Achieving optimal thin film processing of organometallic tri-halide perovskite (CH3NH3PbX3, X= Cl-, Br-, or I-) on conducting polymers or metal oxide surfaces is very critical for making high power conversion efficiency (PCE) planar hetero-junction solar cells. It has been demonstrated that the quality of morphology and crystallinity of perovskite thin films can significantly affect their photo-physical properties. It is very detrimental to device performance if poor morphology is formed because it not only causes electrical shorting but also deleteriously impacts charge dissociation, transport, and recombination. Recently, we have developed a simple and very effective method to enhance the crystallization of solution-processed CH3NH3PbIXCl3-X (MAPbIXCl3-X) by incorporating solvent additives into the precursor solution (3MAI + PbCl2 + additives). 1,8-Diiodooctane (DIO) was found to temporarily chelate with Pb2+ to introduce specific solvent-solute interactions like ligation and chelation that can alter the kinetics of perovskite crystallization, which is evidenced by the improved solubility of PbCl2 in the presence of DIO. As a result, the crystallization kinetics and morphology of the CH3NH3PbIXCl3-X thin-film can be significantly modulated to enhance both light-harvesting and charge collecting efficiency of the devices. Moreover, the molecular structures of the halide additives were found to also play an important role in affecting the dynamics of the relevant solvent-solute interactions. The characteristic peaks in XRD patterns, the absorption band-edge in UV spectra, and the resultant color of the thin films processed from using the MACl + PbCl2 + 1,4-diiodobutane (1,4-DIB) formula are almost identical to those found in conventional CH3NH3PbI3-xClx perovskite (processed from 3MAI + PbCl2). As a result of adding additive, enhanced crystallization and surface coverage of solution-processed perovskite thin films can be achieved to enable significantly increased PCE (from 9.8% in device made from a pristine perovskite to as high as 13.1% in additive-processed perovskites). Our study opens up a new perspective on perovskite preparation for optimizing perovskite active layer properties to enhance device performance.