High thermal conductivity states and enhanced figure of merit in aligned polymer thermoelectric materials

Abstract

Control of morphology in polymer thermoelectric materials is critical to their performance. In this work we study highly aligned polymer thermoelectric materials prepared by mechanical rubbing. We observe a remarkable range of thermal conductivity states from <0.2 W m⁻¹ K⁻¹ to >1 W m⁻¹ K⁻¹ when comparing measurements made parallel and perpendicular to the chain alignment direction and in isotropic films. Nanomechanical analysis reveals that the high thermal conductivity films are stiffer, but this does not fully account for the increase in thermal conductivity. The underlying morphologies of the materials are studied using electron diffraction and Raman spectroscopy and correlated to the electronic and thermal transport states. Despite the discovery of high thermal conductivity states, mechanical rubbing results in a power factor enhancement along the rubbing direction that far outweighs the increase in thermal conductivity, resulting in a 25-fold improvement in the thermoelectric figure of merit, ZT, as compared to the isotropic doped films.