Favorable Contact with Low Interfacial Resistance for n-Type TiCoSb-Based Thermoelectric Devices

In the past decade, significant efforts have been made to develop efficient half-Heusler (HH) based thermoelectric (TE) materials. However, their practical applications remain limited due to various challenges occurring during the fabrication of TE devices, particularly the development of stable contacts with low interfacial resistance. In this study, we have made an effort to explore a stable contact material with low interfacial resistance for an n-type TiCoSb-based TE material, specifically Ti_0.85Nb_0.15CoSb_0.96Bi_0.04 as a proof of concept, using a straightforward facile synthesis route of spark plasma sintering. We tested many metals with compatible coefficients of thermal expansion to TiCoSb, like Fe and Co. Still, we failed to form proper atomic bonds with the TE material. In contrast, Ti metal bonded correctly but showed very high electrical contact resistance (∼300 mΩ at one side), reducing performance due to Ti diffusion and a high potential barrier at the interface. This issue was addressed by highly doped semiconductor (HDS) contact Ti_0.7Nb_0.3CoSb, which matched the TE material in terms of atomic bonding, crystal structure, and stability. The leg with the HDS contact demonstrated superior electronic transport performance and low interface resistance (∼15 mΩ at one side), achieving a maximum output power of 30.7 mW at ΔT = 451 K due to the sharp interface with a low barrier height. These findings suggest that using HDS material as a contact with the same HH TE material would be an effective way to develop a TE device with low interface resistance and high thermal stability.