Technical Field

5 The present invention relates to a solar cell, to a heterojunction, and to methods for preparing the solar cell and the heterojunction.

Technical Background and the Problem Underlying the Invention

10 The conversion of solar energy to electrical current using thin film third generation photovoltaics (PV) is being widely explored for the last two decades. The sandwich/monolithic-type PV devices, consisting of a mesoporous photoanode with an organic/inorganic light harvester, redox electrolyte/solid-state hole conductor, and counter electrode, have gained significant interest due to the ease of fabrication, flexibility in the

15 selection of materials and cost effective production (Gratzel, Acc. Chem. Res. 2009, 42, 1788-1798; Hagfeldt et al., Chem. Rev. 2010, 110, 6595-6663). Recently, bulk layers of organometallic halide perovskite based on tin (CsSnX ₃ , Chung et al., Nature. 2012, 485, 486-489) or lead (CH ₃ NH ₃ PbX _3, Kojima et al., J. Am. Chem. Soc. 2009, 131, 6050-6051; Etgar et al., J. Am. Chem. Soc. 2012, 134, 17396-17399; Kim et al., Sci. Rep. 2012, 2,

20 591: 1-7; Lee et al., Science 2012, 338, 643-647) have been introduced as the light harvester.

The lead perovskite shows a power conversion efficiency (PCE) of 6.54% in liquid electrolyte based devices (Im et al., Nanoscale 2011, 3, 4088-4093), while 12.3% in solid state devices (Noh et al., Nano Lett. 2013, 13, 1764-1769; Ball et al., Environ Sci. 2013, 6, 1739-1743).

25

European patent application EP2693503 Al discloses a solid-state solar cell comprising a conducting support layer, a surface-increasing scaffold structure, one or more organic- inorganic perovskite layers provided on the scaffold structure and a counter electrode. In the solar cells reported in this reference, remarkable conversion efficiencies were achieved in 30 absence of organic hole transporting material or a liquid electrolyte, which rendered the latter optional.

The optimal protocol for the deposition of CH ₃ NH ₃ PbX ₃ on Ti0 ₂ is achieved by the spin- coating of the precursor (CH ₃ NH ₃ X and PbX ₂ , X = CI, Br, I) solution on the mesoporous Ti0 ₂ film, followed by low temperature annealing step. The annealing process results in a crystalline CH ₃ NH ₃ PbX ₃ (Kojima et al.; Lee et al.; Noh et al.).

The present invention addresses disadvantages of devices comprising liquid electrolytes, such as the problem of solvent evaporation and the penetration of water into the solar cell caused by difficulty in long-term sealing especially in temperature cyclic tests.

The present invention also addresses disadvantages of incomplete pore filling which is observed in devices comprising organic hole conductors. In particular, the hole conductor tends not to penetrate equally through the mesoporous film of sensitized solar cells using a porous semiconductor anode. Furthermore, the present invention addresses the problem of low hole mobility observed with conductors used in the prior art, which is low compared to liquid electrolytes. It is a further objective of the invention to provide solar cells, in particular solid state solar cells having yet higher conversion efficiencies than prior art devices. A light to electrical power energy conversion efficiency (η) of about 10% was suggested to be a level necessary for commercial use. The invention seeks to provide an efficient solar cell that can be prepared rapidly in an efficient reproducible way, using readily available, low-cost materials, using a short manufacturing procedure based on industrially known manufacturing steps.

The present invention addresses the problems of stability observed with certain sensitized solar cells.

Summary of the Invention

Remarkably, the present inventors provided novel solar cells. The solar cells differ from previously known solar cells, in particular by way of their simple structure. The novel solar cells generally comprise readily available materials and can be fabricated in an economic manner. The solar cell of this invention can avoid disadvantages encountered in prior art devices. In an aspect, the present invention provides a solar cell comprising a current collector, a surface-increasing structure and one or more perovskite layer.

In an aspect, the present invention provides a solar cell comprising a current collector, a surface-increasing structure comprising a doped semiconductor material, one or more organic-inorganic perovskite layer, and a counter electrode and/or metal layer.

In an aspect, the present invention provides a solar cell comprising a current collector, a surface-increasing structure comprising a doped semiconductor material, one or more perovskite layer, and a counter electrode and/or metal layer in electric contact with said perovskite layer.

In an aspect, the present invention provides a solid-state solar cell comprising a current collector, a surface-increasing structure, one or more organic-inorganic perovskite layer, and a counter electrode and/or metal layer in electric contact with said perovskite layer.

In an aspect, the present invention provides a solid-state solar cell comprising a current collector, a surface-increasing structure comprising a doped semiconductor material, one or more perovskite layer, said perovskite layer being provided on said surface-increasing structure, said solid-state solar cell further comprising a counter electrode and/or metal layer in electric contact with said perovskite layer.

In an aspect, the present invention provides a solar cell having a flat configuration with two major, opposing sides, a first side and a second side, wherein said current collector, said surface-increasing structure comprising a doped semiconductor material, said one or more organic-inorganic perovskite layer, and said a counter electrode and/or metal layer are provided in the form of layers, arranged in this order in a direction extending from said first side to said second side of said solar cell. In an aspect, the present invention provides a solid state heterojunction comprising a layer comprising a doped semiconductor material and a layer comprising an organic-inorganic perovskite layer. The invention also provides a photovoltaic device, in particular a solar cell, comprising the heterojunction of the invention. In an aspect, the present invention provides a method of preparing a solid state solar cell, the method comprising the steps of:

providing a current collector and a layer comprising a doped semiconductor material in electric contact with said current collector;

- applying one or more organic-inorganic perovskite layer on said doped semiconductor material; and,

applying a counter electrode.

In an aspect, the present invention provides a method of preparing a heterojunction comprising the step of applying one or more organic-inorganic perovskite layers on a nanostructured layer comprising a doped semiconductor.

Further aspects and preferred embodiments of the invention are defined herein below and in the appended claims. Further features and advantages of the invention will become apparent to the skilled person from the description of the preferred embodiments given below.

Brief Description of the Drawings

Figure 1 shows current-voltage characteristics of a solar cell according to an embodiment of the invention (0.5 Y-TiO ₂ , closed squares) in comparison to a prior art solar cell (Ti0 ₂ , open squares) under 100 mW cm ^" photon flux (1 Sun).

Figure 2 shows the incident photo-to-electron conversion efficiency spectra of a solar cell according to an embodiment of the invention (0.5 Y-TiO ₂ , closed squares) in comparison to a prior art solar cell (Ti0 ₂ , open squares).

Figures 3 A to 3 G schematically show the structure of solar cells according to embodiments of the present invention. Detailed Description of the Preferred Embodiments

The present invention provides heterojunctions, solar cells and methods of fabricating the heterojunctions and the solar cells. The heterojunction of the invention may be used in a solar cell, in particular in the solar cell of the invention. Herein below, the devices of the invention and their fabrication are described in more detail.

The heterojunctions and solar cells of the invention are preferably flat devices when considered on a macroscopic scale. According to a preferred embodiment, they are layered and/or comprise and/or consist essentially of a plurality of layers. In view of their flat configuration, the devices of the invention preferably have two opposing sides, a first side and a second side, said opposing sides preferably making up the majority of the macroscopic surface of the device of the invention. For the purpose of the present specification, the expression "comprise" and its various grammatical forms, such as "comprising", etc., is intended to mean "includes, amongst other". It is not intended to mean "consists only of".

According to an embodiment, the solar cell of the invention preferably comprises a current collector. The current collector may be provided in the form of a layer, for example. The current collector preferably forms a continuous layer. The current collector is preferably adapted to collect the current (and/or electrons) generated by the solar cell and to conduct it to an external circuit. The current collector preferably provides the electric front contact of the solar cell.

The current collector thus preferably comprises a conducting or semiconducting material, such as a conducting organic material or a conducting inorganic material, such as a metal, doped metal, a conducting metal oxide or doped metal oxide, for example. As shall be shown further below, in some preferred embodiments, the current collector comprises a material selected from indium doped tin oxide (ITO), fluorine doped tin oxide (FTO), ZnO- Ga ₂ 0 ₃ , ZnO-Al ₂ 0 ₃ , tin oxide, antimony doped tin oxide (ATO), SrGe0 ₃ and zinc oxide, or combinations thereof.

The current collector is preferably arranged to collect and conduct the current generated in the working electrode or photoanode. Therefore, the current collector is preferably in electric contact with the working electrode or photoanode.

For the purpose of the present specification, the expression "in electric contact with" means that electrons or holes can get from one layer to the other layer with which it is in electric contact, at least in one direction. In particular, considering the electron flow in the operating device exposed to electromagnetic radiation, layers through which electrons and/or holes are flowing are considered to be in electric contact. The expression "in electric contact with" does not necessarily mean that electrons and/or holes can freely move in any direction between the layers.

According to an embodiment, the solar cell of the invention preferably comprises one or more support layer. The support layer preferably provides the physical support of the device. Furthermore, the support layer preferably provides a protection with respect to physical damage and thus delimits the solar cell with respect to the outside, for example on at least one of the two major sides of the solar cell. According to an embodiment, the solar cell may be constructed by applying the different layers in a sequence of steps, one after the other, onto the support layer. The support layer may thus also serve as a starting support for the fabrication of the solar cell. Support layers may be provided on only one or on both opposing sides of the solar cell.

The support layer, if present, is preferably transparent, so as to let light pass through the solar cell. Of course, if the support layer is provided on the side of the solar cell that is not directly exposed to light to be converted to electrical energy, the support does not necessarily have to be transparent. However, any support layer provided on the side that is designed and/or adapted to be exposed to light for the purpose of energy conversion is preferably transparent. "Transparent" means transparent to at least a part, preferably a major part of the visible light. Preferably, the conducting support layer is substantially transparent to all wavelengths or types of visible light. Furthermore, the conducting support layer may be transparent to non-visible light, such as UV and IR radiation, for example.

Conveniently, and in accordance with a preferred embodiment of the invention, a conducting support layer is provided, said conducting support layer serving as support as described above as well as current collector. The conducting support layer thus replaces or contains the support layer and the current collector. The conducting support layer is preferably transparent. Examples of conducting support layers are conductive glass or conductive plastic, which are commercially available. For example, the conducting support layer comprises a material selected from indium doped tin oxide (ITO), fluorine doped tin oxide (FTO), ZnO-Ga ₂ 03, ZnO-Al ₂ 0 ₃ , tin oxide, antimony doped tin oxide (ATO), SrGe0 ₃ and zinc oxide, coated on a transparent substrate, such as plastic or glass.

According to another embodiment, the current collector may also be provided by a conductive metal foil, such as a titanium, molybdenum or zinc foil, for example. Non- transparent conductive materials may be used as current collectors in particular on the side of the device that is not exposed to the light to be captured by the device. Such metal foils have been used as current collectors, for example, in flexible devices, such as those disclosed by Ito et al., Chem. Commun. 2006, 4004-4006. According to an embodiment, the heterojunction and the solar cell of the invention comprise a surface-increasing structure. Preferably, said surface increasing structure comprises, consists essentially of or consists of a doped semiconductor material. The surface-increasing structure is preferably applied so as to form a layer on the macroscopic scale, for example. The surface-increasing structure may be provided directly on and in physical contact with said current collector.

According to another and preferred embodiment, the surface increasing structure is provided on an underlayer, which may be a compact or electric contact and/or metal oxide layer, as will be described elsewhere in this specification. Preferably, the surface-increasing structure is in electric contact with the current collector layer.

The surface-increasing structure may also be referred to as "scaffold structure" in this specification or as "surface-increasing scaffold", for example.

According to an embodiment of the solar cell and the heterojunction of the invention, the surface-increasing structure is nanostructured and/or nanoporous. The surface-increasing structure is thus preferably structured on a nanoscale. The structures of said surface increasing structure increase the effective surface compared to the surface of the solar cell.

The surface-increasing structure may be made from a large variety of different materials and from combinations of different materials. According to a preferred embodiment, the surface- increasing structure comprises a doped material. According to an embodiment, the surface- increasing scaffold structure of the solar cell and/or the heterojunction of the invention comprises, consists essentially of, or consists of one selected from the group of: a doped semiconductor material, a doped conducting material, a doped insulator and combinations of two or more of the aforementioned. The distinction between "semiconductors" and "insulators" is not that sharp. With bulk, i.e. non-nanoparticulate matter, materials with a band gap of greater than 3 eV are generally considered insulators. For the purpose of this specification, the term "doped semiconductor" shall be applied broadly and include doped materials with band gaps of the undoped materials being substantially greater than 3 eV and the term "insulator" shall be applied for materials with band gaps of the undoped materials being substantially greater than 3.5 eV.

According to an embodiment, said surface-increasing structure is made from and/or comprises one selected from a doped metal oxide, for example a doped transition metal oxide. According to an embodiment, the doped material of the surface-increasing structure comprises, consists essentially of or consists of one or more selected from the group consisting of doped Si, Si0 ₂ , Ti0 ₂ , A1 ₂ 0 ₃ , Zr0 ₂ , Hf0 ₂ , Sn0 ₂ , Fe ₂ 0 ₃ , ZnO, W0 ₃ , Nb ₂ 0 ₅ , In ₂ 0 ₃ , Bi ₂ 0 ₃ , Y ₂ 0 ₃ , Pr ₂ 0 ₃ , Ce0 ₂ and other rare earth metal oxides, CdS, ZnS, PbS, Bi ₂ S ₃ , CdSe, CdTe, MgTi0 ₃ , SrTi0 ₃ , BaTi0 ₃ , Al ₂ Ti0 ₅ , Bi ₄ Ti ₃ 0i ₂ and other titanates, CaSn0 ₃ , SrSn0 ₃ , BaSn0 ₃ , Bi ₂ Sn ₃ 0 ₉ , Zn ₂ SnC"4, ZnSn0 ₃ and other stannates, CaZr0 ₃ , SrZr0 ₃ , BaZr0 ₃ , Bi ₄ Zr ₃ 0i ₂ and other zirconates, combinations of two or more of the aforementioned and other multi-element oxides containing at least two of alkaline metal, alkaline earth metal elements, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, Sc, Y, La or any other lanthanide, Ti, Zr, Hf, Nb, Ta, Mo, W, Ni or Cu.

Si, Fe ₂ 0 ₃ , CdS, PbS, Bi ₂ S ₃ , CdSe, CdTe, are colored semiconductors and are less preferred in accordance with embodiments according to the present invention. W0 ₃ , Bi ₂ 0 ₃ , are yellowish. In accordance with the invention, colourless to slightly yellow materials with a band gap of greater than 2.7 eV are preferred. Colourless materials with a band gap of greater than 3 eV are more preferred, Ti0 ₂ is most preferred.

According to a preferred embodiment, the surface-increasing structure comprises, consists essentially of or consists of one or more selected from doped Ti0 ₂ , Sn0 ₂ , ZnO, W0 ₃ ,

Nb ₂ 05, SrTi0 ₃ , and combinations thereof, for example. Still more preferred doped semiconductor materials are doped Τί(¾, SnC^, ZnO, Ν¾2θ5 and SrTi03, for example. According to a most preferred embodiment, the surface increasing structure comprises doped Τί(¾, in particular Τί(¾ doped with one or more selected from Ta ⁵ +, Nb ⁵ +, La ⁺ , Al ⁺ , Ga ⁺ and Y ⁺ .

According to an embodiment, one or more dopant present in said doped semiconductor material is present at a percentage of 0.01% to 5%, preferably 0.05 to 4%, more preferably 0.1 to 3%, even more preferably 0.2 to 2%, and most preferably 0.3 to 1%. The percentage is the molar percentage of said dopant with respect to atoms other than those of group 16 or 6A (oxygen, sulfur, selenium and/or tellurium atoms) or phosphorus atoms, as applicable, in said semiconductor material. In particular, said percentage is the molar percentage of said dopant with respect to metal and/or transition metal atoms in said semiconductor material. More specifically, said percentage is the molar percentage of said dopant with respect to atoms selected from Al, Si, Ge, Ti, Sb, Sn, Fe, Zn, W, Nb, Cd, Pb, Bi, Cd, Cd, Sr, Ga, In, Cu, Sc, Zr, Hf, Y, Ta, Mo, Ni, La or any other lanthanide in general, alkaline metal in general, alkaline earth metal elements in general.

For example, a 0.5% Y ³⁺ dopant in TiC^ means that one Y ³⁺ ion is present for 200 atoms of Ti in said doped TiC^.

For example, 0.5% Y ³⁺ doped SrTiC^ means that one Y ³⁺ ion is present for 100 atoms of Sr and 100 atoms of Ti. According to an embodiment, the doped semiconductor material of the surface-increasing structure is selected in accordance with its conduction band energy level. Preferably, the conduction band energy level of the doped semiconductor material is below the energy level of the photoexited electron of the organic-inorganic perovskite material disclosed elsewhere in this specification. In accordance with this particular embodiment, the doped semiconductor material is capable of receiving an electron from the photoexited perovskite and to transport this electron to the current collector, or to the underlayer, if applicable.

The doped semiconductor material preferably constitutes the working electrode and/or as photoanode of the solar cell of the invention. According to an embodiment, the doped semiconductor material and the organic-inorganic perovskite material constitute and/or function together as the photoanode and/or as working electrode of the solar cell of the invention. In accordance with this embodiment, the surface-increasing layer not only increases the active surface and/or may serve a support for the perovskite layer, but in addition works as a working electrode and/or photoanode.

According to another embodiment, the surface-increasing structure and the doped semiconductor material are not capable of receiving an electron from the perovskite material. This may apply if the conduction band energy of the material forming the surface-increasing structure is not capable of receiving an electron from the photoexited perovskite material. In accordance with this embodiment, the purpose of the surface-increasing structure is basically and/or exclusively to increase the surface and/or to provide a support layer for the perovskite layer.

In case the surface-increasing structure is made from and/or comprises an "insulator" material, an electric connection between following and preceding layers, for example the perovskite layer and the current collector should be warranted. This may be achieved, for example, by allowing the perovskite layer being in direct contact with the current collector, or, if present, with the underlayer, which may be provided on the current collector. In this regard, it is noted that the surface-increasing structure does not necessarily have to form a layer that completely covers the surface of the current collector or, if present of the underlayer. The surface-increasing structure may be formed by nanoparticles that are applied on the current collector or on said underlayer, wherein said latter layer, as applicable, does not need to be covered completely by said nanoparticles. The perovskite material may thus be in direct physical contact with said current collector or said underlayer.

In accordance with the invention, one can also envisage an "insulator" scaffold structure, which is coated with a layer of an electrically conducting and/or semiconducting material, for example with a doped semiconductor material as disclosed herein. The coating is preferably sufficiently thin so as to substantially retain the original nanostructured and/or nanoporous structure of the surface-increasing scaffold structure. For example, the electrically conducting and/or semiconducting coating may be in electric contact with said current collector and/or underlayer. According to an embodiment, the surface-increasing structure of the solar cell and/or heterojunction of the invention comprises and/or consists of nanoparticles. The nanoparticles are preferably applied and/or fixed on said current collector and/or on an underlayer, if present. The expression "nanoparticles" encompasses particles or particulate elements, which may have any form, in particular also so-called nanosheets, nanocolumns and/or nanotubes, for example. Nanosheets made from anatase Ti0 ₂ have been reported by Etgar et al., Adv. Mater. 2012, 24, 2202-2206, for example. Preferably, the nanoparticles comprise or consist essentially of said doped semiconductor material. The surface increasing structure may also be prepared by screen printing or spin coating, for example as is conventional for the preparation of porous semiconductor (e.g. TiC^) surfaces in heterojunction solar cells, see for example, Noh et al., Nano Lett. 2013, 7, 486-491 or Etgar et al., Adv. Mater. 2012, 24, 2202-2206. Nanoporous semiconductor structures and surfaces have been disclosed, for example, in EP 0333641 and EP 0606453.

According to an embodiment of the invention, said surface-increasing structure comprises and/or is prepared from nanoparticles, in particular nanosheets, nanocolumns and/or nanotubes, which nanoparticles are preferably further annealed. The nanoparticles preferably have average dimensions and/or sizes in the range of 2 to 300 nm, preferably 3 to 200 nm, even more preferably 4 to 150 nm, and most preferably 5 to 100 nm. "Dimension" or "size" with respect to the nanoparticles means here extensions in any direction of space, preferably the average maximum extension of the nanoparticles. In case of substantially spherical or ellipsoid particles, the average diameter is preferably referred to. In case of, nanosheets, the indicated dimensions refer to the length and thickness. Preferably, the size of the nanoparticles is determined by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) as disclosed by Etgar et al., Adv. Mater. 2012, 24, 2202-2206. According to an embodiment, the surface-increasing structure is nanostructured and/or nanoporous.

According to an embodiment, the surface-increasing structure and/or said doped semiconductor material is mesoporous and/or mesoscopic. According to an embodiment, the surface-increasing structure and/or said doped semiconductor material is nanocrystalline.

According to an embodiment, the surface area per gram ratio of said surface-increasing structure is in the range of 20 to 800 m 27g, preferably 25 to 300 m 27g, more preferably 30 to 150 m ² /g, and most preferably 60 to 120 m ² /g. The surface per gram ratio may be determined the BET gas adsorption method.

According to an embodiment, said surface-increasing structure forms a continuous and/or complete, or, alternatively, a non-continuous and/or non-complete layer on said support layer. According to an embodiment, said surface increasing structure forms a layer having an overall thickness of 10 to 3000 nm, preferably 12 to 2000 nm, preferably 15 to 1000 nm, more preferably 20 to 500 nm, still more preferably 50 to 400 nm and most preferably 100 to 300 nm. For the purpose of this specification, a "continuous layer" or a "complete layer" is a layer that covers an adjacent layer, such as the conductive support layer, completely so that there can be no physical contact between the perovskite layer (or, if applicable, the protective layer) and the conductive support, or the underlayer, if applicable. If the surface increasing layer is non-continuously and/or non-completely provided on said conductive support layer, the perovskite layer does or could get in direct contact with said current collector and/or underlayer.

According to an embodiment, the heterojunction and/or solar cells of the invention comprise a perovskite layer, in particular an organic-inorganic perovskite layer. The heterojunction and/or solar cell may comprise one or more perovskite layers, which may each be composed of the same or of different perovskite materials as disclosed elsewhere in this specification.

"Perovskite", for the purpose of this specification, refers to the "perovskite structure" and not to the specific perovskite mineral CaTi0 ₃ . The term "perovskite" includes structures where the ideal cubic unit cell is distorted to some extent. For the purpose of this specification, "perovskite" encompasses and preferably relates to any material that has the same type of crystal structure as calcium titanium oxide and of materials in which the bivalent cation is replaced by two separate monovalent cations. The perovskite structure has the general stoichiometry AMX ₃ , where "A" and "M" are cations and "X" is an anion. The "A" and

"M" cations can have a variety of charges and in the original Perovskite mineral (CaTiC^), the A cation is divalent and the M cation is tetravalent. For the purpose of this invention, the perovskite formulae includes structures having three (3) or four (4) anions, which may be the same or different, one or two (2) organic cations, and a metal atom carrying two or three positive charges, in accordance with the formulae presented elsewhere in this specification.

Organic-inorganic perovskites are hybrid materials exhibiting combined properties of organic composites and inorganic crystalline materials. The inorganic component forms a framework bound by covalent and ionic interactions, which provide high carrier mobility. The organic component helps in the self-assembly process of those materials, it also enables the hybrid materials to be deposited by low-cost technique as other organic materials. An additional property of the organic component is to tailor the electronic properties of the organic-inorganic material by adjusting its dimensionality and the electronic coupling between the inorganic sheets. The structure of some of the organic-inorganic perovskites are analogous to multilayer quantum well structures, with semiconducting inorganic sheets alternating with organic layers having a large energy gap. For example, the conduction band of the inorganic layers is substantially below that of the organic layers, and the valence band of the inorganic layers is similarly above that of the organic layers. Therefore, the inorganic sheets may act as quantum wells for both electrons and holes.

Another option is when the band gaps for the organic and inorganic layers can be offset, leading to a type II hetero structure in which the wells for the electrons and holes are in different layers.

Such structures of the organic-inorganic perovskites permit their use as light absorbers, which can inject electrons to the surface increasing structure, underlayer and/or the current collector and at the same time transport photogenerated charge carriers over considerable distances of several hundred nanometers or over a micron. This latter feature is entirely different from dye solar cells, where photogenerated carriers need to be transported over a one molecular layer only, i.e. over a distance of 1-2 nm only.

According to an embodiment, the organic-inorganic perovskite material that is used in the one or more perovskite layer preferably comprises a perovskite- structure of the formula (I), (II) , (III), or (IV) below, or a mixture comprising two or more perovskites-structures of the formulae (I), (II) , (III), or (IV) below:

AA'MX ₄ (I) AMX ₃ (II) ANX ₄ (III) BMX ₄ (IV) wherein A and A' are monovalent organic cations and B is a bivalent organic cation. Preferably, A, A' and B are independently selected from hydrocarbons comprising up to 60 carbons, and from 1 to 20 heteroatoms (for A and A') and 2 to 20 heteroatoms (for B), in particular one or two positively charged nitrogen atoms, respectively, besides possibly further heteroatoms selected from N, P, O and S. In an embodiment, said further heteroatoms are selected from N, O and S. Furthermore, A, A' and B may be partially or totally halogenated, independently of said 1 to 20 heteroatoms. M is a metal atom, which may be selected from the group consisting of Cu , Ni , Co , Fe ²⁺ , Mn ²⁺ , Cr^, Pd ²⁺ , Zn ²⁺ , Cd ²⁺ , Ge ²⁺ , Sn ²⁺ , Pb ²⁺ , Eu ²⁺ , Yb ²⁺ , and a combination thereof, said combination comprising two or more of said metal cations. In an embodiment, said metal M is selected from the group consisting of Cu ²⁺ , Ni ²⁺ , Co ²⁺ , Fe ²⁺ , Mn ²⁺ , Cr ²⁺ , Pd ²⁺ , Zn ²⁺ , Cd ²⁺ , Ge ²⁺ , Sn ²⁺ , Pb ²⁺ , Eu ²⁺ , Yb ²⁺ and combinations of two or more thereof. Preferably, M is Sn ²⁺ or Pb ²⁺ . N is a trivalent metal, which is preferably selected from the group of Bi ³⁺ and Sb ³⁺ .

X is an anionic compound, and is preferably selected independently from CI ^" , Br ^" , Γ, NCS ^" , CN ^" , NCO ^" , and combinations thereof. As there may be three X in formulae (II), the perovskite material may comprise combinations of different halogens. For example, "X3" may be selected from I ₂ CI- ⁵ -, IiBr- ^5" , C l^ ^' , B^I- ⁵ -, for example. The four anions in "X4" may also be a combination of different halogens. Preferably, X is Br ^" or Γ.

According to a preferred embodiment, all anions in "X3" and "X4" are identical.

According to a preferred embodiment, said organic-inorganic perovskite layer comprises a perovskite- structure of the formula (I), (II), (III) and/or (IV) below,

AA'MX ₄ (I) AMX ₃ (Π)

ANX ₄ (HI)

BMX ₄ (IV) wherein,

A and A' are organic, monovalent cations that are independently selected from primary, secondary, tertiary or quaternary organic ammonium compounds, including N-containing heterorings and ring systems, A and A' having independently from 1 to 60 carbons and 1 to 20 heteroatoms;

B is an organic, bivalent cation selected from primary, secondary, tertiary or quaternary organic ammonium compounds having from 1 to 60 carbons and 2-20 heteroatoms and having two positively charged nitrogen atoms;

M is a divalent metal cation selected from the group consisting of Cu , Ni , Co , Fe , Mn ²⁺ , Cr ²⁺ , Pd ²⁺ , Zn ²⁺ , Cd ²⁺ , Ge ²⁺ , Sn ²⁺ , Pb ²⁺ , Eu ²⁺ , Yb ²⁺ , and a combination of two or more of said metal cations;

NN iiss sseelleecctteedd ffrroomm tthhee group of Bi ³⁺ and Sb ³⁺ ; and,

the three or four X are independently selected from CI ^" , Br ^" , Γ, NCS ^" , CN ^" , and NCO ^" .

In an embodiment, said M is a divalent metal cation selected from the group consisting of Cu ²⁺ , Ni ²⁺ , Co ²⁺ , Fe ²⁺ , Mn ²⁺ , Cr ²⁺ , Pd ²⁺ , Cd ²⁺ , Ge ²⁺ , Sn ²⁺ , Pb ²⁺ , Eu ²⁺ , Yb ²⁺ and a combination of two or more of said metal cations.

M and N are preferably metal ions that can preferably adopt an octahedral anion coordination. According to an embodiment, X is selected from Br ^" and Γ. According to an embodiment, M is Sn ²⁺ and/or Pb ²⁺ . According to an embodiment, X is selected from Br ^" and Γ and M is Sn ²⁺ and/or Pb ²⁺ .

According to an embodiment, A and A' are identical, resulting in perovskite of the formulae A ₂ MX ₄ , A ₂ PbX ₄ , A ₂ SnX ₄ , for formulae (I), (VIII) and (IX), for example. Preferably, A and A' are identical and all X are identical.

According to a preferred embodiment, the perovskite material has the structure selected from one or more of formulae (I) to (III), preferably (II). According to a preferred embodiment, said organic-inorganic perovskite layer (4) comprises a perovskite- structure of any one of the formulae (V), (VI), (VII), (VIII), (IX) and (X), and/or a mixture comprising two or more perovskite- structures of formulae (V), (VI), (VII), (VIII), (IX), (X) and (XI) below:

APbX ₃ (V)

ASnX ₃ (VI)

ABiX ₄ (VII)

AA'PbX ₄ (VIII)

AA'SnX ₄ (IX) BPbX ₄ (X)

BSnX ₄ (XI) wherein A, A', B and X are as defined elsewhere in this specification. Preferably, X is selected from Br ^" and Γ, most preferably X is Γ.

According to a preferred embodiment, said organic-inorganic perovskite layer comprises perovskite- structure of the formulae (V) to (IX), more preferably (V) and/or (VI) above.

According to an embodiment, A and A', in particular in any one of formulae (I) to (III), and (V) to (IX), are monovalent cations selected independently from any one of the compounds of formulae (1) to (8) below:

(i) (2) (3) (4)

(5) (6) (7) (8)

wherein, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from C1-C15 organic substituents comprising from 0 to 15 heteroatoms.

According to an embodiment of said CI -CI 5 organic substituent any one, several or all hydrogens in said substituent may be replaced by halogen and said organic substituent may comprise up to fifteen (15) N, P, S or O heteroatoms, preferably N, S or O heteroatoms, and wherein, in any one of the compounds (2) to (8), the two or more of substituents present (R ¹ ,

R 2 , R 3 and R 4 , as applicable) may be covalently connected to each other to form a substituted or unsubstituted ring or ring system. Preferably, in a chain of atoms of said CI -CI 5 organic substituent, any heteroatom is connected to at least one carbon atom. Preferably, neighboring heteroatoms are absent and/or heteroatom-heteroatom bonds are absent in said CI -CI 5 organic substituent comprising from 0 to 15 heteroatoms.

According to an embodiment any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C15 aliphatic and C3 to C15 aromatic or heteroaromatic substituents, wherein any one, several or all hydrogens in said substituent may be replaced by halogen and wherein, in any one of the compounds (2) to (8), the two or more of the substituents present may be covalently connected to each other to form a substituted or unsubstituted ring or ring system. According to an embodiment, B is a bivalent cation selected from any one of the compounds of formulae (9) and (10) below: wherein,

in the compound of formula (9), L is absent or an organic linker structure having 1 to 10 carbons and 0 to 5 heteroatoms selected from N, P, S, and/or O, preferably from N, S and/or O, wherein any one, several or all hydrogens in said L may be replaced by halogen;

wherein any one of Ri and R ₂ is independently selected from any one of the substituents (20) to (25) below: R ^v

(20) (21) (22) (23)

(24) (25)

wherein the dotted line in the substituents (20) to (25) represents the bond by which said substituent is connected to the linker structure L;

wherein R 1 , R2 , and R 3 are independently as defined above with respect to the compounds of formulae (1) to (8);

wherein Ri and R ₂ , if they are both different from substituent (20), may be covalently connected to each other by way of their substituents R 1 , R2 , and R 3 , as applicable, and wherein any one of R 1 , R2 , and R 3 , if present, may be covalently connected to L or the ring structure of compound (10), independently from whether said substituent is present on Ri or

R ₂ ;

and wherein, in the compound of formula (10), the circle containing said two positively charged nitrogen atoms represents an aromatic ring or ring system comprising 4 to 15 carbon atoms and 2 to 7 heteroatoms, wherein said nitrogen atoms are ring heteroatoms of said ring or ring system, and wherein the remaining of said heteroatoms may be selected independently from N, O and S and wherein R ⁵ and R ⁶ are independently selected from H and from substituents as R ¹ to R ⁴ . Halogens substituting hydrogens totally or partially may also be present in addition to and/or independently of said 2 to 7 heteroatoms.

Preferably, if the number of carbons is in L is impair, the number of heteroatoms is smaller than the number of carbons. Preferably, in the ring structure of formula (10), the number of ring heteroatoms is smaller than the number of carbon atoms.

According to an embodiment, L is absent or an aliphatic, aromatic or heteroaromatic linker structure having from 1 to 10 carbons. If L is absent, said substituents Ri and R ₂ are connected via an N-N bond, as illustrated by compound (34) below. According to an embodiment, in the compound of formula (9), L is an organic linker structure having 1 to 8 carbons and from 0 to 4 N, P, S and/or O heteroatoms, preferably N, S and/or O heteroatoms, wherein any one, several or all hydrogens in said L may be replaced by halogen. Preferably, L is an aliphatic, aromatic or heteroaromatic linker structure having 1 to 8 carbons, wherein any one, several or all hydrogens in said L may be replaced by halogen.

According to an embodiment, in the compound of formula (9), L is an organic linker structure having 1 to 6 carbons and from 0 to 3 N, P, S and/or O heteroatoms, N, S and/or O heteroatoms wherein any one, several or all hydrogens in said L may be replaced by halogen. Preferably, L is an aliphatic, aromatic or heteroaromatic linker structure having 1 to 6 carbons, wherein any one, several or all hydrogens in said L may be replaced by halogen.

According to an embodiment, in the compound of formula (9), said linker L is free of any O, P or S heteroatoms, preferably free of any O, N or S. According to an embodiment, L is free of N, P, O and/or S heteroatoms, preferably free of N, O and/or S.

According to an embodiment, in the compound of formula (10), the circle containing said two positively charged nitrogen atoms represents an aromatic ring or ring system comprising 4 to 10 carbon atoms and 2 to 5 heteroatoms (including said two ring N-atoms).

According to an embodiment, said ring or ring system in the compound of formula (10) is free of any O, P or S heteroatoms, preferably free of any O or S heteroatoms. According to an embodiment, said ring or ring system in the compound of formula (10) is free of any further N, P, O and/or S heteroatoms, preferably free of any N, O and/or S heteroatoms, besides said two N-ring atoms. This does not preclude the possibility of hydrogens being substituted by halogens.

As the skilled person will understand, if an aromatic linker, compound, substituent or ring comprises 4 carbons, it comprises at least 1 ring heteroatom, so as to provide said aromatic compound.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C8 organic substituents comprising, from 0 to 4 N, P, S and/or O heteroatoms, preferably 0 to 4 N, S and/or O heteroatoms, wherein, independently of said N, S or O heteroatoms, any one, several or all hydrogens in said substituent may be replaced by halogen, and wherein two or more of substituents present on the same cation may be covalently connected to each other to form a substituted or unsubstituted ring or ring system. Preferably, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C8 aliphatic, C3 to C8 heteroaromatic and C6 to C8 aromatic substituents, wherein said heteroaromatic and aromatic substituents may be further substituted.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C6 organic substituents comprising, from 0 to 3 N, P, S and/or O heteroatom, preferably 0 to 3 N, S and/or O heteroatom, wherein, independently of said N, P, S or O heteroatoms, as applicable, any one, several or all hydrogens in said substituent may be replaced by halogen, and wherein two or more of substituents present on the same cation may be covalently connected to each other to form a substituted or unsubstituted ring or ring system. Preferably, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C6 aliphatic, C3 to C6 heteroaromatic and C6 to C6 aromatic substituents, wherein said heteroaromatic and aromatic substituents may be further substituted.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C4, preferably CI to C3 and most preferably CI to C2 aliphatic substituents wherein any one, several or all hydrogens in said substituent may be replaced by halogen and wherein two or more of substituents present on the same cation may be covalently connected to each other to form a substituted or unsubstituted ring or ring system.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to CIO alkyl, C2 to CIO alkenyl, C2 to CIO alkynyl, C4 to CIO heteroaryl and C6 to CIO aryl, wherein said alkyl, alkenyl, and alkynyl, if they comprise 3 or more carbons, may be linear, branched or cyclic, wherein said heteroaryl and aryl may be substituted or unsubstituted, and wherein several or all hydrogens in R ^x -R ⁴ may be replaced by halogen. According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C8 alkyl, C2 to C8 alkenyl, C2 to C8 alkynyl, C4 to C8 heteroaryl and C6 to C8 aryl, wherein said alkyl, alkenyl, and alkynyl, if they comprise 3 or more carbons, may be linear, branched or cyclic, wherein said heteroaryl and aryl may be substituted or unsubstituted, and wherein several or all hydrogens in R ^x -R ⁴ may be replaced by halogen.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C6 alkyl, C2 to C6 alkenyl, C2 to C6 alkynyl, C4 to C6 heteroaryl and C6 aryl, wherein said alkyl, alkenyl, and alkynyl, if they comprise 3 or more carbons, may be linear, branched or cyclic, wherein said heteroaryl and aryl may be substituted or unsubstituted, and wherein several or all hydrogens in R ^x -R ⁴ may be replaced by halogen.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C4 alkyl, C2 to C4 alkenyl and C2 to C4 alkynyl, wherein said alkyl, alkenyl and alkynyl, if they comprise 3 or more carbons, may be linear, branched or cyclic, and wherein several or all hydrogens in R -R ⁴ may be replaced by halogen.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C3, preferably CI to C2 alkyl, C2 to C3, preferably C2 alkenyl and C2 to C3, preferably C2 alkynyl, wherein said alkyl, alkenyl and alkynyl, if they comprise 3 or more carbons, may be linear, branched or cyclic, and wherein several or all hydrogens in R -R ⁴ may be replaced by halogen.

According to an embodiment, any one of R ¹ , R ² , R ³ and R ⁴ is independently selected from CI to C4, more preferably CI to C3 and even more preferably CI to C2 alkyl. Most preferably, any one of R ¹ , R ² , R ³ and R ⁴ are methyl. Again, said alkyl may be completely or partially halogenated.

According to an embodiment, A, A' and B are monovalent (A, A') and bivalent (B) cations, respectively, selected from substituted and unsubstituted C5 to C6 rings comprising one, two or more nitrogen heteroatoms, wherein one (for A and A') or two (for B) of said nitrogen atoms is/are positively charged. Substituents of such rings may be selected from halogen and from CI to C4 alkyls, C2 to C4 alkenyls and C2 to C4 alkynyls as defined above, preferably from CI to C3 alkyls, C3 alkenyls and C3 alkynyls as defined above. Said ring may comprise further heteroatoms, which may be selected from O, P, N and S, preferably from O, N and S. Bivalent organic cations B comprising two positively charged ring N-atoms are exemplified, for example, by the compound of formula (10) above. Such rings may be aromatic or aliphatic, for example.

A, A' and B may also comprise a ring system comprising two or more rings, at least one of which being from substituted and unsubstituted C5 to C6 ring as defined as above. The elliptically drawn circle in the compound of formulae (10) may also represent a ring system comprising, for example, two or more rings, but preferably two rings. Also if A and/or A' comprises two rings, further ring heteroatoms may be present, which are preferably not charged, for example.

According to an embodiment, however, the organic cations A, A' and B comprise one (for A, A'), two (for B) or more nitrogen atom(s) but are free of any O, P or S or any other heteroatom, preferably from any O or S heteroatom, with the exception of halogens, which may substitute one or more hydrogen atoms in cation A and/or B.

A and A' preferably comprise one positively charged nitrogen atom. B preferably comprises two positively charged nitrogen atoms.

A, A' and B may be selected from the exemplary rings or ring systems of formulae (30) and

(32) (33) (34)

1 2

in which R and R are, independently, as defined above, and R ₃ , R4, R5, R ₆ , R7, Rs, R9 and Rio are independently selected from H, halogen and substituents as defined above for R ¹ to R ⁴ . Preferably, R ₃ -Rio are selected from H and halogen, most preferably H.

In the organic cations A, A' and B, hydrogens may be substituted by halogens, such as F, CI, I, and Br, preferably F or CI. Such a substitution is expected to reduce the hygroscopic properties of the perovskite layer or layers and may thus provide a useful option for the purpose of the present specification.

In an embodiment, the perovskite material is of ambipolar nature or optionally p-doped or n- doped and can optionally be a ferroelectric material. In the methods of the invention, the perovskite layer may be applied by any one or more selected from drop casting, spin-coating, dip-coating, curtain coating, spray-coating, ink jet coating, and slot die coating for example.

According to an embodiment, the solar cell and/or heterojunction of the invention comprises two or more successive organic-inorganic perovskite layers, wherein said successive perovskite layers may be composed identically or wherein two or more of said layers may have a different molecular structure and/or composition. In this way, the different functions of light absorbing and/or charge carrier transporting, which may be achieved by the perovskite layers, may be optimized and/or fine-tuned. In particular, the perovskite layer that is in contact with the surface-increasing structure, is preferably optimized with respect to its properties as a light absorber. On the other hand, the same or another perovskite layer or layers may be provided, according with some embodiments of this invention, to be in contact with the counter electrode, if an intermediate layer is absent. If there are several, different perovskite layers, the different perovskite structures may be of a different composition. Any one or more of A, A', B, M, N or X in the structures of formulae (I) to (IX) may be changed in order to provide a different perovskite layer having different properties, as desired. In particular, A, B, M, N or X may be changed in a subsequent layer, in order to adjust the band gaps of the material. Different layers comprising different perovskite structures, but preferably still within the general formulae (I) to (XI), may in particular be useful to optimize a respective layer to its function (light absorber or charge carrier conductor).

The solar cell of the invention preferably comprises a counter electrode and/or metal layer. The counter electrode faces the inorganic-organic perovskite layer or, if present, the intermediate layer towards the inside of the cell. The counter electrode may form the outmost layer and thus one of the outer surfaces of the cell. It is also possible that a substrate is present on one side of the solar cell (Figs. 3 A- 3 E, for example).

The counter electrode generally comprises a material that is suitable to provide electrons and/or fill holes towards the inside of the device. This material may be a catalytically active material. The counter electrode may, for example, comprise one or more materials selected from (the group consisting of) Pt, Au, Ni, Cu, Ag, In, Ru, Pd, Rh, Ir, Os, C, including carbon nanotubes, grapheme and graphene oxide, conductive polymer and a combination of two or more of the aforementioned, for example. Conductive polymers may be selected from polymers comprising polyaniline, polypyrrole, polythiophene, polybenzene, polyethylenedioxythiophene, polypropylenedioxy-thiophene, polyacetylene, and combinations of two or more of the aforementioned, for example.

The counter electrode may be applied as is conventional, for example by thermal or electron beam evaporation, sputtering or a printing or spraying process of the counter electrode material, optionally dispersed or dissolved in a water or solvent-based carrier medium, onto the perovskite layer or onto the intermediate layer, if present, and optionally following by a chemical development and/or annealing step.

The counter electrode is preferably connected to a current collector, which is then connected to the external circuit. As detailed with respect to the first side of the device, a conductive support such as conductive glass or plastic may be electrically connected to the counter electrode on the second side (as illustrated in Fig. 3 G). According to an embodiment, the device may have two opposed support layers, which encase the solar cell, for example. The solar cell of the invention is preferably a solid state solar cell. By avoiding an electrolyte, the disadvantages of electrolytes, such as loss due to solvent evaporation, electrolyte leakage, disadvantages associated with the use of redox shuttles, for example, can be avoided. The solar cell of the invention is preferably a hetero junction solar cell, in which said organic-inorganic perovskite is and/or functions as a light absorber and charge carrier transporter.

According to an embodiment, the said surface-increasing structure of said solar cell is nanoporous and said at least one organic-inorganic perovskite layer acts as a light absorber and/or as a charge carrier transporter. In case there are several organic-inorganic perovskite layers, one layer may act as a light absorber and another layer as a charge carrier transporter.

According to an embodiment, the solar cell comprises one or more additional layers. Additional layers may be provided, for example, between said current collector and said surface-increasing structure and/or between said perovskite layer and said counter electrode. For example, the solar cell comprises one or more selected from an intermediate layer and an electric contact and/or metal oxide layer. According to a preferred embodiment, the solar cell of the invention comprises one or more intermediate layer, wherein said one or more said intermediate layer is provided between said one or more perovskite layer and said counter electrode and/or metal layer. Preferably, said intermediate layer comprises one or more selected from (a) a hole transport material and (b) a protective and/or metal oxide layer, (c) an ionic liquid.

Preferably, on one of its two sides and/or surfaces, in particular the side oriented towards said first side of said solar cell, said intermediate layer is in electric contact with said perovskite layer. Preferably, said intermediate layer is in direct or physical contact with said perovskite layer. On the other side, preferably facing said second side of said solar cell, said intermediate layer is preferably in electric contact with said counter electrode. Preferably, said intermediate layer is in direct physical contact with said counter electrode. By "hole transport material", "hole transporting material", "charge transporting material", "organic hole transport material" and "inorganic hole transport material", and the like, is meant any material or composition wherein charges are transported by electron or hole movement (electronic motion) across said material or composition. The "hole transport material" is thus an electrically conductive material. Such hole transport materials, etc., are different from electrolytes. In the latter, charges are transported by diffusion of molecules.

According to a preferred embodiment of the solar cell of the invention, said intermediate layer comprises a hole transport material selected from organic and inorganic hole transport materials.

According to a preferred embodiment, said intermediate layer comprises an organic hole transport material. Preferably, the solar cell of the invention comprises an organic hole transport material, situated between said one or more perovskite layer and said counter electrode.

The skilled person is aware of a large variety of organic hole transport materials, such as the conducting polymers disclosed elsewhere in this specification. For example, in WO2007107961, liquid and non-liquid organic hole conductors are disclosed, which may be used for the purpose of the present invention. Also in EP 1160888 and other publications such as Hsu et al., Phys. Chem. Chem. Phys., 2012, 14, 14099-14109, organic hole transport materials ("organic electrically conducting agent") are disclosed.

Preferred hole transport materials for the purpose of the present invention are Spiro- OMeTAD (2,2 7,7'-tetrakis-N,N-di-p-methoxyphenylamine-9,9'-spirobifluore ne) and PTAA (Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]).

It is noted that the term "organic" in expressions "organic hole transport material", "organic hole transport layer", "organic charge transport material" and the like does not exclude the presence of further components. Further components may be selected from (a) one or more dopants, (b) one or more solvents, (c) one or more other additives such as ionic compounds, and (d) combinations of the aforementioned components, for example. In the organic charge transport material, such further components may be present in amounts of 0-30wt. , 0- 20wt.%, 0-10wt.%, most preferably 0-5wt.%.

Examples of ionic compounds that may be present in organic hole transport materials are TBAPF ₆ , NaCF ₃ S0 ₃ , LiCF ₃ S0 ₃ , LiC10 ₄ and Li[(CF ₃ S0 ₂ ) ₂ N.

According to another embodiment, the intermediate layer comprises and/or consists essentially of an inorganic hole transport material. A wide variety of inorganic hole transport materials is commercially available. Non-limiting examples of inorganic hole transport materials are CuNCS, Cul, NiO, CuA10 ₂ and CsSnI ₃ . The inorganic hole transport material may or may not be doped and may be mixed with an organic hole transport material as described above.

According to an embodiment, the intermediate layer, for example said organic or inorganic hole transport material, removes holes from the perovskite material and/or provides new electrons from the counter electrode to the sensitizer. In other terms, the hole transport material transports electrons from the counter electrode to the perovskite material layer.

The intermediate layer be comprise and/or consist essentially of a protective layer. According to an embodiment, the protective layer preferably comprises a metal oxide. In particular, the protective layer may comprise or consist essentially of a material selected from Mg-oxide, Hf-oxide, Ga-oxide, In-oxide, Nb-oxide, Ti-oxide, Ta-oxide, Y-oxide and Zr-oxide. Ga-oxide is a preferred material for said protective layer. The protective layer preferably has a thickness of not more than 5 nm, preferably 4 nm or less, even more preferably 3 nm or less, and most preferably 2 nm or less. According to preferred embodiments, the protective layer has a thickness of 1.5 nm or less, and even 1 nm or less. Said metal "protective layer" is preferably a "buffer layer".

According to an embodiment, of the solar cell and/or heterojunction of the invention said protective layer is provided by atomic layer deposition (ALD). For example, 2 to 7 layers are deposited by ALD so as to provide said protective layer. Accordingly, said protective layer is preferably a metal oxide multilayer. According to an embodiment, the protective layer is as disclosed in WO 2013084029 Al, which is entirely incorporated herein by reference.

According to another embodiment, the intermediate layer is an ionic liquid or and ionic melt. Exemplary liquids and melts are disclosed in EP1819005 and WO2009/083901A1.

According to another embodiment, the intermediate layer is absent and said counter electrode and/or metal layer is in direct contact with said perovskite layer and/or not separated by any further layer or medium from said perovskite layer.

According to a preferred embodiment, the solar cell of the invention comprises an underlayer and/or metal oxide layer. Preferably, the underlayer is provided between the current collector (on said first side) and said surface-increasing structure. Preferably, the underlayer and/or metal oxide layer is conductive. The underlayer is preferably made from a dense or compact semiconductor material and is thus also referred to as dense or compact semiconductor layer.

The underlayer may facilitate the application of the surface increasing layer. Said underlayer and/or metal oxide layer preferably has a thickness of 1-120 nm (nanometer), preferably 5 to 110 nm, even more preferably 6 to 105 nm, most preferably 10 to 100 nm, in particular 10-60 nm. The underlayer may be applied, for example, by atomic layer deposition (ALD). In this case, the thickness of this layer is preferably 1 nm to 25 nm, more preferably 5 nm to 20 nm.

The underlayer may also be deposited by spray pyrolysis or by a printing process, for example In this case, the thickness is preferably from 10 nm to 120 nm, preferably 20 to 100 nm, for example. The underlayer may comprise the same semiconductor material, doped or non-doped, as disclosed with respect to the surface increasing structure elsewhere in this specification. The underlayer may be selected from materials independently from the surface increasing structure. Preferably, the underlayer does not contain a doped material, or contains a material that is not doped to the same extent or in the same manner as the doped semiconductor material that is preferably comprised in the surface increasing layer. Preferably, underlayer is conducting and/or a semiconductor.

According to another embodiment, the underlayer comprises or consists of a material selected, independently, from the same doped materials as specified with respect to the surface-increasing structure.

According to an embodiment, the underlayer comprises, with respect to the elemental composition, the same material as the surface increasing structure.

Preferably, the underlayer is not doped but otherwise comprises the same semiconductor material as the surface-increasing layer/doped semiconductor material. Preferably, the underlayer comprises or consists of TiC^, preferably non-doped. According to an embodiment, the underlayer is preferably a dense or compact layer, in contrast or compared to the surface increasing layer. Accordingly, the material in the underlayer is denser than in the surface increasing layer. Preferably, the underlayer does not result in such a significant increase of the surface as in case of the surface increasing layer.

Schematically, the solar cell of the invention preferably comprises at least three or more of the following layers, preferably in this order and/or direction from a first side (7) to a second side (8) of the device. Reference numerals are found in Figures 3 A to 3 G:

(12) an optional support layer;

(2) a current collector layer;

(10) an optional underlayer;

(3) a surface increasing structure;

(4) a perovskite layer;

(4.1-4.n) optional n further perovskite layers, n being 0 or an integer of 1 to 10;

(5) an optional intermediate layer;

(6) a counter electrode;

(2.2) an optional current collector layer:

(12.2) an optional support layer. The following preferred embodiments comprise the layers, structures and or components as specified, in the indicated order from the first side 7 to the second side 8 of the device:

Below, embodiments #1 to #32 of solar cells of the invention are listed. These embodiments comprise or consist essentially of the layers as specified by the respective reference numeral, preferably in the indicated order from the first side to the second side of the device.

#1: (2)-(3)-(4)-(6) (Fig. 3 A);

#2: (2)-(3)-(4)-(5)-(6) (Fig. 3 B);

#3: (2)-(10)-(3)-(4)-(6);

#4: (12)-(2)-(3)-(4)-(6) (Fig. 3 C);

#5: (2)-(10)-(3)-(4)-(5)-(6) (Fig. 3 D);

#6: (12)-(2)-(3)-(4)-(5)-(6);

#7: (12)-(2)-(10)-(3)-(4)-(6) (Fig. 3 E);

#8: (12)-(2)-(10)-(3)-(4)-(5)-(6);

#9: (2)-(3)-(4)-(6)-(2.2)

#10 (2)-(3)-(4)-(6)-(12.2) (Fig. 3 F);

#11 (2)-(3)-(4)-(5)-(6)-(2.2)

#12 (2)-(3)-(4)-(5)-(6)-(12.2);

#13 (2)-(3)-(4)-(6)-(2.2)-(12.2) (Fig. 3 G);

#14 (2)-(3)-(4)-(5)-(6)-(2.2)-(12.2);

#15 (12)-(2)-(3)-(4)-(6)-(2.2)

#16 (12)-(2)-(3)-(4)-(6)-(12.2);

#17 (12)-(2)-(3)-(4)-(5)-(6)-(2.2)

#18 (12)-(2)-(3)-(4)-(5)-(6)-(12.2);

#19 (12)-(2)-(3)-(4)-(6)-(2.2)-(12.2);

#20 (12)-(2)-(3)-(4)-(5)-(6)-(2.2)-(12.2);

#21: (2)-(10)-(3)-(4)-(6)-(2.2)

#22: (2)-(10)-(3)-(4)-(6)-(12.2); #23 (2)-(10)-(3)-(4)-(5)-(6)- (2.2)

#24: (2)-(10)-(3)-(4)-(5)-(6)-(12.2);

#25 (2)-(10)-(3)-(4)-(6)-(2.2)-(12.2);

#26 (2)-(10)-(3)-(4)-(5)-(6)-(2.2)-(12.2);

#27 (12)-(2)-(10)-(3)-(4)-(6)-(2.2)

#28 (12)-(2)-(10)-(3)-(4)-(6)-(12.2);

#29 (12)-(2)-(10)-(3)-(4)-(5)-(6)-(2.2)

#30 (12)-(2)-(10)-(3)-(4)-(5)-(6)-(12.2);

#31 (12)-(2)-(10)-(3)-(4)-(6)-(2.2)-(12.2);

#32 (12)-(2)-(10)-(3)-(4)-(5)-(6)-(2.2)-(12.2).

The 32 embodiments depicted above do not preclude the presence of further optional layers, for example between the layers mentioned above, as may be deemed useful. Such additional layers may physically separate otherwise adjacent layers. For example, additional protective layers may be present, for example between the surface-increasing structure and the perovskite layer.

The direction from the first side to the second side of the solar cells of the invention, exemplified by (2)— >(3)— >(4)— >(6) (embodiment #1) is preferably the direction of the flow of holes in the solar cell of the invention, whereas the electrons flow in the opposed direction.

The method of the invention comprises the step of applying one or more organic-inorganic perovskite layer on said surface increasing structure. The perovskite layer may be applied by any suitable process. According to an embodiment, the one or more perovskite layers are applied by any one or a combination of drop casting, spin-coating, dip-coating, curtain- coating, spray-coating, ink jet coating, and slot die coating for example. According to an embodiment, the method of the invention comprises or consists essentially of the steps of providing a conducting and/or current collector layer, applying a surface- increasing structure on said current collector layer and/or on an optional underlayer provided on said current collector layer; applying one or more organic-inorganic perovskite layer on said surface-increasing structure; and, applying a counter electrode. Preferably, these steps are conducted in this order, with further or other steps being conducted before, after, in between and/or in parallel to these steps, without changing the order of the steps.

If the solar cell comprises an intermediate layer, such as an organic hole transport material, the intermediate layer is preferably applied onto said perovskite layer and/or before applying said counter electrode.

Herein below, for the purpose of illustration, several preferred embodiments of solar cells of the invention are discussed with reference to the schematic drawings shown in Figures 3 A to 3 G. These figures do not limit the scope of this invention, which is defined by the appended claims. As the figures are not drawn to scale, they are not suitable to illustrate the actual or relative thickness of the layers and components. However, the figures illustrate the sequence and/or positions of layers, and also show possibilities of combining layers in the solar cell of the invention. Unless further layers are present, the figures also show which layer is in physical contact with which other layer.

Figures 3 A to 3 G show exemplary solar cells 1, 1.1, 1.2, 1.3, 1.4, 1.5 and 1.6 of the present invention. The same layers have the same reference numbers throughout these figures. The solar cell shown in Figure 3 A is encompassed by embodiment #1 disclosed above. Reference numeral 2 represents a current collector and/or a conductive layer. One side of said current collector 2 is oriented towards the bottom and/or outside of the cell and thus forms the first side 7 of the solar cell. The surface increasing structure 3 is provided on said current collector 2. In the preferred embodiments, the surface increasing structure comprises or consists of a doped semiconductor material, such as doped TiC^. Reference numeral 4 represents the perovskite layer, which is in direct contact with and/or on the surface increasing layer 3. The counter electrode 6, which may exemplary be made from a metal, provides the upper or second side 8 of the solar cell, oriented to the outside of the cell. Towards the inside, the counter electrode 6 is in direct contact with the perovskite layer 4. An intermediate layer 5 is absent in the solar cell shown in Figure 3 A. The perovskite layer 4 serves as light absorber and as charge transport material. Upon illumination, electrons are exited in the perovskite layer and injected into the doped semiconductor material of the surface increasing structure 3. From there, the electrons are pushed via the current collector 2 to an external circuit (not shown). New electrons are taken from the external circuit (not shown) connected to the counter electrode 6, which injects the electrons into the perovskite layer 4, thereby closing the electric circuit.

The embodiment shown in Figure 3 B shows solar cell 1.1, encompassed by embodiment #2 above. This cell differs from the embodiment of Figure 3 A in that an additional intermediate layer 5 is provided between said perovskite layer 4 and said counter electrode 6. Preferably, the intermediate layer is a hole transport material, such as an organic hole transport material, and transports holes from the perovskite layer 4 to the counter electrode 6. The embodiment shown in Figure 3 C shows a solar cell 1.2 that differs from the embodiment of Figure 3 A in that a support layer 12 is provided. Layer 12 is preferably transparent. It forms the border to the outside at the first side 7 of the solar cell. Current collector 2 and support layer 12 together may form a conducting glass or plastic layer 13, such as FTO-glass, and the like.

The solar cell 1.3 shown in Figure 3 D comprises an electric contact and/or metal oxide layer 10, between the current collector layer 2 and the surface increasing layer 3. At this occasion, it is mentioned that the surface increasing layer may not completely cover said underlayer 10, so that the perovskite layer may come in contact with said underlayer. The electric contact/dense semiconductor layer 10 may be applied onto the current collector layer as described elsewhere in this specification, and surface increasing structure 3 is applied onto the underlayer 10.

Solar cell 1.4 shown in Figure 3 E (embodiment #7 above) comprises a transparent support layer 12, forming a conductive support layer 13 together with current collector 2. An underlayer 10 and an intermediate layer 5 are present. The intermediate layer preferably comprises an organic hole transport material. This embodiment thus combines structures described in Figures 3 A to 3 D above. The schematic construction shown in Figure 3 E corresponds to the solar cell described in the examples further below.

Figure 3 F shows solar cell 1.5, which comprises a support layer 12.2 on the top of the cell.

Figure 3 G shows solar cell 1.6 comprising a support layer 12.2 as shown in Figure 3 F, wherein a current collector layer 2.2 is present between the support layer 12.2 and the counter electrode 6. A conductive support layer 13.2, for example made from conductive plastic or conductive glass is present on the second side 8 of this solar side.

The present invention will now be further illustrated by way of experimental examples. These examples do not limit the scope of this invention, which is defined by the appended claims.

Examples: SOLAR CELL PREPARATION PROCEDURE

0.5 Y-TiO ₂ was obtained as described in by Chandiran et al. J. Phys. Chem. C. 2011, 115, 9232-9240. A precursor solution of perovskite was prepared by mixing CH ₃ NH ₃ I and Pbl ₂ at a 1: 1 mole ratio in GBL at 60°C for 12h, which was used for the in situ formation of CH ₃ NH ₃ PM ₃ . Fluorine-doped tin oxide (FTO) conductive glass (TEC 7, νΩ/sq, Pilkington) was cleaned with 2% Hellmanex® aqueous solution, acetone, and ethanol, respectively. A 10 nm compact Ti0 ₂ layer was deposited by atomic layer deposition. The mesoporous film was prepared by spin-coating the Ti0 ₂ or 0.5 Y-TiO ₂ paste at 2000 rpm for 30 s, which was then sintered at 500°C for 30 min in air. The prepared perovskite precursor solution was dropped on the semiconductor surface, spin-coating at 1500 rpm for 30 s in a dry air box. The film coated on the Ti0 ₂ or 0.5 Y-TiO ₂ changed its color with annealing under air for 10 min at 100°C, indicating the formation of CH ₃ NH ₃ PbI ₃ .

A mixture consisting of 0.06 M (2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl amine)-9,9- spirobifluorene) (spiro-OMeTAD), 0.03 M lithium bis (trifluoro methyl sulfonyl)imide (LiTFSI), 0.2 M 4-tert-butylpyridine (TBP), and 1% of FK209 Co dopant in chlorobenzene, was spin-coated on the top of the perovskite layer with the spin speed of 4000 rpm. Finally, 70 nm of gold was deposited as the electrical back contact by thermal evaporation under a pressure of 5 x 10 ^"6 Torr. METHODOLOGY FOR PHOTOVOLTAIC CHARACTERIZATION

The current-voltage characteristics were measured by applying an external potential bias to the device, and recording the generated photocurrent with a Keithley model 2400 digital source meter. A 450 W xenon lamp (Oriel) was used as the light source, equipped with a Schott K133 Tempax sunlight filter to reduce the mismatch between the simulated light and AM 1.5G standard. IPCE spectra were measured with a 300 W xenon lamp (ILC technology). The light passed through a Gemini- 180 double monochromator (Jobin Yvon Ltd) before illuminating onto the device. The spectra were recorded with a Keithley 2400 source meter under a constant white light bias of around 5 mW/cm . Both were measured by using a mask with an area of 0.285 cm . COMPARATIVE EXAMPLE

A prior art photovoltaic device with a Ti0 ₂ photoanode based on the structure FTO/10 nm compact Ti0 ₂ / mesoporous Ti0 ₂ / CH ₃ NH ₃ PbI ₃ / spiro-OMeTAD/ Au was constructed based on above solar cell preparation procedure. The open squares in Figure 1 show the current- voltage I-V characteristics measured under AM 1.5G illumination (100 mW/cm ) and the open squares in Figure 2 the incident photon-to-current conversion efficiency (IPCE) spectrum of the corresponding heterojunction solar cell.. EXAMPLE IN ACCORDANCE WITH AN EMBODIMENT OF THE INVENTION

A photovoltaic device according to the present invention with a 0.5 Y-TiO ₂ photoanode based on the structure FTO/lOnm compact T1O ₂ / mesoporous 0.5 Y-TiO ₂ / CH ₃ NH ₃ PM ₃ / spiro-OMeTAD/ Au was constructed based on above solar cell preparation procedure. The closed squares in Figure 1 show the current- voltage I-V characteristics measured under AM 1.5G illumination (100 mW/cm ) and the closed squares in Figure 2 the incident photo-to- current conversion efficiency (IPCE) spectrum of the corresponding heterojunction solar cell.

Figure 1 and Table 1 shows that devices according to the present invention show particularly high photocurrents. Figure 2 reveals that increased device performance occurs over a large fraction of the visible light spectrum. Doping of Ti0 ₂ and other large band gap semiconductors is largely used to extend the photoresponse in photocatalysis and in photoelectrochemical devices (Dou et al., Chem. Mater., 2011, 23, 3938-3945, Hong et al., J. Solid State Chem., 2011, 184, 2244-2249). However, Y-doping of Ti0 ₂ results in only a relatively minor extension of light absorption into the visible part of the spectrum (Chandiran et al. J. Phys. Chem. C. 2011, 115, 9232-9240). Hence the increase of the photoresponse over virtually the entire visible spectrum is unexpected and not due to the same mechanisms as with photocatalytic and photoelectrochemical devices according to prior art. Table 1: Photovoltaic characteristic of perovskite based devices based on Ti0 ₂ and 0.5%Y-

Photoanode Sun Jsc (mA/cm2) Voc (mV) FF PCE (%) material Intensity

Ti0 ₂ 1 sun 15.8 942 0.70 10.5

0.5%Y-TiO ₂ 1 sun 18.1 945 0.66 11.2