Peculiar Bond Length Dependence and its Impact on the Band Gap Bowing in (Ag,Cu)(In,Ga)Se₂ Thin Film Alloys

The incorporation of Ag into Cu(In,Ga)Se₂ absorbers enables a reduced deposition temperature, which benefits the growth of thin film solar cells. Furthermore, Ag alloying leads to a wider optical band gap and improves the open-circuit voltage and thus the photovoltaic conversion efficiency. Contrary to other semiconductor alloys, however, the band gap increase occurs even though the crystal lattice expands with increasing Ag content. Moreover, the Ga-Se bond length of (Ag,Cu)GaSe₂ is predicted by theoretical calculations to decrease with increasing Ag content [1]. This prediction is counterintuitive, since in other chalcopyrite alloys, for example Cu(In,Ga)Se₂, all bond lengths increase as the lattice expands [2]. To unravel this mystery, we studied the atomic-scale structure, especially the element-specific bond lengths, of (Ag,Cu)GaSe₂, (Ag,Cu)InSe₂ and Ag(In,Ga)Se₂ alloys using extended X-ray absorption fine structure spectroscopy (EXAFS). The thin films were grown on Mo-coated soda lime glass by a single stage co-evaporation process [3]. The average Ag-Se and Cu-Se bond lengths determined by EXAFS clearly increase with increasing Ag content in both (Ag,Cu)GaSe₂ and (Ag,Cu)InSe₂, whereas the Ga-Se and In-Se bond length decreases despite the expansion of the chalcopyrite lattice. Thus, (Ag,Cu)GaSe₂ thin film alloys indeed exhibit the counterintuitive bond length dependence predicted by theoretical calculations and (Ag,Cu)InSe₂ shows the same behavior. In contrast, Ag(In,Ga)Se₂ exhibits a bond length dependence similar to that previously observed for Cu(In,Ga)Se₂. This clearly demonstrates that the peculiar behavior of a decreasing bond length with increasing crystal lattice is related to the mixing of the group-I lattice site, independent of the nature of the group-III atom. Furthermore, the element-specific bond lengths determined by EXAFS enable us to model the anion positions of the different local motifs present in the alloys and to estimate their contribution to the band gap bowing. Our study thus provides unique and systematic insight into the local structure of the (Ag,Cu)(In,Ga)Se₂ alloy system, including its impact on a key material property.<br/><br/>[1] S. Chen <i>et al.</i>, Phys. Rev. B 75, 205209 (2007).<br/>[2] C. S. Schnohr <i>et al.</i>, Phys. Rev. B 85, 245204 (2012).<br/>[3] J. H. Boyle <i>et al.</i>, J. Appl. Phys. 115, 223504 (2014).