Heterovalent substitution to enrich electrical conductivity in Cu2 CdSn1-x GaxSe4 series for high thermoelectric performances

Abstract

Serials of Ga doping on Sn sites as heterovalent substitution in Cu 2 CdSnSe 4 are prepared by the melting method and the spark plasma sintering (SPS) technique to form Cu 2 CdSn 1-x Ga x Se 4 (x = 0, 0.025, 0.05, 0.075, 0.01, and 0.125). Massive atomic vacancies are found at x = 0.10 by the heterovalent substitution, which contributes significantly to the increase of electrical conductivity and the decrease of lattice thermal conductivity. The electrical conductivity is increased by about ten times at 300 K after Ga doping. Moreover, the seebeck coefficient only decreases slightly from 310 to 226 μV/K at 723 K, and a significant increase of the power factor is obtained. As a result, a maxium value of 0.27 for the figure of merit (ZT) is obtained at x = 0.10 and at 723 K. Through an ab initio study of the Ga doping effect, we find that the Fermi level of Cu 2 CdSnSe 4 is shifted downward to the valence band, thus improving the hole concentration and enhancing the electrical conductivity at low doping levels. Our experimental and theoretical studies show that a moderate Ga doping on Sn sites is an effective method to improve the thermoelectric performance of Cu2CdSnSe4.