Photocurrents on WO3 electrodes

In Section 7.4 we gave only a brief outline of the photoeffects caused by electron-hole generation by photons with an energy above that of 

Diffusion of carriers that are generated outside the space-charge region. 

Loss of carriers either by electron-hole recombination or by trapping at localized states in the band gap or at the surface. 

The rate of the electrochemical reaction that consumes the carriers. 

When all these factors contribute, the situation becomes almost hopelessly complicated. The simplest realistic case is that in which the photocarriers are generated in the space-charge region and migrate to the surface, where they are immediately consumed by an electrochemical reaction. We consider this case in greater detail. Suppose that light of frequency i/, with hu Eg, is incident on a semiconducting electrode with unit surface area under depletion conditions (see Fig. 8.8). Let Iq be the incident photon flux, and a the absorption coefficient of the semiconductor at frequency v. At a distance x from the surface, the photon flux has decreased to Iq exp(—ax), of which a fraction a is 

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