Semiconductor-electrolyte interface SEI

When the semiconductor and redox solution are brought in contact with each other, their Fermi levels must match at equihbrium. If they differ initially, then charge transfer must take place between these phases to eliminate the disparity in their Fermi levels. For example, if 

the SEI can be modeled as two parallel plate capacitors in series. Because the value of the space charge region capacitor is generally smaller than that of the Helmholtz layer, it dominates the overall capacitance of the SEI. 

One of the important advantages in using a semiconductor as an electrode is that the energetics of the SEI can be modulated by applying a potential bias. 

The capacitance of the space charge region can be related to A(/ by the Mott-Schottky equation 

a plot of 1 / vs. applied potential E (a Mott-Schottky plot) can be used to determine the Efb of the SET 

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Figure 9.26 Schematic diagrams for energy vs. density of states for semiconductor-electrolyte interface (SEI). (a) An ideal interface described by Gerischer s model, where direct transfer of charge between bulk energy states of semiconductor and redox takes place, and (b) transfer of charge mediated through surface states. The hashed curves represent filled states. 2 represents solvent reorganization energy from Marcus theory. Adapted from reference (40).

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