WO2015159192A1

WO2015159192A1 - Process for the production of a solid dye-sensitized solar cell or a perovskite solar cell

Info

Publication number: WO2015159192A1
Application number: PCT/IB2015/052622
Authority: WO
Inventors: Peter Erk; Ingmar Bruder; Herve Dietsch; Matthias Georg SCHWAB; Kitty Chih-Pei Cha; Shinji Nakamichi; Hitoshi Yamato; Ryuichi Takahashi
Original assignee: Basf Se; Basf (China) Company Limited
Priority date: 2014-04-15
Filing date: 2015-04-10
Publication date: 2015-10-22

Abstract

An electrically conductive inter layer is provided between hole transport layer and counter electrode in a solid dye-sensitized solar cell or a Perovskite solar cell and a process is provided for the production of said inter layer. This process can beneficially be integrated in a process for the production of printed solid dye-sensitized solar cells or Perovskite solar cells under ambient conditions.

Description

Process for the production of a solid dye-sensitized solar cell or a Perovskite solar cell

FIELD OF THE INVENTION The present invention relates to a process for the production of a solid dye-sensitized solar cell or a Perovskite solar cell with an electrically conductive inter layer between hole transport layer and counter electrode (cathode).

Solid dye-sensitized solar cells (sDSCs) and Perovskite solar cells (PSCs) have attracted much attention in recent years. They have several advantages compared to silicon-based solar cells such as lower production and material costs because an inexpensive metal oxide semiconductor such as titanium dioxide can be used therefore without the necessity of a purification to a high degree of purity. Other advantages include the flexibility, transparency and light weight of these types of solar cells.

The construction of a solid dye-sensitized solar cell is generally based on a transparent substrate (e.g. glass), which is coated with a transparent conductive layer, the working electrode (anode). Generally, an n-conductive metal oxide is applied to this electrode, for example a nanoporous titanium dioxide (T1O2) layer of approximately 2 to 20 μιτι thickness. On the surface of the metal oxide, in turn, a chromophoric substance, for example a ruthenium complex or an organic dye, which can be converted to an excited state by light absorption, is typically adsorbed to form the photosensitive layer.

The function of solid dye-sensitized solar cells is based on the fact that light is absorbed by the dye and electrons are transferred from the excited dye to the metal oxide and migrate thereon to the anode. In contrast to solid dye-sensitized solar cells, in liquid solar cells the area between the electrodes is filled with a liquid, i.e. a redox electrolyte. A further development are Perovskite solar cells, i.e. solar cells with organometallic Perovskites as light harvesting compounds.

H.-S. Kim et al. describe in Scientific Reports, 2 : 59, DOI: 10.1038/srep00591 , lead iodide Perovskite-sensitized solar cells with an efficiency exceeding 9 %. In those cells, Perovskite nanoparticles of methyl ammonium lead iodide are used as absorber material in combination with mesoporous titanium dioxide as transparent n-type semiconductor and spiro-MeOTAD as p-type hole conductor. M. M . Lee et al. describe in Science 2012, 338, 6107, 643-647, DOI:

10.1 126/science.1228604, hybrid solar cells based on meso-superstructured organometal halide Perovskites. In those cells mesoporous alumina is used instead of titanium dioxide. In those cells AI2O3 does not act as n-type oxide but as a meso-scale "scaffold" upon which the device is structured. Therefore, the authors no longer denote the devices as "sensitized" solar cells but as two-component hybrid solar cells or "meso-superstructured solar cells".

The overall performance of a solid dye-sensitized solar cell or a Perovskite solar cell is characterized by several parameters such as the open circuit voltage (V_oc), the short circuit current (J_sc), the fill factor (FF) and the energy conversion efficiency (η) resulting therefrom.

Solar cells with an inter layer between hole transport layer and counter electrode (cathode) have been described before, in principle.

M. Vogel et al., Appl. Phys. Lett. 2006, 89, 163501 , describe organic photovoltaic cells with a bathocuproine buffer layer between active layer and electrode. Depending on the nature of the active layer and the electrode, it may serve to avoid recombination of excitons for better ohmic contact etc. Nevertheless, in some cases, cells without bathocuproine show a better performance.

H.-L- Yip et al., Adv. Mater. 2008, 20, 2376-2382, describe polymer solar cells that use self-assembled monolayer-modified ZnO/metals as cathodes.

P. B. Boix et al., J. Phys. Chem. Lett. 201 1 , 2, 407-41 1 , describe studies on the role of ZnO electron-selective layers in regular and inverted bulk heterojunction solar cells.

K.-G. Lim et al., J. Mater. Chem. 2012, 22, 25148-25153, describe studies on polymer photovoltaic cells with an insulating interfacial nanolayer.

Z. Ku et al., Sci. Rep. 2013, 3 : 3132, describe a printable mesoscopic heterojunction solar cell with a carbon counter electrode. The solar cells feature a ZrC>2 space layer which is fabricated from a ZrC>2 paste. The purpose of this layer according to Ku et al. is to prevent short circuits due to its insulating properties.

J. Yang et al., Solar Energy Materials & Solar Cells 2013, 109, 47-55, describe organic photovoltaic modules fabricated by an industrial gravure printing proofer wherein the modules can be prepared in air. The hole transport layer PEDOT:PSS and the active layer P3HT:PCBM are printed on ITO electrodes. It is also described that zinc oxide nanoparticle inter layers may be used to improve electrical interfaces. The inventors of the present invention have found that in particular J_sc decreases if a ZnC>2 inter layer is employed.

US 201 1/0030789 A1 describes a method of forming a conducting polymer based photovoltaic device comprising the steps of:

(a) providing a transparent first electrode;

(b) providing the transparent first electrode with a layer of metal oxide nanoparticles, wherein the metal oxide is selected from the group consisting of: Ti02, TiOx and

ZnO;

(c) providing the layer of metal oxide nanoparticles with a bulk heterojunction layer comprising metal oxide nanoparticles and a hole conducting polymer containing thermocleavable groups, wherein the metal oxide is selected from the group consisting of: Ti02, TiOx, Ce02, Nb205 and ZnO;

(d) heating the bulk heterojunction layer to cleave the thermally cleavable groups to produce an insoluble hole containing polymer;

(e) providing the bulk heterojunction layer with a hole transporting layer; and

(f) providing the hole transporting layer with a second electrode.

According to the teaching of this document suitable hole transport layers can be formed from conducting polymers, in particular on the basis of PEDOT. There is not the slightest indication that said hole transport layer may have an additional function in the sense of acting as hole transport layer and increasing the electrical contact with the electrically conductive layer. The conversion efficiencies obtained with the photovaltaic devices of this document are in a range of 0.1 and 0.5 % and thus at such a low level that it can be excluded that the PEDOT layer acts as an interlayer in the sense of the invention.

S. H. Eom et al., Solar Energy Materials & Solar Cells 2008, 92, 564-570, describe the preparation and characterization of nano-scale ZnO as a buffer layer for inkjet printing of silver cathodes in polymer solar cells. Apparently the purpose of the ZnO buffer layer is to provide a hydrophilic surface for the hydrophilic silver ink. The results with ZnO have not been compared with results without ZnO buffer layer and the energy conversion efficiency obtained by the solar cells with ZnO buffer layer is comparatively low. Vacuum processing of layers of organic solar cells often leads to good results in view of the above-mentioned performance parameters, for example the energy conversion efficiency of the solar cells. Nevertheless, vacuum techniques require expensive equipment and are generally not suitable to produce high amounts of solar cell devices in short time.

It is an object of the present invention to provide a process for the production of a solid dye-sensitized solar cell or a Perovskite solar cell. This process should be

economically and easy to perform, especially without the need to exclude oxygen during the process. The process at the same time should have a sufficient energy conversion efficiency.

It was now surprisingly found that this object is achieved by the process according to the invention.

SU MMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a process for the production of a solid dye-sensitized solar cell or a Perovskite solar cell, wherein the solar cell comprises

a) an electrically conductive layer being part of or forming the working electrode (anode),

b1 ) a photosensitive layer comprising a semi-conductive metal oxide and a chromo- phoric substance

or

b2) a photosensitive layer comprising a Perovskite absorber material,

c) a hole transport layer,

d) an electrically conductive inter layer comprising at least one conductor or semiconductor, wherein that electrically conductive inter layer is different from layer c), e) an electrically conductive layer different from layer d) being part of or forming the counter electrode (cathode),

wherein the process comprises contacting layer d) with a liquid medium to provide layer e).

A further aspect of the present invention relates to the use of an inter layer d) as defined herein for increasing the electrical contact between the hole transport layer c) and the electrically conductive layer being part of or forming the counter electrode (cathode) e) in a solid dye-sensitized solar cell or a Perovskite solar cell as defined herein.

A further aspect of the present invention relates to a solid dye-sensitized solar cell or a Perovskite solar cell obtained by the process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The contacting of layer d) with the liquid medium according to the process of the invention is preferably performed under pressure and temperature conditions in which the liquid medium is predominantly in the liquid aggregation state. In other words, the liquid medium features at least one liquid phase.

The contacting of layer d) with the liquid medium according to the process of the invention can principally be performed in a broad temperature range which is preferably below the boiling point of the liquid medium at the given pressure. Preferably, the contacting of layer d) with the liquid medium according to the process of the invention is performed at a temperature in the range of from 0 to 150 °C, more preferably in the range of from 5 to 90 °C, most preferably in the range of from 10 to 50 °C. In particular, the contacting of layer d) with the liquid medium is performed at ambient temperature.

The contacting of layer d) with the liquid medium according to the process of the invention can principally be performed in a broad pressure range which is preferably below the boiling pressure of the liquid medium at the given temperature. Preferably, the contacting of layer d) with the liquid medium is performed in an atmosphere which has a pressure in the range of from 200 mbar to 3 bar, more preferably in the range of from 500 mbar to 2 bar. Most preferably, the contacting of layer d) with the liquid medium is performed at ambient pressure. There is generally no need to exclude oxygen during the process of the invention. The contacting of layer d) with the liquid medium according to the process of the invention is preferably performed in an atmosphere which comprises oxygen, more preferably in an atmosphere comprising or consisting of air. Most preferably, the contacting of layer d) with the liquid medium according to the process of the invention is performed under ambient atmosphere.

The production of layer d) can be performed in principle by the same methods as for the production of layers a), b1 ) or b2) and c) as outlined below. Preferably layer d) is produced by a dry film-forming method or by a printing technique. Suitable dry film- forming methods are physical vacuum deposition methods or CVD methods. The use of printing techniques is especially preferred when the process according to the invention is performed continuously. Suitable methods for the production of inter layer d) are preferably spin coating, slot coating, screen printing, gravure coating, offset printing, spray coating, ink-jet printing, dip coating, drop coating and blade coating, more preferably ink-jet printing, blade coating and spin coating.

According to the invention, layer d) is made of a different material than layer c).

Preferably, layer d) comprises at least one conductor or semi-conductor and has an electrical conductivity in the range of from 1 -10 ¹ S-cnr¹ to 5-10³ S-crrv¹. More preferably, layer d) has an electrical conductivity in the range of from 1 -10¹ S-cnr¹ to 5-10³ S-cm-¹.

Layer d) may comprise metal particles, preferably Ag, Cu and/or Au. If layer d) comprises metal particles, these are generally present in the material of layer d) in an amount of from 20 to 85 wt.-%, preferably in an amount of from 30 to 70 wt.-%. Preferably, the at least one conductor or semi-conductor which the layer d) comprises is selected from carbon materials, poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline, M0S2 and mixtures thereof. The carbon materials are preferably selected from carbon black, carbon nanotubes, fullerenes, graphene, graphite and mixtures thereof.

Generally, the ionization potential of layer d) is lower than the ionization potential of layer c). Preferably, the ionization potential of layer d) is in the range of from 4.5 eV to 5.5 eV. Preferably, the ionization potential of layer c) is in the range of from 4.7 eV to 5.7 eV.

Preferably, the energy difference between the HOMO of layer c) and the HOMO of layer d) is in the range of from 0.1 eV to 1.0 eV. The term "HOMO" is to be understood as being the "highest occupied molecular orbital" of the material constituting layer c) or layer d), respectively.

Generally, layer d) has an average thickness in the range of from 10 nm to 600 μιτι, preferably in the range of from 20 nm to 100 μιτι. According to the process of the invention, layer d) is contacted with a liquid medium to provide layer e).

The liquid medium features at least one liquid phase under the temperature and pressure conditions of the process of the invention. The liquid medium may contain solid ingredients besides the at least one liquid phase, preferably dispersant polymers and/or conductive particles. The amount of liquid, i.e. the at least one liquid phase, is preferably present in the liquid medium in an amount of from 20 to 85 wt.-%, more preferably in an amount of from 25 to 75 wt.-% and most preferably in an amount of from 30 to 70 wt.-%, based on the total weight of the liquid medium.

Preferably, the liquid medium is in the form of an ink, paste, grease, foam or a mixture thereof, more preferably an ink, paste, grease or mixture thereof. The liquid medium preferably is a solution or dispersion of at least one conductor or semi-conductor in at least one solvent.

Preferably, the at least one solvent is selected from Ci-C6-alcohols, water and mixtures thereof, for example methanol, ethanol, n-propanol, isopropanol, n-butanol, sec- butanol, tert-butanol, n-pentanol, n-hexanol and cyclohexanol. More preferably, the at least one solvent is selected from methanol, ethanol, isopropanol, water and mixtures thereof. In particular, the at least one solvent is selected from ethanol, water and mixtures thereof. The liquid medium may comprise conductive particles, in particular particles of Al, Cu, Ag, Au and/or graphene. If these particles are present, they are contained in the liquid medium preferably in an amount of from 20 to 85 wt.-%, more preferably in an amount of from 30 to 70 wt.-%. Preferably, the liquid medium comprises at least one dispersant polymer. The dispersant polymer is preferably selected from polyamides and polyvinylbutyrals.

The contacting of layer d) with the liquid medium can be performed by processes principally known to a person skilled in the art. Preferably, the contacting of layer d) with the liquid medium is performed by spin coating, slot coating, screen printing, gravure coating, offset printing, spray coating, ink-jet printing, dip coating, drop coating or blade coating, more preferably by ink-jet printing, blade coating or spin coating. In particular the contacting of layer d) with the liquid medium is performed by ink-jet printing.

In a further preferred embodiment, the liquid medium is contacted with a substrate to provide layer e) and layer e) is afterwards transferred from the substrate onto layer d).

In this embodiment, the liquid medium is contacted with the substrate and layer e) is formed on the substrate. Afterwards, layer e) is preferably contacted with an adhesive, for example an adhesive foam like REVALPHA from Nitto Denko Corporation, and layer e) is removed from the substrate together with the adhesive. Layer e) is then transferred onto layer d). If an adhesive foam is used for the layer transfer, this procedure preferably comprises pressing the film of adhesive foam and layer e) onto layer d) and short heating of that film, preferably at a temperature of over 90 °C for over 10 seconds. By this treatment, the adhesive foam loses its adhesiveness and can be removed from the solar cell device while the layer e) stays deposited on layer d).

Suitable substrates for this embodiment comprise silicone substrates,

polytetrafluoroethylene (PTFE) and polyethylene terephthalate (PET).

According to the process of the invention, a solid dye-sensitized solar cell or a Perov- skite solar cell is produced.

In an embodiment of the process of the invention, a solid dye-sensitized solar cell is produced. This solid dye-sensitized solar cell comprises

b1 ) a photosensitive layer comprising a semi-conductive metal oxide and a chromo- phoric substance,

c) a hole transport layer,

d) an electrically conductive inter layer comprising at least one conductor or semiconductor, wherein that electrically conductive inter layer is different from layer c), e) an electrically conductive layer different from layer d) being part of or forming the counter electrode (cathode).

In a further embodiment of the process of the invention, a Perovskite solar cell is produced. This Perovskite solar cell comprises

b2) a photosensitive layer comprising a Perovskite absorber material, a hole transport layer,

an electrically conductive inter layer comprising at least one conductor or semiconductor, wherein that electrically conductive inter layer is different from layer c), an electrically conductive layer different from layer d) being part of or forming the counter electrode (cathode).

The Perovskite solar cell which is obtained by the process of the invention is generally in form of a solid state device. In a first embodiment, the photosensitive layer b2) of the Perovskite solar cell comprises a Perovskite absorber material and at least one semi- conductive metal oxide. Suitable semiconductive metal oxides are those described in the following for the solid dye-sensitized solar cells, in particular ΤΊΟ2. Devices according to this first embodiment are described in H.-S. Kim et al., Scientific Reports, 2 : 59, DOI: 10.1038/srep00591 . In a second embodiment, the photosensitive layer b2) of the Perovskite solar cell comprises a Perovskite absorber material and a carrier material that does not act as a n-type oxide, in particular AI2O3. Devices according to this second embodiment are described in M . M. Lee et al., Science 2012, 338, 6107, 643-647, DOI: 10.1 126/science.1228604, and D. Bi et al., RSC Adv. 2013, 3, 18762-18766, DOI: 10.1039/c3ra43228a. As mentioned above, the production of the layers a), b1 ) or b2) and c) of the solid dye- sensitized solar cell or Perovskite solar cell can be performed in a known manner, for example by wet type layer formation methods, coating methods, printing methods, electrolytic deposition methods, electrodeposition techniques or vapor deposition techniques known to the person skilled in the art.

A suitable process for producing a device which comprises layers a), b1 ) or b2) and c) comprises the following steps:

i) providing an electrically conductive layer a),

ii) optionally depositing an undercoating layer thereon,

iii) depositing a photosensitive layer b1 ) or b2) on the electrically conductive layer a) obtained in step i) or, if present, the undercoating layer obtained in step ii), and iv) depositing a hole transport layer c) on the photosensitive layer b1 ) or b2) obtained in step iii). In case of a solid dye-sensitized solar cell the photosensitive layer b1 ) comprises a semi-conductive metal oxide sensitized by a chromophoric substance. In case of a Perovskite solar cell the photosensitive layer b2) comprises a Perovskite absorber and a semiconductive metal oxide or a carrier material that does not act as an n-type oxide. The electrically conductive layer a) may be disposed on a substrate (also called support or carrier) to improve the strength of the solar cell. In the present invention, a layer composed of the electrically conductive layer a) and a substrate on which it is disposed is referred to as conductive support. Preferably, the electrically conductive layer a) and the substrate on which it is optionally disposed are transparent.

Some layers comprised by the solid dye-sensitized solar cell or Perovskite solar cell obtained in the process of the present invention are explained in detail below. a): electrically conductive layer being part of or forming the working electrode (anode)

The electrically conductive layer a) is either as such stable enough to support the remaining layers, or the electrically conductive material forming the electrically conduc- tive layer is disposed on a substrate (also called support or carrier). Preferably, the electrically conductive material forming the electrically conductive layer a) is disposed on a substrate. The combination of electrically conductive material disposed on a substrate is called in the following "conductive support". In the first case, the electrically conductive layer a) is preferably made of a material that has a sufficient strength and that can sufficiently seal the solar cell, for example, a metal such as platinum, gold, silver, copper, zinc, titanium, aluminum and an alloy composed thereof. In the second case, the substrate on which the electrically conductive layer a) containing an electrically conductive material is generally disposed opposite of the photosensitive layer b1 ) or b2), so that the electrically conductive layer a) is in direct contact with the photosensitive layer b1 ) or b2). Preferred examples of the electrically conductive material of layer a) include: metals such as platinum, gold, silver, copper, zinc, titanium, aluminum, indium and alloys composed thereof; carbon, especially in the form of carbon nano tubes; and electrically conductive metal oxides, especially transparent conductive oxides (TCO), such as for example indium-tin composite oxides, tin oxides doped with fluorine, antimony or indi- urn and zinc oxide doped with aluminum. In case of metals, these are generally used in form of thin films, so that they form a sufficiently transparent layer. More preferably, the electrically conductive material of layer a) is selected from transparent conductive oxides (TCO). Among these, tin oxides doped with fluorine, antimony or indium and indi- um-tin oxide (ITO) are preferred, more preferred being tin oxides doped with fluorine, antimony or indium and specifically preferred being tin oxides doped with fluorine. Specifically, the tin oxide is SnC>2. The electrically conductive layer a) preferably has a thickness of 0.02 to 10 μιτι and more preferably from 0.1 to 1 μιτι.

Generally, light will be irradiated from the side of the electrically conductive layer a) (and not from the electrically conductive layer being part of or forming the counter elec- trode (cathode), i.e. layer e)). Thus, as already mentioned, it is preferred that the support which carries the electrically conductive layer a) and preferably the conductive support as a whole is substantially transparent. Herein, the term "substantially transparent" means that the light transmittance is 50% or more to a light in visible region to near infrared region (400 to 1000 nm). The light transmittance is preferably 60% or more, more preferably 70% or more and in particular 80% or more. The conductive support particularly preferably has high light transmittance to a light that the photosensitive layer b1 ) or b2) has sensitivity to.

The substrate may be made of a glass, such as soda glass (that is excellent in strength) and non-alkali glass (that is not affected by alkaline elution). Alternatively, a transparent polymer film may be used as substrate. Used as the materials for the polymer film may be tetraacetyl cellulose (TAC), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), syndiotactic polystyrene (SPS), polyphenylenesulfide (PPS), polycarbonate (PC), polyarylate (PAr), polysulfone (PSF), polyestersulfone (PES), polyimide (PI), polyetherimide (PEI), cyclic polyolefin, brominated phenoxy resin, and the like.

The conductive support is preferably prepared by disposing the electrically conductive material on the substrate by means of liquid coating or vapor deposition.

The amount of the electrically conductive material to be disposed on the substrate is chosen so that a sufficient transparency is secured. The suitable amount depends on the conductive material and the substrate used and will be determined for the single cases. For instance, in case of TCOs as conductive material and glass as substrate the amount may vary from 0.01 to 100 g per 1 m².

It is preferable that a metal lead is used to reduce the resistance of the conductive support. The metal lead is preferably made of a metal such as platinum, gold, nickel, titanium, aluminum, copper, silver, etc. It is preferable that the metal lead is provided on the substrate by a vapor deposition method, a sputtering method or the like, the electrically conductive layer being disposed thereon. The reduction in incident light quantity owing to the metal lead is limited to preferably 10% or less, more preferably 1 to 5% or less.

The electrically conductive layer a) may be coated with an undercoating layer which can also be described as a buffering layer. The purpose of the buffering layer is to avoid a direct contact of the hole transport layer c) with the electrically conductive layer a) and thus to prevent short-circuits.

This "undercoating" or buffering layer material is preferably a metal oxide. The metal oxide is preferably selected from a titanium, tin, zinc, iron, tungsten, vanadium or niobium oxide, such as T1O2, SnC>2, Fe2C>3, WO3, ZnO, V2O5 or Nb20s, and is more prefera- bly Ti0₂.

The undercoating layer may be disposed e.g. by a spray-pyrolysis method as described for example in Electrochim. Acta, 40, 643 to 652 (1995), or a sputtering method as described for example in Thin Solid Films 445, 251 -258 (2003), Surf. Coat. Technol. 200, 967 to 971 (2005) or Coord. Chem. Rev. 248 (2004), 1479.

The thickness of the undercoating layer is preferably 5 to 1000 nm, more preferably 10 to 500 nm and in particular 10 to 200 nm. b1 ) or b2): photosensitive layer comprising a semi-conductive metal oxide and a chro- mophoric substance or photosensitive layer comprising a Perovskite absorber material

In a first embodiment, the photosensitive layer b1 ) contains the semi-conductive metal oxide sensitized with a chromophoric substance (also denoted as dye or photosensitive dye). The dye-sensitized semi-conductive metal oxide acts as a photosensitive substance to absorb light and conduct charge separation, thereby generating electrons. As is generally known, thin layers or films of metal oxides are useful solid semi-conductive materials (n-semi-conductors). However, due to their large band gap they do not absorb in the visible range of the electromagnetic spectrum, but rather in the UV region. Thus, for the use in solar cells, they have to be sensitized with a dye that absorbs in the range of about 300 to 2000 nm. In the photosensitive layer b1 ), the dye molecules absorb photons of the immersive light which have a sufficient energy. This creates an excited state of the dye molecules which inject an electron into the conduction band of the semi-conductive metal oxide. The semi-conductive metal oxide receives and conveys the electrons to the electrically conductive layer a) and thus to the working electrode (see below). In a second embodiment, the photosensitive layer b2) comprises a Perovskite absorber material and a semi-conductive metal oxide or a carrier material that does not act as a n-type oxide.

The solar cell obtained by the process of the invention comprises at least one photosensitive layer b1 ) or b2). The layer b1 ) or b2) may comprise two sublayers, each of which has a homogeneous composition and forms a flat donor-acceptor

heterojunction. The layer b1 ) or b2) may also comprise a mixed layer and form a donor-acceptor heterojunction in the form of a donor-acceptor bulk heterojunction. A preferred embodiment of the invention is a process for the production of a solid dye- sensitized solar cell or a Perovskite solar cell in which layer b1 ) or b2) is a layer in the form of a flat donor-acceptor heterojunction.

(1 ) Semi-conductive metal oxide An n-type semi-conductor is preferably used in the present invention, in which conduction band electrons act as a carrier under photo-excitation condition to provide anode current.

Suitable semi-conductive metal oxides are all metal oxides known to be useful on organic solar cells. They include: oxides of titanium, tin, zinc, iron, tungsten, zirconium, hafnium, strontium, indium, cerium, yttrium, lanthanum, vanadium, caesium, niobium or tantalum. Further, composite semi-conductors such as M¹ _xM²yO_z may be used in the present invention, wherein M¹ and M² independently represent a metal atom, O represents an oxygen atom, and x, y and z represent numbers combined with each other to form a neutral molecule. Examples are T1O2, SnC>2, Fe2C>3, WO3, ZnO, Nb20s, SrTiC , Ta2C>5, CS2O, zinc stannate, complex oxides of the Perovskite type, such as barium titanate, and binary and ternary iron oxides.

Preferred semi-conductive metal oxides are selected from T1O2, SnC>2, Fe2C>3, WO3, ZnO, Nb2C>5, and SrTiC . Of these semiconductors, more preferred are T1O2, Sn02, ZnO and mixtures thereof. Even more preferred are T1O2, ZnO and mixtures thereof, particularly preferred being T1O2.

The metal oxides are preferably present in amorphous or nanocrystalline form. More preferably, they are present as nanocrystalline porous layers. Such layers have a big surface on which a large number of dye molecules can be adsorbed, thus resulting in a high absorption of immersing light. The metal oxide layers may also be present in a structured form, such as nanorods. Nanorods offer the advantage of high electron mobility and an improved filling of the pores with the dye.

If more than one metal oxide is used, the two or more metal oxides can be applied as mixtures when the photosensitive layer is formed. Alternatively, a layer of a metal oxide may be coated with one or more metal oxides different therefrom. The metal oxides may also be present as a layer on a semi-conductor different therefrom, such as GaP, ZnP or ZnS.

T1O2 and ZnO used in the present invention are preferably in anatase-type crystal structure, which in turn is preferably nanocrystalline.

The semi-conductor may or may not comprise a dopant to increase the electron conductivity thereof. Preferred dopants are metal compounds such as metals, metal salts and metal chalcogenides. In the photosensitive layer b1 ) or b2), the semiconductive metal oxide, if present, is preferably porous, particularly preferably nanoporous and specifically mesoporous. Porous material is characterized by a porous, non-smooth surface. Porosity is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume. Nanoporous material has pores with a diameter in the nanometer range, i.e. ca. from 0.2 nm to 1000 nm, preferably from 0.2 to 100 nm. Mesoporous material is a specific form of nanoporous material having pores with a diameter of from 2 to 50 nm. "Diameter" in this context refers to the largest dimension of the pores. The diameter of the pores can be determined by several porosimetry methods, such as optical methods, imbibition methods, water evaporation method, mercury intrusion porosimetry or gas expansion method.

The particle size of the semi-conductive metal oxide used for producing the semi- conductive metal oxide layer is generally in the nm to μιτι range. The mean size of primary semi-conductor particles, which is obtained from a diameter of a circle equivalent to a projected area thereof, is preferably 200 nm or less, e.g. 5 to 200 nm, more preferably 100 nm or less, e.g. 5 to 100 nm or 8 to 100 nm. Two or more of the semi-conductive metal oxides having a different particle size distribution may be mixed in the preparation of the photosensitive layer b1 ) or b2). In this case, the average particle size of the smaller particles is preferably 25 nm or less, more preferably 10 nm or less. To improve a light-capturing rate of the solar cell by scattering rays of incident light, semi-conductive metal oxides having a large particle size, e.g. approximately 100 to 300 nm in diameter, may be used for the photosensitive layer b1 ) or b2).

Preferred as a method for producing the semi-conductive metal oxides are: sol-gel methods described for example in Materia, Vol. 35, No. 9, Page 1012 to 1018 (1996). A method which comprises preparing oxides by subjecting chlorides to a high temperature hydrolysis in an oxyhydrogen salt, is also preferred.

In the case of using titanium oxide as the semi-conductive metal oxide, the above- mentioned sol-gel methods, gel-sol methods and high temperature hydrolysis methods are preferably used. Of the sol-gel methods also preferred are such methods as described by Barbe et al., Journal of American Ceramic Society, Vol. 80, No. 12, page 3157 to 3171 (1997) and Burnside et al, Chemistry of Materials, Vol. 10, No. 9, page 2419 to 2425 (1998).

The semi-conductive metal oxides may be applied onto the electrically conductive layer a), optionally after deposition of an undercoating layer thereon, preferably by the following methods. Preferably, the semi-conductive metal oxides are applied onto the layer a) by a method where the layer a) is coated with a dispersion or a colloidal solution containing the semi-conductive metal oxide particles; the above-mentioned sol-gel method; etc. A wet type layer formation method is generally advantageous for the mass production of the solar cells, for improving the properties of the semi-conductive metal oxide dispersion, and for improving the adaptability of the layer a), etc. As such a wet type layer formation method, coating methods, printing methods, electrolytic deposition methods and electrodeposition techniques are typical examples. Further, the semi-conductive metal oxide layer may be disposed by: oxidizing a metal; an LPD (liquid phase deposition) method where a metal solution is subjected to ligand exchange, etc.; a sputtering method; a PVD (physical vapor deposition) method; a CVD (chemical vapor deposition) method; or an SPD (spray pyrolysis deposition) method where a thermal decomposition-type metal oxide precursor is sprayed on a heated substrate to generate a metal oxide. The dispersion containing the semi-conductive metal oxides may be prepared by the sol-gel methods mentioned above, by crushing the semiconductor in a mortar, by dispersing the semiconductor while grinding it in a mill, by synthesizing and precipitating the semiconductive metal oxides in a solvent, etc.

As a dispersion solvent, water or organic solvents such as methanol, ethanol, isopropyl alcohol, citronellol, terpineol, dichloromethane, acetone, acetonitrile, ethyl acetate, etc., mixtures thereof and mixtures of one or more of these organic solvents with water may be used. A polymer such as polyethylene glycol, hydroxyethylcellulose and carbox- ymethylcellulose, a surfactant, an acid, a chelating agent, etc. may be used as a dispersing agent, if necessary. In particular, polyethylene glycol may be added to the dispersion because the viscosity of the dispersion and the porosity of the semiconductive metal oxide layer can be controlled by changing the molecular weight of the polyeth- ylene glycol, and the semiconductive metal oxide layer containing polyethylene glycol is hardly peeled off.

Preferred coating methods include e.g. roller methods and dip methods for applying the semi-conductive metal oxide, and e.g. air-knife methods and blade methods for cali- brating the layer. Further preferred as a method where the application and calibration can be performed at the same time are wire-bar methods, slide-hopper methods, e.g. such as described in US 2,761 ,791 , extrusion methods, curtain methods, etc. Furthermore, spin methods and spray methods may be used. As to wet type printing methods, relief printing, offset printing, gravure printing, intaglio printing, gum printing, screen printing, etc. are preferred. A preferable layer formation method may be selected from these methods in accordance with the viscosity of the dispersion and the desired wet thickness.

As already mentioned, the semi-conductive metal oxide layer is not limited to a single layer. Dispersions each comprising the semi-conductive metal oxides having a different particle size may be subjected to a multi-layer coating. Further, dispersions each containing different kinds of semi-conductive metal oxides, binder or additives may be subjected to a multi-layer coating. The multi-layer coating is also effectively used in case the thickness of a single layer is insufficient.

Generally, with increasing thickness of the semi-conductive metal oxide layer, which equals the thickness of the photosensitive layer b1 ) or b2), the amount of the dye incorporated therein per unit of projected area increases resulting in a higher light captur- ing rate. However, because the diffusion distances of the generated electrons also increase, higher loss rates owing to recombination of the electric charges are to be expected. Moreover, customarily used dyes such as phthalocyanins and porphyrins have a high absorption rates, so that thin layers or films of the metal oxide are sufficient. Consequently, the preferable thickness of the semi-conductive metal oxide layer is 0.1 to 100 μιτι, more preferably 0.1 to 50 μιτι, even more preferably 0.1 to 30 μιτι, in particular 0.1 to 20 μιτι and specifically 0.5 to 3 μιτι.

The coating amount of the semi-conductive metal oxides per 1 m² of the substrate is preferably 0.5 to 100 g, more preferably 3 to 50 g.

After applying the semiconductive metal oxide(s) onto the electrically conductive layer a), the obtained product is preferably subjected to a heat treatment (sintering step), to electronically contact the metal oxide particles with each other and to increase the coating strength and the adherence thereof with the layer below. The heating temperature is preferably 40 to 700 °C, more preferably 100 to 600 °C. The heating time is preferably 10 minutes to 10 hours.

However, in case the electrically conductive layer a) contains a thermosensitive mate- rial having a low melting point or softening point such as a polymer film, the product obtained after the application of the semi-conductive metal oxide is preferably not subjected to a high temperature treatment because this may damage such a substrate. In this case, the heat treatment is preferably carried out at a temperature as low as possible, for example, 50 to 350 °C. In this case, the semi-conductive metal oxide is prefer- ably one with smaller particles, in particular having a medium particle size of 5 nm or less. Alternatively, a mineral acid or a metal oxide precursor can be heat-treated at such a low temperature.

Further, the heat treatment may be carried out while applying an ultraviolet radiation, an infrared radiation, a microwave radiation, an electric field, an ultrasonic wave, etc. to the semi-conductive metal oxides, in order to reduce the heating temperature. To remove unnecessary organic compounds, etc., the heat treatment is preferably carried out in combination with evacuation, oxygen plasma treatment, washing with pure water, a solvent or a gas, etc.

If desired, it is possible to form a blocking layer on the layer of the semi-conductive metal oxide before sensitizing it with a dye in order to improve the performance of the semi-conductive metal oxide layer. Such a blocking layer is usually introduced after the aforementioned heat treatment. An example of forming a blocking layer is immersing the semi-conductive metal oxide layer into a solution of metal alkoxides such as titanium ethoxide, titanium isopropoxide or titanium butoxide, chlorides such as titanium chloride, tin chloride or zinc chloride, nitrides or sulfides and then drying or sintering the substrate. For instance, the blocking layer is made of a metal oxide, e.g. ΤΊΟ2, S1O2, AI2O3, ZrC>2, MgO, SnC>2, ZnO, EU2O3, Nb20s or combinations thereof, TiCI₄, or a polymer, e.g. poly(phenylene oxide-co-2-allylphenylene oxide) or poly(methylsiloxane). Details of the preparation of such layers are described in, for example, Electrochimica Acta 40, 643, 1995; J. Am. Chem. Soc. 125, 475, 2003; Chem. Lett. 35, 252, 2006; J. Phys. Chem. B, 1 10, 1991 , 2006. Preferably, TiCI₄ is used. The blocking layer is usually dense and compact, and is usually thinner than the semi-conductive metal oxide layer.

As outlined above, it is preferable that the semi-conductive metal oxide layer has a large surface area to adsorb a large number of dye molecules. The surface area of the semi-conductive metal oxide layer is preferably 10 times or more, more preferably 100 times or more higher than its projected area.

(2) Dye

The dye used as chromophoric substance for the photosensitive layer b1 ) is not particularly limited if it can absorb light particularly in the visible region and/or near infrared region (especially from ca. 300 to 2000 nm) and can sensitize the semi-conductive metal oxide. Examples are metal complex dyes (see for example US 4,927,721 , US 5,350,644, EP-A-1 176646, Nature 353, 1991 , 737-740, Nature 395, 1998, 583-585, US 5,463,057, US 5,525,440, US 6,245,988, WO 98/50393), indoline dyes (see for example (Adv. Mater. 2005, 17, 813), oxazine dyes (see for example US 6,359,21 1 ), thiazine dyes (see for example US 6,359,21 1 ), acridine dyes (see for example US 6,359,21 1 ), porphyrin dyes, methine dyes (preferably polymethine dyes such as cyanine dyes, merocyanine dyes, squalilium dyes, etc; see for example US 6,359,21 1 , EP 89241 1 , EP 91 1841 , EP 991092, WO 2009/109499) and rylene dyes (see for example JP-A-10- 189065, JP 2000-243463, JP 2001-093589, JP 2000-100484, JP 10-334954, New J. Chem. 26, 2002, 1 155-1 160 and in particular DE-A-10 2005 053 995 and WO

2007/054470). Particularly preferred dyes are those as described in WO 2013/144177, especially preferred is a dye of the formula (I).

(I)

To make the photoelectric conversion wave range of the solar cell larger, and to increase the photoelectric conversion efficiency, two or more kinds of the dyes may be used as a mixture or in combination thereof. In the case of using two or more kinds of the dyes, the kinds and the ratio of the dyes may be selected in accordance with the wave range and the strength distribution of the light source.

For instance the absorption of the rylene dyes depends on the extent of the conjugated system. The rylene derivatives of DE-A-10 2005 053 995 have an absorption of from 400 nm (perrylene derivatives I) to 900 nm (quaterrylene derivatives I). Terrylene- based dyes absorb from about 400 to 800 nm. In order to obtain absorption over a range of the electromagnetic waves as large as possible it is thus advantageous to use a mixture of rylene dyes with different absorption maxima.

The dye preferably has an interlocking or anchor group, which can interact or adsorb to the surface of the semi-conductive metal oxides. Preferred interlocking groups include acidic groups such as -COOH , -OH , -S0₃H, -P(0)(OH)₂ and -OP(0)(OH)₂, and π- conductive chelating groups such as oxime group, dioxime group, hydroxyquinoline group, salicylate group and a-ketoenolate group. Anhydride groups are also suitable as they react in situ to carboxylic groups. Among them, preferred are acidic groups, par- ticularly preferred are -COOH, -P(0)(OH)₂ and -OP(0)(OH)₂. The interlocking group may form a salt with an alkaline metal, etc. or an intramolecular salt. In the case of polymethine dyes, an acidic group such as squarylium ring group or croconium ring group formed by the methine chain may act as the interlocking group.

Preferably, the dye has on the distal end (i.e. the end of the dye molecule opposite the anchor group) one or more electron donating groups which facilitate the regeneration of the dye after having donated an electron to the semi-conductive metal oxide and which optionally also prevent recombination with the donated electrons.

The dye may be adsorbed to the semi-conductive metal oxides by bringing these com- ponents into contact with each other, e.g. by soaking the product obtained after the application of the semi-conductive metal oxide layer in a dye adsorption solution, or by applying the dye adsorption solution to the semi-conductive metal oxide layer. In the former case, a soaking method, a dipping method, a roller method, an air-knife method, etc. may be used. In the soaking method, the dye may be adsorbed at room tempera- ture, or under reflux while heating as described in JP 7249790. As an applying method of the latter case, a wire-bar method, a slide-hopper method, an extrusion method, a curtain method, a spin method, a spray method, etc. may be used. Further, the dye may be applied to the semi-conductive metal oxide layer by an ink-jet method onto an image, thereby providing a photoelectric conversion surface having a shape of the im- age.

These methods can be used also in the case where the dye is adsorbed on the semi- conductive metal oxide while the semi-conductive metal oxide is treated with an additive, e.g. at least one hydroxamic acid or its salt, as described in WO 2012/001628. An example of a preferred compound from the group of hydroxamic acids or its salts is the compound of the formula (II).

Thus, the dye adsorption solution may contain the additive, in particular one or more hydroxamic acids or their salts. Preferably, the dye, e.g. in the form of a suspension or solution, is brought into contact with the semi-conductive metal-oxide when this is freshly sintered, i.e. still warm. The contact time should be sufficiently long to allow adsorption of the dye to the surface of the metal oxide. The contact time is typically from 0.5 to 24 h. If more than one dye is to be applied, the application of the two or more dyes can be carried out simultaneously, e.g. by using a mixture of two or more dyes, or subsequently by applying one dye after the other.

The dye may also be applied in mixture with the at least one hydroxamic acid or its salt. Additionally or alternatively the dye may be applied in combination with the charge transfer material. The dye not being adsorbed on the semi-conductive metal oxide layer is preferably removed by washing immediately after the dye adsorption process. The washing is preferably carried out by a wet-type washing bath with a polar solvent, in particular a polar organic solvent, for example acetonitrile or an alcohol solvent. The amount of the dye adsorbed on the semi-conductive metal oxides is preferably 0.01 to 1 mmol per 1 g of the semi-conductive metal oxides. Such an adsorption amount of the dye usually effects a sufficient sensitization to the semi-conductors. Too small an amount of the dye results in insufficient sensitization effect. On the other hand, unadsorbed dye may float on the semi-conductive metal oxides resulting in a reduction of the sensitization effect.

To increase the adsorption amount of the dye, the semi-conductive metal oxide layer may be subjected to a heat treatment before the dye is adsorbed thereon. After the heat treatment, it is preferable that the dye is quickly adsorbed on the semi-conductive metal oxide layer having a temperature of 60 to 150 °C before the layer is cooled to room temperature, to prevent water from adsorbing onto the semi-conductive metal oxide layer.

(3) Perovskite absorber material The Perovskite absorber material is preferably an organometallic halide compound. Preferred are compounds of the formula (R^dN H3)PbX^a3, wherein R^d is C1-C4 alkyl and X^a is CI, Br or I. Especially preferred are (CH₃NH₃)Pbl₃, (CH₃CH₂N H3)Pbl3,

(CH₃NH₃)Pbl₂CI and (CH₃CH₂NH₃)Pbl₂CI. (4) non-semiconductive carrier material

Preferred as a non-semiconductive carrier material is Al₂0₃. (5) passivating material

In order to prevent recombination of the electrons in the semi-conductive metal oxide with the hole transport layer, a passivating layer can be provided on the semi- conductive metal oxide. The passivating layer can be provided before the adsorption of the dye and also of the hydroxamic acid or its salt, or after the dye adsorption process and the treatment with the hydroxamic acid or its salt. Suitable passivating materials are aluminium salts, AI2O3, silanes, such as CH3S1CI3, metal organic complexes, especially Al³⁺ complexes, 4-tert-butyl pyridines, MgO, 4-guanidino butyric acid and hexa- decyl malonic acid. The passivating layer is preferably very thin.

(D) hole transport layer material

The hole transport layer c) replenishes electrons to the oxidized dye. Suitable hole- transporting materials are inorganic hole-transporting materials, organic hole- transporting materials or a combination thereof. Those compounds are known to a person skilled in the art.

Particularly suitable hole-transporting materials are small molecule compounds as well as polymer compounds.

Suitable hole-transporting materials are for example described in Y. Shirota et al., Chem. Rev. 2007, 107, 953-1010 and C.-Y. Hsu et al., Phys. Chem. Chem. Phys. 2012, 14, 14099-14109. The disclosure of these two documents is incorporated by reference here.

Suitable small compound hole-transporting materials are particularly 2,2', 7, 7'- tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) and 6,13- bis(triisopropylsilylethynyl)pentacen. Suitable hole-transporting materials based on polymers are particularly triarylamine-based polymers, poly[bis(4-phenyl)(2,4,6- trimethylphenyl)amine] (PTAA), poly(3-alkylthiophene), in particular poly(3- hexylthiophene-2,5-diyl (P3HT), especially regioregular poly(3-hexylthiophene-2,5-diyl (P3HT), poly[2,1 ,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1 - b:3, 4-b']dithiophene-2,6-diyl]] (PCPDTBT), polyaniline, polypyrrole, poly[2-methoxy-5- (2-ethylhexyloxy)-1 ,4-phenylenevinylene] (M EHPPV) and poly(3,4-ethylenedioxythio- phene) (PEDOT). Particularly preferred among the triarylamine-based polymers are linear triangulene oligomers and polymers as disclosed in US 61/752,509 which disclosure is incorporated by reference here. These preferred triarylamine-based compounds are of the formula (III)

(ill) wherein n is 0 to 100,

X¹, X², X³, X⁴, and X⁵ are independently of one another selected from hydrogen, F, CI, Br, I, CN, B(OR^c)₂, hydroxy, mercapto, nitro, cyanato, thiocyanato, formyl, acyl, carboxy, car- boxylate, alkylcarbonyloxy, carbamoyl, alkylaminocarbonyl, dialkyla- minocarbonyl, sulfo, sulfonate, sulfoamino, sulfamoyl, alkylsulfonyl, aryl- sulfonyl, amidino, N E¹E², where E¹ and E² are each independently selected from hydrogen, alkyl, cycloalkyi, heterocycloalkyl, aryl or hetaryl, in each case unsubstituted or substituted alkyl, alkoxy, alkylthio, (monoal- kyl)amino, (dialkyl)amino, cycloalkyi, cycloalkoxy, cycloalkylthio, (monocy- cloalkyl)amino, (dicycloalkyl)amino, heterocycloalkyl, heterocycloalkoxy, heterocycloalkylthio, (monoheterocycloalkyl)amino, (diheterocycloal- kyl)amino, aryl, aryloxy, arylthio, (monoaryl)amino, (diaryl)amino, hetaryl, hetaryloxy, hetarylthio, (monohetaryl)amino and (dihetaryl)amino, wherein R^c is selected from in each case unsubstituted or substituted alkyl, cycloalkyi or aryl, or wherein two radicals R^c may together form a divalent bridging group selected from in each case unsubstituted or substituted C2-Cio-alkylene, C3-C6-cycloalkylene and C6-Ci4-arylene, wherein C2-Cio-alkylene, C3-C6-cycloalkylene and C6-Ci4-arylene may carry one or more identical or different Ci-Ci2-alkyl radicals,

R^a and R^b are independently of one another selected from hydrogen and unsubstituted d-Ce-alkyl,

R¹, R2, R³, R⁴, R⁵, R⁶, R¹³, R¹⁴, R¹⁵, R¹⁶, and, if present, R⁷, R⁸, R⁹, R¹⁰, R¹¹ and R¹² are independently of one another selected from hydrogen, and in each case unsubstituted or substituted alkyl, alkoxy, alkenyl, alkadienyl, alkynyl, cycloalkyi, cycloalkoxy, bicyclo- alkyl, cycloalkenyl, heterocycloalkyl, heterocycloalkyloxy, aryl, aryloxy, heteroaryl and heteroaryloxy.

Particularly preferred hole-transporting materials are selected from 2, 2', 7,7'- tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD), 6,13- bis(triisopropylsilylethynyl)pentacen, triarylamine-based polymers, in particular those of the formula (III), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), poly(3- alkylthiophene), in particular regioregular poly(3-hexylthiophene-2,5-diyl (P3HT), poly[2,1 ,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1 -b:3, 4- b']dithiophene-2,6-diyl]] (PCPDTBT) and polyaniline. Method for forming the hole transport layer

The charge transfer layer c) is preferably directly disposed on the photosensitive layer b1 ) or b2) and the layers d) and e) are disposed afterwards. If the hole-transporting material is a polymer compound, the hole transport layer c) is preferably either formed by disposing the polymer by the following methods onto layer b1 ) or b2) or the precursors of the polymer are disposed by the following methods onto layer b1 ) or b2) and these precursors are polymerized afterwards. The hole transport layer c) may be disposed e.g. by a roller method, a dip method, an air-knife method, an extrusion method, a slide-hopper method, a wire-bar method, a spin method, a spray method, a cast method or a printing method. Suitable methods are similar to those of forming the semi-conductive metal oxide layer or adsorbing a dye to the semi-conductor, mentioned above.

If the hole transport layer c) is composed of at least one solid electrolyte, the solid hole transporting material, etc. may be formed by a dry film-forming method such as a physical vacuum deposition method or a CVD method, followed by disposing the inter layer d) thereon. The hole-transporting material of layer c) may be made to penetrate into the photosensitive layer b1 ) or b2) by a vacuum deposition method, a cast method, a coating method, a spin-coating method, a soaking method, an electrolytic polymerization method, a photo-polymerization method, a combination of these methods, etc.

The solar cells which are obtained by the process of the invention have a short circuit current J_sc which is generally in the range of from 4 to 15 mA-cnr², preferably in the range of from 10 to 15 mA-cnr², more preferably in the range of from 12 to 14 mA-cnr².

The solar cells which are obtained by the process of the invention have an open circuit voltage V_oc which is generally in the range of from 500 to 800 mV, preferably in the range of from 600 to 800 mV, more preferably in the range of from 700 to 780 mV. The solar cells which are obtained by the process of the invention have a fill factor (FF) which is generally in the range of from 25 to 70%, preferably in the range of from 30 to 70%, more preferably in the range of from 50 to 70%.

The solar cells which are obtained by the process of the invention have an energy con- version efficiency η which is generally at least 0.1 %, preferably at least 1 %, more preferably at least 3 % and in particular at least 6 %. Preferably, the energy conversion efficiency η is in the range of from 0.7 to 7.2%, more preferably in the range of from 2 to 7.2% and in particular in the range of from 5 to 7%. The invention is illustrated in detail with reference to the following nonrestrictive examples.

EXAMPLES General procedure for the measurement of physical parameters

The energy conversion efficiencies η, the short-circuit current densities J_sc, the open- circuit voltages V_oc and the fill factors FF of the solar cells were determined with a source meter model 2400 from Keithley Instruments Inc. or a Solartron 1286 from Solartron Analytical under irradiation with artificial sunlight, generated with a solar simulator PEC-L 12 from Peccell Technologies, Inc. (AM 1.5 filter, intensity 100 mW/cm²). A mask with diameters of about 3.5 mm was equipped during the

measurements in order to limit the photoirradiated area.

Preparation examples

Preparation example 1 : copper in ethanol

400 g copper flakes (MP7450 ECKA Granules Germany GmbH; mean particle size between 0.5 and 250 μιτι), 2000 g 50 wt.-% formic acid and 6 g surfactant (alkoxylated, predominantly unbranched fatty alcohols, higher alkene oxides and ethylene oxide) were stirred for 45 min at 60 °C. From the surface of the resulting dispersion, copper(ll)-oxide was removed. The copper flakes were isolated by vacuum filtration under nitrogen atmosphere and subsequently washed with water and acetone. The resulting filter cake was basically free of copper(ll)-oxide. The resulting filter cake was redispersed in a mixture of 250 g acetone and 10 g oleoylsarcosine acid. The resulting dispersion was filtered by vacuum filtration under nitrogen atmosphere and the filter cake was washed with acetone. The filter cake was mixed with about 400 ml. of a 20 wt.-% solution of polyvinylbutyral (Mowital® BH60 from Kuraray) in ethanol and the mixture was treated in a bowl mill (Skandex) for 1 h. The resulting dispersion had a weight ratio of copper to polyvinylbutyral of about 5 : 1. Preparation example 2: graphene in ethanol/water (4 : 1 )

158.4 g ethanol and 39.6 g water were mixed and 2 g Ultramid® 1 C (polyamide from BASF SE) were dissolved in the mixture. In 98 g of the resulting solution, 2 g graphene solid (commercially available, contains over 90 wt.-% graphite) were dispersed over a period of 4 h using a Dispermat® dissolver (from VMA-Getzmann GmbH) and milling pearls at a speed of 4000 rpm. The milling pearls were separated from the resulting disperson before usage.

Preparation example 3: graphene in ethanol

2 g Ultramid® 1 C (polyamide from BASF SE) were dissolved in 198 g ethanol. In 98 g of the resulting solution, 2 g graphene solid (commercially available, contains over 90 wt.-% graphite) were dispersed over a period of 4 h using a Dispermat® dissolver (from VMA-Getzmann GmbH) and milling pearls at a speed of 4000 rpm. The milling pearls were separated from the resulting disperson before usage.

Preparation example 4: PEDOT:PSS in ethanol/isopropanol (1 : 1 )

1 g poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (Clevious P VP Al 4083 from Heraeus) were mixed with 1 g ethanol and 1 g isopropanol. The resulting solution was filtrated with a glass fiber filter (pore size 1 μιτι). Preparation example 5: carbon black in PEDOT:PSS/ethanol/isopropanol (1 : 1 : 1 )

1 g carbon black (#2300 from Mitsubishi Chemical Corp.) was mixed with 9 g of the solution of preparation example 4. The mixture was dispersed with an ultrasonic bath or a homogenizer Q700 from Qsonica for 10 to 60 min.

Application examples: solid dye-sensitized solar cells

General procedure A): procedure for the production of a solid dye-sensitized solar cell without electrically conductive inter layer d) and counter electrode e)

FTO glass substrates (tin oxide doped with fluorine, A1 1 DU80 by AGC Fabritech Co., Ltd., < 12 Ω/sq) were used as the base material. A layer of solid titanium dioxide was deposited on the FTO glass substrate using a spray-pyrolysis method in analogy to Electrochim. Acta 1995, 40, 643-652. Alternatively, the titanium dioxide was deposited by sputtering. On the layer of titanium dioxide, titanium dioxide paste (PST-18NR by Catalysts & Chemicals Ind. Co., Ltd.) was distributed by a screen printing method and sintered for 1 h at 450 °C to afford a mesoporous layer of titanium dioxide having a thickness of about 1 .6 μιτι. The resulting sample was treated with titanium tetrachloride in analogy to M . Gratzel et al., Adv. Mater. 2006, 18, 1202. After the sintering process the sample was cooled to about 60 to 80 °C and treated for 20 min with a 5 mM solution of the compound of the formula (II) in ethanol. With respect to this procedure, reference is made to WO 2012/001628. The sample was washed in a bath of ethanol, briefly dried in a nitrogen stream and subsequently immersed for 2 h in a 0.5 mM solution of the dye of the formula (I) as shown above in a 1 : 1 by volume mixture of acetonitrile and tert-butanol in order to adsorb the dye. With respect to the dye, reference is also made to WO 2013/144177. Afterwards, the sample was removed from the solution, washed with acetonitrile and dried in a nitrogen stream. A 0.165 M solution of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro- MeOTAD, from Merck) in chlorobenzene was prepared and mixed with a 0.3 M solution of LiN(SC>2CF3)2 (from Wako Pure Chemical Industries, Ltd.) in cyclohexanone in a volume ratio of 14 : 1 . The resulting solution was applied onto the sample in amount of approximately 30 μΙ/cm² for about 1 min. Afterwards, the supernatant solution was removed by centrifugation at 2000 rpm and dried in ambient air for 3 h.

Comparative example 1 : Ag counter electrode by metal vapor deposition

The basic sample was prepared according to general procedure A). The Ag counter electrode was prepared by thermal metal vapor deposition in vacuum at a pressure of about 1 ^■ 10^"5 mbar on the sample. The sample was equipped with a mask in order to deposit two separate counter electrodes with a circle shape with diameters of about 5 mm on the surface. The Ag was vaporized at a rate of 0.5 nm/s until a layer with an approximate thickness of 100 nm was formed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 1 .

Comparative example 2: Cu counter electrode by blade coating

The basic sample was prepared according to general procedure A). 1 g of the mixture from preparation example 1 was blade coated with 50 μιτι gap directly onto the sample and immediately dried at 80 °C in an oven or on a hotplate for 30 s to 10 min. A conductive film with a thickness of from 5 to 30 μιτι was obtained. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 1 .

Comparative example 3: carbon black counter electrode by spin-coating

The basic sample was prepared according to general procedure A). 1 ml. of the mixture from preparation example 5 was spin-coated at 1000 to 2000 rpm for 45 s onto the sample. The resulting film was not uniform and had a thickness of from 1 to 10 μιτι. For the measurements, a uniform area on the sample was selected. The surface resistance of this area was about 1 to 4 kQ/sq. The conductivity of this area was about 0.3 to 3 S-crrv¹. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 1. Comparative example 4: polyaniline counter electrode by spin-coating

The basic sample was prepared according to general procedure A). 0.2 ml. of a 5 wt- % dispersion of polyaniline in water (PANW-5 from Kaken Sangyo) were used as is and spin-coated at 1000 to 2000 rpm for 45 s onto the sample. The resulting film had a thickness in the range of from 0.2 to 0.7 μιτι. The surface resistance of the film was about 0.2 to 0.3 ΜΩ/sq. The conductivity of the film was about 0.02 to 0.03 S-cnrr¹. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 1.

Comparative example 5: Ag counter electrode by blade coating with layer transfer

The basic sample was prepared according to general procedure A). 0.5 g silver grease (Heat-Away 651 -EV from Aremco Products, Inc.) were used as is and blade coated with 50 μιτι gap onto indium tinoxide (ITO) film. The film was then attached with double sided tape on the sample so that the Ag grease had contact with the surface of the sample. The Ag film had a thickness of approximately 50 μιτι. The physical parameters of the solar cell were measured according to the general procedure, where the electrical contact was made through the ITO film. The results are shown in table 1. Comparative example 6: PEDOT:PSS counter electrode by spin-coating

The basic sample was prepared according to general procedure A). 0.2 ml. of the mixture from preparation example 4 were spin-coated at 2000 rpm for 45 s. The film was further dried for 10 min at 60 °C in order to obtain a film with a thickness of around 20 nm. The surface resistance of the film was about 4 ΜΩ/sq. The conductivity of the film was about 0.1 S-crrv¹.

Example 1 : Cu counter electrode by blade coating and carbon black/PEDOT:PSS inter layer by spin coating

The basic sample was prepared according to general procedure A). 1 ml. of the mixture from preparation example 5 was spin-coated at 1000 to 2000 rpm for 45 s onto the sample. The resulting film was not uniform and had a thickness of from 1 to 10 μιτι. For a uniform area on the sample, the surface resistance was about 1 to 4 kQ/sq. The conductivity of this area was about 0.3 to 3 S-crrv¹. The mixture from preparation example 1 was blade coated with 50 μιτι gap directly onto the sample and immediately dried at 80 °C in an oven or on a hotplate for 30 s to 10 min. A conductive film with a thickness of from 5 to 30 μιτι was obtained. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 1 .

Example 2: Cu counter electrode by blade coating and polyaniline inter layer by spin coating

The basic sample was prepared according to general procedure A). 0.2 ml. of a 5 wt- % dispersion of polyaniline in water (PANW-5 from Kaken Sangyo) were used as is and spin-coated at 1000 to 2000 rpm for 45 s onto the sample. The resulting film had a thickness in the range of from 0.2 to 0.7 μιτι. The surface resistance of the film was about 0.2 to 0.3 ΜΩ/sq. The conductivity of the film was about 0.02 to 0.03 S-cnrr¹. The mixture from preparation example 1 was blade coated with 50 μιτι gap directly onto the sample and immediately dried at 80 °C in an oven or on a hotplate for 30 s to 10 min. A conductive film with a thickness of from 5 to 30 μιτι was obtained. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 1 .

Example 3: Ag counter electrode by blade coating and PEDOT:PSS inter layer by spin coating

The basic sample was prepared according to general procedure A). 0.2 ml. of the mixture from preparation example 4 were spin-coated at 2000 rpm for 45 s. The film was further dried for 10 min at 60 °C in order to obtain a film with a thickness of around 20 nm. The surface resistance of the film was about 4 ΜΩ/sq. The conductivity of the film was about 0.1 S-crrv¹. Silver grease (Heat-Away 651 -EV from Aremco Products, Inc.) was used as is and blade coated with 50 μιτι gap onto indium tinoxide (ITO) film. The film was then attached with double sided tape on the sample so that the Ag grease had contact with the surface of the sample. The Ag film had a thickness of

approximately 50 μιτι. The physical parameters of the solar cell were measured according to the general procedure, where the electrical contact was made through the ITO film. The results are shown in table 1. Table 1 : solid dye sensitized solar cells with and without inter layer d).

Example layer e) Jsc/ Voc/ fill factor efficienlayer d)

(C = corn- (counter mA-crrr² mV (FF)/% cy/% parative) electrode)

Ag

C 1 - 12.2 730 58 5.17

(evaporated)

C 2 - Cu 1 .9 684 18 0.23

C 3 - carbon black 8.3 721 32 1 .89 neglineglineglinegli¬

C 4 - polyaniline

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C 5 - Ag 1 .1 496 27 0.14 neglineglineglinegli¬

C 6 - PEDOT:PSS

gible gible gible gible carbon black/

1 Cu 10.9 740 37 3.00 PEDOT:PSS

2 polyaniline Cu 4.5 642 27 0.78

3 PEDOT:PSS Ag 8.7 724 37 2.34

As table 1 shows, the solid dye sensitized solar cells with an inter layer d) have an improved photon to current conversion compared to those without inter layer d), except for the vacuum deposited Ag counter electrode.

Application examples: Perovskite solar cells with 2,2',7,7'-tetrakis(N,N-di-p- methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD)

General procedure B): procedure for the production of a Perovskite solar cell with spiro-MeOTAD without electrically conductive inter layer d) and counter electrode e)

As base material, transparent conducting oxide (TCO) glass substrates from Geomatec Co., Ltd. (< 15 Ω/sq) were used. A layer of solid titanium dioxide was deposited on the TCO glass by a spray-pyrolysis method as described in Electrochim. Acta 1995, 40, 643-652. Alternatively, the titanium dioxide was deposited on the TCO glass by sputtering. Titanium dioxide paste (PST-18N R by Catalysts & Chemicals Ind. Co., Ltd.) was diluted with ethanol in a volume ratio of 1 : 3.5 and the resulting mixture was applied onto the sample. The mixture on the sample was dried by spin-coating at 1000 rpm for 45 s and sintered for 1 h at 450 °C in order to afford a mesoporous layer of titanium dioxide with a thickness of about 400 nm. The sample was cooled down to about 100 °C and a Perovskite precursor solution was applied to the sample. The Perovskite precursor solution consisted of 397 mg methylammonium iodide (CH3N H3I) and 231 mg lead(l l) chloride (PbCI₂) in 1 mL N ,N-dimethylformamide (DM F). The

Perovskite precursor solution was allowed to soak in into the titanium dioxide layer for 20 s and the supernatant solution was removed by centrifugation at 2000 rpm for 45 s. The sample was immediately placed in an oven at 100 °C for 40 min in order to form the Perovskite absorber layer. The sample was cooled down to room temperature and a hole-transporting layer was produced on the Perovskite layer. To this end, 98 mg of 2,2',7,7'-tetrakis(N ,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD, from Merck) were mixed with 3 mg LiN(SC>2CF3)2 (from Wako Pure Chemical

Industries, Ltd.), 9.5 μί 4-tert-butylpyridine and 1 mL chlorobenzene. From this mixture, 25 iL per 1 cm² of sample surface were applied to the sample and allowed to act for 20 s. The supernatant solution was then spun off for 45 s at 2000 rpm.

Comparative example 7: Au counter electrode by metal vapor deposition

The basic sample was prepared according to general procedure B). The Au counter electrode was prepared by thermal metal vapor deposition in vacuum at a pressure of about 1 ^■ 10^"5 mbar on the sample. The sample was equipped with a mask in order to deposit two separate counter electrodes with a circle shape with diameters of about 5 mm on the surface. The Au was vaporized at a rate of 0.1 to 0.5 nm/s until a layer with an approximate thickness of 100 nm was formed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2. Comparative example 8: Cu counter electrode by blade coating

The basic sample was prepared according to general procedure B). 1 g of the mixture from preparation example 1 was blade coated with 50 μιτι gap directly onto the sample and immediately dried at 80 °C in an oven or on a hotplate for 30 s to 10 min. A conductive film with a thickness of from 5 to 30 μιτι was obtained. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2. Comparative example 9: graphene counter electrode by blade coating

The basic sample was prepared according to general procedure B). 1 g of the mixture from preparation example 2 was blade coated with 50 to 150 μιτι gap directly onto the sample and immediately dried at 60 °C in an oven with a stream of air for 30 s to 10 min. A conductive film with a thickness of from 3 to 20 μιτι was obtained. The film had a surface resistance between 60 to 600 Ω/sq and a conductivity between 4 to 1 1 S-crrv¹. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Comparative example 10: Cu counter electrode by blade coating with layer transfer

The basic sample was prepared according to general procedure B). 1 g of the mixture from preparation example 1 was blade coated with 100 μιτι gap on releasing film (NP75Z01 AS from PAN AC Corporation) and immediately dried with a stream of air. The obtained counter electrode layer was peeled off from the releasing film with an adhesive foam (REVALPHA from Nitto Denko Corporation). The counter electrode which was sticking to the forming adhesive film was transferred to the device and attached to it by heating the device to 120 °C for 10 s. By this procedure, the forming adhesive film lost its adhesiveness and could be removed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Comparative example 1 1 : Ag counter electrode by blade coating with layer transfer

The basic sample was prepared according to general procedure B). 1 g Ag paste (MP- 301 PAD/5810 from Mino group Co., Ltd.) was blade coated with 50 μιτι gap on releasing film (NP75Z01 AS from PAN AC Corporation) and dried on a hot plate for 30 min at 120 °C. The obtained counter electrode layer was peeled off from the releasing film with an adhesive foam (REVALPHA from Nitto Denko Corporation). The counter electrode which was sticking to the forming adhesive film was transferred to the device and attached to it by heating the device to 120 °C for 10 s. By this procedure, the forming adhesive film lost its adhesiveness and could be removed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Comparative example 12: Ag counter electrode by blade coating The basic sample was prepared according to general procedure B). 1 ml. Ag ink (DOTITE D-550 from Fujikura Ltd.) was blade coated with 50 μιτι gap onto the sample and immediately dried with a stream of air. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Example 4: Cu counter electrode by blade coating and graphene as inter layer by blade coating

The basic sample was prepared according to general procedure B). 1 g of the mixture from preparation example 2 was blade coated with 50 to 150 μιτι gap onto the sample and immediately dried at 60 °C in an oven or with a stream of air for 30 s to 10 min. A conductive film with a thickness of from 3 to 20 μιτι was obtained. The film had a surface resistance between 60 to 600 Ω/sq and a conductivity between 4 to 1 1 S-crrv¹. The mixture from preparation example 1 was blade coated with 50 μιτι gap onto the sample and immediately dried at 80 °C in an oven or on a hotplate for 30 s to 10 min. A conductive film with a thickness of from 5 to 30 μιτι was obtained. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Example 5: Cu counter electrode by blade coating with layer transfer and graphene as inter layer by blade coating

The basic sample was prepared according to general procedure B). 1 g of the mixture from preparation example 3 was blade coated with 50 to 100 μιτι gap onto the sample and immediately dried at 60 °C in an oven or with a stream of air for 30 s to 10 min. A conductive film with a thickness of from 3 to 20 μιτι was obtained. The film had a surface resistance between 60 to 600 Ω/sq and a conductivity between 4 to 1 1 S-crrv¹. The mixture from preparation example 1 was blade coated with 100 μιτι gap on releasing film (NP75Z01 AS from PAN AC Corporation). The obtained counter electrode layer was peeled off from the releasing film with an adhesive foam (REVALPHA from Nitto Denko Corporation). The counter electrode which was sticking to the forming adhesive film was transferred to the device and attached to it by heating the device to 120 °C for 10 s. By this procedure, the forming adhesive film lost its adhesiveness and could be removed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Example 6: Ag counter electrode by blade coating with layer transfer and graphene inter layer by blade coating The basic sample was prepared according to general procedure B). 1 g of the mixture from preparation example 3 was blade coated with 50 to 100 μιτι gap onto the sample and immediately dried at 60 °C in an oven or with a stream of air for 30 s to 10 min. A conductive film with a thickness of from 3 to 20 μιτι was obtained. The film had a surface resistance between 60 to 600 Ω/sq and a conductivity between 4 to 1 1 S-crrv¹. Ag paste (MP-301 PAD/5810 from Mino Group Co., Ltd.) was blade coated with 50 μιτι gap on releasing film (NP75Z01 AS from PAN AC Corporation) and dried on a hot plate at 120 °C for 30 min. The obtained counter electrode layer was peeled off from the releasing film with an adhesive foam (REVALPHA from Nitto Denko Corporation). The counter electrode which was sticking to the forming adhesive film was transferred to the device and attached to it by heating the device to 120 °C for 10 s. By this procedure, the forming adhesive film lost its adhesiveness and could be removed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Example 7: Ag counter electrode by blade coating with layer transfer and graphene inter layer by blade coating

The basic sample was prepared according to general procedure B). 1 g of the mixture from preparation example 3 was blade coated with 50 to 100 μιτι gap onto the sample and immediately dried at 60 °C in an oven or with a stream of air for 30 s to 10 min. A conductive film with a thickness of from 3 to 20 μιτι was obtained. The film had a surface resistance between 60 to 600 Ω/sq and a conductivity between 4 to 1 1 S-crrv¹. Ag ink (DOTITE D-550) was blade coated with 100 μιτι gap on releasing film

(NP75Z01 AS from PANAC Corporation) and immediately dried with a stream of air. The obtained counter electrode layer was peeled off from the releasing film with an adhesive foam (REVALPHA from Nitto Denko Corporation). The counter electrode which was sticking to the forming adhesive film was transferred to the device and attached to it by heating the device to 120 °C for 10 s. By this procedure, the forming adhesive film lost its adhesiveness and could be removed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 2.

Table 2: Perovskite solar cells with spiro-MeOTAD HTM layer with and without inter layer d). Example layer e)

Jsc/ Voc/ fill factor efficien(C = comlayer d) (counter

mA-crrr² mV (FF)/% cy/% parative) electrode)

Au

C 7 - 13.9 756 65 6.78

(evaporated)

C 8 - Cu 1 .6 555 29 0.27

C 9 - graphene 8.5 771 28 1 .85

C 10 - Cu 2.8 327 40 0.37 neglineglineglinegli¬

C 1 1 - Ag

gible gible gible gible

C 12 - Ag 1 .5 213 28 0.09

4 graphene Cu 14.0 682 57 5.45

5 graphene Cu 13.5 667 67 6.07

6 graphene Ag 13.0 518 42 2.82

7 graphene Ag 14.6 703 69 7.1 1

As table 2 shows, the Perovskite solar cells with an inter layer d) have an improved photon to current conversion compared to those without inter layer d), except for the vacuum deposited Au counter electrode.

Application examples: Perovskite solar cells with poly(3-hexylthiophene-2,5-diyl) (P3HT)

General procedure C): procedure for the production of a Perovskite solar cell with P3HT without electrically conductive inter layer d) and counter electrode e)

The procedure was identical with general procedure B), except that the hole transport layer was produced from a solution of 15 mg poly(3-hexylthiophene-2,5-diyl) (P3HT) in 1 ml. chlorobenzene. Comparative example 13: Au counter electrode by metal vapor deposition

The basic sample was prepared according to general procedure C). The Au counter electrode was prepared by thermal metal vapor deposition in vacuum at a pressure of about 1 ^■ 10^"5 mbar on the sample. The sample was equipped with a mask in order to deposit two separate counter electrodes with a circle shape with diameters of about 5 mm on the surface. The Au was vaporized at a rate of 0.1 to 0.5 nm/s until a layer with an approximate thickness of 100 nm was formed. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 3.

Comparative example 14: graphene counter electrode by blade coating The basic sample was prepared according to general procedure C). 1 g of the mixture from preparation example 3 was blade coated with 50 to 100 μιτι gap onto the sample and immediately dried at 60 °C in an oven or with a stream of air for 30 s to 10 min. A conductive film with a thickness of from 3 to 20 μιτι was obtained. The film had a surface resistance between 60 to 600 Ω/sq and a conductivity between 4 to 1 1 S-crrv¹. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 3.

Comparative example 15: Ag counter electrode by blade coating The basic sample was prepared according to general procedure C). 1 ml. Ag ink

(DOTITE D-550 from Fujikura Ltd.) was blade coated with 100 μιτι gap on releasing film NP75Z01 AS from PAN AC Corporation and immediately dried with a stream of air. The obtained layer was peeled off from the releasing film with the help of the forming adhesive REVELPHA from Nitto Denko Corporation. The layer was transferred to the sample by attaching the adhesive film to the sample and heating the forming adhesive at 120 °C for 10 s. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 3.

Example 8: Ag counter electrode by blade coating and graphene inter layer by blade coating

The basic sample was prepared according to general procedure C). 1 g of the mixture from preparation example 3 was blade coated with 100 μιτι gap on releasing film NP75Z01 AS from PAN AC Corporation and immediately dried with a stream of air. The obtained layer was peeled off from the releasing film with the help of the forming adhesive REVELPHA from Nitto Denko Corporation. The layer was transferred to the sample by attaching the adhesive film to the sample and heating the forming adhesive at 120 °C for 10 s. A conductive film with a thickness of from 3 to 20 μιτι was obtained. The film had a surface resistance between 60 to 600 Ω/sq and a conductivity between 4 to 1 1 S-crrv¹. Ag ink (DOTITE D-550 from Fujikura Ltd.) was blade coated with 50 μιτι gap onto the sample and immediately dried with a stream of air. The physical parameters of the solar cell were measured according to the general procedure. The results are shown in table 3.

Table 3: Perovskite solar cells with P3HT HTM layer with and without inter layer d).

As table 3 shows, the Perovskite solar cell with an inter layer d) has an improved photon to current conversion compared to those without inter layer d), except for the vacuum deposited Ag counter electrode.

Claims

A process for the production of a solid dye-sensitized solar cell or a Perovskite solar cell, wherein the solar cell comprises

b1 ) a photosensitive layer comprising a semi-conductive metal oxide and a chromophoric substance

or

b2) a photosensitive layer comprising a Perovskite absorber material, c) a hole transport layer,

d) an electrically conductive inter layer comprising at least one conductor or semi-conductor, wherein that electrically conductive inter layer is different from layer c),

e) an electrically conductive layer different from layer d) being part of or forming the counter electrode (cathode),

The process according to claim 1 , wherein the contacting of layer d) with the liquid medium is performed in an atmosphere comprising or consisting of air.

The process according to either of the preceding claims, wherein the contacting of layer d) with the liquid medium is performed in an atmosphere which has a pressure in the range of from 500 mbar to 2 bar.

The process according to any one of the preceding claims, wherein the contacting of layer d) with the liquid medium is performed at ambient pressure.

The process according to any one of the preceding claims, wherein layer d) has an electrical conductivity in the range of from 1 -10 ¹ S-cnr¹ to 5-10³ S-crrv¹.

The process according to any one of the preceding claims, wherein layer d) has an average thickness in the range of from 20 nm to 100 μιτι.

The process according to any one of the preceding claims, wherein the ionization potential of layer d) is lower than the ionization potential of layer c).

The process according to any one of the preceding claims, wherein the material of layer d) is selected from carbon materials, poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline, M0S2 and mixtures thereof.

The process according to claim 8, wherein the carbon materials are selected from carbon black, carbon nanotubes, fullerenes, graphene, graphite and mixtures thereof.

10. The process according to any one of the preceding claims, wherein layer d) com- prises metal particles, in particular Ag, Cu and/or Au.

1 1 . The process according to any one of the preceding claims, wherein the liquid medium is a solution or dispersion of at least one conductor or semi-conductor in at least one solvent.

The process according to claim 1 1 , wherein the at least one solvent is selected from Ci-C6-alcohols, water and mixtures thereof.

13. The process according to either of claims 1 1 and 12, wherein the liquid medium comprises particles of Al, Cu, Ag, Au and/or graphene.

14. The process according to any one of claims 1 1 to 13, wherein the liquid medium comprises at least one dispersant polymer, in particular at least one polyamide.

15. The process according to any one of the preceding claims, wherein the contacting of layer d) with the liquid medium is performed by spin coating, slot coating, screen printing, gravure coating, offset printing, spray coating, ink-jet printing, dip coating, drop coating or blade coating.

16. The process according to claim 15, wherein the contacting of layer d) with the liquid medium is performed by ink-jet printing, blade coating or spin coating.

17. The process according to any one of the preceding claims, wherein the liquid medium is contacted with a substrate to provide layer e) and layer e) is after- wards transferred from the substrate onto layer d).

18. The use of an inter layer d) as defined in any one of the preceding claims for increasing the electrical contact between the hole transport layer c) and the electri- cally conductive layer being part of or forming the counter electrode (cathode) e) in a solid dye-sensitized solar cell or a Perovskite solar cell as defined in any one of the preceding claims.

A solid dye-sensitized solar cell or a Perovskite solar cell obtained by the process according to any one of claims 1 to 17.