Abstract
The importance of specific ion adsorption and surface charge effects in the design and operation of photoelectrochemical (PEC) devices is demonstrated by experimental data on the n-GaAs/electrolyte and n-Si/electrolyte interface. The electrolyte chosen for the present study was an ambient temperature molten salt comprising mixtures of aluminum chloride and n-butyl pyridinium chloride in varying molar ratios. A direct correlation between specific adsorption effects and photovoltaic output parameters is presented for the n-GaAs PEC system. Evidence for specific adsorption of Cl− ions in this system is found in the systematic shift observed in flat-band potentials Vfb towards negative values with increasing concentration of free Cl− ions in the AlCl3-BPC electrolyte. The magnitude of the slope of Vfb versus pCl (=−log[Cl−]) plots [∼0.13 V or 2(2.3kT/q) V] is consistent with that expected from Esin-Markov adsorption behavior. Anomalous PEC behavior is observed at the n-Si/AlCl3-BPC interface brought about by modifications in electrostatics across the electrode/electrolyte interphasial region. These modifications arise from either specific adsorption effects or by electrodeposition of aluminum on the n-Si electrode surface. Either process results in a net lowering of the band structure in n-Si on the energy scale relative to the redox levels. An examination of literature data on high-efficiency PEC systems reveals that similar shifts in the relative positions of the semiconductor energy levels and redox energies may play an important role in ensuring a more favorable photoresponse than that predicted from idealized models.
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