The research report discusses the development of high-efficiency solution-processed perovskite solar cells with millimeter-scale grains. The authors present a solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites with large, millimeter-scale crystalline grains. This method, which involves casting a hot mixture of lead iodide and methylamine hydrochloride onto a substrate maintained at high temperatures, results in solar cells with efficiencies approaching 18%, with minimal variability. The improved performance is attributed to reduced bulk defects and enhanced charge carrier mobility in large-grain devices. The study also highlights the absence of hysteresis in the solar cell devices, a significant bottleneck for the stable operation of perovskite devices. The findings suggest that this technique could lead to the synthesis of wafer-scale crystalline perovskites, necessary for high-efficiency solar cell fabrication, and have broader applications in other material systems prone to polydispersity, defects, and grain boundary recombination.The research report discusses the development of high-efficiency solution-processed perovskite solar cells with millimeter-scale grains. The authors present a solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites with large, millimeter-scale crystalline grains. This method, which involves casting a hot mixture of lead iodide and methylamine hydrochloride onto a substrate maintained at high temperatures, results in solar cells with efficiencies approaching 18%, with minimal variability. The improved performance is attributed to reduced bulk defects and enhanced charge carrier mobility in large-grain devices. The study also highlights the absence of hysteresis in the solar cell devices, a significant bottleneck for the stable operation of perovskite devices. The findings suggest that this technique could lead to the synthesis of wafer-scale crystalline perovskites, necessary for high-efficiency solar cell fabrication, and have broader applications in other material systems prone to polydispersity, defects, and grain boundary recombination.