The solution-based fabrication of reproducible, high-quality lead iodide perovskite films demands a detailed understanding of the crystallization dynamics, which is mainly determined by the perovskite precursor solution and its processing conditions. A systematic in situ study is conducted during the critical phase before the nucleation in solution to elucidate the formation dynamics of lead iodide perovskite films. Using ultraviolet (UV) absorption spectroscopy during spin coating allows to track the evolution of iodoplumbate complexes present in the precursor solution. It is found that prior to film formation, a novel absorption signature at 3.15 eV arises. This is attributed to the emergence of a PbI₂-DMF solvated (PDS) phase. The amount of PDS phase is closely connected to the concentration of the solution layer during spin coating. It is also proposed that PDS clusters are a predecessor of crystalline perovskite phases and act as nucleation seeds in the precursor solution. In this way, this work provides insights into the early stages of perovskite crystallization.