Time-Resolved and Steady-State Investigations of Carrier Dynamics at the Semiconductor/Liquid Interface

Fundamental investigations of carrier transport and charge transfer at the semiconductor/liquid interface are presented. The application of both steady-state and time-resolved methods to these studies is discussed. Small signal photocurrent transients have been measured for n-Si/CH3OH-dimethylferrocene (Me2Fc)+/0/Pt and n-Si/Au/CH3OH-Me2Fc+/0/Pt interfaces. The photocurrent transients for these interfaces decayed in less than 10 μs, and were limited by the series resistance of the cell in combination with the space charge capacitance of the semiconductor. An equivalent circuit model is presented and physically justified in order to explain this behavior, and to elucidate the conditions under which photocurrent transients at semiconductor electrodes can be expected to yield information regarding the faradaic charge transfer rate across the semiconductor/liquid interface. To provide additional support for the equivalent circuit representation, transient photocurrent responses are also presented for n-Si/Pt/NaOH(aq)/Ni(OH)2//Ni, n-TiO2//NaOH(aq)/Ni(OH)2//Ni and n-TiO2//NaOH(aq)Fe(CN)63-/4-fNi(OH)2/Ni contacts. Studies are reported on the behaviors of thin, nearly intrinsically doped Si electrodes having interdigitated n+ and p+ back contact points. An analysis of the factors governing the photocurrent directionality and photovoltage in these samples is presented. Additionally the back contact geometry has been exploited to perform measurements of the open circuit potential of either electrons or holes while the other carrier type was under electrical control. In combination with current density-voltage measurements of carriers passing through the back contact points, these data allowed a comparison of the behavior of a given carrier type when generated by an applied bias (i.e., as majority carriers) relative to their behavior when generated with band gap illumination of the solid (as minority carriers). The results have been used to validate certain key predictions of the quasi-Fermi level concept in photo-electrochemistry. In addition, digital simulations that include two-dimensional representations of the charge density distribution and of the current fluxes in the solid have been utilized to provide a quantitative understanding of the observed experimental behavior. The application of time-resolved photoluminescence to the study of InP interfaces is described. Photoluminescence decay profiles for etched n-type and p-type InP have been recorded. These data provide support for a bulk-recombination limited PL lifetime in p-InP, while that of n-InP is evidenced to be dominated by radiative recombination. Additional PL decay data are reported for a series of InP/liquid contacts. InP/CH3CN junctions having Me2Fc+/0, decamethylferrocene+/0 (Me10Fc+/0), methyl viologen (MV)2+/+, and cobaltocene (COCp2)+/0 as acceptor species have been studied. Quantitative information on the rate constant for charge transfer could not be obtained from these studies, but upper limits are suggested, and promising systems for further study are identified.