Probing charge-carrier doping effects on interchain thermal transport in edge-on–oriented conjugated polymer thin films

Understanding how charge-carrier states, including polarons and bipolarons, and charge-carrier concentration influence thermal transport along the alkyl side-chain direction in semiconducting conjugated polymer thin films is of both fundamental interest and technological importance. However, the underlying thermal transport mechanisms remain poorly understood. In this research, undoped and doped P3HT thin films with preferred edge-on chain orientation were systematically fabricated on ITO-coated substrates. The charge carrier state and carrier concentration in these films are tuned through electrochemical doping. Thermal transport along the alkyl side-chain direction is then investigated using frequency-domain thermoreflectance technique. The best-fit cross-plane thermal conductivity exhibits a slight decrease with increasing doping level, reaching a reduction of ∼26% at 1.1 V vs Ag/AgCl, while electrical conductivity increases by several orders of magnitude. At the highest applied potential, the cumulative charge reaches ∼0.27 e per thiophene unit. Grazing-incidence wide-angle X-ray scattering measurements reveal anisotropic doping-induced structural changes, including expanded side-chain lamellar spacing and slightly contracted π–π stacking distance. These structural changes indicate enhanced scattering of heat carriers associated with vibrational transport along the alkyl side-chain lamellar direction. The results suggest that charge-carrier-induced structural disorder along side-chain-mediated intermolecular pathways may contribute to limiting thermal transport in doped semiconducting conjugated polymers.