Solution-processable organic-inorganic hybrid perovsldtes have been successfully used as light harvesting materials in efficient photovoltaic cells. Here, an efficient and simple approach is reported for using dimethylsulphoxide (DMSO) to increase crystal size and hydrochloric acid (HCl) to further retard crystallization thereby improving uniformity and crystallinity of the perovskite film. With this synergistic approach, large uniform perovskite crystals over one micron in size with superior quality perovskite film without pinholes resulted as confirmed by both XRD and SEM images. The fabricated planar heterojunction solar cell has reached efficiency of 17.8%. Both the best and average solar cells fabricated with this approach using DMSO reveal that a 10%-11% increase in efficiency as a result of conditioning with HCl which controls crystallization preventing undesirable non-uniform crystal growth, which leads to pinholes and hole-electron recombination sites. Using this method, a uniform perovskite layer that maximizes light harvesting can be readily obtained. Further, the incorporation of chloride using the HCl formulation has demonstrated to have better stability against degradation from moisture, strong solar irradiation, and high temperature. (C) 2017 Elsevier Ltd. All rights reserved.