The simple planar configuration of organic–inorganic hybrid perovskite solar cells produced by a solution coating process has great potential to be a low-cost and high efficiency photovoltaic technology. However planar perovskite films produced by “normal” spin coating usually show a dendritic grain morphology giving many gaps in the film, resulting in poor coverage of the substrate and thus a low power conversion efficiency. Here a facile gas-assisted solution processing technique is reported that has changed the kinetics of nucleation and crystal growth of the perovskite during the spin coating, producing very uniform perovskite thin films consisting of densely packed single crystalline grains. This microstructure is an ideal candidate for the p–i–n solar cell device. Planar perovskite solar cells constructed from these films produced a highly reproducible average power conversion efficiency of 15.7±0.7%. The highest efficiency achieved was 17.0% with a slightly lower steady-state value of 16.5% at the maximum power output of the solar cell.