PROGRESS REPORT Thermoelectrics www.advelectronicmat.de The Thermoelectric Properties of Bismuth Telluride Ian T. Witting, Thomas C. Chasapis, Francesco Ricci, Matthew Peters, Nicholas A. Heinz, Geoffroy Hautier, and G. Jeffrey Snyder* made for efficient thermoelectric cooling Bismuth telluride is the working material for most Peltier cooling devices or temperature management uses Bi2Te 3 and thermoelectric generators. This is because Bi2Te 3 (or more precisely its alloys. Such solid-state devices dominate the market for temperature control in alloys with Sb2Te 3 for p-type and Bi2Se3 for n-type material) has the highest optoelectronics. As the need to eliminate thermoelectric figure of merit,zT , of any material around room temperature. greenhouse-gas refrigerants increases, Since thermoelectric technology will be greatly enhanced by improving Bi2Te 3 Peltier cooling is becoming more attrac- or finding a superior material, this review aims to identify and quantify the tive particularly in small systems where key material properties that make Bi2Te 3 such a good thermoelectric. The efficiencies are comparable to traditional large zT can be traced to the high band degeneracy, low effective mass, high refrigerant based cooling. Such small carrier mobility, and relatively low lattice thermal conductivity, which all devices may enable distributive heating/ cooling (only where and when it is needed) contribute to its remarkably high thermoelectric quality factor. Using literature with higher system level energy efficiency, data augmented with newer results, these material parameters are quantified, for example in electric vehicles where giving clear insight into the tailoring of the electronic band structure of energy for heating/cooling competes with Bi2Te 3 by alloying, or reducing thermal conductivity by nanostructuring. vehicle range. Even for thermoelectric For example, this analysis clearly shows that the minority carrier excitation power generation, e.g., recovery of waste heat, Bi Te alloys are most used because across the small bandgap significantly limits the thermoelectric performance 2 3 of superior efficiency up to 200 °C and the of Bi Te , even at room temperature, showing that larger bandgap alloys are 2 3 technology to make devices with Bi2Te 3 is needed for higher temperature operation. Such effective material parameters most advanced.[1–3] can also be used for benchmarking future improvements in Bi2Te 3 or new While the material and production tech- replacement materials. nology for making Bi2Te 3-based devices has remained essentially unchanged since the 1960s, our understanding of these materials has advanced considerably. Most 1. Introduction recently, the interest in topological insulators (TI) has led to new insights into the complex electronic structure[4,5] revealing Bismuth telluride, Bi2Te 3 and its alloys with Sb2Te 3 and Bi2Se3 that with the accuracy in assessing the band structures avail- are the most important class of thermoelectric materials able today, improvements in the electronic structure by band because they have the highest known efficiency near room engineering should not only be possible but predictable.[6–9] temperature despite numerous attempts to discover better Indeed, the p-type alloys chosen for use in commercial Peltier materials. Being the best material, virtually every Peltier cooler coolers appear to have unintentionally arrived at a composition close to a band convergence. The understanding of defects and doping is also advancing rapidly that will lead to new strategies I. T. Witting, Dr. T. C. Chasapis, Dr. M. Peters, Prof. G. J. Snyder for additional improvements in the electronic properties. Department of Materials Science and Engineering The thermal conductivity of Bi2Te 3-based alloys can also be Northwestern University engineered, where in particular there is much recent interest in 2220 Campus Drive, Cook Hall 2036, Evanston, IL 60208, USA microstructure engineering or nanostructuring.[10–22] Reduced E-mail: [email protected] thermal conductivity has led to numerous reports of exception- Dr. F. Ricci, Prof. G. Hautier Institute of Condensed Matter and Nanosciences ally high efficiency (zT) that would be sufficient to revolutionize Université catholique de Louvain the industry. However, between measurement and material Louvain-la-Neuve 1348, Belgium uncertainties, a revolutionary new Bi2Te 3-based material has Dr. N. A. Heinz not made it to the market. Because even small but reliable National Aeronautics and Space Administration Jet Propulsion improvements could make significant impact, it is worthwhile Laboratory to better understand all the complex, interdependent effects of 4800 Oak Grove Drive, La Cañada Flintridge, CA 91011, USA band engineering and microstructure engineering. To demon- The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.201800904. strate and quantify improvements in thermoelectric properties, it is necessary to have well characterized properties or reliable DOI: 10.1002/aelm.201800904 benchmarks for comparison. Adv. Electron. Mater. 2019, 1800904 1800904 (1 of 20) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advelectronicmat.de It is the goal of this review to provide benchmark proper- ties of Bi2Te 3 to serve as a basis of comparison when improving Ian Witting earned his Master Bi2Te 3 or finding a new material to replace it. While alloys con- of Science in Materials taining bismuth telluride are the state-of-the-art, we intention- Science and Engineering ally focus primarily on the properties of the binary compound. from North Carolina State By doing so, we aim to explain the fundamental properties University in 2008. He is making this material such an outstanding thermoelectric near currently a Ph.D. candidate in room temperature, but also explore its weaknesses which the Department of Materials necessitate alloying and further engineering. Other recent Science and Engineering reviews have focused on the complex electronic structures and at Northwestern University relationship to topological insulating behavior. Here, after a under the direction of discussion of the atomic structure of Bi2Te 3 as it relates to the Dr. G. Jeffrey Snyder. His complex electronic band structure beneficial to thermoelectrics, research interests include the we discuss the dominant charged defects, which determine engineering of electronic properties and microstructure the electronic properties, and their relationship to synthesis of thermoelectric materials, as well as single crystal growth and processing conditions. We then compile and analyze the of thermoelectrics. most reliable thermoelectric data of oriented Bi2Te 3 from past studies and reviews[1,11–13,44,45] to extract the effective mate- Francesco Ricci obtained a rial properties that can be altered by band and microstructure Master’s in Physics from the engineering. University of Cagliari (Italy), with a thesis about chaotic coupled systems. He received 2. Structure and Chemistry his Ph.D. in Physics from the University of Cagliari under 2.1. Crystal Structure the supervision of Vincenzo Fiorentini with research Bismuth telluride Bi Te , along with Sb Te and Bi Se , 2 3 2 3 2 3 focused on multiferroic crystallizes in the tetradymite crystal structure shown in tunnel junctions, ferroelectric Figure 1. The coordination of Bi and Te is octahedral much materials, and transparent like Pb and Te in the lead chalcogenide IV–VI semiconduc- conducting oxides by using ab initio methods. He is currently tors having the rock salt structure. The slightly less electron- a post-doctoral researcher at the Université catholique de egative Bi compared to more electronegative Te justifies the Louvain. The topic of his current research concerns the association of valence as Bi+3 (s2p0) and Te−2. Like rock salt −2 use of high-throughput approaches for discovering new PbTe, the large Te anions of Bi2Te 3 form a close-packed +3 +2 thermoelectric materials. layer with cations (Bi in Bi2Te 3 or Pb in PbTe) in octahe- dral holes. While rock salt PbTe has an FCC cubic packing of close-packed Te layers, Bi2Te 3 has a hexagonal HCP arrange- G. Jeffrey Snyder is a ment of Te layers. Charge balance is maintained in the Professor of Materials Science and Engineering at Northwestern University in Evanston, Illinois. His inter- ests are focused on materials physics and chemistry for thermoelectric engineering, such as band engineering, design of complex Zintl compounds, and use of nanostructured composites. His interdisciplinary approach stems from studies of solid state chemistry at Cornell University and the Max Planck Institute for Solid State Research, applied physics at Stanford University, and thermoelectric materials and device engineering at NASA/Jet Propulsion Laboratory and California Institute of Technology (Caltech). Figure 1. Layered structure of Bi2Te 3. Bismuth atoms are octahedrally coordinated by tellurium atoms residing on two inequivalent sites, tetradymite V–VI semiconductors by the elimination of a distinguished here by tone and notation. Each quintuple layer lamella is metal layer every five close-packed planes to form a rhom- separated by weak van der Waals bonding between adjacent Te(1) layers. bohedral structure.[23] These “quintuple” layers form in the Adv. Electron. Mater. 2019, 1800904 1800904 (2 of 20) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advelectronicmat.de