Structural control over self-assembled crystals of pi--conjugated poly(3,3'''-didodecyl-quaterthiophene) for organic...

Liquid-crystalline (LC) poly(3,3'''-didodecyl-quaterthiophene) (PQT-12) was investigated to optimize its side alkyl ordering and pi-conjugated structure for high performance organic field-effect transistor (OFET) applications. Initially, low-crystal films spun-cast on OTS-treated SiO2 substrates were further crystallized via either thermal or solvent treatments. At temperatures (125-138 degrees C) driving a LC state of PQT-12, mobile chains were better migrated into preformed crystals. The resulting films showed highly crystal nanofibrils, in which pi-conjugated polymer backbones (with an conjugated backbone spacing, d(010) of 3.80 A) and the orientation of self-assembled side-chains were tilted with respect to the film surface, respectively. However, via melting or solvent exposure providing isotropic states, film crystallization generated less ordered crystals with randomly oriented side-chains, increasing d(010) up to 4.16 A. As a result, the usage of LC characteristic allowed us to consistently achieve the desirable crystal structure of PQT-12 and to robustly obtain high field-effect mobility for FET applications.