Roles of anion sites in high-performance GeTe thermoelectrics

Cationic doping is mainly used to improve the performance of rocksalt-structured GeTe thermoelectric materials. However, its counterpart anionic doping is scarcely facilitated. Here, a comprehensive discussion is provided on doping at the anion site of rocksalt-structured thermoelectric materials, surrounding its influence on bonding mechanisms, carrier and phonon transport characteristics, and thermoelectric figure-of-merit. To verify the viewpoint, a modified flux-assist method is adopted to synthesize anionic Iodine (I) doped GeTe samples, which show comparably optimized electrical and thermal properties with that of cationic Antimony (Sb) or Bismuth (Bi) doped GeTe samples. Further combining cationic Bi and anionic I co-doping, an enhanced figure-of-merit from 0.8 in pristine GeTe to 2.5 at 675 K can be realized in 8% (Bi and I) co-doped GeTe, which corresponds to a maximum heat-to-electricity conversion efficiency of 14.6% under a temperature difference of 430 K. This study rationalizes the influence of anionic doping on the electrical and thermal properties of the rocksalt-structured materials, which serves as an effective yet previously neglected strategy toward high-performance GeTe thermoelectrics.