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 (<i>M</i>_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 Stille or Suzuki polycondensation. The fluorinated TTfT and TPfT materials had similar absorption profiles, but exhibited reduced <i>E</i>_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 cm² V^–1 s^–1 were achieved and the fluorine substituted TTfT and TPfT materials exhibited a 2- to 3-fold improvement in hole mobility versus their nonfluorinated analogues

Effect of Pendant Functionality in Thieno[3,4‑<i>b</i>]thiophene-<i>alt</i>-benzodithiophene Polymers for OPVs

The performance of organic photovoltaics (OPVs) is heavily dependent on the structure and functionalization of the conjugated polymer used in the active absorbing layer. Using a set of materials based on poly­(thieno­[3,4-<i>b</i>]­thiophene-<i>alt</i>-benzodithiophene) with different alkyl, aryl, perfluoroalkyl, and perfluoroaryl pendant functionalities, we have studied the correlation between absorbance, morphology, crystallinity, charge mobility, and the OPV performance in an effort to identify structure-performance relationships. The perfluorinated pendants on PTF8B and PTFPB were shown to significantly enhance the <i>V</i>_oc in the OPV devices (by ∼0.2 V), but also induced the formation of larger phase separated PCBM-rich domains. PT8B and PTFPB devices reached average efficiencies of ∼3.2%