Structure And Thermoelectric Properties Of P-Type Cu-Ga(In)-Te Semiconductors

CuMTe2(M=In,Ga) is one of the typical Ⅰ-Ⅲ-Ⅵ2series semiconductor with chalcopyrite structure, and their bandgap are Eg=1.24eV(CuGaTe2) and Eg=1.06eV (CuInTe2) respectively. In these semiconductors there is an inherent Coulomb attraction between charged MCu(Ag)2+and2VCu(Ag)-(a native defect pair, i.e., metal M-on-Cu antisites and two Cu vacancies) which has a profound effect on the electronic and structural properties. In this light, we aim at investigating the relationship among the compositions, structures and thermoelectric properties by using the iso-and non-isoelectronic substitutions of elements in CuGaTe2, and the detailed achievements have been summarized below.1. We prepared the ternary CuGaTe2single phase semiconductors using powder metallurgy and spark plasma sintering (SPS) technology and obtained its direct band gap (Eg=1.0eV) by measurement. The highest thermoelectric figure of merit ZT of CuGaTe2is0.49at701K.2. We prepared quaternary compound CuGa1-xnxTe2(x=0;0.36;0.64;1) through isoelectronic substitution of element of In for Ga in CuGaTe2, and observed that these substituted materials have high Seebeck coefficients (a) with the a value increasing by～70%and relatively low thermal and electrical conductivity. The highest dimensionless figure of merit was0.91for the compound CuGa0.36In0.64Te2at701K, which is about two times that of intrinsic CuGaTe2.3. We prepared quaternary compound Cu1-xGaSbxTe2(x=0;0.02;0.05;0.1) through non-isoelectronic substitution of element of Sb for Cu in CuGaTe2, and calculated the band structures by arranging Sb in the Cu (25.0%) and Te (12.5%) lattice sites respectively. Through calculations and measurements we justified the previous assumption that the element Sb dominantly occupies the Te sites, which allows the Fermi level tend to lie in the valence band. The Seebeck coefficient increases with Sb addition mainly due to the possible increase of the local density of states (DOS) near the Fermi level and the effective mass of the valence band. Besides, we have also proposed that a dual effect which consists of the extra lattice mismatch and crystal structure distortion governs the lattice thermal conductivity kL. The highest ZT value is1.07±0.1for the sample with x=0.02at721K, which is about2.2times that of intrinsic CuGaTe2.4. We annealed the CuGaTe2samples at663K for15days,30days and60days respectively, and coordinately improves the Seebeck coefficient and electrical conductivity. With the annealing time increasing the lattice thermal conductivity KL decreases and the ZT value increases significantly. The highest ZT value of the samples for the samples annealed for30and60days are～0.86at720K, which is about1.7times that of intrinsic CuGaTe2.