Semiconducting/insulating polymer blends with dual phase separation for organic field-effect transistors

In this study, diketopyrrolopyrrole (DPP)-based semiconducting polymers with alkyl chain spacers of different lengths (C24 and C29) were used to compare the effects of crystalline nature in semiconducting polymer on the phase-separation and electrical properties of DPP-based semiconducting polymer/poly(methyl methacrylate) (PMMA) blends. Hierarchical structures containing ultrathin-film (bottom) and hairy rods (top) of DPP-based semiconducting polymers formed on the substrates due to the combined effects of the self-stratification and Marangoni-instability during the phase-separation of polymer blends. In particular, 24-DPP-TVT/PMMA blend film formed-fewer and bigger semiconductor rods, because of coarsening of 24-DPP-TVT phase during solvent evaporation. In contrast, highly crystalline nature of 29-DPP-TVT prevented the coarsening of the 29-DPP-TVT rods, thereby resulted in densely packed semiconductor rods. According to the crystalline natures in DPP-based semiconducting polymers, 29-DPP-TVT phase in 29-DPP-TVT/PMMA blend exhibited lower angular spread in molecular orientation compared to 24-DPP-TVT phase in 24-DPP-TVT/PMMA blend. Accordingly, 29-DPP-TVT/PMMA field-effect transistors (FETs) exhibited significantly higher electrical properties than those of 24-DPP-TVT/PMMA FETs. In addition, field-effect mobility of 29-DPP-TVT/PMMA FETs is comparable to those of homo 29-DPP-TVT FETs, whereas the partial passivation effect lowers turn-on voltage of 29-DPP-TVT/PMMA FETs. This study that utilized dual phase-separation would provide possible guidelines for the selection of semiconducting polymer for the use of semiconducting/insulating polymer blends in FETs.