Thermoelectric Performance of IV–VI Compounds with Octahedral‐Like Coordination

Significance 

At present, the rate of carbon emission is alerting. Consequently, much time, money and research has been directed toward the development of an alternative energy source. Potential fossil fuel substitutes have been found in solar, wind, thermoelectric and hydroelectric power systems – among others. The use of thermoelectric power generators to convert waste heat into electricity could help reducing the carbon footprint of mankind. The constituent materials should have a high thermoelectric figure of merit. Unfortunately, maximization of such a figure of merit provides a significant challenge for material design, since it requires optimization of apparently conflicting properties. This situation favours trial-and-error approaches over the development of simple and predictive design rules.

To establish an alternative approach to identify suitable thermoelectric materials, Matteo Cagnoni (PhD candidate), Daniel Führen, and Professor Matthias Wuttig from the I. Institute of Physics (IA) of RWTH Aachen University inferred a correlation in certain chalcogenides between high thermoelectric performance and metavalent bonding, a novel bonding mechanism identified recently by the same group (Wuttig et al., Advanced Materials 2018, 1803777). Convinced that the assessment of a link between the two phenomena would lead to better understanding and application of the aforementioned materials, they investigated the thermoelectric performance of alloys on the tie line between germanium selenide and germanium telluride. Their work is currently published in the research journal, Advanced Materials.

Samples were prepared by DC magnetron co-sputtering deposition and characterized by EDX and XRD techniques. By measuring Seebeck coefficient, electrical conductivity, carrier concentration and optical reflectivity in the mid-infrared spectral range, the authors could draw important conclusions. First, they observed that doping is not responsible for the superior performance of the metavalent samples. Second, they associated the large thermoelectric figure of merit of the metavalent samples to the remarkably strong anisotropy of the effective mass tensor of the relevant charge carriers. Based on a tight-binding model, they argued that the observed anisotropy stems from the p orbital nature of the electronic states of metavalently bonded materials.

In summary, Professor Matthias Wuttig and his research team identified and explained a link between chemical bonding and thermoelectric performance of crystalline IV–VI compounds. Their study presents insights relating the aforementioned properties and even establishes a map for the same, thanks to the correspondence between tight-binding parameters and chemical coordinates. All in all, the elementary model they derived encompassed simple design rules aimed at optimizing the anisotropy in order to maximize the thermoelectric performance. Altogether, the approach presented can also be applied for other material classes with thermoelectric behaviour, in order to identify new thermoelectric materials.

Thermoelectric Performance of IV–VI Compounds with Octahedral‐Like Coordination: A Chemical‐Bonding Perspective - Advances in Engineering

About the author

Matteo Cagnoni received his M.Sc. in Nanotechnologies for ICTs from the Polytechnic University of Turin, Italy, in December 2014. His thesis was developed at the Research Institute of Electrical Communication of Tohoku University, Japan, where he has been Special Research Student from September 2013 to August 2014. There, he has been working on thin-film solar cells based on methylammonium lead trihalide perovskites. In April 2015, he joined the group of Prof. Wuttig at the I. Institute of Physics (IA) of RWTH Aachen University, Germany, where he has been working as Research Assistant until October 2018. His research topics included charge transport and optical properties of chalcogenides for thermoelectric and optical data storage applications. He is currently completing his Ph.D. studies in Physics at RWTH Aachen University.

His current research interests are physics of materials for photovoltaic and thermoelectric applications, with focus on charge transport and optical properties of chalcogenides, as well as analytical modeling of their electronic structure.

About the author

Daniel Führen received his M.Sc. in Physics (focus on Solid State Physics) from RWTH Aachen, in October 2016. His thesis was developed at I. Institute of Physics (IA) of RWTH Aachen University, Germany, where he has been working as Master Student in the group of Prof. Wuttig from September 2015 to October 2016.

Here, he has investigated charge transport and thermal properties of chalcogenides for thermoelectric applications. Currently, he is working at KEX Knowledge Exchange AG (a spin-off from Fraunhofer IPT in Aachen) as Technology and Innovation Consultant providing structured technology and market information tailored to specific company’s needs as the basis for strategic decisions.

About the author

Matthias Wuttig received his Ph.D. in Physics in 1988 from RWTH Aachen/ Forschungszentrum Jülich. From 1995 to 1997 he worked with a Feodor-Lynen stipend at Bell Labs, Murray Hill, New Jersey. He was a visiting professor at several institutions including Lawrence Berkeley Laboratory, Stanford University, Hangzhou University, IBM Almaden, Bell Labs, DSI in Singapore, CiNAM in Marseilles and the Chinese Academy of Sciences in Shanghai. In 1997, he was appointed Full Professor at RWTH Aachen, where his work focusses on the design of novel functional materials. From 2009 to 2018, he was the speaker of the strategy board of RWTH.

Since 2011, he heads a collaborative research centre on resistively switching chalcogenides (SFB 917), funded by the German Science Foundation DFG. In 2013, he received an ERC Advanced Grant to realize novel functionalities by disorder control. He is a member of Acatech and the North Rhine-Westphalian Academy of Sciences and has written about 320 publications (~16.000 citations).

Reference

Matteo Cagnoni, Daniel Führen, Matthias Wuttig. Thermoelectric Performance of IV–VI Compounds with Octahedral‐Like Coordination: A Chemical‐Bonding Perspective. Advanced Materials 2018, volume 30, 1801787.

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