(Europe Section Heinz Gerischer Award) A Perspective of Photoelectrochemistry: Past Expectations and Present Realities

It was Heinz Gerischer who got me interested in and taught me semiconductor electrochemistry when he visited me just after the Oil Crisis of 1973. He, I and later many others dreamed of a world powered by semiconductor-liquid junction solar cells, hoping that that these would be simpler and cheaper than the silicon p-n junction solar cells that powered at the time communication satellites, remote terrestrial communication relays, wristwatches, calculators and other portable electronic devices.  By 1981 research on the semiconductor -liquid junctions led to the uncovering of truths of nature and to 12 % solar conversion efficiencies, both to electrical power and in the generation of hydrogen. However, I did not see how efficient photo-electrochemical conversion  could provide a technological edge over all-solid silicon and thin-film solar converters, because their packaging cost were lower and their stability was superior.  After 34 years of fine research leading to solar conversion efficiency now at 20 %, packaging cost and stability still prevent photo-electrochemical solar converters from being preferred over all solid solar cells.

In the absence of technologically relevant semiconductor-liquid junction based solar converters we joined Akira Fujishima and Kazuhito Hashimoto in their quest for technologically relevant photo-catalytic surfaces that clean themselves of accumulating organic grime by catalyzing its air oxidation. On the fundamental side, the truth that Heinz Gerischer and I uncovered during his repeated stays at the University of Texas in Austin has been that the rate of the n-TiO₂ –photo-catalyzed O₂ oxidation of organic matter was not controlled by the rate of photo-generation of holes, but by the rate of reduction of adsorbed O₂ by trapped electrons.  On the practical side, we developed photo-stable binders that allowed the incorporation of photo-catalytic n-TiO₂ in coatings; and a method of producing self-cleaning window glass.