Electrode-oxide semiconductor bending interface

The development of photocathode materials for either single- or dual-absorber cells has also received considerable attention [80, 101, 102]. Thermodynamic equilibrium dictates that p-type semiconductors will exhibit upward band bending when in contact with a liquid electrolyte. This behaviour is the opposite to that of n-type semiconductors described previously, and will result in the movement of photogenerated electrons towards the semiconductor-electrolyte interface while the holes are driven into the bulk of the electrode, towards the electrical back contact. At the surface, provided that the energy carried by the electrons is sufficient, H2 is evolved. As discussed previously, one of the electronic properties of metal oxides that makes them suitable for water photo-oxidation purposes is the O 2p character of the valence electrons, which places the VB edge at potentials... 

Figures 2.1 and 2.2. The light-generated minority carriers diiFuse and drift towards the electrolyte interface where charge transfer to the respective species (oxidized electrons reduced holes) occurs. The majority carrier current results in injection of the opposite carrier (here electrons) at the counter electrode-electrolyte interface where the opposite redox reaction takes place. The semiconductor-electrolyte junction shown here is characterized by a photovoltage and a photocurrent, that is, the solar cell is operating at or near its maximum power which, in efficient devices, is rather close to the open circuit condition. This is indicated in the inset of Figure 2.12. Therefore, a residual band bending has been shown and the photovoltage under these conditions is given by the quasi-Fermi levels at the surface. Here, only the quasi-Fermi level for holes is shown because Hf( ) only marginally differs from Ef, the Fermi level without illumination.

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