The roles of poly(ethylene oxide) electrode buffers in efficient polymer photovoltaics

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 ( V _OC) 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 ^-6Torr) 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 V _OC 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.