Effect of Selective Contacts on the Thermal Stability of Perovskite Solar Cells

Thermal stability of CH3NH3PbT3 (MAPbI(3))based perovskite solar cells was investigated for normal structure including the mesoporous TiO2 layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI(3) was found to be intrinsically stable from 85 degrees C to 120 degrees C in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 degrees C. Photoluminescence (PL) intensity and the time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI(3) into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 degrees C-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD, and MAPbI(3) is proved to be thermally stable. Spiro-MeOTAD with additives was crystallized at 85 degrees C due to a low glass transition temperature, and hole mobility was significantly deteriorated, which was responsible for the thermal instability. Thermal stability was significantly improved for the inverted structure with the NiO hole transporting layer, where the power conversion efficiency (PCE) was maintained at 74% of its initial PCE of 14.71% after the 80th thermal cycle (one cycle: heating at 85 degrees C for 2 h and cooling at 25 degrees C for 2 h). This work implies that the thermal stability of perovskite solar cells depends on selective contacts.