Structure-to-property relationships for heat transport in organic semiconductors

The low intrinsic thermal conductivity of organic semiconductors is a limitation for applications in which heat is dissipated and has to be transported away from the active region. Conversely, it is a huge advantage for thermal barriers or thermoelectric usage. Even for non-thermal related applications it is important to know about mechanisms that govern the transport of heat. Despite its importance, the area of thermal transport in organic semiconductors is almost uncharted. The present work aims for a better understanding of heat transport in crystalline organic semiconductors by revealing the corresponding structure-to-property relationships. To that aim, density-functional-tight-binding (DFTB) calculations are employed to study the phonon structure of a series of carefully chosen test systems. The thermal conductivity tensor is obtained by using a quasiharmonic lattice dynamics approach as proposed by Bjerg et. al. (Phys. Rev. B. 89, 024304) that uses the mode Grüneisen parameter. Our results illustrate, how the thermal conductivity of an organic semiconductor is impacted by the material’s chemical structure varying the chain length of the molecules, comparing flexible (biphenyl) and rigid (fluorene) structures, and highlighting the impact of anisotropy in a prototypical conjugated polymer (poly-1,4-phenylene).