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Methylammonium lead halide perovskites are promising absorbers for solution-processed thin film photovoltaics owning to the unique combination of good optical absorption characteristics and long charge cartier diffusion lengths.Here we show that the charge carrier diffusion lengths of two classes of perovskites, CH3NH3PbI3-xClx and CH3NH3PbI3, are both highly sensitive to film processing conditions and optimal processing procedures are critical to preserving the long carrier diffusion lengths of the perovskite films.This understanding, together with the improved cathode interface by integrating ZnO nanocrystal electron-transporting interlayers, lead to the successful fabrication of highly efficient, stable and reproducible planar heterojunction CH3NH3PbI3-xClx solar cells with a remarkably high fill factor of 0.80 and impressive power-conversion efficiencies (PCEs) up to 15.9%.A 1-square-centimeter device yielding a PCE of 12.3% has been realized, demonstrating that this simple planar structure is promising for large-area devices.