Development of Technology for Molecular Thermoelectric Devices Capable of Tolerating High Temperature at Industrial Sites
The thermal stability of molecule-electrode interface was drastically improved by using carbene functional group.

The research results obtained by Professor Hyo Jae Yoon were published in Nano Letters, a renowned journal in nanoscience.

▲Sohyun Park (left, Integrated Master/Ph. D course student, first author) and Yoon Hyo-jae (right, Professor)

Professor Yoon Hyo-jae’s group of the Department of Chemistry, College of Science, developed the world’s first technology for fabricating molecule-based thermoelectric devices that can tolerate the waste heat from industrial sites and exhibit the Seebeck effect.

Thermal energy is ubiquitous, and waste heat is unavoidably generated at industrial sites. If converted to electricity, this waste heat can be an environment-friendly energy source. According to a report from the EU, 90% of the waste heat from industrial processes has a temperature of 300 ℃ (573 K) or less. Hence, in the development of organic material-based thermoelectric devices, it is necessary for realistic application that the devices constantly exhibit the Seebeck effect below 300 ℃.

Many organic materials are vulnerable to structural changes caused by heat. In thermoelectric devices, thermal stability is often lost at the interface with an electrode. The problem is more serious in molecular thermoelectric devices fabricated by using a self-assembled monolayer of thickness 1 to 2 nm. When a SAM is formed on an electrode, chemical bonds are formed to fix the molecules, and a thiol functional group is most frequently employed for this purpose. However, thiol, which is highly vulnerable to heat, is quickly oxidized or induces molecular desorption from an electrode. Therefore, securing the Seebeck effect with high reproducibility and high reliability in molecular thermoelectric devices is a challenge. Thus, studies on molecular thermoelectric devices have been limited to low-temperature ranges until now. The highest temperature attempted in previous studies on thermoelectric devices was approximately 50 ℃, which is far below the temperature required for actual application (300 ℃).

Professor Yoon Hyo-jae’s group of the Department of Chemistry in the College of Science at KU provided an innovative solution to the thermal instability at the molecule-electrode interface by using, instead of a thiol group, a carbene functional group, which forms a strong bond with metals. The carbene group is a ligand molecule that is widely employed in organometallic chemistry to form a strong bond with metals. To test the thermal stability of the carbene group compared to the traditionally used thiol group, the research group obtained thiol and carbene molecules featuring the same molecular skeleton. Then, self-assembled monolayers consisting of the molecules were each prepared to compare their thermoelectric performance (Figure 1). The results showed that when the thiol functional group was incorporated, the Seebeck effect was exhibited at up to 50 to 60 ℃ only, and at higher temperatures, the thermoelectric device was damaged by thermal damage to the thiol groups and the Seebeck effect disappeared. In contrast, when the carbene group was incorporated, the Seebeck effect was continuously exhibited at the much higher temperature of 300 ℃ (Figure 1). Based on these results, it has become possible to fabricate organic-molecule-based nanoscale thermoelectric devices that can tolerate high temperatures while continuously exhibiting the Seebeck effect. Furthermore, because the stability of the molecule-electrode interface can be drastically improved with this approach, the results of the study can generate a large ripple effect on general research into organic thermoelectric devices.

The results of the present study, supported by the National Research Fund of Korea (Basic Science Research Program; Next-Generation Intelligent Semiconductor Program; Priority Research Centers Program), were published online on May 16 in Nano Letters (Nano Letters; https://pubs.acs.org/doi/full/10.1021/acs.nanolett.2c00422), a renowned journal in nanoscience.
- Three authors: Sohyun Park (KU, first author), Seohyun Kang (KU, coauthor) and Yoon Hyo-jae (KU, corresponding author)
- Article title: Thermopower of molecular junction in harsh thermal environments
- Journal: Nano Letters (published online on May 16; https://pubs.acs.org/doi/full/10.1021/acs.nanolett.2c00422)



[ Figures ]


▲ A schematic representation of a thermoelectric device based on a self-assembled monolayer (SAM) prepared using a liquid metal electrode (eutectic Ga-In alloy; EGaIn). The thermoelectric performance of the molecular thermoelectric device fabricated incorporating a nitrogen-heterocyclic carbene (NHC) as an anchor was compared with the device fabricated incorporating the widely used thiol functional group. The use of the carbene group improved the thermal stability such that the device could tolerate the high temperature of 300 ℃ (573 K).