Inorganic−organic hybrid perovskites MAPb(IxBr1−x)3 (0 < x < 1) hold promise for efficient multi-junction or tandem solar cells due to tunable bandgap and improved long-term stability. However, the phase transformation from Pb(IxBr1−x)2 precursors to perovskites is not fully understood which hinders further improvement of optoelectronic properties and device performance. Here, adaptation of the two-step deposition method, which enables the direct probe into the growth dynamics of perovskites using in situ diagnostics, and a detailed view of the effects of solvent, lead halide film solvation, and Br incorporation and alloying on the transformation behavior is presented. The in situ measurements indicate a strong tendency of lead halide solvation prior to crystallization during solution-casting Pb(IxBr1−x)2 precursor from a dimethyl sulfoxide (DMSO) solvent. Highly-efficient intramolecular exchange is observed between DMSO molecules and organic cations, leading to room-temperature conversion of perovskite and high-quality films with tunable bandgap and superior optoelectronic properties in contrast to that obtained from crystalline Pb(IxBr1−x)2. The improved properties translate to easily tunable and a relatively higher power conversion efficiency of 16.42% based on the mixed-halide perovskite MAPb(I0.9Br0.1)3. These findings highlight the benefits that solvation of the precursor phases, together with bromide incorporation, can have on the microstructure, morphology, and optoelectronic properties of these films.