High Thermoelectric Performance of ZnO by Coherent Phonon Scattering and Optimized Charge Transport

• Published in: Advanced functional materials 2021-10, Vol.31 (43), p.n/a
• Main Authors: Acharya, Somnath, Yu, Byung‐Kyu, Hwang, Junphil, Kim, Jiyong, Kim, Woochul
• Format: Article
• Language: English
• Subjects: Bonding strength; Carrier density; Charge transport; coated grain; Coating; coherent phonon scattering; Coherent scattering; Doping; Electrical resistivity; Figure of merit; Heat conductivity; Heat transfer; lattice thermal conductivity; Materials science; oxide thermoelectric; Phonons; Power factor; Thermal conductivity; Thermoelectric materials; Thermoelectricity; Transport properties; Zinc oxide
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202105008

ZnO is identified as a potentially attractive n‐type oxide thermoelectric material due to its abundance, nontoxicity, and a high degree of stability. However, working with ZnO is challenging due to its high thermal conductivity from its strong ionic bonds and low electrical conductivity due to its low charge concentrations. Here, it is demonstrated that the electrical and thermal transport properties of ZnO can be simultaneously improved via the successful doping of Al and ZnS coating. The ZnS coating in Al‐doped ZnO is observed and analyzed through microstructure and spectroscopic studies. The power factor for 1% ZnS‐coated Zn0.98Al0.02O is increased to ≈0.75 mW m−1 K−2 at 1073 K, 161% higher than pure ZnO. This enhancement in the power factor can be explained by the aliovalent Al3+ doping and modifications in intrinsic defects, leading to an increased carrier concentration. Interestingly, ZnS coating significantly reduces lattice thermal conductivity to ≈2.31 W m−1 K−1 at 1073 K for 2% ZnS‐coated Zn0.98Al0.02O, a 62% decrease over pure ZnO. This large reduction in lattice thermal conductivity can be elucidated based on coherent phonon scattering via Callaway's model. Overall, the figure of merit, zT, increases to 0.2 in 2% ZnS‐coated Zn0.98Al0.02O, which is 272% higher than pure ZnO at 1073 K. The employment of a coated grain structure results in a well‐organized approach to enhance the performance of ZnO as a thermoelectric material. Al doping and ZnS coating in ZnO are responsible for the charged defects and enhanced coherent phonon scattering. A significant reduction (≈62%) in lattice thermal conductivity is achieved through the enhanced phonon scattering.