PHOTOELECTROCHEMICAL INVESTIGATIONS OF CHARGE TRANSFER AT THE P-TYPE GALLIUM-ARSENIDE SEMICONDUCTOR-SOLUTION INTERFACE
Abstract
The illuminated semiconductor-solution interface has been suggested as having potential applications as a mechanism for the conversion of solar energy into electrical and/or chemical energy. The goal of this thesis has been to investigate the photoelectrochemical behavior to the p-type GaAs semiconductor-solution interface using continuous and pulsed illumination techniques. Constant potential and cyclic voltammetric techniques were used to characterize the continuous illumination behavior of this system, while both potentiostatic and coulostatic methods were applied to the temporal illumination studies. The specific objectives of this investigation were to gain insight into the nature of charge transfer processes at the illuminated semiconductor-solution interface and to apply the recently developed technique of flash-induced coulostatic chronopotentiometry to the study of a p-type semiconductor-solution interface, and evaluate its utility as a method for probing rapid photoelectrochemical processes. Part I of this thesis provides a general introduction to the investigation. The renewed interest in the semiconductor-solution interface, due to its potential applications in solar energy conversion, is discussed. The use of photoelectrochemical techniques for the characterization of this interface is briefly described. Background information concerning semiconductors, semiconductor electrochemistry and photoelectrochemical techniques is presented in Part II. The band theory of solids is briefly described as a prelude to a discussion of the formation of semiconductor-solid-state junctions. Once this introduction is provided, the formation and properties of the semiconductor-liquid junction are described. Special emphasis is placed on the characteristics of an ideal interface between a semiconductor and a conducting solution. The interaction of light with this junction is described, and light-induced electron transfer at this juction is compared with that at a metal-solution interface. Deviations from the ideal model, due to the influence of surface energy states, are discussed in terms of recent models of the imperfect interface. This section concludes with a discussion of the development and application of electrochemical techniques for the investigation of the illuminated semiconductor-solution interface. Part III details the experimental considerations and approaches used in this investigation. Instrumental design, experimental procedures, and the preparation of solutions and electrodes are described. The results of the photoelectrochemical investigations of the p-type GaAs semiconductor-solution interface are presented and discussed in Part IV. The results from the continuous illumination studies show a dependence of light-induced charge transfer on illumination intensity and potential scan rate. A mechanism of charge transfer involving surface state mediators is proposed. Results from the transient illumination studies are presented to support this proposal, and also demonstrate the applications and limitations of pulsed illumination techniques for the study of rapid electrode processes. The conclusions formed from this thesis are summarized in Part V. Surface energy states at the p-type GaAs semiconductor-solution interface are shown to play a significant role in light-induced charge transfer processes. Pulsed illumination techniques are shown to provide unique information about the semiconductor-liquid junction at the submicrosecond time scale. The combination of continuous and pulsed illumination photoelectrochemical techniques allows for the examination of a broad range of time domains.
Degree
Ph.D.
Subject Area
Analytical chemistry|Energy
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