Gradient Doping Strategy for Sn─Pb Mixed Perovskite Solar Cells with High Efficiency and Stability.

Sn─Pb hybrid perovskite has attracted more attention due to its ideal bandgap and excellent photoelectric properties. However, easy oxidation and poor crystallinity caused by the introduction of Sn²⁺ have become two major problems. In this study, Sn²⁺ is doped in the Pb-based perovskite to prepare high crystalline Sn─Pb mixed perovskite with larger grain size by using the solvent engineering technique and the cooperation optimization with HCOOH. The reducibility of HCOOH and its inhibition of deprotonation significantly prevent the oxidation of Sn²⁺ and the decomposition of A-site cations. The experimental and theoretical results show that the interactions between HCOOH and Sn²⁺ and Pb²⁺, which reduce the defect density and improve the crystallinity and stability of the film with excellent photoelectric properties. In addition, the compact SnO₂ prepared by atomic layer deposition as electronic transformation layer to further improve the stability of devices. The photoelectric conversion efficiency of the Sn─Pb hybrid perovskite solar cells (PSCs) prepared by the dopant growth method can reach 21.53% and the stability is significantly better than that of the Sn─Pb PSCs prepared by the traditional method.