Preparation of Low Band Gap Fibrillar Structures by Solvent-Induced Crystallization.

Solvent-induced crystallization of the low band gap polymer poly[-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) was shown to give fibril-like structures of 40-60 nm width and ∼0.5 μm length. These structures, formed by heating and cooling PCDTBT in a marginal solvent for the polymer, were characterized by AFM, TEM, GI-WAXS, and steady state absorption and emission spectroscopy. The width of the PCDTBT structures suggests that the polymer chains are oriented perpendicular to the fiber axis, while the observed undulated structure, as revealed by AFM, suggests that the nanostructures may be composed of smaller crystalline units that associate preferentially on specific faces of the crystals. The spectroscopic signatures of the suspended PCDTBT fibrils resembled that of the polymer in solution, in contrast to features associated with the fibril formation of the well-known conjugated polymer poly(3-hexyl thiophene) (P3HT). The solution-based crystallization of PCDTBT reported herein offers insight into the self-assembly of conjugated polymers toward better understanding of their role in photovoltaic devices.