Methyl group in asymmetric DPP n-type copolymers impedes aggregation and charge transport anisotropy

A comparative study of the impact of a single methyl group attached to dithienyl-diketopyrrollopyrrole (ThDPPTh)/tetrafluorobenzene (F4) copolymers on optoelectronic and structural properties is presented. Two monomers with and without a methyl group attached are prepared and used for the direct arylation polycondensation with dibromo-F4 to get two n-type copolymers. The asymmetric monomer with the methyl group is prepared on a ∼8 g scale. The presence of the methyl group leads to significant backbone torsion, strongly improved solubility, and reduced aggregation in solution. The methyl group furthermore reduces the melting point by ∼80 K but does not eliminate crystallinity. Thin films of both materials probed by GIWAXS feature lamellar morphologies with pronounced edge-on orientations, indicating their propensity to order. However, in thin films of the methylated copolymer directionally processed by off-center spin-coating, complete loss of charge transport anisotropy is found along with moderately reduced electron mobilities of 10^-2-10^-1 cm²/(V s). The study shows that single methyl groups attached to conjugated polymers can have a dramatic impact on properties and that careful placement within the copolymer structure opens up an efficient handle to balance properties related to processing and performance.