Inexpensive solar cells with a low energy budget are interesting due to the need for alternative energy sources in the future.[1] Organic solar cells may become a low-cost alternative to Si-based solar cells because they are low weight, flexible, and could be produced by roll-to-roll printing techniques for large-area deposition. [2][3][4] Polymer solar cells are still hindered by their low efficiency, measured by the multiplication of short-circuit current (J sc ), open-circuit voltage (V oc ), and fill factor (FF) under solar illumination. J sc depends on the optical and electrical properties of the active layer, such as, absorption spectrum and charge mobility. V oc is proportional to the energy gap between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, but also influenced by the work function difference between electrodes as well as the thickness of the active layers. It is hard to increase V oc and J sc simultaneously in polymer solar cells, because larger photocurrents require a low bandgap absorber for maximum overlap with the solar spectrum, but to enhance V oc a high bandgap absorber is needed. It is common to use a thin layer of LiF between the active layer and the cathode to improve the V oc in polymer solar cells. [5][6][7][8] The role of the submonolayer of LiF has been suggested to be that of protecting the organic layer during metal deposition, modifying the work function of the cathode, and/ or introducing a dipole at the cathode interface, affecting charge injection. [9][10][11][12][13] The possibility to use roll-to-roll printing techniques is the biggest advantage of polymer electronic devices compared with their inorganic counterparts. However, LiF is vapor deposited in vacuum and is impossible to use in printing techniques.In order to increase V oc recently many efforts have been made to build tandem cells. The V oc is increased by stacking two cells on top of each other. [14][15][16][17][18][19] However, the significant loss of J sc makes the overall performance lower than that of a single cell. [19] So far few polymer tandem cells give higher efficiency than single cells. [18] In addition, the procedure to build tandem cells is very complicated involving multilayer deposition both via a solution process and a vapor process in vacuum, [18,19] which is not easy in printing methods.Here, we report a simple method to increase the V oc up to 200 mV and the power conversion efficiency (PCE) by 50 %, by using a thin layer of poly(ethylene oxide) (PEO) to modify the cathode in polymer solar cells. It was reported that a thin layer of PEO between the active layer and the Al or LiF/Al cathode improves the performance of polymer light-emitting diodes (PLED).[20] An enhanced photocurrent of solar cells based on poly[2-methoxy-5-(2'-ethyl-hexoxy)-1,4-phenylene vinylene] by blending a small amount of PEO and LiCF 3 SO 3 into the active layer was also demonstrated. [21] In this Communication, we demonstrate that a thin layer o...