Four-electron negative- U vacancy defects in antimony selenide

The phenomenon of negative- U behavior, where a defect traps a second charge carrier more strongly than the first, has been established in many host crystals. Here, we report the case of four-carrier transitions for both vacancy defects in Sb₂ Se₃ . A global structure searching strategy is employed to explore the defect energy landscape from first principles, revealing large atomic reconfigurations which facilitate a major charge redistribution. A thermodynamic analysis of the accessible charge states reveals a four-electron negative- U transition (Δq = 4) for both V_Se and V_Sb which, combined with previous calculations for antisites and interstitials, now demonstrates amphoteric behavior for all intrinsic point defects in Sb₂ Se₃ , with an impact on its usage in solar cells. The unusual behavior is facilitated by valence alternation, a reconfiguration of the local bonding environments, characteristic of both Se and Sb.