Thermoelectric Thin Films for Thermal Energy Harvesting

Dear Colleagues,

One of the biggest challenges of the coming years will be meeting energy demand (+30% in 2040) in a cost-effective and environmentally responsible manner [1], which has attracted a huge amount of interest in materials with applications in energy production systems. In addition to the well-known materials for photovoltaic systems, there is another class of materials under intense investigation, thermoelectric ones [2]. These can convert thermal differences into electrical energy. In the case of metal oxide films, by adequately reducing oxygen concentration or by doping these materials with other elements, it is then possible to achieve a very promising figure of merit (ZT > 0.1) [3–10]. From the number of recent publications on the subject, it is clear that there has been increasing interest in materials with thermal properties and associated technologies for power generation and cooling [11], especially for converting temperature gradients into electrical energy, so as to meet the need for alternative energy resources. In order to maximize the figure of merit of these thermoelectric materials, the metric used to characterize their performance, a high Seebeck coefficient, good electrical conductivity, and low thermal conductivity are required [12]. For the case of thin films, it is advantageous in many application scenarios to render these materials with a very high optical transmittance in the visible and near-infrared region of the electromagnetic spectrum. Currently, devices that use this technology already exist on the market, using titanium oxide in the bulk form, and the results are encouraging [13]. However, for certain applications, such as in photovoltaic systems, namely, dye-sensitized solar cells (DSSC), for glass surfaces and for facades of buildings, it is crucial that the material with intrinsic thermoelectric properties be in the form of a very thin coating and transparent, in order not to hinder solar absorption [14,15]. Other potential applications are for touch displays, where thermal heat from the environment and from the user’s touch can be converted to electricity and thus render these devices more sustainable.

This Coatings Special Issue will focus on the recent developments of thermoelectric thin films for thermal energy harvesting, from theoretic fundamentals to applications, covering a wide range of production methods. Insight will be given with regard to increasing the Seebeck coefficient and electrical conductivity and reducing thermal conductivity, with the objective of optimizing their performance. To achieve this, beforehand, it is necessary to study and understand the inherent physical–chemical properties. The optimization of these materials may involve a selective doping of these metal oxides with cations with a larger ionic radius, so that on one hand, it increases the concentration of charge carriers, and on the other hand, it provides phonon dispersion mechanisms that reduce thermal conductivity. Authors are invited to share their knowledge on the inherent physics involved, deposition methods, and strategies; full comprehensive studies of optical and electrical properties are of interest. Additionally, the theoretical and experimental methods for the determination of thermal conductivity need to be understood, so studies on this subject are especially welcome.

1. S. Energy Information Administration. Annual Energy Outlook 2020. Available online: https://www.eia.gov/outlooks/aeo/pdf/AEO2020%20Full%20Report.pdf.
2. Martín-González, M.; Caballero-Calero, O.; Díaz-Chao, P. Nanoengineering thermoelectrics for 21st century: Energy harvesting and other trends in the field. Sustain. Energy Rev. 2013, 24, 288–305.
3. Tritt, T.M.; Subramanian, M.A. Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View. MRS Bulletin 2006, 31, 188–198.
4. Kim, S.; Kim, D.; Byeon, J.; Lim, J.; Song, J.; Park, S.; Park, C.; Song, P. Transparent amorphous oxide semiconductor as excellent thermoelectric materials. Coatings 2018, 8, 462.
5. Du, F.P.; Cao, N.N.; Zhang, Y.F.; Fu, P.; Wu, Y.G.; Lin, Z.D.; Shi, R.; Amini, A.; Cheng, C. PEDOT:PSS/graphene quantum dots films with enhanced thermoelectric properties via strong interfacial interaction and phase separation. Rep. 2018, 8, 6441.
6. Zhao, B.; Chen, K.; Buddhiraju, S.; Bhatt, G.; Lipson, M.; Fan, S. High-performance near-field thermophotovoltaics for waste heat recovery. Nano Energy 2017, 41, 344–350345.
7. Miller, S.A.; Gorai, P.; Aydemir, U.; Mason, T.O.; Stevanović, V.; Toberer, E.S.; Snyder, G.J. SnO as a potential oxide thermoelectric candidate. Mater. Chem. C 2017, 5, 8854—8861.
8. Patil, P.S.; Kadam, L.D. Preparation and characterization of spray pyrolyzednickel oxide (NiO) thin films. Surf. Sci. 2002, 199, 211–221
9. Ohta, H.; Sugiura, K.; Koumoto, K. Recent progress in oxide thermoelectric materials: p-Type Ca₃Co₄O₉ and n-Type SrTiO₃. Chem. 2008, 47, 8429–8436.
10. Sheng, X.; Li, Z.; Cheng, Y. Electronic and thermoelectric properties of V₂O₅, MgV₂O₅ and CaV₂O₅. Coatings 2020, 10, 453.
11. Park, N.W.; Ahn, J.Y.; Cho, N.K.; Park, J.S.; Umar, A.; Lee, S.K. All in-plane thermoelectric properties of atomic layer deposition-grown Al₂O₃/ZnO superlattice film in the temperature range from 300 to 500 K. Adv. Mater. 2017, 9, 1296–1301.
12. Sootsman, J.R.; Chung, D.Y.; Kanatzidis, M.G. New and old concepts in thermoelectric materials. Chem. Int. Ed. 2009, 48, 8616–8639.
13. Conze, S.; Poenicke, A.; Martin, H.P.; Rost, A.; Kinski, I.; Schilm, J.; Michaelis, A. Manufacturing processes for TiO_x-based thermoelectric modules: From suboxide synthesis to module testing. Mater. 2014, 43, 3765–3771.
14. Su, S.; Liu, T.; Wang, Y.; Chen, X.; Wang, J.; Chen, J. Performance optimization analyses and parametric design criteria of a dye-sensitized solar cell thermoelectric hybrid device. Energy. 2014, 120, 16–22.
15. Wang, N.; Han, L.; He, H.; Park, N.H.; Koumoto, K. A novel high-performance photovoltaic–thermoelectric hybrid device. Energy Environ. Sci. 2011, 4, 3676.

Prof. Dr. Carlos Jose Macedo Tavares
Guest Editor

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• thermoelectric
• thermal energy
• energy harvesting
• thin films
• metal oxide
• superlattice
• graphene
• thermal conductivity
• thermoreflectance
• scanning thermal microscopy
• 2 omega
• 3 omega
• thermal wave
• frequency domain
• pulse technique
• phonon scattering
• CVD
• PVD
• ALD