Recently, the Department of Materials Science and Engineering Associate Professor Guo Xugang’s undegraduate Wang Yu Lun became the first SUSTech author in the field of international materials when the first-class journal “Chemistry of Materials” (Chemistry of Materials, Impact Factor: 9.407) published the student’s results on the topic of dithiophene polymer semiconductors, entitled “Alkynyl-Functionalized Head-to-Head Linkage Containing Bithiophene as a Weak Donor Unit for High-Performance Polymer Semiconductors”.

Wang Yu Lun will enter the research group from the next semester to carry out further research work. In November 2016, Wang Yu Lun was invited to present the relevant results on this study in an oral presentation at the 2016 Materials Research Society Fall Meeting in Boston in the USA.

Building blocks having a high degree of backbone planarity, good solubilizing characteristics, and well-tailored physicochemical properties are highly desirable for constructing high-performance polymer semiconductors. Due to the detrimental steric hindrance created by alkyl chain substituents at the 3- and 3′-positions of bithiophene, “head-to-head” linkage containing 3,3′-dialkyl-2,2′-bithiophenes (BTR) are typically avoided in materials design. Replacing alkyl chains with less steric demanding alkynyl chains should greatly reduce steric hindrance by eliminating two H atoms at the sp-hybridized carbon center. Here we report the synthesis of a novel electron donor unit, 3,3′-dialkynyl-2,2′-bithiophene (BTRy), and its incorporation into conjugated polymer backbones. The alkynyl-functionalized head-to-head bithiophene linkage yields polymers with good solubility without sacrificing backbone planarity; the BTRy-based polymers show a high degree of conjugation with a narrow bandgap of ∼1.6 eV. When incorporated into organic thin-film transistors, the polymers exhibit substantial hole mobility, up to 0.13 cm2 V–1 s–1 in top-gated transistors. The electron-withdrawing alkynyl substituents lower the frontier molecular orbitals, imbuing the difluorobenzothiadiazole and difluorobenzoxadiazole copolymers with remarkable ambipolarity: electron mobility > 0.05 cm2 V–1 s–1 and hole mobility ∼0.01 cm2 V–1 s–1 in bottom-gated transistors. In bulk-heterojunction solar cells, the BTRy-based polymers show promising power conversion efficiencies approaching 8% with very large Voc values of 0.91–1.04 V, due to the weak electron-withdrawing alkynyl substituents. In comparison to the tetrathiophene-based polymer analogues based on the unsubstituted π-spacer design, the BTRy-based polymers have comparable light absorption but with 0.14 V larger open-circuit voltage, translating to enhanced optoelectronic properties for this attractive design strategy. Thus, alkynyl groups are versatile semiconductor substituents, offering good solubility, substantial backbone planarity, optimized optoelectronic properties, and film crystallinity, for materials innovation in organic electronics. See the full details of the paper at http://pubs.acs.org/doi/abs/10.1021/acs.chemmater.7b01052

Contribution: Department of Materials Science and Engineering