Defect formation energetics and carrier lifetime limitations in Pb-Sn mixed perovskites: a first-principles study

Abstract

Abstract Lead–tin (Pb–Sn) halide perovskites are promising absorbers for high efficiency solar cells due to their narrow bandgap (∼1.3 eV) and extended near-infrared absorption. However, performance and stability are critically limited by intrinsic point defects. Using first principles calculations with the HSE06 hybrid functional and spin–orbit coupling, defect formation energetics, migration pathways, and their influence on carrier recombination in MAPb 0.5 Sn 0.5 I 3 were systematically investigated. Results show that tin vacancies are the most favorable defects under iodine-rich conditions, introducing deep mid-gap states that reduce carrier lifetimes to 10–100 ns, corresponding to open-circuit voltage deficits of ∼0.2–0.3 V and limiting achievable efficiencies to below ∼18% without passivation. Halide vacancies, while more mobile, act as efficient non-radiative recombination centers and contribute to the ionic instability commonly observed in mixed Pb–Sn devices. Migration barrier calculations reveal facile iodine diffusion, whereas Sn vancancy defects are less mobile but significantly more detrimental when present. By correlating defect energetics with Shockley–Read–Hall lifetime modeling, the study establishes a unified first-principles framework that couples defect energetics, migration kinetics, and carrier lifetime limitations in Pb–Sn alloys. This framework clarifies why Pb–Sn perovskites suffer stronger recombination losses than Pb-only analogues and provides predictive design rules for overcoming these limitations: stabilizing Sn 2+ to suppress V Sn formation, introducing reducing additives (e.g., SnF 2 , KI), and applying surface passivation strategies to limit halide migration. This work presents a unified first principles workflow that connects defect formation energetics, charge transition levels, and SRH carrier lifetimes in mixed Pb–Sn perovskites, enabling a quantitative assessment of defect limited device performance, providing predictive design rules for next-generation single-junction and tandem perovskite solar cells.