A WORKING ELECTRODE FOR A PHOTOVOLTAIC DEVICE, AND A PHOTOVOLTAIC DEVICE INCLUDING THE WORKING ELECTRODE

Abstract

The present invention relates to a working electrode (1a) for a photovoltaic device, comprising a light absorbing layer (3) and a conductive layer (6) arranged in electrical contact with the light absorbing layer (3), and the light absorbing layer (3) comprises a light absorbing photovoltaic material consisting of a plurality of dye molecules. The light absorbing layer (3) is formed by a layer of a plurality of clusters (7), whereby each cluster (7) is formed by dye molecules and each dye molecule in the cluster (7) is bonded to its adjacent dye molecules.

Claims

1. A working electrode for a photovoltaic device, comprising a light absorbing layer and a conductive layer arranged in electrical contact with the light absorbing layer, and the light absorbing layer comprises a light absorbing photovoltaic material consisting of a plurality of dye molecules, wherein the light absorbing layer is formed by a layer of a plurality of clusters, whereby each cluster is formed by dye molecules and each dye molecule in the cluster is bonded to its adjacent dye molecules, wherein the light absorbing layer is essentially a monolayer of the clusters, and wherein spaces are formed between the clusters, and the working electrode comprises a conducting medium that fills the spaces between the clusters.

2. The working electrode according to claim 1, wherein the light absorbing layer does not contain dye molecules disposed or absorbed on surfaces of semiconducting particles.

3. (canceled)

4. (canceled)

5. The working electrode according to claim 1, wherein at least 80% of the clusters forming the light absorbing layer comprises more than 100 dye molecules, preferably more than 1,000 dye molecules, and preferably more than 10,000 dye molecules.

6. The working electrode according to claim 1, wherein the thickness of the light absorbing layer is between 20 nm and 2 μm.

7. The working electrode according to claim 1, wherein at least 40% of the clusters forming the light absorbing layer are crystalline clusters, where the dye molecules within the clusters are arranged in a defined and repeatable way, and preferably at least 50% of the clusters are crystalline clusters, and most preferably at least 70% of the clusters are crystalline clusters.

8. The working electrode according to claim 1, wherein the dye molecules are organic dye molecules, or organometallic dye molecules, or natural dye molecules.

9. The working electrode according to claim 1, wherein said clusters are substantially evenly distributed in the light absorbing layer.

10. The working electrode according to claim 1, wherein the clusters forming the light absorbing layer are in physical and electrical contact with the conductive layer and the clusters are bonded to the conductive layer.

11. The working electrode according to claim 1, wherein the working electrode comprises a reflective layer disposed between the light absorbing layer and the first conductive layer, and the reflective layer comprises semiconducting particles in electrical contact with said clusters forming the light absorbing layer and the first conductive layer.

12. The working electrode according to claim 11, wherein the size of at least 80% of the semiconducting particles in the reflective layer is larger than 0.1 μm, and preferably larger than 0.2 μm.

13. The working electrode according to claim 11, wherein the thickness of the reflective layer is between 0.1 μm and 10 μm.

14. The working electrode according to claim 11, wherein the reflective layer is porous, and the porosity of the reflective layer is between 40%-70%.

15. A photovoltaic device comprising: a working electrode according to claim 1, a counter electrode, and a conducting medium for transferring charges between the counter electrode and the working electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

[0076] FIG. 1 shows one example of a working electrode including a light absorbing layer.

[0077] FIG. 2 shows another example of a working electrode including a light absorbing layer and a reflective layer.

[0078] FIG. 3 shows an example of a photovoltaic device including the working electrode shown in FIG. 1.

[0079] FIG. 4 shows an enlarged part of the light absorbing layer and a conductive layer of the photovoltaic device shown in FIG. 3.

[0080] FIG. 5 shows another example of a photovoltaic device including the working electrode shown in FIG. 1.

[0081] FIG. 6 shows an example of a photovoltaic device including the working electrode shown in FIG. 2.

DETAILED DESCRIPTION

[0082] Like numbers in the figures refer to like elements throughout the description.

[0083] FIG. 1 shows a schematic drawing of a working electrode 1a including a light absorbing layer 3 made of a light absorbing photovoltaic material and a conductive layer 6 in electrical contact with the light absorbing layer 3. The light absorbing layer 3 has an upper surface 5 for receiving incoming light. The conductive layer 6 is arranged on an opposite side of the light absorbing layer 3 with respect to the upper surface 5. In this example, the light absorbing layer 3 is disposed directly on the conductive layer 6. The light absorbing photovoltaic material consists of a plurality of dye molecules. The dye molecules form clusters 7. The dye molecules within a cluster 7, are arranged so that each dye molecule is bonded to its adjacent dye molecules. The clusters 7 are disposed on the surface of the conductive layer 6 and essentially every cluster 7 is bonded to the conductive layer 6. The clusters 7 shall cover a large part of the area of the light absorbing layer 3 and need not be bonded to each other. Preferably, the light absorbing layer 3 is porous to allow a conducting medium to pass through the light absorbing layer. To achieve sufficient light absorption most of the clusters may comprise more than 100 dye molecules, preferably more than 1000 dye molecules, and most preferably more than 10 000 dye molecules. Each dye molecule in a cluster is bonded to its adjacent dye molecules.

[0084] The dye can be any type of dye with the ability to absorb photons, and to generate photo-excited electron. There exist several thousands of known types of dyes with the ability to absorb photons and generate photo-excited electron. The dye molecules can be organic dye molecules, organometallic dye molecules, or natural dye molecules. Metal organic dyes are well known photovoltaic materials having good light absorption and which can be tailor made for efficient absorption of visible light.

[0085] Examples of organic dyes: tetrahydroquinolines, pyrolidine, diphenylamine, triphenylamine (TPA), coumarin dyes, indole dyes, aryl amine dyes, porphyrine dyes, fluorine dyes, carbazole dyes (CBZ), phenothiazine dyes (PTZ), phenoxazine dyes (POZ), hemicyanine dyes, merocyanine dyes, squaraine dyes, perylene dyes, anthraquinone dyes, boradiazaindacene (BODIPY) dyes, oligothiophene dyes, and polymeric dyes, fluorinated quinoxaline dyes. It has been found that organic dyes can improve the performance of DSSC devices. By using clusters of crystalline organic dyes, the band gap can be reduced resulting in light absorption in a broader wavelength range and more efficient light absorption of longer wavelengths of light.

[0086] Examples of metal organic dyes: ruthenium-based complexes, or other metal complexes such as iron complexes or platinum complexes.

[0087] Examples of natural dyes: betalain dyes, anthocyanin dyes [268], chlorophyll dyes [269], flavonoid dyes [270], carotenoid dyes.

[0088] The dye molecules suitable for use in the present invention, is not limited to the examples given above. Further, the dye molecules in the clusters can be a mixture of two or more dyes.

[0089] Suitably, the clusters 7 are substantially evenly distributed in the light absorbing layer 3 to achieve an even conversion of incident light over the entire surface of the light absorbing layer 3. The clusters can be in physical contact with each other, but they do not need to be bonded to each other. The clusters 7 are typically bonded to another layer arranged underneath the light absorbing layer 3, for example, the conductive layer 6. The conductive layer 6 is arranged in electrical contact with the clusters 7. In this example, the conductive layer 6 is arranged in electrical as well as physical contact with the clusters 7.

[0090] The desired size of the clusters 7 depends on the type of dye and its absorptions coefficient. The larger size of the clusters, the better light absorption. The shape and size of the clusters 7 may be varied by the method used for producing the clusters. To achieve a good ability to absorb light, the size of at least 80% of the clusters preferably is more than 5 nm along a straight line through the cluster connecting two points on the surface of the cluster. For example, the line is the diameter of the clusters. More preferably, the size of at least 80% of the clusters along a straight line through the cluster is more than 10 nm, and most preferably more than 20 nm. Suitably, the size of at least 80% of the clusters is between 5 nm and 2 μm at a straight line connecting two point on the surface of the cluster. Preferably, the size of at least 80% of the clusters is between 10 nm and 1 μm at a straight line connecting two point on the surface of the cluster. The size of the clusters is, for example, measured by using SEM “Scanning Electron Microscopy”.

[0091] For example, the clusters 7 are arranged so that they form a monolayer of clusters 7 in the light absorbing layer 3, as shown in FIG. 1. Each of the clusters 7 in a monolayer has an upper surface facing the incoming light and accordingly can contribute to the light conversion.

[0092] The optimal thickness for an efficient light absorbing layer depends both on the light absorption spectrum of the dye and the light emission spectrum of the light source. For example, the thickness of the light absorbing layer 3 is less than or equal to 2 μm, and preferably less than or equal to 1 μm. For example, the thickness of the light absorbing layer is larger than 20 nm. The thickness of the light absorbing layer mainly depends on the thickness of the clusters 7. Suitably, the thickness of the light absorbing layer is between 20 nm and 2 μm.

[0093] The light absorbing layer 3 may further include a conducting medium 9, as shown in FIG. 3. Spaces 8 are formed between the clusters 7 for housing the conducting medium. For example, the conducting medium 9 can be a liquid electrolyte, or a solid charge conducting material, such as a conducting polymer. The conducting medium 9 is disposed in the spaces 8 between the clusters 7. For example, the clusters 7 can be partly covered with the charge conducting material 42, as shown in FIG. 5. Preferably, the conductive layer 6 is also porous to allow the conducting medium 9 to penetrate through the conductive layer 6. The conductive layer 6 is made of a conducting material. For example, the conductive layer 6 is made of porous Ti.

[0094] The working electrode may comprise a connection element 46 electrically connected to the conductive layer 6 for connecting the conductive layer to an external load as shown in FIG. 3.

[0095] In the example of FIG. 1, the clusters 7 are disposed on the conductive layer 6. The conductive layer 6 extracts the photo-generated electrons from the light absorbing layer 3.

[0096] The clusters 7 are bonded to the conductive layer 6. The clusters 7 can be in physical contact with each other, but they are not bonded to each other. In this example, the clusters are disposed on the first conductive layer 6 so that they form a monolayer of clusters 7 on the conductive layer 6. The clusters 7 have an upper surface facing the light and a lower surface being in direct mechanical and electrical contact with the conductive layer 6. In a monolayer of clusters, each of the clusters are in direct physical and electrical contact with another layer arranged underneath the light absorbing layer 3, for example, the first conductive layer 6.

[0097] FIG. 2 shows another example of a working electrode 1b including the light absorbing layer 3, the conductive layer 6 and a reflective layer 9a arranged between the light absorbing layer 3 and the conductive layer 6. The reflective layer 9a is arranged on an opposite side of the light absorbing layer 3 with respect to the upper surface 5. The light absorbing layer is arranged on top of the reflective layer 9a, and the reflective layer 9a is arranged on top of the conductive layer 6. The reflective layer 9a is arranged so that it reflects light having passed from the light absorbing layer 3 back to the light absorbing layer 3. The reflective layer 9a comprises semiconducting particles 10 in electrical contact with the clusters 7 and with the conductive layer 6. It is important that the reflective layer forms a good electric contact with the light absorbing layer so that the light absorbing layer can transfer photoexcited charges to the reflective layer without significant electrical energy losses.

[0098] The semiconducting 10 particles are made of a reflective material, i.e. a material that reflects light. The semiconducting particles 10 are in electrical contact with the conductive layer 6 as well as the light absorbing layer 3. Thus, the clusters 7 are in electrical contact with the conductive layer 6 via the semiconducting particles 10. The semiconducting particles 10 are bonded to each other and to the conductive layer. The semiconducting particles are, for example, made of TiO.sub.2, ZnO, or Nb2O5. Suitably, the size of at least 80% of the semiconducting particles 10 is between 10 nm and 2 μm. For example, the semiconducting 10 particles are made of titanium dioxide (TiO.sub.2). The reflective layer act as a mirror that scatters incident light back into the light absorption layer thereby increasing the effective absorption path length, and accordingly increases the light absorption of the light absorbing layer. The light scattering effect of the reflective layer is wavelength dependent. The light scattering effect depends strongly on the sizes of the semiconducting particles 10 in the reflective layer. Thus, the light scattering can be tuned and optimized by choosing semiconducting particles with adequate particle sizes to suit the application at hand.

[0099] In this example, the clusters 7 are disposed on the reflective layer 9a. At least some of the semiconducting particles 10 are in physical contact with at least some of the clusters 3. In this example, the clusters 7 are bonded to the semiconducting particles 10 of the reflective layer 9a. For example, the clusters are disposed on the reflective layer 9a so that they form a monolayer of clusters 7 on the reflective layer, as shown in FIG. 2. Preferably, the reflective layer is porous to allow the conducting medium to pass through the reflective layer. For example, the porosity of the reflective layer is between 35%-80% or 40%-70%. The thickness of the reflective layer is between 0.1 μm and 10 μm, and preferably between 1 μm and 10 μm.

[0100] It is also possible that some clusters 7 are placed within pores of the reflective layer 9a. These clusters 7 are prepared to be clusters 7 in accordance with the description above and are not formed by for example excess dye forming an agglomeration as dye is infused into a semi-conducting structure.

[0101] In all possible embodiments of a working electrode 1a the main part of the light absorbing layer 3 is the monolayer of clusters 7 disposed on the surface of the conductive layer 6 or the reflective layer 9a.

[0102] FIG. 3 shows an example of a photovoltaic device 20 comprising the working electrode 1a, as shown in FIG. 1. The photovoltaic device comprises a counter electrode comprising a second conductive layer 24 electrically insulated from the first conductive layer 6, and a conducting medium 9 for transferring charges between the counter electrode and the working electrode. The conducting medium 9 is disposed in the spaces 8 between the clusters 7.

[0103] The photovoltaic device 20 further comprises an insulating substrate 26 arranged between the first and second conductive layers 6, 24. The first conductive layer 6 is disposed on one side of the insulating substrate 26, the second conductive layer 6 is disposed on the opposite side of the insulating substrate 26. The light absorbing layer 3 is disposed on the first conductive layer 6. The light absorbing layer 3 is positioned on a top side of the photovoltaic device facing the sun to allow the sunlight to hit the clusters 7 and to generate photo-exited electrons. The first conductive layer 6 serves as a back contact that extracts the photo-generated electrons from the light absorbing layer 3. Preferably, the first conductive layer 6 is porous for housing the conducting medium. For example, the first conductive layer 6 comprises a plurality of conducting particles 28 made of a conducting material, as shown in FIG. 4. The conductive particles 28 of the first conductive layer are bonded to each other and are in electrical contact with each other. The first and second conductive layers 6, 24 are, for example, made of Ti, Ti alloys, Ni alloys, graphite, or amorphous carbon. Preferably, the first and second conductive layers 6, 24 are made of porous Ti.

[0104] FIG. 4 shows an enlarged part of the light absorbing layer and the first conductive layer 6 of the photovoltaic device shown in FIG. 3. The conductive particles 28 of the first conductive layer 6 form a network for conducting electrical charges and for having a sufficient mechanical stability for the photovoltaic device. The clusters 7 of the light absorbing layer are in physical and electrical contact with some of the conducting particles 28 of the first conductive layer 6.

[0105] It is possible that some of the clusters 7 partly protrudes into the first conductive layer 6. In this example, the clusters 7 are larger than the conducting particles 28. However, the clusters 7 and the conducting particles 28 can also be of substantially equal size.

[0106] The photovoltaic device 20 further comprises a conducting medium for transferring charges from the light absorbing layer 3 to the second conductive layer 24. In this example, the conducting medium is a liquid electrolyte and not shown in the figure. However, the conducting medium can be any suitable type of conducting medium, such as a gel or a solid conductor. The liquid electrolyte is, for example, a redox electrolyte capable of transferring charges i.e. electrons or holes to or from the clusters 7. The redox electrolyte is also capable of transferring charges to or from the second conductive layer 24. Examples of electrolytes include the I.sup.−/I.sub.3.sup.− redox couple or ferrocene compound containing electrolytes, however also other electrolytes such as copper based electrolytes or cobalt based electrolytes can be used. The electrolyte may be selected from a group comprising or consisting of Iodine/iodide-based electrolytes such as:

[0107] LiI/I2, NaI/I2, KI/I2, PMII/I2,

[0108] or cobalt-based electrolytes such as:

[0109] Tris(1,10-phenanthroline)cobalt bis(hexafluorophosphate)/Tris(1,10-phenanthroline)cobalt tris(hexafluorophosphate), or

[0110] Bis(6-(1H-pyrazol-1-yl)-2,2′-bipyridine)cobalt bis(hexafluorophosphate)/Bis(6-(1H-pyrazol-1-yl)-2,2′-bipyridine)cobalt tris(hexafluorophosphate), or

[0111] Tris-(2,2′-bipyridine)cobalt(II) di(tetracyanoborate)/Tris-(2,2′-bipyridine)cobalt(III) tri(tetracyanoborate),

[0112] Or copper-baser electrolytes such as

[0113] bis-(2,9-dimethyl-1,10-phenanthroline)copper(I) bis(trifluoromethanesulfonyl)imide/bis-(2,9-dimethyl-1,10-phenanthroline)copper(II)bis(trifluoromethanesulfonyl)imide chloride, or bis-(4,4′,6,6′-tetramethyl-2,2′-bipyridine)copper(I) bis(trifluoromethanesulfonyl)imide/bis-(4,4′,6,6′-tetramethyl-2,2′-bipyridine)copper(II) bis[bis(trifluoromethanesulfonyl)imide], or

[0114] Bis(1,1-Bis(2-pyridyl)ethane)copper(I) hexafluorophosphate/Bis(1,1-Bis(2-pyridyl)ethane)copper(II) bis(hexafluorophosphate).

[0115] Also hole transport materials can be used (HTM) as the conducting medium.

[0116] The porosity of the insulating substrate 26 will enable ionic transport through the insulating substrate. The porosity of the first conductive layer 6 will enable ionic transport through the first conducing layer. For example, the substrate 26 and the applied layers 3, 6, 24 is immersed in a liquid electrolyte and encapsulated. The liquid electrolyte is filled in the pores of the first porous conductive layer 6, in pores of the porous insulating substrate 26, and in the spaces between the clusters 7 in the light absorbing layer 3. The first and second conductive layers 6, 24 are separated physically and electrically by the insulating substrate 26 and therefore the conductive layers 6, 24 are not in direct physical or electrical contact. However, the first and second conductive layers 6, 24 are electrically connected via electrolyte ions penetrating the porous insulating substrate.

[0117] The photovoltaic device 20 also comprises a casing or other means for enclosing the photovoltaic device for protection of the device and to prevent leakage of the electrolyte. For example, the photovoltaic device 20 comprises a first sheet 30 covering a top side of the photovoltaic device and a second sheet 32 covering a bottom side of the photovoltaic device and acting as liquid barriers for the electrolyte. The first sheet 30 on the topside of the photovoltaic device needs to be transparent to allow light to pass through. The sheets 30, 32 are, for example, made of a polymer material. An additional layer may be added between the counter electrode 24 and the bottom sheet covering 32, in order to further support the mechanical stability of the photovoltaic device. The photovoltaic device 20 comprises at least one connection element 46 electrically connected to the first conductive layer 6 for connecting the first conductive layer to an external circuit L, and at least one connection elements 47 electrically connected to the second conductive layer 24 for connecting the second conductive layer to the external circuit L. For example, the connection elements 46, 47 are busbars. The first and second conductive layers 6, 24 are connected to each other through the external circuit L. Thus, an electrical circuit is formed, where one type of charge carrier, i.e. electrons or holes, are transported from the first conductive layer 6 to the second conductive layer 6 via the external circuit, and the other type of charge carrier, i.e. electrons or holes, are transported from the first conductive layer 6 to the second conductive layer 24 via the charge conducting medium.

[0118] FIG. 5 shows another example of a photovoltaic device 40 including the working electrode 1a. The photovoltaic device 40 includes a porous insulation substrate 26, and a counter electrode including a second conductive layer 24. In this example, the conducting medium is a solid charge conductor 42. The light absorbing layer 3 comprises the clusters 7 of dye molecules and the solid charge conductor 42. The charge conductor 42 can be a hole conductor or an electron conductor. For example, the charge conductor 42 is a conductive polymer, such as PEDOT, poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate) called PEDOT:PSS. The clusters 7 are essentially evenly distributed in the light absorbing layer 3, and the solid charge conductor 42 is located on the clusters 7 and in the spaces between the clusters. The photovoltaic device 40 further comprises a plurality of charge conducting paths 44 of a charge conducting material disposed between the light absorbing layer 3 and the second conductive layer 24 to enable charges, i.e. holes or electrons, to travel between the light absorbing layer 3 and the second conductive layer 24. The conducting paths 6 penetrate through the first conductive layer 6 and the porous insulating substrate 26. Suitably, the first conductive layer 6 is porous to allow the charge conductor to penetrate through the first conductive layer 6.

[0119] FIG. 6 shows an example of a photovoltaic device 50 including the working electrode 1b shown in FIG. 2.

[0120] The light absorbing layer can be manufactured in many different ways. For example, the clusters can be manufactured beforehand, and a solution containing the clusters is deposited on the conductive layer of the photovoltaic device. The clusters can, for example, be crystals of dye produced beforehand. Alternatively, a solution containing dye molecules is deposited on the conductive layer of the photovoltaic device and the clusters are formed during drying of the conductive layer covered with the solution. The dye molecules are bonded to each adjacent dye molecule and form clusters during the drying. If the conductive layer covered with the solution is heated during the drying, the dye molecules can be bonded to each other so that they form clusters of dye crystals on the surface of the conductive layer.

[0121] In one aspect, the method comprises producing a solution including dye molecules and/or clusters of dye molecules distributed in a solvent, distributing the solution on the conductive layer, and drying the conductive layer provided with the solution until the solvent has evaporated. For example, the coating may be made by spraying. Alternatively, the coating can be made by electro-spraying. The method may comprise heating the conductive layer provided with the solution to achieve crystallization of the clusters of dye molecules. This method for producing the light absorbing layer is simple, fast and provides an even distribution of the clusters on the surface of the conductive layer. The solution may contain dye molecules solved in the solvent. For example, dye powder is dissolved in the solvent to form a solution comprising dye molecules. In such case, the dye molecules will bond to each other and form clusters during the drying. Alternatively, clusters of a desired size can be manufactured beforehand. The clusters are then added to the solvent to form the solution. The clusters are distributed on the surface of the conductive layer during the coating. Alternatively, the solution comprises clusters of dye molecules as well as dye molecules solved in the solvent. This can be advantageous since the dye molecules may act as a glue between the clusters, and between the clusters and the conductive layer so that the clusters will attach to each other and to the conductive layer.

Example 1

[0122] In this example, the clusters are formed directly on top of the conductive layer 6.

[0123] In a first step, a dye solution is manufactured by dissolving a solid dye, e.g., in the form of a powder of dye in a suitable solvent that dissolves the solid dye. Consequently, a solution of dye molecules dissolved in a solvent is being formed. In one example the dye is an arylamine dye, for example, (E)-3-(5-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (also abbreviated as D35). The solvent may be any organic solvent that has the capability to dissolve the dye, such as for example methylene chloride, acetonitrile, NMP, DMF, THFA, butyrolactone, or DMSO, methanol.

[0124] In a second step, an upper surface of a conductive layer comprising porous Ti is coated with the solution. For example, the coating of the upper surface of the conductive layer is carried out by spraying the solution on the conductive layer.

[0125] In a third step, the conductive layer provided with the solution is dried until the solvent has been evaporated and a plurality of clusters of dye molecules is formed on the conductive layer. In this example, the clusters are boned to the conductive layer during the formation of the clusters on the conductive layer, i.e. during the evaporation of the solvent.

[0126] In this example, the solution comprises dye molecules solved in the solvent, and the clusters are achieved after the solution has been applied to the surface of the conductive layer.

Example 2

[0127] In a first step, a dye solution is manufactured by dissolving a solid dye, e.g., in the form of a powder of dye in a suitable solvent that dissolves the solid dye. Consequently, a solution of dye molecules dissolved in a solvent is being formed. The dye and the solvent can be the same as in example 1.

[0128] In a second step, the dye molecules in the solution are being precipitated into crystals consisting of crystalline clusters of dye molecules. The crystallization can be achieved in several ways. For example, the solvent can be removed to a level where the dye starts to precipitate because the solubility of the dye is too low. Alternatively, it is possible to precipitate the dye by adding precipitating agents like, e.g., salts.

[0129] In a third step, the solution including the crystalline clusters is deposited onto the conductive layer 6. It is advantageous to add dye molecules to the solution including the crystalline clusters before the solution is deposited onto the conductive layer 6.

[0130] In a fourth step, the conductive layer 6 provided with the solution is dried until the solvent has been evaporated and the crystalline clusters are distributed on the surface of the conductive layer. The added dye molecules will serve as a glue between the clusters and the conductive layer after the the solvent has been evaporated so that the clusters are attached to the conductive layer.

[0131] In this example, the solution comprises clusters distributed in a solvent.

Example 3

[0132] The crystalline clusters can also be formed directly on top of the conductive layer 6.

[0133] In a first step a dye solution is manufactured by dissolving a solid dye, e.g., in the form of a powder of dye in a suitable solvent that dissolves the solid dye. The dye and the solvent can be the same as in example 1.

[0134] In a second step, an upper surface of the conductive layer is coated with the solution. For example, the coating of the upper surface of the conductive layer is carried out by spraying the solution on the conductive layer.

[0135] In a third step, the conductive layer provided with the solution is subjected to heating (annealing) for a certain amount of time, for example at 70° C. during three hours, so that the solvent evaporates to precipitate the solid dye into clusters on top of the conductive layer 6, and to achieve crystallization of the clusters. The annealing can be performed in air or in inert atmosphere like, e.g., argon or in vacuum. The solvent is evaporated during the heating.

[0136] The spraying and heating procedure can be repeated several times in order to achieve a layer of clusters, where the clusters are thick enough to efficiently absorb the light. It is possible to vary the concentration of the dye solution or the temperature during drying to achieve different qualities of the cluster layer. For example, a fast drying can result in smaller clusters and therefore a high drying temperature can result in fast evaporation of solvent, which can result in small clusters. By allowing the solvent to evaporate slowly it is possible to grow larger clusters on the conductive layer.

Example 4

[0137] Firstly, clusters are manufactured by precipitating dye in a crystalline structure from a dye solution by adding cations to the dye solution. The cations make the dye insoluble in the solvent and as a result the dye precipitates in the solution in the form of crystalline structure. The crystalline clusters are then separated from the solution by sedimentation and decantation. The crystalline clusters can also be separated from the solution more efficiently by centrifugation followed by decantation. Alternatively, the crystalline clusters can be separated from the solution by filtration through a filter, preferably by applying vacuum and sucking the liquid crystal mixture through the filter. Alternatively, the crystalline clusters can be separated from the solution by filtration and applying overpressure to the crystal liquid mixture, and thereby pressing the liquid through the filter leaving the crystals on the filter.

[0138] The crystalline clusters can, for example, be deposited on the conductive layer by spraying, vacuum suction, or electro-spraying.

Example 5

[0139] This example describes a method for manufacturing a working electrode having a reflective layer.

[0140] In a first step, a first solution is manufactured comprising TiO.sub.2 and a solvent.

[0141] In a second step, an upper surface of a conductive layer made of porous Ti is coated with the first solution. For example, the coating of the upper surface of the conductive layer is carried out by spraying or printing the first solution on the conductive layer.

[0142] In a second step, the conductive layer provided with the first solution is dried until the solvent has been evaporated at a temperature between 50-80° C. and a layer of TiO.sub.2 particles is formed on the first conductive layer. Further, the conductive layer provided with the TiO.sub.2 particles are sintered, for example, for 15 minutes in about 500° C., to bond the TiO.sub.2 particles to the conductive layer and to achieve electrical contact between the TiO.sub.2 particles and the conductive layer.

[0143] In a third step, a second solution is manufactured by dissolving a solid dye, e.g., in the form of a powder of dye in a suitable solvent that dissolves the solid dye. The dye and the solvent can be the same as in example 1.

[0144] In a fourth step, the layer of TiO.sub.2 particles is coated with the dye solution. For example, the coating is carried out by spraying the dye solution on the layer of TiO.sub.2 particles.

[0145] In a fifth step, the conductive layer provided with TiO.sub.2 particles and the dye solution is dried in between 50-80° C. until the solvent has been evaporated and a plurality of clusters of dye molecules are formed on the layer of TiO.sub.2 particles. Further, the conductive layer provided with TiO.sub.2 particles and the dye solution can be subjected to heating (annealing) for a certain time to increase the crystallinity of the precipitated clusters of dye molecules.

[0146] In another example, seeds of another material then the dye can be used during manufacturing of the clusters to start the crystallisation process. The crystals are grown on the seeds to form crystalline clusters. An advantage of using a seed during the manufacturing process is that the clusters can be spherical and of substantially equal size. This facilitates the manufacturing of the light absorbing layer and makes it possible to achieve a more homogeneous layer.

[0147] The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For example, the light absorbing layer may include small amounts of a second light absorbing photovoltaic material.