Ideal behavior of the open circuit voltage of semiconductor/liquid junctions

Using simple bimolecular kinetic expressions for interfacial charge-transfer rates at semiconductor/liquid junctions, the authors present an analysis of the dependence of the open circuit voltage, V_oc, on the solution redox potential E(A⁺/A). It is found that if E(A⁺/A) is varied by using a series of redox couples with identical heterogeneous rate constants and the concentrations [A] and [A⁺] are held constant, V_oc should change as E(A⁺/A) is varied. However, if E(A⁺/A) is manipulated for a given redox system by changing [A⁺] and holding [A] constant, 'ideal' behavior predicts no change in V_oc at different solution redox potentials. These considerations are of importance both in diagnosing the presence of Fermi level pinning at semiconductor electrodes and in the design of photoelectrochemical cells for use in solar energy conversion.