Systematic Variation of Fluorinated Diketopyrrolopyrrole Low Bandgap Conjugated Polymers: Synthesis by Direct Arylation Polymerization and Characterization and Performance in Organic Photovoltaics and Organic Field-Effect Transistors

The synthesis of four different diketopyrrolopyrrole (DPP) low bandgap polymers by direct arylation polymerization (DArP) is reported. These materials were designed for use in organic photovoltaic (OPV) and organic field-effect transistor (OFET) devices. While the DPP conjugated unit was held constant for each of the materials, the alternating unit of the copolymer was varied from thiophene (TTT), to phenyl (TPT), to 3,4-difluorothiophene (TTfT), to 2,5-difluorophenyl (TPfT) creating a series of DPP materials that can be used to study structure-property-performance relationships. Molecular weights (M-w) of 17-110 kg/mol were achieved by DArP and the resulting polymers displayed excellent optical and electrical properties, comparable to previous reports of similar materials synthesized by Stifle or Suzuki polycondensation. The fluorinated TTfT and TPfT materials had similar absorption profiles, but exhibited reduced E-homo levels (by 0.1-0.2 eV) relative to TTT and TPT, which is due to the incorporation of the highly electron withdrawing fluorine atoms. OPVs fabricated with the TTT and TPT materials reached average power conversion efficiencies of nearly 4%. Additionally, OFET hole mobilities on the order of 10(-2) cm(2) V-1 s(-1) were achieved and the fluorine substituted TTfT and TNT materials exhibited a 2- to 3-fold improvement in hole mobility versus their nonfluorinated analogues.