Severe nonradiative recombination loss occurs in ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$-based solar cells, which is related to the prevalent Cu-Zn cation disorder and the deep defect states. However, due to the inherent complexity of site-dependent defect energetics in disordered systems, the synergistic mechanism between these crucial defects remains elusive. Using rigorous first-principles calculations, we demonstrate that the prevalent cation disorder markedly enhances defect-assisted nonradiative carrier capture, increasing the capture coefficient by approximately 2 orders of magnitude compared to the ordered lattice. We show that cation disorder affects the nonradiative capture primarily by altering the transition energy through band fluctuations, softening the lattice, and amplifying the lattice relaxations during the defect charge-state transitions. Notably, we find that the lattice softening can suppress the nonradiative capture, whereas its effect is surpassed by the transition energy and lattice relaxations that have even stronger effects on promoting the capture. This work elucidates the impacts of cation disorder on defect-assisted nonradiative capture, and the mechanism presented here is crucial for defect characterization and efficiency improvement of kesterite-based photovoltaic devices.

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• Defects
• Electronic structure
• Doped semiconductors
• First-principles calculations