P-type dopability in Half-Heusler thermoelectric semiconductors

Half-Heusler (HH) semiconductors with high valence band degeneracy are promising p-type thermoelectric (TE) materials. However, effective p-type doping in HH semiconductors remains challenging, hindering further the exploration of high-performance p-type TE materials. In this work, we conduct first-principles calculations to identify the dominant native defects and potential p-type dopants in three representative HH compounds, e.g., NbFeSb, NbCoSn, and ZrNiSn. Our findings reveal that 4d interstitials underline the p-type dopability. By systematically investigating the extrinsic doping at the three Wyckoff positions in NbFeSb, NbCoSn, and ZrNiSn, respectively, we highlight that the pinned Fermi level serves as an indicator of p-type dopability. The calculation results identify Hf as a p-type dopant in NbCoSn under the Co-poor condition, which is further validated by experiments. A significantly improved p-type TE performance is obtained in Hf-doped NbCoSn. These results could guide the dopant selection and experimental optimization of the p-type TE performance of HH semiconductors.