WO2015189551A1

WO2015189551A1 - Heterojunction device

Info

Publication number: WO2015189551A1
Application number: PCT/GB2015/000183
Authority: WO
Inventors: Henry Snaith; Antonio Abate; Sandeep PATHAK; Simone GUARNERA; Annamaria PETROZZA
Original assignee: Isis Innovation Limited; Fondazione Instituto Italiano Di Tecnologia
Priority date: 2014-06-12
Filing date: 2015-06-12
Publication date: 2015-12-17
Also published as: GB201410542D0

Abstract

The invention provides a solid-state heterojunction comprising a p-type material, a perovskite material and a cathode, wherein the p-type material and the perovskite material are in contact, p- type material and the cathode are in contact and a permeable barrier layer comprising at least one insulating material is located between the perovskite material and the cathode.

Description

Hetero junction Device

Statement on Funding

The work leading to this invention has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 279881.

Field of the Invention

The present invention relates to a solid-state heterojunction, in particular solid- stateheterojunctions comprising a perovskite material, and to the use of such heteroj unctions in optoelectronic devices such as solid-state solar cells and corresponding light sensing devices. Most particularly, the present invention relates to optoelectronic devices having an improved stability under anaerobic conditions and/or enhanced light to electrical power conversion efficiency.

Background of the Invention

The junction of an n-type semiconductor material (known as an electron transporter) with a p- type semiconductor material (known as a hole-transporter) is perhaps the most fundamental structure in modern electronics. This so-called " heteroj unction" forms the basis of most modern diodes, transistors and related devices including optoelectronic devices such as light emitting diodes (LEDs), photovoltaic cells, and electronic photo-sensors. The additional insertion of a layer of intrinsic ("i-type" or simply "i") semiconductor between the p-type and n-type materials can also have advantageous properties in some optoelectronic applications such as solar cells or light emitting diodes, where the optoelectronic properties of the intrinsic photoactive material may be superior when the material is not heavily doped. Such so-called "p-i-n heteroj unctions" are particularly relevant in the context of perovskite solar cells.

A realization of the pressing need to secure sustainable future energy supplies has led to a recent explosion of interest in photovoltaics (PV). Conventional semi-conductor based solar cells are reasonably efficient at converting solar to electrical energy. However, it is generally accepted that further major cost reductions are necessary to enable widespread uptake of solar electricity generation, especially on a larger scale. Dye-sensitized solar cells (DSCs), for example, offer a promising solution to the need for low-cost, large-area photovoltaics. Typically, DSCs are composed of mesoporous Ti0₂ (electron transporter) sensitized with a light-absorbing molecular dye, which in turn is contacted by a redox-active hole-transporting medium (electrolyte). Photo- excitation of the sensitizer leads to the transfer (injection) of electrons from the excited dye into the conduction band of the Ti0₂. These photo-generated electrons are subsequently transported to and collected at the anode. The oxidized dye is regenerated via hole-transfer to the redox active medium with the holes being transported through this medium to the cathode.

The functioning of a solar cell relies initially on the collection of solar light energy in the form of capture of solar photons by a sensitizer (typically a molecular, metal complex, or polymer dye). The effect of the light absorption is to raise an electron into a higher energy level in the sensitizer. This excited electron will eventually decay back to its ground state, but in a solid state cell, the n-type material in close proximity to the sensitizer provides an alternative (faster) route for the electron to leave its excited state, viz. by "injection" into the n-type semiconductor material. This injection results in a charge separation, whereby the n-type semiconductor has gained a net negative charge and the dye a net positive. Since the dye is now charged, it cannot function to absorb a further photon until it is "regenerated" and this occurs by passing the positive charge ("hole") on to the p-type semiconductor material of the junction (the "hole transporter"). In a solid state device, this hole transporter is in direct contact with the dye material, while in the more common electrolytic dye sensitised photocells, a redox couple (typically iodide/triiodide) serves to regenerate the dye and transports the "hole species"

(triiodide) to the counter electrode. Once the electron is passed into the n-type material, it must then be transported away, with its charge contributing to the current generated by the solar cell.

While the above is a simplified summary of the ideal working of a dye-sentitized solar cell, there are certain processes which occur in any practical device in competition with these desired steps and which serve to decrease the conversion of sunlight into useful electrical energy. Decay of the sensitizer back to its ground state was indicated above, but in addition to this, there is the natural tendency of two separated charges of opposite sign to re-combine. This can occur by return of the electron into a lower energy level of the sensitizer, or by recombination of the electron directly from the n-type material to quench the hole in the p-type material. In an electrolytic DSC, there is additionally the opportunity for the separated electron to leave the surface of the n-type material and directly reduce the iodide/iodine redox couple. Evidently, each of these competing pathways results in the loss of potentially useful current and thus a reduction in cell energy-conversion efficiency.

The varying kinetics of the steps in the energy conversion process have greater and lesser effects upon the overall efficiency and stability of a DSC. For example, the dye is most stable in its neutral state and so the very rapid dye regeneration (such as that provided by a solid-state hole- transporter rather than an ionic electrolyte) not only helps to avoid recombination of the negative charge with the dye, but also renders it more stable to long term use. Similarly, varying the energy level changes and thus kinetics of the various electron transfers involved can improve injection efficiency but care must be taken that recombination with the dye is not also enhanced. Many of these processes have been studied in detail in various types of cell.

In DSCs, the Ti0₂ is often in contact with a redox active liquid electrolyte as the hole- transporting medium. Those incorporating an iodide/triiodide redox couple in a volatile solvent can convert over 12% of the solar energy into electrical energy. However, liquid electrolyte devices suffer from a number of shortcomings: the efficiency is far from optimal; the systems are typically optimised to operate with sensitizers which predominantly absorb in the visible region of the spectrum thereby losing out on significant photocurrent and energy conversion; and liquid electrolytes are normally corrosive and often prone to leakage, factors which become particularly problematic for larger-scale installations or over longer time periods.

More recent work has focused on creating gel or solid-state electrolytes, or entirely replacing the electrolyte with a solid-state polymeric or molecular hole-transporter (p-type material) which is much more appealing for large scale processing and durability. Of these alternatives, the use of a molecular hole-transporter appears to be the most promising. Though these solid-state DSCs (SDSCs) are a proven concept, the most efficient Ti0₂-based solid-state DSCss still only convert around 7-8% of the solar energy into usable electrical power.

In another recent approach, solid-state solar cells employing perovskite materials (sometimes referred to as perovskite solar cells (PSSCs)) have been developed . Perovskites are

advantageous in that these are able to act not only as a sensitizer (light absorber) but also as an n- type, p-type or intrinsic (i-type) semiconductor material (charge transporter). A perovskite can therefore act both as a sensitizer and as the n-type semiconductor material. In this way the perovskite may assume the roles both of light absorption and long range charge transport.

Perovskites are often more strongly absorbing over a broader wavelength range than dyes, thus offering the opportunity for enhanced light absorption in thinner films than are feasible with solid-state DSCs, where a thickness limitation of around 2 μιη is often imposed by external factors. Typically, perovskite cells are "meso-superstructured solar cells" (MSSCs) in which the perovskite is coated onto a mesoporous insulating scaffold, normally AI2O3 or Ti0₂.

Alternatively, perovskite cells may be planar heteroj unctions such as those described by Liu, M., Johnston, M. B. & Snaith, H. J., Nature 501, 395-398 (2013), in which a thin crystalline film of perovskite material is sandwiched between positive-type (p-type) and negative-type (n-type) charge collection layers. Conventionally, such devices employ a solid-state (e.g. organic semiconductor) p-type material. In the current most efficient embodiment, which achieves power conversion efficiencies of around 16%, a flat compact layer of Ti0₂ typically of about 50nm thickness is also employed as an electron selective contact forming a planar heterojunction with the perovskite film In some embodiments the mesoporous scaffold is omitted and the perovksite forms a solid dense homogeneous layer sandwiched between the n-type and p-type charge collection layers such as the device structure described by Liu et al. (Nature, vol. 501, pp. 398 ff., 2013) In some embodiments the perovskite layer is processed on top of the p-type charge collection layer in what is termed an "inverted" structure. In this inverted configuration both the p- and n-type layers are often organic semiconductors, such as the device architecture described by Docampo et al. (Nature Communications, vol. 4, 2761, 2013, published online November 2013 with DOI 10.1038/ncomms3761).

Solid-state heterojunction devices employing solid-state p-type materials (e.g. solid-state organic p-type materials such as spiro-OMeTAD or solid-state inorganic p-type materials such as CuSCN) are advantageous in that they do not suffer from electrolyte leakage, nor does the hole- transporting material (HTM) possess corrosive qualities. Currently-known devices are also relatively cheap to manufacture and have achieved performance comparable to or much greater than liquid electrolyte dye-sensitized solar cell systems. However, higher power conversion efficiencies are still desirable. A significant factor limiting the commercial viability of solid- state heteroj unctions at present is also their photostability, both at early times under illumination and over the long term. For a solar cell to be economically viable, it must have an operational lifetime of at least 10 years, and more typically at least 20 to 25 years under typical outdoor full sunlight conditions. One of the most important features that will affect the possibility of the perovskite solar cells to break into the market of photovoltaic technology is the long term stability. Despite the extensive efforts recently reported to improve the efficiency of this kind of solar cells and the importance of stability to commercialisation, there are few papers published on the topic of stability. This appears to be because no useful advance in the stability of perovskite based solar cell has yet been made.

The UV-Vis absorption spectra of perovskite solar cells stored in a non-reacting environment shows that there are no significant changes when they are continuously exposed at full sunlight. This means that the absorbing material, the perovskite, does not degrade when exposed to the intense ultraviolet radiation present in the solar spectrum.

Although the potential of perovskite technology has been recognised by researchers, until now it has not been possible to realize cells which maintain a constant efficiency for a long time. The best result achieved up to now showed an 20% loss in the power conversion efficiency (PCE) of one cell exposed for 500h to an intense white light, without the UV components.

After further experimentation, the present inventors have now established that by physical separation of the perovskite material from the cathode, particularly by means of a permeable (e.g. porous, preferably mesoporous) insulating structure (referred to as a "buffer" or "barrier" layer) which allows penetration of the p-type hole transporting material, solid-state perovskite solar cells may be created with an improved stability. Remarkably, the buffer layer significantly improves the early time stability of the solar cells. A dispersion of A1₂0₃ nanoparticles in isopropanol has been found to be a particularly suitable material for formation of such a buffer layer of insulating material. An unexpected advantage of the approach was that the overall thickness of the hole-transporter layer could be considerably reduced, leading to a significant increase in the solar cell fill factor and efficiency. An improvement in efficiency is therefore also a further unexpected advantage of the use of a buffer layer.

Without wishing to be bound by theory, it is believed that solar cells suffer the presence of shunting pathways that arise, during the operation of the cell, from the migration of metallic contacts through the hole transporting material (HTM). The contact regions between the electrode and the perovskite are low resistance shunting pathways. Conventionally this can be avoided by employing a thick p-type material layer. However, the use of a protective buffer layer not only avoids the need to use a thick p-type layer, but also increases the fill factor, leading to an improvement in the power conversion efficiency.

Overview of the Invention

In a first aspect, the present invention therefore provides a solid-state heterojunction comprising a p-type material, a perovskite material and a cathode, wherein the p-type material and the perovskite material are in contact, and a permeable barrier layer comprising at least one insulating material is located between the perovskite material and the cathode.

In a second aspect, the invention provides an optoelectronic device comprising at least one solid- state heterojunction as herein described.

In a further aspect, the invention provides the use of a permeable barrier layer in a solid-state heterojunction comprising a perovskite material.

In a further aspect, the invention provides a method for reducing and/or eliminating the light- induced drop in shunt resistance in a solid-state heterojunction comprising a perovskite material, a p-type material and a cathode under anaerobic conditions, said method comprising

(i) providing at least one insulating material

(ii) incorporating said insulating material as a permeable barrier layer in the solid-state heterojunction between the perovskite material and the cathode..

In a further aspect, the invention provides a method for the manufacture of a solid-state heterojunction comprising a cathode separated from a perovskite material by a permeable barrier layer of at least one insulating material, said method comprising: a) coating an anode, preferably a transparsnt anode (e.g. a Fluorine doped Tin Oxide - FTO anode) with a compact layer of an n-type semiconductor material;

b) forming a layer of a perovskite semiconductor material on said compact layer, c) optionally surface sensitizing said compact layer and/or said layer of perovskite material with at least one sensitizing agent; d) forming a permeable barrier layer comprising at least one insulating material;

e) forming a layer of a p-type material permeating the permeable barrier layer and in contact with said layer of perovskite material; and

f) forming a cathode, preferably a metal cathode (e.g. a silver or gold cathode) on said permeable barrier layer, in contact with said p-type semiconductor material, wherein in steps d) and e) said permeable barrier layer may be comprised in said layer of p-type material or may be a separate layer, penetrated by said p-type material.

In a further aspect, the present invention provides a solid-state inverted heteroj unction comprising a n-type material, a perovskite material and an anode, wherein the n-type material and the perovskite material are in contact, and a permeable barrier layer comprising at least one insulating material is located between the perovskite material and the anode.

In a further aspect, the invention provides a meihod for the manufacture of an inverted solid-state heteroj unction comprising an anode separated from a perovskite material by a permeable barrier layer of at least one insulating material, said method comprising:

a) coating a cathode, preferably a transparent cathode (e.g. a Fluorine doped Tin Oxide (FTO) cathode) with a layer (preferably a compact layer) of a p-type material;

b) forming a layer of a perovskite semiconductor material on said compact layer, c) optionally surface sensitizing said compact layer and/or said layer of perovskite material with at least one sensitizing agent;

d) forming a permeable barrier layer comprising at least one insulating material;

e) forming a layer of an n-type material permeating the permeable barrier layer and in contact with said layer of perovskite material; and

f) forming an anode, preferably a metal anode (e.g. a silver or gold anode) on said permeable barrier layer, in contact with said n-type material, wherein in steps d) and e) said permeable barrier layer may be comprised in said layer of an n- type material or may be a separate layer, penetrated by said n-type material. In a further aspect, the invention provides a method of stabilising and/or improving the power conversion efficiency of a solid-state heterojunction comprising a p-type material in contact with aperovskite material under anaerobic conditions, said method comprising providing a permeable barrier layer comprising at least one insulating material between perovskite material and a cathode.

In a further aspect, the invention provides a solid-state heterojunction formed or formable by the methods described herein.

In a further aspect, the invention provides an optoelectronic device comprising at least one solid- state heterojunction formed or formable by the methods described herein.

Brief Description of the Drawings

Figure 1 illustrates an SEM image and schematic representation of device structures: a) standard MSSC and b) MSSC with A1₂0₃ buffer layer. " K330" in this figure represents perovskite material CH₃NH₃PbI_3-xCl_x.

Figure 2 illustrates schematic representations of two buffer layer structures: a) nanoparticles dispersed in the HTM material and b) nanozeolites buffer layer.

Figure 3 (a)-(d) illustrates JV parameters of cells without and with the buffer layer.

Figure 4 illustrates JV curves of the two best solar cells realized without and with buffer layer.

Figure 5 illustrates the normalized efficiency of most robust cells with buffer layer (black squares, top line) and standard cells (grey squares, lower line) under AM1.5G illumination condition. The cells were encapsulated and maintained at Voc condition.

Figure 6 a)-d) illustrates statistics of normalized JV parameters time evolution of encapsulated cells with buffer layer (black squares, top line) and standard cells (grey squares, lower line) under AMI .5G illumination condition (4 cells for each type). The cells were maintained at Voc condition during the aging.

Figure 7 illustrates absorbance spectra of a cell without buffer layer sealed in nitrogen before exposure and when exposed to AMI .5 sunlight for 400 hours.

Figure 8 illustrates statistics of dark JV curve over before and after aging over 4 different devices a) with buffer layer and b) without buffer layer.

Figures 9a and 9b illustrate embodiments of the buffer layer according to the invention in combination with a hole transporting layer.

Figure 10 illustrates the absorbance of alumina buffer layers on glass infiltrated with spiro- OMeTAD with concentrations of 2wt%, 4wt%, 6wt%, 8wt% and 10wt%.

Figure 1 1 illustrates the conductivity of mesoporous alumina layers infiltrated with spiro- OMeTAD with concentrations of 10 wt %, 8.5 wt %, 7 wt %, 5.5 wt %, 4 wt %, 2.5 wt % and 1 wt % spiro-OMeTAD on glass substrates, measured by four-point probe method.

Figure 13 illustrates the shunt leakage of hole only diodes with structure of FTO/ mesoporous alumina/ spiro-OMeTAD/ gold, as a function of spiro-OMeTAD concentration.

Figure 14 illustrates current density-voltage curves of devices with alumina buffer layers infiltrated with spiro-OMeTAD with a concentration of 10 wt %, 8 wt %, 7 wt %, 6 wt %, 5 wt % and of a control cell without a buffer layer and with a 10 wt % spiro-OMeTAD layer, all measured under AMI .5 simulated sunlight (100 m W.cm ).

Detailed Description of the Invention

References herein to the abbreviations "DSC", "SDSC", "DSSC", "SSC" and any of the foregoing abbreviations prefixed by "ss" may be taken to refer equally to a solid-state dye- sensitized heterojunction device, including referring to an optoelectronic device such as a solar cell or a photo-detector where context allows. Similarly, all references to a heterojunction herein may be taken to refer equally to an optoelectronic device including referring to a solar cell or to a photo-detector where context allows. References to heterojunctions encompass both DSCs and perovskite-based cells where context allows and unless otherwise specified.

All references herein to a "compact" or "planar" layer are interchangeable and are intended to distinguish from "mesoporous" or "meso-superstructured" layers as described in further detail below. Compact layers do not comprise a mesoporous scaffold material.

While photovoltaic cells and, in particular, solid-state DSCs are frequently used herein as illustration, it will be appreciated that the heterojunctions of the invention may equally be applied to other corresponding optoelectronic devices including all those described in all sections herein. In particular, the following discussion is generally applicable, amongst other devices, to photovoltaic cells (solar cells), photo-detectors, and light-emitting diodes, including but not limited to dye-sensitized solid-state heterojunction devices comprising perovskites and to those which are perovskite-free.

References to "solid state" heterojunctions refer to heterojunctions not containing a liquid hole- transporting species (electrolyte), e.g. an ionic solution or ionic liquid. As an example, the heterojunction and/or corresponding devices, such as DSCs should not contain any iodine/iodide redox couples, such as I₂/I^3". The use of the term "electrolyte" is not entirely consistent in the prior art, with some authors using the term "electrolyte" to include non-ionic charge transporting species such as molecular hole transporters. The general convention, however, which is adopted herein, is that the term "electrolyte" refers to a medium through which charge is transported by the movement of ions. Thus, a polymer or gel electrolyte is distinct from the hole transporters (p-type materials) employed in the present invention because the former relies on the movement of ions to carry charge, while the latter conducts by passage of electrons. Thus, the solid state heterojunctions, solar cells and related aspects of the present invention relate to non-electrolyte devices in that they preferably do not contain any "electrolyte", being any species which conducts by ion movement, but rather contain a solid-state "hole transporter" which conducts electrons.

In addition to the rutile, anatase and brookite crystal structures, Ti0₂ has three known metastable phases which can be produced synthetically (monoclinic, tetragonal and orthorombic), and five high-pressure forms ( -Pb0₂-like, baddeleyite-like, cotunnite-like, orthorhombic, and cubic phases) also exist. For use in the heteroj unctions of the present invention, Ti0₂ having any phase structure may be used. The anatase structure is particularly preferred.

Certain values or parameters may be referred to herein is "about", "around" or "substantially" some value or range of values. Typically this is used to represent a value of up to 10% greater or less than the specified value. More preferably this will represent the value ± 5% or more preferably ± 2%. The exact value specified evidently forms a preferred embodiment.

In a first aspect, the present invention provides a solid-state heteroj unction comprising a p-type material, a perovskite material and a cathode, wherein the p-type material and the perovskite material are in contact, and a permeable barrier layer comprising at least one insulating material is located between the perovskite material and the cathode.

The p-type material (which may also be referred to as a "hole transporter", "hole transporting material" or "HTM") may be organic or inorganic. Suitable organic p-type materials and inorganic p-type materials are described herein. Preferably the p-type material is an organic p- type material as herein described.

All references herein to a "permeable barrier layer", "insulating layer", "insulating barrier layer", "permeable layer" or the like may be taken to refer equally to a permeable barrier layer comprising at least one insulating material.

Preferably, said perovskite material and said cathode are separated by a distance of no less than 1 nm at their closest point, by said barrier layer. The said barrier layer will cover substantially the whole of the area between the cathode and the anode. For example, the barrier layer may cover at least 95%, preferably at least 99% and most preferably at least 99.9% of the area of overlap between the perovskite material and the cathode.

Preferably, the permeable barrier layer is porous, e.g. mesoporous. By "mesoporous" is meant a material having pores with diameters in the range 2 to 50 run. A mesoporous material is conventionally defined as having pores with diameters in the range 2 nm to 50 nm and that meaning is adopted herein. Particularly preferably, mesoporous materials for use in the heteroj unctions of the invention have a pore size in the range 20 nm to 50 nm or 30 nm to 50 nm. By way of comparison, other conventional pore size naming conventions include "nanoporous" or "microporous" (pore diameters < 2 nm) and "macroporous" (pore diameters in the range 50 to 1000 nm and references herein to such terms adopt this conventional meaning. Examples of such definitions may be found, for instance, in J. Rouquerol et al. , "Recommendations for the Characterization of Porous Solids", Pure & Appl. Chem., vol. 66, no. 8, pp. 1739-1758, 1994; Sing et al, Pure & Appl. Chem., vol. 57, no. 4, pp. 603-919, 1985; and "Manual on Catalyst Characterization", J. Haber, Pure & Appl. Chem., vol. 63, pp. 1227-1246, 1991.

The general term "porous" is intended to refer to materials having pores on the mesoporous or macroporous level, and therefore "non-porous" layers may nevertheless contain nanopores (micropores), i.e. pores typically having a diameter of 2 nm or less. Compact layers (also called planar layers) as described herein are normally non-porous.

In one embodiment, the permeable barrier layer is a substantially continuous capping layer, e.g a layer extending over substantially all of the perovskite material layer.

In an embodiment, the permeable barrier layer is not conformal with the perovskite layer.

In an embodiment, the permeable barrier layer contacts the perovskite layer across less than or equal to about 20% of the surface area of either layer, preferably less than or equal to about 15%, e.g. less than or equal to about 10% or less than or equal to about 5% of the surface area. In one such embodiment there is negligible contact (e.g. no contact, substantially no contact, or contact only at isolated points) between the perovskite layer and the permeable barrier layer. In embodiments such as those discussed herein where the insulating material is in the form of particles dispersed within the p-type material there may however be contact between the p-type material and the perovskite material (including contact between the p-type material and the perovskite material across substantially all of the surface area of the perovskite material) provided that the insulating material dispersed within the p-type material does not itself contact the perovskite material in excess of the limitations discussed above. Optionally, the insulating material may be provided in the form of solid particles dispersed in the p-type material. Preferably such particles are nanoparticles having a particle size of 1 to 100 nm. The permeable barrier layer of insulating material should preferably be sufficiently porous so as to permit a conduction path from the perovskite material to the cathode by means of the p-type material (hole transporter) but sufficiently insulating to prevent electrical contact (i.e. direct charge transfer) between the cathode and the perovskite material. Similarly, the barrier layer should be sufficiently thick to reliably insulate the perovskite material from the cathode but should not be thicker than necessary due to the increase in resistance caused by a longer path of conduction through the hole transporter.

In this respect, it will be preferable that the permeable barrier layer is formed from at least one insulating material with a resistivity of greater than 10⁹ Qcm. Suitable barrier layers are described in detail herein.

It is particularly preferable that the insulating barrier layer has a porosity of 10 to 90%.

Preferably, the insulating material used in the heteroj unctions described herein comprises at least one insulating metal oxide, either alone or in combination with other metal salts. Suitable insulating metal oxides may for example be selected from A1₂0₃, Si0₂, ZrO, MgO, Hf0₂, Ta₂0₅, Nb₂0₅, Nd₂0₃, Sm₂0₃, La₂0₃, Sc₂0₃, Y₂0₃, NiO, Mo0₃, MnO, SiA10₃,₅, Si₂A10₅,₅, SiTi0₄, AlTi0₅, zeolites, and mixtures thereof. Particularly preferably, the insulating metal oxide is A1₂0₃.

In addition to the permeable barrier performing the role of a buffer layer, the permeable barrier may have other additional properties, such as being highly moisture absorbent, or able to absorb other gases such as oxygen or halide ions. As such it could additionally act as a "getter" in order to enhance the long term stability of the solar cells. Materials with such properties include nanozeolites, for example microcrystalline alumina silicate. Such materials are a further preferable material for use as the permeable barrier layer.

Insulating polymers, either alone or in combination with each other or with other materials are also highly suitable materials for forming a permeable barrier layer. Examples of suitable polymers include: polystyrene, acrelate, methacrylate, methylmethacrylate, ethelene oxide, ethelene glycol, cellulose and/or imide polymers or mixtures thereof. Block copolymers are a subset of insulating polymers which are highly suitable either alone or in combination with each other, with other polymers and/or with other materials. Examples of suitable block copolymers include: polyisoprene-block-polystyrene, poly(ethylene glycol)- block-poly(propylene glycol)-block-poly(ethylene glycol), polystyrene-block-polylactide, and/or polystyrene-block-poly(ethylene oxide) or mixtures thereof.

The thickness of the insulating barrier layer should be sufficient to provide an insulating effect without being so thick that a significant increase in the internal resistance is caused. Suitable thicknesses will be readily established by those of ordinary skill depending upon the barrier material, the porosity and the nature of the hole transporter. In a preferred embodiment, the permeable barrier layer has a thickness of 1 to 1000 nm, preferably less than 500 nm, e.g. 450 nm or less, 400 nm or less, 350 nm or less, 300 nm or less, 250 nm or less, 200 nm or less, 150 nm or less, 100 nm or less, or 50 nm or less. Barrier layer thicknesses of 50 to 200 nm are particularly preferred, especially 50 to 150 nm, e.g. 80 to 140 nm, particularly 50 to 100 nm or 100 to 130 nm. Thicknesses of 50 nm or below may be attained if the barrier layer is in the form of nanoparticles dispersed in the hole-transporting material as discussed in further detail below, particularly if such nanoparticles have an average size of about 20 nm or less.

The use of a barrier layer in accordance with the present invention allows the thickness of the hole-transporter layer to be considerably reduced. In an embodiment the hole-transporting material layer may have a thickness of 50 to 500 nm, preferably 50 to 400 nm, 50 to 300 nm or 50 to 200 nm, particularly 50 to 150 nm, 50 to 100 nm or 100 to 150 nm, for example 100 to 130 nm.

In an embodiment the combined thickness of the hole transporting material and the barrier layer is in the range 50 to 500 nm, preferably 50 to 400 nm, 50 to 300 nm or particularly preferably 50 to 200 nm.

The porosity of the insulating barrier layer should be such as to allow this to be impregnated with the hole transporter with a sufficient degree of penetration that a reliable conduction path from the point of charge separation to the cathode is provided. Examples of suitable porosities range from 10 to 90%, preferably 25 to 75%, more preferably 40 to 60%. Such porosities allow the hole transporting material to infiltrate the permeable barrier layer such that the pores of the permeable barrier layer are at least partially filled by the p-type material. The optimal extent of pore filling will depend on, amongst other factors, the materials used for the p-type material and insulating material and the pore size of the porous insulating material. For a mesoporous insulating material with a pore size of about 20 to about 30 nm, for example, preferably about 30 to about 90% of the pores should be filled by the p-type material, e.g. about 40 to about 85%, about 45 to about 80%, about 50 to about 75%, about 55 to about 70%, or about 60 to about 65%.

Preferably the material of the barrier layer is not itself disposed on a substrate (e.g. a mesoporous substrate) in order to confer the property of porosity, but rather the porosity is an innate characteristic of the material which is used to form the barrier layer.

In an embodiment the barrier layer has the structure of sintered particles (preferably porous particles, e.g. mesoporous particles, optionally nanoparticles such as mesoporous nanoparticles).

In an embodiment the barrier layer is formed or formable by the sintering of particles (preferably porous particles, e.g. mesoporous particles, optionally nanoparticles such as mesoporous nanoparticles).

The material for forming the insulating barrier layer should evidently have a low conductivity and thus high resistivity. A suitable conductivity will be established by routine testing by one of skill in the art, such that the resulting device is effective and capable of encapsulation without any significant light-induced drop in shunt resistance. Typically, however, a suitable insulator will have a conductivity of less than 10^"9 Scm^"1. Correspondingly, a suitable insulator will typically have a resistivity of greater than 10⁹ Qcm. Conductivity and/or resistivity may be measured by standard techniques, such as by a 4 point probe conductivity measurement, as described in S. M. Sze in "Semiconductor Devices Physics and Technology", 2nd Edition, Wiley Page 54.

The buffer layer is optionally a substantially continuous capping layer as described herein.

Alternatively, the buffer layer may be provided in the form of nanoparticles which are dispersed in the p-type material. Such nanoparticles may suitably be deposited by spin-coating together with the p-type material, for example.

In an embodiment, the p-type material and the barrier layer of insulating material may together form a substantially homogeneous layer, i.e. a layer in which the p-type material and insulating material are substantially uniformly intermixed with one another, preferably in which the p-type material infiltrates the pores of the insulating material. For example, where the insulating material is in the form of solid particles dispersed in the p-type material, the insulating material may be uniformly dispersed throughout the bulk of the p-type material. An example of such an embodiment is illustrated schematically in Figure 9a in which a mesoporous insulating material 1 is dispersed uniformly throughout p-type material 2 and in which p-type material 2 infiltrates the pores of insulating material 1.

In a preferred embodiment, the barrier layer of insulating material is within a layer of p-type material such that the layer of p-type material comprises a first region and a second region, wherein in the first region the p-type material and the insulating material are substantially uniformly intermixed with one another, preferably in which the p-type material infiltrates the pores of the insulating material, and in the second region the p-type material is substantially free of insulating material, such that the second region forms a substantially continuous capping layer of p-type material which caps the first region. An example of such an embodiment is illustrated schematically in Figure 9b in which a mesoporous insulating material 3 is dispersed throughout a region 4 of a p-type material 5 while a further region 6 of p-type material 5 forms a capping layer, wherein in region 6 (i.e. the capping layer) the p-type material 5 does not have insulating material 3 dispersed throughout it.

In embodiments of the invention where a first region having a homogeneous mixture of p-type material and insulating material is capped by a second region of p-type material, it is particularly preferred that the overall thickness of the first and second regions is only slightly thicker than the thickness of the first region, e.g. the overall thickness of the first and second regions is greater than the thickness of the first region by about 50 nm or less. For example where the first region has a thickness of about 50 to about 100 nm the combined thickness of the first and second regions may be about 50 to about 150 nm.

The amount of the p-type material present must be sufficient to provide an acceptable level of conductivity for charge transport through the p-type material from the perovskite material to the cathode. Suitably the conductivity of the p-type material should be at least about 4 ^χ 10^"5 S/cm, e.g. about 4 ^χ 10^"5 S/cm to about l 10^"4 S/cm, e.g. about 5 ^χ 10^"5 S/cm to about 9 ^χ 10^"5 S/cm or about 6 x 10^"5 S/cm to about 8 ^χ 10^"5 S/cm. In embodiments where the p-type material infiltrates the porous insulating material this conductivity may be informally characterised or referred to as the conductivity of the barrier layer; however, it should be borne in mind that such conductivity is effectively a statement of the conductivity arising from the presence of the p-type material which will typically be infiltrated within the barrier layer and this should not be confused with the conductivity of the insulating material per se as discussed above

Too low an amount of p-type material will result in discontinuity in the hole-transporting material (e.g. formation of discrete, disconnected "islands" of hole-transporting material in the pores with no continuous path for charge transport), increasing the number of pinholes in the layer of hole-transporting material and therefore increasing the number of available shunting pathways and decreasing the efficiency of the heterojunction. The greater the amount of p-type material the better the conductivity through the hole transporting material and the greater the reduction in shunting pathways. However, the greater the amount of p-type material used, the thicker the p-type material layer (e.g. the combined layer of p-type material and insulating material, and/or any capping layer of p-type material over a combined layer of p-type material and insulating material) will become. This results in a greater extent in so-called parasitic absorption, i.e. optical absorption which does not generate electron/hole pairs and therefore reduces the available photocurrent. It is desirable to minimise parasitic absorption, not only in order to ensure that the available photocurrent and efficiency of the device are optimised, but also in order to increase the transparency of the heterojunction. Transparency is particularly important in the context of tandem cells in which the heterojunction of the invention is employed as a top cell, since it is necessary for at least a portion of light to reach the lower cell in the tandem junction to allow the lower cell to generate photocurrent.

In light of the foregoing considerations the thicknesses of the buffer layer and p-type layer should therefore be selected in order to provide sufficient conductivity and sufficient resistance to shunting pathways whilst also minimising the extent of parasitic absorption. Thicknesses as discussed herein have been found to be suitable for this purpose. In particular, it is preferable that the thickness of the p-type material layer should be at least the same as the thickness of the barrier layer in order to ensure that sufficient continuity of the hole-transporting material and sufficient pore-filling of the buffer layer are achieved. In a further embodiment, the buffer layer may comprise nanoparticulate nanozeolites such as LTA or LTL type zeolites. These may suitably be deposited by spin coating prior to deposition of the hole transporting material. Such embodiments protect the pervoskite not only from the shunting of the electrode but also from water molecules that infiltrates through the hole transporter (especially "spiro") in the presence of a humid environment. The foregoing discussions of buffer layer structures and thicknesses self-evidently apply to such embodiments also.

A key part of the present invention is the presence of a perovskite material. References herein to perovksites are to be understood as referring to materials having a substantially similar crystal structure to CaTi0₃. Such a crystal structure may be described by the general formula ABX₃ or A₂B¾„ wherein A and B are cations having differing sizes, X is an anion, and the A cations are larger than the B cations. Examples of perovksites having general formula ABX₃ include those wherein A is CH₃NH₃, formadmidinium, Cs, or K, B is selected from among Pb, Sn and Ge, and X is F, CI, Br or I or a pseudohalide such as BF₄, PF₆ or SCN. In the ideal perovskite structure, the crystal lattice has cubic symmetry with the B cation in 6-fold coordination, surrounded by an octahedron of anions, and the A cation in 12-fold cuboctahedral coordination. Depending on the relative sizes of the ions in the lattice, slight buckling and distortion can produce several lower- symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced. This leads to the existence of "distorted perovskite" structures having

orthorhombic, tetragonal or trigonal symmetry. For the avoidance of doubt, the term

"perovskite" as employed herein is intended to encompass materials having the ideal structure or a distorted, e.g. orthorhombic, tetragonal or trigonal, perovskite-based structure.

Examples of perovksites suitable for use in heterojunctions and optoelectronic devices according to the invention include ammonium trihalogen plumbates such as CH₃NH₃Pbl3, CH₃NH PbCl₃, CH₃NH₃PbF₃ and CH₃NH₃PbBr₃; mixed-halide ammonium trihalogen plumbate perovskites with general formula CH₃NH₃Pb[Hall]₃-_x[Hal2]_x wherein [Hall] and [Hal2] are independently selected from among F, CI, Br and I and the pseudohalides with the proviso that [Hall ] and [Hal2] are non-identical and wherein 0 < x < 3, preferably wherein x is an integer (e.g. 1, 2 or 3, preferably 1 or 2); CsSnX₃ perovskites wherein X is selected from among F, CI, Br and I, preferably I; organometal trihalide perovskites with the general formula (RNH₃)BX₃ where R is CH₃, C„H 2n or C_nH2n₊i, n is an integer in the range 2<n<10, preferably 2<n<5, e.g. n— 2, n— 3, or n=4, most preferably n=2 or n=3, X is a halogen (F, I, Br or CI), preferably I, Br or CI, and B is Pb or Sn; CH3NH₃SnX₃ perovskites wherein X is selected from among F, CI, Br and I, preferably I; formamidinium-SnX3 perovskites wherein X is selected from among F, CI, Br and I, preferably I; and combinations thereof. CH₃NH₃PbI₃ is especially preferred. A preferred mixed-halogen compound is CH₃NH₃PbI₃._xCl_x especially where x= 1 or x=2.

Perovskites may suitably be selected to be photoactive, i.e. light-absorbing or light-emitting. A layer comprising such photoactive perovskite may therefore also be light-absorbing or light- emitting. The term "light-absorbing" in relation to the perovskite (and by extension the layer comprising said perovskite) refers to its role in absorbing light, e.g. visible light, so as to act as a light absorbing material which converts the light into electrical energy when used in photovoltaic cells, photo-detectors and other such optoelectronic devices whose function is reliant upon the absorption of light, and therefore the perovskite may be selected whereby to be capable of performing this function. The term "light-emitting" in relation to the perovksite (and by extension the layer comprising said perovskite) refers to its function as an active light-emitting (e.g. visible-light-emitting) region and thus the term "light-emitting perovskite" may

equivalently be employed in order to reflect this purpose. Suitable light-absorbing or light- emitting perovskites will readily be identified by the skilled person.

The perovskites which may be employed in the heteroj unctions of the invention are preferably undoped but may if desired be doped with p-type or n-type dopants. Thus the perovksites referred to herein may be essentially intrinsic semiconductor (e.g. having only unavoidable impurities). Alternatively they may be doped (throughout and/or at the surface) with at least one dopant material of greater valency than the bulk (to provide n-type doping) and/or may be doped with at least one dopant material of lower valency than the bulk (to provide p-type doping), n- type doping will tend to increase the n-type character of the semiconductor material while p-type doping will tend to reduce the degree of the natural n-type state (e.g. due to defects). Such doping of perovskites may be made with any suitable element including F, Sb, N, Ge, Si, C, In, InO and/or Al. Suitable dopants and doping levels for use in connection with perovskites will be evident to those skilled in the art.

In one embodiment, the perovskite material may comprise only one type of material . In an embodiment, the perovskite material may comprise a mixture of different perovskite materials. In the heteroj unctions of the invention, it is preferred that the heteroj unction is a

mesosuperstructured heterojunction comprising a mesoscaled scaffold. By this is meant that the perovskite material is coated onto a mesoporous scaffold material. Suitable materials for use as the mesoporous (also called mesoscaled) scaffold include mesoporous AI2O3, mesoporous Ti0₂ and mixtures thereof. Mesoporous A1₂0₃ is especially preferred as it has been found that this leads to an increase in open-circuit voltage compared to the case where Ti0₂ is used.

Heteroj unctions incorporating a mesoscaled scaffold may be referred to as meso-superstructured solar cells (MSSC).

In an embodiment appropriate to all aspects of the invention, the perovksite is present as a conformal coating layer on the surface of a mesoporous scaffold material.

In an embodiment appropriate to all aspects of the invention, the perovskite is not a conformal coating layer on the surface of a mesoporous scaffold material. In such embodiments the perovskite may form a compact and/or continuous layer or may, for example, have a mesoporous structure in which a mesoporous scaffold is embedded within an otherwise continuous compact layer of perovskite.

Alternatively, the perovskite material may be present in the form of a compact (planar) layer. In a further alternative, the perovskite material may be present in the form of a layer comprising a microstructured array of "islands". In such an embodiment the perovksite material is provided as a discontinuous film of discrete deposits of compact perovskite material on a length scale small enough to appear continuous to the eye yet large enough to enable unattenuated

transmission of light between the islands. Such layers are particularly beneficial for the formation of semitransparent films. Such islands, and methods for their formation, are described in Eperon et ai, ACS Nano, 2014, 8(1), pp 591-598, the contents of which are hereby

incorporated by reference. Such "island"-containing films may equivalently be described as compact layers having macroporous or larger-than-macroporous (i.e. > 1000 nm) pore diameters.

The heteroj unctions of the invention will preferably comprise a solid p-type material (hole transporter) in the form of an organic semiconductor, such as a molecular, oligomeric or polymeric hole transporter. In one embodiment the p-type material is an optionally amorphous molecular organic compound. Alternatively the hetereoj unctions of the invention will comprise a solid p-type material (hole transporter) in the form of an inorganic semiconductor. The hole transporter is such that the conduction occurs by means of electron (hole) transport and not by the movement of charged ions through the material.

Having discovered the benefit of employing a buffer layer in perovskite solar cells in the "regular" device architecture as described above, it is logical to deduce that an "inverted" perovskite heteroj unction would also benefit from the employment of a buffer layer as herein described. Thus in a further aspect the present invention provides a solid-state inverted heteroj unction comprising a n-type material, a perovskite material and an anode, wherein the n- type material and the perovskite material are in contact, and a permeable barrier layer comprising at least one insulating material is located between the perovskite material and the anode.

The layers of such an "inverted" heteroj unction are as herein described. In particular, the n-type material may comprise or consist of an inorganic or organic material. Suitable materials for use as n-type materials may for instance be selected from among the inorganic and organic materials described herein for use as p-type materials.

In a further related aspect, the invention provides a method for the manufacture of an inverted solid-state heteroj unction comprising an anode separated from a perovskite material by a permeable barrier layer of at least one insulating material, said method comprising:

f) forming an anode, preferably a metal anode (e.g. a silver or gold anode) on said permeable barrier layer, in contact with said n-type material, wherein in steps d) and e) said permeable barrier layer may be comprised in said layer of an n- type material or may be a separate layer, penetrated by said n-type material.

Preferably the barrier layer does not contact the compact layer of p-type material.

Preferably the perovskite layer separates the compact layer of p-type material and the n-type material.

One of the key features of the heteroj unctions of the present invention is that they are less susceptible to a light-induced decrease in shunt-resistance than previously known

heterojunctions. In all appropriate aspects of the invention, therefore, the solid-state

heteroj unctions and related devices may be resistant to light-induced decrease in shunt resistance. For example, they may maintain not less than 75% of their initial energy conversion efficiency when exposed to full sun illumination under anaerobic conditions for at least 20 minutes. Such features are described in greater detail herein below.

The solid-state heterojunctions of the present invention are particularly suitable for use in solar cells, photo-detectors and other optoelectronic devices. In a second aspect, the present invention therefore provides an optoelectronic device comprising at least one solid state heteroj unction of the invention, as described herein. Such devices will optionally be encapsulated. Such encapsulation will preferably substantially isolate the device(s) from atmospheric oxygen (particularly molecular oxygen such as atmospheric oxygen) and/or water (particularly atmospheric water, e.g. water vapour). Substantial isolation may indicate that the concentration of molecular oxygen and/or water within the encapsulation is less than 10%, preferably less than 1 % ot the concentration in the surrounding atmosphere.

All references to a heteroj unction herein may be taken to refer equally to an optoelectronic device, including referring to a solar cell or to a photo-detector where context allows. Similarly, while solid-state solar cells are frequently used herein as illustration, it will be appreciated that such heterojunctions may equally be applied to other corresponding optoelectronic devices including all those described in all sections herein.

In a further aspect, the invention provides the use of a permeable barrier layer in a solid-state heterojunction comprising a perovskite material. In a corresponding further aspect, the present invention additionally provides the use of a permeable barrier layer to reduce the light-induced drop in shunt resistance in a solid-state he teroj unction comprising a perovskite material under anaerobic conditions. This will preferably be a heteroj unction of the present invention as described herein. All preferred features of the heteroj unctions described herein apply correspondingly to the use aspect of the invention.

The use in all appropriate aspects of the invention will preferably be in an optoelectronic device such as any of those described herein, e.g. in a solar cell or photodetector, particularly a solar cell.

In an embodiment, an optoelectronic device may comprise a heterojunction as herein described as a top cell in a tandem junction. In a further embodiment, the optoelectronic device of the invention may comprise a solid-state heterojunction as herein described as the top cell in a multiple junction and a further solid-state heterojunction as herein described as the bottom cell in said multiple junction. Suitable tandem junctions include, for example, those comprising a perovskite top cell as herein described with a silicon photovoltaic, copper indium gallium (di)selenide (CIGS), copper indium sulfide (CIS), copper zinc tin sulphide and/or CdTe bottom cell. A tandem junction comprising a heterojunction as herein described as a top cell may for example be a 4-terminal tandem junction. Suitable bottom cells include silicon solar cells, which are well known in the art.

The use of the present invention will preferably be to maintain the efficiency of a device (e.g. SDSC) at no less than 75% of its initial efficiency for a period of no less than 100 hours under full sun illumination in the substantial absence of oxygen.

In a related further aspect, the invention thus provides a method for reducing and/or eliminating the light-induced drop in shunt resistance in a solid-state heterojunction comprising a perovskite material, a p-type material and a cathode under anaerobic conditions, said method comprising

(i) providing at least one insulating material

(ii) incorporating said insulating material as a permeable barrier layer in the solid-state heterojunction between the perovskite material and the cathode. In a further aspect, the invention provides a method for the manufacture of a solid-state heteroj unction comprising a cathode separated from a perovskite material by a permeable barrier layer of at least one insulating material, said method comprising: a) coating an anode, preferably a transparent anode (e.g. a Fluorine doped Tin Oxide - FTO cathode) with a compact layer of an n-type semiconductor material (e.g. a compact Ti0₂ layer); b) forming a layer (optionally meso superstructured layer as described herein) of a perovskite semiconductor material on said compact layer,

c) optionally surface sensitizing said compact layer and/or said layer of perovskite material with at least one sensitizing agent;

e) forming a layer of a p-type material permeating the barrier layer and in contact with said layer of perovskite material; and

Preferably the barrier layer does not contact the compact layer of n-type semiconductor material.

Preferably the perovskite layer separates the compact layer of n-type semiconductor material and the p-type material.

Typically one or more of the layers of the heteroj unctions of the invention (including both inverted and non-inverted embodiments as herein described) may be deposited by spin-coating of suitable solutions or dispersions of the relevant material for any given layer onto its respective substrate. Suitable spin-coating techniques and equipment, solvents, and parameters such as rotation speeds are within the knowledge of the skilled person. In spin-coating methods the thickness / of any layer deposited is inversely proportional to the square root of the angular velocity ω , i.e. t « (ω)^{ι 2}. Thus, all other factors (e.g. speed of rotation and duration of coating) being equal, the speed of rotation (and hence the angular velocity) can be varied as appropriate to achieve a desired thickness. Alternatively the angular velocity may be maintained at a constant value and the concentration of the solution employed for spin-coating may be varied in order to achieve a desired thickness, with the thickness being approximately linear in dependence upon the solution concentration.

By way of illustration, spin-coating of hole-transporting material using a speed of about 2000 to 3000 rpm for about 45 seconds (e.g. 30 to 60 seconds) results in a 300 to 400 nm layer thick of hole transporting material when a spin-coating solution having about 10 wt.% hole transporter is used; if the concentration is reduced to about 8 wt.% the layer thickness is about 240 to 320 nm; at about 6 wt% the thickness is 180 to 240 nm; at about 4 wt% the thickness is about 120 to 160 nm; at about 2 wt% the thickness is about 60 to 80 nm; and at about 1 wt% the thickness is about 30 to 40 nm.

In selecting a solution concentration and conditions for spin-coating it should be remembered that the amount of hole-transporting material deposited must be sufficient to achieve good conductivity and also minimise the available shunting pathways, therefore in the illustrative case where a buffer layer of thickness 50 nm is employed, the hole transporting material layer should also have a thickness of at least about 50 nm in order to achieve sufficient continuity and pore- filling, and therefore a solution of about 1.5 wt% or more, preferably at least about 2 wt% or 2.5 wt% of hole-transporting material should be used for spin-coating of the hole-transporting material layer at a speed of 2000 to 3000 rpm. The skilled person will readily be able to determine the spin-coating conditions required to achieve any other desired level of thickness having regard to the required relationships between layer thicknesses.

The surface sensitizing of the layer (e.g. compact layer, meso-superstructured layer or porous layer) of perovskite and/or n-type semiconductor material is optionally achieved by surface absorption of the sensitizing agent. This sensitizing agent may be absorbed by contact of the surface with a solution of the desired sensitizing agent. The addition of the sensitizing agent or agents may occur before the formation of the barrier layer and/or after the formation of that layer.

Preferably no sensitizing agent is employed, but rather the perovskite material and/or p-type material are selected whereby to be capable of absorbing light, e.g. visible light, so as to act as a sensitizer. The term "hybrid" ("hybrid solar cell") is typically employed to refer to a combination of organic and inorganic semiconductors and that meaning is adopted herein. In case of perovskite solar cells such as those of the invention, the crystalline perovskites may themselves be described as "hybrid" in nature due to their containing both inorganic (metal and halide) and organic

(ammonium cation) components. Similarly, a solar cell containing an inorganic perovskite material and an organic p-type material may also be termed "hybrid".

In a further aspect, the invention provides a method of stabilising and/or improving the power conversion efficiency of a solid-state heteroj unction comprising comprising a p-type material in contact with a perovskite material under anaerobic conditions, said method comprising providing a permeable barrier layer comprising at least one insulating material between the perovskite material and a cathode.

The use of the term "electrolyte" is not entirely consistent in the prior art, with some authors using the term "electrolyte" to include non-ionic charge transporting species such as molecular hole transporters. The general convention, however, which is adopted herein, is that the term "electrolyte" refers to a medium through which charge is transported by the movement of ions. Thus, a polymer or gel electrolyte is distinct from the hole transporters employed in all aspects of the present invention because the former relies on the movement of ions to carry charge, while the latter conducts by passage of electrons. Thus, the heteroj unctions, solar cells and related aspects of the present invention relate to non-electrolyte devices in that they preferably do not contain any "electrolyte", being any species which conducts by ion movement, but rather contain a solid-state "hole transporter" which conducts electrons.

Hole transport by the use of electron conductors rather than ionic movement within electrolytes is believed to offer significant potential benefit in the long term stability of heterojunction devices because the dye molecules are regenerated orders of magnitude faster using molecular hole transporters and thus spend a much shorter proportion of their time in their less stable charged form.

The heterojunctions of the invention, as well as those used with or generated by alternative aspects of the invention are light sensitive and as such may include at least one light sensitizing agent (sensitizer). Perovskites themselves are able to act as sensitizers and hence normally no further sensitizing agent will be necessary. However, if desired, one or more additional sensitizing agent besides the perovskite may be included.

Where present, the sensitizing agent may be one or more dyes or any material which generates an electronic excitation as a result of photon absorption and which is capable of electron injection into the n-type and/or perovskite (e.g. n-type perovskite) material. The most commonly used light sensitising materials in electrolytic DSCs are organic or metal-complexed dyes. These have been widely reported in the art and the skilled worker will be aware of many existing sensitizers, all of which are suitable in all appropriate aspects of the invention. Common categories of organic dye sensitizers are indolene based dyes, which are discussed, for example, in Horiuchi et al. J Am. Chem. Soc. 126 12218-12219 (2004), which is hereby incorporated by reference; ruthenium metal complexes, particularly those having two bipyridyl coordinating moieties, which are discussed in many published documents including, for example, uang et al. Nano Letters 6 769-773 (2006), Snaith et al. Angew. Chem. Int. Ed. 44 6413-6417 (2005), Wang et al. Nature Materials 2, 402-498 (2003), Kuang et al. Inorganica Chemica Acta 361 699-706 (2008), and Snaith et al. J Phys, Chem. Lett. 1 12 7562-7566 (2008), the disclosures of which are hereby incorporated herein by reference, as are the disclosures of all material cited therein;

Metal-Phthalocyanine complexes such as zinc phthalocyanine PCH001 , the synthesis and structure of which is described by Reddy et al. (Angew. Chem. Int. Ed. 46 373-376 (2007)), the complete disclosure of which (particularly with reference to Scheme 1), is hereby incorporated by reference; Metal-Porphyrin complexes, Squarine dyes, Thiophene based dyes, fluorine based dyes, molecular dyes and polymer dyes. Examples of Squarine dyes may be found, for example in Burke et al., Chem. Commun. 2007, 234, and examples of polyfluorene and polythiothene polymers in McNeill et al., Appl. Phys. Lett. 2007, 90, both of which are incorporated herein by reference.

It is envisaged that, where a dye sensitizer is employed in the heterojunctions herein described, only a single sensitizing agent will be used. However, two or more dye sensitizers may nevertheless be used. For example, all aspects of the present invention are suitable for use with co-sensitisation using a plurality of (e.g. at least 2, such as 2, 3, 4 or 5) different dye sensitizing agents. If two or more dye sensitizers are used, these may be chosen such that their respective emission and absorption spectra overlap. In this case, resonance energy transfer (RET) results in a cascade of transfers by which an electron excitation steps down from one dye to another of lower energy, from which it is then injected into the n-type and/or perovskite (e.g. n-type perovksite) material. However, it is preferable that the emission and absorption spectra of the individual dyes do not overlap to any significant extent. This ensures that all dye sensitizers are effective in the injection of electrons into the n-type and/or perovskite (e.g. n-type perovskite) material. Where two or more dye sensitizers are used, these will preferably have complimentary absorption characteristics. Some complimentary pairings include, for example, the near-infra red absorbing zinc phthalocyanine dyes referred to above in combination with indoline or ruthenuim-based sensitizers to absorb the bulk of the visible radiation. As an alternative, a polymeric or molecular visible light absorbing material may be used in conjunction with a near IR absorbing dye, such as a visible light absorbing polyfluorene polymer with a near IR absorbing zinc phthalocyanine or squarine dye.

In all aspects of the present invention a solid state hole transporter is a key constituent, since this forms the p-type material of the heterojunction. The hole transporter will preferably be a molecular p-type material rather than an inorganic material such as a salt, and more preferably will be an organic molecular material. Suitable materials will typically comprise an extended pi- bonding system through which charge may readily pass. Suitable materials will also preferably be amorphous or substantially amorphous solids rather than being crystalline at the appropriate working temperatures (e.g. around 30-70°C). The organic hole-transporter would preferably have a high energy HOMO to LUMO transition, rendering its predominant function dye- regeneration and hole-transport. However, it may optionally have a narrow HOMO to LUMO transition, with its additional function being to absorb solar light, and subsequently transfer an electron to the n-type and/or perovskite (e.g. n-type perovskite) material, or its excited state energy to a dye molecule tethered to the n-type and/or perovskite (e.g. n-type perovskite) material surface. The then excited dye molecule would subsequently transfer an electron to the n- type and/or perovskite (e.g. n-type perovskite) material and the hole to the hole-transporter, as part of the photovoltaic conversion process. According to a preferred embodiment, the solid state hole transporter is a material comprising a structure according to any of formulae (tl) , (til), (till), (tlV) and/or (tV) below:

in which N, if present, is a nitrogen atom;

n, if applicable, is in the range of 1-20;

A is a mono-, or polycyclic system comprising at least one pair of a conjugated double bond (-

C=C-C=C-), the cyclic system optionally comprising one or several heteroatoms, and optionally being substituted, whereby in a compound comprising several structures A, each A may be selected independently from another A present in the same structure (til - tV);

each of A₁-A4, if present, is an A independently selected from the A as defined above;

v in (til) recites the number of cyclic systems A linked by a single bond to the nitrogen atom and is 1 , 2 or 3;

(R)w is an optional residue selected from a hydrocarbon residue comprising from 1 to 30 carbon atoms, optionally substituted and optionally comprising 1 or several heteroatoms, with w being 0, 1 or 2 provided that v + w does not exceed 3, and, if w = 2, the respective Rwi or bei ng the same or different; Ra represents a residue capable, optionally together with other Ra present on the same structure (tl-tV), of decreasing the melting point of an organic compound and is selected from a linear, branched or cyclic alkyl or a residue comprising one or several oxygen atoms, wherein the alkyl or the oxygen comprising residue is optionally halogenated;

x is the number of independently selected residues Ra linked to an A and is selected from 0 to a maximum possible number of substituents of a respective A, independently from the number x of other residues Ra linked to another A optionally present;

with the proviso that per structure (tl-tV) there is at least one Ra being an oxygen-containing residue as defined above; and, if several Ra are present on the same structure (I-V), they are the same or different; and wherein two or more Ra may form an oxygen-containing ring;

Rp represents an optional residue enabling a polymerisation reaction with compounds comprising structure (tl - tV) used as monomers, and/or a cross-linking between different compounds comprising structures (tl - tV);

z is the number of residues Rp linked to an A and is 0, 1, and/or 2, independently from the number z of other residues Rp linked to another A optionally present;

Rp may be linked to an N-atom, to an A and/or to a substituent Rp of other structures according (tl - tV), resulting in repeated, cross-iinked and/or polymerised moieties of (tl - tV);

(R^{a p})x/_z and (Ri_-4 ^{a p})_x/z , if present, represent independently selected residues Ra and Rp as defined above.

Preferably, the charge transporting material comprises compounds having the structures (tl) - (tV).

General reference to the several structures, such as in the references "(tl-tV)", "(tVII-tXVI)", or "A 1 -A4", for example, means reference to any one selected amongst (tl), (til), (till), (tlV), or (tV), any one selected amongst (tVII), (tVIII), (tlX), (tX), (tXI), (tXII), (tXIII), (tXIV), (tXV) or (tXVI), or any one selected amongst Ai, A₂, A;, or A₄, respectively. In addition, in the charge transporting material for use in the invention, for example, different compounds of structures (tl- tV) may be combined and, if desired cross-linked and/or polymerised. Similarly, in any structure (tl-tV), different structures for A may be selected independently, for example from (tVII-tXVI).

According to a preferred embodiment, the organic charge transporting material of the device of the invention comprises a structure according to formula (tVI):

in which Ral, Ra2 and Ra3 and xl , x2 and x3 are defined, independently, like Ra and x, respectively, above;

Rpl , Rp2 and Rp3 and zl , z2 and z3 are defined, independently, like Rp and z, respectively, above. Formula (tVI) thus represents a specimen of formula (til) above, in which v is 3, and in which R(w) is absent.

Preferably, A is a mono- or polycyclic, optionally substituted aromatic system, optionally comprising one or several heteroatoms. Preferably, A is mono-, bi- or tricyclic, more preferably mono-, or bicyclic. Preferably, if one or more heteroatoms are present, they are independently selected from O, S, P, and/or N, more preferably from S, P and/or N, most preferably they are N- atoms.

According to a preferred embodiment, A is selected from benzol, naphthalene, indene, fluorene, phenanthrene, anthracene, triphenylene, pyrene, pentalene, perylene, indene, azulene, heptalene, biphenylene, indacene, phenalene, acenaphtene, fluoranthene, and heterocyclic compounds such as pyridine, pyrimidine, pyridazine, quinolizidine, quinoline, isoquinoline, quinoxaline, phtalazine, naphthyridine, quinazoline, cinnoline, pteridine, indolizine, indole, isoindole, carbazole, carboline, acridine, phenanthridine, 1 , 10-phenanthroline, thiophene, thianthrene, oxanthrene, and derivatives thereof, each of which may optionally be substituted.

According to a preferred embodiment, A is selected from structures of formula (tVII-tXIV) given below:

in which each of Z¹, Z² and Z³ is the same or different and is selected from the group consisting of O, S, SO, SO₂, NR¹, N⁺CR' X^1"), C(R²)(R³), Si(R^2')(R^3') and P(0)(OR⁴), wherein R¹, R^1' and R¹ are the same or different and each is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxy groups, alkoxyalkyl groups, aryl groups, aryloxy groups, and aralkyl groups, which are substituted with at least one group of formula -N⁺(R⁵)₃ wherein each group R⁵ is the same or different and is selected from the group consisting of hydrogen atoms, alkyl groups and aryl groups, R², R³, R² and R³ are the same or different and each is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxy groups, halogen atoms, nitro groups, cyano grcups, alkoxyalkyl groups, aryl groups, aryloxy groups and aralkyl groups or R² and R³ together with the carbon atom to which they are attached represent a carbonyl group, and R⁴ is selected from the group consisting of hydrogen atoms, alkyl groups, haloalkyl groups, alkoxyalkyl groups, aryl groups, aryloxy groups and aralkyl groups.

Preferred embodiments of, structure (tXV) for A may be selected from structures (tXVI) and (tXVIa) below:

(tXVI), (tXVIa). Preferably, in any structure of (tl-tV) all A are the same, but differently substituted. For example, all A are the same, some of which may be substituted and some of which are not. Preferably, all A are the same and identically substituted.

Any A may be substituted by other substituents than Ra and/or Rp. Other substituents may be selected at the choice of the skilled person and no specific requirements are indicated herein with respect to them. Other substituents may thus correspond to (R)w in (til) defined above. Other substituents and R(w) may generally be selected from linear, branched or cyclic hydrocarbon residues comprising from 1 to 30 carbon atoms, optionally substituted and optionally comprising 1 or several heteroatoms, for example. The hydrocarbon may comprise C-C single, double or triple bonds. For example, it may comprise conjugated double bonds. For example, optional other residues on A may be substituted with halogens, preferably -F and/or -CI, with -CN or -N0₂, for example.

One or more carbon atoms of other substituents of A may or may not be replaced by any heteroatom and/or group selected from the group of -0-, -C(O)-, -C(0)0-, -S-, -S(O)-, S0₂-, - S(0)₂0-, -N=, -P-, -NR'-, -PR'-, -P(0)(OR')-, -P(0)(OR')0-, -P(0)(NR'R')-, -P(0)(NR'R')0-, P(0)(NR'R')NR'-, -S(0)NR\ and -S(0)₂NR^>, with R' being H, a C1-C6 alkyl, optionally partially halogenated.

According to a preferred embodiment, any A may optionally be substituted with one or several substituents independently selected from nitro, cyano, amino groups, and/or substituents selected from alkyl, alkenyl, alkynyl, haloalkyl, alkoxy, and alkoxyalkyl groups, including substituted substituents. Alkyl, alkenyl, alkynyl, haloalkyl, alkoxy and alkoxyalkyl are as defined below.

Preferably, further residues optionally present on A, such as R(w) in (til), for example, are selected from C4-C30 alkenes comprising two or more conjugated double bonds.

Ra may be used as a residue capable of controlling the melting point of an organic, charge- transporting compound. The reference with respect to the ability to control the melting point is the same charge transporting material devoid of the at least one residue Ra. In particular, the function of Ra is to provide a charge transporting material that adopts the desired phase at the temperatures indicated herein. The adjustment of the melting point to obtain the desired characteristics in the temperature ranges indicated above may be brought about by a single residue Ra or a combination of identical or different residues Ra, present in any of the structures (tl)-(tV).

At least one linear, branched or cyclic residue containing one or several oxygen atoms may be used for lowering the melting point, and thus the absence of such residues or alternative residues may be used to correspondingly raise melting points, thus obtaining the desired characteristics. Other residues, include for example alkyls as defined below, may assist in the adjustment of the melting point and/or phase characteristics.

Ra may be halogenated and/or perhalogenated in that one, several or all H of the residue Ra may be replaced with halogens. Preferably, the halogen is fluorine.

If Ra is oxygen containing compound, it is preferably a linear, branched, or cyclic saturated Cl- C30 hydrocarbon comprising 1 - 15 oxygen atoms, with the proviso that the number of oxygen atoms does preferably not exceed the number of carbons. Preferably, Ra comprises at least 1.1 to 2 as much carbon as oxygen atoms. Preferably, Ra is a C2 - C20, saturated hydrocarbon comprising 2-10 oxygen atoms, more preferably a C3-C10 saturated hydrocarbon comprising 3-6 oxygen atoms.

Preferably, Ra is linear or branched. More preferably Ra is linear.

Preferably, Ra is selected from a C1-C30, preferably C2-C15 and most preferably a C3-C8 alkoxy, alkoxyalkyl, alkoxyalkoxy, alkylalkoxy group as defined below.

Examples of residues Ra may independently be selected from the following structures:

with A indicating any A in formula (tl-V) above. Any Ra present may be linked to a carbon atom or a heteroatom optionally present in A. If Ra is linked to a heteroatom, it is preferably linked to a N-atom. Preferably, however, any Ra is linked to a carbon atom. Within the same structure (tl-tV), any Ra may be linked to a C or a heteroatom independently of another Ra present on the same A or in the same structure.

Preferably, every structure A, such as A, Aj, A2, A3 and A4, if present in formulae (tl-tV) above comprises at least one residue Ra. For example, in the compound according to structure (tl-tV), at least one structure A comprises an oxygen containing residues Ra as defined above, whereas one or more other and/or the same A of the same compound comprise an aliphatic residue Ra, for example an alkyl group as defined below, preferably a C2-C20, more preferably C3-C15 alkyl, preferably linear.

The following definitions of residues are given with respect to all reference, to the respective residue, in addition to preferred definitions optionally given elsewhere. These apply specifically to the formulae relating to hole transporters (tN formulae) but may optionally also be applied to all other formulae herein where this does not conflict with other definitions provided.

An alkoxyalkoxy group above is an alkoxy group as defined below, which is substituted with one or several alkoxy groups as defined below, whereby any substituting alkoxy groups may be substituted with one or more alkoxy groups, provided that the total number of 30 carbons is not exceeded.

An alkoxy group is a linear, branched or cyclic alkoxy group having from 1 to 30, preferably 2 to 20, more preferably 3-10 carbon atoms.

An alkoxyalkyl group is an alkyl group as defined below substituted with an alkoxy group as defined above.

An alkyl group is a linear, branched and/or cyclic having from 1-30, preferably 2-20, more preferably 3-10, most preferably 4-8 carbon atoms. An alkenyl groups is linear or branched C2- C30, preferably C2-C20, more preferably C3-C10 alkenyl group. An alkynyl group is a linear or branched C2-C30, preferably C2-C20, more preferably C3-C10 linear or branched alkynyl group. In the case that the unsaturated residue, alkenyl or alkynyl has only 2 carbons, it is not branched. A haloalkyl groups above is an alkyl groups as defined above which is substituted with at least one halogen atom.

An alkylalkoxy group is an alkoxy group as defined above substituted with at least one alkyl group as defined above, provided that the total number of 30 carbons is not exceeded.

The aryl group above and the aryl moiety of the aralkyl groups (which have from 1 to 20 carbon atoms in the alkyl moiety) and the aryloxy groups above is an aromatic hydrocarbon group having from 6 to 14 carbon atoms in one or more rings which may optionally be substituted with at least one substituent selected from the group consisting of nitro groups, cyano groups, amino groups, alkyl groups as defined above, haloalkyl groups as defined above, alkoxyalkyl groups as defined above and alkoxy groups as defined above.

The organic charge transporting material may comprise a residue Rp linked to an A. According to a preferred embodiment, Rp is selected from vinyl, allyl, ethinyl, independently from any other Rp optionally present on the A to which it is linked or optionally present on a different A within the structures (tl) and/or (til).

The charge transporting material comprised in the device of the invention may be selected from compounds corresponding to the structures of formulae (tl-tV) as such. In this case, n, if applicable, is 1 and the charge transporting material comprises individual compounds of formulae (tl-tV), or mixtures comprising two cr more different compounds according formulae (tl-tV).

The compounds of structures (tl-tV) may also be coupled (e.g. dimerised), olilgomerised, polymerized and/or cross-linked. This may, for example, be mediated by the residue Rp optionally present on any of the structures (tl-tV). As a result, oligomers and/or polymers of a given compound selected from (tl-tV) or mixtures of different compounds selected from structures (tl-tV) may be obtained to form a charge transporting material. Small n is preferably in the range of 2-10.

A particularly preferred organic molecular hole transporter contains a spiro group to retard crystallisation. A most preferred organic hole transporter is a compound of formula tXVII below, and is described in detail in Snaith et al. Applied Physics Letters 89 2621 14 (2006), which is herein incorporated by reference.

Formula tXVII

wherein R is alkyl or O-alkyl, where the alkyl group is preferably methyl, ethyl, n-propyl, iso- propyl, n-butyl, sec-butyl or tert-butyl, preferably methyl. Particularly preferably, R is O- methyl.

In an alternative embodiment, the hole transporter (p-type material) may be an inorganic material. As the heterojunctions of the invention are solid-state heterojunctions, such inorganic materials are provided in solid form (e.g. in the form of a compact layer) rather than as electrolyte solutions. Suitably, the inorganic p-type material may comprise or consist of an inorganic hole transporter comprising an oxide of nickel, vanadium, copper or molybdenum; Cul, CuBr, CuSCN, Cu₂0, CuO or CIS; a perovskite; amorphous Si; a p-type group IV semiconductor, a p-type group III-V semiconductor, a p-type group II-VI semiconductor, a p- type group I-VII semiconductor, a p-type group IV-VI semiconductor, a p-type group V-VI semiconductor, and a p-type group II -V semiconductor, which inorganic material may be doped or undoped. CuSCN is preferred. The p- type layer may be a compact layer of said inorganic- hole transporter. It is common practice in the art to generate heteroj unctions, especially for optical applications, from a mesoporous layer of the n-type material so that light can interact with the junction at a greater surface than could be provided by a flat junction. In the present case, this mesoporous layer may be conveniently generated by sintering of appropriate semiconductor particles using methods well known in the art and described, for example in Green et al. (J. Phys. Chem. B 109 12525-12533 (2005)) and Kay et al. (Chem. Mater. 17 2930-2835 (2002)), which are both hereby incorporated by reference. With respect to the surface coatings, where present, these may be applied before the particles are sintered into a film, after sintering, or two or more layers may be applied at different stages.

Typical particle diameters for the semiconductors will be dependent upon the application of the device, but might typically be in the range of 5 to 1 OOOnm, preferably 10 to 100 nm, more preferably still 10 to 30 nm, such as around 20 nm. Surface areas of 1-1000 m²g^"' are preferable in the finished film, more preferably 30-200 m g^" , such as 40 - 100 m g^" . The film will preferably be electrically continuous (or at least substantially so) in order to allow the injected charge to be conducted out of the device. The thickness of the film will be dependent upon factors such as the photon-capture efficiency of the photo-sensitizer, but may be in the range 0.05 - 100 μπι, preferably 0.5 to 20 μπι, more preferably 0.5 -10 μηι, e.g. 1 to 5 μιη. In one alternative embodiment, the film is planar or substantially planar rather than highly porous, and for example has a surface area of 1 to 20 m²g^_1 preferably 1 to 10 m²g^"'. Such a substantially planar film may also or alternatively have a thickness of 0.005 to 5 μπι, preferably 0.025 to 0.2 μηι, and more preferably 0.05 to 0.1 μιτι.

Where the n-type and/or perovskite (e.g. n-type perovskite) material is surface coated, materials which are suitable as the coating material (the "surface coating material") may have a conduction band edge closer to or further from the vacuum level (vacuum energy) than that of the principal n-type semiconductor material, depending upon how the property of the material is to be tuned. They may have a conduction band edge relative to vacuum level of at around -4.8 eV, or higher (less negative) for example -4.8 or -4.7 to -1 eV, such as -4.7 to -2.5 eV, or -4.5 to -3 eV

Suitable coating materials, where present, include single metal oxides such as MgO, A1₂0₃, ZrO, ZnO, Hf0₂, Ti0₂, Ta₂0_5, Nb₂0₅, W0₃, W₂0₅, ln₂0₃, Ga₂0₃, Nd₂0₃, Sm₂0₃, La₂0₃, Sc₂0₃, Y₂0₃, NiO, Mo0₃, PbO, CdO and/or MnO; compound metal oxides such as Zn_xTi_yO_z, ZrTi0₄, ZrW₂Os, SiA10_3i5> S^AlOs^ SiTi0₄ and/or AlTi0_5; doped metal oxides such as any of the single or compound metal oxides indicated above doped with at least one of Al, F, Ge, S, N, In, Mg, Si, C, Pb and/or Sb; carbonates such as Cs₂C₅; sulphides such as PbS, CdS, CuS; selenides such as PbSe, CdSe; telurides such as CdTe; nitrides such as TiN; and/or multicompound

semiconductors such as CIGaS₂. Some suitable materials are discussed in Gratzel (Nature 414 338-344 (2001)). The most preferred surface coating material is MgO.

Where present, the coating on the n-type and/or perovskite (e.g. n-type perovskite) material will typically be formed by the deposition of a thin coating of material on the surface of the n-type and/or perovskite (e.g. n-type perovskite) semiconductor film or the particles which are to generate such a film. In most cases, however, the material will be fired or sintered prior to use, and this may result in the complete or partial integration of the surface coating material into the bulk semiconductor. Thus although the surface coating may be a fully discrete layer at the surface of the semiconductor film, the coating may equally be a surface region in which the semiconductor is merged, integrated, or co-dispersed with the coating material.

Since any coating on the n-type and/or perovskite (e.g. n-type perovskite) material may not be a fully discrete layer of material, it is difficult to indicate the exact thickness of an appropriate layer. The appropriate thickness will in any case be evident to the skilled worker from routine testing, since a sufficiently thick layer will retard electron-hole recombination without undue loss of charge injection into the n-type and/or perovskite (e.g. n-type perovskite) material. Coatings from a monolayer to a few nm in thickness are appropriate in most cases (e.g. 0.1 to 100 ran, preferably 1 to 5 nm).

The bulk or "core" of the n-type and/or perovskite (e.g. n-type perovskite) material in all embodiments of the present invention may be essentially pure semiconductor material, e.g.

having only unavoidable impurities, or may alternatively be doped in order to optimise the function of the heteroj unction device, for example by increasing or reducing the conductivity of the n-type and/or perovskite (e.g. n-type perovskite) semiconductor material or by matching the conduction band in the n-type and/or perovskite (e.g. n-type perovskite) semiconductor material to the excited state of the chosen sensitizer.

Thus the n-type and/or perovskite (e.g. n-type perovskite) semiconductor and oxides such as Ti0₂, ZnO, Sn0₂ and WO referred to herein (where context allows) may be essentially pure semiconductor (e.g. having only unavoidable impurities). Alternatively they may be doped throughout with at least one dopant material of greater valency than the bulk (to provide n-type doping) and/or may be doped with at least one dopant material of lower valency than the bulk (to give p-type doping), n-type doping will tend to increase the n-type character of the

semiconductor material while p-type doping will tend to reduce the degree of the natural n-type state (e.g. due to defects).

Such doping may be made with any suitable element including F, Sb, N, Ge, Si, C, In, InO and/or Al. Suitable dopants and doping levels will be evident to those of skill in the art. Doping levels may range from 0.01 to 49% such as 0.5 to 20%, preferably in the range of 5 to 15%. All percentages indicated herein are by weight where context allows, unless indicated otherwise.

Preferably the heteroj unction devices of the invention further comprise a flat compact layer of an n-type material as an electron sensitive contact forming a planar heterojunction with the perovskite film. Preferably in such embodiments a flat compact layer of Ti0₂ typically of about 50nm thickness is employed as an electron selective contact forming a planar heterojunction with the perovskite film. Where a compact Ti0₂ n-type layer is provided, it is particularly preferred that this is doped with A1₂0₃ in an amount of less than 10 mol%, preferably less than 5 mol%, particularly preferably less than 1.5 mol%, e.g. 0.2 to 0.5 mol%. 0.3 mol% is particularly preferred.

Thus in a preferred embodiment, the present invention provides a solid-state heterojunction comprising a p-type material, a perovskite material, a flat compact layer of an n-type material, a cathode, and a permeable barrier layer comprising at least one insulating material, wherein the p- type material and the perovskite material are in contact, the permeable barrier layer comprising at least one insulating material is located between the perovskite material and the cathode, and the flat compact layer of an n-type material and the perovskite material are in contact.

Preferably the flat compact layer of an n-type material is located between the perovskite material and an anode.

The various aspects of the present invention include the use of an insulating barrier layer to inhibit the light induced drop in shunt resistance (and thus drop in efficiency) in a heterojuncti on such as a solar cell. Such a use will typically be in the absence of oxygen, since this is where the photo-induced drop in efficiency is most significant. This use thus applies particularly to any of the devices of the present invention, including those which are encapsulated, such as in the absence or substantial absence of oxygen.

The use of the present invention will preferably be to maintain the efficiency of a heteroj unction at no less than 75%, preferably no less than 85% and more preferably no less than 95% of its initial efficiency for a period of no less than 20 minutes, preferably no less than 1 hour and more preferably no less than 12 hours under full sun illumination in the substantial absence of oxygen. As used herein, "substantial absence of oxygen ' may be taken to indicate a level of oxygen of less than 10 ppm, preferably less than lppm in the surrounding atmosphere.

In one embodiment, any of the heteroj unctions or optoelectronic devices (e.g. solar cells) of the present invention may be encapsulated. Such encapsulation will be such that the heteroj unctions or devices are maintained in the substantial absence of oxygen.

The step of forming an insulating barrier layer may be performed by any method suitable for generating a permeable insulating layer from the material to be employed. Such methods include formation of a polymeric porous sheet and placing that between the perovskite material and the cathode, depositing the material by evaporation, spraying (e.g. spray pyrolysis) or sputter deposition, or by forming a paste of insulating particles, followed by heating/sintering. Suitable methods are described in the Examples below.

The invention is illustrated further in the following non-limiting examples. Example 1

The structure of standard perovskite meso-superstructured solar cell is shown in figure 1 a. In the SEM picture is clearly shown the thick (350 nm) spiro-OMeTAD capping layer required to avoid detrimental shunting pathways. In figure 1 b is shown the structure of a device with the porous A1₂0₃ capping layer. In presence of the buffer layer, the spiro-OMeTAD layer is much thinner (100-130 nm). Example 2

Figure 3 demonstrates the improvement in fill factor achieved in devices according to the invention. A thin A1203 buffer layer is employed (Fig 3a). Figure 1 shows the device performance parameters as extracted from the current- voltage (JV) curves that we obtained by spin coating the buffer layers starting from two different dilutions.

The overall power conversion efficiency with the buffer layers is on average 2-3% higher compared to the efficiency of the standard cells without the buffer layer.

The improvement in the FF is even more clear in the JV curve of the best devices produced (Fig. 4). In this graph is evident that the buffer layer increase the fill factor of the cell keeping the same short-circuit current and open-circuit voltage.

Figures 4b,c,d show the other parameters extracted from the JV curves. The photocurrent generated by the cells is kept unchanged, this suggest that the photogeneration is not affected by the presence of the buffer layer.

The open circuit voltage is somewhat lower (less than 0.05V) when the buffer layer is in between the perovskite and the HTM.

The fill factor is the mostly affected parameter. It increases significantly in presence of the buffer layer.

The same trends were observed in 3 different batches.

An aging test was performed to check the effect of the buffer layer. The devices were sealed with an epoxy resin and then subjected to 1 sun of continuous illumination up to 350 hours. The results are reported in figure 5.

The solar cells without the buffer layer show ε rapid deterioration of the performances in the first 200 hours. This is believed to be due to a rapid drop of both the short circuit current density and the open circuit voltage. On the contrary cells with the buffer layer result more stable during time. The most robust one show an efficiency drop of only 5% after 350 hours of continuous illumination with unfiltered 100 W/cm2 intense light (Fig 5).

The change in the photocurrent cannot be ascribed to a degradation of the perovskite absorber material since the absorption spectra (Fig. 7) demonstrated good optical stability under constant illumination under full spectrum simulated sunlight.

We are confident that cells with buffer layer, with a proper sealing, could reach up to 1000 hours of continuous illumination without losing significantly their power conversion efficiency.

During the aging test we monitored not only the JV parameters, but also the JV dark currents. The results obtained are shown in figure 8a for MSSCs with buffer layer and in figure 8b for standard MSSCs. In these figures are shown the statistics obtained from 8 different solar cells, 4 with buffer layer and the other 4 without the buffer layer.

A bigger change in the shape is observed at low voltages (between 0 to 0.8V) in control cells as compared to the cells with the buffer layers. Therefore it can be inferred that the shunt resistance has reduced more in the control cells than in the cells that incorporate the A1₂0₃ buffer layer.

Without wishing to be bound by theory, it is believed that the A1₂0₃ buffer layer protects the perovskite in presence of pin-holes defects that originate during the deposition of the HTM and from electrode migration (metal filament growth for instance) that could arise under the aging process.

Example 3

Tandem solar cells made from silicon solar cells and perovskite cells are ideal structures for low cost high efficiency solar cells. 4-terminal tandems require a semi-transparent top cell, e.g. a semi-transparent perovskite cell. The metal electrode (typically the cathode) in conventional perovskite solar cells is opaque. In order to have a semi-transparent perovskite cell a transparent top electrode is needed. The parasitic absorption in the top cell should also be reduced, which mostly occurs in the hole transporter material layer. Reducing the thickness of the hole transporter layer would increase the number of pinholes in the layer and subsequently the shunting paths. A mesoporous buffer layer infiltrated with spiro-OMeTAD hole transporting material between the perovskite and the metal electrode was therefore prepared which allowed reduction in the amount of the spiro-OMeTAD in the cell without the decrease in shunt resistance which would otherwise result from introducing pinholes to the hole transport layer. This was compared to cells without a buffer layer.

Solar cells were fabricated on fluorine doped tin oxide (FTO) coated glass substrates (Pilkington, TEC 15). Initially FTO was patterned by etching with zinc powder and HCl. The substrates were then washed with Hellmanex and deionized water, acetone, and ethanol, followed by oxygen plasma etching for 10 minutes.

Compact layers were deposited by spin-coating slightly acidic solution of titanium isopropoxide in anhydrous ethanol (0.254 M). Substrates were sintered at 500 °C for 45 min.

A mesostructured A1₂0₃ layer was deposited by spin-coating a colloidal dispersion of ~20 nm AI2O3 nanoparticles at 8 wt % in isopropanol at 2500 rpm, followed by drying at 150°C for 30 min. Then the substrates were transferred to a nitrogen-filled glovebox and a solution of

CH3NH3I and PbCl₂ (3: 1 molar ratio) in dimethylformamide (40 wt %) was spin-coated on them at 2000 rpm, followed by drying at 50°C for 10 min and then annealing at 100^°C for 90 min.

Subsequently, diluted solution of iodopentafluorobenzene (IPFB) in chlorobenzene was spin coated on formed perovskite layers in order to passivate the perovskite surface.

The hole transport material (HTM) solution was prepared using 10 wt % 2,2',7,7'-tetrakis(N,N- dipmethoxyphenylamine)9,9'-spirobifluorene (spiro-OMeTAD) solution in anhydrous chlorobenzene and adding 80 mM tert-butylpyridine and 25 mM lithium- bis(trifluoromethanesulphonyl)imide then spin coated on samples without an A1₂0₃ buffer layer at 2000 rpm.

In cells with the buffer layer, the previous HTM solution was diluted with anhydrous

chlorobenzene to obtain 8.5 wt %, 8 wt %, 7 v t %, 6 wt %, 5.5 wt %, 5 wt %, 4 wt %, 2.5 wt % and 1 wt % spiro-OMeTAD solution to study devices performances changing the thickness o f the capping layer. The AI2O3 buffer layer was deposited in the glovebox by dynamic spin-coating a colloidal dispersion of ~20 nm A1₂0₃ nanoparticles at 2 wt % in anhydrous isopropanol at 3000 rpm for 45 s, followed by drying at 100°C for 10 min. Then the spiro-OMeTAD solutions with different concentrations were deposited by spin-coating at 2000 rpm on alumina buffer layers. Finally, devices were completed with thermal evaporation of gold electrodes, through a shadow mask in a chamber with pressure of ~10-6 bar.

Further samples with the structure of hole-only diodes were fabricated on FTO coated glass substrates. The 100 nm-thick mesostructured A1₂0₃ layer was deposited by spin-coating a colloidal dispersion of ~20 nm A1₂0₃ nanoparticles at 2 wt % in isopropanol at 3000 rpm, followed by drying at 150°C. Then spiro-OMeTAD solutions with different concentrations were spin-coated on alumina mesoporous layers. Finally a 50 nm-thick gold layer was thermally evaporated on the samples in a chamber with pressure of ~ 10-6 Torr.

Current density-voltage (J-V) measurements were carried out using a Keithley 2400 sourcemeter under AM 1.5 simulated sunlight at 100 mW.cm^"2 irradiance (ABET Technologies Sun 2000, calibrated with an NREL calibrated KG5 filtered Si reference cell). Solar cells were masked with a metal aperture for all the current voltage measurements. The active area of the device was 0.092 cm².

Reflectance and Transmittance measurements were carried out using a LAMBDA 1050

UV Vis/NIR Spectrophotometer. The absorbance of the films was calculated as (l-T-R)xlOO.

Figure 10 shows the absorbance spectra of alumina buffer layers on glass infiltrated with different concentrations of spiro-OMeTAD. In the 325 nm to 425 nm region, the absorbance of the layers is highly sensitive to the concentration of the spiro-OMeTAD, exhibiting a

proportional increase with increasing concentration of spiro-OMeTAD.

In order to have an idea about the optimized spiro-OMeTAD concentration, the conductivity of mesoporous alumina layers infiltrated with different concentrations of the spiro-OMeTAD were measured. As shown in Figure 1 1 except for very low concentrations of spiro-OMeTAD, the conductivity of films was not significantly affected by spiro-OMeTAD concentration. To reduce the thickness of the spiro-OMeTAD another important aspect would be the shunt leakage through the hole transport layer. To investigate that, hole only diodes were made with the structure of FTO, mesoporous alumina/spiro-OMeTAD and gold as described above and hole only I-V curves were measured. Figure 12 shows the slope of I-V cures in the linear region around zero as a function of spiro-OMeTAD concentration which represents the shunt leakage. Introducing the alumina buffer layer significantly reduced the shunting. Even a very thick spiro- OMeTAD flat layer had lower shunt resistance than most of the layers with the buffer layer. The shunt resistance of the 10 wt % flat spiro-OMeTAD layer was comparable to 2.5wt% spiro- OMeTAD with the buffer layer.

Finally full perovskite cells were made with spiro-OMeTAD concentration from 5 wt % to 10 wt %. Figure 13 shows the J-V curves. The main parameters (V_oc, J_sc , fill factor and efficiency) of the same curves are shown in Figure 14. The efficiencies of cells with buffer layers and spiro- OMeTAD concentration more than 6 wt% were equal to or higher than the standard cells without buffer layers.

Overall it was concluded that by using an alumina buffer layer, reducing the spiro-OMeTAD concentration to even half of the standard concentration did not significantly change the photovoltaic properties of the cell.

The results therefore showed that by introducing a buffer layer between the perovskite and the metal electrode it is possible to control and reduce the amount of spiro-OMeTAD in the cells which results in lower parasitic absorption without losing the solar cell performance.

Claims

Claims:

1. A solid-state heteroj unction comprising a p-type material, a perovskite material and a cathode, wherein the p-type material and the perovskite material are in contact, p-type material and the cathode are in contact and a permeable barrier layer comprising at least one insulating material is located between the perovskite material and the cathode.

2. A solid-state heterojunction as claimed in claim 1 wherein said permeable barrier layer is a substantially continuous capping layer.

3. A solid-state heterojunction as claimed in claim 1 or claim 2 wherein said permeable barrier layer covers substantially all of the area of overlap between the perovskite material and the cathode.

4. A solid-state heterojunction as claimed in any one of claims 1 to 3 wherein said permeable barter layer is porous, preferably mesoporous.

5. A solid-state heterojunction as claimed in any one of claims 1 to 3 wherein said insulating material is provided in the form of particles (e.g. beads) dispersed in the p-type material.

6. A solid-state heterojunction as claimed in claim 5 wherein said particles are nanoparticles (e.g. nanozeolites) having a particle size of 1 to 100 nm.

7. A solid-state heterojunction as claimed in any preceding claim wherein said insulating material comprises at least one insulating metal oxide.

8. A solid-state heterojunction as claimed in claim 7 wherein said insulating metal oxide is selected from A1₂0₃, Si0₂, ZrO, MgO, Hf0₂, Ta₂0₅, Nb₂0₅, Nd₂0₃, Sm₂0₃, La₂0₃, Sc₂0₃, Y₂0₃, NiO, Mo0₃, MnO, SiA10₃,5, Si₂A10₅,₅, SiTiC^ AlTi0₅, zeolites, and mixtures thereof.

A solid-state heteroj unction as claimed in claim 7 wherein said insulating metal oxide

10. A solid-state heterojunction as claimed in any preceding claim wherein said insulating material comprises at least one insulating polymer.

1 1. A solid-state heterojunction as claimed in any preceding claim wherein said perovskite material and said cathode are separated by a distance of no less than 1 nm at their closest point.

12. A solid-state heterojunction as claimed in any preceding claim wherein said permeable barrier layer has a thickness of 1 to 1000 nm.

13. A solid-state heterojunction as claimed in any preceding claim wherein said permeable barrier layer has a porosity of 10 to 90%.

14. A solid-state heterojunction as claimed in any preceding claim wherein said permeable barrier layer is composed of material having a resistivity of greater than 10⁹ Ω cm.

15. A solid-state heterojunction as claimed in any preceding claim, wherein said perovskite material is an organometal halide perovskite.

16. A solid-state heterojunction as claimed in claim 14 wherein said organometal halide perovskite is selected from among ammonium trihalogen plumbates (e.g. CH₃NH₃PbI₃, CH₃NH₃PbCl₃, CH₃NH₃PbF₃ and CH₃NH₃PbBr₃); mixed-halide ammonium trihalogen plumbates with general formula CH₃NH3Pb Hal I Jj. x[Hal2]_x, wherein [Hal l ] and [Hal2] are independently selected from among F, CI, Br and I and the pseudohalides, with the proviso that [Hall] and [Hal2] are non-identical, and wherein 0_<x <3;

CsSnX₃ perovskites wherein X is selected from among F, CI, Br and I, preferably I; organometal trihalide perovskites with the general formula (RNH₃)BX₃ where R is CH₃, C_nH_2n or C_nH_2n+], 2 <n <10, X is selected from among F, CI, Br and I, and B is Pb or Sn;

CH₃NH₃SnX₃ perovskites wherein X is selected from among F, CI, Br and I, preferably I; formamidinium-SnX₃ perovskites wherein X is selected from among F, CI, Br and I, preferably I;

and combinations thereof.

17. A solid-state heteroj unction as claimed in any preceding claim wherein the perovskite material is porous, preferably mesoporous.

18. A solid-state heterojunction as claimed in any preceding claim wherein said

heteroj unction is a mesosuperstructured heterojunction comprising a mesoscaled scaffold.

19. A solid-state heterojunction as claimed in claim 18 wherein said mesoscaled scaffold comprises mesoporous Al₂0₃ or mesoporous Ti0₂.

20. A solid-state heterojunction as claimed in any preceding claim wherein the p-type material is an organic hole-transporter, preferably a molecular, oligomeric or polymeric organic hole transporter.

21. A solid state heterojunction as claimed in claim 20 wherein said organic hole-transporter is at least one optionally oligomerised, polymerized and/or cross-linked compound of formula (tl) , (til), (till), (t!V) and/or (tV) below,

(til);

in which N, if present, is a nitrogen atom;

n, if applicable, is in the range of 1-20;

A is a mono-, or polycyclic system comprising at least one pair of a conjugated double bond (-C=C-C=C-), the cyclic system optionally comprising one or several heteroatoms, and optionally being substituted, whereby in a compound comprising several structures A, each A may be selected independently from another A present in the same structure (tl-tV);

each of A₁-A4, if present, is an A independently selected from the A as defined above; v in (til) recites the number of cyclic systems A linked by a single bond to the nitrogen atom and is 1 , 2 or 3;

(R)w is an optional residue selected from a hydrocarbon residue comprising from 1 to 30 carbon atoms, optionally substituted and optionally comprising 1 or several heteroatoms, with w being 0, 1 or 2 provided that v + w does not exceed 3, and, if w = 2, the respective Rwi or Rw₂ being the same or different;

Ra represents a residue capable, optionally together with other Ra present on the same structure (tl-tV), of decreasing the melting point of an organic compound and is selected from a linear, branched or cyclic alkyl or a residue comprising one or several oxygen atoms, wherein the alkyl and/or the oxygen comprising residue is optionally halogenated;

with the proviso that per structure (I-V) there is at least one Ra being an oxygen containing residue as defined above; and, if several Ra are present on the same structure (tl-tV), they are the same or different; and wherein two or more Ra may form an oxygen-containing ring;

Rp may be linked to an N-atom, to an A and/or to a substituent Rp of other structures according (tl - tV), resulting in repeated, cross-linked and/or polymerised moieties of (tl - tV);

(R^{a p})x/_z and (R]_-4 ^{a p})_x/z , if present, represent independently selected residues Ra and Rp as defined above;

22. A solid state heteroj unction as claimed in claim 20 or claim 21 wherein said organic hole- transporter is a compound of formula tXVII below:

(Formula tXVII) wherein R is C1 -C6 alkyl or C1-C6 O-alkyl.

23. A solid-state heteroj unction as claimed in any one of claims 1 to 19, wherein the p-type material is an inorganic hole-transporter, preferably CuSCN.

24. A solid-state heteroj unction as claimed in any preceding claim, wherein the perovskite material and/or n-type material has at least one surface-coating of a surface coating material having a conduction band edge closer to vacuum level and/or a higher band-gap than the perovskite material or n-type material.

25. A solid-state heteroj unction as claimed in any preceding claim wherein said

heteroj unction is sensitised by at least one sensitizing agent.

26. A solid-state heteroj unction as claimed in claim 25 wherein said sensitizing agent comprises a perovskite and/or at least one dye selected from a ruthenium complex dye, a metal- phthalocyanine complex dye, a metal-porphryin complex dye, a squaraine dye, a thiophene based dye, a fluorine based dye, a polymer dye, and mixtures thereof.

27. A solid-state heteroj unction as claimed in any preceding claim further comprising a flat compact layer of an n-type material in contact with the perovskite material, preferably wherein said flat compact layer of an n-type material is located between the perovskite material and an anode.

28. A solid-state heteroj unction as claimed in claim 27 wherein said flat compact layer of an n-type material comprises Ti0₂.

29. A solid-state heteroj unction as claimed in claim 27 wherein said flat compact layer of an n-type material consists of Ti0₂.

30. An optoelectronic device comprising at least one solid-state heteroj unction as defined in any of claims 1 to 29.

31. An optoelectronic device as claimed in claim 30 wherein said device is a solar cell or photo-detector.

32. An optoelectronic device as claimed in claim 30 or 31 wherein said device is encapsulated.

33. An optoelectronic device as claimed in any one of claims 30 to 32 wherein said solid- state heteroj unction is a top cell in a tandem junction.

34. An optoelectronic device as claimed in any one of claims 30 to 33 wherein said solid-state heteroj unction is a top cell in a multiple junction, said multiple junction further comprising a second solid-state heteroj unction as defined in any one of claims 1 to 29 as a bottom cell in said multiple junction.

35. An optoelectronic device as claimed in any one of claims 30 to 33 wherein said solid- state heteroj unction is a top cell in a tandem solar cell, said tandem solar cell further comprising a silicon solar cell.

36. Use of a permeable barrier layer in a solid-state heteroj unction comprising a perovskite material.

37. Use as claimed in claim 36 wherein the barrier layer is used to provide physical separation of said perovskite material from a cathode (p 5 1 19).

38. Use as claimed in claim 36 or claim 37wherein said heteroj unction is a solid-state heteroj unction as claimed in any one of claims 1 to 29.

39. A method for reducing and/or eliminating the light-induced drop in shunt resistance in a solid-state heteroj unction comprising a perovskite material, an p-type material and a cathode under anaerobic conditions, said method comprising

(i) providing at least one insulating material

(ii) incorporating said insulating material as a permeable barrier layer in a solid-state heteroj unction comprising a perovskite material.

40. The method of claim 39 wherein said barrier layer physically separates siad perovskite material from said cathode.

41. A method as claimed in claim 39 or claim 40 wherein said permeable barrier layer is a substantially continuous capping layer.

42. A method as claimed in any of claima 39 or claim 41 wherein said insulating material is provided in the form of particles (e.g. beads) dispersed in the p-type material.

43. A method as claimed in any one of claims 39 to 42 wherein said insulating material comprises at least one insulating metal oxide.

44. A method for the manufacture of a solid-state p-n heterojunction comprising a cathode separated from a perovskite material by a permeable barrier layer of at least one insulating material, said method comprising: a) coating an anode, preferably a transparent anode (e.g. a Fluorine doped Tin Oxide - FTO anode) with a compact layer of an n-type semiconductor material;

b) forming a layer of an perovskite semiconductor material on said compact layer, c) optionally surface sensitizing said compact layer and/or said layer of perovskite material with at least one sensitizing agent;

f) forming a cathode, preferably a metal cathode (e.g. a silver or gold cathode or a transparent cathode) on said permeable barrier layer, in contact with said p-type semiconductor material, wherein in steps d) and e) said permeable barrier layer may be comprised in said layer of p-type material or may be a separate layer, penetrated by said p-type material.

45. A method as claimed in claim 44 further comprising an encapsulating step.

46. A method of stabilising and/or improving the power conversion efficiency of a solid-state heterojunction comprising comprising a p-type material in contact with a perovskite material under anerobic conditions, said method comprising providing a permeable barrier layer comprising at least one insulating material between the perovskite material and a cathode.

47. A method according to claim 46 wherein said method further comprises encapsulating said heterojunction.

A solid-state heterojunction formed or formable by the method of any of claims 44 to 47.

49. An optoelectronic device comprising at least one solid-state heterojunction formed or formable by the method of any of claims 44 to 47.