Donor–acceptor (D–A) conjugated polymers are of great interest for their tunable energy levels, absorption spectra and high performance in organic thin-film transistors (OTFTs) and organic photovoltaics (OPVs). Intensive research effort has been devoted to the structural optimization of D-A polymers in order to achieve higher charge motilities in OTFTs and power conversion efficiency in OPVs. Recently, introducing fluorine (F) onto the polymer backbone has been used as a strategy to lower the energy levels (HOMO/LUMO) of the polymers to supress the hole transport and increase the open-circuit voltage (Voc) in OPVs. 2,2’-bithiophene (BT) is a frequently used donor building block in D-A polymers and its corresponding polymers generally showed good charge transport properties. In this work, we studied the effect of fluorination of the BT unit on its corresponding polymers’ morphological and electronic properties. We chose a diketopyrrolopyrrole (DPP) and BT copolymer, PDQT, as the pristine reference, since it has been well studied and demonstrated excellent OTFT performance in literature. Then we introduced the fluorine atoms on the 3,3’ positions and the 4,4’ positions of the BT respectively. We found that the introduction of fluorine increased the Voc and fill factor (FF) in the OPV devices. But X-ray diffraction analysis showed that it also disrupted the ordered packing of the polymer chain, which led to an inferior charge transport properties in OTFTs.

Effect of fluorination on morphological and electronic properties of donor-acceptor polymers for organic thin-film transistors and organic photovoltaics

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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