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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 Cu 2 CdSnSe 4 .

Heterovalent Substitution to Enrich Electrical Conductivity in Cu2CdSn1-xGaxSe4 Series for High Thermoelectric Performances