Organic Thermoelectrics

Figure 1: Schematic representation of a thermoelectric generator (TEG) showing both the hole transporting p-type and the electron transporting n-type leg.

Fossil fuels (e.g. oil, natural gas and coal) are currently the backbone of the world’s energy production, contributing around two-thirds of the total energy output. However, fossil fuel supplies are limited, and the combustion of hydrocarbons has led to a significant increase in anthropogenic carbon dioxide in the atmosphere, leading to a continuous rise in surface temperatures on Earth over the last century. It is therefore paramount to develop new energy sources, able to produce energy without contributing to the greenhouse gas emission responsible for the occurring climate change.

Considering that of all the energy produced only 40% is used to perform actual work, around 60% (± 325×1018 J) of the energy input is lost after conversion as waste heat; the equivalent of burning an additional 8.1×109 tons of oil. Recovering this wasted heat and to converting it back into usable energy, i.e. electricity, would be a significant step forward to increase the energy output without releasing additional greenhouse gases. Thermoelectric generators (TEG) convert a heat differential into an electrical current, a phenomenon known as the Seebeck effect. The most basic TEG is comprised of two legs (one p-type, one n-type) which are connected thermally in parallel, but electrically in series (See Figure 1). Most thermoelectric generators employed these days focus on the recovery of high-temperature waste heat (>500°C) because the used materials are expensive and therefore need to be operated at large temperature differentials to maximise device performance and economic return. However, over 50% of wasted heat originates from low-temperature sources (<200°C), which is more difficult to recover economically.

Organic thermoelectric generators (OTEG) have the potential to bridge this gap because the thermoelectric materials are based on conducting polymers, which are both more economical to produce and solution processable. The processing ease is of particular importance because it allows the use of conventional printing techniques to apply the active materials over large areas, thus increasing the power output at low operating temperatures, while keeping the TEG device production costs low. Furthermore, conjugated polymers have lower Young’s moduli than crystalline inorganic materials, allowing the fabrication of wearable OTEG on flexible substrates. The possibility to fabricate light-weight conformal OTEG opens the opportunity to harvest low-temperature waste heat from unconventional sources (e.g. human body, windows, complex shapes, …), which so far has been impossible to recover with inorganic TEG.

In contrast to inorganic thermoelectrics, the research on organic thermoelectric materials is still in its infancy, despite the considerable potential of OTEG for low-temperature waste heat valorisation. Our group is interested in developing new thermoelectric materials and in understanding the underlying complex relationships between chemical structure, material morphology, charge and phonon transport.