Structural and chemical evolution of methylammonium lead halide perovskites during thermal processing from solutionElectronic supplementary information (ESI) available: Complete details of the precursor solutions used in these studies, detailed XRD analysis of the 1 : 1 MAI : PbI2 precursor phase and the Cl− intermediate phase, the XRD peak positions of the various crystal phases studied, control TPD-MS experiments of MACl, MAI, and PbI2, mass spectral analysis, analysis of the perovskite (200)

Following the prominent success of CH 3 NH 3 PbI 3 in photovoltaics and other optoelectronic applications, focus has been placed on better understanding perovskite crystallization from precursor and intermediate phases in order to facilitate improved crystallinity often desirable for advancing optoelectronic properties. Understanding of stability and degradation is also of critical importance as these materials seek commercial applications. In this study, we investigate the evolution of perovskites formed from targeted precursor chemistries by correlating in situ temperature-dependent X-ray diffraction, thermogravimetric analysis, and mass spectral analysis of the evolved species. This suite of analyses reveals important precursor composition-induced variations in the processes underpinning perovskite formation and degradation. The addition of Cl − leads to widely different precursor evolution and perovskite formation kinetics, and results in significant changes to the degradation mechanism, including suppression of crystalline PbI 2 formation and modification of the thermal stability of the perovskite phase. This work highlights the role of perovskite precursor chemistry in both its formation and degradation. Perovskites are processed from solution; understanding the influence of solution composition on crystallization and degradation is critical to their success.