Luminescence of Zinc Oxide Crystals Controlled by Electrode Potential and Electrochemical Reactions

Abstract

The voltage dependence of the green luminescence of ZnO crystals used as electrodes in an electrochemical cell was investigated and compared with the voltage dependence of the photocurrent. When the exciting light is of shorter wavelength than the band edge of ZnO, this luminescence can be varied from its maximal value to complete extinction by applying small positive voltages. The luminescence resulting from excitation within the long wavelength tail of absorption, however, cannot be influenced. The potential dependence of the luminescence has been investigated with differently doped crystals under various conditions. The addition of formic acid to the electrolyte causes an increase of luminescence; this is found to be a consequence of photoelectrochemically induced radical reaction by which electrons are injected into the conduction band of the electrode. The potential dependence of the green luminescence is explained by a mechanism in which the trapping of the holes in recombination centers competes with their extraction, which leads to the generation of the photocurrent. The average concentration of photogenerated holes in the crystal is therefore rate limiting for the intensity of luminescence, provided that an excess of electrons is available for recombination. At sufficiently large positive potentials, however, when electrons are being depleted in the space charge layer, their concentration also becomes limiting, and electron injection can stimulate luminescence. The experimentally observed potential dependence of luminescence and photocurrent, as well as their interrelation, is shown to be consistent with results which were derived from a theoretical treatment of these effects. It is suggested that this green luminescence may be used as a probe for the study of the space charge layer and of electrochemical reactions.