Interplay of Thermoelectric and Mechanical Properties of Doped Conjugated Polymers

Doktorsavhandling, 2025

The rapid growth of interconnected and wearable devices, which together make up the so-called Internet of Things, is increasing the demand for autonomous and on-site energy supplies. One promising solution relies on harvesting heat, an abundant and often wasted energy source, using thermoelectric generators (TEGs) that can directly convert heat into electricity. Wearable small devices, such as health-monitoring sensors and GPS, may be powered by the heat dissipated from the human body using TEGs made of organic semiconductors, such as conjugated polymers, which offer the advantages to be lightweight and flexible. However, to design effective organic TEGs, a good thermoelectric perfomance of conjugated polymers alone is not enough. Long-term stability, bulk processability and mechanical robustness are also essential for their use in wearable electronics.

This thesis explores the structure–property relationships governing the thermoelectric and mechanical behavior of p- and n-type conjugated polymers for their potential use in flexible and wearable electronics. Here, side-chain engineering is used as a tool to improve the thermoelectric performance of thiophene-based conjugated polymers. Shorter oligoether side chains enhance solid-state order, leading to improved charge-carrier mobility and thus a p-type conductor with high electrical conductivity. Additionally, the formation of lateral doping gradients, achieved through the drift of dopant counterions in an electric field, is explored. In turn, gradients are proposed as a viable strategy to improve the thermoelectric performance of non-optimized doped polymers and can serve as a screening tool for new materials. Moreover, the effect of chemical doping on the nanostructure and mechanical behavior of conjugated polymers with oligoether side chains is investigated. Doping enhances solid-state order of these materials, increases the temperatures associated with the onset of polymer relaxation, and raises their elastic modulus. The extent of these changes depends on the type of dopant counterion, suggesting that counterion selection offers a strategy for tailoring the mechanical properties, enabling the design of soft conductors needed for wearable electronics. Finally, the mechanical properties of a n-type conjugated polymer are studied. The suitability of this n-type polymer as coating material for the preparation of conductive multifilaments is assessed, which show a promising stability with a half-life of more than 3 years. The importance of air stability and mechanical robustness for the development of wearable organic TEGs is highlighted throughout.