Method of forming a crystalline or polycrystalline layer of an organic-inorganic metal halide perovskite

Abstract

The present invention provides a method of forming a crystalline or polycrystalline layer of an organic-inorganic metal halide perovskite material comprising a three-dimensional crystal structure represented by the formula AMX.sub.3, in which A represents an organic cation or a mixture of two or more different cations, at least one of which is an organic cation, M represents a divalent metal cation or a mixture of two or more different divalent metal cations, and X represents halide anions which are the same or different, the method comprising the steps of: (i) forming a first layer on the surface of a substrate, the first layer comprising an organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure (ii) reacting the first layer with one or more organic halides to form the crystalline or polycrystalline layer comprising an organic-inorganic metal halide perovskite material having the formula AMX.sub.3. Also provided is an optoelectronic or photovoltaic device including an active layer comprising an organic-inorganic metal halide perovskite material comprising a three-dimensional crystal structure represented by the formula AMX.sub.3, wherein the material is obtainable using the above defined method.

Claims

1. A method of forming a crystalline or polycrystalline layer of an organic-inorganic metal halide perovskite material comprising a three-dimensional crystal structure represented by the formula AMX.sub.3, in which A comprises a monovalent organic cation, M comprises at least one divalent inorganic cation selected from Pb.sup.2+ and Sn.sup.2+, and X comprises one or more halide anions independently selected from fluoride, chloride, bromide, and iodide, the method comprising the steps of: (i) forming a first layer on the surface of a substrate, the first layer comprising an organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure, and (ii) reacting the first layer with one or more organic halides to form the crystalline or polycrystalline layer comprising an organic-inorganic metal halide perovskite material having the formula AMX.sub.3.

2. The method of claim 1, wherein step (ii) comprises: a) depositing on the first layer a material comprising one or more organic halides; and b) reacting the first layer and the material deposited thereon to form the crystalline or polycrystalline layer comprising an organic-inorganic metal halide perovskite material having the formula AMX.sub.3.

3. The method according to claim 1, wherein in step i) the first layer comprises a material having a formula selected from L.sub.2L′.sub.n−1M.sub.nX3.sub.n+1 or L.sub.2K.sub.n−1M.sub.nX.sub.3n+3, LL′.sub.n−1M.sub.nX3.sub.n+1 or L.sub.2L′.sub.nM.sub.nX.sub.3n+2, in which L and L′ represent organic or inorganic cations which can be the same or different, K represents an organic cation and is different from L, M represents a divalent metal cation or two or more different divalent metal cations selected from Pb.sup.2+ and Sn.sup.2+, and X represents halide anions independently selected from fluoride, chloride, bromide, and iodide.

4. The method as claimed in claim 3, wherein L, L′ and K are independently selected from the group consisting of: napthylethylammonium, n-propylammonium, benzylammonium, phenylammonium, n-pentylammonium, 2-thiopheneethylammonium, t-butylammonium, diethylammonium, iso-butylammonium, cyclohexylammonium, cyclohexylmethylammonium, and ethylammonium.

5. The method as claimed in claim 1, wherein the first layer is formed by physical or chemical vapour deposition of the organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure.

6. The method as claimed in claim 1, wherein the first layer is formed by deposition of the organic-inorganic metal halide perovskite material from a solution.

7. The method as claimed in claim 1, wherein the first layer is formed by depositing a precursor material on the surface of the substrate comprising one or more metal halide materials, followed by converting the metal halide precursor material into the first layer comprising said organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure.

8. The method as claimed in claim 7, wherein the metal halide precursor material is converted into the first layer by depositing one or more organic halide materials onto the metal halide precursor material and reacting the organic halide and metal halide materials to form the first layer comprising said organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure.

9. The method as claimed in claim 8, wherein the one or more organic halide materials are selected from the group consisting of: napthylethylammonium iodide, n-propylammonium iodide, benzylammonium iodide, phenylammonium iodide, n-pentylammonium iodide, 2-thiopheneethylammonium iodide, t-butylammonium iodide, diethylammonium iodide, iso-butylammonium iodide, cyclohexylammonium iodide, cyclohexylmethylammonium iodide, and ethylammonium iodide.

10. The method as claimed in claim 1, wherein the first layer is formed by depositing a precursor material on the surface of the substrate comprising one or more organic halide materials, followed by depositing one or more metal halide materials, and subsequently converting the organic halide and metal halide materials into the first layer comprising said organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure.

11. The method as claimed in claim 7, wherein the precursor materials are deposited from a solution.

12. The method as claimed in claim 8, wherein the one or more organic halide materials are deposited from a solution of one or more organic ammonium halides dissolved in a solvent onto the one or more metal halide materials followed by removal of the solvent.

13. The method as claimed in claim 12, wherein the solvent comprises one or more solvents selected from the group consisting of 1-butanol, 1-propanol, isopropyl alcohol, 1-methoxy-2-propanol, 2-methoxy-methanol, 2-ethoxy-ethanol, ethylene glycol mono-tert-butyl ether, and toluene.

14. The method as claimed in claim 12, wherein the organic ammonium halide is selected from the group consisting of: napthylethylammonium iodide, n-propylammonium iodide, benzylammonium iodide, phenylammonium iodide, n-pentylammonium iodide, 2-thiopheneethylammonium iodide, t-butylammonium iodide, diethylammonium iodide, iso-butylammonium iodide, cyclohexylammonium iodide, cyclohexylmethylammonium iodide, and ethylammonium iodide.

15. The method as claimed in claim 2, wherein: the material in step a) is deposited by exposing the first layer to a vapour comprising the halides AY and AZ, wherein Y and Z are halide anions which are the same or different, and the cations A are the same or different and each comprise an organic monocation selected from the group consisting of: Formamidinium, Guanidinium, R.sub.3NH.sup.+ where R are selected from the group consisting of H, unsubstituted or substituted aryl groups, and unsubstituted or substituted alkyl groups, and H.sup.+R.sub.2NR′NR.sub.2H.sup.+ where R is selected from the group consisting of H, unsubstituted or substituted aryl groups, and unsubstituted or substituted alkyl groups, and R′ is selected from the group consisting of unsubstituted or substituted aryl groups, and unsubstituted or substituted alkyl groups, and optionally wherein the cations A further comprise a monovalent inorganic cation selected from Cs and Rb.

16. The method as claimed in claim 2, wherein: the material in step a) is deposited from a solution of the halides AY and AZ, wherein Y and Z are halide anions which are the same or different, and the cations A are the same or different and each comprise an organic monocation selected from the group consisting of: Formamidinium, Guanidinium, R.sub.3NH.sup.+ where R are selected from the group consisting of H, unsubstituted or substituted aryl groups, and unsubstituted or substituted alkyl groups, and H.sup.+R.sub.2NR′NR.sub.2H.sup.+ where R is selected from the group consisting of H, unsubstituted or substituted aryl groups, and unsubstituted or substituted alkyl groups, and R′ is selected from the group consisting of unsubstituted or substituted aryl groups, and unsubstituted or substituted alkyl groups, and optionally wherein the cations A further comprise a monovalent inorganic cation selected from Cs and Rb.

17. The method as claimed in claim 1, wherein the reaction in step ii) is achieved by heating the materials above room temperature.

18. The method as claimed in claim 17, wherein the heating is performed at a temperature of less than 200 degrees centigrade.

19. A method of making an optoelectronic or photovoltaic device including an active layer comprising an organic-inorganic metal halide perovskite material comprising a three-dimensional crystal structure represented by the formula AMX.sub.3, in which A comprises a monovalent organic cation, M comprises at least one divalent inorganic cation selected from Pb.sup.2+ and Sn.sup.2+, and X comprises one or more halide anions independently selected from fluoride, chloride, bromide, and iodide, wherein the method comprises forming the material comprising the active layer by the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows conversion of a 2D perovskite structure to a 3D perovskite structure having MX.sub.6 corner octahedra;

(3) FIG. 2A illustrates a schematic representation of the process flow of the present invention;

(4) FIG. 2B illustrates a schematic representation of an alternative process flow of the present invention;

(5) FIG. 3a-d illustrates n=2 Ruddlesden Popper, Aurivillius, Dion-Jacobson, and {110} parallel type 2D perovskites;

(6) FIG. 4a-c illustrates the n=1, n=2 and n=3 members of the Ruddelsden-Popper 2D perovskite family and {110} parallel type 2D perovskites;

(7) FIG. 5 shows XRD patterns of the same 2D perovskite material prepared by different routes; (a) conversion by spin coating of a solution containing the A′ cation on a metal halide precursor layer, (b) conversion by chemical bath in a solution containing the A′ cation of a metal halide precursor layer and (c) 1-step formation of the 2D material by spin coating of a solution containing the metal halide and the A′ cation;

(8) FIG. 6a shows overlapping XRD patterns for a 3D perovskite material i) made using the process of the present invention, and ii) made using a two-step prior art process;

(9) FIG. 6b shows the full width at half maximum (FWHM) value and number of counts for the 3D perovskite material made by the process of the present invention;

(10) FIG. 6c shows the full width at half maximum (FWHM) value and number of counts for the 3D perovskite material made by a two-step prior art process;

(11) FIG. 7a shows the XRD patterns for 3 3D-perovskite films made using the process according to the present invention, with 3 different ratios of halide anion Y:Z;

(12) FIG. 7b shows the UV-Vis spectra for the same 3 films;

(13) FIG. 8a shows two Atomic Force Microscope (AFM) images of a 3D-perovskite material made using the process according to the present invention;

(14) FIG. 8b shows two Atomic Force Microscope (AFM) images of a 3D-perovskite material made using a two-step prior art process;

(15) FIG. 9a shows the top plan view (upper plot) and the cross-sectional view (lower plot) produced from Scanning Electron Microscopy (SEM) images of a 3D perovskite material produced using the process according to the present invention;

(16) FIG. 9b shows the top plan view (upper plot) and the cross-sectional view (lower plot) produced from Scanning Electron Microscopy (SEM) images of a 3D perovskite material produced using a 2-step prior art process;

(17) FIG. 10 shows overlapping Photoluminescence measurements for a 3D perovskite i) made using the process of the present invention (grey), and ii) made using a two-step prior art process (black);

(18) FIG. 11 shows overlapping Time-Resolved Photoluminescence (TRPL) measurements showing the charge carrier lifetime for a 3D perovskite material made using the process of the present invention compared against a 3D perovskite material made using a 2-step prior art process;

(19) FIG. 12A shows overlapping X-ray diffraction patterns for 3D perovskite materials obtained using the process of the present invention involving direct chemical deposition of 2D materials with A′=an aryl ammonium ion (grey) or A′=an alkyl ammonium ion (black);

(20) FIG. 12B shows an XRD of a perovskite, made following the method of the present invention in which there is not a second halide deposition step;

(21) FIG. 13 illustrates a plot of the current-voltage sweep for single-junction devices prepared by the 2D route and 2-step process, where we see a Voc gain via the 2D route, as would be expected from higher PLQE;

(22) FIG. 14 illustrates schematic representation of the overall process flow for a method using the process of the present invention, and the overall process flow for a 2-step prior art process.

DETAILED DESCRIPTION OF THE INVENTION

(23) Definitions

(24) The term “photoactive”, as used herein, refers to a region, layer or material that is capable of responding to light photoelectrically. A photoactive region, layer or material is therefore capable of absorbing the energy carried by photons in light that then results in the generation of electricity (e.g. by generating either electron-hole pairs or excitons).

(25) The term “perovskite”, as used herein, refers to a material with a three-dimensional crystal structure related to that of CaTiO.sub.3 or a two-dimensional material comprising a layer of material, which layer has a structure related to that of CaTiO.sub.3. The structure of CaTiO.sub.3 can be represented by the formula AMX.sub.3, wherein A and M are cations of different sizes and X is an anion. In the unit cell, the A cations are at (0,0,0), the M cations are at (1/2, 1/2, 1/2) and the X anions are at (1/2, 1/2, 0). The A cation is usually larger than the M cation. The skilled person will appreciate that when A, M and X are varied, the different ion sizes may cause the structure of the perovskite material to distort away from the structure adopted by CaTiO.sub.3 to a lower-symmetry distorted structure. The symmetry will also be lower if the material is in the form of a layer that has a structure related to that of bulk CaTiO.sub.3. Materials comprising a layer of perovskite material are well known. For instance, the K.sub.2NiF.sub.4 perovskites adopt the n=1 RP structure (as defined in the introduction). 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 comprises more than one A cation, the different A cations may be 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 be distributed over the M sites in an ordered or disordered way. When the perovskite comprises more than one X anion, the different X anions may be distributed over the X sites in an ordered or disordered way.

(26) 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, 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. Both types of perovskites have been described in detail in the introduction (FIGS. 1, 3 and 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 invention is preferably a perovskite of the first category, (a), i.e. a perovskite which has a three-dimensional crystal structure, particularly when the optoelectronic device is a photovoltaic device such as a solar cell.

(27) The 3D perovskite material produced using the method of this invention has general formula (I):
[A][M][X].sub.3  (I)

(28) wherein [A] is an organic cation or a mixture of two or more different cations, at least one of which is an organic cation; [M] is a divalent metal cation or a mixture of two or one or more different divalent metal cations; and [X] is one or more halide anions.

(29) [X] preferably comprises one or more halide anions selected from fluoride, chloride, bromide, and iodide, and preferably selected from chloride, bromide and iodide. More preferably [X] comprises one or more halide anions selected from bromide and iodide. In some examples, [X] preferably comprises two different halide anions selected from fluoride, chloride, bromide, and iodide, and preferably selected from chloride, bromide and iodide, and more preferably comprises bromide and iodide.

(30) Preferably, [A] is selected from one or more monovalent cations. [A] preferably comprises one or more organic cations selected from methylammonium (CH.sub.3NH.sub.3.sup.+), formamidinium (HC(NH).sub.2).sub.2.sup.+), and ethyl ammonium (CH.sub.3CH.sub.2NH.sub.3.sup.+), and preferably comprises one organic cation selected from methylammonium (CH.sub.3NH.sub.3.sup.+) and formamidinium (HC(NH.sub.2).sub.2.sup.+). [A] may comprise one or more inorganic cations selected from Cs.sup.+, Rb.sup.+, Cu.sup.+, Pd.sup.+, Pt.sup.+, Ag.sup.+, Au.sup.+, Rh.sup.+, and Ru.sup.+.

(31) [M] preferably comprises at least one divalent inorganic cation selected from Pb.sup.2+ and Sn.sup.2+, and preferably comprises Pb.sup.2+.

(32) In preferred examples, the perovskite material has the general formula:
A.sub.xA′.sub.1−xM(X.sub.yX′.sub.1−y).sub.3  (IA)

(33) wherein A is formamidinium (FA), A′ is a caesium cation (Cs.sup.+), M is Pb.sup.2+, X is iodide and X′ is bromide, and wherein 0<x≤1 and 0<y≤1. In these preferred embodiments, the perovskite material can therefore comprise a mixture of two monovalent cations. In addition, in the preferred embodiments, the perovskite material can therefore comprise either a single iodide anion or a mixture of iodide and bromide anions. The present inventors have found such perovskite materials can have band gaps in from 1.50 eV to 1.75 eV and that layers of such perovskite materials can be readily formed with suitable crystalline morphologies and phases. More preferably, the perovskite material is FA.sub.1−xCs.sub.xPbl.sub.3−yBr.sub.y.

(34) The perovskite material employed in the present invention is one which is capable of absorbing light and thereby generating free charge carriers. Thus, the perovskite employed is a light-absorbing perovskite material. However, the skilled person will appreciate that the perovskite material could also be a perovskite material that is capable of emitting light, by accepting charge, both electrons and holes, which subsequently recombine and emit light. Thus, the perovskite employed may be a light-emitting perovskite for use in light emitting devices.

(35) The perovskite material produced in this invention is crystalline or polycrystalline, e.g. comprised of more than one crystal.

(36) As the skilled person will appreciate, the perovskite material employed in the present invention may be a perovskite which acts as an n-type, electron-transporting semiconductor when photo-doped. Alternatively, it may be a perovskite which acts as a p-type hole-transporting semiconductor when photo-doped. Thus, the perovskite may be n-type or p-type, or it may be a nominally intrinsic semiconductor. In preferred embodiments, the perovskite employed is one which acts as an n-type, electron-transporting semiconductor when photo-doped. The perovskite material may exhibit ambipolar charge transport, and therefore act as both n-type and p-type semiconductor. In particular, the perovskite may act as both n-type and p-type semiconductor depending upon the type of junction formed between the perovskite and an adjacent material.

(37) Typically, the perovskite semiconductor used in the present invention is a photosensitizing material, i.e. a material which is capable of performing both photogeneration and charge transportation.

(38) The term “mixed-anion”, as used herein, refers to a compound comprising at least two different anions. The term “halide” refers to an anion of an element selected from Group 17 of the Periodic Table of the Elements, i.e., of a halogen. Typically, halide anion refers to a fluoride anion, a chloride anion, a bromide anion, an iodide anion or an astatide anion. Perovskite compounds defined herein may have more than one halide anion. Suitable combinations of halide anions include Br/I.

(39) The term “metal halide perovskite”, as used herein, refers to a perovskite, the formula of which contains at least one metal cation and at least one halide anion. The term “organometal halide perovskite”, as used herein, refers to a metal halide perovskite, the formula of which contains at least one organic cation.

(40) The term “organic cation” refers to a cation comprising carbon. The cation may comprise further elements, for example, the cation may comprise hydrogen, nitrogen or oxygen. The term “inorganic cation” refers to a cation that is not an organic cation. By default, the term “inorganic cation” refers to a cation that does not contain carbon.

(41) The term “n-type”, as used herein, refers to a region, layer or material that comprises an extrinsic semiconductor with a larger concentration of electrons than holes. In n-type semiconductors, electrons are therefore majority carriers and holes are the minority carriers, and they are therefore electron transporting materials. The term “n-type region”, as used herein, therefore refers to a region of one or more electron transporting (i.e. n-type) materials. Similarly, the term “n-type layer” refers to a layer of an electron-transporting (i.e. an n-type) material. An electron-transporting (i.e. an n-type) material could be a single electron-transporting compound or elemental material, or a mixture of two or more electron-transporting compounds or elemental materials. An electron-transporting compound or elemental material may be undoped or doped with one or more dopant elements or compounds.

(42) The term “p-type”, as used herein, refers to a region, layer or material that comprises an extrinsic semiconductor with a larger concentration of holes than electrons. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers, and they are therefore hole transporting materials. The term “p-type region”, as used herein, therefore refers to a region of one or more hole transporting (i.e. p-type) materials. Similarly, the term “p-type layer” refers to a layer of a hole-transporting (i.e. a p-type) material. A hole-transporting (i.e. a p-type) material could be a single hole-transporting compound or elemental material, or a mixture of two or more hole-transporting compounds or elemental materials. A hole-transporting compound or elemental material may be undoped or doped with one or more dopant elements or compounds.

(43) The term “layer”, as used herein, refers to any structure which is substantially laminar in form (for instance extending substantially in two perpendicular directions, but limited in its extension in the third perpendicular direction). A layer may have a thickness which varies over the extent of the layer. Typically, a layer has approximately constant thickness. The “thickness” of a layer, as used herein, refers to the average thickness of a layer. The thickness of layers may easily be measured, for instance by using microscopy, such as electron microscopy of a cross section of a film, or by surface profilometry for instance using a stylus profilometer.

(44) Reaction Process

(45) The present invention relates to a process for making a 3D organic-inorganic hybrid halide perovskite material which process involves the conversion of a 2D organic-inorganic hybrid halide perovskite material to the desired 3D perovskite material. Preferably, the 3D perovskite is a highly crystalline material. Further preferably, the resulting 3D perovskite material is suitable for use as a high-quality photovoltaic absorber material.

(46) FIG. 2 shows a schematic representation of several possible process flows of the method according to the present invention, and FIG. 14 illustrates one of these process flows for the overall reaction process of the present invention and compares it to the prior art route. The process of the present invention involves forming a layer of a 2D perovskite material onto a substrate and subjecting this 2D perovskite material layer to a reaction step to convert the 2D perovskite material to a 3D perovskite analogue.

(47) In its broadest aspect the present invention provides a method of forming a crystalline or polycrystalline layer of an organic-inorganic metal halide perovskite material comprising a three-dimensional crystal structure represented by the formula AMX.sub.3,

(48) in which A represents an organic cation or a mixture of two or more different cations, at least one of which is an organic cation,

(49) M represents a divalent metal cation or a mixture of two or more different divalent metal cations, and X represents halide anions which are the same or different,

(50) the method comprising the steps of: (i) forming a first layer on the surface of a substrate, the first layer comprising an organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure (ii) reacting the first layer with one or more organic halides to form the crystalline or polycrystalline layer comprising an organic-inorganic metal halide perovskite material having the formula AMX.sub.3.

(51) There are various ways of forming the first layer comprising an organic-inorganic metal halide perovskite material having a planar, layered two-dimensional crystal structure. In one aspect, the present invention provides a method of forming a crystalline or polycrystalline layer of an organic-inorganic metal halide perovskite material comprising a three-dimensional crystal structure represented by the formula AMX.sub.3, in which A represents one or more cations which are the same or different, at least one comprising an organic cation, M represents one or a mixture of more than one divalent metal, and X are halide anions which are the same or different, the method comprising the steps of: forming a first layer on the surface of a substrate, the first layer comprising an organic-inorganic halide perovskite material having a planar, layered two-dimensional crystal structure; depositing on the first layer a material comprising one or more organic halides; and reacting the first layer and the mixture deposited thereon to form the polycrystalline layer comprising an organic-inorganic metal halide perovskite material having the formula AMX.sub.3 as defined previously.

(52) In other words, in the method of claim 1, step (ii) comprises

(53) a) depositing on the first layer a material comprising one or more organic halides; and

(54) b) reacting the first layer and the material deposited thereon to form the crystalline or polycrystalline layer comprising an organic-inorganic metal halide perovskite material having the formula AMX.sub.3.

(55) In other embodiments, there is not a separate step of depositing on the first layer a material comprising one or more organic halides. The organic halides may already be present while step (i) is being carried out and therefore the organic halides need not be added separately. This is illustrated in FIG. 2B.

(56) In step i), the two-dimensional crystal structure typically has general formula L.sub.2MX.sub.4, in which L is a ligand which does not appear in the final perovskite product. Preferably L is an organic or inorganic cation or a mixture of organic or inorganic cations which may be the same or different. M is a divalent metal cation or a mixture of two or more different divalent metal cations. X is one or more different halide anions.

(57) Preferably, the 2D material has the formula L.sub.2L′.sub.n−1M.sub.nX.sub.3n+1 or L.sub.2K.sub.n−1M.sub.nX.sub.3n+3 or LL′.sub.n−1M.sub.nX.sub.3n+1 or L.sub.2L′.sub.nM.sub.nX.sub.3n+2, in which L and L′ represent organic or inorganic cations which can be the same or different, K represents an organic cation different from L, M represents one or more divalent metals, and X represents one or more different halide anions.

(58) Ideally, the 3D organic-inorganic hybrid halide perovskite is essentially free of 2D perovskite material. By this, we mean that the 3D organic-inorganic hybrid halide perovskite product layer typically comprises less than 5 wt %, preferably less than 2 wt %, even more preferably less than 1 wt % 2d perovskite material.

(59) The step (i) of forming a first layer on a substrate of a two-dimensional organic-inorganic hybrid halide perovskite as described above may be achieved using any convenient method. Suitable methods include, but are not limited to: i) physical or chemical vapour deposition, ii) deposition from solution (this may optionally include spin coating or dipping techniques), and iii) deposition of a precursor compound, for example one or more metal halides, by physical vapour deposition or from solution, followed by conversion of this precursor compound to the desired L.sub.2MX.sub.4 material.

(60) This first step in the reaction process may be represented by general reaction scheme:
MX.sub.2+2LX.fwdarw.L.sub.2MX.sub.4

(61) In a first method according to the present invention the first layer is formed by physical or chemical vapour deposition of the 2D-perovskite material.

(62) In an alternative method of the present invention the first layer is formed by deposition of the 2D-perovskite material from a solution.

(63) In a further alternative method according to the present invention the first layer is formed by physical vapour deposition of a precursor layer comprising at least one metal halide precursor material, followed by conversion of the precursor layer into the first layer comprising 2D-perovskite material.

(64) In a variation of the above further alternative method the first layer is formed by deposition from solution of a precursor layer comprising at least one metal halide precursor material, followed by conversion of the precursor layer into the first layer comprising the 2D-perovskite material.

(65) The first layer of two-dimensional organic-inorganic hybrid halide perovskite may comprise a plurality of 2D perovskite layers in any structural form. Preferably the structural form will be of a type discussed in the introduction, for example “Ruddelsden-Popper perovskites” which preferably adopt the general formula A.sub.n−1A.sub.2′M.sub.nX.sub.3n+1; “{110} parallel perovskites” which preferably adopt the general formula A′.sub.2A.sub.nM.sub.nX.sub.3n+2; “Aurivillius perovskites” which preferably adopt the general formula A′.sub.2A.sub.n−1M.sub.nX.sub.3n+3 and “Dion-Jacobson perovskites” which preferably adopt the general formula CA.sub.n−1M.sub.nX.sub.3n+1. An example of each of these families for n=2 is shown in FIG. 4.

(66) It is especially preferred that the first layer of two-dimensional organic-inorganic hybrid halide perovskite, comprises a plurality of 2D perovskite layers which adopt at least one of: a Ruddelsden-Popper structural form, a structural form similar to or derived from a Ruddlesden-Popper form, and a structural form which produces 2D perovskites which behave in the same, or a substantially similar, manner to those in a Ruddelsden-Popper or similar structural form.

(67) In particular embodiments, A and L′ represent the same cation which is present in the final 3D layer, and are selected from: Cs, Rb, Methylammonium, Ethylammonium, Formamidinium or Guanidinium.

(68) L is a cation in the initial 2D precursor layer but not the final 3D layer, and is selected from: Cs, Rb, Formamidinium, Guanidinium, or organic ions R.sub.3NH.sup.+ where R are selected from H, unsubstituted or substituted aryl groups or unsubstituted or substituted alkyl groups, for example methylammonium or ethylammonium. Specific examples include:—phenethylammonium, n-propylammonium, n-butylammonium, n-pentylammonium, n-hexylammonium, n-heptylammonium, n-octylammonium, napthylethylammonium, 2-(2-naphthyl)ethananammonium, benzylammonium, phenylammonium, 2-thiopheneethylammonium, t-butylammonium, diethylammonium iso-butylammonium, cyclohexylammonium, cyclohexylmethylammonium, ethylammonium, 1,4-butanediammonium, 2,2′-Dithiobis(ethylammonium), 3-(ammoniomethyl)piperidine, 4-(ammoniomethyl)piperidine, 2-(4-Biphenyl)ethylammonium, 1,4-butanediammonium, 1,5-pentanediammonium, 1,6-hexanediammonium, 1,7-heptyldiammonium, 1,8-octodiammonium, 1,9-nonadiammonium, 1,4-benezene diammonium, and 4-Methoxy-phenethylammonium. K is another cation in the initial 2D layer but not the final 3D layer, and has the formula H.sup.+R.sub.2NR′NR.sub.2H.sup.+ where R is selected from H, unsubstituted or substituted aryl groups or unsubstituted or substituted alkyl groups, and R′ can be unsubstituted or substituted aryl groups or unsubstituted or substituted alkyl groups.

(69) In step ii) of the process according to one aspect of the invention, when a material comprising one or more halides is deposited on the first layer, the first layer may be exposed to a vapour comprising the organic halides AY and AZ, wherein Y and Z are halide anions which are the same or different, and cations A are the same of different and are selected from the group consisting of cations of the following metals: Cs, Rb, Formamidinium, Guanidinium, or organic ions R.sub.3NH.sup.+ where R are selected from H, unsubstituted or substituted aryl groups or unsubstituted or substituted alkyl groups, or H.sup.+R.sub.2NR′NR.sub.2H.sup.+ where R can be H, unsubstituted or substituted aryl groups or unsubstituted or substituted alkyl groups and R′ can be unsubstituted or substituted aryl groups or unsubstituted or substituted alkyl groups. The Y and Z halide anions become the anions represented by group X in the final 3D perovskite product.

(70) Preferred groups for the A groups in the halides AY and AZ correspond to those listed above in the final AMX.sub.3 perovskite product.

(71) Step ii) of the reaction according to this invention can be represented by general reaction scheme:
L.sub.2MX.sub.4+AY+AZ.fwdarw.AMXZY

(72) Step ii) of the reaction according to this invention is generally performed by heating to a temperature of less than 200 degrees, preferably by heating to a temperature between 100 and 180 degrees centigrade.

EXAMPLES

(73) An overview of the reaction scheme of the process of the present invention is given in FIG. 2.

Example 1 (a)—Formation of [FA]Pbl.SUB.2.Br

(74) (a). A layer of Pbl.sub.2 300 nm thick is evaporated onto a glass substrate. A solution of phenethylammonium iodide (PEAI) dissolved in 1-methoxy-2-propanol (PGME) with 10 percent PEAI by weight in the solution is made and filtered through a 0.2 μm filter. This solution is then spin coated onto the layer of Pbl.sub.2, typically at a spin speed of 7000 rpm in an air ambient. The substrate is then transferred to an oven and baked at 90° C. for 30 minutes. This forms the first layer comprising an organic-inorganic halide perovskite material having a planar, layered two-dimensional crystal structure in which the planar layers are oriented perpendicular to the normal of the surface of the substrate and having the formula (PEA).sub.2Pbl.sub.4. This layer will be approximately 400 nm thick.

(75) (b). A solution of Formamidinium iodide (FAI) and Formamidinium bromide (FABr) dissolved in anhydrous isopropyl alcohol (IPA), with 5 percent by weight of solids in the solution, and a 0.7:1 molar ratio of FAI:FABr is made up. The solution is filtered through a 0.2 μm filter, and spin coated onto the first layer prepared in step (a) above at a spin speed of 5000 rpm in a dry nitrogen ambient. The substrate is then transferred to a hotplate and baked at 50° C. for 1 minute, then transferred to an oven and baked at 160° C. for 60 minutes. This forms a layer of material having a three-dimensional perovskite structure having the formula [FA]Pbl.sub.2Br. This layer will be approximately 400 nm thick.

Example 1 (b)—Formation of [FA]Pbl.SUB.2.Br

(76) The method is as described above, but the initial layer of Pbl.sub.2 is deposited from solution.

Example 2—Formation of [FA].SUB.(1−x).Cs.SUB.(x).Pbl.SUB.2.Br

(77) (a). A layer comprising a mixture of Pbl.sub.2 and Csl 300 nm thick is deposited by co-evaporation onto a glass substrate. The mixture is controlled to have a desired molar ratio of Cs:Pb depending on the target composition of the final 3D perovskite. A solution of phenethylammonium iodide (PEAI) dissolved in 1-methoxy-2-propanol (PGME) with 10 percent PEAI by weight in the solution is made, and filtered through a 0.2 μm filter. This solution is then spin coated onto the layer of Pbl.sub.2+Csl, typically at a spin speed of 7000 rpm in an air ambient. The substrate is then transferred to an oven and baked at 90° C. for 30 minutes. This forms the first layer comprising an organic-inorganic halide perovskite material having a planar, layered two-dimensional crystal structure in which the planar layers are oriented perpendicular to the normal of the surface of the substrate and having the formula ((PEA).sub.1−xCs.sub.(x))Pbl.sub.4. This layer will be approximately 400 nm thick.

(78) (b). A solution of Formamidinium iodide (FAI) and Formamidinium bromide (FABr) dissolved in anhydrous isopropyl alcohol (IPA), with 5 percent by weight of solids in the solution, and a 0.7:1 molar ratio of FAI:FABr is made up. The solution is filtered through a 0.2 μm filter, and spin coated onto the first layer prepared in step (a) above at a spin speed of 5000 rpm in a dry nitrogen ambient. The substrate is then transferred to a hotplate and baked at 50° C. for 1 minute, then transferred to an oven and baked at 160° C. for 60 minutes. This forms a layer of material having a three dimensional perovskite structure having the formula [FA].sub.(1−x)Cs.sub.(x)Pbl.sub.2Br. This layer will be approximately 400 nm thick.

Example 3—Formation of [MA]Pbl.SUB.2.Br

(79) (a). A layer of Pbl.sub.2 300 nm thick is evaporated onto a glass substrate. A solution of phenethylammonium iodide (PEAI) dissolved in 1-methoxy-2-propanol (PGME) with 10 percent PEAI by weight in the solution is made, and filtered through a 0.2 μm filter. This solution is then spin coated onto the layer of Pbl.sub.2, typically at a spin speed of 7000 rpm in an air ambient. The substrate is then transferred to an oven and baked at 90° C. for 30 minutes. This forms the first layer comprising an organic-inorganic halide perovskite material having a planar, layered two-dimensional crystal structure in which the planar layers are oriented perpendicular to the normal of the surface of the substrate and having the formula (PEA).sub.2Pbl.sub.4. This layer will be approximately 400 nm thick.

(80) (b). A solution of methylammonium iodide (MAI) and methylammonium bromide (MABr) dissolved in anhydrous isopropyl alcohol (IPA), with 5 percent by weight of solids in the solution, and a 0.7:1 molar ratio of MAI:MABr is made up. The solution is filtered through a 0.2 μm filter, and spin coated onto the first layer prepared in step (a) above at a spin speed of 5000 rpm in a dry nitrogen ambient. The substrate is then transferred to a hotplate and baked at 50° C. for 1 minute, then transferred to an over and baked at 160° C. for 60 minutes. This forms a layer of material having a three-dimensional perovskite structure having the formula [MA]Pbl.sub.2Br. This layer will be approximately 400 nm thick.

Example 4

(81) The first layer comprising an organic-inorganic halide perovskite material having a planar, layered two-dimensional crystal structure in which the planar layers are oriented perpendicular to the normal of the surface of the substrate mentioned in step (a) in examples 1 to 3 above may, in an alternative embodiment, be grown directly from the vapour phase as described, for example, in Masanao Era et al., “Self-Organised Growth of Pbl-based Layered Perovskite Quantum Well by Dual-Source Vapour Deposition” Chem. Mater. Vol. 9, No. 1, pp 8-10 [1997].

Example 5

(82) In Examples 1 to 3 above, the organic halides (FAI and FABr) may be added to the initial reaction mixture together with the PEAI Thus the organic halides are not added in a separate step, (b). Both the organic halides and the PEAI are delivered to the metal halide film, and the 2D perovskite structure forms with the FA in situ. The 3D perovskite structure is formed via heating in the same manner as described for the Examples above.

Example 6

(83) 18 functioning devices were constructed and tested as described below. The devices were divided into two groups: the first having the perovskite layer produced according to the 2-D process of the present invention as described above in Example 2, and the second having the perovskite layer produced according to the two-step process of the prior art.

(84) The devices were n-i-p single junctions built on a glass substrate and contacted with gold electrodes, with the same device structure used in both groups, with only perovskite deposition altered. Current-voltage (JV) characteristics of the solar cells were recorded in ambient air under simulated AM 1.5 solar light (1-Sun, 100 mW cm.sup.−2) generated by an ABET Class AAB sun 2000 simulator. The solar simulator was calibrated for a 1.6 eV perovskite bandgap using a certified KG3 filtered silicon reference cell. Each device was measured with a 20 mV voltage step using a Keithley 2400 source meter. All devices were masked with a 0.0919 cm.sup.2 metal aperture to define the active area and eliminate edge effects.

(85) The average efficiencies over 18 functioning devices per split were 14.84 for the 2-step process and 15.93 for the 2D process of the invention.

(86) Extracted parameters and comparative data are shown in the table below, wherein the figures are derived from the plots in FIG. 13:

(87) TABLE-US-00001 2D Route 2 step process η (%) 17.3 16.7 V.sub.oc (V) 1.13 1.10 FF (%) 78.8 79.3 J.sub.sc (mA .Math. cm.sup.−2) 19.43 19.19

(88) The data is illustrated in FIG. 13 which shows a plot of the current-voltage sweep for single-junction devices prepared by the 2D route and 2-step process, where we see a Voc gain via the 2D route, as would be expected from the higher photoluminescence quantum efficiency (PLQE).