Fast Photocurrent Transients in Photoelectrochemical Cells with Semiconductor and Insulator Electrodes

Abstract

Photocurrent transients have been measured with better than 1 nanosecond time resolution in photoelectrochemical cells with insulator and semiconductor electrodes. The shape of the transients observed with semiconductor electrodes in the time window of several nanoseconds can be deduced from a simple equivalent circuit. The transient observed in the external circuit consists of two contributions, the first is the time integral and the second is directly proportional to the actual photoresponse. At low light intensity good semiconductor electrodes show only the first contribution. There is no direct influence of charge carrier discharge or of the Helmholtz‐layer as such on the fast transient in the nano‐second range.

Conditions Under Which Heterogeneous Charge-Transfer Rate Constants Can Be Extracted from Transient Photoluminescence Decay Data of Semiconductor/Liquid Contacts As Determined by Two-Dimensional Transport Modeling

“…An underlying assumption in the analysis of the data leading to these very large apparent charge-transfer rate constants is that the real-time photoluminescence decay data is sensitive to, and can be used to evaluate quantitatively, the values of S min and k ct for a semiconductor under the experimental conditions of interest. This assumption appears to be in contradiction to prior experimental work, which has demonstrated that the transient photoluminescence decay signals, , and the transient photocurrent signals, , of a strongly depleted semiconductor/liquid interface under low- or moderate-level injection conditions are insensitive to the value of k ct under most experimentally accessible conditions.…”

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

“…For conventional photoelectrodes under low level injection conditions, the photovoltage exhibits an extremely rapid rise time which is usually limited by the bandwidth of the measuring instrumentation. This limitation occurs because the photovoltage under low level injection is produced predominantly by charge separation across the depletion region, driven by drift, which occurs in <1 ps. , In the absence of a significant interfacial electric field to separate charge, however, diffusion processes are required for effective charge movement. This mode of transport should result in a relatively slow rise time of the photovoltage, with a functional form predictable by standard diffusional models.…”

“…Real-time measurements of the photovoltage rise and decay at the back of the sample were performed in response to a variety of spatial and temporal carrier generation impulses (Figure ). The functional form of the rising portion of the photovoltage signal is sensitive to charge transport processes, , and this signal was therefore 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 additional information concerning the surface recombination velocity, S f , of the various Si/CH 3 OH contacts.…”

Modeling of Electrical Transients in the Semiconductor/Electrolyte Cell for Photogeneration of Charge Carriers in the Bulk

“…In the simplest approach the RC-controlled time-dependent behavior of the semiconductor/electrolyte system can be modeled with an equivalent circuit, where the spatial separation of photogenerated electron-hole pairs is represented by two capacitors, the first for the depletion layer in the semiconductor and the second for the Helmholtz layer at the solution side of the interface. Several authors [2][3][4][5][6][7][8] have confirmed the usefulness of this approximation for modeling the RC-controlled time-dependent behavior of the system. However, the time dependence of the electrical response caused by the most important physical processes occurring in this system, e.g.…”