Simulation of Ultra-thin Solar Cells Beyond the Limits of the Semi-classical Bulk Picture

The photovoltaic characteristics of an ultra-thin GaAs solar cell with gold back reflector are simulated using the standard semi-classical 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 semi-classical 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 semi-classical model, revealing the strong impact of the large built-in potential gradients in ultra-thin 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 semi-classical model, pointing at an essentially bulk-like transport mechanism.