Highly Stable Hybrid Perovskite Solar Cells Modified with Polyethylenimine via Ionic Bonding

Abstract Organic‐inorganic hybrid perovskites are highly promising materials for photovoltaic applications, yet their rapid degradation remains a significant challenge. Here, three highly stable methylammonium lead iodide (MAPbI 3 ) perovskite films modified with polyethylenimines (PEIs) of different molecular weights (Mw=600, Mw=10000 and Mw=70000) were successfully obtained. The PEIs were used as an additive in the perovskite to determine their influences on the structure, film quality, and performance of the perovskite. The perovskite films modified with PEIs showed excellent stability, when studied over 20 days under ambient air conditions with a medial humidity of 60%. The results of XRD, UV‐Vis absorption and fluorescence lifetime measurements showed that the PEI could not substitute for MA within the perovskite crystal lattice. The solar power conversion efficiency (PCE) η of the pristine perovskite film and those modified with PEI (Mw=600), PEI (Mw=10000) and PEI (Mw=70000) were 9.69%, 6.84%, 6.43% and 6.94%, respectively. Even though the PCE is lower for the PEI‐modified solar cells, their stability is substantially improved. The improved stability is attributed to PEI‐H + cations in the grain boundary of the perovskites, preventing them from direct contact with water molecules that otherwise cause fast degradation. This demonstrates that PEI modification is a simple and powerful strategy for stabilizing perovskite films.