Nonlinear effects in modeling thin-film graded-bandgap solar cells

We model the effect of concentrated sunlight on CIGS thin-film graded-bandgap solar cells using an optoelectronic numerical model. For this purpose it is necessary first to solve the time-harmonic Maxwell equations to compute the electric field in the device due to sunlight and so obtain the electron-hole-pair generation rate. The generation rate is then used as input to a drift-diffusion model governing the flow of electrons and holes in the semiconductor components that predicts the current generated. The optical submodel is linear; however, the electrical submodel is nonlinear. Because the Shockley-Read-Hall contribution to the electron-hole recombination rate increases almost linearly at high electron/hole densities, the efficiency of the solar cell can improve with sunlight concentration. This is illustrated via a numerical study.