Exploring Co(Sb,Ge)(S,Se,Te) and CuCr(S,Se)2 for enhanced thermoelectric performance.

Thermoelectric energy harvesters convert temperature gradient directly into electrical energy and vice versa. These materials have several compelling advantages and applications, making them a subject of active research and development. The efficiency of a thermoelectric material is assessed using the dimensionless figure of merit, defined as ZT = S2σT/kt, where S is Seebeck coefficient, σ is electrical conductivity, kt is total thermal conductivity and T is absolute temperature. For efficient thermoelectric conversion, the material should exhibit a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Optimization of thermoelectric materials involves balancing these interdependent parameters. Doping, nano-structuring, and band engineering are essential strategies for optimization. However, current state-of-the-art thermoelectric materials like Bi2Te3 and PbTe contain either scarce or toxic elements. Recently, CoSbS has emerged as a promising thermoelectric candidate with a favorable elemental composition, high Seebeck coefficient and reasonable electrical conductivity. An exciting feature of CoSbS is that it can exhibit both p-type and n-type conductivity by adjusting sulfur stoichiometry, simplifying thermoelectric module design, and enhancing performance by addressing material compatibility issues. Exploring CoSbX (X = S, Se, Te) analogues is promising due to the excellent performance of Se and Te-based thermoelectric materials. However, CoSbSe and CoSbTe were found metallic with low Seebeck coefficient compared to CoSbS. CoSbS indeed shows promise as an efficient thermoelectric material, but its high thermal conductivity and low electrical conductivity remain major challenges that need to be addressed to fully realize its potential. Current research highlights that adding point defects through germanium (Ge) doping can effectively address these issues simultaneously. The thermal conductivity in CoSbS is primarily due to lattice thermal conductivity, and introducing Ge, creates point defects within the CoSbS lattice and softens the phonon modes, leading to a massive reduction in lattice thermal conductivity. The combined effect of reduced thermal conductivity and increased electrical conductivity has improved the thermoelectric performance, close to room temperatures. Layered CuCrS₂, initially known for magnetic properties, shows promise as a thermoelectric material due to its low thermal conductivity. Introducing Se in CuCr(S1-xSex)2 further reduces thermal conductivity and potentially improves the ZT.