Efficiency improvement in fullerene-layer-inserted organic bulk-heterojunction solar cells

We have improved power conversion efficiencies of organic bulk-heterojunction solar cells with the active layer composed of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) by inserting a PCBM layer between the active layer and adjacent Al cathode. This improvement is mainly due to the increase in short-circuit current, which prevails over the changes in other parameters. The largest improvement of 11.7% is obtained at the PCBM thickness of 10 nm, while further increasing of the thickness degrades the fill factor and eventually induces S-kinks in the current-voltage characteristics. We discuss possible models explaining the efficiency variations with the PCBM thickness and the occurrence of the S-kinks.