GB2501119A

GB2501119A - Dye-sensitized solar cell counter electrode

Info

Publication number: GB2501119A
Application number: GB1206546.2A
Authority: GB
Inventors: Mazhar Bari; David Jeng; Marin Gheorghe; Iftikhar Ahmed; Yurii Gounko
Original assignee: College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin; Solarprint Ltd
Current assignee: College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin; Solarprint Ltd
Priority date: 2012-04-13
Filing date: 2012-04-13
Publication date: 2013-10-16
Also published as: GB201206546D0

Abstract

The invention relates to an electrode suitable for a dye sensitized solar cell DSSC having a layer comprising carbon nanotubes (CNT), graphene and poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate) (PEDOT:PSS) and its uses. A process for preparing an electrode and a dye-sensitized solar cell are also described. The materials are formed within a dispersion and the dispersion is applied to a substrate. This counter electrode negates the need to fabricate the DSSC within a vacuum chamber. The electrode may also comprise of platinum. Typically the graphene is a powder wherein the graphene particles are graphene nanoplatelets having a mean particle size of 20nm or less.

Description

DYE-SENSITIZED SOLAR CELL COUNTER ELECTRODE

Field of the invention

The invention relates to an electrode for a dye-sensitized solar cell and to a process for preparing the electrode. The invention further relates to the use of the electrode in a dye-sensitized solar cell (DSSC) and to a dye-sensitized solar cell comprising the electrode.

Background to the invention

Dye-sensitized solar cells (DSSCs) were invented in 1991 by M. Grätzel. DSSCs generally comprise an electrolyte that is sandwiched between a working electrode (WE) and a counter electrode (CE). The counter electrode (CE) is commonly made up of a thin layer of a platinum catalyst that has been deposited onto a transparent and electrically conductive substrate.

Platinum is an expensive material. In addition to its material cost, the manufacture of large DSSC5 containing platinum often requires the use of expensive equipment, such as a large-sized sputtering facility. There is a need for a low cost alternative to platinum for use as a catalyst in the counter electrode (CE) of a DSSC.

Several phases of carbon have been reported as showing similar catalytic behaviour to platinum. However, DSSCs formed from such carbon-based CEs tend to suffer from one or more drawbacks. The CE usually has poor mechanical properties that cause reliability problems or have to be made by an expensive process, which often cannot be used on an industrial scale. Some DSSCs containing carbon-based CEs also show low cell efficiency compared with DSSCs containing platinum-based CEs.

US 2008/0087322 describes a counter electrode (CE) that has a layer formed of a porous carbon material that is either carbon black, activated carbon or graphite. Carbon materials are typically difficult to adhere onto a substrate. During preparation of the CE, an adhesive layer is printed onto the substrate of the CE before application of the porous carbon material in powder form. A heat treatment of up to 500 °C is employed to dissociate a polymer resin in the adhesive layer, which places the substrate under thermal stress and may cause it to crack or warp. Due to the presence of the adhesive layer, the manufacturing process also employs additional printing and pressing steps, which increase production costs. There is also a further problem associated with the use of a carbon material in powder form in that any non-adhered powder is difficult to remove from the surface of the substrate. This may cause problems with sealing the DSSC and electrolyte leakage or water ingress may occur.

US 2010/0313938 describes a counter electrode (CE) having a porous carbon layer comprising carbon nanotubes (CNTs). The carbon nanotubes (CNTs) are prepared by a chemical-vapour deposition (CVD) method. The CNTs are either grown directly by CVD onto a surface of the substrate of the CE or by coating the substrate with a solution of the CNTs and then evaporating the solvent. The mechanical properties of the CE are not discussed, but itis likely that the CNTs are only weakly adhered to the surface of the substrate. Furthermore, the CVD method for depositing the CNTs onto the substrate requires the use of a vacuum chamber and is unlikely to provide low cost CEs that can be manufactured on an industrial scale.

Summary of the invention

It has been found that an electrode having a layer comprising carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) has a number of advantageous properties. The electrode of the invention has robust mechanical properties and excellent electrical characteristics, and can be manufactured easily, especially on an industrial scale.

The invention therefore provides an electrode suitable for a dye sensitized solar cell having a layer comprising carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate).

The invention also provides a dye-sensitized solar cell comprising: (a) a counter electrode; (b) a working electrode comprising a semiconductor and a sensitized dye; and (c) an electrolyte; wherein the counter electrode is an electrode of the invention.

A further aspect of the invention relates to the use of an electrode of the invention as a counter electrode in a dye-sensitized solar cell.

The invention further provides a process for preparing an electrode suitable for a dye-sensitized solar cell, which process comprises the steps of: (i) forming a dispersion comprising carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate); and (ii) applying the dispersion onto a substrate.

Brief description of drawings

Figure 1 is an exploded view showing the structure of a DSSC of the invention.

Figure 2 is a graph showing the % efficiency (q) of some test cells.

Detailed description of the invention

The adhesion of carbon nanotubes, graphene or mixtures thereof to conventional substrates for electrochemical cells, such as DSSCs, is generally very poor. It has unexpectedly been found that the combination of carbon nanotubes (CNTs), graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) show good adhesion to such substrates. This was a surprising finding because the inventors have found that mixtures consisting only of CNTs and PEDOT-PSS generally show poor adhesion to the same substrates. It is believed that poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) acts a binder when specifically used in combination with carbon nanotubes and graphene.

The electrode of the invention has a layer comprising CNTs, graphene and PEDOT-PSS. The electrode of the invention could in principle be used in any type of electrochemical cell, such as a fuel cell. However, the electrode of the invention is particularly suitable for use in a DSSC. The layer comprising CNTs, graphene and PEDOT-PSS is referred to herein as the "carbon layer" unless otherwise stated.

Typically, the layer comprises a mixture of CNTs, graphene and PEDOT-PSS.

The CNTs, graphene and PEDOT-PSS are generally randomly distributed throughout the layer.

Typically, the layer is supported on a substrate. The layer may partially or completely cover a surface of the substrate. When the layer partially covers a surface of the substrate, then the surface of the substrate or an underlying film or other layer may be partially exposed between the layer comprising CNTs, graphene and PEDOT-PSS. For example, the layer may have a patterned arrangement (see, for example, Figure 1).

Generally, the substrate is an optically transparent material. Preferably, the substrate has a transmittance of 90% or more in the range of visible light (light ot wavelength 400 to 800 nm).

Examples of suitable optically transparent materials include glass, polyethylene terephthalate, polyethylenenaphthalate, syndiotactic polystyrene, polyphenylene sulphide, polyarylate, polycarbonate, polypropylene, polyimide, polyetherimide, triacetylcellulose, polysulfone, polyestersulfone and polyethersulfone. When the electrode is a counter electrode of a DSSC, it may be necessary to select a substrate material that is chemically compatible or tolerant to the electrolyte solution.

Preferably, the substrate is glass, polyethylene terephthalate or polyethylenenaphthalate. More preferably, the substrate is glass.

In one embodiment, the substrate is stainless steel.

Typically, the substrate is coated with a transparent conductive film. Generally, the transparent conductive film is present on one surface of the substrate. The transparent conductive film may partially or completely cover at least one surface of the substrate.

When at least one surface of the substrate is partially covered by the transparent conductive film, then the surface of the substrate may be partially exposed between the transparent conductive film, such as when the substrate is coated with a transparent conductive film in a patterned arrangement. The transparent conductive film provides an electrically conductive surface without significantly impairing transparency.

The substrate may be coated with a laminate of different transparent conductive films, such as a laminate of two, three or four transparent conductive films. It is, however, preferable that the substrate is coated with a single transparent conductive film.

The transparent conductive film typically comprises a transparent conducting oxide (TCO), preferably a transparent conducting metal oxide. Examples of suitable TCOs include a tin-doped indium oxide (ITO), a fluorine-doped tin oxide (FTO), a tin oxide (e.g. SnO2), ZnO-Ga2O3, ZnO-A12O3 and SnO2-Sb2O3. The TCO is preferably an ITO or a ETO.

Films formed of an ITO or a FTO are easy to manufacture at low cost. More preferably, the TCO is an FTO.

It is preferred that the carbon layer is supported on a surface of a substrate coated with a transparent conductive film. Generally, the carbon layer is in electrical contact with the transparent conductive film. For example, the carbon layer may be directly supported on the transparent conductive film, such that the film is either partially or completely covered by the layer.

Typically, the carbon layer shows an adhesion of from 3 to 5. The adhesion is measured in accordance with ASTM D3359-97 using test method A or B at 25 00, typically with an adhesive tape of 50 mm width having an adhesive strength of 6 N per 25 mm width (as measured in accordance with BSI ISO 2409). Preferably, the carbon layer shows an adhesion of 4 or 5, more preferably 5.

Typically, the electrode of the invention is obtainable by forming a dispersion comprising carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), and applying the dispersion onto a substrate. Further details of steps for preparing the electrode are described below.

The electrode may have at least one additional layer supported on the substrate.

Each additional layer may be an intervening layer that is arranged between the surface of the substrate and the carbon layer. Each additional layer may then be completely or partially covered by the carbon layer. When the carbon layer has a patterned arrangement, an additional layer may be present between spaces in the patterned arrangement.

Each additional layer may have a patterned arrangement. When the carbon layer has a patterned arrangement, then an additional layer may have a complimentary patterned arrangement. The complimentary patterned arrangement may allow the carbon layer and the additional layer to fit together on the surface of the substrate (see, for

example, Figure 1).

The electrode may complise a plurality of caibon layers.

Typically, the electrode further comprises a layer of an electrically conducting material. The electrically conducting material may be metallic. Suitable metallic, electrically conducting mateiials include stainless steel, silver, copper and gold.

Preferably, the metallic, electrically conducting material is silver.

When the electrode comprises a layer of an electrically conducting material and the carbon layer has a patterned arrangement, then preferably the layer of the electrically conducting material has a complimentary patterned arrangement.

In an embodiment of the invention, the electrode may further comprise an adhesive. Typically, the adhesive is located between the carbon layer and the substrate.

If an adhesive is included, then it is to piovide supplementaiy adhesion to secure the layer to the substrate. The adhesive may be an additional layer oi part of an additional layei, such as pad of an additional layer with one or more other materials. Examples of suitable adhesives include a phenol resin, an epoxy resin, an ethylene-vinyl acetate copolymer and combinations thereof.

In another embodiment of the invention, the electrode does not include an adhesive. In particular, the adhesive is not present as an additional layer or part of an additional layer, such as part of an additional layer with one or more other materials.

Preferably, the electrode does not include an adhesive selected from a phenol resin, an epoxy resin, an ethylene-vinyl acetate copolymei and combinations thereof.

Typically, the electrode is a laminate. The substrate generally forms the base layer of the laminate and the carbon layer is supported on a surface of the substrate.

The shape of the electiode is geneially not important. It is usually selected to match the shape of a counterpart electrode in the electrochemical cell (e.g. the working electrode in a DSSC) and for manufacturing convenience. The shape of the electrode is usually determined by the shape of the substiate.

Generally, the substrate is plate-shaped. The substrate typically has a square or substantially rectangular cross-section.

Typically, the substrate has a thickness of 0.5 to 5 mm. Preferably, the substrate has a thickness of 1 to 4 mm, more preferably 2 to 3 mm. Many commercially available substrates have a veiy flat surface. Thus, the reference to a "thickness" in the context of the substrate preferably refers to the mean thickness of the substrate with a maximum deviation in thickness of ± 50 rJrn, particularly ± 10 pm, from the mean thickness.

In general, the electrode is plate-shaped. The electrode typically has a square or substantially rectangular cross-section.

Typically, the carbon layer has a thickness of ito 100 pm. Preferably, the electrode has a thickness of 2 to 50 pm. It is difficult to form a uniform layer having a thickness lower than 2 pm. In contrast, it is relatively easy to form layers having a thickness greater than 50 pm, but the performance of such layers is generally no better than thinner layers and their formation is therefore wasteful of material. The thickness of the carbon layer also affects the overall size of an electrochemical cell comprising the electrode. When the electrochemical cell is a DSSC, then the quantity of sealant that must be used to seal the DSSC increases with the thickness of the layer, which affects manufacturing costs. More preferably, the electrode has a thickness of 5 to 25 pm, particularly a thickness of 7.5 to 15 pm, and even more preferably a thickness of 10 to 12 pm. The reference to a "thickness" in the context of the layer comprising CNTs, graphene and PEDOT-PSS preferably reters to the mean thickness of the layer with a maximum deviation in thickness of ± 20%, particularly ± 10%, from the mean thickness.

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) is a mixture of the two ionomers polystyrene sulfonate and poly(3,4-ethylenedioxythiophene). The sulfonyl groups of polystyrene sulfonate carry a negative charge. The other ionomer, poly(3,4-ethylenedioxythiophene), is a conjugated polymer that carries a positive charge. When the ionomers are combined they form a macromolecular salt. The acronym PEDOT-PSS is used herein to represent poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate).

Carbon nanotubes (CNTs) are an allotrope of carbon, which have a cylindrical nanostructure. CNTs permit one dimensional electron transfer along their axes. CNTs are commercially available or may be prepared using standard methods known in the art, which involve the use of arc discharge, laser ablation, high-pressure carbon monoxide or chemical vapour deposition.

Typically, the carbon nanotubes are single walled carbon nanotubes, (SWCNT5), double walled carbon nanotubes (DWCNTs), multi-walled carbon nanotubes (MWCNTs), graphenated carbon nanotubes (gCNTs), carbon nanohorns or mixtures thereof It is preferable that the carbon nanotubes are SWCNTs, DWCNTs, MWCNTs or mixtures thereof. More preferably, the carbon nanotubes are MWCNTs. The inventors have found that an electrode having good electrical properties can be obtained when the CNTs are MWCNTs.

Typically, the CNTs have diameters in the range of 0.5 to 100 nm (as measured by high-resolution transmission electron microscopy (HRTEM)). Preferably, the CNT5 have diameters in the range of 1 to 50 nm. In general, SWCNTs have diameters in the range of ito 10 nm, particularly in the range of 2 to 7.5 nm. MWCNT5 typically have diameters in the range of 5 to 100 nm, particularly in the range of 10 to 50 nm.

Generally, the CNTs have lengths in the range of 10 nm to 100 pm (as measured by transmission electron microscopy). Preferably, the CNTs have lengths in the range of 50 nm to 50 pm. More preferably, the CNTs have lengths in the range of 100 nm to 1 pm.

Typically, the CNTs have a purity of at least 60 wt. %, preferably at least 80 wt. %, more preferably at least 90 wt. %, still more preferably 95 wt. %, and most preferably at least 99 wt. %. The purity of the CNTs may be determined by transmission electron microscopy or by thermal gravimetric analysis using standard techniques. The purity of commercially available carbon nanotubes is generally specified with the product.

Methods known in the art for preparing an electrode that comprises CNTs often aim to deposit a layer of CNT5 where their axes are aligned in a particular direction, such as perpendicular or parallel to the surface of the substrate. It is believed that the alignment of the CNTs in a particular direction may affect the electrical characteristics of the material.

In one embodiment of the invention, the axes of the ONTs have an alignment that is perpendicular or parallel to the plane of the surface of the substrate on which the carbon layer is supported.

In another embodiment of the invention, the axes of the CNTs have a random orientation.

Graphene is an allotrope of carbon that consists of a one-atom thick planar sheet of sp2-bonded carbon atoms. Graphene is a different form of carbon to graphite, where the planar sheets are stacked on top of one another at a regular interplanar spacing. The structure of graphene permits two dimensional transfer over its planar sheet. Graphene is commercially available or may be prepared using standard methods known in the art.

Typically, the graphene is a powder, crystalline or flake form of graphene.

Preferably, the graphene is a powder or flake form of graphene.

The graphene is typically present as graphene nanoplatelets. The graphene nanoplatelets generally have a thickness (or mean particle size) of 20 nm or less (as measured by HRTEM). Preferably, the graphene nanoplatelets have a thickness (or mean particle size) of 0.5 to 10 nm. More preferably, the graphene nanoplatelets have a thickness (or mean particle size) of ito 8 nm, especially 2 to 5 nm.

Typically, the graphene has a purity of at least 60 wt. %, preferably at least 80 wt.

more preferably at least 90 wt. %, still more preferably 95 wt. %, and most preferably at least 99 wt. %. The purity of graphene may be determined by transmission electron microscopy or by thermal gravimetric analysis using standard techniques. The purity of commercially available graphene is generally measured and provided with the product.

Generally, the FEDOT-PSS and the CNTs are piesent in a ratio by weight of 0.1 to 20: 1. Preferably, the PEDOT-PSS and CNTs are present in a ratio by weight of 0.25 to 10:1, more preferably 0.5 to 7.5 1, still more preferably 0.75 to 6.5:1, and even more preferably ito 5:1.

It is particularly preferable for the % weight of CNTs to be less than or equal to the % weight of PEDOT-PSS (in terms of the % weight of the carbon layer). It has been found that cells having excellent efficiency are obtained when the carbon layer of the electrode has more PEDOT-PSS than CNTs in the carbon layer have excellent efficiencies.

Typically, the PEDOT-PSS and the CNT5 are present in a ratio by weight of ito 20: 1. Preferably, the PEDCT-PSS and CNTs are present in a ratio by weight of 1.1 to 10 :1, more preferably 1.2 to 7.5: 1, still more preferably 1.3to 5:1! and even more preferably 1.5 to 3.5:1.

The ratios preferably relate to the amount of PEDOT-PSS and the CNTs in the carbon layer.

Generally, the graphene and the PEDOT-PSS are present in a ratio by weight of 0.1 to 20: 1. Preferably, the graphene and the PEDOT-PSS are present in a ratio by weight of 0.25 to 10:1, more preferably 0.5 to 7.5: 1, still more preferably 0.75 to 6.5:1, and even more preferably 1 to 5:1.

It is particularly preferable for the % weight of PEDOT-PSS to be less than or equal to the % weight of graphene (in terms of the % weight of the carbon layer). Cells comprising an electrode of the invention have been found to show excellent efficiency when there is more graphene than PEDOT-PSS.

Typically, the graphene and the PEDOT-PSS are present in a ratio by weight of 1 to 20:1. Preferably, the PEDOT-PSS and CNTs are present in a ratio by weight of 1.1 to 10:1, more preferably 1.2 to 7.5: 1, still more preferably 1.25 to 5:1, and even more preferably 1.3 to 3.5:1.

The ratios preferably relate to the amount of graphene and PEDOT-PSS in the carbon layer.

Each of the above ratios may be expressed in terms of a weight fraction of the total weight of PEDOT-PSS, CNTs and graphene, preferably in the carbon layer.

Typically, the ratio by weight fraction of PEDOT-PSS to CNTs to graphene (WPEDOT.PSS: WCNTS: Wgraphene) is 1: 0.1 to 1.5: 0.5 to 5. Preferably, the ratio of WpEDoTpss WONTS Wgraphone is 1: 0.25 to 1: ito 3.5, more preferably 1: 0.5 to 0.75: 1.1 to 2.5.

Generally, at least 50 wt. % of the carbon layer is comprised of CNT5, graphene and PEDOT-PSS. Preferably, at least 60 wt. % of the carbon layer is comprised of ONTs, graphene and PEDOT-PSS, more preferably at least 75 wt. %, even more preferably at least 90 wt. %, and even more preferably at least 95 wt. %.

In one embodiment, the carbon layer comprises no more than 5 wt. % of platinum.

Preferably, the carbon layer comprises no more than 3 wt. % of platinum. More preferably, the carbon layer comprises no more than 1 wt. % of platinum. Still more preferably, the carbon layer comprises no more than 0.1 wt. % of platinum. Even more preferably, the carbon layer is substantially free from platinum.

In another embodiment, the electrode further comprises platinum. It has been found that the combination of CNTs, graphene and PEDOT-PSS with platinum can provide an electrode having excellent electrode characteristics. Preferably, the carbon layer further comprises platinum. The combination of CNTs, graphene and PEDOT-PSS can act as a replacement for some of the platinum present in conventional counter electrodes thereby reducing cost. The platinum is typically present in an amount of 0.01 to 5 wt. %. Preferably, the platinum is present in an amount of 0.1 to 2 wt. %. More preferably, the platinum is present in an amount of 0.5 to 1.5 wt. %. The amount of platinum is preferably in terms of the wt. % of the carbon layer.

The carbon layer may consist of CNTs, graphene and PEDOT-PSS.

The invention also relates to the use of an electrode of the invention in an electrochemical cell, particularly as a counter electrode in a dye-sensitized solar cell. The invention also provides a dye-sensitized solar cell comprising: (a) a counter electrode; (b) a working electrode comprising a semiconductor and a sensitized dye; and (c) an electrolyte; wherein the counter electrode is an electrode of the invention. The DSSC5 of the invention possess excellent electrical characteristics, but also have good reliability that results from the mechanical properties of the counter electrode.

The working electrode (also referred to as the photoelectrode) may be any conventional working electrode of a DSSC that is known in the art.

Typically, the working electrode comprises a substrate having a transparent conductive film on a surface of the substrate.

Generally, the substrate is an optically transparent material. Preferably, the substrate has a transmittance of 90% or more in the range of visible light (light of wavelength 400 to 800 nm). Examples of suitable optically transparent materials include glass, polyethylene terephthalate, polyethylenenaphthalate, syndiotactic polystyrene, polyphenylene sulphide, polyarylate, polycarbonate, polypropylene, polyimide, polyetherimide, triacetylcellulose, polysulfone, polyestersulfone and polyethersulfone.

Preferably, the substrate is glass, polyethylene terephthalate or polyethylenenaphthalate.

More preferably, the substrate is glass.

The transparent conductive film typically comprises a transparent conducting oxide (TCO), preferably a transparent conducting metal oxide. Examples of suitable ICOs include a tin-doped indium oxide (ITO), a fluorine-doped tin oxide (FTO), a tin oxide (e.g. SnO2), ZnO-Ga2O3, ZnO-A1203 and Sn02-Sb2O3. The TCO is preferably an ITO or a FTO.

Films formed of an ITO or a FTO are easy to manufacture at low cost. More preferably, the TCO is an ETO.

The working electrode comprises a semiconductor, which is typically present in a layer supported on the substrate. Preferably, the layer has a thickness of 5 to 50 pm, more preferably 7.5 to 30 pm, and even more preferably 10 to 15 pm.

The semiconductor is generally a metal oxide. Preferably, the layer of the metal oxide is electrically conductive. More preferably, the layer is mesoporous.

Examples of suitable metal oxides include TiO2, SnO2, WO3, ZnO, Nb205, ln2O3, ZrO2, Ta2O5, La2O3, SrTiO3, Y203, Bi2O3, CeO2, A12O3 and mixtures thereof. Preferably the metal oxide is Ti02.

The working electrode comprises a sensitized dye. The sensitized dye is generally an organic dye, such as eosin, rhodamine, or merocyanine, or is a ruthenium complex or an iron complex, which each have at least one ligand with a bipyridine or a terpyridine structure. Preferably, the sensitized dye is a ruthenium complex. More preferably, the sensitized dye is [Ru(2,2'-bipyridyl-4,4-dicarboxylic acid)2(NCS)2].2(tetra-n-butylammonium).

Typically, the sensitized dye is on a surface of the semiconductor.

Electrolytes for DSSC5 are well known in the art.

Typically, the electrolyte is a solution or a gel.

Generally, the electrolyte may comprise iodine, iodide ions or tertiary butylpyridine in an organic solvent. The electrolyte may be an ionic liquid, such as an ionic liquid comprising an imidazolium cation, a pyridinium cation or an ammonium cation.

When the electrolyte is a solution, the electrolyte comprises an organic solvent.

Examples of suitable organic solvents include ethylene carbonate and methoxyacetonitrile.

When the electrolyte is a gel, the electrolyte comprises a gelling agent. Examples of suitable gelling agents include polyvinylidene fluoride and polyethylene oxide.

Preferably, the electrolyte comprises lVl.

Typically, the counter electrode faces the working electrode, and the electrolyte is present in the space between the counter electrode and the working electrode.

Generally, the DSSC further comprises a sealant. The sealant prevents both the electrolyte from leaking out of the DSSC and contaminants from entering the cell.

The sealant is generally a resin. Examples of suitable resins include an ionomer, ethylene-vinyl acetate-anhydride copolymer, an ethylene-methacrylic copolymer, an ethylene-vinyl alcohol copolymer, an ultraviolet cured resin and a vinyl alcohol polymer.

The invention also provides a process for preparing an electrode for a dye-sensitized solar cell, which process comprises the steps of: (i) forming a dispersion comprising carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate); and (ii) applying the dispersion onto a substrate. The electrode can be prepared using simple, conventional techniques, which can be used on an industrial scale. Furthermore, it is not necessary to heat the substrate at temperatures as high as 500 °C in the process of the invention to obtain an electrode having good mechanical properties.

The term "dispersion" as used herein refers to a system in which particles are dispersed in a continuous phase. The continuous phase is typically a liquid.

Generally, the dispersion is paste, foam, gel, emulsion, or a solution comprising a suspension of CNTs, graphene and PEDOT-PSS. Preferably, the dispersion is a solution comprising a suspension of CNTs, graphene and PEDOT-PSS.

Typically, the dispersion comprising CNTs, graphene and PEDOT-PSS is an aqueous dispersion. Thus, the continuous phase of the dispersion is water.

In one embodiment, the dispersion is formed by sonicating for at least 1 hour, preferably for at least 4 hours, an aqueous mixture comprising CNTs, graphene and PEDOT-PSS.

The dispersion may be applied onto the substrate using conventional techniques.

For example, the step of applying the dispersion onto a substrate may be by drop casting.

Drop casting is a method where the dispersion is dropped, poured or placed onto the substrate.

Typically, the step of applying the dispersion onto the substrate includes a step of drying the dispersion on the substrate. This drying step generally follows the initial application of the dispersion onto the substrate. The drying conditions used are selected to remove the liquid from the dispersion. For example, the step of drying the dispersion on the substrate may be performed at9O to 150 °C, preferably 100 to 130 °C, for at least minutes. The conditions may be selected to avoid placing the substrate under thermal stress.

Typically, the process includes a step of pressing the dispersion onto the substrate to form a layer. Preferably, the step of pressing the dispersion onto the substrate follows the step of applying the dispersion onto the substrate. More preferably, the step of pressing the dispersion onto the substrate follows the step of applying the dispersion onto the substrate and the step of drying the dispersion onto the substrate. The dispersion is then dry before pressing.

Generally, the step of pressing the dispersion onto the substrate may be performed by printing the dispersion onto the substrate. Methods for printing films onto substrates are well known in the art and any conventional method can used in the process of the invention. Preferably, the step of pressing the dispersion onto the substrate is by screen printing the dispersion onto the substrate.

When the dispersion is pressed onto the substrate by screen printing, a screen or mask can used to control the shape, size and/or thickness of the layer that is to be formed. The use of a screen or a mask allows the shape and dimensions of the carbon layer to be accurately controlled.

The electrode may comprise platinum. Platinum can be applied to the electrode using conventional methods known in the art.

Typically, the dispersion further comprises a platinum salt.

A convenient way of preparing an electrode that additionally comprises platinum is by mixing a platinum salt into the dispersion comprising CNTs, graphene and PEDOT-P55. This step may be performed before applying the dispersion onto the substrate. The dispersion can then be applied to the substrate in the normal way.

Examples of suitable platinum salts include hexachloroplatinic (IV) acid, hexabromoplatinic (IV) acid, hydrogen hexahydroplatinate (IV), PtCI2, PtBr2, Pt12, and Pt(NH3)2C12. Preferably the platinum salt is hexachloroplatinic (IV) acid.

When the electrode is for use as a counter electrode of a DSSC, then the process may comprise a final step of creating a hole in the electrode. The dimensions and location of the hole should be suitable to allow the DSSC to be filled with an electrolyte after the DSSC has been assembled.

The invention will now be illustrated by the following, non-limiting examples.

Examples

Preparation of the counter electrode (CE) A series of formulations comprising multi-walled carbon nanotubes [MER Corporation], graphene [graphene nanoplatelets; Cheap Tubes Inc.] and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) [Sigma-Aldrich] were prepared by adjusting the weights of two dispersions (see Table 1). The first dispersion was a dispersion of MWCNT5 in a PEDOT-PSS solution that was prepared by sonication for 4 hours. The second dispersion was a dispersion of graphene in a PEDOT-PSS solution, which was also prepared by sonication for 4 hours. After mixing the first and second dispersions together, the resulting mixture was sonicated for a further 24 hours in a closed glass vial.

Each counter electrode was prepared by transferring a few drops of the formulation onto FTC glass [Pilkington] using a pipette, then leaving it to dry at room temperature for a few hours followed by heating it to dryness at 110 °C in a furnace for 30 mm.

Mixtures consisting only of MWCNTs and PEDOT-PSS showed poor adhesion to the FTO glass substrate.

The thickness of the resulting composite film can be controlled by concentration and measured by a surface profilometer. The film thickness can vary from 6 to 50 pm without difficulty within a variation of less than 10%. A 1 x 1 cm2 pattern was used for the counter electrode. In order to control the shape and size of the casted film, Scotch tape was used as a mask. A hole was pre-drilled on the counter electrode of each cell, which was used to fill the cell with the liquid electrolyte after assembly.

Preparation of the working electrode (WE) A Ti02 paste [nanoparticulate titania paste; Dyesol Ltd.] was printed onto a TCO substrate [Pilkington] using a screen printer and was then sintered in air at 550 °C for 1 hour. The coated substrate was then soaked in a dye solution for 24 hours. The dye solution consisted of the dye [Ru(2,2'-bipyridyl-4,4'-dicarboxylic acid)2(NCS)2].2(tetra-n-butylammonium) [Nil 9; Dyesol Ltd.] dissolved in ethanol. After soaking for 24 hours, the working electrodes were removed from the dye solution and rinsed in ethanol and then dried in air.

A 1 x 1 cm2 pattern was used for the working electrode. In order to control the shape and size of the casted film, Scotch tape was used as a mask.

Production of a DSSC A 50 pm thick thermal plastic sealing foil [Surlyn; DuPont] was trimmed to a square frame that has a 1.1 cm span for its inner width, and a 1.5 cm span for its outer width.

The frame had 2 mm on each side for the sealing of electrolyte all around.

The trimmed thermal plastic frame was then placed around the counter electrode.

The working electrode was then placed on top of the counter electrode and moved into alignment with the counter electrode. The cell was then loaded into a thermal press for heat selling at a temperature of 160 °C. Both sides of the cell were pressed.

After assembly, the cell was filled with electrolyte [organic-base electrolyte; Dyesol Ltd]. A drop of the liquid phase electrolyte was placed over the pre-drilled hole in the counter electrode. The cell was then loaded into a vacuum chamber to suck the drop of the electrolyte into the cell. The cell was then removed from the vacuum chamber and residue electrolyte on the cell outer surface was then cleaned away with acetone.

A small piece of glass, together with a thermal plastic sheet cut into the same size, was used to seal the hole. Cells made by this process can produce 3.2 pW!cm2/100 lux of light intensity.

Results A series of DSSCs were prepared using the method above where the relative amount of multi-walled carbon nanotubes (MWCNT5), graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) in the counter electrode was varied. The electrical characteristics, namely the short circuit current (J3), the open circuit voltage (V0), fill factor (FF) and solar efficiency (q), of the DSSC5 were measured using standard methods and the results are shown in Table 1 below.

Test ID (PEDOT-PSS):CNT:Graphene V0 FF q (%) (weight fraction ratio) (mA/cm2) (V) 1 1:0:0 9.63 0.71 0.34 2.32 2 2:1:0 9.38 0.76 0.39 2.81 3 3:4:2 9.74 0.73 0.34 2.47 4 3:3:3 8.61 0.76 0.48 3.20 3:2:4 11.35 0.70 0.51 4.10 6 2:0:1 9.82 0.77 0.33 2.53 7 Pt (reference) 13.21 0.67 0.44 3.90

Table 1

The test cells having a counter electrode comprising PEDOT-PSS alone (Test ID 1) or PEDOT-PSS mixed with only either MWCNTs (Test ID 2)or graphene (Test ID 6) showed no evidence of an improvement in efficiency compared to the reference Pt cell.

The counter electrodes of Test ID numbers 3, 4 and 5 each consist of the three components PEDOT-PSS, CNTs and graphene. The weight fraction ratio of PEDOT-PSS was fixed, whereas the weight fraction ratio of CNT to graphene varied from 2:1, 1:1 and 1:2. The cell efficiency was found to increase monotonically with increasing graphene content. The highest efficiency (see Test ID 5) was found to exceed that of the reference cell, which uses Pt as the catalyst.

The cell efficiencies are shown graphically in Figure 2 and the results for Test ID numbers 3, 4 and 5 have been highlighted.

Production of a DSSC module by printing a carbon film The materials used to prepare the counter electrode! the working electrode and the DSSC described above were used to prepare a DSSC, where the glass size was larger. The TCO also required laser patterning. Screen printing was used to manufacture a larger area module DSSC, instead of drop casting the carbon formulation as mentioned above.

The formulation of multi-walled carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) was used as the printed paste for screen printing. A stainless screen with mesh count of about 90 per inch was used for printing.

The concentration of the formulation and the printing parameters were used to control the thickness of the printed composite film, which was measured by a surface profilometer.

The thickness of the film can vary from 6 pm to 50 pm with a variation of less than 10%.

A 60 x 60 mm2 pattern was used for both the working electrode and the counter electrode to provide a large sized module. Within this 60 x 60 mm2 area, the module was divided into eight strip cells. Each cell was composed of a strip size of 0.5 x 6 mm2 active area in rectangular shape, and they were connected in series. Metal lines were printed and protected for interconnections, and they occupied part of the spaces between the cells.

To fill the electrolyte, eight holes were pre-drilled near the edge of the counter electrode on each module. The Ti02 paste for the working electrode was printed on the TCO glass substrate using a screen printer and was then sintered in air at 550 °C for 1 hour. The shape, size and the thickness were accurately controlled by the design of the screen.

After sintering, the working electrode was soaked in the dye solution. The dye solution consists of the dye [Ru(2,2'-bipyridyl-4,4-dicarboxylic acid)2(NCS)2].2(tetra-n-butylammonium) [N719] dissolved in ethanol. After soaking for 24 hours, the working electrodes were removed from the dye solution, rinsed in ethanol and then dried in air.

In contrast to the thermal plastic that was used to seal the small DSSCs above, a UV curable sealant was applied by screen printing. A mask was used to protect the Ti02 dye-soaked area to avoid UV exposure when the sealant was cured by UV light. The module was exposed to a UV lamp for curing for ito 5 minutes. The UV light intensity can vary from 70mW/cm2 and 500 mW/cm2.

After assembling the module, it was ready for filling with an electrolyte. The electrolyte was filled through the holes that were pre-drilled on the counter electrode side using vacuum equipment that was similar to that used for liquid crystal filling. The module was then removed from the vacuum equipment for final sealing. The module was cleaned and the holes were sealed with a glass sheet laminated through a piece of thermal plastic.

A module fabricated by this process was found to deliver 3.0 to 3.2 pW/crn2/iOO lux with 3.2 V output voltage. It may be used for energy harvesting to charge batteries for indoor sensor applications.

Preparation of a counter electrode (CE) comprising platinum A counter electrode containing a relatively small amount of platinum was prepared.

Formulations containing multi-walled carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) were prepared as described above. A 5 to 10 mM solution of hexachloroplatinic (IV) acid hexahydrate in anhydrous isopropanol was mixed into the formulation. The counter electrode was then prepared using the drop casting method described above.

Claims

CLAIMS1. An electrode suitable for a dye sensitized solar cell having a layer comprising carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate).
2. An electrode according to claim 1, which is obtainable by forming a dispersion comprising carbon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), and applying the dispersion onto a substrate.
3. An electrode according to claims 1 or 2, wherein the PEDOT-PSS and the CNTs are present in a ratio by weight of 1 to 5:1.
4. An electrode according to any one of claims 1 to 3, wherein the graphene and the PEDOT-PSS are present in a ratio by weight of 1 to 5:1.
5. An electrode according to any one of claims 1 to 4, wherein the carbon nanotubes are multi-walled carbon nanotubes.
6. An electrode according to any one of claims 1 to 5, wherein the layer further comprises platinum in an amount of 0.01 to 5 wt. %.
7. An electrode according to any one of claims 1 to 6, wherein the layer has a thickness of ito 100 pm.
8. Use of an electrode according to any one of claims 1 toY as a counter electrode in a dye-sensitized solar cell.
9. A dye-sensitized solar cell comprising: (a) a counter electrode; (b) a working electrode comprising a semiconductor and a sensitized dye; and (c) an electrolyte; wherein the counter electrode is an electrode as defined in any one of claims 1 to 7.
10. A process for preparing an electrode suitable for a dye-sensitized solar cell, which piocess complises the steps of: (i) foiming a dispersion complising caibon nanotubes, graphene and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate); and (U) applying the dispersion onto a substrate.
11. A process according to claim 10 further comprising the step of printing the dispersion onto the substrate.