The crystallization of MAPbI3 perovskite films was purposefully engineered to investigate the governing factors which determine their morphological properties and moisture stability. By modulating nucleation, we obtained a single layer perovskite film with controlled crystal facet orientation and grain size. The lack of perovskite nucleation sites during crystallization allowed us to tailor the resulting crystallization phase. Theoretical calculations indicated that the nucleation sites for perovskite growth are related to the electron density around the oxygen atom (C[double bond, length as m-dash]O and S[double bond, length as m-dash]O) in a Lewis base. A single layer of micrometer-sized and (110)-oriented perovskite crystals was achieved in the optimized MAPbI3 films via suppressing the formation of nucleation sites. We fabricated inverted perovskite solar cells with the structure of glass/ITO/PEDOT:PSS/MAPbI3/PC61BM/Al which exhibited a high power conversion efficiency of 17.5% and a high fill factor over 83%. In addition, a study of the moisture stability indicated that the (110) facet orientation of the perovskite grains plays a more important role in film degradation than grain size.