New organic thermoelectric device can produce energy even at room temperature

Researchers at Kyushu University in Japan have designed a new organic device to harvest energy at room temperature. This paves the way for building non-toxic, flexible, and even large-scale devices that could be powered by ambient temperatures in the future.

At the core of this technology is an older approach that has enabled even the Voyager probe to source power from an onboard device. Called thermoelectric generators, these devices can convert heat into electricity if they have a thermal gradient – one end of the material is hot while the other is cool.

The approach can also be used to tap into waste heat and repurpose it, making it a ‘hot’ topic for research and development. NASA even used a thermoelectric generator on its Curiosity Rover, which contained a radioactive isotope that generated heat. The gradient thus generated was used to power onboard instruments.

However, creating devices with radioactive isotopes for general use is expensive, inefficient, and even hazardous. So, a research team led by Chihaya Adachi, a professor at Kyushu University, worked on developing a thermoelectric device that uses organic materials and works at ambient temperature.

“Just as solar cells generate electric current by absorbing photons, the mechanism of this thermoelectric device generates electric current by absorbing phonons,” Adachi told Interesting Engineering in an email. A phonon is a quantum unit of vibration or sound.

How was the energy harvester device made?

The researchers working at the University’s Center for Organic Photonics and Electronics Research (OPERA) were inspired by organic LEDs and solar cells , which use organic compounds.

In their application, the key was to find compounds that could play the role of a charge transfer interface. “The basic idea is to combine a donor molecule that can easily donate electrons to an acceptor molecule that has a strong ability to withdraw electrons,” explained Adachi in the email to IE . “There are only a limited number of acceptor materials that satisfy this requirement.”

After going through a database of over 3,000 candidates, the team optimized the device design using copper phthalocyanine (CuPc) and copper hexadecafluoro phthalocyanine (F16CuPc) and also included fullerenes and BCP to improve the device’s thermoelectric properties.

“These are known to be good facilitators of electron transport. Adding these compounds together significantly enhanced the device’s power,” Adachi said.

Output of the device

The researchers’ optimized device design included a 180 nm layer of CuPc, 320 nm of F16CuPc, 20 nm of fullerene, and 20 nm of BCP.

“Although the role of fullerenes and BCPs is still unclear, I can say that they serve to extract electrons generated at the organic-CT interface to the electrode. These materials have relatively long electron diffusion coefficients and are used universally in organic solar cells,” Adachi told IE .

The device’s open circuit voltage was 384 mV, and its short-circuit current density was 1.1 uA per cm square, with a maximum output of 94 nW/ cm square, the statement added. “The current value is less than micro-amperes/cm2, but it is relatively easy to improve the current value by increasing the large area,” Adachi told IE .

The team will also work on optimizing the device using different materials.

The research findings were published in Nature Communications today.