Effect of hole modulation on the electrical transport properties and thermal stability of single crystal α-Cu2Se prepared by Bi-flux method

In this study, alpha-Cu2Se single crystal with space group C2/c was prepared by the Bi-flux method according to the stoichiometric ratio of Cu2Se(1+x)Bi3 (x = 0, 0.01, 0.02, 0.03). All samples involve hole conduction and carrier concentration increases with the increase of Se content. The valence of Bi atom is +3 in the sample, and the solubility limit of Bi atom is approximately 0.012% in Cu2Se. The crystal cell volume of all samples is larger than that of standard alpha-Cu2Se for the larger ion radius of Bi3+ than Cu+. In addition, the crystal cell volume of the samples decreases with the increase of Se content because of more Cu vacancies caused by excessive Se. The transmission electron microscopy image and the corresponding area electron diffraction pattern show that the prepared samples have a complete crystal structure. Eventually, the single crystal alpha-Cu2Se1.01 sample obtains the highest power factor of 1.53 x 10(-3) W m(-1) K-2 at 690 K. Thermogravimetric-differential thermal analysis shows that no weight loss occurred below the melting point. After three continuous thermal cycles, the value of power factor drops down to similar to 0.04 x 10(-3) W m(-1) K-2 at 660 K. In comparison with polycrystalline sample prepared by hot pressing, the thermal stability is improved obviously. (C) 2019 Elsevier B.V. All rights reserved.