Abstract
As a promising solar energy harvesting technology, solution-processed metal halide perovskites (MHPs) are of great current interest in developing low-cost and efficient photovoltaic cells. Despite their excellent optoelectronic properties and the nascent advancements in compositional tailoring and interfacial engineering to develop high-performance MHPs, issues associated with the long-term environmental stability of these materials are yet to be addressed. Here we examine the moisture-induced cascade degradation reactions over a year for methylammonium lead iodide (MAPbI3)- and formamidinium-rich [Cs0.05(MA0.17FA0.83)0.95Pb(Br0.17I0.83)3] formulations at 40 and 85% relative humidity (RH) in the air. The transformative reactions at 85% RH lead to chemical degradation process in both MA-rich and FA-rich perovskites, yielding to the different organic and inorganic byproducts within a few hours, but the exposure to 40% RH retains the longevity of these materials up to several months. The defect passivation by the tetrapropylammonium cation (TPA+) imparts enhanced stability of MAPbI3 particles, irrespective of the exposure conditions to water vapor. By resolving thin-film morphology at sub-nanometer to nanometer resolution using solid-state (ss)NMR spectroscopy and X-ray diffraction techniques, kinetics of degradation reactions and structural insights into the inorganic/organic interfaces and degradation products are obtained and compared. Our findings provide mechanistic details into the cascade degradation reactions in pristine and defect-passivated MHPs, enabling guidance for novel passivating and interfacial engineering strategies to further improve the robustness of the MHPs with respect to environmental stressors.