Unpinning of Energy Bands during Illumination

The accumulation of minority carriers and the resulting shift of energy bands can be avoided if a suitable redox system is added to the electrolyte. For instance. 

The same type of Fermi level pinning also occurs when the surface of InP is reduced by cathodic polarization leading to a thin In layer, or when another metal, such as Pt, is deposited [49]. 

If all surface states are occupied by electrons their density is given by A, = AQs/e (Eq. 5.35). Assuming a Helmholtz capacity of Ch 10F cm one obtains /V, in the order of about lO cm 2 for a shift of ACfb = 0.2 V. It is interesting to note that the shift of the flatband potential occurs mostly at very low light intensities, and it saturates at higher light intensities because then all surface states are filled [45]. In Fig. 5.20 the shift of energy bands is indicated for some semiconductors. 

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