The influence of sodium in Cu(In,Ga)Se2 solar cells investigated by quantum efficiency measurements

Thin film solar cells based on Cu(In,Ga)Se2 (CIGSe) absorbers can be deposited on flexible plastic foils reaching efficiencies up to 17.6 %. The addition of sodium plays a key role for enhancing CIGSe solar cell perfomance. As polyimide is a sodium free substrate (in contrast to soda-lime glass) extrinsic incorporation of sodium is indispensable to achieve high efficiencies. The aim of this work is to analyze the influence of different sodium contents on photocurrent and defect characteristics by quantum efficiency (QE) measurements. We show that the sodium content influences the CIGSe bandgap, carrier collection in the long wavelength region of the QE spectra and the Urbach energy as a measure of structural disorder. Furthermore, we show how luminescence spectra can be calculated from QE data and how the sodium content affects peak intensities and energetic peak positions in those spectra.