C–H σ-Dopants Mediated n-Doping of Conjugated Polymers: Mutual Designs and Multiscale Characteristics

Abstract

Conjugated polymers have gained significant interest in recent decades, offering complementary advantages over traditional inorganic electronic materials in ways such as solution processability, mechanical flexibility, and structural diversity afforded via bottom-up organic synthesis. Doping is a crucial aspect for advancing these materials as it adjusts the energies, spatial distributions, and occupancies of the orbitals, affecting the carrier density and mobility. Compared to their p-doping counterparts, n-doping strategies lag behind in their performances. The combination of p-type and n-type polymers is essential for many organic optoelectronic applications, which signified the importance of developing novel n-doping solutions. Traditional electron transfer-type n-dopants, which rely on a low ionization potential for their reactivity, face challenges in processability, reversibility, and sensitivity to ambient conditions. In contrast, C–H σ-dopants operate through different mechanisms. This could open up new avenues to reconcile these conflicts.

In this Account, we present our recent efforts to establish a multiscale model for understanding the determinants of n-doping conjugated polymers with C–H σ-dopants. Central to the doping process are the molecular structures of the dopant and the polymer repeating unit, as their interactions dictate reaction kinetics and lay the foundation for the electronic structures of the doped polymers. Backbone conformation is pivotal for orbital delocalization and π–π stacking, affecting the intrachain charge transport process and interactions between polymer backbones. Beyond single-molecule behaviors, achieving ordered polymer–polymer stacking structures is crucial for enhanced electrical performance. This requirement coexists with the need for solubility and efficient doping, influenced by polymer-dopant or polymer–solvent interactions. Fine-tuning these interactions involves considerations of the polymer conformation, side-chain structure, dopant design, and solvent selection. Induced disorders in the side-chain packing region can accommodate the orderly arrangement of polymer backbones, while tolerating dopant molecules, preventing phase segregation, and reducing the tendency for the cationic dopant byproducts to interact with the oppositely charged polymer backbones. We also demonstrated that polarized side chains could minimize transport barriers caused by electrostatic interactions and reduce the impact of dopant cations on backbone packing. Finally, we explored the effects of polymer microstructures including phase segregation and crystallization behaviors. The practical importance of considering these multifaceted factors is illustrated through the construction of some of the best-performing flexible thermoelectric generators. From theories and molecular designs to characterizations and applications, this Account provides a comprehensive framework for further exploration in the field of n-doping of conjugated polymers, paving the way for the next generation of organic electronic devices.