Enhancing charge transport in isoindigo-based donor–acceptor copolymers by combining ionic doping with polar alkoxy side chains

Abstract

Side chains of polymers play a crucial role in manipulating polymer interchain interactions, especially polar side chains that promote strong molecular stacking and facilitate ionic diffusion. Here, we first synthesize a series of isoindigo-based donor–acceptor copolymers with varying ratios of linear alkoxy and branched alkyl side chains (PII2TC8C10, PII2TPEO25, PII2TPEO40, PII2TPEO63). Among this group of polymers, PII2TPEO25 with partial alkoxy chains exhibits a high field-effect mobility of 1.98 ± 0.05 × 10⁻¹ cm² V⁻¹ s⁻¹, comparable to PII2TC8C10 with all branched alkyl side chains. After doping with an ionic additive, i.e., tetrabutylammonium perchlorate (TBAP), the hole mobility of PII2TPEO25 was significantly increased up to 0.37 cm² V⁻¹ s⁻¹, which is almost twice the mobility of its counterpart without the ionic additive. The increase in charge mobility with the addition of TBAP is mainly attributed to the fact that the presence of the ionic species inhibits the torsion of the isoindigo-based copolymer backbone, which is induced by the alkoxy side chains. Polymers with alkoxy side chains have smaller torsion angles and larger crystal sizes than polymers without alkoxy side chains. This study demonstrates that the binding of polar alkoxy side chains to ionic additives is important for charge transport in donor–acceptor copolymers.