Photocell using covalently-bound dyes on semiconductor surfaces

Abstract

ONLY TiO₂,¹ SnO₂,² and SrTiO₃,^3,4 which do not undergo decomposition upon irradiation when used as photo-electrodes in aqueous media, are stable enough to be used as practical solar energy conversion and hydrogen production devices. The band gaps of these n-type semiconductors correspond to the energy of photons in the ultraviolet region. Solar energy reaching the Earth's surface has, however, a spectrum distribution in the longer wavelength range and cannot, therefore, be used effectively by these semiconductors^5,6. Spectral sensitisation would solve this problem by extending the sensitivity of the electrochemical photocells to longer wavelengths. In applying this technique to a real photocell another problem may arise in that most of the solar energy is absorbed by the dye molecules in the solution rather than by the dye molecules adsorbed at the electrode–solution interface, though only the excited adsorbed dye molecules can contribute to sensitisation^7–9. To cope with this we have developed a new type of photocell¹⁰. In this, the chemically modified SnO₂ or TiO₂ electrode, with rhodamine B as a sensitiser, was in contact with the transparent electrolyte solution containing a reducing agent as a supersensitiser. In this communication, further developments of this subject are described, especially, the correlation between the efficiency of sensitisation and the structure of the modified layer, which gives us the criteria for more successful chemical modifications of electrode surfaces.