The inclusion of iodide additives in hybrid perovskite precursor solutions has been successfully exploited to improve the solar cell efficiency but their impact on perovskite formation, morphology and photovoltaic performance is still not clear. Here an extensive analysis of the effect of iodide additives in the solution-phase and during the perovskite film formation, as well as their effect on device performance is provided. The results demonstrate that in the solution-phase the additives promote the formation of lead poly-iodide species resulting in the disaggregation of the inorganic lead iodide framework and in the formation of smaller nuclei inducing the growth of uniform and smooth perovskite films. Most importantly, the complexation capability of different iodide additives does not only directly affect film morphology but also influences the density of defect states by varying the stoichiometry of precursors. These findings demonstrate that the fine control of the interactions of the chemical species in the solution-phase is essential for the precise control of the morphology at the nanoscale and the growth of the perovskite films with a reduced density of defect states. Therefore, the in-depth understanding of all the processes involved in the solution-phase is the first step for the development of a facile and reproducible approach for the fabrication of hybrid perovskite solar cells with enhanced photovoltaic performance.