Tuning the Properties of Poly(3-alkylthiophene)s via Side Chain Engineering

Regioregular poly(3-alkylthiophene)s (P3ATs) constitute an important class of conjugated polymers (CPs) for organic electronic applications. In particular, poly(3-hexylthiophene) (P3HT) is one of the most studied and accessible CPs due to it’s simple synthesis that enables kilogram scale production. One way to potentially tune CPs properties without affecting electronic properties is to introduce functional groups several carbons away from the conjugated backbone. However, such modifications can affect CPs self-assembly, crystallinity and π−π stacking, making it challenging to optimize properties while maintaining good charge carrier mobility. It is thus essential to better understand how side chain modifications affect desirable properties. Here, we explore using side chain engineering to increase the relative dielectric constant (ε_r) and to improve the mechanical properties of P3ATs. Results give us guidelines to tune properties while maintaining favorable film morphology for charge carrier mobility.<br/><br/>Conjugated polymers typically have low ε_r (3-4), which limits their application. Increasing ε_r could enhance device performance by promoting exciton dissociation, reducing bimolecular charge carrier recombination, and enhance charge carrier mobility via charge screening. Here, we introduced highly polar methylsulfinyl and methylsulfonyl groups at the end of P3AT side chains. This placement should allow the polar groups to efficiently rotate and increase orientational polarization. High ε_r values were achieved for these functionalized P3ATs based on an accurate capacitance measurement using a gold/ semiconducting polymer/SiO₂/n-doped Si configuration. The ε_r at megahertz and room temperature increased from 3.8 for P3HT to 7.4 for the sulfinylated and 8.1−9.3 for sulfonylated P3ATs. These values are among the highest ε_r reported for CPs. Grazing-incident wide-angle X-ray diffraction results showed that these polar groups decreased the crystallinity for the polythiophene backbones and interfered with the π−π stacking in the crystalline structure, especially the methylsulfonyl groups. From this study, we conclude that the methylsulfinylated polymers may be promising to provide a balance between high ε_r and preserving favorable π−π stacking structure for device applications.<br/><br/>Like most CPs, P3ATs tend to be brittle, limiting their application. Interestingly, one of the polymers with high dielectric constant had unusual mechanical properties. We hypothesized that the improved properties were due to the ester group six carbon away from the backbone. To better understand the effect of ester groups on both mechanical and electronic properties, we synthesized two random copolymer series: poly(3-alkylthiophene-2,5-diyl)-<i>ran</i>-(3-(6-pentanoatehexyl)thiophene-2,5-diyl) where alkyl is either hexyl (P3HT series) or dodecyl (P3DDT series). In both series, the ester-functionalized side chain content was varied. The copolymer’s optical, thermal, structural, electrical and mechanical properties were investigated. Optimal mechanical and electrical properties were obtained with 10% ester content, and the mechanical property improvements were maximized when the co-side chain was shorter (hexyl) than the ester-functionalized side chain. The best combination of robustness and charge carrier mobility was thus obtained for the P3HT random copolymer with 10% ester-functionalized side chain: a high fracture strain (29±6%) combined with a high tensile strength (3.9±0.6 MPa) was achieved while maintaining the same high charge carrier mobility than a similar MW P3HT (0.12±0.01cm²/Vs). These exciting results demonstrate the feasibility of using ester-functionality in optimizing both mechanical and electrical properties of poly(3-alkylthiophene)s.