Further mechanistic studies of n-type silicon photoelectrodes: behavior in contact with methanol-dimethylferrocene^(+/0) and in contact with aqueous electrolytes

Abstract

Mechanistic studies of n-Si photoelectrodes have been conducted using aqueous and nonaqueous electrolytes. In contact with the CH₃OH-dimethylferrocene (Me₂Fc)^(+/0) electrolyte, the dark current and open-circuit voltage (V_(oc)) were not dependent on acceptor concentration for [Me₂Fc⁺] < 0.010 M and were only weakly dependent on acceptor concentration for [Me₂Fc⁺] 0.010 M. For [Me₂Fc⁺] between 0.3 mM and 0.050 M, the temperature dependence of V_(oc) indicated that bulk-diffusion/recombination was the rate-limiting recombination process. In all of these experiments, conventional Shockley diode theory provided an excellent description of the solid/liquid junction properties. In a related set of experiments, HF-etched n-Si photoelectrodes ((100)- and (11l)-oriented samples) and n-Si samples that had been metalized by several different methods all showed passivation in contact with Fe(CN)₆^(3-/4-) (aq) Br₂/Br⁻(aq) electrolytes. For several metalized Si samples, etches that had been reported to produce metal islands on the Si surface instead yielded Si surfaces free of metal. All Si samples that had been metalized by filament evaporation yielded I-V behavior in contact with the CH₃OH-Me₂Fc^(+/0) redox system that was characteristic of a pinned surface Fermi level, even for coverages of metal as low as 9 Â. This indicates that proposed metal-insulator-semiconductor junctions with anomalously high barrier heights are difficult to obtain by such metalization and etching methods.

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