Thermoelectric materials can convert a temperature difference into electricity. Organic thermoelectric materials could be used to power portable electronics or sensors. However, the output power is still very low. An international team led by Jan Anton Koster, Professor of Semiconductor Physics at the University of Groningen, has now produced an n-type organic semiconductor with superior properties that brings these applications a big step closer. Their results were published on November 10 in the journal Nature Communications.
The thermoelectric generator is the only man-made power source outside of our solar system: both Voyager space probes, launched in 1977 and now in interstellar space, are powered by generators that convert heat (in this case from a radioactive source) into an electric current. “The great thing about generators like this is that they are solid-state devices with no moving parts,” explains Koster.
However, the inorganic thermoelectric material used in Voyager’s generators is not suitable for more everyday applications. These inorganic materials contain toxic or very rare elements. In addition, they are usually rigid and brittle. “This is why there is increasing interest in organic thermoelectric materials,” says Koster. However, these materials have their own problems. The optimal thermoelectric material is a phonon glass with a very low thermal conductivity (so that it can maintain a temperature difference) and an electron crystal with high electrical conductivity (to transport the electricity generated). Koster: “The problem with organic semiconductors is that they usually have low electrical conductivity.”
Nonetheless, over a decade of experience developing organic photovoltaic materials at the University of Groningen has put the team on the path to a better organic thermoelectric material. They focused their attention on an n-type semiconductor that carries a negative charge. Both n-type and p-type (with positive charge) semiconductors are required for a thermoelectric generator, although the efficiency of p-type organic semiconductors is already quite good.
The team used fullerenes (buckyballs made up of 60 carbon atoms) to which a double triethylene glycol-type side chain was added. In order to increase the electrical conductivity, an n-type dopant was added. “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,” says Koster. “In addition, these chains also contain the dopant and create a very ordered structure during annealing.” The latter makes the material an electrical crystal with an electrical conductivity similar to that of pure fullerenes.
“We have now made the first electric crystal from organic phonon glass,” says Koster. “But the most exciting thing for me is the thermoelectric properties.” These are expressed by the ZT value. The T refers to the temperature at which the material works, while Z contains the other material properties. The new material increases the highest ZT value in its class from 0.2 to over 0.3, a significant improvement.
“A ZT of 1 is considered an economically viable efficiency, but we believe our material can already be used in applications that require low power,” says Koster. For example, powering sensors requires a few microwatts of power that could be generated by a few square centimeters of the new material. “Our people in Milan are already making thermoelectric generators from fullerenes with a single side chain that have a lower ZT value than we do now.”
The fullerenes, side chain, and dopant are all readily available, and production of the new material can probably be increased without too many problems, according to Koster. He is very pleased with the results of this study. ‘The paper has twenty authors from nine different research groups. We have used our combined knowledge of synthetic organic chemistry, organic semiconductors, molecular dynamics, thermal conductivity, and X-ray structure studies to achieve this result. And we already have some ideas on how we can further increase efficiency. ‘
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