Rational Design of Highly Soluble and Crystalline Conjugated Polymers for High-Performance Field-Effect Transistors

Abstract

Conjugated polymers based on cyclopentadithiophene (CPDT) moieties are widely used in optoelectronics applications but rarely in field-effect transistors (FETs). This is due to the steric hindrance at the tetrahedral center of the CPDT core, which is a critical problem requiring resolution without diminishing the polymer crystallinity and solubility. A bulky CPDT-based high-mobility copolymers are synthesized here, which shows excellent pi-pi stacking in the film states with good solubility. The addition of bulky ethylhexyl side chains to CPDT does not interfere with the molecular ordering and promotes the intercalation of linear alkyl chains on the co-monomeric units. The resulting polymer forms bimodal packing structures with strong lamellar ordering and pi-pi interactions. Furthermore, the free volume generated by the bulky ethylhexyl side chains allows for a small amount of fullerene additive to be contained in the film state without interfering with the molecular ordering, leading to enhanced on/off ratios and hole mobilities of the resulting FETs. As a result, promising hole mobilities of up to 1.08-1.64 cm(2) V-1 s(-1) are achieved with the optimized FETs, which are the highest mobilities reported for CPDT-based conjugated polymers containing branched alkyl side chains.