High-efficiency solution-processed perovskite solar cells with millimeter-scale grains have been developed using a solution-based hot-casting technique. This method enables the growth of continuous, pinhole-free thin films of organometallic perovskites with large crystalline grains. The resulting solar cells achieved efficiencies approaching 18% with minimal cell-to-cell variability. The devices demonstrated hysteresis-free photovoltaic response, a critical factor for stable operation. Improved performance is attributed to reduced bulk defects and enhanced charge carrier mobility in large-grain devices. The hot-casting technique involves casting a hot mixture of lead iodide and methylamine hydrochloride onto a substrate maintained at high temperatures, followed by spin-coating to obtain a uniform film. This method produces large, millimeter-scale crystalline grains with a unique leaf-like pattern. The x-ray diffraction pattern shows sharp and strong perovskite peaks, indicating a highly oriented crystal structure. The grain size increases with higher substrate temperatures or the use of high-boiling point solvents. Optical and scanning electron microscopy images confirm that the perovskite thin films are uniform, pinhole-free, and cover the entire substrate. The hot-casting method is applicable for both pure and mixed-halide perovskite combinations and may lead to the realization of industrially scalable large-area crystalline thin films from other materials. The current density-voltage (J-V) curves for devices fabricated at various temperatures show a marked increase in short-circuit current density, open-circuit voltage, and fill factor with increasing grain size. The J-V curves for large-grain devices are in good agreement with the calculated values. The devices exhibited hysteresis-free performance with negligible change in photocurrent density with voltage sweep direction or scan rate. Self-consistent optoelectronic simulations support the hypothesis that improved material quality for larger grains leads to higher efficiency. The energy band diagram of the large-grain device shows a fully depleted absorber region, allowing efficient charge collection. The experimental results can be interpreted only if mobility is correlated to grain size. Direct optical characterization of the material characteristics supports the hypothesis that material crystalline quality is correlated to grain size. Microphotoluminescence, absorption spectroscopy, and time-resolved photoluminescence measurements confirm the high crystalline quality of large-grain crystals. The results are consistent with earlier measurements of open-circuit voltage as a function of light intensity, suggesting reduced trap-assisted recombination in large-area grains. These results indicate that the hot-casting technique can lead the field toward the reproducible synthesis of wafer-scale crystalline perovskites, necessary for the fabrication of high-efficiency solar cells.High-efficiency solution-processed perovskite solar cells with millimeter-scale grains have been developed using a solution-based hot-casting technique. This method enables the growth of continuous, pinhole-free thin films of organometallic perovskites with large crystalline grains. The resulting solar cells achieved efficiencies approaching 18% with minimal cell-to-cell variability. The devices demonstrated hysteresis-free photovoltaic response, a critical factor for stable operation. Improved performance is attributed to reduced bulk defects and enhanced charge carrier mobility in large-grain devices. The hot-casting technique involves casting a hot mixture of lead iodide and methylamine hydrochloride onto a substrate maintained at high temperatures, followed by spin-coating to obtain a uniform film. This method produces large, millimeter-scale crystalline grains with a unique leaf-like pattern. The x-ray diffraction pattern shows sharp and strong perovskite peaks, indicating a highly oriented crystal structure. The grain size increases with higher substrate temperatures or the use of high-boiling point solvents. Optical and scanning electron microscopy images confirm that the perovskite thin films are uniform, pinhole-free, and cover the entire substrate. The hot-casting method is applicable for both pure and mixed-halide perovskite combinations and may lead to the realization of industrially scalable large-area crystalline thin films from other materials. The current density-voltage (J-V) curves for devices fabricated at various temperatures show a marked increase in short-circuit current density, open-circuit voltage, and fill factor with increasing grain size. The J-V curves for large-grain devices are in good agreement with the calculated values. The devices exhibited hysteresis-free performance with negligible change in photocurrent density with voltage sweep direction or scan rate. Self-consistent optoelectronic simulations support the hypothesis that improved material quality for larger grains leads to higher efficiency. The energy band diagram of the large-grain device shows a fully depleted absorber region, allowing efficient charge collection. The experimental results can be interpreted only if mobility is correlated to grain size. Direct optical characterization of the material characteristics supports the hypothesis that material crystalline quality is correlated to grain size. Microphotoluminescence, absorption spectroscopy, and time-resolved photoluminescence measurements confirm the high crystalline quality of large-grain crystals. The results are consistent with earlier measurements of open-circuit voltage as a function of light intensity, suggesting reduced trap-assisted recombination in large-area grains. These results indicate that the hot-casting technique can lead the field toward the reproducible synthesis of wafer-scale crystalline perovskites, necessary for the fabrication of high-efficiency solar cells.