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.

The cyanobiphenyl mesogen tails can induce the C60-bp-CN to form interdigitated bilayer lamellar structure with a tilted orientation in the assemblies, and fine-tune the energy levels of C60-bp-CN to be a suitable acceptor for PSCs. The orientation of the mesogens not only favors the self-organization of the C60-bp-CN acceptor, but also induces the assembly behavior of conjugated polymer donors, consequently, controlling the microstructure to form the well-ordered interpenetrating bicontinous networks to favor the charge mobilities and collection. Thermal treatment has large impact great on the efficiency and the thermal annealing from the liquid state leads to an overall enhanced photovoltaic performance, including Jsc, FF, and PCE.Figure optionsDownload as PowerPoint slideHighlights
► Modified fullerene derivative, C60-bp-CN, with cyanobiphenyl mesogen tails.
► Mesogens induce the assembly behavior of C60-bp-CN acceptor and conjugated polymer donors.
► Well-ordered interpenetrating bicontinous networks favor the charge mobilities and collection.
► Thermal annealing from the liquid state leads to overall enhanced photovoltaic performance.