The Concept of Fermi Level Pinning at Semiconductor/Liquid Junctions. Consequences for Energy Conversion Efficiency and Selection of Useful Solution Redox Couples in Solar Devices

Fermi level pinning refers to a situation where the band bending in a semiconductor contacting a metal is essentially independent of the metal even for large variation in the work function of the metal. We find that a similar situation sometimes results for a semiconductor contacting liquid electrolyte solutions containing redox couples having very different electrochemical potentials. When Fermi level pinning obtains, illumination of the semiconductor can result in an output photovoltage which is independent of the solution potential. Fermi level pinning is brought about by semiconductor surface states resulting in a situation where the potential drop across the semiconductor (band bending) is essentially fixed and the potential drop across the Helmholtz layer in the solution is thus the variable. Recently, workers in the field of semiconductor photoelectrochemistry have emphasized a limiting case of the model of the semiconductor/liquid interface where the drop across the semiconductor depends on applied potential; at equilibrium with the solution, the band bending is generally regarded as varying with changes in the solution potential by virtue of changes in the redox couple or simply changing the ratio of oxidized and reduced material. Fermi level pinning results in semiconductor/liquid interfaces which can be viewed as analogous to a Schottky barrier photocell in series with an electrochemical cell in that the extent to which a given redox process can be driven uphill is independent of the potential of the redox couple. Quantitative considerations show that a surface state density as low as ~10¹² cm^-2 is sufficient to result in Fermi level pinning. n-GaAs, p-GaAs, and p-Si are semiconductors that exhibit Fermi level pinning in liquid electrolyte solutions (CH₃CN/[n-Bu_4N]ClO₄) of redox reagents and these are among the materials known to exhibit Fermi level pinning when contacted by metals. Fermi level pinning has the disadvantage in practical terms of limiting photovoltage in optical energy conversion applications, but such a phenomenon allows the use of a very wide range of solution couples. Since Fermi level pinning results from surface states, changes in the surface brought about by deliberate surface chemistry may change the surface states and hence the photovoltage in solid-state and liquid-junction solar devices.