Studies of Charge Transfer at Semiconductor/Liquid Junctions

Steady-state current-voltage measurements on two types of semiconductor/liquid junction solar cells are presented. Results of these and other experiments on single-crystal electrodes are discussed in terms of present kinetic theory on charge transfer at these junctions. Nanoporous titanium dioxide electrodes have been sensitized with a series of dyes with varying ground state reduction potentials. The spectral response and current-voltage properties of these electrodes have been measured. The trends can be explained by the differences in absorption spectra and ground state potentials. The effect of pH on the conduction band edge position of nanoporous titanium dioxide has been examined through current-voltage measurements and luminescence studies on dye-sensitized electrodes. The results are discussed in the framework of current theories on increasing efficiency of these electrodes. Studies involving charge separation at the back electrode contact were also performed. Predictions of the Marcus/Gerischer theory for photoelectrode stability have been investigated experimentally for n-Si/CH3OH photoelectrochemical cells. Specifically, a semiconductor electrode is predicted to be more stable if the reorganization energy of the stabilizing agent is decreased (in the normal region of the Marcus behavior), thereby increasing the rate of minority carrier capture by the stabilizer. This prediction was quantified experimentally by monitoring the branching ratio between two competing reactions at a semiconductor/liquid interface. The experimental observations can be consistently explained through this predicted influence of the minority carrier acceptor reorganization energy on the interfacial charge transfer rate constant. Interfacial charge transfer rate constants at indium phosphide/liquid junctions were investigated using current density-potential and differential capacitance-potential methods. The measured electron transfer rate constants, ket, for these systems were ≈10-16 cm4 s-1, in excellent agreement with previous theoretical predictions. Contacts which did not display bimolecular kinetic behavior still yielded upper bounds on the charge transfer rate constants, which were also consistent with limits predicted by theory. The current density potential behavior of n-InP and p-InP/Fe(CN)63-/4-(aq) junctions was also examined in order to assess the validity of prior kinetic measurements on these interfaces.