Effects of Oxygen and Water on the Formation and Degradation Processes of (CH3NH3)PbI3 Thin Films

Thin films of lead halide perovskites have attracted considerable interest for optoelectronic applications, especially photovoltaics. The influence of oxygen and water have been well studied on fully formed films in an effort to improve and understand material stability. Here, we study the influence of oxygen and water exposure on unconverted, precursor films prior to the conversion to the (CH3NH3)­PbI3 perovskite phase. We find that, for the PbCl2 synthesis route studied here, extended oxygen exposure can lead to increased PbI2 in the final film. Additionally, a reversible transformation into the perovskite structure can be driven by exposure to high humidity, which we hypothesize is due to the deliquescent nature of methylammonium chloride in the precursor film. Upon prolonged exposure to humidity, the film undergoes an irreversible phase transformation to a previously unknown crystalline phase of (CH3NH3)2PbI3Cl, the structure of which is solved using X-ray powder diffraction.