Abstract
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 (MAPbI3) grain boundaries (GBs) to be more heat-resistant. It is found that hot-casted MAPbI3 films contain GBs composed of hydrated (CH3NH3)4PbI4·H2O, while grain interiors (GIs) are mainly composed of CH3NH3PbI3. Upon heating bare MAPbI3 film at 85 °C up to 1000h in a nitrogen environment, thermal degradation of MAPbI3 started at GBs and extended into GIs. Such degradation pathway can be explained by hydrated (CH3NH3)4PbI4·H2O structures where moisture at GBs acts as a catalyst for thermal degradation at GBs. Conversely, when PMMA was applied to MAPbI3, a new level of thermal stability of MAPbI3/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)4PbI4·H2O GBs and driving them out through GB channels.
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