Improving performance of organic solar cells using amorphous tungsten oxides as an interfacial buffer layer on transparent anodes

Thermally evaporated tungsten oxide (WO3) films are investigated as a buffer layer on anodes to improve the performance of bulk-heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM-60). Characterization of the WO3 film under study shows that it is amorphous with the conductivity on the order of 10−6 S/cm and can effectively planarize an originally rough ITO surface down to the RMS roughness of 0.88 nm. Insertion of 5–30 nm-thick WO3 layers resulted in power conversion efficiency and fill factor as high as 3.1% and 0.70, respectively, showing an improvement with a good batch-to-batch consistency over conventional cells. The observed improvement is related mainly to the relatively large shunt resistance of WO3-based cells under illumination that is linked to a low carrier recombination resulting in high fill factor and open-circuit voltage.