  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.
