Self-assembled mesogens modified fullerene for efficiently stable bulk heterojunction solar cells

A facile approach to develop a novel soluble liquid-crystalline fullerene derivative, N-methyl-2- [(4-(4′-cyano)biphenyloxy)hexyloxyphenyl)]-3,4-fulleropyrrolidine, C60-bp-CN is reported. Compared to C60, the modified C60-bp-CN shows broadened light harvesting and lower LUMO level (−4.2 eV), ascribing to the strong intermolecular interaction induced by self-assembled cyanobiphenyl mesogens, especially after thermal annealing from liquid crystalline states. When blended with poly(3-hexylthiophene) (P3HT), the absorption of the blend film is red-shifted to 700 nm, and the intensity is also remarkably increased. The results indicate that the modified C60-bp-CN is a promising acceptor for polymer solar cells (PSCs) based on BHJ active layers. Therefore, solar cell devices based on an ITO/PEDOT:PSS/P3HT:C60-bp-CN/LiF/Al configuration are fabricated. Clearly, the performances of all the devices are dramatically enhanced relative to corresponding species based on P3HT/C60 active layers. Among all the thermal treatments the mesophase annealing achieves the best performance with a short-circuit current density (Jsc) of 5.5 mA/cm2, an open-circuit voltage (Voc) of 0.52 V, a calculated fill factor (FF) of 0.23, and a power conversion efficiency (PCE) of 0.65%, which is 3 times higher than that of the untreated one.