Moisture-Induced Structural Degradation in Methylammonium Lead Iodide Perovskite Thin Films

Methylammonium lead iodide (CH3NH3PbI3, MAPbI(3)) belongs to the group of organic-inorganic halide perovskites (OIHPs) that exhibit exceptional electrical and optical behavior suitable for photovoltaic applications. However, improving its structural and chemical stability and robustness remains a challenge for OIHPs to be considered a feasible active material in optoelectronic devices. This is due to the fact that this material is very susceptible to degrade under various intrinsic and extrinsic factors. Therefore, it is crucial to understand the mechanisms through which MAPbI(3) undergoes chemical degradation under operating conditions such as a relatively humid environment. We present the structural characteristics of MAPbI(3) under working conditions that suggest the routes of phase segregation as a result of exposing to highly moist media. We use dark pulse discharge behavior and current-voltage (I-V) variations of MAPbI(3) under different moisture levels to investigate the nature of structural degradation in OIHPs. We show that while relatively lower levels of humidity (similar to 60% RH) have a limited impact on the structural stability of MAPbI(3), exposure to higher levels of moisture (similar to 100% RH) results in the formation of PbI2 and aqueous CH3NH3I, which fundamentally change the charge transport characteristics in MAPbI(3). Our findings explain the ongoing debate on the presence of a threshold for the humidity that triggers irreversible structural transformation and leads to full degradation of MAPbI(3).