Enhancement of thermoelectric properties by partial substitution of Ge sites in anion ring [Ge₂S₂]^4- found in Co₂Ge₃S₃ skutterudite-based material

In an effort to discover new thermoelectric materials, p-type Co₂Ge_3-xM_xS₃ (M ​= ​Zn, Cd, Bi; x ​= ​0, 0.01, 0.03, 0.05) skutterudite-based chalcogenides are prepared using a traditional solid-state reaction. The samples are then compacted into pellets by hot pressing for thermoelectric characterization. A comparative investigation is performed by partial substitution of Ge sites with various dopants to assess the influence of substitution on the thermoelectric properties of Co₂Ge₃S₃. Electrical and thermal transport properties, i.e., electrical conductivity, carrier concentration, Seebeck coefficient, and thermal conductivity, of the substituted Co₂Ge₃S₃ samples are measured. Substitution has a moderate positive effect on the transport properties of these ternary skutterudites. All samples exhibit p-type behavior. A maximum power factor value of 10.3 ​μW ​cm⁻¹ ​K⁻² is obtained at approximately 518 ​K for Co₂Ge_2.99Zn_0.01S₃. Compared to Co₂Ge₃S₃, the lattice thermal conductivity of substituted Co₂Ge₃S₃ is substantially reduced due to the atomic disorder at the Ge sites, and a minimum lattice thermal conductivity value of 2.19 ​W ​m⁻¹ ​K⁻¹ is obtained at approximately 675 ​K for Co₂Ge_2.95Cd_0.05S₃. Compared with the referred Co₂Ge₃S₃ sample, the Bi-substituted skutterudite obtained the 21% reduction of the lattice thermal conductivity and 38% enhancement of power factor, resulting in a peak ZT value of ~0.22 ​at 763 ​K of Co₂Ge_2.97Bi_0.03S₃.