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Thermoelectric materials can transform a temperature difference into electricity. Organic thermoelectric materials could be used to power wearable 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 organic n-type semiconductor with superior properties that takes these applications a big step forward. Their findings were published in the journal Nature Communications on November 10.
The thermoelectric generator is the only man-made energy source outside 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, supplied by a radioactive source) into an electric current. “The great thing about these generators is that they are solid-state devices with no moving parts,” explains Koster.
Conductivity
However, the inorganic thermoelectric material used in Voyager’s generators is not suitable for more mundane applications. These inorganic materials contain toxic or very rare elements. In addition, they are generally rigid and brittle. “This is why interest in organic thermoelectric materials is on the rise,” says Koster. However, these materials have their own problems. The optimal thermoelectric material is a phononic glass, which has a very low thermal conductivity (so that a temperature difference can be maintained) and also an electron crystal with high electrical conductivity (to carry the generated current). Koster: “The problem with organic semiconductors is that they usually have low electrical conductivity.”
However, over a decade of experience developing organic photovoltaic materials at the University of Groningen led the team on a path towards 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 semiconductors (which carry positive charge) are required, although the efficiency of organic p-type semiconductors is already quite good.
Buckyballs
The team used fullerenes (buckyballs, consisting of sixty carbon atoms) with the addition of a double triethylene glycol side chain. To increase the electrical conductivity, a dopant No. was added. “Fullerenes already have low thermal conductivity, but the addition of the side chains makes it even lower, so the material is a very good phononic glass,” says Koster. “Furthermore, these chains also incorporate the dopant and create a very ordered structure during annealing.” The latter makes the material an electric crystal, with an electrical conductivity similar to that of pure fullerenes.
“We have now made the first electric crystal out of organic phononic glass,” says Koster. “But the most exciting part for me is its thermoelectric properties.” These are expressed by the ZT value. T refers to the temperature at which the material operates, while Z incorporates the other properties of the material. The new material raises the highest ZT value in its class from 0.2 to over 0.3, a notable improvement.
Sensors
“A ZT value of 1 is considered a commercially viable efficiency, but we believe our material can already be used in applications that require low performance,” says Koster. To power the sensors, for example, a few microwatts of power are required, and these could be produced from a couple of square centimeters of the new material. “Our collaborators in Milan are already making thermoelectric generators using fullerenes with a single side chain, which have a lower ZT value than we have now.”
The fullerenes, the side chain, and the dopant are all readily available, and production of the new material can likely be increased without too much trouble, according to Koster. He is extremely pleased with the results of this study. “The paper has twenty 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 accomplish this. And we already have some ideas on how. further increase efficiency. ‘
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