Impedance analysis of semiconductor electrodes in the accumulation region†

Given the interest in solar fuels production through electron transfer from the conduction band of semiconductor electrodes to reduce CO₂ or produce H₂, a theoretical and experimental examination has been made of these electrodes under an accumulation bias. This has been done with the use of a general model from the solid state physics literature that encompasses degeneracy situations in the electrode, a scope that is greater than the present model in use that assumes all donors are fully ionized. In an illustration of the aspects of these two models, experimental capacitance measurements with p-Si, n-Si, and n-InP have been made with a TBAPF₆ electrolyte in acetonitrile. A variation of the TBAPF₆ concentration under 0.50 M at the Si semiconductor electrode was used to control the capacitance of the Helmholtz layer and revealed that moderately doped semiconductors can only be biased −200 mV into accumulation before the applied potential induces band edge shifts with respect to a reference electrode. At degenerate n-InP electrodes, this shift begins at a lower potential negative of a flatband condition. The fully ionized model was found to fail with increasing bias in the accumulation region. The general model also describes expected behavior for the inversion region of these electrodes and the implications of its predictions in this regime are discussed.