Ge-alloying-regulated band-edge dynamics suppress nonradiative recombination in kesterite Cu2Zn(GexSn1-x)S4.

Cu2ZnSn(S,Se)4 kesterite photovoltaics suffer from severe voltage losses associated with cation disorder, band-tail fluctuations, and nonradiative carrier recombination. Ge-alloying has been reported to improve absorber quality and reduce recombination losses, but the underlying carrier-dynamics mechanism remains unclear. Here, density functional theory and nonadiabatic molecular dynamics are used to investigate Cu2Zn(GexSn1-x)S4 in Cu/Zn ordered and disordered configurations. The calculations show that Cu/Zn disorder introduces local structural distortion, electrostatic perturbations, and low-frequency anharmonic phonon channels, thereby enhancing electron-vibrational coupling and accelerating nonradiative recombination. Because the conduction band minimum is dominated by Ge/Sn-s and S-3p antibonding states, Ge/Sn-S bond fluctuations regulate conduction band-edge dynamics more strongly than the Cu-3d/S-3p-derived valence band maximum. Increasing Ge content generally widens the bandgap and reduces nonadiabatic coupling, whereas the carrier lifetime varies nonmonotonically owing to the coupled effects of band-edge fluctuations, phonon spectral density, electronic delocalization, and dephasing-mediated dynamics. These results provide a microscopic explanation for Ge-alloying-suppressed recombination in kesterite photovoltaics.