Ester-End-Capped Semiconducting Polymers for High-Performance Field-Effect Transistors Processed from Nonhalogenated Solvents

The environmentally friendly processing of high-performance polymer semiconductors necessitates overcoming poor solubility in nonhalogenated solvents and challenges in optimizing molecular packing. In this study, we present an ester-end-capping strategy for developing high-performance and nonhalogenated solvent processable diketopyrrolopyrrole (DPP)-based polymer semiconductors. The incorporation of benzoate ester groups as terminal units significantly enhances the solubility of the polymers, exceeding 12 mg·mL^-1 in nonhalogenated solvents such as toluene and <i>p</i>-xylene. Moreover, the introduced end-capped ester groups critically enable effective control over the molecular arrangement in solid-state films, resulting in tighter π-π stacking and increased crystallite coherence length (CCL). By further engineering the alkyl chain structure (linear versus branched) of the end-capped ester groups, solution-state aggregation can be finely tuned, leading to more ordered molecular packing and larger CCL for DPPTT-Ester-L with a linear alkyl ester in comparison to DPPTT-Ester-B with a branched alkyl ester. In addition, cryogenic transmission electron microscopy images reveal that this ordered arrangement initially forms in the solution state and is retained in the fabricated thin films. Organic field-effect transistors based on DPPTT-Ester-L processed in <i>p</i>-xylene solution exhibit a high hole mobility of 3.19 cm²V^-1s^-1. This represents a 5.7-fold increase compared with unmodified polymer DPPTT and a 2.3-fold enhancement over devices processed using halogenated solvents. Our findings underscore the significant potential of end-group engineering for tuning solubility and molecular packing to enable nonhalogenated solvent processing of high-performance polymer semiconductors.