WO2012100930A1

WO2012100930A1 - Flexible dye sensitised solar cell and a method for manufacturing the same

Info

Publication number: WO2012100930A1
Application number: PCT/EP2012/000280
Authority: WO
Inventors: Timothy Harold English; Deborah Hammond; Paolo MELGARI; Sivasambu BÖHM
Original assignee: Tata Steel Uk Limited; Mc GETTRICK, James
Priority date: 2011-01-24
Filing date: 2012-01-23
Publication date: 2012-08-02
Also published as: GB2504414B; WO2012100930A8; GB2504414A; GB201318801D0

Abstract

A dye sensitised solar cell comprising a metal carrier substrate, an organic electrically insulating layer on the metal carrier substrate, a conductive layer on the organic electrically insulating layer, a photoabsorber layer comprising a dye sensitised metal oxide, a transparent counter electrode and an electrolyte encapsulated between the photoabsorber layer and the transparent counter electrode, wherein a layer of a semi-conductive material is present between the photoabsorber layer and the conductive layer, said conductive layer comprising an organic polymeric material and an allotrope of carbon as a conductor.

Description

FLEXIBLE DYE SENSITISED SOLAR CELL AND A METHOD FOR

MANUFACTURING THE SAME

FIELD OF THE INVENTION

The present invention relates to a dye sensitised solar cell, to a method for manufacturing the dye sensitised solar cell and to the use of the dye sensitised solar cell in a module of series connected dye sensitised solar cells. BACKGROUND OF THE INVENTION

Third generation or Organic' PV devices use low-cost materials on flexible carrier substrates such as metal or plastic foil. Dye Sensitised solar Cells (DSC) fall into this category and are typically based on metal oxide semi-conductors such as titania (Ti0₂) impregnated with a light-absorbing dye. Due to their lower production cost per unit electric power, DSCs are now seen as viable alternatives to silicon based solar cell devices.

A typical dye sensitised solar cell consists of a working electrode and a counter electrode, which when sealed encapsulates an electrolyte that contains a suitable redox couple. A representative example of such a dye sensitised solar cell was first developed by Dr. Michael Gratzel and co-workers (Nature, 353 (24) 737-740).

Under the irradiation of sunlight dye molecules become photo-excited and inject electrons into the conduction band of the metal-oxide semiconductor. Electrons from the conduction band then diffuse to the conductive layer of the working electrode and travel via an external circuit to the conductive layer of the counter electrode. From this layer, electrons pass through a layer of catalytic material to reduce the oxidised part of the redox couple that diffuses to the counter electrode upon reducing the photo-oxidised dye.

Although dye sensitised solar cells are interesting alternatives to conventional photovoltaic devices, work still needs to be done to improve DSC efficiency. Factors that currently limit cell efficiency include cell and module design, incident photon losses and internal and external resistive losses associated with the cell. Charge recombination is also an issue which needs to be addressed.

The charge recombination reaction, otherwise known as the back reaction, is thought to be a consequence of cell design, and in particular a consequence of the electrolyte containing the redox couple being in contact with both the conductive layer of the working electrode and the metal oxide semi-conductor. With respect to charge recombination, significant losses in cell efficiency are observed when photo-electrons that have been injected into the conduction band of the metal oxide semiconductor reduce the oxidised part of the redox couple instead of being conducted to the conductive layer and then around the cell to perform electrical work. Efficiency losses are also observed when charge recombination occurs at the electrolyte - conductive layer interface of the working electrode. In this respect, photoelectrons are injected into the conduction band of the metal oxide semi-conductor and then conducted to the conductive layer before charge recombination between the oxidised part of the redox couple and the photoelectron takes place. Charge recombination occurring at the electrolyte interface with either the metal oxide semi-conductor or the conductive layer of the working electrode is shown in Figure 2 (C) and (D) respectively.

An object of the present invention is to improve DSC conversion efficiency.

Another object of the present invention is to reduce the detrimental effects of the back reaction at the electrolyte— conductive layer interface.

DETAILED DESCRIPTION OF THE INVENTION

According to a first aspect of the invention there is provided a dye sensitised solar cell comprising a metal carrier substrate, an organic electrically insulating layer on the metal carrier substrate, a conductive layer on the organic electrically insulating layer, a photoabsorber layer comprising a dye sensitised metal oxide, a transparent counter electrode and an electrolyte encapsulated between the photoabsorber layer and the counter electrode, wherein a layer of a semi-conductive material is present between the photoabsorber layer and the conductive layer, said conductive layer comprising an organic polymeric material and an allotrope of carbon as a conductor. The inventors found that dye sensitised solar cells which comprise the layer of semi-conductive material on the conductive layer exhibit significantly improved cell efficiencies relative to cells in which the layer of semi-conductive material is absent. Such an improvement has been attributed to an improved electrical matching between the Fermi energy levels of the dye sensitised metal oxide and the layer of semi-conductive material, thereby facilitating electron transfer towards the conductive layer instead of towards the redox couple within the electrolyte. For DSCs where the layer of semi-conductive material is absent, the electrical match between the dye sensitised metal oxide and the conductive layer is reduced and therefore the likelihood of electrons reducing the oxidised part of the redox couple increases. In a preferred embodiment of the invention the layer of semi-conductive material has a band gap between 3.0 and 3.7 eV. The inventors found that further improvements in DSC efficiency are possible if the semi-conductive material has a band gap between 3.0 and 3.7 since band gaps in this range afford the best electrical match between the semi-conductive material and the dye sensitised metal oxide. Consequently charge recombination via the back reaction is significantly reduced.

In a preferred embodiment of the invention the layer of semi-conductive material is titania. Advantageously titania is inexpensive, readily available, non-toxic, possesses good stability under visible radiation e.g. visible light and has an extremely high surface area that is suitable for dye adsorption. Another advantage of titania is that it is stable in its photo-excited state and recombination of the electrons and positive holes occurs slowly relative to other semi-conductors. The band gap of titania is between 3.0 and 3.2 eV although this is dependent upon the polymorph that is used, i.e. rutile or anatase.

In a preferred embodiment of the invention the layer of semi-conductive material has a dry film thickness of 10-600 nm, preferably 30-200 nm. In order to maximise cell efficiency it is necessary to control the dry film thickness of the layer of semi- conductive material. Dye sensitised solar cells comprising semi-conductive layers having a dry film thickness above 600 nm exhibit reductions in DSC efficiency relative to semi-conductive layers having dry film thicknesses within the specified range. One factor that may contribute to the observed efficiency reduction is an increase in resistive losses within the layer as a consequence of increasing the layer thickness. However, if the dry film thickness of the layer of semi-conductive material is below 10 nm then the beneficial effect of including the layer will not be recognised due to the limited surface area of the layer. A dry film thickness between 30 and 200 nm is therefore preferred since within this range there exists a good balance of properties that leads to improved cell efficiencies, i.e. between surface area, electrical resistance and mechanical properties. Very good properties that lead to improved cell efficiencies are also observed when the dry film thickness of the layer is between 60 and 100 nm.

In a preferred embodiment of the invention the allotrope or allotropes of carbon are selected from the group consisting of graphite, carbon nanotubes, amorphous carbon, carbon black, graphenes and fullerenes. The incorporation of such carbonaceous conductors provides the conductivity of the conductive layer. The selection of one allotrope as a conductor is possible although it has been demonstrated that a combination of allotropes leads to improved sheet resistivities, i.e. the electrical sheet resistance of the layer decreases. Advantageous combinations include 1) graphite and carbon black, 2) graphite and carbon nanotubes and 3) graphite, carbon black and carbon nanotubes.

In a preferred embodiment of the invention the first conductive layer has a carbon allotrope content between 10 and 80 wt %, preferably between 10 and 60 wt %. It is preferred to have a carbon allotrope content of at least 10 wt% otherwise the insulating properties of the polymeric material will dominate causing the electrical sheet resistance of the conductive layer to be too high, i.e. above 200 Ohms/square. On the other hand the carbon allotrope content should not exceed 80 wt % otherwise the conducive layer may crack after the step of sintering at a temperature between 350 and 550°C. Moreover, conductive layers with insufficient polymeric material, i.e. 20 wt % or less tend to be porous and consequently absorb the electrolyte.

Preferably the conductive layer comprises a carbon nanotube content between 0 and 10 wt %. While conductive layers comprising carbon nanotubes are characterised by electrical sheet resistances as low as 12 Ohms/square, it has also become apparent that the inclusion of carbon nanotubes could play a role in promoting the back reaction. The inclusion of carbon nanotubes in the conductive layer should therefore not exceed 10 wt %. A preferred carbon nanotube content is between 1 and 4 wt % since within this range a balance is struck between promoting conduction within the conductive layer and limiting its surface area to reduce the effects of the back reaction. Preferably the conductive layer comprises a graphite content between 10 and 80 wt % since conductive layers comprising graphite exhibit reductions in electrical sheet resistivity. However, a graphite content above 80 wt % should not be exceeded otherwise the physical properties of the conductive layer may become compromised. It has been demonstrated that conductive layers comprising 80 wt % graphite exhibit an electrical sheet reduction as low as 26 Ohms/square. On the other hand conductive layers comprising less than 10 wt % graphite do not possess sufficient conductivity. A preferred graphite range is between 10 and 50 wt %.

Preferably the conductive layer comprises a carbon black content between 10 and 60 wt %. Conductive layers comprising a carbon black content below 10 wt % is less preferred since the conductivity of such layers may be compromised and the insulating properties of the polymeric material may start to dominate. The mechanical properties of the conductive layer may also start to become compromised if the carbon black content exceeds 60 weight %. A preferred carbon black content is between 10 and 50 wt%.

In a preferred embodiment of the invention the conductive layer itself comprises 1-3 conductive sub-layers, each comprising the polymeric material and the allotrope of carbon. The inventors found that the provision of multiple conductive sub-layers further reduced the electrical sheet resistance of 'the conductive layer'. In this respect the 'conductive layer' can comprise a first conductive sub-layer and a second conductive sub-layer layer or the first conductive sub-layer, the second conductive sub-layer and a third conductive sub-layer. The first, second and third conductive layers can have the same composition or each conductive layer can be different. However, each conductive layer should comprise the polymeric material and allotrope or allotropes of carbon.

In a preferred embodiment of the invention the conductive sub-layer adjacent to the layer of semi-conductive material comprises a carbon allotrope content that is equal to or below the carbon allotrope content of the conductive sub-layer below it. If two conductive sub-layers are used then the second conductive sub-layer should possess a carbon allotrope content that is equal to or less than the carbon allotrope content of the first conductive sub-layer; similarly if three conductive sub-layers are used then the carbon allotrope content of the third conductive sub-layer should be equal to or less than the first and/or second conductive sub-layers. The inventors found it particularly advantageous to limit the carbon allotrope content of the conductive sub-layer that is in direct contact the with layer of semi-conductive material and electrolyte, as in doing so, the mechanical properties of the conductive sub-layer or sub-layers below it could be maintained during subsequent processing while the surface roughness of the conductive sub-layer in contact with the layer of semi-conductive material can be reduced.

The preferred carbon allotrope for the third conductive sub-layer is carbon black due to its reduced catalytic effect on the back reaction compared to other possible allotropes. In this respect, the third conductive sub-layer or the second conductive sub-layer (if in contact with the layer of semi-conductive material) comprises a carbon black content between 10 and 50 wt %

In a preferred embodiment of the invention the surface roughness, Sa, of the conductive layer adjacent to the layer of semi-conductive material is less than 3.0 microns, preferably between 0.1 and 3.0 microns and more preferably between 0.1 and 1.0 microns. Advantageously conductive layers having a surface roughness of less than 3.0 microns result in improved DSC efficiencies since the detrimental effects of the back reaction are reduced. The inventors found that conductive layers having a surface roughness between 0.1 and 1.0 μηι were particularly advantageous.

In a preferred embodiment of the invention the sheet resistance of the conductive layer is 200 Ohms/square or below, preferably 30 Ohm/square or below and more preferably below 15 Ohms/square. The electrical sheet resistance of the conductive layer should be as low as possible so that the resistive losses in the conductive layer are minimised.

According to a second aspect of the invention there is provided a method of providing the dye sensitised solar cell according to the first aspect of the invention, which comprises the steps of:

(i) providing a metal carrier substrate;

(ii) providing an organic electrically insulating layer on the metal carrier substrate;

(iii) providing a conductive layer on the organic electrically insulating layer, said conductive layer comprising an organic polymeric material and an allotrope of carbon as a conductor;

(iv) providing on the conductive layer a layer of a semi-conductive material; (v) providing on the layer of semi-conductive material a photo-absorber layer comprising a dye sensitised metal oxide;

(vi) providing the electrolyte on the photo-absorber layer and providing the counter electrode thereon to encapsulate the electrolyte.

Strip metals such as steel and aluminium are very suitable as metal carrier substrates for dye sensitised solar cells since both metals can be used in a continuous roll-to-roll process. Particularly suitable metal carrier substrates include carbon steel and Electrolytic Chromium Coated Steel" (ECCS). If the surface is to be divided into cells that are connected in series, the metal carrier substrate needs to be electrically insulated from the PV layers. The electrically insulating layer comprises an organic polymeric material such as polyimide which can be roller coated onto the metal carrier substrate. This layer should not thermally degrade when exposed to temperatures in the range of 150 and 550°C, since such temperatures are required in subsequent processing steps.

The conductive layer, also called the back contact can be applied onto the electrically insulating layer by roller coating or by screen printing, thereby negating the need to use vacuum based processes that lead to a batch manufacturing route. Polyimide is a very suitable polymeric material since the conductive layer also needs to be resistant to subsequent high temperature processing steps such as sintering. Conductive layers comprising graphite, carbon nanotubes, carbon black or a mixture thereof are preferred since conductive layers having an electrical sheet resistance equal to or below 200 Ohms/square can be obtained. The conductive layer can be one layer or comprise multiple layers.

The layer of semi-conductive material can be provided on the conductive layer by spray pyrolysis. Spray pyrolysis is a technique that takes advantage of spray as a delivery method so that the pre-cursor is deposited/delivered as a fine mist having a large surface area. A consequence of the increased surface area is that the precursor is more likely to react at the substrate interface and therefore improvements in coating adhesion can be achieved. This also includes the situation where the pre-cursor undergoes chemical reaction close to the substrate surface to produce the coating.

Prior to applying the semi-conductive material precursor by spray pyrolysis it is necessary to heat the underlying substrate to a temperature between 150 and 450°C to facilitate the thermal breakdown of the semi-conductive material precursor. In this connection the underlying substrate comprises the metal carrier substrate, insulating layer and conductive layer. By using spray pyrolysis a dense substantially continuous layer is formed which acts as a physical barrier against the electrolyte. Moreover, the layer readily conforms to the substrate being coated. A second advantage is that the spray pyrolysis can be used in a continuous production line. Preferably the layer of semi-conductive material has a dry film thickness between 10 and 600 nm. The semi-conductive material precursor formulation may be based on a metallo-organic compound in organic solvent or a metallo-inorganic compound in water, optionally provided with a stabiliser. Preferably the metallo-organic compound and metallo- inorganic compound comprise titanium, with titanium isoproxide and titanium oxichloride being particularly preferred respectively.

The photo-absorber layer is provided by providing a paste on the layer of semi- conductive material using screen printing, gravure printing or by roller coating. The paste comprises the metal oxide, a solvent and a binder. Suitable metal oxides include Sn0₂, ZnO and Ti0₂. When the paste is subjected to temperatures between 200 and 550°C the solvent and binder are removed leaving behind a mesoporous layer of metal oxide having a large surface area. Between 350 and 550°C the metal oxide is sintered increasing the number of electrical interconnections between the metal oxide particles, which increases the overall conductivity of this layer. The metal oxide layer is subsequently dyed with the photo-sensitive dye.

The electrolyte is then provided on the dye sensitised metal oxide, which electrolyte comprises a redox couple such as iodide/tri-iodide and a suitable solvent such as acetonitrile or proprionitrile. The counter electrode is then fed into a production line, registered with the working electrode and laminated at a temperature preferably no greater than 100°C to avoid thermal degradation of the dye. The counter electrode otherwise known as the 'top window electrode' comprises a transparent film, a layer of catalytic material, typically platinum, and a transparent conductive layer disposed therebetween. The counter electrode is arranged so that the layer of catalytic material is closest to the electrolyte.

In a preferred embodiment of the invention the dye sensitised solar cell is manufactured in a continuous reel to reel production line.

The use of the dye sensitised solar cell according to the first aspect of the invention in a module of series connected dye sensitised solar cells. The output voltage of a single dye sensitised solar cell is not sufficient for commercial application and therefore it is necessary to interconnect a number of dye sensitised solar cells in series to generate a useful output voltage. To provide a module it is necessary to divide the conductive layer of the working electrode into a number of separate cells. Similarly, it is also necessary to divide the transparent conductive layer and layer of catalytic material of the counter electrode. Bus bars and current collectors can be provided on the divided conductive layer or on the divided layer of catalytic material to reduce resistive losses. Interconnects are provided on the conductive layer or on the layer of catalytic material to electrically interconnect two or more dye sensitised solar cells. Alternatively interconnects can be provided on the bus bars. The counter electrode is then laminated on the working electrode to encapsulate the electrolyte. The module is then provided with an external connection on a interconnect that allows energy produced by the module to be utilised.

EXAMPLES

Embodiments of the present invention will now be described by way of example. These examples are intended to enable those skilled in the art to practice the invention and do not in anyway limit the scope of the invention as defined by the claims.

Figure 1 shows a DSC having a reverse design wherein there is provided a metal carrier substrate (1), an organic electrically insulating layer (2) on the metal carrier substrate, a conductive layer (3) on the organic electrically insulating layer, a layer of a semi- conductive material (4) on the conductive layer, a photoabsorber layer (5) comprising a dye sensitised metal oxide on the layer of semi-conductive material and an electrolyte (6) encapsulated between the photoabsorber layer and a counter electrode (7). Figure 2 shows the interface between the photoabsorber layer (5) and the underlying conductive layer (3) with potential electron pathways highlighted. It also shows that pores of the metal oxide are filled with electrolyte (6) and that the electrolyte is in contract with both the porous photoabsorber and the underlying conductive layer. In the ideal case, any photo-electron injected into the conduction band of the metal oxide, such as at point (A), will travel through the photoabsorber layer and the conductive layer to pass into an external circuit and perform electrical work (B). Point (C) shows a possible site for charge recombination occurring at the electrolyte-metal oxide interface, whereas point (D) shows another point for possible charge recombination taking place at the electrolyte-conductive layer interface of the working electrode. The inventors found that the charge recombination reaction occurring at point (D) can be reduced by providing the layer of semi-conductive material (4) (not shown in Figure 2) on the conductive layer.

A standard cell geometry was retained for all cells. Substrates were produced in a reel- to-reel process on an electro chromium coated steel (ECCS) substrate. The insulating polyimide layer was applied by roller coating the full width of the ECCS substrate with a poly(amic acid) precursor to a dry film thickness of 20 μπι. Two coats of a polyimide resin containing graphite and carbon black were then applied by roller coating the full width of the ECCS - insulating polymer substrate to form a conductive substrate stack. The substrate stack was then heated to 300 °C and subsequently coated with a titania using the technique of spray pyrolysis. The spray was generated by air-brush and the air pressure at the air-brush inlet was 20 psi. The air-brush needle and nozzle size was 0.3 mm and the distance between the conductive substrate stack surface and the nozzle was 30 mm. Alternatively, the spray may be generated using a spray nozzle with fan control. The air pressure at the air-nozzle was 20 psi and fan pressure was 30 psi. The nozzle size was 1.1 mm in diameter and the distance between the conductive substrate stack surface and the nozzle was 100 mm.

Titania paste (DSL 18NR-AO) was flatbed screen printed in a rectangular pattern (12 x 56 mm, rounded corners, 6.69cm² area). Two layers of Ti0₂ paste were applied, with each layer being dried for 10 min at 150 °C, to obtain an 18 μπι thick layer. The conductive substrate stack comprising the pyrolysed titania and the dried Ti0₂ paste was then sintered along a 7 m long infrared belt furnace at a temperature of 550 °C and speed of 2 m/min. Dyeing took place over night in a solution of N719 dye (Dyesol) in solvent. After dyeing busbars were added to the cell perimeter using RS silver conductive paint (RSI 86-3600). The cells were then sealed using thermoplastic perimeter gaskets to a prepared counter electrode to give a cell depth of 50 μηι. The counter electrode itself is a transparent conductive ITO on a PET substrate. This had been platinised via a hexachloroplatinic acid reduction route in order to improve electron transfer between the ITO and electrolyte.

The electrical sheet resistivity of the conductive layer can be expressed in 'Ohms per square'; this value gives the resistance in Ohms of current passing from one side of a square region in the conductive layer to the opposite side, regardless of the size of the square. The electrical sheet resistivity was measured by a commercially available resistivity meter that is manufactured by Guardian in Florida, model SRM-232-100. Tables 1 and 2 show the electrical sheet resistivity of conductive layers having different compositions. The components of the conductive layer are expressed in terms of % by weight of the dry conductive layer, i.e., after the conductive layer has been cured. The sheet resistances are measured after the material had been sintered.

Cells were tested in dark conditions and under simulated solar emission in a Dyesol UPTS instrument. Typically the cell is clamped under a class C light source (ASTM AM1.5G) filtered to the appropriate sun level. The current response of the cell is then measured over a range of externally applied potentials (typically -0.6V to 0.8V), and the data plotted to produce I-V curves and power-voltage curves. The conversion efficiency is the power produced by the cell relative to the incident power supplied by the light source. Efficiency results are reported at 1/3 sun.

Table 1 shows the electrical sheet resistivities of conductive layers comprising polyimide and allotropes of carbon selected from carbon nanotubes and graphite. BYK is a chemical company which produces polyether modified polydimethylsiloxane in butyl glycol (BYK341) and an alkylammonium salt of an unsaturated acidic carboxylic ester in isobutanol (BYK ES 80). BYK ES 80 can be used to increase the conductivity of the conductive layer, whereas BYK 341 can be used to increase the wetting of the conductive layer.

Table 1

Table 2 shows the electrical sheet resistivities of conductive layers comprising polyimide and allotropes of carbon selected from carbon nanotubes, graphite and carbon black.

26 4 10 50 36 0 0 14 15

27 4 20 40 36 0 0 16 14

28 4 30 30 36 0.02 0.02 18 15

29 4 40 0 56 0.03 0 10 53

30 4 40 10 46 0 0 15 28

31 4 40 15 41 0 0 16 22

32 4 40 20 36 0 0 14 25

33 4 50 0 46 0 0 14 48

34 0 30 30 40 0.00 0.00 17 15

1 1

35 0 25 0 75 0.00 0.00 4 230

1 1

Table 2

Table 3 shows DSC conversion efficiencies at 1/3 sun light for DSCs comprising a layer of semi-conductive material (pyrolysed titania "blocking layer") between the dye sensitised metal oxide and conductive layer. Comparative examples have been provided wherein the titania blocking layer is absent. In the examples below the conductive sublayers of the conductive layer have the same composition, i.e. the composition of example 25. Although the composition according to example 25 has been used in this instance it should be clear that the invention is not limited to such a composition and any one of compositions (1-35) could be used in combination with the titania blocking layer. In the case of multiple sub-layers, each sub-layer could have the same composition or a different composition.

Table 3

The results indicate that DSCs comprising a layer of semi-conductive material (titania blocking layer) exhibit improved DSC conversion efficiencies relative to DSCs wherein the titania blocking layer is absent. It can be seen from examples 38 and 39 that the DSC conversion efficiency is doubled when the DSC comprises the titania blocking layer. The same effect can be seen when comparing examples 36 and 37, and 40 and 41 albeit to a lesser extent.

The results also reveal that the electrical sheet resistance can be reduced by providing conductive layers comprising conductive sub-layers. It can be seen from conductive layer example 25 (one layer) that the electrical sheet resistance of this layer is 39 Ohms/square. The provision of a second layer, i.e. to form a conductive layer comprising two conductive sub-layers reduces the electrical sheet resistance to 20 Ohms/square (examples 36 and 37). The electrical sheet resistance can be reduced still further to a value of 15 Ohms/square) if the conductive layer comprises three conductive sub-layers (examples 38 and 39).

Claims

A dye sensitised solar cell according to claim 1 wherein the layer of semi- conductive material has a band gap between 3.0 and 3.7 eV.

A dye sensitised solar cell according to claim 1 or claim 2 wherein the layer of semi-conductive material is titania.

A dye sensitised solar cell according to any one of the preceding claims wherein the layer of semi-conductive material has a dry film thickness of 10-600 nm, preferably 30-200 nm.

A dye sensitised solar cell according to any one of the preceding claims wherein the conductive layer itself comprises 1-3 conductive sub-layers, each comprising the polymeric material and the allotrope of carbon.

A dye sensitised solar cell according to claim 5 wherein the conductive sub-layer adjacent to the layer of semi-conductive material comprises a carbon allotrope content that is equal to or below the carbon allotrope content of the conductive sublayer below it.

A dye sensitised solar cell according to any one of the preceding claims wherein the first conductive layer has a carbon allotrope content between 10 and 80 wt %, preferably between 10 and 60 wt %.

A dye sensitised solar cell according to any one of the preceding claims wherein the allotrope or allotropes of carbon are selected from the group consisting of graphite, carbon nanotubes, amorphous carbon, carbon black, graphenes and fullerenes.

A dye sensitised solar cell according to any one of the preceding claims wherein the surface roughness, Sa, of the conductive layer adjacent to the layer of semi- conductive material is less than 3.0 microns.

10. A dye sensitised solar cell according to any one of the preceding claims wherein the sheet resistance of the conductive layer is 200 Ohms/square or below, preferably 30 Ohm/square or below and more preferably below 15 Ohms/square. 11. A method of providing the dye sensitised solar cell according to any one of claims 1-10, which comprises the steps of:

(i) providing a metal carrier substrate;

(iv) providing on the conductive layer a layer of a semi-conductive material;

(v) providing on the layer of semi-conductive material a photo-absorber layer comprising a dye sensitised metal oxide;

12. A method of manufacturing a dye sensitised solar cell according to claim 11 wherein the dye sensitised solar cell is manufactured in a continuous reel to reel production line. 13. The use of the dye sensitised solar cell according to any one of claims 1-10 in a module of series connected dye sensitised solar cells.