Intrinsic Point Defects and Frenkel Pair Formation in Photovoltaic Absorber Zn3P2: Regulating p-type Conductivity

Abstract

This study investigates the ground-state energetics and thermodynamics of intrinsic point defects in zinc phosphide Zn₃P₂ using ab initio density functional theory combined with an extensive potential energy landscape search. Our analysis reveals that the defect chemistry is dominated by zinc vacancies V_Zn and zinc interstitials Zn_i, with equilibrium concentrations significantly surpassing those of other intrinsic species. Notably, we find that phosphorus interstitials P_i, previously suggested to be significant, possess high formation energies and likely exist only in negligible quantities. The characteristic p-type conductivity of undoped Zn₃P₂ is shown to be a direct consequence of zinc vacancies,which act as shallow acceptors and pull the Fermi level toward the valence band. Furthermore, we identify a positive binding energy between V_Zn and Zn_i, leading to the formation of electrically benign Frenkel pairs that partially compensate the intrinsic p-type conductivity. Our results suggest that achieving n-type conductivity is fundamentally limited by these thermodynamic constraints. We conclude that hole densities can be optimized through phosphorus-rich growth conditions and high-temperature annealing, and suggest that future photovoltaic strategies should prioritize interface engineering over bulk n-type doping.