Electron Mobility of Diketopyrrolopyrrole Copolymers Is Robust against Homocoupling Defects

Structural defects in semiconducting conjugated polymers are usually suspected to deteriorate their properties and device performance and therefore complicate batch-to-batch reproducibility. This study investigates homocoupling (hc) defects in copolymers made from dithiazolyldiketopyrrolopyrrole (TzDPPTz) and tetrafluorobenzene (F4) by direct arylation polycondensation (DAP). Hc defects are quantified by H-1 NMR spectroscopy with good accuracy, and the effect of several reaction parameters on the TzDPPTz hc content in the resulting copolymers PTzDPPTzF4 is investigated in detail. A range of polymers with hc contents between 0.6 and 12.4% is used for a detailed structure-function relationship study. Experimentally, it is observed that TzDPPTz hc defects cause bathochromically shifted absorption spectra, decrease photoluminescence, and lower the lowest unoccupied molecular orbital (LUMO) energy level. Thin film morphology, nanostructure, and electron mobility probed by field-effect devices is marginally or not affected. The latter result is explained by theoretical calculations that suggest a localization of the highest occupied molecular orbital on the hc defect, but not that of the LUMO, the latter being relevant for electron transport. Thus, under these conditions, the hc content is not limiting device performance, which makes PTzDPPTzF4 a robust electron-transporting copolymer. These results are promising in the context of batch-to-batch reproducibility and further guide efforts toward a more detailed understanding of hc-function relationships.

Automated summary

This study investigates homocoupling defects in copolymers made from TzDPPTz and F4, finding that these defects cause bathochromically shifted absorption spectra, decreased photoluminescence, and lower LUMO energy levels. However, thin film morphology, nanostructure, and electron mobility were minimally impacted. Theoretical calculations suggest that the localization of the highest occupied molecular orbital on the hc defect does not affect electron transport, making PTzDPPTzF4 a robust electron-transporting copolymer.