The power conversion efficiency (PCE) of solution-processed organic-inorganic hybrid perovskite solar cells has been drastically improved. Despite this considerable progress, systematic research on precursor solution chemistry and its effects on photovoltaic parameters has been limited thus far. Herein, we report on the tracking of changes in chemical species in a precursor solution under solar illumination and investigate the correlation between the equilibrium change and the corresponding perovskite film formation. The illuminated perovskite precursors display a higher density of high-valent iodoplumbate, where the resulting perovskite film exhibits reduced defect density with uniform film formation. Conclusively, the perovskite solar cells prepared by the photoaged precursor solution demonstrate not only improved average PCE but also enhanced reproducibility with a narrow PCE distribution. This discovery shows robust control of perovskite precursor solutions from a simple treatment and suggests that the resulting uniform film may be applicable to various halide perovskite-based devices.