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Pure formamidinium lead triiodide (α-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 δ-FAPbI 3 polymorph ('yellow' phase) is a major hurdle in the path towards the development of α-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 δ-to-α phase transformation to induce exceptional grain coarsening (up to 50-fold) in 0.3-μm thickness FAPbI 3 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-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 α-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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The lineal-intercept method is commonly used to estimate the average grain size, where the 'Mendelson' multiplying factor (1.60) that is typically used explicitly assumes tetrakaidecahedral grains. While this is appropriate for bulk equiaxed microstructures, grains in thin films where the grain size exceeds the film thickness, are closer to prismatic polyhedrons in shape. Thus, assuming log-normal distribution of hexagonal-prism-shaped grains, a new multiplying factor of 1.20 is derived which relates the average grain size and measured intercept size. This model is successfully validated using experimental measurements of three different coarse-grained thin films of halide perovskites used in solar cells. Image, graphical abstract [ABSTRACT FROM AUTHOR]