Effects of Side Chain Branch Point on Self Assembly, Structure, and Electronic Properties of High Mobility Semiconducting Polymers

Abstract

Semiconducting polymers exhibiting lyotropic liquid-crystalline (LC) mesophases allow for precise control over crystallinity and crystallite orientation using scalable equilibrium processing techniques. The ability to control morphologies from molecular to macroscopic scales is desirable for optimizing polymer electronic devices such as field-effect transistors. LC mesophases can form in semiconducting polymers with branched side chains, but steric repulsion from the side chain structure increases π-stacking distances. This change in unit cell negatively impacts electronic transport, negating any improvements gained by achieving more favorable mesostructures. Herein, we show that the π-stacking distance in cyclopenta­dithiophene-benzo­thiadiazole donor–acceptor copolymers is successively shortened by moving the branch point further away from the conjugated backbone. These materials retain their favorable LC properties, and the change in unit cell is accompanied by a monotonic increase in charge carrier mobility in field effect transistors. Mobility values span from ∼10–4 to 0.41 cm2 V–1 s–1, reaching their highest value when the branch point is furthest from the polymer backbone. In addition, these LC materials can be aligned across large length scales on rubbed polyimide, resulting in an improvement of charge carrier mobility by a factor of 56 when comparing transistors fabricated parallel and perpendicular to the alignment direction.

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

“…Branching alkyl chains are usually attached to the donor and/or acceptor moieties of conjugated D–A polymers to improve their solubilities in organic solvents. Many groups have investigated the effect of the branching point position of branching alkyl chain on the solid state structures and charge transporting behaviors of conjugated polymers ( Scheme ) . In most cases, the polymer backbones become more planar, and the interchain packing and charge mobilities are improved after moving the branching points away from the backbones.…”

“…The effect of branching point of side chains on the interchain packing and semiconducting performance was also explored for DPP‐based conjugated polymers . Chung and co‐workers prepared DPP‐based conjugated polymers P57 – P60 with either TVT or the selenophene analog as the electron acceptors .…”

The Polymer Physics of Multiscale Charge Transport in Conjugated Systems

“…Similarly, Bridges et al . systematically shortened the π‐stacking distance in cyclopentadithiophene‐benzothiadiazole copolymers by moving the branch point further away from the conjugated backbone . As the π‐stacking distance decreases from 3.8 to 3.5 å, the mobility of the resulting transistor increases from ~10 −4 to 0.4 cm 2 V −1 s −1 .…”

Liquid Crystal Ordering on Conjugated Polymers Film Morphology for High Performance

“…Rachel A. Segalman and coworkers reported a self‐synthesized polymer based on the PCDTPT main chain, which exhibited nematic ordering in chlorobenzene solution, and polymer alignment was achieved by heating the film deposited on a polyimide alignment layer to 200 °C. A DR of 3~4 was measured using polarized UV–vis absorption measurements, and the anisotropy of the carrier mobility was as high as 57 …”

“…Molecular design principles were proposed to solve this contradiction. According to a recent work by Segalman's group, variation of the branch site in the side chains could reduce the π–π stacking distance while maintaining the nematic liquid crystalline texture in solution; thus, enhanced charge mobility was achieved …”