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

Pure formamidinium lead triiodide (α-FAPbI3) 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 δ-FAPbI3 polymorph (‘yellow’ phase) is a major hurdle in the path towards the development of α-FAPbI3-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 FAPbI3, and use solvent-vapor-assisted δ-to-α phase transformation to induce exceptional grain coarsening (up to 50-fold) in 0.3-μm thickness FAPbI3 thin films, resulting in an unprecedented average grain size of up to ~9 μm. 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 α-FAPbI3 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 α-FAPbI3 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.