Influence of Compensating Defect Formation on the Doping Efficiency and Thermoelectric Properties of Cu_2-ySe_1-xBr_x

The superionic conductor Cu_2-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 Cu_2-δSe is in fact possible. Furthermore, we provide profound insight into the complex defect chemistry of bromine doped Cu_2-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.