Simulation of Ultrathin Solar Cells Beyond the Limits of the Semiclassical Bulk Picture

The photovoltaic characteristics of an ultrathin GaAs solar cell with a gold back reflector are simulated using the standard semiclassical drift-diffusion-Poisson model and an advanced microscopic quantum-kinetic approach based on the nonequilibrium Green's function (NEGF) formalism. For the standard assumption of flat-band bulk absorption coefficient used in the semiclassical model, substantial qualitative and quantitative discrepancies are identified between the results of the two approaches. The agreement is improved by consideration of field-dependent absorption and emission coefficients in the semiclassical model, revealing the strong impact of the large built-in potential gradients in ultrathin device architectures based on high-quality crystalline materials. The full quantum-kinetic simulation results for the device characteristics can be reproduced by using the NEGF generation and recombination rates in the semiclassical model, pointing at an essentially bulk-like transport mechanism.