Organo-lead halide perovskite solar cells have gained enormous significance and approached the power conversion efficiencies of commercialized c-Si solar cells and some other thin film photovoltaic solar cells recently. However, one major concern comes from the potential toxicology of the soluble lead. Efficiency boosts for the lead-free perovskite solar cells are thus highly desirable in this regard. Here we show that the crystallization of the methylammonium tin triiodide (CH3NH3SnI3) perovskite can be controlled through the choice of solvents used in the spin coating process, thus enabling a faster crystallization rate in comparison with the CH3NH3PbI3 analog. Careful evaporation of the solvents and the convective self-assembly process during spinning effectively assist the formation of well-crystallized perovskite films due to the favorable interactions between the solvent molecules and the solvated CH3NH3+ and [SnI3]-. The crystallization processes in the selected polar solvents (N,N-dimethylmethanamide (DMF), ?-butyrolactone(GBL) and dimethylsulfoxide (DMSO)) have been investigated and compared. Intermmediate compounds of the perovskite withcrystallized solvent molecules were recognized to be important for the resultant film morphology. Efficient lead-free pervoskite solar cells have been realized with a conductive poly(triarylamine) hole-transporting material. Our results provide important progress towards the understanding of the role of crystallization engineering in the realization of low-cost and highly efficient lead-free perovskite solar cells.