Mechanisms of Photocurrent Generation at Metal and Semiconductor Electrodes

Citation

Finklea, Harry Osborn (1976) Mechanisms of Photocurrent Generation at Metal and Semiconductor Electrodes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5d1f-5821. https://resolver.caltech.edu/CaltechTHESIS:06272025-221955987

Abstract

An attempt to measure the electrochemical properties of an excited molecule is made. Polyaromatics and Ru(bipy)₃²⁺are photoexcited in nonaqueous electrolytes via a monochromatic beam passing through a thin-metal film electrode. It is concluded that observation of charge transfer from an excited molecule is not possible because of rapid quenching by energy transfer to the electrode. All observed photocurrents in the presence of a light-absorbing solute are explained by heating of the solution near the interface or the photochemical generation of electroactive products. Anodic and cathodic photocurrents are observed in the absence of a dye. The mechanism of the latter process entails the formation of excited holes and electrons within the metal with their subsequent reaction at the interface. Photoelectron ejection into the solution is also proven by the effects of an electron scavenger.

A related problem is the mechanism of dye-sensitization at a semiconductor electrode. The concentration dependence of the photo induced oxidation of the dye rhodamine 6G at a polycrystalline SnO₂ electrode is determined in aqueous and nonaqueous electrolytes. In water, the concentration study reveals a slow irreversible buildup of an adsorbed dye layer. Photocurrent spectra suggest that the sensitizing species is the monomer within the adsorbed layer, slightly perturbed by its environment. The adsorbed layer structure is hypothesized to be localized in clumps. In acetonitrile, onset of sensitization does not occur until 10^-4 M dye concentrations are reached. Over the next decade of concentration, the photocurrent caused by excitation of the electrode decreases by a factor of two, indicating the formation of a uniform monolayer. Evidence for sensitization by non-adsorbed molecules is obtained. Again, dilution reveals an irreversible adsorption process. The observation of fluorescence quenching of the adsorbed dye as a function of electrode potential is attempted. The negative results are attributed to the low quantum yield of the sensitized photocurrent.

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