LIGHT-INDUCED ELECTRON TRANSPORT ACROSS SEMICONDUCTOR ELECTRODE/REACTION-CENTER FILM/ELECTROLYTE INTERFACES

Abstract— Reaction center (RC) complexes isolated from the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 were dried as a film onto platinum and semiconductor (SnO₂) electrodes. The light-induced primary charge separation which occurs across the biological complex couples electrically with the SnO₂ but not with the metal electrode on the time scale of observation. As the working electrode in a two-electrode photoelectrochemical cell, RC-coated SnO₂ generated photovoltages as high as 80 mV and photocurrents as high as 0.5µA·cm² when exposed to light of λ >600nm. The number of quinone molecules per RC strongly influences the photovoltage and photocurrent observed. Photo-effects generated by RC electrodes persist after several days of storage; however, the kinetics and polarity of the effects are subject to change. The potential use of RC electrodes lies more as a new probe of photosynthetic electron transport rather than as a solar energy conversion device because modification to the RCs and their environment affect the electrical properties of the cell. An energy-level model is proposed to explain how the photoelectrochemical cell functions.