Effect of Air Exposure on Electron-Beam-Induced Degradation of Perovskite Films

Organic-inorganic halide perovskites are interesting candidates for solar cell and optoelectronic applications owing to their advantageous properties such as a tunable band gap, low material cost, and high charge carrier mobilities. Despite making significant progress, concerns about material stability continue to impede the commercialization of perovskite-based technology. In this article, we investigate the impact of environmental parameters on the alteration of structural properties of MAPbI₃ (CH₃NH₃PbI₃) thin films using microscopy techniques. These characterizations are performed on MAPbI₃ thin films exposed to air, nitrogen, and vacuum environments, the latter being possible by using dedicated air-free transfer setups, after their fabrication into a nitrogen-filled glovebox. We observed that even less than 3 min of air exposure increases the sensitivity to electron beam deterioration and modifies the structural transformation pathway as compared to MAPbI₃ thin films which are not exposed to air. Similarly, the time evolution of the optical responses and the defect formation of both air-exposed and non-air-exposed MAPbI₃ thin films are measured by time-resolved photoluminescence. The formation of defects in the air-exposed MAPbI₃ thin films is first observed by optical techniques at longer timescales, while structural modifications are observed by transmission electron microscopy (TEM) measurements and supported by X-ray photoelectron spectroscopy (XPS) measurements. Based on the complementarity of TEM, XPS, and time-resolved optical measurements, we propose two different degradation mechanism pathways for air-exposed and non-air-exposed MAPbI₃ thin films. We find that when exposed to air, the crystalline structure of MAPbI₃ shows gradual evolution from its initial tetragonal MAPbI₃ structure to PbI₂ through three different stages. No significant structural changes over time from the initial structure are observed for the MAPbI₃ thin films which are not exposed to air.