Possible Charge Density Wave and Enhancement of Thermoelectric Properties at Mild-Temperature Range in n‑Type CuI-Doped Bi 2 Te 2.1 Se 0.9 Compounds Hyunyong Cho, † Jae Hyun Yun, † Jin Hee Kim, † Song Yi Back, † Ho Seong Lee, ‡ Sung Jin Kim, § Seokyeong Byeon, ∥ Hyungyu Jin, ∥ and Jong-Soo Rhyee* ,† † Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Gyung-gi 17104, Korea ‡ School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Korea § Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 03760, Korea ∥ Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 37673, Korea ABSTRACT: Bi 2 Te 3 -based compounds have long been studied as thermoelectric materials in cooling applications near room temperature. Here, we investigated the thermoelectric properties of CuI-doped Bi 2 Te 2.1 Se 0.9 compounds. The Cu/I codoping induces the lattice distortion partially in the matrix. We report that the charge density wave caused by the local lattice distortion affects the electrical and thermal transport properties. From the high-temperature specific heat, we found a first-order phase transitions near 490 and 575 K for CuI-doped compounds (CuI) x Bi 2 Te 2.1 Se 0.9 (x = 0.3 and 0.6%), respectively. It is not a structural phase transition, confirming from the high-temperature X-ray diffraction. The temperature-dependent electrical resistivity shows a typical behavior of charge density wave transition, which is consistent with the temperature-dependent Seebeck coefficient and thermal conductivity. The transmission electron microscopy and electron diffraction show a local lattice distortion, driven by the charge density wave transition. The charge density wave formation in the Bi 2 Te 3 -based compounds are exceptional because of the possibility of coexistence of charge density wave and topological surface states. From the Kubo formula and Boltzmann transport calculations, the formation of charge density wave enhances the power factor. The lattice modulation and charge density wave decrease lattice thermal conductivity, resulting in the enhancement of thermoelectric performance simultaneously in CuI-doped samples. Consequently, an enhancement of thermoelectric performance ZT over 1.0 is achieved at 448 K in the (CuI) 0.003 Bi 2 Te 2.1 Se 0.9 sample. The enhancement of ZT at high temperature gives rise to a superior average ZT avg (1.0) value than those of previously reported ones. KEYWORDS: Bi 2 Te 3 , codoping, thermoelectric, charge density wave, lattice modulation, phase transition 1. INTRODUCTION Thermoelectric (TE) materials that can directly exchange heat and electricity and vice versa are widely used in various devices such as heat generators and thermoelectric coolers. The performance of TE materials is characterized by the dimensionless figure of merit ZT, defined as ZT = S 2 σT/(κ lat + κ el ), where S, σ, κ lat , κ el , and T are Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal conductivity, and absolute temperature, respectively. 1,2 When the thermoelectric materials have good performance, it can be used in various devices such as alternative energy-saving systems and solid-state cooling. Generally, more than 60% of the primary energy in the industry is lost to waste heat, and much of the waste heat is in the low-temperature region near 100−300 °C. Therefore, there have been much efforts to increase thermoelectric performance at the low-mid temper- ature range up to 300 °C. 3−7 Typically, Bi 2 Te 3 -based materials are well-known for their excellent TE material near room Received: October 28, 2019 Accepted: December 18, 2019 Published: December 18, 2019 Research Article www.acsami.org Cite This: ACS Appl. Mater. Interfaces 2020, 12, 925-933 © 2019 American Chemical Society 925 DOI: 10.1021/acsami.9b19398 ACS Appl. Mater. Interfaces 2020, 12, 925−933 Downloaded via KYUNG HEE UNIV on April 27, 2023 at 00:26:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.