FLASH PHOTOLYSIS STUDIES OF THE SEMICONDUCTOR/ELECTROLYTE INTERFACE

JEFFREY ROSENTHAL, Purdue University

Abstract

The semiconductor/electrolyte interface, as applicable to solar energy conversion, was studied using the technique of flash photolysis with electrochemical monitoring. A recent suggestion that semiconductor-liquid junction solar cells may prove more useful than solid state solar cells has led to an intense investigation of the semiconductor/electrolyte interface. To date, most researchers have studied the interface only under conditions that simulate a solar cell in use. Generally, this is done by using continuous irradiation. Under such conditions, however, little can be learned about the kinetics which control the conversion process. The kinetics are important because the system is to be operated at steady state and not equilibrium. It has been demonstrated that flash photolysis with electrochemical monitoring is a viable technique for kinetic studies. Systems that can be studied by the technique are those that are inititated by light and electrochemical in nature, as is the case with the semiconductor/electrolyte interface. The interaction of light with the semiconductor excites electrons to energy lvels which facilitate charge transfer between the seminconductor and a redox couple in the electrolyte. Electrochemistry at a semiconductor electrode is more complex than at a metal electrode so that certain precautions must be taken in the interpretation of the results. As will be pointed out, however, complementary information to continuous irradiation studies can be obtained by flash photolysis. Two important observations have come out of these studies. First, the intermediates or products of a light induced reaction at a semiconductor/electrolyte interface are electrochemically detectable at the semiconductor. This was demonstrated by the detection of a photo-oxidation product shown to exist under continuous irradiation by cyclic voltammetry. Also detected were the light induced decomposition products of n-type and p-type semiconductors, at conditions consistent with the literature observations. Second, charge transfer between a semiconductor electrode and a species in solution can be so slow that detection on the time scale of a flash photolysis experiment is not possible. This is information complementary to that obtained by continuous irradiation experiments, since at steady state conditions the same species are detectable.

Degree

Ph.D.

Subject Area

Analytical chemistry

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