Surface charge and specific ion adsorption effects in photoelectrochemical devices

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.