Selenium and the role of defects for photovoltaic applications

We present first-principles calculations of the electronic properties of trigonal selenium with emphasis on photovoltaic applications. The band gap and optical absorption spectrum of pristine selenium is calculated from many-body perturbation theory yielding excellent agreement with experiments. We then investigate the role of intrinsic as well as extrinsic defects and estimate the equilibrium concentrations resulting from realistic synthesis conditions. The intrinsic defects are dominated by vacancies and we show that these do not result in significant nonradiative recombination. The charge balance remains dominated by vacancies when extrinsic defects are included, but these may give rise to sizable nonradiative recombination rates, which could severely limit the performance of selenium based solar cells. Our results thus imply that the pollution by external elements is a decisive factor for the photovoltaic efficiency, which will be of crucial importance when considering synthesis conditions for any type of device engineering.