We developed a facile,but efficient,layer-by-layer approach to uniform CH3NH3PbI3-xClx perovskite flat films for high-performance perovskite solar cells (Figure 1,left).1 Briefly,a thin layer of Lead Chloride (PbCl2) was first deposited by thermal evaporation onto a PEDOT:PSS layer pre-coated on an ITO/Glass substrate.Subsequent dipping of the PbCl2 into a solution of CH3NH3I in 2-propanol transferred it into a layer of CH3NH3PbI3-xClx perovskite.