An organic lead halide perovskite film proved to be an excellent light harvesting material for solar cells. The ability to prepare high quality perovskite films, with large areas, for solar modules using simple methods is essential for commercializing this promising photovoltaic technology. By combining the composition of the halogens (Cl can improve the morphology of the film by reducing the crystallization rate, and Br can increase the Voc and stabilize the crystal lattice) and film casting engineering (hot solution spin-coating (HSS) and solvent annealing (SA)), high quality single-phase mixed-halide perovskite films can be prepared using a one-step method. This new method can not only prepare high quality, small area films reproducibly but also be adopted readily to prepare high quality homogeneous films with large areas for perovskite solar modules. An inverted mixed-halide perovskite solar module (active area of 25.2 cm2) achieves the highest efficiency of 14.3% with a remarkably high FF of 74.4% and has no current hysteresis.