Chemistry: electrical current producing apparatus – product – and – Deferred action type – Responsive to light
Patent
1992-10-15
1994-09-27
Weisstuch, Aaron
Chemistry: electrical current producing apparatus, product, and
Deferred action type
Responsive to light
H01M 636
Patent
active
053506448
DESCRIPTION:
BRIEF SUMMARY
The invention relates to new transition metal dyestuffs and to their use in photovoltaic cells. These dyes can be coated on titanium dioxide films rendering such devices effective in the conversion of visible light to electric energy.
BACKGROUND OF THE INVENTION
Titanium dioxide films (layers) are known for their semiconductive properties and this property renders them useful for photovoltaic cells. However, titanium dioxide has a large band gap and therefore it does not absorb light in the visible region of the spectrum. For solar applications it is important that the titanium dioxide film be coated with a photosensitiser which harvests light in the wavelength domain where the sun emits light, i.e. between 300 and 2000 nm. Thermodynamic considerations show that conversion of solar energy into electricity is achieved in the most efficient fashion when all the emitted photons with wavelengths below 820 nm are absorbed by the photosensitizer. The optimal dye for solar conversion should therefore have an absorption onset around 800 nm and the absorption spectrum should be such that it covers the whole visible domain.
A second requirement for efficient solar light energy conversion is that the dyestuff after having absorbed light and thereby acquired an energy-rich state is able to inject with practically unit quantum yield, an electron into the conduction band of the titanium dioxide film. This requires that the dyestuff be attached to the surface of the titanium dioxide through suitable interlocking groups. The function of the interlocking group is to provide electronic coupling between the chromophoric group of the dyestuff and the conduction band of the semiconductor. This type of electronic coupling is required to facilitate electron transfer between the excited state of the dyestuff and the conduction band. Suitable interlocking groups are .pi.-conducting substituents such carboxylate groups, cyano groups, phosphate groups or chelating groups with .pi.-conducting character such as oximes, dioximes, hydroxy quinolines, salicylates and alpha keto enolates. The electrons, photoinjected by the dyestuff, generate electrical current in the external circuit when the photovoltaic cell is operated.
SUMMARY OF THE INVENTION
According to the invention there is provided a photovoltaic cell comprising:
a light transmitting electrically conductive layer deposited on a glass plate or a transparent polymer sheet to which one or more preferably porous high suface area titanium dioxide layers have been applied, in which at least the last titanium dioxide layer (optionally also the second to last and the third to last layer) may be (preferably are) doped with a metal ion which is selected from a divalent or trivalent metal.
Still further according to the invention there is provided a photovoltaic cell comprising:
i) two electrodes, at least one of which is transparent and has a visible light transmittance of at least 60%, the plates being arranged so as to define a receptacle between them, in which receptacle an electrolyte is located, said one transparent electrode having, in addition, an electrolyte-contacting film of titanium dioxide; and
ii) means for permitting the passage of an electrical current generated by the cell;
wherein the improvement comprises the electrolyte-contacting surface of said film being doped with at least one ion selected from divalent and trivalent metals and boron.
For the purposes of this invention it is essential that the dopant be confined to the surface of the titanium dioxide, that is, to the titanium dioxide/electrolyte interface or very close thereto. The preferred way of achieving this is by the application of a series of layers of titanium dioxide casting, one on top of the other, with up to the last three layers containg the dopant. Preferably the last four layers and most preferably the last layer contain the dopant.
Preferably a photosensitiser dye is applied to the doped TiO.sub.2 layer. Preferably such a photosensitiser is a ruthenium, osmium or iron complex or a combination
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"Extension of the Optical Absorption Range of TiO.sub.2 Thin Films by Chromium and Cadmium Doping", F. A. Soliman, H. J. J. Seguin, Thin Solid Films, 145 (1986) pp. 61-67, Dec., No. 1.
Graetzel Michael
Nazeeruddin Mohammad K.
O'Regan Brian
Ecole Polytechnique Federale de Lausanne
Weisstuch Aaron
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