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

Using in situ electrical biasing transmission electron microscopy, structural and chemical modification to n-i-p-type MAPbI₃ solar cells are examined with a TiO₂ electron-transporting layer caused by bias in the absence of other stimuli known to affect the physical integrity of MAPbI₃ such as moisture, oxygen, light, and thermal stress. Electron energy loss spectroscopy (EELS) measurements reveal that oxygen ions are released from the TiO₂ and migrate into the MAPbI₃ under a forward bias. The injection of oxygen is accompanied by significant structural transformation; a single-crystalline MAPbI₃ grain becomes amorphous with the appearance of PbI₂ . Withdrawal of oxygen back to the TiO₂ , and some restoration of the crystallinity of the MAPbI₃ , is observed after the storage in dark under no bias. A subsequent application of a reverse bias further removes more oxygen ions from the MAPbI₃ . 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 MAPbI₃ under a forward bias. This study identifies a new degradation mechanism intrinsic to n-i-p MAPbI₃ devices with TiO₂ .