We have developed a simple and robust process to prepare efficient perovskite solar cells. Pinhole free thin films of CH3NH3PbI3-xClx can be coated on high conductivity poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS), (Clevios PH1000) when the PEDOT:PSS is deposited together with dimethyl sulfoxide (DMSO) and Zonyl as additives. This process enables the fabrication of perovslcite solar cells using [6,6] -phenyl-C-61-butyric acid methyl ester (PCBM) as electron transport layer with > 12% power conversion efficiency, low hysteresis and excellent operational stability. We have performed a detailed opto-electronic characterization of these solar cells and identified the main loss mechanism limiting the device performance. (C) 2016 Elsevier B.V. All rights reserved.