Investigation of the Mobility–Stretchability Relationship of Ester-Substituted Polythiophene Derivatives

Abstract

Poly(3-hexylthiophene) (P3HT) has been one of the most important organic semiconductors in the past few decades. To date, considerable effort has been dedicated toward improving its charge mobility, air sensitivity, and even stretchability. In this work, through a systematic investigation, the ester-substituted side chain is manifested to effectively improve the charge mobility, air stability, and stretchability of polythiophenes. Herein, three series of polythiophenes with different side chains and backbones are synthesized and compared. First, phenol, 4,4′-(3H-1,2-benzoxathiol-3-ylidene)bis[2,6-dibromo-3-methyl-S,S-dioxide, monosodium salt] (PBTBO) consisting of a common butyloctyl side chain is developed and compared with poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT) comprising an ester-substituted side chain. Second, PDCBT is compared with thieno[3,2-b]thiophene (PDCTT), for which the bithiophene-based backbone is changed to a thienothiophene-based backbone. Finally, backbone fluorination is imposed by copolymerization with a fluorinated bithiophene moiety to obtain PBTBO-F, PDCBT-F, and PDCTT-F. Our results reveal that the aggregation behaviors and solid-state stacking patterns of polythiophenes can be finely modulated by these side-chain and backbone modifications. The ester-substituted side chain is found to effectively improve the charge mobility of the derived polymers and it can be further enhanced by backbone planarization, as evidenced by the best hole mobility (μh) (0.44 cm2 V–1 s–1) observed for PDCTT. Moreover, the ester side-chain substitution also results in air-stable mobility and enriches the stretchability of polythiophenes. As a result, PDCBT exhibits the best mobility–stretchability property among the prepared polythiophenes. It not only yields air-stable hole mobility (μh) (>0.1 cm2 V–1 s–1 and stable for one month under ambient conditions), outperforming the representative P3HT, but also retains μh up to 0.018 cm2 V–1 s–1 with 100% strain (two orders higher than the value of P3HT with the same strain). Moreover, it can preserve 81% of the initial mobility (with the mechanical strain perpendicular to the charge-transporting direction) after 400 stretch–release cycles with 100% strain, showing good mechanical stability. Our work proposes an efficient structural design strategy to enhance the mobility, air stability, and intrinsic stretchability of polythiophenes.

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