A comparison of the interface energetics for n-type cadmium sulfide/ and cadmium telluride/nonaqueous electrolyte junctions

Interface energetics for n-type CdS (E_g = 2.4 eV) and CdTe (E_g = 1.4 eV) have been compared in CH₃CN/0.1 M [n-Bu₄N]ClO₄ solutions containing low concentrations of fast, one-electron reagents having a wide range of formal potentials, E°. We find that n-type CdTe is blocking to the (dark) oxidation of the reduced form of any couple having E° in the range -2.0 to +0.7 V vs. SCE. Illumination of the CdTe with ≥E_g light, however, results in the oxidation of the reduced form of the couple and the photoanodic peak occurs ∼0.6 V more negative than the anodic peak at a reversible electrode such as Pt. The ∼0.6 V more negative anodic current peak is a measure of the open-circuit photovoltage that would be expected for an n-type CdTe-based photoelectrochemical cell employing some A⁺/A redox couple. A significant and essentially constant photovoltage independent of the E° of the couples studied leads to the conclusion that "Fermi level pinning" occurs for n-type CdTe/liquid junctions, as is also found for n-type CdTe/metal Schottky barriers. The crucial finding is that a photovoltage, insensitive to E°, is found for redox couples having E°'s that span a range of potential greater than the separation of the valence and conduction band, E_g. Fermi level pinning refers to a semiconductor/liquid or metal interface where photovoltage is insensitive to the contacting material. The effect is brought about by a sufficiently large density of surface states. By way of contrast, data for n-type CdS are consistent with those previously reported and indicate that CdS behaves as would be expected for a semiconductor essentially free of surface states. Photovoltage does depend on the E° of the contacting couple; only E°'s positive of the flat-band potential, E_FB ≈ -1-0 V vs. SCE, give a photovoltage. Surface states, though, do appear to play a role in facilitating reduction of the oxidized form of a couple at potentials positive of E_FB.