Cu–Zn Cation Disorder in Kesterite Cu₂ZnSn(S_xSe_1–x)₄ Solar Cells

Cu–Zn cation disorder plays a vital and controversial role in kesterite CuZnSn(S_1–xSe_x)₄ solar cells. We demonstrate using density functional theory and nonadiabatic molecular dynamics simulations that the Cu–Zn disorder across different planes (i.e., Cu–Sn and Cu–Zn planes) is significantly more detrimental to device performance than the case when disorder is confined only to the Cu–Zn planes. The main reason is that different plane disorder induces a significant elongation of Sn–S/Se bond lengths, leading to a downshift of the conduction band minimum, decreasing the band gap, and reducing the optical absorption. Moreover, Cu–Zn disorder across different planes accelerates nonradiative electron–hole recombination and decreases charge carrier lifetime due to the reduction of the band gap and enhanced electron-vibrational interaction. Our results provide a theoretical explanation for the influence of Cu–Zn disorder on material performance and offer valuable insight into the design of more efficient solar cells.