Surface Iodide Defects Control the Kinetics of the CsPbI Perovskite Phase Transformation.

Halide perovskites are technologically interesting across a wide range of optoelectronic devices, especially photovoltaics, but material stability has proven to be challenging. One degradation mode of note is the meta stability of the perovskite phase of some material compositions. This was studied by tracking the change of CsPbI from its optoelectronically relevant perovskite phase to its thermodynamically stable nonperovskite phase, δ-CsPbI. We explore kinetics as a function of surface chemistry and observe a quantitatively similar, ∼5-fold, reduction in the phase transition rate between neat films and those treated with CsI and CdI. Using XPS to explore surface chemistry changes across samples, we link the reduction in the phase transition rate to the surface iodide concentration. When informed by previous theoretical studies, these experiments point to surface iodide vacancies as the nucleation sites for δ-CsPbI growth and show that phase nucleation is the rate limiting step in δ-CsPbI formation for CsPbI perovskite thin films.