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 similar to 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 similar to 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) cm(2)/(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.