Supramolecular Engineering of Oligothiophene Nanorods without Insulators: Hierarchical Association of Rosettes and Photovoltaic Properties

Supramolecular rosettes of oligothiophenes that do not bear long aliphatic tails have been designed as semiconducting nanomaterials for solution-processable bulk heterojunction solar cells. The rosettes consist of six barbiturated thienyl[oligo(hexylthiophene)] units (Bar-T-hT(n); n=3,4,5) aggregated by multiple hydrogen bonds, which have been directly visualized by scanning tunneling microscopy (STM) at a solid-liquid interface. (HNMR)-H-1 spectroscopy in [D-8]toluene showed that Bar-T-hT(n) exists as a mixture of monomers and small hydrogen-bonded aggregates. Hierarchical organization of the hydrogen-bonded aggregates took place through - stacking interactions upon casting their toluene solutions, resulting in the growth of highly ordered nanorods whose widths are consistent with the diameters of the rosettes. The nanorods could be generated in the presence of soluble fullerene derivatives via solution casting or the annealing of the resulting thin films. The solar cells fabricated based on these bulk heterojunction films showed power conversion efficiencies of 1-3%, which are far higher than those of the non-hydrogen-bonded reference oligothiophene and the derivative that possesses long aliphatic tails.