Enhancement of thermoelectric properties over a wide temperature range by lattice disorder and chemical potential tuning in a (CuI)(y)(Bi(2)Te(3))(0.95−x)(Bi(2)Se(3))(x)(Bi(2)S(3))(0.05) quaternary system

Bi(2)Te(3)-based compounds have received attention as thermoelectric materials for room-temperature cooling and waste heat recovery applications. With potential application prospects, quaternary compounds of Bi(2)Te(3)–Bi(2)Se(3)–Bi(2)S(3) composites can be used for mid-temperature power generation under 500 °C. Herein, we investigated the thermoelectric properties of (CuI)(y)(Bi(2)Te(3))(0.95−x)(Bi(2)Se(3))(x)(Bi(2)S(3))(0.05) (x = 0.05, 0.2; y = 0.0, 0.003) compounds. Through X-ray diffraction and transmission electron microscopy, we confirmed that the lattice disorder in (Bi(2)Te(3))(0.95−x)(Bi(2)Se(3))(x)(Bi(2)S(3))(0.05) (x = 0.2) was due to multiple element substitutions. Disorder carrier scattering induced the localized nature of electrical resistivity, as confirmed by variable range hopping at low temperature. The temperature-dependent Seebeck coefficient of (Bi(2)Te(3))(0.95−x)(Bi(2)Se(3))(x)(Bi(2)S(3))(0.05) showed a carrier-type change from p- to n-type behaviour in the intermediate temperature range (525 K for x = 0.05 and 360 K for x = 0.2). Even though strong carrier localization increased electrical resistivity, resulting in degradation of the power factor and thermoelectric performance, when the chemical potential was increased to the conduction band minimum through CuI co-doping into the (CuI)(0.003)(Bi(2)Te(3))(0.95−x)(Bi(2)Se(3))(x)(Bi(2)S(3))(0.05) (x = 0.05, 0.2) compounds, the carriers were delocalized and showed n-type behaviour in the Seebeck coefficient. The temperature-dependent thermal conductivity shows the suppression of bipolar conduction behaviour. The simultaneous effect on carrier optimization through chemical potential tuning and lattice disorder caused a high ZT value of 0.85 at 523 K for CuI-doped (Bi(2)Te(3))(0.75)(Bi(2)Se(3))(0.2)(Bi(2)S(3))(0.05), which was comparatively high for n-type thermoelectric materials in the mid-temperature range.