  We discuss the effects of built-in fields and contact configuration on the
photovoltaic characteristics of ultrathin GaAs solar cells. The investigation
is based on advanced quantum-kinetic simulations reaching beyond the standard
semi-classical bulk picture concerning the consideration of charge carrier
states and dynamics in complex potential profiles. The thickness dependence of
dark and photocurrent in the ultra-scaled regime is related to the
corresponding variation of both, the built-in electric fields and associated
modification of the density of states, and the optical intensity in the films.
Losses in open-circuit voltage and short-circuit current due to leakage of
electronically and optically injected carriers at minority carrier contacts are
investigated for different contact configurations including electron and hole
blocking barrier layers. The microscopic picture of leakage currents is
connected to the effect of finite surface recombination velocities in the
semi-classical description, and the impact of these non-classical contact
regions on carrier generation and extraction is analyzed.
