Flux enhancement and efficiency for photovoltaic cells

Radiation flux enhancement in a thin film with a rough upper surface and a diffusive perfectly reflecting lower surface is considered. In the case of a completely random surface, taken as a collection of randomly oriented smooth microscopic surfaces or facets, the flux enhancement factor is shown to be n²in the absence of absorption, where n is the refractive index. This result is derived from thermodynamic arguments as well as by making use of the Fresnel equations. In the cases where the upper surface is not completely rough but may be approximated by a set of facets whose orientations follow a Gaussian distribution, the flux enhancement factor is found numerically to be generally less than n². A general expression for efficiency is derived in the presence of absorption and numerical results are displayed graphically for silicon-type materials.