Organic semiconductors harness waste heat for electricity

Electronic organic materials offer the potential to support alternative and green energy sources to meet escalating global energy demands and strict environmental regulations. An international research team, led by the King Abdullah University of Science and Technology (KAUST), has now developed electron-transporting, n-type organic semiconductors that could help generate electricity from waste heat released by industrial processes and homes.

Thermoelectric generators that can convert temperature changes or gradients into electricity are highly suitable for harnessing waste heat. These readily scalable devices are environmentally friendly and do not have any moving parts, which makes them resistant to wear. Their efficiency in energy conversion hinges on minimising the thermal conductivity of their components while maximising their electrical conductivity and Seebeck coefficient, a direct measure of their ability to produce a thermoelectric current.

At the heart of thermoelectric generators are two electronically different materials, an n-type semiconductor and a hole-transporting (or p-type) semiconductor, which are joined at their ends to form a circuit. The conversion efficiency of the generators depends on both types of semiconductor delivering optimal performance.

Organic thermoelectric materials have recently emerged as easier to process and less toxic than their cheaper and more abundant conventional inorganic counterparts. These new materials also present lower thermal conductivity, but their thermoelectric performance remains inadequate. Typically, doped n-type organic semiconductors are not stable in ambient conditions and display lower electrical conductivities than their p-type equivalents, which have been widely investigated.

“One important challenge is to find n-type organic materials with comparable performance to the best p-type semiconductors,” said KAUST research scientist Hu Chen, who led the new study within the research group of Iain McCulloch.

The KAUST team devised a systematic approach to synthesise air-stable doped n-type organic semiconductors with high thermoelectric performance. The monomers comprised cyclic amides, or lactams, fused with naphthalene and anthracene cores, generating rigid conjugated polymers by a non-toxic metal-free acid-catalysed polymerisation.

“There is no rotational freedom along the backbone, which reduces energetic disorder and subsequently enhances electrical conductivity,” McCulloch said.

In this design, the electron withdrawing lactam groups produced a highly electron-deficient backbone, stabilising the polymer under ambient conditions. Additionally, smaller cores led to larger electron affinity and, consequently, better thermoelectric performance in the polymers, “which had not been so strikingly demonstrated before this work”, McCulloch said. Chen explained that larger cores have a lower density of electron withdrawing groups, which cumulatively decrease the electron affinity.

Described in the Journal of the American Chemical Society, the air-stable polymers are believed to have good commercial potential. The team is now planning to develop scalable processes to allow these materials to be integrated into thermoelectric generators.

Image credit: ©stock.adobe.com/au/miklyxa

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