Control of Defect-Mediated Charge Recombination in Kesterite Absorbers through Oxygen–Sodium Interplay

The performance of kesterite Cu₂ZnSnS₄ (CZTS) solar cells is limited by severe open-circuit voltage losses arising from defect-mediated nonradiative recombination. Using ab initio nonadiabatic molecular dynamics simulations, we elucidate how oxygen and sodium jointly regulate sulfur-vacancy-induced charge recombination at the atomistic level. Doubly positively charged sulfur vacancies induce outward relaxation of neighboring Sn cations, creating deep donor-like trap state through enhanced antibonding Sn-5s/S-3p hybridization and dramatically reducing carrier lifetime. Under oxygen-poor conditions, atomic oxygen passivates sulfur vacancies by restoring local tetrahedral coordination, eliminating deep traps, and extending carrier lifetimes by approximately 3-fold. In contrast, under oxygen-rich conditions, molecular oxygen dissociation produces interstitial oxygen that introduces mid-gap states via antibonding Cu-3d/O-2p interactions, accelerating recombination to subnanosecond time scales. Sodium stabilizes oxygen configurations by forming Na-O complexes, suppresses Cu-O antibonding, and restores carrier lifetimes to near-pristine values. These findings establish general principles for rational defect passivation in kesterite photovoltaics.