Atomic scale structure and its impact on the band gap energy for Cu₂Zn(Sn,Ge)Se₄ kesterite alloys

Abstract

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.

Atomic Scale Structure of (Ag,Cu)2ZnSnSe4 and Cu2Zn(Sn,Ge)Se4 Kesterite Thin Films

“…The average cation-Se bond lengths of the Cu 2 Zn(Sn,Ge)Se 4 thin films are qualitatively very similar to the previously published results on Cu 2 Zn(Sn,Ge)Se 4 powders shown in Figure 2 as open symbols (Ritter et al, 2020). The fact, that most thin film results are slightly lower than their corresponding powder values, may stem both from experimental differences like fluorescence vs. transmission EXAFS measurements or from physical differences between the samples like their composition.…”

“…Element specific average cation-Se bond lengths obtained from the low temperature EXAFS measurements of (Ag,Cu) 2 ZnSnSe 4 and Cu 2 Zn(Sn,Ge)Se 4 thin film samples are plotted in Figure 2 as full symbols. Additionally, the results from a previous EXAFS study on Cu 2 Zn(Sn,Ge)Se 4 powder samples are plotted as open symbols (Ritter et al, 2020). For most of the elements, the overall uncertainties are in the order of the symbol size in Figure 2 and are therefore not plotted.…”

“…The estimated overall uncertainties comprise the influence of all analysis parameters as well as the fitting uncertainty and they are lower than 0.0025 Å whenever they are not depicted explicitly. A major difference between the two sample sets is residual sulfur in the thin film samples (S/(Se+S)< 10 %) (Hages et al, 2013), which is not present in the powder samples (Ritter et al, 2020). To account for this effect, a modified structural model was tested for all thin film samples.…”

“…The influence of structural parameters smaller than the unit cell dimensions has been proven to be important in Cu(In,Ga)(S,Se) 2 (Wei et al, 1998;Vidal et al, 2010) as well as in kesterite type compounds (Botti et al, 2011). For Cu 2 Zn(Sn,Ge)Se 4 powder samples a significant part of the band gap bowing parameter could be attributed to the structural changes due to alloying (Ritter et al, 2020). The elements sharing a lattice site in the (Ag,Cu) 2 ZnSnSe 4 and Cu 2 Zn(Sn,Ge)Se 4 alloys are very different in size (Shannon, 1976), which must impose changes in both the long range structure and the atomic scale structure of the material.…”

“…This work uses Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) to determine the element specific average bond lengths for all cations in technologically relevant (Ag,Cu) 2 ZnSnSe 4 and Cu 2 Zn(Sn,Ge)Se 4 thin films. This method has been successfully applied to both kesterite (Zalewski et al, 2010;Ritter et al, 2020) and chalcopyrite (Eckner et al, 2013;Schnohr, 2015) materials to probe for subtle differences in atomic scale structure as a function of elemental concentrations. Additionally, X-ray diffraction was employed to measure the lattice parameters of the samples and thus to evaluate the change of the long range crystallographic structure with changing alloy composition.…”