Investigation of the structure–property relationship of thiadiazoloquinoxaline-based copolymer semiconductors via molecular engineering

Abstract

Six thiadiazoloquinoxaline (TQ) based copolymers (P1–P6) have been synthesized using Stille coupling reaction upon varying donor moieties, substitution positions and architectures of polymer side chains. UV-vis-NIR absorption spectra indicated that all of these polymers exhibited low optical bandgaps from 0.96 to 0.75 eV. The electron affinities of these six polymers with values from −3.92 to −3.99 eV slightly changed due to the contribution of the same TQ core. However, the ionization potentials were tuned from −4.95 eV in P1 to −5.28 eV in P3 by introducing different donors. Comparing polymers P1–P4 with different donor parts, two dimensional wide-angle X-ray scattering measurements implied that P4 had a higher crystallinity because its coherence length (5.8 nm) was 2–3 times higher than those of P1–P3 (1.7–2.9 nm). This led to a best field-effect hole mobility of 0.1 cm² V⁻¹ s⁻¹ for P4 among the four polymers. The polymers P4–P6 had identical molecular formulae of side chains but significant differences in device performance. In comparison with P4, the two linear alkyl chains were moved onto head to head positions of bithiophene in P5, resulting in a hole mobility of only 0.007 cm² V⁻¹ s⁻¹. However, a pair of 2-decyl-tetradecyl alkyl chains was grafted onto thiophene units adjacent to the TQ core in P6, leading to the highest hole mobility up to 0.24 cm² V⁻¹ s⁻¹ in this series of polymers.