Thermoelectric properties of Ag-doped Cu2Se and Cu2Te

Abstract

Cu₂Se, Cu₂Te and Ag-overstoichiometric compounds Cu_1.98Ag_0.2Se and Cu_1.98Ag_0.2Te were prepared by melting, annealing, followed by spark plasma sintering compaction. Low and high temperature thermoelectric properties were investigated by measuring the electrical conductivity, Seebeck coefficient, thermal conductivity and Hall coefficient between 2 K and 900 K. Structural analyses were performed by PXRD and SEM-EDX analyses. The Hall and Seebeck coefficients show that holes are the dominant carrier in all compounds. High temperature α–β phase transition in Cu₂Se and Cu_1.98Ag_0.2Se between 350 and 400 K and multiple phase transitions (α–β, β–γ, γ–δ, δ–∈) in Cu₂Te and Cu_1.98Ag_0.2Te between 350 K and 900 K were observed in measurements of heat capacity, temperature dependent PXRD data, and transport coefficients. Low temperature transport measurements (Hall coefficient, electrical conductivity, carrier mobility) strongly suggest the presence of yet another phase transition in Cu₂Se, Cu_1.98Ag_0.2Se, and Cu_1.98Ag_0.2Te compounds at temperatures between 85 K and 115 K, reported here for the first time. Based on the transport data and structural analysis we conclude that doping Cu₂Se and Cu₂Te by Ag reduces the density of holes and strongly suppresses the thermal conductivity not only due to a smaller electronic contribution but also due to enhanced point defect scattering of phonons that reduces the lattice portion of the thermal conductivity. Moreover, the phase transition temperature is shifted to lower temperatures upon doping with Ag. The presence of Ag enhances thermoelectric performance of Cu₂Te at all temperatures and Cu₂Se benefits from Ag doping over a broad range of temperatures up to 700 K. The maximum ZT value of 1.2 at 900 K; 0.52 at 650 K; 0.29 at 900 K; and 1.0 at 900 K were achieved for Cu₂Se, Cu_1.98Ag_0.2Se, Cu₂Te and Cu_1.98Ag_0.2Te, respectively, between 2 K and 900 K.