Chemical Doping of Conjugated Polymers for Thermoelectric Applications

Licentiate thesis, 2023

The ongoing development of interconnected and wearable small devices, which together make up the so-called Internet of Things, is driving a rising need for autonomous and on-site energy supplies. Heat, an abundant and often wasted energy source, can be harnessed through thermoelectric generators that can directly convert heat into electricity. Small devices, such as health-monitoring sensors, may be powered by the heat dissipated from the human body using flexible thermoelectric generators made of organic semiconductors, like conjugated polymers.

This thesis discusses polythiophenes with oligoether side chains, which are a promising class of conjugated polymers for the design of flexible, easily processable and light-weight thermoelectric generators.
At first, this thesis investigates the solid-state order of these polymers and how is affected by varying the length of the oligoether side chains and by chemical doping with an oxidizing agent, F4TCNQ, and an acid, H-TFSI. It is showed that inducing order, either by shortening the side-chain length or by doping, enhances the transport properties of the materials, and so their thermoelectric performance.
Secondly, this thesis studies the relationship between the solid-state order of these polymers and their mechanical properties. It is found that electrical and mechanical properties are usually correlated. The induced solid-state order not only improves the electrical conductivity but also enhances the stiffness of the materials. Partial decoupling of the electrical and mechanical properties is found to be possible if a suitable dopant is selected.