Trace MA ⁺ Accelerates the Degradation of Perovskite Solar Cells at Elevated Temperatures

Methylammonium (MA + ) is the most widely adopted A‐site cation for high‐efficiency perovskite solar cells. Here, we demonstrate that although MA + enhances initial power‐conversion efficiency (PCE), it also creates a photo‐instability channel and speeds up the device degradation at elevated temperature. By systematically comparing MA‐containing triple‐cation (Cs 0.05 MA 0.05 FA 0.9 PbI 3 ) and MA‐free double‐cation (Cs 0.05 FA 0.95 PbI 3 ) devices and thin films, we find that removing MA + more than doubles the operational lifetime under 1‐sun illumination at 85°C. Comprehensive characterization reveals that MA + deprotonation leads to the formation of ionic defects that initiate perovskite decomposition. These findings suggest that the removal of MA + is crucial for achieving intrinsically stable perovskite photovoltaics, particularly under conditions of elevated temperature.