Design, Synthesis, and Characterization of Electron Transporting DPP-based Copolymers for Application in Organic Electronics
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The complete design, synthesis, characterization, and electrical performance of n-type donor-acceptor polymers based on diketopyrrolopyrrole (DPP) moieties was conducted for application in OPVs, OFETs, and OECTs. First, a family of polymers was designed to determine the impact of two key structural features, thiazole-flanked DPP and thioalkyl side chains, on the stabilization of frontier molecular orbital energy levels and electrical properties. When the two structural features are used together, they have been shown to have a combinatorial effect on energy level stabilization. Likewise, it was determined that the thiazole-flanked DPP is essential in achieving the desired n-type performance. Pump-probe transient absorption spectroscopy was used to probe the efficiency of polaron and exciton formation, leading to a potential explanation for poor solar performance. Second, a thiazole-flanked DPP containing a modifiable linear methyl ester side chain was copolymerized with bithiophene to yield an organic soluble pOMeDPPTz-BTh. The organic side chain demonstrated an average electron mobility of 1.2 x 10-2 cm2/V·s in OFETs but did not have the desired electron affinity for electrochemical stability in aqueous environments. Despite that, saponification of the methyl ester into a potassium salt formed pK+DPPTz-BTh, which is water soluble. Once filmed, an acid soak resulted in pCOOH-DPPTz-BTh which has stronger hydrogen bonding and could result in the desired OECT performance.