We herein report on the improved performance of inverted polymer solar cells (PSCs) that consist of a bulk-heterojunction (BHJ) photovoltaic (PV) layer with a water-processable electron-collecting interlayer of a hybrid poly(ethylene oxide) (PEO) and caesium carbonate (Cs2CO3) compound. For the BHJ PV layers of poly(3-hexylthiophene):phenyl C-61-butyric acid methyl ester (P3HT:PCBM), we found that the introduction of a solution-processed hybrid PEO:Cs2CO3 interlayer dramatically enhanced the performance of the cells, resulting in improved power conversion efficiencies (PCEs) of 3.8% for the inverted PSCs on glass substrates. These improvements may be attributed to a reduction in the work function of the ITO cathode together with the dual functions of electron-selective/hole-blocking effects of the PEO:Cs2CO3 interlayer. Moreover, the influence of the introduction of the PEO:Cs2CO3 interlayer on device performance was also examined even for flexible inverted PSCs on plastic substrates, and a high PCE of 2.8% was obtained for these inverted structures. The use of this approach in the design of electron-collecting interlayers in flexible inverted PSC structures could be of considerable benefit in the future development of all-solution- and roll-to-roll processed PSCs. (C) 2012 Elsevier B.V. All rights reserved.