Thermal conductivity in organic semiconducting polymers

Abstract

The thermoelectric effect in materials, which accounts for the direct conversion between thermal energy and electrical energy, has been practically used to build devices for many purposes, including thermoelectric generators for producing clean electricity from temperature difference and thermoelectric coolers for precisely controlling temperature using electricity.

The efficiency of these devices would benefit from using materials with higher electrical conductivity, higher Seebeck coefficient and lower thermal conductivity; certain organic semiconducting polymers were considered to be potential candidates, and in this work thermal conductivity in these materials was particularly explored.

This thesis started with accurate experimental measurements of in-plane thermal conductivity for selected polymers in the form of thin films, using the recently implemented 3ω-Volklein method. It was found that values of thermal conductivity for different polymers were relatively similar, around 0.4 Wm⁻¹K⁻¹, comparable to many traditional polymers. In addition, these measurements suggested that degree of order in a polymer thin film might play an important role in deciding its thermal conductivity.

Then persistence length, which quantifies the stiffness, of these polymers in dilute solution was determined both experimentally and computationally; although this was an interesting topic on its own, no explicit correlation was found between persistence length in dilute solution and thermal conductivity in thin films with these examined samples.

Previous research has also shown that doping these polymers could lead to a significant increase of electrical conductivity and a relatively less influential change of Seebeck coefficient; practically a low thermal conductivity would be also desired at the same time for achieving the highest possible efficiency of devices. Therefore in this work, thermal conductivity for selected doped samples was quantitatively examined. The Lorenz number calculated using classic Wiedemann-Franz law was found to be much higher in doped polymers with relatively low electrical conductivity than the value for typical metals; however, this number would drop significantly at higher electrical conductivity, making these doped polymers excellent materials for thermoelectric applications.

This work would deepen the current understanding of thermal transport mechanism in organic semiconducting polymers, as well as provide insights for designing and selecting suitable polymers in the future, for the use of various thermoelectric devices.