Improving Thermal Stability of Perovskite Solar Cells by Suppressing Ion Migration Using Copolymer Grain Encapsulation

Thermal stability of organic-inorganic hybrid perovskites (OIHPs) remains as one of the critical challenges against the stable operation of perovskite solar cells (PSCs) in direct sunlight with elevated temperatures. Here, we show that the addition of a polystyrene-co-polyacrylonitrile (SAN) copolymer can significantly enhance thermal stability of OIHPs and improve the stability of the corresponding PSCs by suppressing the migration of organic cations in OHIP. The methylammonium lead iodide (MAPI) with SAN incorporated within the perovskite layer featured a superior thermal stability compared to pure MAPI without SAN, only displaying an average of 5-15% decrease in PCE even after continuous thermal aging for 24 h at 100 degrees C. The secondary ion mass spectrometry revealed that the thermal degradation of the pure MAPI was largely associated with MA(+) out-migration. Conducting atomic force microscopy analysis further indicated that the incorporated SAN led to a suppression of ionic currents present at the grain boundaries of the perovskite film, which was understood by high immiscibility between SAN and MA(+) components as confirmed by the experimentally estimated Flory-Huggins parameter between them. This study newly identifies a promising potential of using polymer grain encapsulation for enhancing thermal stability of OIHPs and their solar cell performance by suppressing the out-diffusion of cationic organic components.