Ni and Se co-doping increases the power factor and thermoelectric performance of CoSbS

We present a new investigation on paracostibite (CoSbS), a potential thermoelectric material for power generation in the intermediate temperature range (700-900 K) consisting of earth abundant elements. We report an improved synthesis of single phase materials of Co_1-xNi_xSbS_1-ySe_y (x = 0-0.07, y = 0-0.09) using vacuum melting and annealing followed by SPS processing. The effects of Ni and Se co-doping on the electronic and thermal transport properties of CoSbS were investigated. Doping with Ni on the Co site increases the carrier concentration and moves the Fermi level deep into the conduction band, producing an enhanced power factor. First-principles electronic band structure calculations indicate a multi valley nature of the conduction band where Ni doping leads to n-type behaviour and a sharp increase of the density of states effective mass, enhancing the Seebeck coefficient. Unlike the Ni doping which has little effect on thermal transport, alloying Se on the S site intensifies the alloying scattering resulting in a lower lattice thermal conductivity of 2.4 W m^-1 K^-1 at 900 K for Co_0.93Ni_0.07SbS_0.93Se_0.07. Alloying Se on the S site of the structure further boosts the power factor to 22 μW cm^-1 K^-2 and leads to a maximum ZT of 0.58 at 900 K, which is the best ZT value achieved in the system.