On The Thermal Conductivity of Conjugated Polymers for Thermoelectrics

Abstract

AbstractThe thermal conductivity (κ) governs how heat propagates in a material, and thus is a key parameter that constrains the lifetime of optoelectronic devices and the performance of thermoelectrics (TEs). In organic electronics, understanding what determines κ has been elusive and experimentally challenging. Here, by measuring κ in 17 π‐conjugated materials over different spatial directions, it is statistically shown how microstructure unlocks two markedly different thermal transport regimes. κ in long‐range ordered polymers follows standard thermal transport theories: improved ordering implies higher κ and increased anisotropy. κ increases with stiffer backbones, higher molecular weights and heavier repeat units. Therein, charge and thermal transport go hand‐in‐hand and can be decoupled solely via the film texture, as supported by molecular dynamics simulations. In largely amorphous polymers, however, κ correlates negatively with the persistence length and the mass of the repeat unit, and thus an anomalous, albeit useful, behavior is found. Importantly, it is shown that for quasi‐amorphous co‐polymers (e.g., IDT‐BT) κ decreases with increasing charge mobility, yielding a 10‐fold enhancement of the TE figure‐of‐merit ZT compared to semi‐crystalline counterparts (under comparable electrical conductivities). Finally, specific material design rules for high and low κ in organic semiconductors are provided.