Planar CH 3 NH 3 PbI 3 perovskite solar cells with constant 17.2% average power conversion efficiency irrespective of the scan rate are described. These properties are attributed to the formation of pure CH 3 NH 3 PbI 3 thin film by introduction of HI solution. Thereby charge injection/ separation efficiency, charge collection efficiency, diffusion coefficient, carrier’s life time, and traps are improved. J. H. Heo, D. H. Song, H. J. Han, S. Y. Kim, J. H. Kim, D. Kim, H. W. Shin, T. K. Ahn, C. Wolf, T.-W. Lee, S. H. Im*....................... X–XX Planar CH 3 NH 3 PbI 3 Perovskite Solar Cells with Constant 17.2% Average Power Conversion Efficiency Irrespective of the Scan Rate Solar Cells COMMUNICATIONS