Film uniformity and large crystalline size are indispensable and critical for obtaining high performance planar perovskite solar cells. Perovskite precursor composition plays a key role in the formation of high quality perovskite films. In this work, excess PbCl2 is deliberately introduced, and is found to facilitate the solubility of CH3NH3I in concentrated solution at room temperature. The excess PbCl2 is found to alter the formation pathway of perovskite thin films. Accordingly, the formation pathway is proposed. Analysis of the perovskite films and associated devices revealed that extremely uniform perovskite films consisting of large grains of several microns can be obtained with a small amount of excess PbCl2. The associated planar perovskite solar cells showed a reliable and reproducible efficiency of 16.1%. (C) 2015 Elsevier B.V. All rights reserved.