  In order to optimize the optoelectronic properties of novel solar cell
architectures, such as the amorphous-crystalline interface in silicon
heterojunction devices, we calculate and analyze the local microscopic
structure at this interface and in bulk a-Si:H, in particular with respect to
the impact of material inhomogeneities. The microscopic information is used to
extract macroscopic material properties, and to identify localized defect
states, which govern the recombination properties encoded in quantities such as
capture cross sections used in the Shockley-Read-Hall theory. To this end,
atomic configurations for a-Si:H and a-Si:H/c-Si interfaces are generated using
molecular dynamics. Density functional theory calculations are then applied to
these configurations in order to obtain the electronic wave functions. These
are analyzed and characterized with respect to their localization and their
contribution to the (local) density of states. GW calculations are performed
for the a-Si:H configuration in order to obtain a quasi-particle corrected
absorption spectrum. The results suggest that the quasi-particle corrections
can be approximated through a scissors shift of the Kohn-Sham energies.
