Role of PMMA to make MAPbI₃ grain boundary heat-resistant

Thermal instability of perovskite films is one of the important issues limiting the outdoor application of perovskite solar cells because perovskite films are intrinsically thermally unstable under the normal operation temperature. In this work, we explore the new role of poly (methyl methacrylate) (PMMA) that alters CH3NH3PbI3 (MAPbI(3)) grain boundaries (GBs) to be more heat-resistant. It is found that hot-casted MAPbI(3) films contain GBs composed of hydrated (CH3NH3)(4)PbI4 center dot H2O, while grain interiors (GIs) are mainly composed of CH3NH3PbI3. Upon heating bare MAPbI(3) film at 85 degrees C up to 1000 h in a nitrogen environment, thermal degradation of MAPbI(3) started at GBs and extended into GIs. Such degradation pathway can be explained by hydrated (CH3NH3)(4)PbI4 center dot H2O structures where moisture at GBs acts as a catalyst for thermal degradation at GBs. Conversely, when PMMA was applied to MAPbI(3), a new level of thermal stability of MAPbI(3)/PMMA was achieved where PMMA altered the perovskite GB to be thermally resistant. Remarkably, the high thermal stability of perovskite GBs is attributed to the newly discovered role of PMMA in absorbing moisture from hydrated (CH3NH3)(4)PbI4 center dot H2O GBs and driving them out through GB channels.