Amine Additive Reactions Induced by the Soft Lewis Acidity of Pb2+ in Halide Perovskites. Part II: Impacts of Amido Pb Impurities in Methylammonium Lead Triiodide Thin Films

The performance of lead halide perovskite optoelectronic devices continues to improve, yet the efficiencies are still well below the radiative limit. To approach the radiative limit, detailed understanding of impurities/defects and precise control over their concentrations are required. In Part I, we demonstrated that the soft Lewis acidity of Pb²⁺ induces a chemical reaction between PbI₂ and aliphatic amines producing Pb-alkylamide bonds which can be subsequently incorporated into thin films. Here, we investigate the consequences of these impurities in methylammonium lead triiodide (MAPbI₃) thin films. In particular, we link Pb-alkylamide impurities to an extrinsic degradation pathway resulting in Pb⁰ formation. The proposed mechanisms proceeds via β-C-H proton transfer reactions of the amido Pb species. Metallic Pb⁺/Pb⁰ defects acting as non-radiative recombination centers may limit the performance of many perovskite layers. However, optimal concentrations of Pb-methylamide impurities in sub-stoichiometric (slight excess of PbI₂) MAPbI₃ films passivate Pb⁰ defect formation, shown here to simultaneously correlate to improvements in photoluminescence lifetime. These results elucidate the beneficial properties of Pb-amide impurities in low concentrations and the sensitivity of halide perovskite materials to extrinsic defect chemistry.