Fermi Level Splitting in the Semiconductor-Electrolyte Junction

5 Fermi Level Splitting in the Semiconductor-Electrolyte Junction 

The first term in Equation (18.14) corresponds to diffusion, and the second term to migration. The steady-state profile of holes across the space charge region is the result of a balance between the effect of the electric field, which pushes holes towards the surface, and the effect resulting in a steep gradient of concentration which tends to drive holes back in the other direction.  

The quasi-thermodynamic treatment outlined here leads to the prediction that a minimum light intensity is required to drive the minority carrier Fermi level to the point where electron transfer becomes thermodynamically feasible. However, as pointed out by Gregg and Nozik [22], such a threshold would be almost impossible to detect since the initial redox Fermi level for water splitting will be far from the standard value because the concentration of product (oxygen in the case of a photoanode) will be very small. 

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