Mixed Sulfur/Selenium Anions Weaken Electron-Vibrational Interaction in Cu2ZnSn(S,Se)4 Photoabsorber

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar absorbers have attracted intensive investigations for next-generation photovoltaic applications. Here, by using ab initio static and molecular dynamics simulations, we investigated the anion compositional dependence of electron-vibration interaction in CZTSSe materials. We found that the conduction band fluctuates more than the valence band, and as a result, the band gap variation is more sensitive to the change of the former, which can be understood in terms of p-d hybridization in the valence bands. Electron-phonon coupling is smaller in CZTSSe alloy compared to pure S- or Se-containing structures, as evidenced by the smaller fluctuation of excitation energy, and can be attributed to the weaker structural dynamics of the metal-anion bond. Small electron-phonon coupling strength may lead to better charge transport in these materials. We also elucidated the interplay between disordered structures and S/Se stoichiometry through analysis of optical line width. The results highlight the importance of anion composition engineering and provide new insights into the rational design of high-performing kesterite absorbers for solar cells.