Influence of Compensating Defect Formation on the Doping Efficiency and Thermoelectric Properties of Cu2‐ySe1−xBrx

The superionic conductor Cu2−δSe has been shown to be a promising thermoelectric at higher temperatures because of very low lattice thermal conductivities, attributed to the liquid-like mobility of copper ions in the superionic phase. In this work, we present the potential of copper selenide to achieve a high figure of merit at room temperature, if the intrinsically high hole carrier concentration can be reduced. Using bromine as a dopant, we show that reducing the charge carrier concentration in Cu2−δSe is in fact possible. Furthermore, we provide profound insight into the complex defect chemistry of bromine doped Cu2−δSe via various analytical methods and investigate the consequential influences on the thermoelectric transport properties. Here, we show, for the first time, the effect of copper vacancy formation as compensating defects when moving the Fermi level closer to the valence band edge. These compensating defects provide an explanation for the often seen doping inefficiencies in thermoelectrics via defect chemistry and guide further progress in the development of new thermoelectric materials. ■ INTRODUCTION On the basis of Seebeck and Peltier effects, thermoelectric materials either convert thermal to electrical energy or vice versa, providing a perspective for power generation and refrigeration applications, respectively. The thermoelectric efficiency is governed by the thermoelectric figure of merit zT = S2(ρκ)−1T, and the past decade has seen the discovery of many suitable materials for high temperature power generation. One of these materials is Cu2−δSe, which exhibits a maximum zT of 1.6 at 1000 K, making it a competitive and cheap alternative for PbTe, the leading thermoelectric material in the hundreds of kelvin temperature range. Because of a small, intrinsic copper deficiency δ, copper selenide (Cu2−δSe) is a p-type semiconductor with a band gap of 1.23 eV. Above 410 K, copper selenide transforms to a Cu conducting phase, wherein the copper ions are mobile and described as “liquid like” because their diffusion coefficient is about 10−5 cm2s−1, comparable to the value for water molecules in liquid water. This superionic state results in extremely low lattice thermal conductivity values as low as 0.4 W(Km)−1 in the superionic phase and in turn a high figure of merit. Starting with the superionic Cu2−δSe, many other superionic copper and silver selenides have been found to be interesting thermoelectrics; for instance, the argyrodites Cu7PSe6, 7 Ag2Se, 8−10 CuAgSe, and quaternary copper chalcogenides Cu2MM′Q4 (M = Zn, Fe,...; M′ = Zn, Ge, Q = S, Se, Te) which also exhibit interstitial copper ions and even show band convergence. While copper selenide has potential as a thermoelectric material at high temperatures, its ionic conductivity has caused long-term stability problems for use in thermoelectric generators. This spurs interest in the properties of copper selenide below the superionic phase transition at 410 K, where the ion migration would not be of concern. In this work, we investigate the potential for copper selenide to achieve high zT values at room temperature. Using bromine as a dopant, we show the effect of carrier scattering on the thermoelectric quality factor and how the dopant bromine itself prevents possibly high figures of merit at room temperature. Furthermore, we explore the Received: June 24, 2015 Revised: September 24, 2015 Published: September 24, 2015 Article