Enhancing Doping Efficiency of Diketopyrrolopyrrole-Copolymers by Introducing Sparse Intramolecular Alkyl Chain Spacing

A synthetic approach that endows high doping efficiency on high-mobility diketopyrrolopyrrole (DPP)-copolymers is described in this paper. Typical DPP-copolymers are not suitable for molecular doping, which relies on intermolecular charge transfer, presumably because of their relatively high ionization potential or weak intermolecular charge transfer characteristics between the dopant and acceptor of copolymers. Herein, we introduce new DPP-copolymers with strategically designed sparse intramolecular alkyl chain spacing where dopant molecules can reside. The resulting two type A1-D1-A2-D1 and A1-D2-A2-D2 terpolymers consist of cyclopentadithiophenes or cyclopentadithiophenyl thiophene as alternative donors, diketopyrrolopyrrole as an acceptor, and difluorinated benzothiadiazole as another acceptor. These new DPP-copolymers have significantly enhanced doping efficiency compared to conventional DPP-copolymers, exhibiting typical features of integer charge transfer, as confirmed by ultraviolet-visible-near-infrared absorption spectroscopy, electron paramagnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and thin film conductivity analyses. Moreover, doping-induced solubility control, which is not possible with other conventional DPP-copolymers due to low doping efficiency, is applicable, thus resulting in arrays of high-mobility organic field-effect transistors and complementary all-polymer inverters. This study sheds light on the possibility of tuning the doping efficiency of donor-acceptor copolymers without sacrificing their intrinsic properties, such as their well-ordered molecular packing and high charge carrier mobility.