A Neutral Barrier at CGS Grain Boundaries - Compositional and Structural Dependencies

Abstract

AbstractUnlike other photoactive layers (e.g. Si) polycrystalline chalcopyrite layers achieve higher efficiency than monocrystalline ones. Although grain boundaries may play an important role for this phenomenon, there is currently no commonly accepted model for the electronic structure of grain boundaries. First experimental results on CuGaSe2 absorbers indicated a small neutral barrier for majority carriers (20-40meV) at sigma 3 grain boundaries, which is the predominant grain boundary in polycrystalline chalcopyrite absorbers [1]. These results are in discrepancy with theory, which predicts a 10 times higher barrier; we suspect that the copper excess might reduce the barrier height.Here we present a study using Hall-effect and Kelvin Probe Force Microscopy (KPFM) measurements to investigate the composition dependence of the barrier height of epitaxially grown CuGaSe2 layers containing a sigma 3 grain boundary as a function of the Cu/Ga ratio. First results show that the barrier height is independent of the copper content. In addition, we present initial results on sigma 9 grain boundaries which are observed to occur in high efficiency absorbers.