PT Unknown AU Seeger, J Piesch, U Kiowski, O Hariskos, D Witte, W Powalla, M Eraerds, P Lechner, R Dalibor, T Kalt, H Hetterich, M TI Suppression of interference effects in electroreflectance spectroscopy on Cu(In,mGa)(S, Se)₂ solar cell buffer layers PY 2018 LA english ER