Understanding thermal transport in polymer semiconductors via two-channel mechanism

The vibrational thermal conductivity of polymer semiconductors is critical to the performance of organic electronic devices, yet its underlying mechanisms remain elusive. This work presents a 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, driven by the substantial difference in force constants between the backbone and side chains. This gives rise to two distinct dispersive longitudinal branches: a backbone branch, characterized by phonon-like propagons that govern thermal conductivity, and a side chain branch, consisting of non-propagating diffusons that make minimal contributions to heat transport. The reduced maximum thermal conductivity of these polymer semiconductors, compared to their side chain-free counterparts, is attributed to phonon scattering by the low-frequency optical modes of the side chains. Our findings establish a foundation for strategically modifying side chains as a potent approach to fine-tuning thermal conductivity.