  A physical model is presented for a semiconductor electrode of a
photoelectrochemical (PEC) cell, accounting for the potential drop in the
Helmholtz layer. Hence both band edge pinning and unpinning are naturally
included in our description. The model is based on the continuity equations for
charge carriers and direct charge transfer from the energy bands to the
electrolyte. A quantitative calculation of the position of the energy bands and
the variation of the quasi-Fermi levels in the semiconductor with respect to
the water reduction and oxidation potentials is presented. Calculated
current-voltage curves are compared with established analytical models and
measurement. Our model calculations are suitable to enhance understanding and
improve properties of semiconductors for photoelectrochemical water splitting.
