with a dual shaft—not illustrated) and components to convert rotation to linear translation and an integrated syringe pump (not illustrated) to feed solution directly from syringe to nozzle without any tubes. The stepper motor is controlled to rotate a feeding screw in the syringe pump to dispense fluid through the slot nozzle 105. This type of 3D printer can control the xyz positions with acceleration and speed control. The temperature of the nozzle 100 and the bed can also be controlled. For gas quenching, the printing head 101 comprises a first slot die head 102 and a second gas quenching head 104 connected to high-pressure nitrogen to quickly dry the applied first precursor solution. The order of the heads 102 and 104 ensures that the nitrogen quenching gas is applied to the first precursor solution (or second precursor solution) immediately after application to the substrate when the slot printing head 101 is moving in printing direction D (arrow D in FIG. 2(B)). It should be noted that unlike conventional slot-die coating, curved stripes could be also prepared using the illustrated modified 3D printing set up 100. This demonstrates another advantage of the 3D printing platform—photoactive cells with nonconventional designs can be fabricated.

[0160] In some embodiments, as will be demonstrated in some of the following examples, the coating layer formed by the first precursor solution (MX.sub.2 coating layer) after the first deposition step of the process of the present invention can be converted to a more reactive form by using a solvent vapour soaking technique. This technique can be particularly effective, if used after the first precursor solution is rapidly dried, for example using a gas quenching technique (discussed above). Solvent vapour soaking process comprises storing the wet MX.sub.2 coating layer in an enclosure, preferably a small enclosure as soon as the MX.sub.2 coating is applied to the substrate, or where gas quenching is used, following gas quenching step. This additional solvent vapour soaking technique can create micro-size cracks in the MX.sub.2 coating layer which allow AX to penetrate into MX.sub.2. Whilst not wishing to be limited to any one theory, the Inventors consider that during solvent vapour soaking, a gas-quenched first precursor solution (MX.sub.2) layer formed a dense, dried skin on the film, which trapped solvent inside. Therefore, the trapped solvent resulted in mobile ions that could form small crystals. Gas quenching assists in removing this solvent. Overall, solvent vapour soaking MX.sub.2 coating layers shows a much faster conversion to perovskite after application of the second precursor solution as compared to non-solvent vapour soaking samples.

[0161] The perovskite precursor solutions can include additional additives. For example, in one at least one of the first perovskite precursor solution or the second precursor solution further comprises a polymer additive which is soluble in the respective first coating solvent or second coating solvent. Here the polymer acts as an additional crystallisation retardant in the perovskite crystallisation process. The Applicant expects that the use of an additional polymer crystallisation retardant may assist in the formation of defect free perovskite layers. The use of polymers as a crystallisation retardant in the perovskite crystallisation process is described in detail in the Applicant's pending international patent application No. PCT/AU2015/000100, published as WO2015127494A1 the disclosure of which should be understood to be incorporated into this specification by this reference. As discussed above and in WO2015127494A1 a large number of polymers additives can be used in the present invention. In some embodiments, the polymer is selected from the group consisting of poly vinyl alcohol, poly vinyl acetate (PVAc), ABS, poly amides, poly acrylics, poly imide, poly acrylonitrile, poly butyl methacrylate, poly butadiene, poly carboxy methyl cellulose, poly ethers, poly ethylene acrylates, poly glycols, poly isocyanates, poly methacrylates, poly vinyl butyral, poly vinyl fluoride, poly vinyl methyl ethers, poly amines, polyethylene oxide, polyethylene glycol, Poly(2-ethyl-2-oxazoline) and combinations thereof. In some embodiments, the polymer comprises polyethylene oxide. In other embodiments, the polymer comprises Poly(2-ethyl-2-oxazoline) (PEOXA). In a preferred embodiment, the polymer additive comprises poly vinyl acetate, polyethylene glycol or combinations thereof. In another embodiment, the polymer comprises a poly amine or a hydrochloride salt thereof, such as polyethyleneimine, polyallylamine, a hydrochloride salt thereof or combinations thereof.

[0162] As discussed previously, the perovskite precursor solutions can be used to form a large number of different perovskite layers in accordance with the present invention. In an exemplary embodiments, illustrated in the following examples, the form perovskite layer comprises an organo-lead Iodide perovskite, preferably comprising at least one of CH.sub.3NH.sub.3PbI.sub.3 or HC(NH.sub.2).sub.2PbI.sub.3. It should however be understood that the present invention is not limited to those specific components, but rather can comprise a wide range of components as covered above.

[0163] It should be appreciated that the photoactive layer of the present invention can be incorporated into the layered structure of a variety of optoelectronic and photoactive devices having both conventional and inverted structures discussed above. The photoactive layer of the present invention can also be incorporated into multijunction structures, for example tandem photoactive structures including two stacked layers of photoactive structures as discussed previously. A photoactive device such as a solar cell which includes the perovskite photoactive layer of the present invention could be one or both of the bottom cell or top cell of a tandem device. In some embodiments, the other solar cell could comprise any other type of photoactive cell including organic solar cells and even other perovskite solar cells. For example, a device structure could comprise one of: [0164] Glass/TCO/ETL/perovskite I/HTL/TCO/ETL/perovskite II/HTL/metal electrode; or [0165] PET/organic solar cell (TCO/ZnO/P3HT:PCBM/PEDOT:PSS)/ETL/perovskite/HTL/metal electrode.

EXAMPLES

Example 1—Mx.SUB.2 .Coating/Layer Morphology

Photoactive Layer Fabrication:

[0166] ITO-coated glass (Shenzhen Display, 5Ω sq.sup.−1) was successively sonicated for 5 min each in Deconex 12PA detergent solution, distilled water, acetone and 2-propanol. The substrates were then exposed to UV-ozone (Novascan PDS-UVT) cleaning at room temperature for 15 min. For the electron transporting layer, ZnO nanoparticle solution (20 mg/ml in ethanol) was coated onto ITO glass using a slot die head with 50 μm shim and 200 μm-thick meniscus guide at 3 mm/s coating speed and 1 μL/cm.sup.2 solution feed. The shim with 13 mm channel was used with the same width of meniscus guide, and the gap between the meniscus guide and the substrate was fixed at 100 μm. The ZnO films were then annealed at 120° C. for 10 min in air.

[0167] For the perovskite layer, a PbI.sub.2 (99%, Sigma-Aldrich) precursor solution (0.7 M, 322 mg/mL) in N,N-dimethylformamide was prepared by stirring at 70° C. for 1 hr in air. Various amount of CH.sub.3NH.sub.3I was added based on molar ratio. The solutions were transferred to the slot die head without filtration.

[0168] The first layer (PbI.sub.2 solution) was applied using a slot die coating (without gas quenching), as shown and described in relation to FIG. 2 and taught in Hwang et al, Toward Large Scale Roll-to-Roll Production of Fully Printed Perovskite Solar Cells DOI: 10.1002/adma.201404598 and D. Vak, K. Hwang, A. Faulks, Y.-S. Jung, N. Clark, D.-Y. Kim, G. J. Wilson, S. E. Watkins, Adv. Energy Mater. 2014, DOI: 10.1002/aenm.201401539, again the contents of which are to be understood to be incorporated into this specification by this reference. A range of different PbI.sub.2 solution compositions were applied with and without an AX(CH.sub.3NH.sub.3I) additive, including (A) PbI.sub.2 solution (no AX); (B) PbI.sub.2 with 5% CH.sub.3NH.sub.3I; (C) PbI.sub.2 with 10% CH.sub.3NH.sub.3I; (D) PbI.sub.2 with 25% CH.sub.3NH.sub.3I; (E) PbI.sub.2 with 50% CH.sub.3NH.sub.3I; (F) PbI.sub.2 with 100% CH.sub.3NH.sub.3I (i.e. stoichiometric concentrations). For these experiments a 200 μm gap between a meniscus guide and the substrate was used to maximize the wet film thickness. Coating was carried out at speed of 5 mm/s with 1 μL/cm.sup.2 solution feed. The wet film was then dried naturally at room temperature. Samples were kept in air for more than 10 min or instantly transferred to an enclosed sample carrier and kept in the carrier for 10 min. Optical microscope images were taken of each layer as shown in FIG. 3.

Results

[0169] FIG. 3 provides optical microscope images of the various MX.sub.2 (PbI.sub.2) and MX.sub.2 with various amount of AX films prepared. FIG. 3 clearly shows effect of AX additive in MX.sub.2 solution. Pure PbI.sub.2 film prepared by same process showed micron size pinholes. With small amount of AX(CH.sub.3NH.sub.3I), film is more defect free. No pinhole is found in PbI.sub.2 with 5-10% CH.sub.3NH.sub.3I films even without using a gas quenching process to rapidly dry the PbI.sub.2 layer once applied.

[0170] With non-stoichiometric amount of AX, the films can have more amorphous phase as it is difficult to form either MX.sub.2 or AMX.sub.3 crystal. Amorphous phase is thermodynamically less favourable compared to crystal from. This indications that the a PbI.sub.2 film can be more easily converted to perovskite crystal in dipping process due to more thermodynamic driving force.

Example 2—Perovskite Formation Reaction Speed

Photoactive Layer Fabrication:

[0171] The perovskite photoactive layers were fabricated using a similar methodology as set out in Example 1. However, here the range of different PbI.sub.2 solution compositions were applied with and without an AX(CH.sub.3NH.sub.3I) additive, comprises (A) PbI.sub.2 solution (no CH.sub.3NH.sub.3I); (B) PbI.sub.2 with 30% CH.sub.3NH.sub.3I.

[0172] CH.sub.3NH.sub.3I solution (10 mg/mL) in 2-propanol was prepared for perovskite conversion. The PbI.sub.2 layers were dipped into the CH.sub.3NH.sub.3I solution for 3 min, rinsed with 2-propanol before the film dried, and then the solvent was quickly removed by N.sub.2 gas-blowing. The reaction speed of PbI.sub.2 films without and with 30 mol % CH.sub.3NH.sub.3I were tested by timing the formation of perovskite layer formation by monitoring absorbance change during conversion. A white LED was illuminated to a beaker with CH.sub.3NH.sub.3I solution and light transmission through the beaker was monitored by a photodiode. PbI.sub.2 films with and without an AX(CH.sub.3NH.sub.3I) additive were dipped to the solution while monitoring absorbance as shown in FIG. 4.

Results

[0173] The results of the reaction speed tests are shown in FIG. 4 which show absorbance change during dipping process for conversion of PbI.sub.2 with and without CH.sub.3NH.sub.3I additive to perovskite. The films having the CH.sub.3NH.sub.3I additive were observed to have a very fast initial reaction. The initial increase was considered to be due to reaction on surface. Both the films (PbI.sub.2 and PbI.sub.2 with 30% CH.sub.3NH.sub.3I) showed very rapid increase at start. However, amount of change at the initial change is significantly different. After initial increase, the film with the additive also showed significantly faster absorbance increase and reached almost saturated absorbance.

Example 3—Slot Die Coated Perovskite Solar Cell Devices

Photoactive Layer Fabrication:

[0174] The perovskite photoactive layers were fabricated using a similar methodology as set out in Example 1. However, in this run the PbI.sub.2 layers were fabricated by slot die coating with gas quenching as described in relation to FIG. 2 above and taught in Hwang et al, Toward Large Scale Roll-to-Roll Production of Fully Printed Perovskite Solar Cells DOI: 10.1002/adma.201404598 again, the contents of which are to be understood to be incorporated into this specification by this reference. Furthermore, in this experimental run, the PbI.sub.2 layer were formed using four different techniques: [0175] A). Slot die coated PbI.sub.2 layer without CH.sub.3NH.sub.3I, air dried for 10 minutes; [0176] B). Slot die coated PbI.sub.2 layer without CH.sub.3NH.sub.3I, stored in a small chamber for solvent vapour soaking for 10 minutes; [0177] C). Slot die coated PbI.sub.2 layer with CH.sub.3NH.sub.3I, air dried for 10 minutes; and [0178] D). Slot die coated PbI.sub.2 layer with CH.sub.3NH.sub.3I, stored in a small chamber for solvent vapour soaking for 10 minutes.

[0179] For B) and D) a solvent vapour soaking technique was used in the film was stored in a small enclosed sample carrier as soon as PbI.sub.2 layer was coated. In presence of solvent vapour, ions in Pb.sup.2+ and I.sup.− ions become mobile and can form thermodynamically stable crystal. The process also create sub-micro-size crack which allow AX(CH.sub.3NH.sub.3I) to penetrate into PbI2 layer more rapidly and deeply. So full conversion can be done.

Device Fabrication

[0180] The perovskite layer formed on the coated substrates from Example 1 were coated with a hole transporting layer immediately after perovskite formation to minimize exposure to moisture. For the hole transporting layer, 1 mL of P3HT (Merck) solution (15 mg/mL in chlorobenzene), 6.8 μL of Li-bis(trifluoromethanesulfonyl) imide (28.3 mg/mL in acetonitrile) and 3.4 μL of 4-tert-butylpyridine were mixed, and then transferred to the slot die head without filtration. The solution was coated onto the perovskite film at 7 mm/s speed with 3 μL/cm.sup.2 solution feed without thermal treatment, and the gap between the meniscus guide and the substrate was also fixed at 100 μm. It is noteworthy that all slot die coating processes were carried out in air. Temperature and relative humidity were typically 25 to 30° C. and 30 to 40%, respectively. For an evaporated electrode, the samples were carried to a vacuum evaporator and 100 nm of Ag was deposited through a shadow mask to produce a 10 mm.sup.2 active area.

Results

[0181] The device results are shown in FIG. 5 which provide output curves of perovskite solar cells with a device configuration of ITO/ZnO/perovskite (CH.sub.3NH.sub.3PbI.sub.3)/P3HT/Ag fabricated by sequential deposition process. As shown in this Figure, there was significant difference between perovskite layers fabricated from PbI.sub.2 with and without soaking process. The vapour soaked films showed a much faster conversion to perovskite when it was dipped in MAI solution. In contrast, devices made from PbI.sub.2 with the additive showed similar performance with about 9% of power conversion efficiency.

Example 4—Slot Die Coated Perovskite Solar Cells with Different Amount of CH.SUB.3.NH.SUB.3.I Additive

[0182] Photoactive Layer Fabrication:

[0183] The perovskite photoactive layers were fabricated using a similar methodology as set out in Example 1. However, in these experiments the range of different amount of CH.sub.3NH.sub.3I was added to 0.7 M PbI.sub.2 solution comprising (a) no additive; (b) 10 mol %; (c) 20 mol %; (d) 30 mol %; (e) 40 mol %. For these experiments a 200 μm gap between a meniscus guide and the substrate was used to maximize the wet film thickness. Coating was carried out at speed of 5 mm/s with 1 μL/cm.sup.2 solution feed with gas quenching as described in relation to FIG. 2 above and as taught in Hwang et al (see Example 1). Films were kept in open environment, i.e. no soaking process to produce mesoporous PbI.sub.2 film. CH.sub.3NH.sub.3I solution (10 mg/mL) in 2-propanol was prepared for perovskite conversion. The PbI.sub.2 layers were dipped into the CH.sub.3NH.sub.3I solution for 3 min, rinsed with 2-propanol before the film dried, and then the solvent was quickly removed by N.sub.2 gas-blowing.

Device Fabrication

[0184] The perovskite layer formed on the coated substrates was coated with a hole transporting layer immediately after perovskite formation to minimize exposure to moisture. For the hole transporting layer, 1 mL of P3HT (Merck) solution (15 mg/mL in chlorobenzene), 6.8 μL of Li-bis(trifluoromethanesulfonyl) imide (28.3 mg/mL in acetonitrile) and 3.4 μL of 4-tert-butylpyridine were mixed, and then transferred to the slot die head without filtration. The solution was coated onto the perovskite film at 7 mm/s speed with 3 μL/cm.sup.2 solution feed without thermal treatment, and the gap between the meniscus guide and the substrate was also fixed at 100 μm. It is noteworthy that all slot die coating processes were carried out in air. Temperature and relative humidity were typically 25 to 30° C. and 30 to 40%, respectively. For an evaporated electrode, the samples were carried to a vacuum evaporator and 100 nm of Ag was deposited through a shadow mask to produce a 10 mm.sup.2 active area.

Results

[0185] The device results are shown in FIG. 6 which provide average power conversion efficiency perovskite solar cells with a device configuration of ITO/ZnO/perovskite (CH.sub.3NH.sub.3PbI.sub.3)/P3HT/Ag fabricated by sequential deposition process. As shown in FIG. 6, the effect of CH.sub.3NH.sub.3I was significant. Devices made from pure PbI.sub.2 with soaking process showed less than 1%. In contrast, devices with optimized amount of the additive showed power conversion efficiency over 10% which is similar to that of device via soaking process used in Hwang et al.

Example 5—Slot Die Coated Perovskite Solar Cells with Extra Additive

Photoactive Layer Fabrication:

[0186] The perovskite photoactive layers were fabricated using a similar methodology as set out in Example 1. However, here extra polymer additive was added to PbI.sub.2 with 40 mol % CH.sub.3NH.sub.3I to improve processability further as described above and also in the Applicants pending international patent application No. PCT/AU2015/000100, published as WO2015127494A1, the contents of which are incorporated into this specification by this reference.

[0187] 5 mg of polyvinlyacetate was added to 1 ml of 0.7 M PbI.sub.2 with 40 mol % CH.sub.3NH.sub.3I as a crystallization retardant. For these experiments a 200 μm gap between a meniscus guide and the substrate was used to maximize the wet film thickness. Coating was carried out at speed of 5 mm/s with 1 μL/cm.sup.2 solution feed with gas quenching as described in relation to FIG. 2 above and as taught in Hwang et al (see Example 1). Films were kept in open environment, i.e. no soaking process to produce mesoporous PbI.sub.2 film. CH.sub.3NH.sub.3I solution (10 mg/mL) in 2-propanol was prepared for perovskite conversion. The PbI.sub.2 layers were dipped into the CH.sub.3NH.sub.3I solution for 1 min, rinsed with 2-propanol before the film dried, and then the solvent was quickly removed by N.sub.2 gas-blowing.

Device Fabrication

[0188] The perovskite layer formed on the coated substrates was coated with a hole transporting layer immediately after perovskite formation to minimize exposure to moisture. For the hole transporting layer, 1 mL of P3HT (Merck) solution (15 mg/mL in chlorobenzene), 6.8 μL of Li-bis(trifluoromethanesulfonyl) imide (28.3 mg/mL in acetonitrile) and 3.4 μL of 4-tert-butylpyridine were mixed, and then transferred to the slot die head without filtration. The solution was coated onto the perovskite film at 7 mm/s speed with 3 μL/cm.sup.2 solution feed without thermal treatment, and the gap between the meniscus guide and the substrate was also fixed at 100 μm. It is noteworthy that all slot die coating processes were carried out in air. Temperature and relative humidity were typically 25 to 30° C. and 30 to 40%, respectively. For an evaporated electrode, the samples were carried to a vacuum evaporator and 100 nm of Ag was deposited through a shadow mask to produce a 10 mm.sup.2 active area.

Results

[0189] The device results are shown in FIG. 7 which provide output curves of perovskite solar cells with a device configuration of ITO/ZnO/perovskite (CH.sub.3NH.sub.3PbI.sub.3) with and without polyvinylacetate/P3HT/Ag fabricated by sequential deposition process. As shown in FIG. 7, the effect of polymer was significant. Devices made from PbI.sub.2 with 40 mol % CH.sub.3NH.sub.3I showed no photovoltaic behaviour as shown in FIG. 6. Additional process improving additive further retards crystallization. Without soaking process over 10% power conversion efficiency was achieved.

[0190] The perovskite photoactive layers were fabricated using a similar methodology as set out in Example 5. However, here the range of different PbI.sub.2 solution compositions were applied with additives comprising (A) PbI.sub.2 solution (no CH.sub.3NH.sub.3I); (B) PbI.sub.2 with 30% CH.sub.3NH.sub.3I; (c) PbI.sub.2 with 30% CH.sub.3NH.sub.3I and 5 mg/ml of polyvinylacetate; (D) PbI.sub.2 with 40% CH.sub.3NH.sub.3I and 5 mg/ml of polyvinylacetate. Polymer additive made it possible to process higher content of CH.sub.3NH.sub.3I. Therefore, more rapid conversion could be achieved.

Example 6—Slot Die Coated Perovskite Solar Cells Via Roll-to-Roll Printer

Photoactive Layer Fabrication:

[0191] ITO-coated PET substrate (0050, Solutia) was used as a substrate to produce flexible perovskite solar cells using Mini-Labo coater (Yasui Seiki). ZnO nanoparticle solution (20 mg/ml in ethanol) was coated onto ITO glass using a slot die head with 50 μm shim and 200 μm-thick meniscus guide at 0.2 m/min coating speed and 10 μL/min solution feed. The film was dried by hot air at 120° C. for 20 sec and on a curved hot plate at 140° C. for 20 sec in the roll-to-roll coater.

[0192] The perovskite photoactive layers were fabricated using a similar methodology as set out in Example 1. However, here different substrate and printing machine is used. For these experiments a 200 μm gap between a meniscus guide and the substrate was used to maximize the wet film thickness. Coating was carried out at speed of 0.2 m/min with 20 μL/min solution feed with gas quenching as described in relation to FIG. 2 above and as taught in Hwang et al (see Example 1). CH.sub.3NH.sub.3I solution (40 mg/mL) in 2-propanol was deposited on the film at 70° C. with coating speed of 0.2 m/min with 40 μL/min solution feed. The conversion process is shown in FIGS. 9a and 9b which show the a photograph of the roll-to-roll conversion process of CH.sub.3NH.sub.3I added intermediate 200 (FIG. 9a) and a photograph of the intermediate 200 and formed perovskite 210 during roll-to-roll process (FIG. 9b).

Device Fabrication

[0193] The perovskite layer formed on the coated substrates was coated with a hole transporting layer. For the hole transporting layer, 1 mL of P3HT (Merck) solution (15 mg/mL in chlorobenzene), 6.8 μL of Li-bis(trifluoromethanesulfonyl) imide (28.3 mg/mL in acetonitrile) and 3.4 μL of 4-tert-butylpyridine were mixed, and then transferred to the slot die head without filtration. The solution was coated onto the perovskite film at 0.3 m/min speed with 40 μL/min solution feed without thermal treatment, and the gap between the meniscus guide and the substrate was also fixed at 100 μm. It is noteworthy that all slot die coating processes were carried out in air. Temperature and relative humidity were typically 25 to 30° C. and 30 to 40%, respectively. For an evaporated electrode, the samples were carried to a vacuum evaporator and 10 nm of MoOx and 100 nm of Ag were deposited through a shadow mask to produce a 10 mm.sup.2 active area.

Results

[0194] The device results are shown in FIG. 10 which provide average power conversion efficiency perovskite solar cells with a device configuration of ITO/ZnO/perovskite (CH.sub.3NH.sub.3PbI.sub.3)/P3HT solution/MoOx/Ag fabricated by sequential deposition process. The device made on plastic substrate using industrial process showed 8.6% power conversion efficiency.

[0195] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

[0196] Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.