Compound defects and thermoelectric properties in ternary CuAgSe-based materialsElectronic supplementary information (ESI) available: Cycle measurements of electrical conductivity and thermopower for Cu0.98AgSe and CuAg0.98Se (Fig. S1); magnetic field dependence of longitudinal electrical resistivity ρxx, Hall resistivity ρxy, the calculated longitudinal conductivity σxx and Hall conductivity σxy of five samples from 300 K to 400 K (Fig. S2-S6); fitting parameters for six samples (Tables S1-S6);

CuAgSe is a narrow band gap semiconducting material with superior carrier mobility and low lattice thermal conductivity, which are important and useful for high thermoelectric performance. However, its electrical and thermal transport properties are greatly affected by ionic deficiencies or compositional non-stoichiometry, which lead to a low thermoelectric figure of merit near room temperature. In this work, we systematically studied the compound defects in CuAgSe by tuning its starting chemical composition. We found that its phase purity is very sensitive to nominal chemical compositions. Only a small amount of Ag deficiency is allowed in CuAgSe to maintain phase purity, while the other non-stoichiometric compositions lead to impurity phases. Thermoelectric properties are weakly affected by these compound defects or impurity phases at 300 K, but greatly change at high temperatures. A single type carrier conduction is observed in CuAgSe, but a noticeable two-type carrier conduction is observed in the non-stoichiometric samples. This leads to an evident n to p conduction transition. Consequently, zT values in CuAgSe continuously increase to 0.6 at 450 K while the non-stoichiometric samples display considerably low values due to the contribution from both electrons and holes. The high zT value in n-type CuAgSe suggests that it is a promising thermoelectric material near room temperature. Carrier concentrations dominated the TE properties in CuAgSe-based materials, which is a promising thermoelectric material near room temperature.