Behavior of Si Photoelectrodes under High Level Injection Conditions. 3. Transient and Steady-State Measurements of the Quasi-Fermi Levels at Si/CH3OH Contacts

Abstract

Real-time measurements of the photovoltage rise and decay at the back of lightly doped, thin, long lifetime Si photoelectrodes were recorded subsequent to a variety of spatial and temporal carrier generation impulses. The functional form of the rising portion of the photovoltage signal is sensitive to charge transport processes, and this signal was used to validate experimentally the hypothesis that charge transport in these samples under high level injection is primarily driven by diffusion, as opposed to drift. The decay of the photovoltage signal back to its equilibrium value yielded information concerning the surface recombination velocity, Sf, of the various Si/CH3OH redox couple contacts. These data validated the relatively high surface quality of the Si/liquid interface in contact with a variety of redox species. Furthermore, the low surface recombination velocities are in agreement with prior theoretical and experimental estimates of interfacial charge-transfer rate constants for semiconductors in contact with nonadsorbing, outer-sphere, redox species. The front surface recombination velocity data also provided a needed boundary condition for modeling the carrier concentration dynamics and allowed quantification of the difference between the quasi-Fermi levels at the back and front surfaces of the samples at all times of experimental interest. Digital simulation and analytical modeling were performed to compute the gradients in the quasi-Fermi levels for samples operated under steady-state, open-circuit, high level injection conditions. In no case was the difference between the quasi-Fermi level value at the back of the sample and its value at the solid/liquid contact greater than 10 meV. These data, combined with those described in parts 1 and 2, comprise a relatively complete picture of the transport and recombination processes that occur at these types of semiconductor/liquid contacts.