The Mechanism of Cell Operation

All potentials given in Fig. 3 are referenced to the potential at the interface between the semiconductor and the current collector. This choice of reference potential is arbitrary, and is used here to emphasize the degree of band bending and straightening in the semiconductor. A number of researchers (see, e.g., Refs. 17 and 19) have reported that the potential of the solution is independent of current and illumination intensity when referenced to an external quantity such as the Fermi energy of an electron in vacuum. This concept does not have strict thermodynamic validity because it depends upon the calculation of individual ionic activity coefficients38 however, it has proved useful for the prediction of the interaction among semiconductors and a variety of redox 

Illumination under open-circuit conditions produces electron-hole pairs, which are separated by the potential gradient (see Fig. 3). The concentration of holes increases near the interface, and the concentration of electrons increases near the current collector (curve b in Fig. 4). Under steady-state conditions, the rate of generation of electron-hole pairs is balanced by the rate of homogeneous and interfacial recombination. As the system without kinetic limitations approaches short circuit (curve c in Fig. 4), the concentrations of holes and electrons approach the equilibrium distributions. 

The potential and concentration distributions described for the system with no kinetic limitations to interfacial reactions are constrained by the rates of generation and mass transfer in the semiconductor. More generally, kinetic limitations to interfacial reactions are compensated by the increased interfacial potential and concentration driving forces required to allow passage of electrical current. In contrast to the results shown as curve c in Fig. 4, the surface concentration of holes under kinetic limitations to interfacial reactions can increase with increasing current density.37 The presence of these limitations may be inferred from experimental data by inflection points in the current-potential curve. 

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