Influence of side chain length and bifurcation point on the crystalline structure and charge transport of diketopyrrolopyrrole-quaterthiophene copolymers (PDQTs)

The influence of the side chain on the molecular organization and charge transport performance of diketopyrrolopyrrole-quaterthiophene copolymers (PDQTs) was studied. It was found that, by increasing the side chain length from 2-octyldodecyl (PDQT-20) to 2-decyltetradecyl (PDQT-24), the mobility increased from 2.10 cm(2) V-1 s(-1) up to 3.37 cm(2) V-1 s(-1) in organic thin film transistors (OTFTs). The increase was found to be due to the improved surface morphology, rather than the changes in crystallinity and pi-pi stacking distance. A new side chain substituent, 4-decylhexadecyl, was developed for studying the effects of the bifurcation point of the branched side chains in comparison with 2-octyldodecyl and 2-decyltetradecyl. The 4-decylhexadecyl substituted PDQT (PDQT-26) showed a surge in mobility up to 6.90 cm(2) V-1 s(-1). The remarkably enhanced charge transport performance observed for PDQT-26 was believed to originate from its much shorter pi-pi distance (3.68 angstrom) than those of PDQT-20 (3.79 angstrom) and PDQT-24 (3.86 angstrom). The improvement was the result of a farther distance of the bifurcation point of 4-decylhexadecyl from the polymer backbone, which could effectively minimize the steric interference of the bulky side chain branches with the backbone to facilitate the co-facial pi-pi stacking.