Stability of Halide Perovskite Solar Cell Devices: In Situ Observation of Oxygen Diffusion under Biasing

Abstract Using in situ electrical biasing transmission electron microscopy, structural and chemical modification to n–i–p‐type MAPbI 3 solar cells are examined with a TiO 2 electron‐transporting layer caused by bias in the absence of other stimuli known to affect the physical integrity of MAPbI 3 such as moisture, oxygen, light, and thermal stress. Electron energy loss spectroscopy (EELS) measurements reveal that oxygen ions are released from the TiO 2 and migrate into the MAPbI 3 under a forward bias. The injection of oxygen is accompanied by significant structural transformation; a single‐crystalline MAPbI 3 grain becomes amorphous with the appearance of PbI 2 . Withdrawal of oxygen back to the TiO 2 , and some restoration of the crystallinity of the MAPbI 3 , is observed after the storage in dark under no bias. A subsequent application of a reverse bias further removes more oxygen ions from the MAPbI 3 . Light current–voltage measurements of perovskite solar cells exhibit poorer performance after elongated forward biasing; recovery of the performance, though not complete, is achieved by subsequently applying a negative bias. The results indicate negative impacts on the device performance caused by the oxygen migration to the MAPbI 3 under a forward bias. This study identifies a new degradation mechanism intrinsic to n–i–p MAPbI 3 devices with TiO 2 .