# Cocrystal Engineering of Conjugated Polymer Blends via External Electric Field for Enhanced Charge Transport

## TL;DR

This paper introduces a new method using electric fields to improve charge transport in polymer blends by forming cocrystals.

## Contribution

The first use of an external electric field to induce cocrystallization in conjugated polymer blends, enhancing charge mobility.

## Key findings

- EEF induces co-aggregation and cocrystal formation in P3BT/PQT blends, improving charge mobility.
- P3BT/PQT-C10 retains phase-separated structures despite EEF application.
- Cocrystal formation correlates with solution aggregation behavior and film structure.

## Abstract

Cocrystal engineering that combines different components into cocrystals renders the newly formed materials with innovative and multifunctional properties. However, this strategy is rather limitedly explored in conjugated polymer blends. Herein, we report, for the first time, the investigation into the external electric field (EEF)‐induced cocrystallization (i.e., cocrystals of two components) in conjugated polymer blends: poly(3‐butylthiophene) (P3BT) and poly[3,3'''‐dialkyl‐quaterthiophene]s (PQTs) with various alkyl side‐chain lengths (PQT‐C6, PQT‐C8, and PQT‐C10) for significantly improved charge mobilities, and correlate their different cocrystalline and phase‐separated structures (i.e., producing two different crystals from two components) strongly to the aggregation in the solution. Specifically, all three P3BT/PQT blends display bimodal distributed aggregates in the solution and form phase‐separated structures in the as‐cast film. Upon EEF strategy, P3BT/PQT‐C6 and P3BT/PQT‐C8 co‐aggregate in the solution and self‐assemble into cocrystals in the film at the increased EEF strength, demonstrating the cocrystal‐facilitated charge transport in organic field‐effect transistors (OFETs). Conversely, P3BT/PQT‐C10 retain bimodal distribution of aggregates in the solution and thus phase‐separated structures in the film throughout the EEF process. As such, this work demonstrates the robustness of EEF to craft cocrystals in conjugated polymers for the enhancement of charge mobilities, which may facilitate their application in a wide range of optoelectronic devices.

We report, for the first time, an external electric field (EEF) strategy to achieve co‐aggregation of conjugated polymer blends (i.e., poly(3‐butylthiophene) and poly[3,3'''‐dialkyl‐quaterthiophene]s) in the solution and thus their cocrystals in thin films, demonstrating cocrystal‐improved charge mobilities due to the synergy of two components.