Understanding thermal transport in polymer semiconductors via two-channel mechanism

The vibrational thermal conductivity of polymer semiconductors is crucial for the performance of organic electronic devices, yet its underlying mechanisms remain elusive. This work presents a novel two-channel model to elucidate the vibrational heat transport in semiconducting polymers with extended side chains. We reveal a striking splitting of vibrational modes along the polymer chain, which is driven by the significant difference in force constants between the covalent bonds of the backbone and the van der Waals forces between the side chains. This leads to the emergence of two distinct longitudinal acoustic-like branches: a dominant backbone branch, featuring phonon-like propagons that predominantly dictate thermal conductivity, and a side chain branch, primarily composed of non-propagating diffusons, contributing minimally to heat transport. The reduced thermal conductivity of these polymer semiconductors relative to their side chain-free counterparts is attributed to phonon scattering with the low-frequency optical modes of the side chains, as validated by both first-principles calculations and inelastic neutron scattering experiments. Our findings highlight that the strategic modification of side chains offers a potent approach for fine-tuning thermal conductivity, as demonstrated by substituting side chain atoms with heavier elements. This finding opens new pathways for improving the thermoelectric performance of polymer semiconductors.