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 estersubstituted 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(2butyloctyl)-(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 (μ h ) (0.44 cm 2 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 (μ h ) (>0.1 cm 2 V −1 s −1 and stable for one month under ambient conditions), outperforming the representative P3HT, but also retains μ h up to 0.018 cm 2 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.

Achieving Ultralow, Zero, and Inverted Tunneling Attenuation Coefficients in Molecular Wires with Extended Conjugation

“…Polythiophenes are the most widely investigated π‐conjugated semiconducting polymers, which exhibit tunable as well as effective conjugation lengths and HOMO–LUMO gaps. [ 28 ] Therefore, polythiophenes present high charge mobility (up to 0.41 cm 2 V −1 s −1 ) [ 29 ] and are utilized for a variety of applications in (opto)electronics.…”

Optimizing Morphology to Trade Off Charge Transport and Mechanical Properties of Stretchable Conjugated Polymer Films

“…Generally, the conjugated polymers possess heterogeneous structures with rigid backbones and flexible side chains, where the backbones determinate the optoelectronic properties, and the flexible side chains increase the solubility in processing solvents . Thus, different molecular design strategies can be employed to balance the mechanical and electrical properties of conjugated polymers, that is, preparing triblock copolymers with soft building blocks, − partially breaking the conjugation in backbones, − and incorporating long flexible side chains − or self-healed cross-linking moieties. , These strategies have greatly improved the ductility of conjugated polymers without significantly decreasing the charge mobility (Table ). For example, Bao et al successfully fabricated intrinsically stretchable conjugated polymer films with high mobility of 1.12 cm 2 V –1 s –1 at 100% strain by introducing alkyl spacers with hydrogen-bonding moiety into backbones of conjugated polymers.…”

Intrinsically Stretchable n-Type Polymer Semiconductors through Side Chain Engineering

“…These side chains could significantly impact the mechanical properties of the resulting conjugated polymers since the flexible side chains disperse energy under stress and protect the main chains. Many reports have indicated that the mechanical properties of conjugated polymers can be improved by varying the length, , size, , branched point, and grafted density of the side chains. Additionally, many new side chains have been designed and developed to improve the stretchability of conjugated polymers, including carbosilane, , alkyl-thienyl, poly­(butyl acrylate), and poly­(acrylate amide) …”