Atomic scale structure and its impact on the band gap energy for Cu 2 Zn(Sn,Ge)Se 4 kesterite alloys

Kesterite based materials gain more and more relevance in the pursuit of affordable, efficient and flexible absorber materials for thin film photovoltaics. Alloying Cu 2 ZnSnSe 4 with Ge could allow controlled band gap engineering as already established for Cu(In,Ga)(S,Se) 2 based solar cells. This study investigates the local atomic arrangements of Cu 2 Zn(Sn,Ge)Se 4 alloys by means of low temperature Extended x-ray Absorbtion Fine Structure Spectroscopy. The element specific bond lengths are used together with x-ray diffraction data to derive the anion positions of the different local configurations. Ab initio theoretical calculations are performed to predict the influence of structural parameters such as anion position and lattice constants on the band gap energy. Combining the results of the experimental and theoretical studies suggests that the overall influence of the structural changes on the band gap bowing due to alloying is significant yet smaller than the total non-linear change of the band gap energy. Consequently, it is concluded, that band gap bowing stems from both structural and electronic changes.