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Formamidinium lead triiodide (FAPbI₃) is considered as an alternative to methylammonium lead triiodide (MAPbI₃) because of its lower band gap and better thermal stability. However, owing to the large size of FA cations, it is difficult to synthesize high‐quality FAPbI₃ thin films without the formation of an undesirable yellow phase. Smaller sized cations, such as MA and Cs, have been successfully used to suppress the formation of the yellow phase. Whereas FA and MA lead triiodide perovskite solar cells (PVSCs) have achieved power conversion efficiencies (PCEs) higher than 20 %, the PCEs of formamidinium and cesium lead triiodide (FA_1−xCs_xPbI₃) PVSCs have been only approximately 16.5 %. Herein, we report our examination of the main factors limiting the PCEs of (FA_1−xCs_xPbI₃) PVSCs. We find that one of the main limiting factors could be the small grain sizes (≈120 nm), which leads to relatively …