Patent Application
20120240993
1. Field of the Invention
This invention relates to the field of dye-sensitised solar cells (DSSC) and to a method for the low temperature platinisation of the counter-electrode which is applicable to a wide range of substrates.
2. Description of the Related Art
Solar cells are traditionally prepared using solid state semiconductors. Cells are prepared by juxtaposing two doped crystals, one with a slightly negative charge, thus having additional free electrons (n-type semiconductor) and the other with a slightly positive charge, thus lacking free electrons (p-type semiconductor). When these two doped crystals are put into contact with each other, extra electrons from the n-type semiconductor flow through the n-p junction to reduce the lack of electrons in the p-type semiconductor. At the p-n junction, charge carriers are depleted on one side and accumulated on the other side thereby producing a potential barrier. When photons produced by sunlight strike the p-type semiconductor, they induce transfer of electrons bound in the low energy levels to the conduction band where they are free to move. A load is placed across the cell in order to transfer electrons, through an external circuit, from the p-type to the n-type semiconductor. The electrons then move spontaneously to the p-type material, back to the low energy level they had been extracted from by solar energy. This motion creates an electrical current.
Typical solar cell crystals are prepared from silicon because photons having frequencies in the visible light range have enough energy to take electrons across the band-gap between the low energy levels and the conduction band. One of the major drawbacks of these solar cells is that the most energetic photons in the violet or ultra-violet frequencies have more energy than necessary to move electrons across the band-gap, resulting in considerable waste of energy that is merely transformed into heat. Another important drawback is that the p-type layer must be sufficiently thick in order to have a chance to capture a photon, with the consequence that the freshly extracted electrons also have a chance to recombine with the created holes before reaching the p-n junction. The maximum reported efficiencies of the silicon-type solar cells are thus of 20% to 25% or lower for solar cell modules, due to losses in combining individual cells together.
Another important problem of the silicon-type solar cell is the cost in terms of monetary price and also in terms of embodied energy, that is the energy required to manufacture the devices. Dye-sensitised solar cells (DSSC) have been developed in 1991 by O'Regan and Grätzel (O'Regan B. and Grätzel M., in Nature, 1991, 353, 737-740). They are produced using low cost material and do not require complex equipment for their manufacture. They separate the two functions provided by silicon: the bulk of the semiconductor is used for charge transport and the photoelectrons originate from a separate photosensitive dye. The cells are sandwich structures as represented in
In these cells, photons strike the dye moving it to an excited state capable of injecting electrons into the conducting band of the titanium dioxide from where they diffuse to the anode. The electrons lost from the dye/TiO2 system are replaced by oxidising the iodide into triiodide at platinised counter electrode, whose reaction is sufficiently fast to enable the photochemical cycle to continue. Platinum coating the counter electrode catalyses the reduction of the iodide/triodide redox couple; if Pt is not present the efficiency of the cell is severely limited.
The DSSC generate a maximum voltage comparable to that of the silicon solar cells, of the order of 0.7 V. An important advantage of the DSSC, as compared to the silicon solar cells, is that they inject electrons in the titanium dioxide conduction band without creating electron vacancies nearby, thereby preventing quick electron/hole recombinations. They are therefore able to function in low light conditions where the electron/hole recombination becomes the dominant mechanism in the silicon solar cells. The present DSSC are however not very efficient in the lower part of the visible light frequency range in the red and infrared region, because these photons do not have enough energy to cross the titanium dioxide band-gap or to excite most traditional ruthenium bipyridyl dyes.
A major disadvantage of the prior art DSSC resides in the high temperature necessary for depositing and calcining the platinum on the counter electrode. The high temperature needed for sintering the metal oxide paste used on the photoelectrode is also a problem. Another drawback of the dye-sensitised solar cells lies in the long time necessary to dye the titanium dioxide nanoparticles: it takes between 12 and 24 hours to dye the layer of titanium dioxide necessary for solar cell applications. Another major difficulty with the DSSC is the electrolyte solution: the cells must be carefully sealed in order to prevent liquid electrolyte leakage and therefore cell deterioration In classical solar cells preparation, the heat necessary for the decomposition of [PtCl6]2− is of about 400° C. Such high temperature limits the nature of transparent material useable for the substrate to glass. If the temperature can be brought down to at most 150° C., transparent polymers such as polyethylene terephthalate (PET) or polyethylene napthalate (PEN) can also be used.
There is thus a need to prepare robust solar cells that can be prepared rapidly at reduced cost.
It is an aim of the present invention to reduce the temperature necessary for the deposition and calcination of the platinum particles coating the counter electrode of dye-sensitised solar cells.
It is another aim of the present invention to ensure good adhesion of platinum particles to the substrate.
It is also an aim of the present invention to produce high transparency for the deposited platinum layer
It is a further aim of the present invention to ensure even coverage of the platinum layer on the widest range of conducting substrates.
In accordance with the present invention, the foregoing aims are realised as defined in the independent claims. Preferred embodiments are defined in the dependent claims.
Accordingly, the present invention discloses a method for low temperature deposition of platinum coating the counter electrode of dye-sensitised solar cells that comprises the steps of:
a) providing a counter electrode prepared from an electro-conducting substrate;
b) coating the counter electrode with one or more layers of a transparent conducting oxide;
c) cleaning the counter-electrode with an organic solvent optionally along with ultra-sound;
d) optionally pre-treating the counter electrode to prepare the surface for platinisation and to ensure good adhesion of the platinum film;
e) activating the counter electrode with a solution containing a metal salt and reducing said metal salt on the counter-electrode surface;
f) treating the electrode from step e) with aqueous platinum solution of either Pt (II) or Pt(IV), said solution having a pH of at least 3;
g) adding a reducing agent;
h) retrieving the platinum coated counter electrode.
Platinisation of the counter-electrode is an important step in the preparation of the dye-sensitised solar cells. The platinum catalyses electron transfer from the counter electrode to the iodide/triiodide redox couple which, in turn, regenerates the ground state dye from its excited state. Without an efficiently platinised counter electrode, DSSC device efficiencies are very severely limited.
In the prior art, platinisation of the counter-electrode is carried out by the application of an aqueous solution of hexachloro-platinate(IV) followed by heating to 400° C. for at least 30 minutes. This is suitable for the application of platinum to counter electrodes prepared from transparent conducting oxide (TCO) coated glass but not to temperature sensitive substrates such as polymers.
The electroconducting substrate or counter electrode can be prepared from transparent glass or polymer selected for example from a polyester based film such as polyethylene naphthalate (PEN) or polyethylene terephthalate (PET). it can also be prepared from an opaque substrate such as aluminium, titanium or steel. Preferably it is conducting and transparent and preferably it is prepared from TCO-coated glass, more preferably from TCO-coated polymer.
The transparent conducting oxide (TCO) is preferably zinc oxide doped with aluminium or fluorine, or it is cadmium stannate, or it is tin oxide, more preferably, tin oxide doped with fluorine, indium or antimony, preferably it is doped with fluorine or indium. Several layers of conducting oxide can be applied to the counter-electrode.
In addition, one or more layers of titania particles having a diameter ranging between 300 and 500 nm, preferably about 400 nm, can further be added as scattering layer. This scattering layer further improves the cell efficiency.
The organic solvent used as cleaning solution from step c) has the additional role of reducing the surface tension of the TCO to increase surface wettability and interactions with subsequent aqueous solutions. It is preferably selected from acetone, ethanol or diethyl ether. The cleaning solution can be selected from aqueous solutions of hydrochloric acid, ammonium hydroxide, sodium hydroxide, potassium hydroxide or ammonia/hydrogen peroxide. Preferably it is aqueous ammonia/hydrogen peroxide. The cleaning solution has the additional effect of removing any surface material from the TCO surface. The cleaning step is carried out at a temperature between 20 and 80° C. for a period of time of 3 to 5 minutes. The substrate is preferably placed face down in the solution in order to ensure that oxygen is kept in contact with the substrate's surface.
An optional pre-treatment of step d) can be included. It is preferably carried out with an aqueous tin(II) chloride solution or an aqueous zinc (II) chloride, tin (II) chloride being preferred.
The activation of step e) improves platinum nucleation on the electrode surface and reduces the production of Pt particles in solution. It can advantageously be carried out with a solution of water and isopropanol containing a metal salt selected from PdCl2 or NiCl2 or CuCl2, preferably PdCl2. It is important to control the pH of this solution to ensure that it is not too low. The pH can be controlled by the addition of a base such as sodium hydroxide, ammonium hydroxide, potassium hydroxide or a buffer solution to keep a pH of at least 3 but preferably of at most 7.
The activating agent is preferably reduced after it has been applied to the surface. The reducing agent is preferably selected from citrate, hydrazine, phosphate hydride, hydrogen or borohydride. More preferably, it is hydrazine, phosphate hydride or hydrogen. Most preferably, it is hydrogen. It is carried out at a temperature between room temperature and 100° C. More preferably it is carried out at room temperature as it replaces the alternative standard heat treatment at 400° C. This extends the range of possible substrates to include polymeric materials such as PEN or PET which are thermally unstable above 150° C.
The aqueous platinum solution is selected from potassium hexachloroplatinate (II), potassium hexachloroplatinate (IV), hexachloroplatinic acid (IV), hexahydroxoplatinate, or a hexahaloplatinate salt such as a hexafluoroplatinate salt, preferably potassium hexachloroplatinate (IV). It is important to control the pH of this solution to ensure that it is not too low. The pH can be controlled by the addition of a base such as sodium hydroxide, ammonium hydroxide, potassium hydroxide or a buffer solution to keep a pH of at least 3 but preferably of at most 7.
The reducing agent from step g) is preferably selected from citrate, hydrazine, phosphate hydride, hydrogen or borohydride. More preferably, it is hydrazine, phosphate hydride or hydrogen. Most preferably, it is hydrogen. It is carried out at a temperature between room temperature and 100° C. More preferably it is carried out at room temperature as it replaces the alternative standard heat treatment at 400° C. This extends the range of possible substrates to include polymeric materials such as PEN or PET, which are thermally unstable above 150° C.
In the present invention, a series of substrate pre-treatment steps are included to clean the substrate surface, to wet the TCO surface to enhance interactions between aqueous solutions and the TCO surface, to improve nucleation of platinum particles on the TCO surface, to improve the efficiency of platinum deposition on the TCO surface over Pt particle formation within solution, to ensure even coverage of Pt over the TCO surface and to improve adhesion of the Pt particles to the TCO-coated counter electrode. Reduction of the platinum precursor in solution is then carried out at a temperature of at most 100° C., preferably of at most 90° C. The thermal treatment is followed by cooling, down to room temperature. The counter electrode is then ready for DSSC device manufacture.
The cleaning, activation, reduction and deposition processes can all be carried out rapidly in less than 3 to 5 minutes for each step, at room temperature. Increasing the cleaning, activation and deposition time can further decrease the platinisation temperature required and vice versa. In an alternative process, cleaning, activation, reduction and deposition can be assisted by exposure to ultra-violet or microwave radiation. The microwave radiation, if present, can be provided by a commercial or a conventional microwave oven, the commercial oven being preferred because it delivers a constant radiation. The power ranges between 600 and 1000 watts, preferably, it is of about 800 watts.
The platinum layer applied to the counter-electrode using the method of the present invention is very thin, very transparent and very homogeneous contrary to that of the prior art. The platinum particles have a size ranging between 5 and 20 nm.
The substrates can be cleaned, activated, reduced and chemicals deposited thereupon by dipping or spraying followed by wiping to remove excess solution.
The temperature is held at less than 100° C. throughout the entire process.
In a particularly preferred embodiment according to the present invention the platinisation of the counter-electrode comprises the steps of:
Dye-sensitised solar cells are then prepared according to any method known in the art. They have a better efficiency and fill factor than those of the prior art when prepared at low temperature and they can use polymers as a substrate. Preferably they are prepared according to a fast-dyeing method and/or a low temperature sintering method.
According to the preferred method, dye-sensitised solar cells are prepared by the steps of:
The steps of cleaning, activation and platinisation can advantageously be carried out in a continuous “roll-to-roll” process, very useful for industrial applications.
The present invention also discloses a continuous process for industrially producing dye-sensitised solar cells in the form of a roll or sheet that comprises the steps of:
The dye-sensitised solar cells prepared according to the preferred method here-above have a higher efficiency and fill factor than those of the prior art.
Solar panels can then be prepared by connecting individual solar cells.
Sandwich-type DSC cells devices were prepared following the structure described in
The metal oxide films were dipped into ethanolic dye solution containing the di-ammonium salt of cis-bis(4,4′-dicarboxy-2,2′-bipyridine)dithiocyanato ruthenium(II), commonly known as N719 (1 mM) for time periods of 18-24 h. After dyeing, a thermoplastic polymer gasket (Surlyn® from Du Pont) was placed around the photoelectrode and a commercial counter electrode (Dyesol Ltd, Australia) was placed on top and the electrodes sealed together at a temperature of 120° C. The commercial counter electrode was transparent-conducting glass coated electrode with a platinum layer which had been activated at a temperature of 400° C. for a period of time of 30 minutes prior to being used. A commercial liquid electrolyte containing iodine/tri-iodide in nitrile solvent (Dyesol Ltd, Australia) was added through a hole in the counter electrode which was then sealed using thermoplastic polymer (Surlyn®). Table 1 displays typical efficiency data and fill factors along with the open circuit voltage Voc and the short circuit current Jsc for comparative cells having also a working area of approximately 0.88 cm2.
Data shown in Table 1 is for DSC devices prepared using titania photoelectrodes. PEN=polyethylene nathphthalene and PET=polyethylene terephthalate. Illumination side is not applicable for plastic substrates as they are not thermally stable enough to survive platinisation at 400° C.
It must be noted that in the conventional method wherein platinisation is carried out at a temperature of about 450° C., a thermoplastic polymer cannot be used to prepare the counter electrode because it would be destroyed by the high temperature treatment. Hence the efficiencies quoted in Table 1 are zero.
Sandwich-type DSC cells devices were prepared following the structure described in
The metal oxide films were dipped into ethanolic dye solution containing the di-ammonium salt of cis-bis(4,4′-dicarboxy-2,2′-bipyridine)dithiocyanato ruthenium(II), commonly known as N719 (1 mM) for time periods of 18-24 h. After dyeing, a thermoplastic polymer gasket (Surlyn® from Du Pont) was placed around the photoelectrode and a TCO-coated counter electrode which has been coated with platinum was placed on top and the electrodes sealed together at a temperature of 120° C. The commercial counter electrode was commercial transparent-conducting polymer (PEN from CP Films Ltd). The TCO-coated PEN polymer was first cleaned in a solution of ammonia and hydrogen peroxide in distilled water at a temperature of 70° C. for a period of time of 3 minutes. The TCO surface was then activated with a 1:2 (v/v) solution of water and isopropanol containing PdCl2 and HCl for 5 mins. Hydrogen gas was then bubbled through a solution of isopropanol for a period of time of 30 s and the PdCl2 coated substrate was dipped into the hydrogen containing isopropanol solution for a period of time of 3 minutes in order to reduce Pd2+ to Pd. Finally, the Pd coated substrate was dipped into an aqueous solution of potassium hexachloroplatinate and hydrogen gas bubbled through this solution from for a period of time of 1 minute and then left to stand for a period of time of 3 minutes prior to washing in deionised water and drying in air. A commercial liquid electrolyte containing iodine/tri-iodide in nitrile solvent (Dyesol Ltd, Australia) was added through a hole in the counter electrode which was then sealed using thermoplastic polymer (Surlyn®). Table 2 displays typical efficiency data and fill factors along with the Voc and Jsc for comparative cells having also a working area of approximately 0.88 cm2.
In Table 2, illumination side=titania means that the light is striking the cell from the titanium side generally called the ‘normal side’. Illumination side=Pt CE means that the light is striking the cell from the counter-electrode side and is generally called the ‘reverse side’.
In further examples according to the invention, sandwich-type DSC cells devices were prepared following the structure described in
The metal oxide films were dipped into ethanolic dye solution containing the di-ammonium salt of cis-bis(4,4′-dicarboxy-2,2′-bipyridine)dithiocyanato ruthenium(II), commonly known as N719 (1 mM) for time periods of 18-24 h. After dyeing, a thermoplastic polymer gasket (Surlyn® from Du Pont) was placed around the photoelectrode and a TCO-coated counter electrode which had been coated with platinum was placed on top and the electrodes sealed together at a temperature of 120° C. The commercial counter electrode was commercial transparent-conducting polymer (PET from Optical Filters Ltd). The TCO-coated PET polymer was first cleaned in a solution of ammonia and hydrogen peroxide in distilled water at a temperature of 70° C. for a period of time of 3 minutes. The TCO surface was then activated with a 1:2 (v/v) solution of water and isopropanol containing PdCl2 and HCl for 5 mins. Hydrogen gas was then bubbled through a solution of isopropanol for a period of time of 60 s and the PdCl2 coated substrate was dipped into the hydrogen containing isopropanol solution for a period of time of 3 minutes in order to reduce Pd2+ to Pd. The dipping in the palladium solution and the hydrogen solution was then repeated. Finally, the Pd coated substrate was dipped into an aqueous solution of potassium hexachloroplatinate and hydrogen gas bubbled through this solution from for a period of time of 90 s and then left to stand for a period of time of 4 minutes prior to washing in deionised water and drying in air. A commercial liquid electrolyte containing iodine/tri-iodide in nitrile solvent (Dyesol Ltd, Australia) was added through a hole in the counter electrode which was then sealed using thermoplastic polymer (Surlyn®).
Table 3 shows efficiency data and fill factors along with the Voc and Jsc for comparative cells having a working area of 0.92 cm2. The DSC devices in Table 3 were manufactured from small pieces of ITO-coated PET film (3×1.5 cm) which had been platinised in small containers. These samples were used as counter electrodes in DSC devices with FTO-coated glass titania photo-electrodes to assess the reproducibility and consistency of the method between electrodes.
In Table 3, illumination side=titania means that the light is striking the cell from the titanium side generally called the ‘normal side’. Illumination side=Pt CE means that the light is striking the cell from the counter-electrode side and is generally called the ‘reverse side’.
Table 4 shows efficiency data and fill factors along with the Voc and Jsc for comparative cells having a working area of approximately 0.94 cm2. The DSC devices in Table 4 were manufactured from a large piece of ITO-coated PET film (15×16 cm) from which sub-samples had been cut. These smaller sub-samples were used as counter electrodes in DSC devices with FTO-coated glass titania photo-electrodes to assess the reproducibility and consistency of the method across a larger sample area.
In Table 4, illumination side=titania means that the light is striking the cell from the titanium side generally called the ‘normal side’. Illumination side=Pt CE means that the light is striking the cell from the counter-electrode side and is generally called the ‘reverse side’.
It can be seen that the efficiency, Voc and Jsc of the present invention is systematically higher than that of conventional cells prepared at the same temperature.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0915251.3 | Sep 2009 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/062648 | 8/30/2010 | WO | 00 | 6/12/2012 |
