Evaluating the Electronic Structure of Cu₂GeSe₃ for Energy Conversion Applications

Thermoelectrics offer a promising avenue for sustainable energy solutions by enabling the harnessing of waste heat for power generation. Not only do they have applications in various sectors such as wearable medical devices, aerospace and automotives, but they also provide a reliable and low maintenance power source for remote locations¹. The success of a thermoelectric is measured by the dimensionless figure of merit. The highest figure of merit achieved to date is 3.1 by hole doped polycrystalline SnSe at 783K². However, there are issues surrounding mechanical fragility and brittleness, as well as concerns around the long-term chemical and thermal stability. It has classically been difficult to dope, causing problems with optimisation of the material. This is also observed in other high performing thermoelectric materials such as Bi₂Te₃³. PbTe, another high performer, shows less mechanical and stability problems, however, contains highly toxic elements⁴. As a result, developing new thermoelectric materials is still an important area of research.<br/>Cu₂GeSe₃ has been explored as a thermoelectric for many years, it contains mostly cheap, non-toxic and abundant materials, with good thermal and chemical stability. It has shown poor performance as a pure material with a maximum performance of zT 0.3 but has had a recent resurgence in doped systems because of these benefits with elements such as bismuth and zinc ^5,6. However, little is known about the structure and properties of Cu₂GeSe₃. Once believed to be ordered, Cu₂GeSe₃ has now been shown to be more complex and intriguing⁷. It is essential to comprehensively characterise the Cu₂GeSe₃ endmember to ensure its suitability as an energy material.<br/>This work aims to explore the structure and properties of Cu2GeSe3 for use as a thermoelectric by employing computational techniques such as Density Functional Theory via the Vienna Ab initio Simulation Package (VASP) and Cluster Expansions with Monte Carlo sampling.<br/><br/>(1) d’Angelo, M.; Galassi, C.; Lecis, N. Thermoelectric Materials and Applications: A Review. Energies 2023, 16, 6409. https://doi.org/10.3390/en16176409<br/>(2) Zhou, C., Lee, Y.K., Yu, Y. <i>et al.</i> Polycrystalline SnSe with a thermoelectric figure of merit greater than the single crystal. <i>Nat. Mater.</i> <b>20</b>, 1378–1384 (2021).<br/>(3) Tanner Q. Kimberly, Kamil M. Ciesielski, Xiao Qi, Eric S. Toberer, and Susan M. Kauzlarich. High Thermoelectric Performance in 2D Sb2Te3 and Bi2Te3 Nanoplate Composites Enabled by Energy Carrier Filtering and Low Thermal Conductivity. ACS Applied Electronic Materials 6 (5), 2816-2825 (2024).<br/>(4) Shtern, Y., Sherchenkov, A., Shtern, M., Rogachev, M. and Pepelyaev, D., 2023. Challenges and perspective recent trends of enhancing the efficiency of thermoelectric materials on the basis of PbTe. Materials Today Communications, p.107083.<br/>(5) M. Ibáñez, R. Zamani, W. Li, D. Cadavid, S. Gorsse, N.A. Katcho, A. Shavel, A.M. López, J.R. Morante, J. Arbiol, A. Cabot. Chem. Mater., 24 (2012), pp. 4615-4622<br/>(6) Hu, Z., Wei, R., Yan, C., Wan, H., Liu, Y., Cheng, L., Li, Z. and Song, J., 2023. Inducing high power factor in Cu2GeSe3-based bulk materials via (Bi, Zn)-co-doping. ACS Applied Energy Materials, 6(5), pp.2962-2972.<br/>(7) N. B. A.V. Chichagov, M.A. Symonov, Doklady Akademii Nauk, SSSR, 1961, 137, 68–71.