Photovoltaic properties of anisotropic relaxation semiconductors

A relaxati\`on-case theory for the photovoltaic effect which occurs in anisotropic semiconductors is developed. Expressions for the short-circuit photocurrent and open-circuit photovoltage, valid for linear conductivity-locked transport, are obtained for the case of constant (depth-independent) conductivity anisotropy. These show that anisotropic relaxation semiconductors can generate large open-circuit photovoltages in some cases. A material in which the anisotropy factor a varies with depth is also considered and several specific cases are analyzed in detail. The properties of a new photovoltaic structure, an isojunction, emerge from this analysis. The isojunction, a structure composed of two different regions characterized by anisotropy factors of opposite sign and containing an isoplane at which $a=0$, possesses certain new and novel properties. This structure can, for example, generate photovoltages of opposite polarity when illuminated with light of different wavelengths. A new theory for the widely observed larger-than-bandgap "anomalous" photovoltages exhibited by many obliquely deposited polycrystalline semiconducting films is proposed. The theory is obtained by considering this class of photovoltaic films to be examples of anisotropic relaxation semiconductors. In a simple and self-consistent way, the new theory accounts in detail for the various aspects of observed behavior and yields good numerical estimates of photovoltages and photocurrents when relaxation-case values for parameters of the theory are employed.