The Energetics of p/n Photoelectrolysis Cells

The energetics of p/n photoelectrochemical cells, containing simultaneously illuminated p-type photocathodes and n-type photoanodes, have been investigated by using appropriate combinations of n-WSe₂, n-MoSe₂, n-WS₂, n-TiO₂, p-InP, p-GaP, and p-Si semiconductor electrodes. The open-circuit photovoltages (E_OCV) of the p/n cells were measured as a function of the redox reactions in the cell and as function of light intensity. For many semiconductor electrode combinations, the sum of E_OCV plus the standard cell voltage for the net cell reaction (ΔE°) was found to be constant for a given pair of n-and p-type electrodes at a given light intensity. This constancy was shown to be equivalent to the constancy of the sum of the band bending in the semiconductor depletion layers and the sum of the electrode overvoltages. These results are explained by movement of the semiconductor band edges with changes in the redox reactions that occur at the semiconductor electrode. If special care is taken to produce semiconductor electrodes that do not show surface charging effects, E_OCV was found to be independent of the redox reaction. This is the expected behavior for pinned band edges. The dependence of E_OCV on light intensity for p-InP/n-MoSe₂ cells was either about 60 mV or about 130 mV per decade increase in light intensity. The latter value occurred when both the p-InP and n-MoSe₂ band edges were pinned, while the former value occurred when only the p-InP band edge was pinned. These conditions for band-edge pinning depended upon light intensity and the nature of the electrolyte.