Oxide semiconductors in photoelectrochemical conversion of solar energy

A wide range of oxides is examined for use as anodes in photoelectrochemical cells for the conversion of solar energy into electrical power or hydrogen. The Schottky barrier model of the semi-conductor-electrolyte interface is used throughout. Type (a) oxides, not containing partly-filled d-levels, are found to conform to the relationship between flat band potential, Vfb(SHE), and band gap, Eg, VFB(SHE) = 2.94 − Eg which essentially rules out the possibility of finding type (a) oxides with simultaneously the small band gap and large negative flat band potential required for efficient operation in the unbiased photoelectrolysis of water. Incorporation of this realationship into the Schottky barrier formula for photocurrent enables the calculation of efficiencies of conversion for type (a) oxides. For air mass 1 radiation, the predicted maximum efficiency of conversion to hydrogen is 3.4 per cent for Eg = 3 eV for unbiased photoelectrolysis; 6.3 per cent for Eg = 2.4 eV for series voltage biased photoelectrolysis; and 4.7 per cent for Eg = 2.2 eV for pH biased photoelectrolysis. For power cells with redox operation, type (a) oxides are predicted to give 5–6 per cent efficiency for Eg = 2.4 eV, with a redox couple having standard potential not less than 0.8 V. High efficiency operation of photoelectrochemical cells with type (a) oxide anodes appears to be possible only in some special cases. Comparisons of the observed stabilities for a wide range of oxides with their calculated thermodynamic stabilities towards photoanodic dissolution indicate that oxygen overpotentials at the oxide anodes in photoelectrolysis may be small, and that the thermodynamic method of estimation of long term stability in oxide anodes is useful. The prospects of obtaining satisfactory efficiencies with oxides containing partly-filled d-levels are also examined, and found to be severely limited by the simultaneous requirements of stability, flat band potential, and band gap. Some suggestions for further research are made.