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.

Study of Charge Carrier Dynamics at Illuminated ZnO Photoanodes

“…In addition, the results of electrical and optoelectrical admittance spectroscopy show that surface recombination plays an important role; the difference between j/eφ and 1 -I/I M is the recombination current density j r divided by eφ. As a consequence, the claim, made in ref 17, that the luminescence vs potential and the photocurrent quantum yield vs potential measurements are fully complementary, is not correct.…”

“…Light emission from ZnO photoanodes has been observed in the potential range in which holes recombine with majority carriers. , Since it is generally agreed that surface recombination is nonradiative, photoluminescence indicates that at least a part of the photogenerated holes recombine in the bulk solid. Petermann et al claimed a strict correlation between the photocurrent and the green luminescence at ZnO single-crystal electrodes. This result indicated that photoluminescence might be a useful tool for the study of the competition between bulk recombination and anodic charge transfer.…”

Excited-state processes of relevance to photoelectrochemistry

“…The effects which the passage of photocurrent has on PL properties have been examined with a number of n-type semiconductors exhibiting edge emission and/or impuritybased subband-gap emission. The former category includes ZnO and CdS.,-Sei-x (0 < .v < 1) samples; in the latter category are Cu-doped ZnO (ZnO:Cu) and CdS:Te (8)(9)(10)(11). In all cases examined, passage of anodic photocurrent from ultraband-gap excitation quenched the PL intensity but did not appreciably alter the spectral distribution.…”

Photoelectrochemical and Photoluminescence (PL) Properties of p-Type Porous Silicon/Electrolyte Solution Interface: Potential Dependent PL Spectra as a Result of Size Dependent Quenching

“…The luminescence properties of the semiconductor/electrolyte solution interface are known to provide useful information of the mechanism of electrochemical reactions as well as the potential distribution at the interface .…”