Cross‐Plane Thermal Transport in Acceptor‐Doped Thiophene‐Based Polymer Thin Films Investigated by the 3‐Omega Method

Abstract

Thermal transport properties in acceptor‐doped thiophene‐based polymer films such as poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) and poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT) are investigated using the cross‐plane 3‐omega method. A bilayer structure consisting of polymer semiconductors sequentially doped with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and poly(methylmethacrylate) (PMMA) on a Si substrate is originally prepared by spin coating and employed to prevent leakage current from a heater wire during the 3‐omega measurements. Nondoped PBTTT thin film reveals an intrinsic cross‐plane thermal conductivity of 0.49 W m−1 K−1, which is higher than that for nondoped P3HT film (0.20 W m−1 K−1). X‐ray diffraction (XRD) analysis suggests that higher crystallinity of PBTTT films may result in higher thermal conductivity. The intrinsic thermal conductivity of P3HT and the interfacial thermal resistance between PMMA and P3HT are slightly increased at a low doping concentration (0.01 mg mL−1) and decreased at a higher doping concentration of 0.1 mg mL−1, whereas F4TCNQ doping reduces these values monotonously in the case of PBTTT layers. Furthermore, excess doping occurs only for P3HT and increments both intrinsic thermal conductivity and thermal resistance at PMMA/P3HT interface. The observed trend in the cross‐plane thermal transport caused by the sequential doping can be attributed to the differences between P3HT and PBTTT in their crystallinity and effective doping levels.