The role of poly(ethylene oxide) polymer is investigated as an effective buffer with Al electrodes to markedly improve the electrode interface and enhance the open-circuit voltage (VOC) and the power conversion efficiency (PCE, η) of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM)-based bulk-heterojunction (BHJ) solar cells. A unique process is developed by thermally co-evaporating the poly(ethylene glycol) dimethyl ether (PEGDE, Mn ca. 2000) polymer with Al metal simultaneously at different ratios in vacuum (10−6 Torr) to prepare the electrode buffers. The instant formation of a carbide-like junction at the ethylene oxide/Al interface during the thermal evaporation is of essential importance to the extraction of electrons through the Al electrode. The performance of P3HT:PCBM-based solar cells can be optimized by modulating the co-evaporation ratios of the PEGDE polymer with Al metal due to the changes in the work functions of the electrodes. The VOC and η for devices fabricated with Al electrode are 0.44 V and 1.64%, respectively, and significantly improve to 0.58 V and 4.00% when applying the PEGDE:Al(2:1)/Al electrode. This research leads to a novel electrode design – free of salts, additives, complicated syntheses, and having tunable work function – for fabricating high-performance photovoltaic cells.