Influence of Cu-Zn disorder in Cu2ZnSnSe4 absorbers on optical transitions: A spectroscopic ellipsometry study

Abstract Spectroscopic ellipsometry is applied to investigate the effect of Cu/Zn disorder in a Cu2ZnSnSe4 crystal on the optical transitions. We observe that the degree of order has an impact not only on the first optical transition E0 at the gamma point, which is the commonly discussed direct band gap in kesterites. It also varies the further optical transition E1 away from the gamma point. While E0 shows an increasing trend with higher degree of order, the transition E1 shows an opposite behavior, indicating a fundamental impact of disorder on the shape of the electronic bands of kesterite material.