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, S f, 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.

Behavior of Si Photoelectrodes under High Level Injection Conditions. 2. Experimental Measurements and Digital Simulations of the Behavior of Quasi-Fermi Levels under Illumination and Applied Bias

“…[2][3][4][5][6][7][8][9] This paper describes experimental data and digital simulations of the carrier generation, transport, and recombination processes in such systems. The simulations have been performed to confirm quantitatively some of the approximations advanced in the preceding and present papers 1,[10][11][12] and to provide insight into some unexpected aspects of the behavior of quasi-Fermi levels that have been observed during the course of this study.…”

“…These results are relevant to the controversy regarding the thermodynamic interpretation of the quasi-Fermi levels in an operating, illuminated photoelectrochemical cell and are of fundamental interest with respect to the behavior of the quasi-Fermi levels under an applied bias. − This paper describes experimental data and digital simulations of the carrier generation, transport, and recombination processes in such systems. The simulations have been performed to confirm quantitatively some of the approximations advanced in the preceding and present papers ,− and to provide insight into some unexpected aspects of the behavior of quasi-Fermi levels that have been observed during the course of this study.…”

“…The bulk low-level minority carrier lifetime of these Si samples was 1 ms, and this was represented in the simulation by setting the nonradiative (Shockley−Read−Hall) bulk lifetime τ n = τ p = 1 ms . The front surface recombination velocity, describing recombination mediated by surface states, was chosen to be 100 cm s -1 , which corresponds to the experimentally measured value for these Si/CH 3 OH contacts. ,,, Other specific input parameters for the simulations of this work are provided in the appropriate sections of the paper.

2 Schematic representation of the band diagram of the high purity Si/liquid junction under high level injection conditions.

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Reactions of Etched, Single Crystal (111)B-Oriented InP To Produce Functionalized Surfaces with Low Electrical Defect Densities

“…D. Surface Recombination Velocity Measurements. Time-resolved photoluminescence measurements were performed using a time-correlated single-photon-counting apparatus that has been described previously. , Rhodamin 6G (Exciton) was used in the dye laser to provide an excitation at 600 nm. The repitition rate was set to 152 kHz to ensure that the delay between pulses was substantially larger than the time scale for the observed InP luminescence decay (6 ms).…”

Effects of Polyfluorene Polyelectrolyte Interfacial Layers on Selectivity and Recombination Measured Using the Interdigitated Back-Contact Solar Cell

“…The high-purity silicon absorber does not contribute to the selectivity because under normal operating conditions it is driven into high injection conditions, where the concentrations of electrons and holes, unlike in a doped semiconductor, are equal. The use of the IBC solar cell in a three contact arrangement was introduced by Lewis and coworkers in the fundamental study of electron- and hole-transfer processes at semiconductor/liquid redox couple interfaces. − A focus of their work was to describe the relative roles of built-in electric fields and kinetic asymmetries in the development of photocurrent and photovoltage at silicon photoelectrodes. More specifically, they studied the kinetic asymmetry associated with the relative rates of electron and hole transfer from the semiconductor electrode to liquid-phase redox couples, which as defined below, is a measure of carrier selectivity.…”