Working electrode for a photovoltaic device and photovoltaic device comprising the same

Abstract

The invention relates to a working electrode (1a) for a photovoltaic device comprising a light-absorbing layer (3) and an electrically conductive layer (6) 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) consists of a layer of a plurality of clusters (7), wherein each cluster (7) is formed of dye molecules, and each dye molecule in a cluster (7) is bonded to its adjacent dye molecules.

Description

TECHNICAL FIELD

[0001] The present invention relates to a working electrode comprising a light absorbing layer for a photovoltaic device. The present invention also relates to a photovoltaic device comprising the working electrode. BACKGROUND

[0002] A photovoltaic device converts light into electricity by using a semiconductor material that has a photovoltaic effect.

[0003] A photovoltaic device, such as a solar cell, is a device that directly converts sunlight into electricity. Light incident on the surface of the photovoltaic device generates electricity. The photovoltaic device comprises a working electrode having a light absorbing layer. The light absorbing layer comprises a light absorbing photovoltaic material that has the ability to absorb light and generate photo-excited electrons. When the energy of a photon is equal to or greater than the band gap of the light absorbing material, the photon is absorbed by the material and generates a photo-excited electron.

[0004] The energy of a photon depends on the type of light source. For example, the energy of a photon of indoor light is less than the energy of a photon of sunlight. Furthermore, the energy of a photon from outdoor sunlight depends on the latitude. If the energy of a photon reaching the light absorbing material is less than the band gap of the light absorbing material, the photon cannot be absorbed by the material and therefore the energy cannot be converted into electricity. If the energy of a photon reaching the light absorbing material is equal to or greater than the band gap of the light absorbing material, the photon is absorbed by the material but only the amount of energy corresponding to the band gap is converted into electricity. Therefore, it is important that the band gap of the light absorbing material in a photovoltaic device matches the light source that the device will use.

[0005] Therefore, it would be desirable to be able to tune the band gap of a light absorbing photovoltaic material to allow efficient conversion of light energy of the light spectrum of a weak light source, such as indoor light and outdoor sunlight, into electrical energy.

[0006] Currently, silicon is the most commonly used light absorbing photovoltaic material in solar cells. Silicon has some advantages, for example, it is chemically stable and environmentally friendly. Silicon can efficiently convert light energy of sunlight into electrical energy. However, silicon has a low conversion efficiency for weak light sources.

[0007] It is also known to use dye molecules as light absorbing photovoltaic materials in solar cells. Such solar cells are known as dye-sensitized solar cells (DSSC). In a DSSC solar cell, the light absorbing layer comprises a semiconducting scaffolding layer comprising a network of sintered particles (e.g. metal oxide particles such as Ti02 particles) that are dyed with a light absorbing dye. The light absorbing dye is also known as a sensitizing dye. The dye molecules are disposed on the surface of the semiconducting particles. The dye absorbs incident light and excites an electron with the energy in the light. The semiconducting particles act as a material to transport the excited electron to a conductive layer.

[0008] A dye-sensitized solar cell is described in EP2533352. In EP2533352, a porous semiconductor layer is placed on a porous conductive metal layer that serves as a current collector electrode. The porous semiconductor layer absorbs dye. Dye molecules will attach to the surface of the porous semiconductor layer. The porous semiconductor layer has a dual function and acts as a scaffolding structure to hold the dye molecules to spread out in a three-dimensional structure. The other function of the semiconductor layer is to transfer the excited electrons (or holes) to the current collector. EP2533352 further describes that due to the metal property of the current collector, high conversion efficiency can be obtained even if the thickness of the porous semiconductor layer is made to be greater than 14 pm (thicker semiconductor layer can absorb more dye). EP2533352 mentions several common dyes such as ruthenium dye, phthalocyanine dye or cyanine dye.

[0009] EP2834823 describes another configuration of a dye-sensitized solar cell, wherein the dye-sensitized solar cell comprises a working electrode, a porous insulating layer, a porous first conductive metal layer, a counter electrode and a conductive medium, wherein the porous first conductive metal layer is formed on top of the porous insulating layer and in electrical and physical contact with the light absorbing layer; the counter electrode comprises a second conductive metal layer; the conductive medium is in the form of a liquid electrolyte for transferring charge between the counter electrode and the working electrode. The working electrode comprises a porous Ti02 layer deposited on the first conductive layer. Dye molecules are adsorbed onto the surface of the Ti02 particles of the Ti02 layer in a conventional manner.

[0010] The dye molecules preferably form a monolayer of dye molecules on the particles of the scaffolding semiconductor structure. To ensure sufficient light absorption, there should be a certain amount of sensitizing dye in the light absorbing layer. The number of molecules per square centimeter required to achieve sufficient light absorption depends on the absorption coefficient of the dye. To obtain sufficient sensitizing dye in the light absorbing layer, the light absorbing layer typically comprises hundreds of layers of dyed semiconductor particles. The number of layers will determine the thickness of the light absorbing layer, and increasing the amount of sensitizing dye will increase the thickness of the light absorbing layer. Therefore, the problem with dye-sensitized solar cells is that in order to increase the amount of dye, the porous semiconductor layer becomes thicker. A thick semiconductor layer will reduce the efficiency of the solar cell due to longer electron diffusion length.

[0011] Many documents (e.g. KR101469570, JP2016207919) describe various methods for preventing the aggregation of dye molecules in the semiconductor layer of a dye-sensitized solar cell. Aggregation on the surface of the semiconductor is described as reducing the efficiency of the DSSC.

[0012] Lei Zhang et al. "Dye aggregation in dye-sensitized solar cells" (Journal of materials chemistry A, vol. 5, no. 37, 5 September 2017) discloses a review on dye aggregation on Ti02coated in solar cells. On page 19542, part 2, it is specifically mentioned that dye aggregates severely compromise the functioning of DSSC devices and that dye aggregation can affect photovoltaic DSSC performance. Dye aggregation in DSSCs is considered a phenomenon that should be best avoided.

[0013] WO2018 / 021952 describes another type of solar cell. A multitude of crystalline grains of doped semiconductor material in electrical contact with a conductive layer are described and these grains are at least partially surrounded by a charge conductive material, such as PEDOT. The grains are described to be made of silicon, or of CdTe, CIGS, CIS, GaAs or perovskite.

[0014] In recent years, there is an increasing interest in organic sensitizing dyes for use in dye-sensitized solar cells (DSSC) because it has been found that organic dyes in combination with ionic-based electrolytes can improve the performance of DSSC devices, in particular for indoor applications. There are a large number of different types of sensitizing dyes with different light absorption capabilities. SUMMARY

[0015] It is an object of the present invention to at least partly overcome some of the above-mentioned problems and to provide an improved working electrode for a photovoltaic device, and an improved photovoltaic device comprising the same.

[0016] This object is achieved by a working electrode as defined in claim 1.

[0017] The working electrode for a photovoltaic device comprises a light absorbing layer and a conductive layer, the conductive layer being arranged in electrical contact with the light absorbing layer, and the light absorbing layer comprising a light absorbing photovoltaic material consisting of a plurality of dye molecules. The light absorbing layer is formed by a plurality of layers of clusters, wherein each cluster is formed by dye molecules, and each dye molecule in a cluster is bonded to its adjacent dye molecules.

[0018] The dye molecules form a plurality of clusters of dye molecules. The clusters form a light absorbing layer, which is in electrical contact with the conductive layer. The individual layers of clusters do not disperse onto the scaffold layer of semiconductor material.

[0019] By arranging the dye molecules into clusters, forming individual light absorbing layers, the semiconductor scaffold layer of a conventional dye-sensitized solar cell can be omitted. Thus, for the same amount of dye molecules, a solar cell comprising a light absorbing layer of clusters can be made thinner than a conventional dye-sensitized solar cell.

[0020] The light absorbing layer does not contain dye molecules disposed or absorbed on the surface of the semiconductor particles. The dye molecules forming the clusters are not disposed on the semiconductor particles within the semiconductor layer. These clusters can be directly stacked on the surface of the conductive layer without the support of a semiconductor underlayer, thereby forming the working electrode of the solar cell that can be directly connected to an external circuit.

[0021] The production time of the solar cell is significantly reduced when the step of absorbing the dye by the semiconductor layer is omitted in the preparation of the dye-sensitized solar cell. The absorption of the dye can take several hours to complete. The production time is further reduced by not having to form a semiconductor underlayer in the production of the dye-sensitized solar cell.

[0022] Another advantage of the light absorbing layer as a separate layer of clusters of dye molecules is that more dye molecules per square meter can be inserted compared to a dye-sensitized solar cell where the dye molecules are absorbed by the semiconductor layer.

[0023] The solar cell comprising a working electrode with a light absorbing layer made of clusters of dye molecules where the clusters form a separate layer can be made thinner than a dye-sensitized solar cell comprising a semiconductor underlayer into which the dye is injected. Thinner solar cells have many advantages. The shorter distance between the electrodes of the solar cell improves the efficiency of the solar cell. Thinner solar cells can also find new applications where it is desirable to have a thin and light weight solar cell.

[0024] The clusters of dye molecules are formed by arranging the dye molecules into a lattice or a random, amorphous structure or a combination thereof. The term "lattice" means that the clusters have a defined and repeatable arrangement of dye molecules.

[0025] The dye molecules within the clusters are bonded to their adjacent dye molecules. The bond between the dye molecules can be an electrostatic bond, or a covalent bond, or a van der Waals bond, etc.

[0026] The term "dye" is understood to mean a dye having a photovoltaic effect, i.e. having the ability to absorb light and generate photo-excited electrons.

[0027] The dye molecules of the clusters can be dye molecules of different chemical composition. The amount of dye required per unit volume in the light absorbing layer depends on the type of dye as different dyes have different absorption coefficients and therefore different abilities to absorb light. The size of the clusters can be controlled during the process of making the clusters. Therefore, clusters of different sizes can be prepared depending on the amount of dye required in the light absorbing layer in order to efficiently absorb the incoming light. If the clusters are prepared large enough, only one cluster monolayer is required to achieve the same effect as a prior art dye-sensitized solar cell. Therefore, the light absorbing layer of the present invention can be prepared significantly thinner.

[0028] In the past decades, thousands of different dyes have been synthesized and tested in DSSC devices. Known dye molecules can be used to form clusters and crystalline clusters with a band gap different from the HOMO / LUMO band gap of the individual dye molecules. This makes it possible to make light absorbing layers with different band gaps. For example, clusters with a band gap in the interval 1.0-1.6 eV can be made. Thus, the band gap of the light absorbing layer of a photovoltaic device can be optimized for the specific spectrum of light to be converted into electricity.

[0029] In one aspect, a mixture of two or more dyes is used in the clusters of the light absorbing layer.

[0030] In one aspect, voids are formed between the clusters, and the working electrode comprises an electrically conductive medium filling the voids between the clusters in the light absorbing layer. The electrically conductive medium transfers charge to / from the clusters in the working electrode.

[0031] In one aspect, the clusters forming the light absorbing layer are essentially a cluster monolayer. The clusters absorb incoming light and excite electrons with the energy in the light. In this aspect, the clusters are arranged in a single layer such that each cluster is directly facing the incoming light without restriction. Incoming light without restriction means that the light comes directly from a light source, such as the sun or a lamp, and that the light is not obstructed by other clusters arranged on top of the clusters in the single layer. It is advantageous to have a cluster monolayer, since each cluster will face the incoming light and will contribute to the conversion of the incoming light into electricity. If the light absorbing layer comprises multiple layers of clusters arranged on top of each other, the clusters in the upper layers will obstruct the clusters in the lower layers, so that their contribution to the light conversion is less. Furthermore, if the light absorbing layer only has one cluster monolayer, the thickness of the light absorbing layer can be reduced. The thickness of the light absorbing layer is essentially equal to the thickness of the clusters in the cluster monolayer.

[0032] In one aspect, at least 40% of the clusters forming the light absorbing layer are crystalline clusters, wherein the dye molecules within the clusters are arranged in a determined and reproducible manner, preferably at least 50% of the clusters are crystalline clusters, and most preferably at least 70% of the clusters are crystalline clusters.

[0033] Clusters of dye molecules with a crystalline structure or at least a partially crystalline structure, hereinafter referred to as crystalline clusters, have some specific advantages. When dye molecules are arranged in a crystal, the properties of the individual dye molecules change. For example, the individual dye molecules have a HOMO / LUMO band gap. However, the band gap of a crystalline cluster depends on the type of dye molecules in the cluster. A band gap that is smaller than the HOMO / LUMO gap of the individual dye molecules will broaden the light absorption spectrum of the crystalline cluster compared to the individual dye molecules. A broader light absorption spectrum means that the crystalline cluster has the ability to absorb light in a wider range of wavelengths compared to the individual dye molecules. The light capturing ability of the crystalline cluster is thus increased compared to a structure where the same number and type of dye molecules are arranged as individual molecules in a scaffold structure.

[0034] In working electrodes where the light absorbing layer comprises crystalline clusters, at least 40%, 45% or 50% of the clusters should be crystalline clusters. Preferably, at least 70% or 80% of the clusters in the light absorbing layer should be crystalline clusters, and most preferably at least 90% of the clusters are crystalline clusters.

[0035] In addition to the advantages listed above, one advantage of crystalline clusters is that solar cells with a specific light absorption spectrum can be designed. This enables solar cells to be tailored for specific uses with specific light conditions. The possibility of having different types of dye molecules in the crystalline clusters will further increase the flexibility of designing solar cells.

[0036] In some aspects, the conductive layer is in direct physical and electrical contact with the light absorbing layer. This means that at least some of the clusters are in physical contact with the conductive layer.

[0037] In some aspects, the clusters forming the light absorbing layer are in physical and electrical contact with the conductive layer and the clusters are bound to the conductive layer. The light absorbing layer is arranged in electrical contact with the conductive layer such that the conductive layer receives photo-generated charges from the clusters. In an embodiment where one cluster is in direct physical and electrical contact with the conductive layer, the cluster is bound to the conductive layer. The clusters in the light absorbing layer do not need to be bound to each other.

[0038] In one aspect, the clusters are arranged in a single layer along the conductive layer, and each cluster is in physical and electrical contact with the conductive layer.

[0039] The conductive layer is preferably a porous layer of sintered metal particles. The particles can also be other types of conductive particles, such as particles of conductive glass, carbon or semiconductor material.

[0040] In some aspects, at least 80% of the clusters comprise more than 100 dye molecules per cluster. Preferably, at least 80% of the clusters comprise more than 1000 dye molecules per cluster. Most preferably, at least 80% of the clusters comprise more than 10000 dye molecules per cluster. Thus, for most dyes, the light absorbing layer will comprise a sufficient amount of dye to absorb most of the incident light into electricity.

[0041] A sufficient number of dye molecules ensures sufficient light absorption. The larger the number of dye molecules, the better the light absorption obtained. In order to achieve the same light absorption as known dye-sensitized solar cells, the light absorbing layer should comprise approximately the same number of dye molecules per unit surface area as known dye-sensitized solar cells. If the clusters comprise a smaller number of molecules, the light absorbing layer can comprise more than one layer of clusters to ensure that the light absorbing layer will comprise a sufficient amount of dye. The larger the clusters, the fewer layers of clusters that are needed in the light absorbing layer.

[0042] In some aspects, at least 80% of the clusters with more than 100 dye molecules have a size along a straight line through two points on the surface of the cluster that is larger than 5 nm. The straight line is for example the diameter of a circular cluster or the z-axis of a cubic lattice of a crystalline cluster.

[0043] For larger clusters, at least 80% of the clusters can have a size along a straight line passing through two points on the surface of the cluster connecting the cluster that exceeds 10 nm. Most preferably, at least 80% of the clusters have a size along a straight line passing through two points on the surface of the cluster connecting the cluster that is greater than 20 nm. The size of the clusters required depends on the type of dye and its absorption coefficient. The larger the size of the clusters, the better the light absorption.

[0044] In certain aspects, for larger clusters, at least 80% of the clusters have a size along a straight line passing through two points on the surface of the cluster connecting the cluster that is less than 2 μιη. Preferably, at least 80% of the clusters have a size less than 1 μιη. Thus, the thickness of the light absorbing layer can be made thinner. The thickness of the light absorbing layer depends on the size of the clusters. The light absorbing layer is essentially a monolayer of clusters. For a light absorbing layer comprising one monolayer of clusters, the thickness of the light absorbing layer is essentially equal to the thickness of the clusters.

[0045] Suitably, at least 80% of the clusters have a size along a straight line passing through two points on the surface of the cluster connecting the cluster that is between 5 nm and 2 μιη. Preferably, at least 80% of the clusters have a size along a straight line passing through two points on the surface of the cluster connecting the cluster that is between 10 nm and 1 μιη. Thus, the light absorbing layer can be designed to contain a sufficient amount of dye to absorb a large portion of the incident light under various light conditions to convert into electricity, and the light absorbing layer will be thin.

[0046] According to some aspects, the dye molecules are organic dye molecules, organometallic dye molecules, or natural dye molecules.

[0047] In one aspect, the dye is selected from the group consisting of organic dyes or selected from organic dyes, wherein the organic dyes are, for example, tetrahydroquinoline, pyrrolidine, diphenylamine, triphenylamine (TPA), coumarin dyes, indole dyes, arylamine dyes, porphyrin 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, and fluorinated quinoxaline dyes. It has been found that organic dyes can improve the performance of DSSC devices. By using crystalline organic dye clusters, the band gap can be reduced, thereby absorbing light over a wider range of wavelengths and more efficiently absorbing longer wavelengths of light.

[0048] In another aspect, the dye is selected from the group consisting of natural dyes or selected from natural dyes, wherein the natural dyes are, for example, betalain dyes, anthocyanin dyes

[268] , chlorophyll dyes

[269] , flavonoid dyes

[270] , and carotenoid dyes.

[0049] Metal organic dyes are well known photovoltaic materials with good light absorption properties and are tailored for efficient absorption of visible light. Examples of organic metal dyes can be the commonly used ruthenium (Ru) bipyridine derivative dyes (N3: cis-diisothicyano-di(2,2'-bipyridine-4,4'-dicarboxylic acid) ruthenium(II); N719: bis-tetrabutylammonium cis-di(isothicyano) di(2,2'-bipyridine-4,4'-dicarboxylic acid) ruthenium(II); Z907: cis-di(isothicyano)(2,2'-bipyridyl-4,4'-dicarboxylic acid)(4,4'-dinonyl-2'-bipyridyl).

[0050] In a third aspect, the dye is selected from the group of organic metal dye molecules or from the group of organic metal dye molecules, which are for example metal organic complexes, such as ruthenium based complexes or other metal complexes (such as iron complexes or platinum complexes).

[0051] Other types of dyes can be organic dyes without metals, such as eosin Y, aniline blue, bromophenol blue, alcian blue, methyl orange, crystal violet, fast green and carbol fuchsin.

[0052] Furthermore, it can be considered to use natural dyes in the solar cell, such as anthocyanins, carotenoids, flavonoids or chlorophyll pigments.

[0053] The dye molecules suitable for use in the present application are not limited to the examples given above.

[0054] One way to determine which dye is suitable for the desired use in a solar cell is to measure the luminescence of the dye. A light "echo" that does not attenuate the light intensity corresponding to the incident light or that does not shift the light can be used as an indicator of suitability of the dye.

[0055] In some aspects, the clusters can comprise an inner core or seed of a different material. The clusters can contain a small inner core of another material used to initiate the crystallization process during the manufacture of the clusters. The crystals grow on the seed / core to form the clusters of crystals. One advantage of using an inner core during the manufacturing process is that the clusters can be spherical and substantially the same size. This facilitates the manufacturing of the light absorbing layer and a more uniform layer can be obtained. Another advantage of using an inner core is that a narrower size distribution of the clusters can be achieved. A narrow size distribution is useful in cases where the size of the clusters needs to be controlled very precisely.

[0056] In some aspects, the clusters are substantially uniformly distributed in the light absorbing layer. This enables uniform conversion of the incident light over the entire surface of the light absorbing layer. The uniform distribution of the clusters also creates a large active surface area for the conversion of light to electricity. By uniform distribution, it is meant that the number of clusters per square centimeter is the same or substantially the same over the entire area of the light absorbing layer. By substantially uniform distribution, it is meant that the number of clusters per square centimeter can vary by ±10% between different parts of the light absorbing layer.

[0057] In some aspects, the thickness of the light absorbing layer is less than or equal to 2 pm, and preferably less than or equal to 1 pm. The present invention provides an effective light absorbing layer with a thickness of less than or equal to 2 pm. The optimal thickness of the effective light absorbing layer depends on the light absorption spectrum of the dye and the light emission spectrum of the light source. This increases the flexibility of use of the photovoltaic device, as the light absorbing layer can be designed to achieve the optimal balance between the light absorption spectrum of the dye and the light emission spectrum of the light source.

[0058] In some aspects, the thickness of the light absorbing layer is from 20 nm to 2 pm.

[0059] In some aspects, the working electrode comprises a reflective layer disposed on the opposite side of the light absorbing layer relative to the upper surface. The reflective layer is disposed between the light absorbing layer and the first conductive layer. The light absorbing layer comprising the clusters is disposed on top of the reflective layer. The reflective layer comprises semiconductor particles in electrical contact with the clusters and the first conductive layer. The semiconductor particles are made of a reflective material, i.e. a material that can reflect light. The reflective layer reflects light back into the light absorbing layer. The semiconductor particles adhere to each other and form the reflective layer. The reflective layer acts as a mirror, scattering the incident light back into the light absorbing layer, thereby increasing the effective absorption path length and thus the light absorption of the light absorbing layer. The light scattering effect of the reflective layer depends on the wavelength, and the light scattering effect depends largely on the size of the semiconductor particles in the reflective layer. Therefore, the light scattering can be adjusted and optimized by selecting semiconductor particles with a sufficient particle size to suit the application of the existing photovoltaic device. The semiconductor particles are in electrical contact with the conductive layer and the light absorbing layer. Therefore, the clusters are in electrical contact with the conductive layer through the semiconductor particles.

[0060] The semiconductor particles in the reflective layer are designed so that they can reflect light. In some aspects, at least 80% of the semiconductor particles in the reflective layer have a size or the size of the agglomerates of the semiconductor particles is greater than 0.1 pm, and preferably greater than 0.2 pm. The larger the particles, the better they reflect light. If the semiconductor particles are smaller than 0.1 pm, they have a poor ability to reflect light.

[0061] In one aspect, at least 80% of the semiconductor particles have a size of from 0.1 pm to 2 pm. This will improve the light scattering ability of the semiconductor particles.

[0062] In some aspects, the semiconductor particles are made of titanium dioxide (Ti02). The use of titanium dioxide is advantageous because it can reflect light well without absorbing light. Titanium oxide has a high refractive index and a large enough band gap to avoid absorbing light. Furthermore, titanium dioxide has sufficient electrical conductivity so that the reflective layer can efficiently transfer photo-excited charges received from the light-absorbing layer to the conductive layer.

[0063] In some aspects, the reflective layer has a thickness of 0.1 pm to 10 pm. Preferably, the reflective layer has a thickness of 1 pm to 10 pm. Thus, the reflective layer is thin enough to achieve small electrical energy loss during the transfer of photo-excited charges from the light-absorbing layer to the conductive layer.

[0064] Preferably, the reflective layer is porous to allow the conductive medium to pass through the reflective layer.

[0065] In some aspects, the reflective layer has a porosity of 40% - 70%. It is important that the reflective layer is porous enough so that the conductive medium can form a continuous conductive path through the pores inside the reflective layer.

[0066] In some aspects, the light-absorbing layer and the reflective layer overlap so that part of the clusters are disposed in the pores formed between the semiconductor particles. One advantage of the light-absorbing layer penetrating inside the reflective layer is that it can increase the effective light-absorbing path length, resulting in higher light absorption.

[0067] In this case, the use of titanium dioxide (Ti02) in the semiconductor particles is particularly advantageous because titanium dioxide is partially transparent and allows light to reach the clusters disposed within the pores of the reflective layer.

[0068] The clusters disposed in the pores between the semiconductor particles are pre-formed clusters of dye molecules that are not dye molecules that are injected into the semiconductor structure to attach to the semiconductor scaffold structure, whether some of these dye molecules form aggregates or not.

[0069] In some aspects, the reflective layer has a pore size of 10 nm to 1 pm. Thus, the pores in the reflective layer are large enough to accommodate the clusters in the pores of the reflective layer and the conductive medium.

[0070] In another aspect, the object of the present application is achieved by a photovoltaic device comprising a working electrode according to the present application.

[0071] The photovoltaic device comprises a working electrode according to the present application, a counter electrode, and a conductive medium for transferring electrical charges between the counter electrode and the working electrode. The photovoltaic device can be a solar cell.

[0072] In some aspects, the photovoltaic device includes a porous insulating substrate, where the conductive layer is a porous conductive layer formed on one side of the porous insulating substrate. The counter electrode can also include a second conductive layer disposed on the opposite side of the porous insulating substrate. In some aspects, the clusters are uniformly distributed over the surface of the first conductive layer.

[0073] In some aspects, the conductive medium is an ionic-based electrolyte that transfers photo-excited electrons from the counter electrode to the working electrode, which can also include a gel polymer electrolyte. A hole-conducting medium is also a possible conductive medium.

[0074] In addition to the several advantages of the photovoltaic device having a working electrode according to the present invention described above, the surface of the light absorbing layer can also become rougher, which increases the probability of reflected light being absorbed. This in turn reduces the efficiency loss due to surface reflection. The rougher surface provides multiple angles for the incident light, the efficiency of the photovoltaic device does not strictly depend on the angle of incidence of the light with respect to the layer. Therefore, the optical losses are reduced compared to known dye-sensitized solar cells. BRIEF DESCRIPTION OF DRAWINGS

[0075] The present invention will now be explained in greater detail by a description of different embodiments of the invention, with reference to the accompanying drawings.

[0076] Figure 1 An example of a working electrode including a light absorbing layer is shown.

[0077] Figure 2 Another example of a working electrode including a light absorbing layer and a reflective layer is shown.

[0078] Figure 3 An example of a working electrode including Figure 1 An example of a photovoltaic device including the working electrode shown.

[0079] Figure 4 An example of a working electrode including Figure 3 An example of a photovoltaic device including the working electrode shown.

[0080] Figure 5 An example of a working electrode including Figure 1 Another example of a photovoltaic device including the working electrode shown.

[0081] Figure 6 An example of a working electrode including Figure 2 An example of a photovoltaic device including the working electrode shown. DETAILED DESCRIPTION

[0082] The same numbers in different drawings refer to the same elements.

[0083] Figure 1A schematic of a working electrode 1a is shown, comprising a light absorbing layer 3 made of a light absorbing photovoltaic material and an electrically 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 electrically conductive layer 6 is provided on the opposite side of the light absorbing layer 3, opposite the upper surface 5. In this example, the light absorbing layer 3 is provided directly on the electrically conductive layer 6. The light absorbing photovoltaic material is composed of a plurality of dye molecules. The dye molecules form clusters 7. The dye molecules within a cluster 7 are arranged such that each dye molecule is bonded to its neighbouring dye molecules. The clusters 7 are provided on the surface of the electrically conductive layer 6 and substantially each cluster 7 is bonded to the electrically conductive layer 6. The clusters 7 should cover a large portion 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 the electrically conductive medium to pass through the light absorbing layer. To obtain sufficient light absorption, most clusters can contain more than 100 dye molecules, preferably more than 1000 dye molecules and most preferably more than 10000 dye molecules. Each dye molecule in a cluster is bonded to its neighbouring dye molecules.

[0084] The dye can be any type of dye capable of absorbing a photon and generating a photo-excited electron. There are thousands of known types of dyes with the ability to absorb a photon and generate a 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 with good light absorption and can be tailored for efficient absorption of visible light.

[0085] Examples of organic dyes: tetrahydroquinoline, pyrrolidine, diphenylamine, triphenylamine (TPA), coumarin dye, indole dye, arylamine dye, porphyrin dye, fluorine dye, carbazole dye (CBZ), phenothiazine dye (PTZ), phenoxazine dye (POZ), hemicyanine dye, cyanine dye, squarine dye, perylene dye, anthraquinone dye, boron-dipyrromethene (BODIPY) dye, oligomeric thiophene dye and polymeric dye, fluorinated quinoxaline dye. 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, allowing absorption of light over a wider range of wavelengths and more efficient absorption of longer wavelengths of light.

[0086] Examples of metal organic dyes: ruthenium-based complexes, or other metal complexes (e.g. iron complexes or platinum complexes).

[0087] Examples of natural dyes: betalain dye, anthocyanin dye

[268] , chlorophyll dye

[269] , flavonoid dye

[270] , carotenoid dye.

[0088] The dye molecules suitable for use in the present application are not limited to the examples given above. Furthermore, the dye molecules in a cluster can be a mixture of two or more dyes.

[0089] Suitably, the clusters 7 are substantially uniformly distributed in the light absorbing layer 3 to achieve uniform conversion of the incident light over the entire surface of the light absorbing layer 3. The clusters can be in physical contact with each other, but they need not be bonded to each other. The clusters 7 are typically bonded to another layer, such as the conductive layer 6, disposed below the light absorbing layer 3. The conductive layer 6 is disposed in electrical contact with the clusters 7. In this example, the conductive layer 6 is disposed in electrical and physical contact with the clusters 7.

[0090] The desired size of the clusters 7 depends on the type of dye and its absorption coefficient. The larger the size of the cluster, the better the light absorption. The shape and size of the clusters 7 can be varied by the method used to produce the clusters. To achieve good light absorption capabilities, at least 80% of the clusters have a size greater than 5 nm along a straight line passing through the cluster connecting two points on the surface of the cluster. For example, the line is the diameter of the cluster. More preferably, at least 80% of the clusters have a size greater than 10 nm, and most preferably greater than 20 nm, along a straight line passing through the cluster connecting two points on the surface of the cluster. Suitably, at least 80% of the clusters have a size of 5 nm to 2 μιη along a straight line passing through the cluster connecting two points on the surface of the cluster. Preferably, at least 80% of the clusters have a size of 10 nm to 1 μιη along a straight line passing through the cluster connecting two points on the surface of the cluster. The size of the clusters is measured, for example, by using a SEM "scanning electron microscope".

[0091] For example, the clusters 7 are disposed such that they form a monolayer of clusters 7 in the light absorbing layer 3, as shown in Figure 1 Each cluster 7 in the monolayer has an upper surface facing the incident light, and thus can facilitate light conversion.

[0092] The optimal thickness of the effective light absorbing layer depends 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 μιη, and preferably less than or equal to 1 μιη. For example, the thickness of the light absorbing layer is greater than 20 nm. The thickness of the light absorbing layer depends primarily on the thickness of the clusters 7. Suitably, the thickness of the light absorbing layer is 20 nm to 2 μιη.

[0093] As shown in Figure 3 The light absorbing layer 3 can also include a conductive medium 9. Voids 8 are formed between the clusters 7 for accommodating the conductive medium. For example, the conductive medium 9 can be a liquid electrolyte or a solid charge conductive material, such as a conductive polymer. The conductive medium 9 is disposed in the voids 8 between the clusters 7. For example, the clusters 7 can be partially covered by the charge conductive material 42, as shown in Figure 5 Preferably, the conductive layer 6 is also porous to allow the conductive medium 9 to pass through the conductive layer 6. The conductive layer 6 is made of a conductive material. For example, the conductive layer 6 is made of porous Ti.

[0094] The working electrode can include a connecting element 46 electrically connected to the conductive layer 6 to connect the conductive layer to an external load, as shown in Figure 3 The working electrode can include a connecting element 46 electrically connected to the conductive layer 6 to connect the conductive layer to an external load, as shown in

[0095] In Figure 1 In this example, the clusters 7 are provided on the conductive layer 6. The conductive layer 6 extracts photo-generated electrons from the light absorbing layer 3. The clusters 7 are bonded to the conductive layer 6. The clusters 7 can be in physical contact with each other but are not bonded to each other. In this example, the clusters are provided on the first conductive layer 6 such that they form a single layer of clusters 7 on the conductive layer 6. The clusters 7 have an upper surface facing the light and a lower surface in direct mechanical and electrical contact with the conductive layer 6. In the single layer of clusters, each cluster is in direct physical and electrical contact with another layer provided below the light absorbing layer 3, for example the first conductive layer 6.

[0096] Figure 2 Another example working electrode 1 b is shown, the working electrode 1 b comprising a light absorbing layer 3, a conductive layer 6 and a reflective layer 9a provided between the light absorbing layer 3 and the conductive layer 6. The reflective layer 9a is provided on the opposite side of the light absorbing layer 3 to the upper surface 5. The light absorbing layer is provided on top of the reflective layer 9a and the reflective layer 9a is provided on top of the conductive layer 6. The reflective layer 9a is provided such that it reflects light that has passed from the light absorbing layer 3 back into the light absorbing layer 3. The reflective layer 9a comprises semiconductor particles 10 in electrical contact with the clusters 7 and the conductive layer 6. It is important that the reflective layer forms a good electrical contact with the light absorbing layer so that the light absorbing layer can transfer photo-excited charges to the reflective layer without significant electrical energy loss.

[0097] The semiconductor particles 10 are made of a reflective material, i.e. a material capable of reflecting light. The semiconductor 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 through the semiconductor particles 10. The semiconductor particles 10 are bonded to each other and to the conductive layer. The semiconductor particles are made of, for example, Ti02, ZnO or Nb205. Suitably, at least 80% of the semiconductor particles 10 have a size of 10 nm to 2 pm. For example, the semiconductor particles 10 are made of titanium dioxide (Ti02). The reflective layer acts as a mirror, scattering the incoming light back into the light absorbing layer, thereby increasing the effective absorption path length and in turn increasing the light absorption of the light absorbing layer. The light scattering effect of the reflective layer depends on the wavelength. The light scattering effect depends to a large extent on the size of the semiconductor particles 10 in the reflective layer. Thus, the light scattering can be adjusted and optimised to suit existing applications by selecting semiconductor particles with a sufficient particle size.

[0098] In this example, the clusters 7 are provided on the reflective layer 9a. At least some of the semiconductor particles 10 are in physical contact with at least some of the clusters 3. In this example, the clusters 7 are bonded to the semiconductor particles 10 of the reflective layer 9a. For example, the clusters are provided on the reflective layer 9a such that they form a single layer of clusters 7 on the reflective layer, as Figure 2As shown. Preferably, the reflective layer is porous to allow a conductive medium to pass through it. For example, the porosity of the reflective layer is 35%-80% or 40%-70%. The thickness of the reflective layer is 0.1 μm to 10 μm, and preferably 1 μm to 10 μm.

[0099] Alternatively, some clusters 7 can be placed within the holes of the reflective layer 9a. These clusters 7 are prepared as described above and are not formed by, for example, the formation of agglomerates from excess dye when the dye is implanted into the semiconductor structure.

[0100] In all possible implementations of the working electrode 1a, the main part of the light-absorbing layer 3 is a single layer of clusters 7 disposed on the surface of the conductive layer 6 or the reflective layer 9a.

[0101] Figure 3 It shows including, for example Figure 1 An example of a photovoltaic device 20 with a working electrode 1a is shown. The photovoltaic device includes a counter electrode comprising a second conductive layer 24 electrically insulated from a first conductive layer 6, and a conductive medium 9 for transferring charge between the counter electrode and the working electrode. The conductive medium 9 is disposed in the gaps 8 between the clusters 7.

[0102] The photovoltaic device 20 also includes an insulating substrate 26 disposed between the first conductive layer 6 and the second conductive layer 24. The first conductive layer 6 is disposed on one side of the insulating substrate 26, and the second conductive layer 6 is disposed on the opposite side of the insulating substrate 26. A light-absorbing layer 3 is disposed on the first conductive layer 6. The light-absorbing layer 3 is located on the top side of the photovoltaic device, facing the sun, to allow sunlight to irradiate the cluster 7 and generate photo-excited electrons. The first conductive layer 6 serves as a back contact layer for extracting photogenerated electrons from the light-absorbing layer 3. Preferably, the first conductive layer 6 is porous to accommodate a conductive medium. For example, the first conductive layer 6 includes a plurality of conductive particles 28 made of a conductive material, such as... Figure 4 As shown. The conductive particles 28 of the first conductive layer are bonded to each other and in electrical contact with each other. The first conductive layer 6 and the second conductive layer 24 are made of, for example, Ti, Ti alloys, Ni alloys, graphite, or amorphous carbon. Preferably, the first conductive layer 6 and the second conductive layer 24 are made of porous Ti.

[0103] Figure 4 It shows Figure 3 The diagram shows the amplified portion of the light-absorbing layer and the first conductive layer 6 of the photovoltaic device. The conductive particles 28 of the first conductive layer 6 form a network in the photovoltaic device for conducting charge and possessing sufficient mechanical stability. Clusters 7 of the light-absorbing layer are in physical and electrical contact with some of the conductive particles 28 of the first conductive layer 6. The clusters 7 may partially protrude into the first conductive layer 6. In this example, the clusters 7 are larger than the conductive particles 28. However, the clusters 7 and the conductive particles 28 may also have substantially equal dimensions.

[0104] The photovoltaic device 20 also comprises an electrically conductive medium for transferring electrical charge from the light absorbing layer 3 to the second conductive layer 24. In this example, the electrically conductive medium is a liquid electrolyte (not shown in the figure). However, the electrically conductive medium can be any suitable type of electrically conductive medium, such as a gel or a solid conductor. The liquid electrolyte is, for example, a redox electrolyte capable of transferring electrical charge (i.e. electrons or holes) to or from the clusters 7. The redox electrolyte is also capable of transferring electrical charge to or from the second conductive layer 24. Examples of electrolytes include I - / I3 - redox couples or ferrocene-containing compounds, but other electrolytes (e.g. copper-based electrolytes or cobalt-based electrolytes) can also be used. The electrolyte can be selected from the group consisting of, or from, the following:

[0105] iodine / iodide-based electrolytes, for example:

[0106] LiI / I2, NaI / I2, KI / I2, PMII / I2,

[0107] or cobalt-based electrolytes, for example:

[0108] tris(1,10-phenanthroline)cobalt di(hexafluorophosphate) / tris(1,10-phenanthroline)cobalt tri(hexafluorophosphate), or

[0109] bis(6-(1 H-pyrazol-1 -yl)-2,2'-bipyridine)cobalt di(hexafluorophosphate) / bis(6-(1 H-pyrazol-1 -yl)-2,2'-bipyridine)cobalt tri(hexafluorophosphate), or

[0110] tris-(2,2'-bipyridine)cobalt(II) di(tetracyanoborate) / tris-(2,2'-bipyridine)cobalt(III) tri(tetracyanoborate),

[0111] or copper-based electrolytes, for example

[0112] bis-(2,9-dimethyl-1,10-phenanthroline)copper(I) di(trifluoromethylsulfonyl)imide / bis-(2,9-dimethyl-1,10-phenanthroline)copper(II) di(trifluoromethylsulfonyl)imide chloride, or

[0113] bis-(4,4',6,6'-tetramethyl-2,2'-bipyridine)copper(I) di(trifluoromethylsulfonyl)imide / bis-(4,4',6,6'-tetramethyl-2,2'-bipyridine)copper(II) di[di(trifluoromethylsulfonyl)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] A hole transport material (HTM) can also be used as the conductive medium.

[0116] The porosity of the insulating substrate 26 will enable ion transport through the insulating substrate. The porosity of the first conductive layer 6 will enable ion transport through the first conductive layer. For example, the substrates 26 and applied layers 3, 6, 24 are immersed in a liquid electrolyte and encapsulated. The liquid electrolyte fills the pores of the first porous conductive layer 6, the pores of the porous insulating substrate 26, and the interstices between the clusters 7 in the light absorbing layer 3. The first conductive layer 6 and the second conductive layer 24 are physically separated and electrically separated by the insulating substrate 26, so the conductive layer 6, conductive layer 24 do not directly physically or electrically contact. However, the first conductive layer 6 and the second conductive layer 24 are electrically connected by the electrolyte that penetrates the porous insulating substrate.

[0117] The photovoltaic device 20 also includes a housing or other means for encapsulating the photovoltaic device to protect the device and prevent electrolyte leakage. For example, the photovoltaic device 20 includes a first sheet 30 that covers the top surface of the photovoltaic device and a second sheet 32 that covers the bottom surface of the photovoltaic device and serves as a liquid barrier for the electrolyte. The first sheet 30 on the top side of the photovoltaic device needs to be transparent to allow light to pass through. The sheets 30, 32 are made of, for example, a polymeric material. Additional layers can be added between the counter electrode 24 and the bottom covering sheet 32 to further support the mechanical stability of the photovoltaic device. The photovoltaic device 20 includes at least one connection element 46 electrically connected to the first conductive layer 6 to connect the first conductive layer to an external circuit L and at least one connection element 47 electrically connected to the second conductive layer 24 to connect the second conductive layer to the external circuit L. For example, the connection elements 46, 47 are busbars. The first conductive layer 6 and the second conductive layer 24 are connected to each other through the external circuit L. Thus, a circuit is formed in which one type of charge carrier (i.e., either electrons or holes) is transported from the first conductive layer 6 to the second conductive layer 24 through the external circuit and the other type of charge carrier (i.e., either electrons or holes) is transported from the first conductive layer 6 to the second conductive layer 24 through the charge conductive medium.

[0118] Figure 5Another example of a photovoltaic device 40 is shown that includes a working electrode 1a. The photovoltaic device 40 includes a porous insulating substrate 26 and a counter electrode that includes a second conductive layer 24. In this example, the conductive medium is a solid charge conductor 42. The light absorbing layer 3 includes 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(styrenesulfonate) known as PEDOT:PSS. The clusters 7 are substantially uniformly distributed in the light absorbing layer 3, with the solid charge conductor 42 located on the clusters 7 and in interstitial spaces between the clusters. The photovoltaic device 40 also includes a plurality of charge conductive pathways 44 of charge conductive material disposed between the light absorbing layer 3 and the second conductive layer 24 to enable movement of charges (i.e., holes or electrons) between the light absorbing layer 3 and the second conductive layer 24. The conductive pathways 6 pass 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 pass through the first conductive layer 6.

[0119] Figure 6 An example of a photovoltaic device 50 is shown that includes a working electrode 1b. Figure 2 An example of a photovoltaic device 50 is shown that includes a working electrode 1b.

[0120] The light absorbing layer can be fabricated in many different ways. For example, the clusters can be pre-fabricated and a solution containing the clusters is deposited on a conductive layer of the photovoltaic device. For example, the clusters can be pre-fabricated dye crystals. Alternatively, a solution containing dye molecules is deposited on a conductive layer of the photovoltaic device and the clusters are formed during drying of the solution-coated conductive layer. The dye molecules bond with each other and form clusters during the drying process. If the solution-coated conductive layer is heated during the drying process, the dye molecules can bond with each other to form clusters of dye crystals on the surface of the conductive layer.

[0121] In one aspect, the method comprises preparing a solution comprising dye molecules and / or clusters of dye molecules distributed in a solvent, distributing the solution on an electrically conductive layer, and then drying the electrically conductive layer provided with the solution until the solvent evaporates. For example, the coating can be achieved by spraying. Alternatively, the coating can be achieved by electro-spraying. The method can comprise heating the electrically conductive layer provided with the solution to achieve crystallization of the clusters of dye molecules. This method of manufacturing the light absorbing layer is simple, fast and provides a uniform distribution of clusters on the surface of the electrically conductive layer. The solution can contain dye molecules dissolved in the solvent. For example, a dye powder is dissolved in a solvent to form a solution comprising dye molecules. In this case, the dye molecules will bond to each other and form clusters during the drying process. Alternatively, clusters of the desired size can be manufactured beforehand. The clusters are then added to the solvent to form the solution. During the coating process, the clusters are distributed on the surface of the electrically conductive layer. Alternatively, the solution comprises clusters of dye molecules as well as dye molecules dissolved in the solvent. This can be advantageous because the dye molecules can act as glue between the clusters and between the clusters and the electrically conductive layer, such that the clusters will be connected to each other and to the electrically conductive layer.

[0122] Example 1

[0123] In this embodiment, the clusters are formed directly on top of the electrically conductive layer 6.

[0124] In a first step, a dye solution is prepared by dissolving a solid dye, for example in the form of a dye powder, in a suitable solvent that dissolves the solid dye. Thus, a solution of dye molecules dissolved in a solvent is formed. In one example, the dye is a arylamine dye, for example (E)-3-(5-(4-(di(2',4'-dibutoxy-[1,1'-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (also abbreviated as D35). The solvent can be any organic solvent that is able to dissolve the dye, for example dichloromethane, acetonitrile, NMP, DMF, THFA, butyrolactone or DMSO, methanol.

[0125] In a second step, the upper surface of the electrically conductive layer comprising porous Ti is coated with the solution. For example, the coating of the upper surface of the electrically conductive layer is performed by spraying the solution on the electrically conductive layer.

[0126] In a third step, the electrically conductive layer provided with the solution is dried until the solvent evaporates and a plurality of clusters of dye molecules is formed on the electrically conductive layer. In this embodiment, the clusters are bonded to the electrically conductive layer during the formation of the clusters on the electrically conductive layer, i.e. during the evaporation of the solvent.

[0127] In this embodiment, the solution comprises dye molecules dissolved in a solvent and the clusters are obtained after applying the solution to the surface of the electrically conductive layer.

[0128] Example 2

[0129] First step, a dye solution is prepared by dissolving a solid dye (e.g. in the form of a dye powder) in a suitable solvent that dissolves the solid dye. The dye and solvent can be the same as in example 1.

[0130] Second step, the dye molecules in the solution are precipitated into crystals consisting of clusters of dye molecules. Crystallization can be achieved in several ways. For example, the solvent can be removed to a level at which the dye starts to precipitate because the solubility of the dye is too low. Alternatively, the dye can be precipitated by adding a precipitant such as a salt.

[0131] Third step, the solution containing the clusters of crystals is deposited onto the conductive layer 6. It is advantageous to add dye molecules to the solution containing the clusters of crystals before the solution is deposited onto the conductive layer 6.

[0132] Fourth step, the conductive layer 6 provided with the solution is dried until the solvent evaporates and the clusters of crystals are distributed over the surface of the conductive layer. After evaporation of the solvent, the added dye molecules will act as glue between the clusters and the conductive layer, thereby attaching the clusters to the conductive layer.

[0133] In this embodiment, the solution comprises clusters distributed in a solvent.

[0134] Example 3

[0135] The clusters of crystals can also be formed directly on top of the conductive layer 6.

[0136] First step, a dye solution is prepared by dissolving a solid dye (e.g. in the form of a dye powder) in a suitable solvent that dissolves the solid dye. The dye and solvent can be the same as in example 1.

[0137] Second step, the upper surface of the conductive layer is coated with the solution. For example, the coating of the upper surface of the conductive layer is performed by spraying the solution onto the conductive layer.

[0138] Third step, the conductive layer provided with the solution is heated (annealed) for a certain time, for example 3 hours at 70°C, to evaporate the solvent, 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 an inert atmosphere (e.g. argon or vacuum). The solvent evaporates during the heating.

[0139] The spraying and heating process can be repeated multiple times to form a layer of clusters in which the thickness of the clusters is sufficient to efficiently absorb light. The concentration of the dye solution or the temperature during the drying process can be varied to obtain different qualities of the layer of clusters. For example, fast drying results in smaller clusters, so a high drying temperature results in fast evaporation of the solvent and thus small clusters. By slowly evaporating the solvent, larger clusters can be grown on the conductive layer.

[0140] Example 4

[0141] First, clusters are produced by precipitating the dye from the dye solution in a crystalline structure by adding cations to the dye solution. The cations make the dye insoluble in the solvent, so the dye precipitates in the solution in a crystalline structure. The crystalline clusters are then separated from the solution by sedimentation and decanting. The crystalline clusters can also be more efficiently separated from the solution by centrifugation followed by decanting. Alternatively, the crystalline clusters can be separated from the solution by filtering through a filter, preferably by applying a vacuum and sucking the liquid crystal mixture through the filter from the solution. Alternatively, the crystalline clusters can be separated from the solution by filtering and applying an overpressure to the liquid crystal mixture, thereby squeezing the liquid through the filter, leaving the crystals on the filter.

[0142] For example, the crystalline clusters can be deposited on the conductive layer by spraying, vacuum suction or electro-spraying.

[0143] Example 5

[0144] This example describes a method of manufacturing a working electrode with a reflective layer.

[0145] First step, a first solution comprising TiO2and a solvent is manufactured.

[0146] Second step, the upper surface of a conductive layer made of porous Ti is coated with the first solution. The coating of the upper surface of the conductive layer is performed, for example, by spraying or printing the first solution on the conductive layer.

[0147] In the second step, the conductive layer containing the first solution is dried at a temperature of 50-80°C until the solvent evaporates and a layer of TiO2particles is formed on the first conductive layer. Furthermore, the conductive layer containing the TiO2particles is sintered at, for example, about 500°C for 15 minutes to bond the TiO2particles to the conductive layer and to achieve electrical contact between the TiO2particles and the conductive layer.

[0148] Third step, a second solution is prepared by dissolving a solid dye (for example in the form of a dye powder) in a suitable solvent capable of dissolving the solid dye. The dye and the solvent can be the same as in example 1.

[0149] Fourth step, the layer of TiO2particles is coated with the dye solution. The coating is performed, for example, by spraying the dye solution on the layer of TiO2particles.

[0150] Fifth step, the conductive layer containing the TiO2particles and the dye solution are dried between 50-80°C until the solvent evaporates, forming a plurality of clusters of dye molecules on the layer of TiO2particles. Furthermore, the conductive layer containing the TiO2particles and the dye solution can be heated (annealed) for a certain time to increase the crystallinity of the precipitated clusters of dye molecules.

[0151] In another embodiment, a seed of another material can be used during the fabrication of the cluster followed by a dye to initiate the crystallization process. The crystal grows on the seed to form the crystal cluster. One advantage of using a seed during the fabrication process is that the clusters can be spherical and substantially the same size. This facilitates the fabrication of the light absorbing layer and a more uniform layer can be obtained.

[0152] The present application is not limited to the disclosed embodiments, but can vary and modify within the scope of the appended claims. For example, the light absorbing layer can include a small amount of a second light absorbing photovoltaic material.

Claims

1. A working electrode (1a; 1b; 1c) for a photovoltaic device, comprising a light-absorbing layer (3) and a conductive layer (6) electrically in contact with the light-absorbing layer (3), wherein the light-absorbing layer (3) comprises a light-absorbing photovoltaic material composed of a plurality of dye molecules, characterized in that, The light-absorbing layer (3) is composed of a layer of multiple clusters (7), wherein each cluster (7) is formed by dye molecules and each dye molecule in the cluster (7) is bonded to its adjacent dye molecules, wherein the light-absorbing layer (3) is a cluster monolayer and wherein a void (8) is formed between the clusters (7), and the working electrode (1a; 1b; 1c) comprises a conductive medium (9; 42) filling the void (8) between the clusters (7); The light-absorbing layer (3) does not contain dye molecules adsorbed on the surface of semiconductor particles.

2. The working electrode according to claim 1, wherein at least 80% of the clusters (7) forming the light-absorbing layer (3) contain more than 100 dye molecules.

3. The working electrode according to claim 1, wherein at least 80% of the clusters (7) forming the light-absorbing layer (3) contain more than 1,000 dye molecules.

4. The working electrode according to claim 1, wherein at least 80% of the clusters (7) forming the light-absorbing layer (3) contain more than 10,000 dye molecules.

5. The working electrode according to claim 1, wherein the thickness of the light absorption layer (3) is 20 nm to 2 μm.

6. The working electrode according to any one of claims 1 to 5, wherein at least 40% of the clusters (7) forming the light-absorbing layer are crystal clusters, wherein the dye molecules within the clusters (7) are arranged in a defined and repeatable manner.

7. The working electrode according to any one of claims 1 to 5, wherein at least 50% of the clusters (7) forming the light-absorbing layer are crystal clusters, wherein the dye molecules within the clusters (7) are arranged in a defined and repeatable manner.

8. The working electrode according to any one of claims 1 to 5, wherein at least 70% of the clusters (7) forming the light-absorbing layer are crystal clusters, wherein the dye molecules within the clusters (7) are arranged in a defined and repeatable manner.

9. The working electrode according to any one of claims 1 to 5, wherein the dye molecule is an organic dye molecule, an organometallic dye molecule, or a natural dye molecule.

10. The working electrode according to any one of claims 1 to 5, wherein the clusters (7) are substantially uniformly distributed in the light-absorbing layer (3).

11. The working electrode according to any one of claims 1 to 5, wherein the cluster (7) forming the light absorption layer (3) is in physical and electrical contact with the conductive layer (6), and the cluster (7) is bonded to the conductive layer (6).

12. The working electrode according to any one of claims 1 to 5, wherein the working electrode (1b) comprises a reflective layer (9a) disposed between the light-absorbing layer (3) and the conductive layer (6), the reflective layer (9a) comprising semiconductor particles (10) in electrical contact with the cluster (7) forming the light-absorbing layer (3) and the conductive layer (6).

13. The working electrode according to claim 12, wherein at least 80% of the semiconductor particles (10) in the reflective layer (9a) have a size greater than 0.1 μm.

14. The working electrode according to claim 12, wherein at least 80% of the semiconductor particles (10) in the reflective layer (9a) have a size greater than 0.2 μm.

15. The working electrode according to claim 12, wherein the thickness of the reflective layer (9a) is from 0.1 μm to 10 μm.

16. The working electrode according to claim 13, wherein the thickness of the reflective layer (9a) is from 0.1 μm to 10 μm.

17. The working electrode according to claim 14, wherein the thickness of the reflective layer (9a) is from 0.1 μm to 10 μm.

18. The working electrode according to claim 12, wherein the reflective layer (9a) is porous and the porosity of the reflective layer (9a) is 40% to 70%.

19. A photovoltaic device (20; 40; 50), comprising: The working electrode (1a) according to any one of claims 1 to 18; 1b)， Counter electrode (24), and A conductive medium for transferring charge between the counter electrode (24) and the working electrode (1a; 1b).

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