In contrast, water inhibits nucleation of dimethylammonium lead iodide until the system reaches higher supersaturation, resulting in decreased grain lengths. Despite the different crystallization mechanisms, information obtained about free-standing single crystal formation by ITC can be used to identify systems where additives modify substrate-based grain growth by spin-coating. In summary, water can promote or inhibit perovskite crystallization, depending on the organoammonium cation species present. The latter observation contrasts with previous ITC studies, which attributed the role of water to creation of nucleation sites (from vaporization) or changes in protonation equilibrium (through formation of acid decomposition products), both of which only promote crystal formation. Qualitatively consistent trends in both ITC and thin-film systems suggests a common underlying water-organoammonium interaction mechanism. A practical implication is that trace amounts of water can provide an additional experimental parameter to produce the compact thin films of desired grain sizes needed for stable and efficient devices.