Modification of CH3NH3PbI3 and related hybrid organic–inorganic semiconductors has become an increasingly important effort because of the need to control fundamental material properties. Herein, we closely study material growth to identify the most significant controlling variables determining morphological evolution in a new class of hybrid perovskite alloy. Specifically, drop-casting based perovskite analysis shows that CH3NH3Pb(Mn)yI3, CH3NH3Pb(Fe)yI3, CH3NH3Pb(Co)yI3, and CH3NH3Pb(Ni)yI3 constitute a unique class of hybrid organic–inorganic perovskite in which growth route most strongly determines morphology. Mn, Fe, Co, and Ni consistently modify CH3NH3PbI3 growth, enabling direct perovskite nucleation to compete with growth through solvent induced intermediate states. We show unambiguously that solvent-perovskite co-crystal formation is responsible for the rod-like thin-film morphology that a great deal of work optimizing perovskite growth in planar heterojunction solar cells endeavors to circumvent. In addition to providing insight into the role of growth route in morphological evolution, we also identity the impact of CH3NH3I stoichiometry and the impact of magnetic properties on growth as secondary variables that significantly affect optoelectronic properties. Leveraging this understanding to minimize the impact of morphological phenomena on performance, we closely analyze the compositional impact of these transition metals on optoelectronic quality using CH3NH3Pb(Fe)yI3 as a model system showing that transition metal inclusion of this type leads to trap-assisted recombination within the perovskite bulk that both sharply limits Jsc and causes significant hysteresis. By comparing device performance of Mn, Fe, Co, and Ni based systems, we show that Mn relieves this sharp limitation on Jsc and almost completely eliminates hysteresis. CH3NH3Pb(Mn)yI3 thus allows the implementation of direct perovskite nucleation while minimizing the deleterious impact of transition metal inclusion. PL analysis shows that this material is also more emissive than CH3NH3PbI3, making it ideal for light production as well. Methodology and insights developed herein outline a generalizable approach for navigating complexity of perovskite compositional modification.