Microstructural evolution dominates the changes in the thermal conductivity of conjugated polymers upon doping

Abstract

In this study, the correlation between the changes in microstructure of conjugated polymers upon doping and their concomitant thermal transport properties is elucidated. Eight conjugated polymers across distinct doping systems are examined: molecular dopants, Lewis acid type dopants, and ion-exchange systems. These findings indicate that, upon doping, there is a decrease in out-of-plane thermal conductivity for five polymers characterized by high structural order, such as those based on thiophene, diketopyrrolopyrrole, and terthiophene-naphthalimide copolymers. Conversely, for polymers with less ordered structures, including regioregular poly(3-hexylthiophene) and thiophene-based polymers with oligoether side chains, an increase in out-of-plane thermal conductivity is observed. To elucidate these trends, several hypotheses are examined: i) enhanced intrinsic thermal anisotropy originating from backbone orientation, ii) variations in the crystalline to amorphous fraction, and (iii) alloying effects resulting from dopant incorporation. Grazing incidence wide-angle X-ray scattering reveals that in-plane alignment exerts a direct influence on both in-plane and out-of-plane thermal conductivities. Photooxidation experiments provide further insights into the role of alloy scattering. Ultimately, the in-plane thermoelectric figure of merit is ascertained for two diketopyrrolopyrrole-based polymers, underscoring the critical importance of measuring thermal and electrical properties in the same orientation to ensure precise thermoelectric evaluation.