GB2536862A

GB2536862A - Solar cells

Info

Publication number: GB2536862A
Application number: GB1422729.2A
Authority: GB
Inventors: James Holliman Peter; Lloyd Davies Matthew; Connell Arthur
Original assignee: Bangor University
Current assignee: Bangor University
Priority date: 2014-12-19
Filing date: 2014-12-19
Publication date: 2016-10-05

Abstract

An electrode comprising a conducting substrate and a layer of mesoporous titanium oxide. The titanium oxide layer is coated with a dye and a pigment. The dye may comprise ruthenium bipyridyl, coumarin, pthalocyanine,squaaine, or indoline. The pigment may comprise perovskite. The combination of a dye and a pigment coating allows the solar cell to convert multiple spectral ranges of solar radiation into electrical energy thereby increasing the device conversion efficiency.

Description

Solar Cells The present invention relates to solar cells comprising mesoporous TiO2, methods of manufacturing such solar cells and uses of dyes to improve the properties of such cells.

First generation (crystalline silicon) and second generation (amphorous silcon, thin film CdTe or copper indium gallium di(selenide) which is commonly known as CIGS) solar cells (or photovoltaic cells -PV cells) generate electricity from sunlight through the photovoltaic effect. The photovoltaic effect is observed when incident sunlight promotes electrons in a semi-conducting material from the valence band to the conduction band. This effect allows electrons to flow through a circuit attached to the cell. Solar cells based on silicon semiconductors have been known for many years and extensive research has led to the development of relatively efficient versions (20-25 % conversion of incident solar energy into electricity). However, the high cost of the materials and complicated production methods have hindered large scale production of silicon-based PV modules with the result that such silicon-based solar cells, and the electricity generated from them, have remained expensive.

Second generation (thin-film) and third generation (dye-sensitised or organic photovoltaic -OPV) solar cells have been developed as an alternative to silicon-based cells. Dye-sensitised solar cells (DSSC), developed in 1991 by O'Regan and Gratzel (O'Regan B. and Gratzel M., in Nature, 1991, 353, 737-740), can be produced using low cost materials and relatively simple manufacturing techniques which can be scaled up using roll-to-roll processing. A typical DSSC includes an anode comprising a dyed TiO2 layer on an indium-tin oxide (ITO) coated conducting substrate; a metallic cathode on a second conducting substrate; an electrolyte separating the cathode and anode; and a circuit connecting the anode and cathode. The two conducting substrates form two sides of a sealed unit enclosing the electrolyte and the electrodes. At least one of the conducting substrates is optically transparent to allow solar energy to enter the cell.

Such a DSSC operates when incident light strikes the dye and promotes electrons within the dye to an excited state. The excited state electrons are then transferred ("injected") to the conduction band of the TiO2 where they move through the anode, around the circuit and to the cathode. The electrons transferred from the dye are either replaced with electrons from a redox within the electrolyte or via a hole conductor. In the case of a redox couple electrolyte the electron deficient electrolyte species then diffuse to the cathode where they are regenerated by electrons which have flowed around the circuit.

DSSCs can generate a maximum voltage which is comparable or greater to that of silicon-based solar cells, around 0.7-0.9V. A disadvantage of silicon-based solar cells is that in low light conditions the electron/hole recombination mechanism becomes dominant. An advantage of DSSCs is that they transfer electrons into the TiO2 conduction band without creating electron vacancies (holes) nearby, thereby reducing quick electron/hole recombinations. DSSCs are therefore able to function relatively more efficiently in low light conditions where the electron/hole recombination mechanism would otherwise become dominant.

DSSC devices are also more efficient than p-n junction PV devices when sunlight is incident on the device at non-perpendicular angles. This is because DSSC electrodes include a metal oxide film which scatters light within it. This makes the initial angle of incidence less important than for p-n junction PV devices.

However, many existing DSSCs are not very efficient in the longer wavelength of the visible light frequency range, in the red and infrared region, because these photons do not have enough energy to cross the TiO2 band-gap or to excite the commonly used ruthenium bipyridyl dyes.

It is an aim of the present invention to overcome at least one of the disadvantages of the prior art.

According to a first aspect of the present invention there is provided an electrode comprising a conducting substrate and a layer of metal oxide, wherein the layer of metal oxide comprises a dye and a pigment.

Any suitable conducting substrate can be used. Suitable conducting substrates include metals, glass, polymers, graphite, alloys, metal oxides and semi-metals. The conducting substrate may be optically transparent or optically non-transparent. Suitable non-transparent conducting substrates include metals or alloys, for example steel, aluminium and titanium; and polymers, for example polyimide (PI) or poly ether ketone (PEEK) which have been coated with a conducting layer of polyaniline, and/or graphite layers.

Preferably the conducting substrate is optically transparent. Suitably the conducting substrate is selected from polymers and glass which have been coated with an optically transparent conducting layer.

Suitably conducting layers include transparent conducting oxides, for example fluorine-doped tin oxide, indium-doped tin oxide or aluminium-doped zinc oxide. Other suitable conducting layers include metal grids coated onto the substrate surface either alone or in combination with a transparent conducting oxide layer. For plastic substrates, these are preferably made from polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) which is either coated with a metal grid and/or indium tin oxide. Most preferably the conducting substrate is a glass substrate which has a low iron content to reduce optical losses. This may be coated with a metal grid and/or indium tin oxide and/or fluorine-doped tin oxide. Preferably it is coated with fluorine doped tin oxide coated glass.

The electrode further comprises a layer of metal oxide. The layer of metal oxide is suitably coated across the whole surface of one face of the conducting substrate. Suitably the layer of metal oxide forms a coherent film on the substrate surface.

Suitably the layer of metal oxide has a thickness of at least 10nm, suitably at least 50nm, preferably at least 100nm. The layer of metal oxide may have a thickness of up to 50pm, suitably up to 1 Opm, for example up to 5pm. The thickness may depend on other components of the cell comprising the electrode.

Preferred metal oxides include transition metal oxides and rare earth metal oxides, including mixed metal oxides. Most preferred are transition metal oxides.

Examples of suitable metal oxides include aluminium oxide, fin oxide, niobium oxide, zinc oxide and titanium dioxide.

Titanium dioxide is especially preferred.

Preferably the metal oxide is coated onto the conducting substrate in particulate form and then sintered to form a porous substrate of high surface area. Suitably the layer of formed from nanoparticles of metal oxide. Suitably the metal oxide nanoparticles have an average particle size of 1 to 500nm.

Preferably the layer of metal oxide is mesoporous. Mesoporous materials typically have a pore diameter of from 2 to 50nm. Most preferably the layer of metal oxide is a layer of mesoporous titanium dioxide.

The provision of a porous metal oxide (preferably mesoporous) enables the dye and the pigment to be dispersed throughout the metal oxide layer. The dye and/or pigment may also be adsorbed onto the surface of the metal oxide layer.

The layer of metal oxide comprises a dye. Suitable dyes are compounds capable of absorbing light in the visible region of the solar spectrum. Absorption of light promotes electrons in the dye molecules to an excited state. Suitable dyes are also capable of transferring the electrons in the excited state to the conduction band of the metal oxide.

The layer of metal oxide may comprise a single dye. The layer of metal oxide may comprise a mixture of two or more dyes. Suitable dyes include ruthenium bipyridyl complex dyes, coumarin dyes, phthalocyanine dyes, squaraine dyes and indoline dyes. Suitable squarine dyes include blue squaraine dye SQ1. Suitable indoline dyes include red indoline dye commonly known as D102. Suitable ruthenium bipyridyl complex dyes include di-ammonium salt of cis-bis(4,4'-dicarboxy-2,2'-bipyridine)dithiocyanato ruthenium(II) N719. Other suitable dyes will be known to the person skilled in the art.

The layer of metal oxide comprises a pigment. The pigment is suitably a light harvesting pigment which absorbs light in the visible region of the solar system. Suitable pigments are capable of absorbing light to form an electron/hole pair. Suitable pigments include quantum dots and perovskite pigments. Perovskite pigments are especially preferred.

The layer of metal oxide may comprise a single pigment for example single perovskite pigment. The layer of metal oxide may comprise a mixture of two or more pigments, for example two or more perovskite pigments.

Suitable quantum dots include combinations of cations of gallium or cadmium or lead with anions including arsenide, sulphide, selenide or telluride.

Suitable perovskite pigments include organometal halide perovskites. Suitable organometal halide perovskites include lead halide-methyl amine perovskite salts. Suitable lead halide-methyl amine perovskite salts include CH3NH3PbBr3 and CI-131\11-13Pb12C12.

Preferably the perovskite pigment is provided in crystalline form. Crystalline forms include cubic and tetragonal forms. Crystals present in tetragonal form tend to have lower band gaps and absorb further across the solar spectrum than perovskite crystals in the cubic form. These tend to have larger band gaps and so only absorb higher energy radiation. Ideally the crystals are nano-sized and form a thin, continuous film on the surface of a meso-porous support..

The dye and the perovskite may be selected to have at least partially complimentary light absorption. For example a dye may be selected which strongly absorbs light in a region of the UV/visible spectrum where the selected pigment weakly absorbs light. A pigment may selected which strongly absorbs light in a region of the UV/visible spectrum where the selected dye weakly absorbs light. Suitable dyes and pigments have an excited state which is higher in energy than the conduction band of the metal oxide.

The dye and the pigment may be selected to provide a specific colour of solar cell. For example the solar cell may comprise blue squarine dye SQ1 and yellow perovskite pigment CH3NH3PbBr3, providing a solar cell with a light green colour when viewed through the glass substrate. The solar cell may comprise red indoline dye D102 and yellow perovskite pigment CH3NH3PbBr3, providing a solar cell with a red colour when viewed through the glass substrate.

The surface of the metal oxide may further comprise a surface modifying agent. This surface modifying agent may be able to link to the metal oxide surface, preferably through a covalent bond, for example a silane or ester linkage. The surface modifying agent suitably also includes an alkyl chain which may or may not be fluorinated. The surface modifying agent suitably increases hydrophobicity. Suitable surface modifying agents include chloro-dimethyl-alkyl silanes, fatty acids, fluoro-alkylcarboxylic acids, phosphates and alkyl sulfonates.

According to a second aspect of the present invention there is provided a solar cell comprising an electrode of the first aspect.

The solar cell of the second aspect of the present invention suitably comprises an electrode of the first aspect; a second electrode comprising a conducting substrate; an electrolyte; and means for completing an electrical circuit between the electrodes; wherein the conducting substrate of at least one of the electrodes is optically transparent.

Thus the second aspect of the present invention suitably provides a solar cell comprising: a first electrode comprising a conducting substrate and a layer of metal oxide; a second electrode comprising a second conducting substrate; an electrolyte; and means for completing a circuit between the electrodes; wherein the conducting substrate of at least one of the electrodes is optically transparent; and wherein the layer of metal oxide comprises a dye and a pigment.

The solar cell of the second aspect of the present invention comprises two electrodes. Suitably the electrode of the first aspect is the anode and the second electrode is the cathode. Preferred features of the anode are as defined in relation to the first aspect.

Preferably the conducting substrate of the anode is optically transparent.

The structure of a solar cell is generally known to those skilled in the relevant art.

The cathode comprises a conducting substrate. This may or may not be optically transparent. The conducting substrates are typically joined to form a sealed enclosure comprising the anode, the cathode and the electrolyte. The solar cell suitably comprises an electrical circuit connecting the anode and the cathode.

The conducting substrates of the anode and the cathode may be made form the same or different materials. In some embodiments the anode and cathode both comprise glass substrates.

The cathode may be formed from conducting metals or metal compounds; for example platinum, gold and metal sulphides, for example cobalt sulfide.

The cathode may be formed from a conducting organic polymer. Suitable non-limiting examples of conducting polymers include PEDOT and OMETaD.

Suitable electrolytes for use in the solar cells of the second aspect of the present invention will be known to those skilled in the art.

The electrolyte may be a liquid electrolyte. Suitable liquid electrolytes are typically based on dipolar aprotic organic solvents, for example nitriles such as acetonitrile or methoxypropionitrile. The electrolyte solution must also contain a redox couple capable of carrying charge between the counter and working electrodes. Suitable redox couples include iodine/tri-iodide salts, bromine/tribromide salts, cobalt 11/111 complexes, ferrocene/ferrocenium, SCINP(SCN)3 SeCNI/(SeCN-)3. Other electrolyte solutions may include an ionic liquid, for example 1-methyl 3-propyl imidazolium iodide (PMII) and an optional gelling agent, for example polyethylene oxide or fumed silica to control rheology and solvent volatility.

Suitable solid electrolytes include solid metal salts, for example Cul or CuSCN and/or hole conducting polymers. Hole conducting polymers are known to the person skilled in the art and include molecular hole transport materials such as 2,2',7,7'-tetrakis(N,N'-di-pmethoxyphenylamine)-9,9'-spirobifluorene (spiro-OMETaD) or polymeric hole transport materials for example poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). There may be doped to improve conductivity for instance with lithium salts or an antioxidant.

The present invention may further provide a solar panel comprising a plurality of solar cells of the second aspect.

According to a third aspect of the present invention there is provided a method of producing an electrode for a solar cell, the method comprising the steps of (a) providing a conducting substrate; (b) forming a layer of metal oxide on the conducting substrate; (c) contacting the metal oxide with a solution comprising a dye; and (d) contacting the metal oxide with a pigment or a pigment precursor.

Suitably the method of the third aspect provides an electrode for a solar cell according to the first aspect.

Preferred features of the third aspect are as defined in relation to the first and second aspects.

Step (a) involves providing a conducting substrate. In some preferred embodiments the conducting substrate is a glass substrate. Step (a) may involve treatment of a glass substrate to improve its suitability for use in the production of an electrode for a solar cell.

Treating the glass substrate in step (a) may involve etching the glass substrate. Etching may be carried out with Zn powder and a solution of HCI.

Treating the glass substrate in step (a) may involve cleaning the glass substrate. Cleaning may be carried out using an aqueous surfactant solution, acetone, isopropanol or a combination thereof, for example a sequential addition of these liquids.

Treating the glass substrate in step (a) may involve depositing a metal oxide blocking layer onto the glass substrate, for example a layer of titanium dioxide. Depositing a blocking layer of metal oxide may be carried out by any suitable means, for example by screen printing or gravure printing. In the case of TiO2 it may be deposited by spin coating the glass substrate with a solution titanium isopropoxide, or by spray coating a solution of titanium diisopropoxide bis(acetylacetonate).

Step (b) involves forming a layer of metal oxide on the conducting substrate. This may comprise contacting the conducting substrate with a composition comprising the metal oxide. The composition comprising the metal oxide may be a colloidal composition comprising metal oxide particles, preferably metal oxide nanoparticles. Any suitable solvent may be used.

The layer of metal oxide may be deposited on the substrate by any suitable means. Such means will be known to the person skilled in the art and include gravure printing, screen printing and spin coating.

Step (b) may be followed by a step of sintering the metal oxide. Suitable sintering is carried out at a temperature of between 400°C and 600°C.

As mentioned above the thickness of the metal oxide film may suitably be from 100 nm to 50 pm. Suitable thicknesses depend on whether the electrolyte is solid or liquid and the relative balance between dye and pigment absorbers. For liquid electrolytes allied to dye absorbers the metal oxide thickness is preferably from 5 to 20 pm and most preferably 7 to 10 pm. For solid electrolytes allied to pigment absorbers the metal oxide thickness is preferably from 100 nm to 5 pm and most preferably 300 nm to 2 pm.

Steps (b), (c) and (d) could be carried out in any order. For example, the metal oxide could be first mixed with the pigment and the dye and the resultant mixture then be deposited onto the conducting substrate.

In some embodiments a mixture of dye and metal oxide could be deposited and then contacted with the pigment.

In some embodiments a mixture of pigment and metal oxide could be deposited and then contacted with dye.

Preferably step (b) is carried out before steps (c) and (d).

Step (c) suitably involves contacting the layer of metal oxide with a solution containing a dye.

Step (d) suitably involves contacting the layer of metal oxide with a pigment.

In some embodiments step (c) is carried out before step (d). In some embodiments step (d) is carried out before step (c).

Step (c) involves contacting the layer of metal oxide with a solution comprising a dye. Suitable dyes are as defined in relation to the first aspect.

Suitably step (c) provides a layer of metal oxide comprising a dye. Step (c) may involve immersing the layer of metal oxide in the solution comprising a dye. Suitably step (c) involves forming covalent bonds between the metal oxide and the dye. Typically this is believed to involve a chemisorption process which results in the formation of a covalent ester bond between a carboxylate-like linker group of the dye and surface hydroxyl groups on the metal oxide surface. Any suitable method of contacting the solution comprising the dye with the layer of metal oxide may be used. Such methods will be known to the person skilled in the art and include spraying and pumping. One suitable method involves spin coating the layer of metal oxide with the solution comprising a dye. Step (c) may involve pumping the solution comprising a dye over the layer of metal oxide. Step (c) may be carried out according to the fast-dyeing method described in the Applicant's co-pending patent application EP 09152316.7.

Step (d) involves contacting the metal oxide with a pigment or a pigment precursor. Suitable pigments including perovskite pigments are as defined in relation to the first aspect.

Suitably step (d) provides a layer of metal oxide comprising a pigment, preferably a perovskite pigment. In some embodiments step (d) involves spin coating the layer of metal oxide with pigment or a pigment precursor.

In some preferred embodiments step (d) involves contacting the metal oxide with a perovskite precursor solution. Suitable perovskite precursor solutions comprise a lead halide salt and a methylamine halide salt. Suitably the methylamine halide salt is selected from CH3NH3CI, CH3NH3Br, CH3NH3I and mixtures thereof. Suitably the lead halide salt is selected from PbBr2, Pb12 and mixtures thereof.

Suitably steps (b), (c) and (d) provide a layer of metal oxide, preferably a layer of mesoporous TiO2 comprising a dye and a pigment, preferably a perovskite pigment. Suitably the method of the third aspect further comprises a step (e) of heating the layer of metal oxide. Suitably step (e) is performed after steps (b), (c) and (d) and therefore step (e) involves heating a layer of metal oxide comprising a dye and a pigment. Preferably step (e) involves heating at a temperature of between 20°C and 120°C. . According to a fourth aspect of the present invention there is provided a method of providing a solar cell comprising the steps of: (i) providing an electrode according to the method of the third aspect; (ii) providing a second electrode; (iii) providing an electrolyte between the electrodes; and (iv) providing means for connecting the electrodes to complete a circuit.

Suitably the solar cell provided by the present invention absorbs more light at a specific wavelength than a comparable solar cell in which the layer of metal oxide comprises a pigments, for example a perovskite pigment and does not comprise a dye.

Suitably the solar cell provided by the present invention absorbs more light overall across the UV/visible spectrum than a comparable solar cell in which the layer of metal oxide comprises a pigment, for example a pigment and does not comprise a dye.

Suitably the solar cell provided by the present invention absorbs more light at a specific wavelength than a comparable solar cell in which the layer of metal oxide comprises a dye and does not comprise a pigment, for example a perovskite pigment.

Suitably the solar cell provided by the present invention absorbs more light overall across the UV/visible than a comparable solar cell in which the layer of metal oxide comprises a dye and does not comprise a pigment, for example a perovskite pigment.

According to a fifth aspect of the present invention there is provided the use of a dye to improve the absorbance of UV/visible light of a solar cell comprising a layer of perovskite-coated mesoporous TiO2.

According to a sixth aspect of the present invention there is provided the use of a dye to adjust the colour of a solar cell comprising a layer of perovskite-coated mesoporous TiO2.

The solar cells of the present invention are particularly advantageous because they have an improved absorbance of UV/visible light at specific wavelengths compared to known DSSCs. The solar cells solar cells of the present invention may have an improved overall absorbance of UV/visible light compared to known DSSCs. Furthermore the solar cells of the present invention may have an improved efficiency of conversion of incident sunlight to electrical energy compared to known DSSCs.

The solar cells of the present invention are particularly advantageous because they can be prepared in a range of colours.

The invention will now be further described with reference to the following non-limiting

examples.

Comparative Example

Solar cells comprising a layer of mesoporous TiO2 and a perovskite pigment were prepared according to the following procedure. A substrate consisting of fluorine-doped tin oxide coated glass (NSG-Pilkington TEC 7) with a sheet resistance of 7 OIL was patterned by etching with Zn powder and 4 M HCloq). The etched substrate was then cleaned sequentially with an aqueous surfactant solution, acetone and isopropanol before drying with N2@) before treating with an 02 plasma for 20 min. A compact titanium dioxide blocking layer was then deposited onto this cleaned substrate by spin coating 1 ml of a titanium isopropoxide solution diluted in ethanol. A mesoporous layer consisting of TiO2 nanoparticles (approximately 500 nm thick) was then deposited by spin-coating (2000 RPM) a TiO2 containing colloid (Dyesol-AO) which had been diluted (1:3) in anhydrous ethanol. The layers were then sintered in air at 500 °C for minutes. Once this had cooled to room temperature, a perovskite precursor solution (40% wt) was dispensed onto the mesoporous electrode film by spin-coating at 2000 RPM for 60 seconds. The perovskite precursor solution contained a lead halide salt (for example, PbBr2 or Pb12) and a methylamine halide salt (for example, CH3NH3X where X = CI, Br or I). The perovskite-coated metal oxide films were then heated at 100 °C for 45 minutes in air. A hole-conducting material was then deposited onto the perovskite-coated mesoporous metal oxide; for example by spin coating a 100 pl chlorobenzene solution containing 68 mM SpiroOMeTAD, 55 mM tertiary-butyl pyridine and 9 mM lithium bis(trifluoromethylsyfonyl)imide salt onto the perovskite-coated substrate at 2000 RPM for 60 seconds. Solar cells were left in the dark in air overnight prior to thermal evaporation of a gold back contact approximately 60 nm thick.

The external quantum efficiency (EQE) may be defined as the percentage of the photons of a given wavelength shining on the solar cell which are collected by the solar cell. Table 1 shows solar cell data for two different perovskite pigment precursors. Solar cell A shows data for the yellow, bromide-based perovskite CH3NH3PbBr3 showing a short circuit current of 3.46 mA cm 2 and efficiency of 0.74%. The colour and current of this solar cell are in line with the EQE which is shown in Fig. 1 and shows light harvesting up to 550 nm. Solar cell B shows the data for the brown/black, mixed halide-based perovskite CH3NH3PbI2CI showing increased short circuit current of 12.72 mA cm -2 and efficiency of 4.93% due to the increased light harvesting out to 800 nm which is shown in Figure 2.

Table 1

Comp. Voc/ J.. / 11/ Example Perovskite Colour mA cm-

V F

A CH3NH3PbBr3 Yellow 0.38 3.46 0.56 0.74 B CH3NH3Pb12CI Brown/black 0.76 12.72 0.51 4.93 Solar cells according to the first aspect were prepared as outlined above. However, in these examples prior to the deposition of the perovskite pigments, the sintered mesoporous TiO2 layers were first immersed in dye solution for 5 minutes. After rinsing and drying, perovskite precursor solution was deposited onto the dyed electrode using spin coating at 2000 RPM for 60 seconds as described above. The perovskite material was crystallised at 100 °C for 45 minutes before a hole conducting later such as spiro-OMeTAD was spin coated onto the sensitized TiO2 at 2000 RPM for 40 seconds followed by the thermal evaporation of a gold back contact as described above.

Table 2 shows solar cell data for a series of mesoporous TiO2 films sensitized with three different dyes followed by the yellow perovskite pigment -CH3NH3PbBr3. Solar cell C shows data for a solar cell which uses the blue squaraine dye SQ1 which absorbs light around 650 nm. This solar cell has changed colour from yellow to light green due to the combination of the yellow pigment and blue dye and shows very similar data to solar cell A showing no loss of solar cell performance despite the change in colour. Figure 3 shows the external quantum efficiency for this solar cell showing a clear response from both the perovskite pigment and the SQ1 dye. Solar cell D shows data for a combination of the yellow perovskite CH3NH3PbBr3 with the red indoline dye D102 again showing very similar performance to Solar cell A for the perovskite alone although the solar cell colour has changed to bright red. Figure 4 shows the external quantum efficiency for solar cell D showing a clear extension of light harvesting from < 550 nm out to <650 nm when the D102 dye is present. Solar cell E shows the effect of combining the blue squaraine SQ1 dye with the brown/black mixed-halide perovskite CH3NH3Pb12CI. The solar cell shows a similar performance to solar cell B for the perovskite alone. However, Figure 5 shows signals for both the perovskite and the SQ1 dye to prove that both contribute to the external quantum efficiency of this solar cell with the SQ1 being more efficient at approximately 650 nm where the perovskite light harvesting becomes less efficient.

Table 2

voc/ Jse / n/ Example Perovskite Dye Colour V mA cm-2 FF % C CH3NH3PbBr3 SQ1 Light green 0.40 3.13 0.57 0.72 D CH3NH3PbBr3 D102 Red 0.40 3.82 0.51 0.79 E CH3NH3Pb12C1 SQ1 Brown/black 0.77 9.68 0.64 4.78

Claims

Claims: 1. An electrode comprising a conducting substrate and a layer of metal oxide, wherein the layer of metal oxide comprises a dye and a pigment.
2. An electrode according to claim 1 wherein the conducting substrate is optically transparent.
3. An electrode according to claim 1 or claim 2 wherein the metal oxide is selected from aluminium oxide, tin oxide, niobium oxide, zinc oxide and titanium dioxide.
4. An electrode according to any preceding claim wherein the dye is selected from ruthenium bipyridyl complex dyes, coumarin dyes, phthalocyanine dyes, squaraine dyes and indoline dyes.
5. An electrode according to any preceding claim wherein the pigment is a perovskite pigment.
6. A solar cell comprising an electrode as claimed in any preceding claim.
7. A solar cell comprising: - a first electrode comprising a conducting substrate and a layer of metal oxide; - a second electrode comprising a second conducting substrate; an electrolyte; and means for completing a circuit between the electrodes; wherein the conducting substrate of at least one of the electrodes is optically transparent; and wherein the layer of metal oxide comprises a dye and a pigment.
8. A solar cell according to claim 7 wherein the conducting substrate of the anode is optically transparent.
9. A solar panel comprising a plurality of solar cells as claimed in any of claims 6 to 8.
10. A method of producing an electrode for a solar cell, the method comprising the steps of: (a) providing a conducting substrate; (b) forming a layer of metal oxide on the conducting substrate; (c) contacting the metal oxide with a solution comprising a dye; and (d) contacting the metal oxide with a pigment or a pigment precursor.
11. A method according to claim 10 wherein step (b) is carried out before steps (c) and (d).
12. A method of providing a solar cell comprising the steps of (i) providing an electrode according to the method of claim 10 or claim 11; (ii) providing a second electrode; (iii) providing an electrolyte between the electrodes; and (v) providing means for connecting the electrodes to complete a circuit.
13. The use of a dye to improve the absorbance of UV/visible light of a solar cell comprising a layer of perovskite-coated mesoporous TiO2.
14. The use of a dye to adjust the colour of a solar cell comprising a layer of perovskite-coated mesoporous TiO2.