Significant Thickness Dependence of In-Plane Thermal Conductivity in Poly(3-hexylthiophene-2,5-diyl) Thin Films

Conjugated polymers are promising materials for flexible electronics, with poly(3-hexylthiophene) (P3HT) standing out for its ease of solution-based processing. However, the influence of the molecular arrangement on thermal transport remains less understood, particularly with respect to thin-film thickness. Here, we investigate the in-plane thermal conductivity of P3HT thin films with thicknesses ranging from 12 to 136 nm using transient thermal grating (TTG) spectroscopy, a precise noncontact optical technique. A pronounced thickness dependence is observed, with the highest thermal conductivity of 0.88 W/m·K in the thinnest film and the lowest value of 0.25 W/m·K in the thickest film. The enhancement in thermal conductivity in thinner films is attributed to denser lamellar packing in face-on configurations, an increased contribution of face-on domains, and spatial confinement-induced improvements in short-range ordering within the amorphous regions of thinner films. These findings demonstrate that the film thickness is an effective parameter for tuning in-plane thermal conductivity in P3HT thin films.