Analysis of time-resolved photocurrent transients at semiconductor/liquid interfaces

Small signal photocurrent transients have been measured for n-Si/CH₃OH-Me₂Fc^+/0/Pt, n-Si/Au/ CH₃OH-Me₂Fc^+/0/Pt, n-Si/Pt/NaOH(aq)/Ni(OH)₂/Ni, n-TiO₂/NaOH(aq)/Ni(OH)₂/Ni, and H-TiO₂/ NaOH(aq)-Fe(CN)₆ ^3-/4-/Pt cells. Even though the radio-frequency and microwave conductivity signals for photoexcited n-Si/CH₃OH-Me₂Fc^+/0 contacts persist for > 100 μs, the photocurrent transients for these interfaces decayed in <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. The model is also used 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.