This paper reports a synergistic strategy to enhance the power conversion efficiency (PCE) of flexible planar perovskite solar cells (PSCs) by controlling the thickness of the ZnO electron transport layer (ETL), wPbl(2) phase, and size/morphology of the perovskite (MAPbI(3)) absorber layer. To optimize the size/morphology of MAPbI(3) via a two-step spin coating process, various volumes of CH3NH3I precursor solutions with a constant concentration were continuously coated, which greatly affected the grain growth condition of the MAPbI(3). In addition, the remnant PbI2 phase in the MAPbI3, which acted as a recombination barrier, was simultaneously controlled. This strategic method to synergistically combine the major factors affecting the final PCE resulted in the best efficiency of 123%, which is the highest efficiency among ZnO-ETL-based flexible planar PSCs to date. (C) 2016 Published by Elsevier B.V.