Legislative Documents

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Enhancing Polymer Photovoltaic Performance via Optimized Intramolecular Ester-Based Noncovalent Sulfur···Oxygen Interactions

Head-to-head (HH) bithiophenes are typically avoided in polymer semiconductors since they engender undesirable steric repulsions, leading to a twisted backbone. While introducing electron-donating alkoxy chains can lead to intramolecular noncovalent S···O interactions, this comes at the cost of elevating the HOMOs and compromising polymer solar cell (PSC) performance. To address the limitation, a novel HH bithiophene featuring an electron-withdrawing ester functionality, 3-alkoxycarbonyl-3′-alkoxy-2,2′-bithiophene (TETOR), is synthesized. Single crystal diffraction reveals a planar TETOR conformation (versus highly twisted diester bithiophene), showing distinctive advantages of incorporating alkoxy on promoting backbone planarity. Compared to first-generation 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR), TETOR contains an additional planarizing (thienyl)­S···O­(carbonyl) interaction. Consequently, TETOR-based polymer (TffBT-TETOR) has greatly lower-lying FMOs, stronger aggregation, closer π-stacking, and better miscibility with fullerenes versus the TRTOR-based counterpart (TffBT-TRTOR). These characteristics are attributed to the additional S···O interaction and electron-withdrawing ester substituent, which enhances backbone planarity, charge transport, and PSC performance. Thus, TffBT-TETOR-based PSCs exhibit an increased PCE of 10.08%, a larger <i>V</i>_oc of 0.76 V, and a higher <i>J</i>_sc of 18.30 mA cm^–2 than the TffBT-TRTOR-based PSCs. These results demonstrate that optimizing intramolecular noncovalent S···O interactions by incorporating electron-withdrawing ester groups is a powerful strategy for materials invention in organic electronics

Alkynyl-Functionalized Head-to-Head Linkage Containing Bithiophene as a Weak Donor Unit for High-Performance Polymer Semiconductors

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 (<b>BTR</b>) 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 (<b>BTRy</b>), 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 <b>BTRy</b>-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 <b>BTRy</b>-based polymers show promising power conversion efficiencies approaching 8% with very large <i>V</i>_oc 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 <b>BTRy</b>-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