Photoelectrochemical Behavior of n-GaAs Electrodes in Ambient-Temperature Molten-Salt Electrolytes

Article abstract of DOI:10.1021/ja00534a020

Photoelectrochemical (PEC) characterization of n-GaAs electrodes was carried out in room-temperature molten-salt electrolytes by using the aluminium chloride- butylpyridinium chloride (AlCl3-BPC) system as a representative example.The working potential limits for the above electrodes in the melts, containing varying ratios of AlCl3 and BPC, were established by cyclic voltammetry.Flat-band potential (V_fb) measurements on n-GaAs in the same melts enabled location of the semiconductor band edge positions relative to the melt stability windows.In electrolytes containing AlCl3 and BPC in the 1:1 molar ratio, the available range of potential was wide enough to probe the entire band-gap region.On the other hand, the potentials corresponding to the conduction band edges of n-GaAs were beyond the cathodic stability limit of both the 2:1 and 0.75:1 AlCl3-BPC compositions.The electrode dissolution behavior of illuminated n-GaAs electrodes was investigated by cyclic voltammetry in melts of varying composition containing no intentionally added electroactive species.The onset of photoanodic corrosion currents was significantly positive of the values observed in aqueous electrolytes.The redox behavior of ferrocene-ferricenium ion couple (Fe(Cp)2/Fe(Cp)2⁺) was studied by cyclic voltammetry on vitreous carbon electrodes in the 2:1, 0.75:1, and 1:1 AlCl3-BPC electrolytes.This couple was electrochemically reversible in the above melts.Photogenerated holes on n-GaAs electrodes were competitively captured by the electrochemically active ferrocene species in the 1:1 AlCl3-BPC electrolyte.This redox reaction occurred quite efficiently at the expense of the parasitic electrode dissolution process as judged by the constancy of photocurrents in a n-GaAs<*>1:1 AlCl3-BPC<*><*>Fe(Cp)2/Fe(Cp)2⁺<*>C PEC cell under short-circuit conditions for periods up to ca. 1 month.The uderpotentials developed for the photoanodic process on n-GaAs relative to the reversible (dark) thermodynamic values on vitreuos cabon were direct evidence for the sustained conversion of light energy to electrical energy.Rectifying behavior was observed on n-GaAs electrodes, with the reduction waves corresponding to the dark reduction of ferricenium chloride.The occurence of this process at potentials positive of V_fb indicated the mediation of surface states in the electron transfer process.Nonoptimized n-GaAs<*>1:1<*< AlCl3-BPC<*><*>Fe(Cp)2/Fe(Cp)2⁺<*>C PEC cells typically showed open-circuit potentials of 680 mV, fill factors around 0.49, and a net optical-to-electrical conversion efficiency of ca. 1percent under illumination with a tungsten lamp at 40 mW/cm².