Kesterite based materials gain more and more relevance in the pursuit of affordable, efficient and flexible absorber materials for thin film photovoltaics. Alloying Cu2ZnSnSe4 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 Cu2Zn(Sn,Ge)Se4 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.

Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both (Ag,Cu)2ZnSnSe4 and Cu2Zn(Sn,Ge)Se4 based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X-ray diffraction and low temperature X-ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu2Zn(Sn,Ge)Se4, the thin film absorbers show a very similar behavior as Cu2Zn(Sn,Ge)Se4 powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn-S bonds instead of Ge-S bonds. In (Ag,Cu)2ZnSnSe4, the dependence of the Ag-Se and Cu-Se bond lengths on Ag/(Ag+Cu) might indicate an energetic advantage in the formation of certain local configurations.