Electron transfer at illuminated semiconductor electrodes

When a photon with energy exceeding that of the bandgap is absorbed in a semiconductor, an electron is promoted from the valence band to the conduction band, creating an electron hole pair. In an n-type semiconductor, the electron is the majority carrier and the hole is the minority carrier the converse is true for a p-type semiconductor. The minority 

The Gartner equation (equation (8.6)) provides a satisfactory description of the photocurrent voltage curves measured at many semiconductor electrodes, but it contains no information about electron transfer kinetics because its derivation involves an a priori assumption about the boundary conditions. 

Under steady state conditions, the concentration of holes at the surface is determined by the rate of their arrival from the bulk and on the rate of their removal by electron transfer and surface recombination. It is usually assumed that the potential distribution across the semiconductor/electro-lyte junction is not affected by illumination under potentiostatic conditions, 

Many photoelectrochemical reactions of interest involve multistep electron transfer reactions. Examples that have been studied are hydrogen evolution on InP [29] and Si [30], oxygen evolution on Ti02 [31] and photodecomposition of CdS [32]. These reactions involve the formation of charged surface bound intermediates, and the accumulation of surface charge modified the potential distribution across the interface. For example, the photodecomposition of n-CdS is believed to occur by the route 

The Cd+ state corresponds to a surface trapped hole, and its concentration under steady state conditions will depend on illumination intensity and the rate constants for the two steps. 

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