A simple model of a graded band-gap pn-heterojunction cell

Alloy semiconductors with spatially varying composition represent a very general class of semiconductors having position dependent material parameters. The current-voltage characteristics of a pn-junction is constructed with the aid of drift-diffusion theory. The one dimensional analysis is based on the integration of the differential equations for the quasi-fermi levels. The result is a formally exact, simple equation for the current density, which can be easily used for practical calculations. The treatment introduces a spatial collection efficiency function which gives the probability that a carrier generated at a given point in the cell will contribute to the current. The collection efficiency is closely related to the spectral response function. The results predicted by the model are compared to those obtained from the full numerical simulation