Anharmonic Effects Revealed by Temperature-Dependent Phonon Lifetimes and Thermal Conductivities in a π-Conjugated...

Anharmonicity plays a central role in many key phenomena in solids. Here, we investigate its impact on the organic semiconductor [1]benzothieno[3,2-b]-benzothiophene (BTBT), whose Raman spectra exhibit signatures of phonon mixing revealed by polarization-dependent measurements. Using molecular dynamics simulations, we compute the spectral energy density to reproduce the experimentally observed temperature-dependent phonon frequencies and lifetimes. To quantify interactions between vibrational modes, we introduce cross-correlation analysis as a tool to characterize phonon mixing across temperatures, revealing that mode coupling persists even at low temperatures. We further explore how these anharmonic effects influence macroscopic thermal transport. Because the thermal conductivity of anharmonic organic semiconductors cannot be captured by the particle-like Peierls-Boltzmann formalism, we solve the Wigner Transport Equation to account for phonon wavelike tunneling effects. Our results show that wave-like transport dominates thermal conductivity, leading to a temperature-activated conductivity along all crystallographic directions, a trend also confirmed by approach-to-equilibrium molecular dynamics simulations.