Side-Chain Branching Dictates the σ-S Coupling in Conjugated Polymer Thermoelectrics

Abstract

Understanding the intrinsic coupling between electrical conductivity (sigma) and the Seebeck coefficient (S) remains a central challenge in organic thermoelectrics, where energetic disorder and charge transport are highly sensitive to molecular design. Here, we show that precise control over the side-chain branching position provides an effective structural lever to tune the sigma-S relationship in conjugated polymers. Two DPP-selenophene copolymers with identical backbones but branched at distinct positions exhibit markedly different molecular packing, charge-carrier delocalization, and density-of-states (DOS) widths. Polymers with more distant branching points form tighter pi-pi stacks, yielding enhanced carrier mobility and a narrower DOS that collectively boost sigma to 129.3 S cm- 1. In contrast, closer branching induces greater energetic disorder and broader DOS distributions, resulting in a substantially higher S of 160 & micro;V K- 1. Despite their contrasting transport characteristics, both polymers deliver similar peak power factors owing to complementary changes in sigma and S. These results identify side-chain branching as a previously underappreciated design parameter that mechanistically governs the coupling between conductivity and Seebeck coefficient in organic thermoelectric materials.