Mechanisms of exceptional grain growth and stability in formamidinium lead triiodide thin films for perovskite solar cells

Pure formamidinium lead triiodide (alpha-FAPbI(3)) organic-inorganic halide perovskite (OIHP) semiconductor is very attractive for use as light absorber in the new thin-film perovskite solar cells (PSCs) technology. This is primarily because of its superior thermal stability, more suitable bandgap, and compositional simplicity. However, the existence of the photo-inactive non-perovskite delta-FAPbI(3) polymorph ('yellow' phase) is a major hurdle in the path towards the development of alpha-FAPbI(3)-based PSCs. Also, there is general consensus that the fine-grained nature of OIHP thin films is detrimental to the environmental stability and performance of the resulting PSCs. In this context, here we take advantage of the polymorphism in FAPbI(3), and use solvent-vapor-assisted delta-to-alpha phase transformation to induce exceptional grain coarsening (up to 50-fold) in 0.3-mu m thickness FAPbI(3) thin films, resulting in an unprecedented average grain size of up to similar to 9 mu m. The underlying mechanisms are elucidated based on the results from a combination of some key experiments, which involve studying systematically the effects of time, temperature, initial grain size, and solvent polarity index (PI). The ultra-coarse-grained alpha-FAPbI(3) thin films show dramatically improved environmental stability over their medium-grained counterparts, which is explained based on grain-boundary density arguments. PSCs made using the ultra-coarse-grained alpha-FAPbI(3) thin films have improved photovoltaic (PV) performance, but it is somewhat modest. This is attributed to the underestimation of the effective grain size relevant to photocarrier dynamics. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.