Electrons and Holes under Illumination

While at real thermal equilibrium the Fermi levels of electrons and holes coincide, they differ in the steady state of illumination. This is shown in Fig. III.1 for an n-type semiconductor. One sees that the quasi-Fermi level deviates drastically for the minority carriers, while the effect on the majority carriers is small. 

Two different situations must be considered. One in which photoelectrolysis is done with an externally fixed potential of the semiconductor electrode. In this way, photocurrent voltage curves are normally measured in galvanic cells with semiconductor electrodes. 

The other situation corresponds to the photoelectrochemical cells for voltage generation. These contain semiconductor electrodes in contact with a redox system in which a photovoltage is obtained under illumination. 

If the Fermi level in the bulk of an n-type semiconductor is controlled by means of a potentiostat against a reference electrode in the electrolyte and such a potential is applied that a depletion layer is formed in the dark, the generation of electron hole pairs 

If a redox reaction with some components of the electrolyte can occur, the accumulation of minority carriers and the shift of the band edges due to their accumulation goes on until the photocurrent generated by the electron hole pair separation is compensated by the rate of the surface reaction. Such a situation is shown in Fig. III.3, where part of the photocurrent is lost by recombination via the anodic and cathodic process of the redox reaction. 

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