Theory of High Photovoltages in Semiconducting Films

A theory of photo-e.m.f. much exceeding the energy gap is given for various semiconducting films based on the assumption of periodicity of the film parameters. The equations describing space charge, transport, and generation-recombination of current carriers are solved rigorously under periodic boundary conditions. The layer can be treated as an array of photocells, forming effectively an integrated circuit. Expressions for photovoltage are given and discussed in the limits of low and high illumination. Some integral inequalities being the conditions for the solutions to have a physical meaning are discussed. It is found that a phase shift between space charge and generation-recombination distributions as functions of x is essential for the occurrence of high photovoltage. The space charge may be due either to charged donor and acceptor centres and occur already in dark or may be due to light induced trapped charge. Rise and decay of photovoltage are discussed and the possibility of obtaining fast effects is indicated.