High Mobility Preservation of Near Amorphous Conjugated Polymers in the Stretched States Enabled by Biaxially-Extended Conjugated Side-Chain Design

• Published in: Chemistry of materials Vol. 32; no. 17; pp. 7370 - 7382
• Main Authors: Huang, Yen-Wen, Lin, Yan-Cheng, Yen, Hao-Chi, Chen, Chun-Kai, Lee, Wen-Ya, Chen, Wen-Chang, Chueh, Chu-Chen
• Format: Journal Article
• Language: English
• Published: American Chemical Society 08.09.2020
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.0c02258

Polymers with an abundant amorphous domain should facilitate energy dissipation upon stretching, making near amorphous π-conjugated polymers have immense potential in realizing intrinsically stretchable field-effect transistor (FET) devices. In this study, high mobility preservation under the stretched state is attempted by replacing typical alkyl-monothienyl (T-R) on a benzo­[1,2-b:4,5-b’]­dithiophene-difluorobenzothiadiazole backbone with three other biaxially extended side-chains, including alkyl-dithienyl (2T-R), branching alkyl-trithienyl (3T-R), and alkyl-benzotrithienyl (B3T-R) groups. Despite showing near amorphous features, the semi-2D BDT-based polymers with bulkier biaxially extended side chains (PBDT-2T, PBDT-3T, and PBDT-B3T) still present comparable mobility to the reference semicrystalline polymer (PBDT-T). Although these four polymers yield comparable mobility, they show distinctly different mobility retention in the stretched state. From the study of their mobility-stretchability relationship, the interdigitating and/or entanglement of these biaxially extended conjugated side chains are shown to play a nontrivial role in the resultant mechanical robustness against the stretching force. Owing to the proper spatial mobility and geometry, the branched 3T-R side chain possesses a more intense interdigitating and/or entanglement capability than the linear 2T-R one and the fused B3T-R one, providing better mechanical strength under stretched states. Meanwhile, it maintains sufficient interchain connectivity for intermittent interchain hopping to compensate for the 1D charge transport along the backbone, ensuring good charge transport even in the stretched state. As a result, the printed PBDT-3T film is demonstrated to deliver a high mobility retention of 73% at a 60% strain exerted parallel to the charge-transporting direction and a very stable mobility retention of 88% after 1000 stretching-releasing cycles at a 60% strain, being one of the best stretchable near amorphous conjugated polymers reported thus far. Our result underlines the effectiveness of using biaxially extended conjugated side chains to realize high-performance stretchable polymers.