The chemical structure of the fullerene derivative improves the ordering of the molecules (right), while the dopant increases the electrical conductivity. Image: J.A. Koster, University of Groningen.
The chemical structure of the fullerene derivative improves the ordering of the molecules (right), while the dopant increases the electrical conductivity. Image: J.A. Koster, University of Groningen.

Thermoelectric materials can turn a temperature difference into electricity. Organic thermoelectric materials could thus be used to power wearable electronics or sensors, but their power output is still very low.

An international team led by Jan Anton Koster, professor of semiconductor physics at the University of Groningen in the Netherlands, has now produced an n-type organic semiconductor with superior properties that brings these applications a big step closer. The team reports its work in a paper in Nature Communications.

A thermoelectric generator is the only human-made power source outside our solar system. Both Voyager space probes, which were launched in 1977 and are now in interstellar space, are powered by generators that convert heat (in this case, provided by a radioactive source) into an electric current. "The great thing about such generators is that they are solid-state devices, without any moving parts," explains Koster.

However, the inorganic thermoelectric material used in the Voyager generators is not suitable for more mundane applications, because these inorganic materials contain toxic or very rare elements. Furthermore, they are usually rigid and brittle.

"That is why interest in organic thermoelectric materials is increasing," says Koster. Yet, these materials have their own problems. The optimal thermoelectric material is both a phonon glass with a very low thermal conductivity (to maintain a temperature difference) and an electron crystal with high electrical conductivity (to transport the generated current).

"The problem with organic semiconductors," Koster says, "is that they usually have a low electrical conductivity."

Nevertheless, over a decade of experience in developing organic photovoltaic materials at the University of Groningen has put the team on a path to a better organic thermoelectric material. They focused their attention on an n-type semiconductor, which carries a negative charge. For a thermoelectric generator, both n-type and p-type (carrying a positive charge) semiconductors are needed, although the efficiency of organic p-type semiconductors is already quite good.

The team used fullerenes (spherical molecules made up of 60 carbon atoms) with a double-triethylene glycol-type sidechain added to them. To increase the electrical conductivity, they added an n-dopant.

"The fullerenes already have a low thermal conductivity, but adding the side chains makes it even lower, so the material is a very good phonon glass," explains Koster. "Furthermore, these chains also incorporate the dopant and create a very ordered structure during annealing." The dopant makes the material an electric crystal, with an electrical conductivity similar to that of pure fullerenes.

"We have now made the first organic phonon glass electric crystal," Koster says. "But the most exciting part for me is its thermoelectric properties." These are expressed by the ZT value – the T refers to the temperature at which the material operates, while Z incorporates the other material properties. This new material increases the highest ZT value for its class from 0.2 to over 0.3, a sizeable improvement.

"A ZT value of 1 is considered a commercially viable efficiency, but we believe that our material could already be used in applications that require a low output," says Koster. For example, power sensors only require a few microwatts of power, which could be produced by a couple of square centimetres of the new material. "Our collaborators in Milan are already creating thermoelectric generators using fullerenes with a single side chain, which have a lower ZT value than we now have."

According to Koster, the fullerenes, side chain and dopant are all readily available, and the production of the new material can likely be scaled up without too many problems. "The paper has 20 authors from nine different research groups. We used our combined knowledge of synthetic organic chemistry, organic semiconductors, molecular dynamics, thermal conductivity and X-ray structural studies to get this result. And we already have some ideas on how to further increase the efficiency."

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.