Dual Band Optimization and Defect Engineering Enable Ultrahigh Thermoelectric Performance in Filled Skutterudites

ABSTRACT Skutterudites are promising thermoelectric candidates for medium‐temperature energy conversion owing to their excellent mechanical strength and thermal stability, yet their practical deployment is hindered by their moderate thermoelectric performance. Herein, we report a synergistic optimization of the electronic band structure and phonon scattering in Yb‐filled skutterudites through simultaneous indium filling and Te doping. This dual strategy delivers an exceptional figure of merit ( zT ) of ∼1.72 at 773 K, arising from a high power factor of 5.63 mW m −1 K −2 and a markedly suppressed total thermal conductivity of 2.23 W m −1 K −1 . First‐principles calculations reveal that In filling and Te doping cooperatively converge electronic bands, thereby enhancing electrical transport, while advanced electron microscopy uncovers the formation YbTe nanoprecipitates and associated lattice distortions that intensify phonon scattering. A 7‐pair single‐stage thermoelectric device fabricated from the optimized materials achieves an ultra‐high energy conversion efficiency of 8.2% under a temperature difference of 380 K. These findings establish a clear structure‐property‐performance correlation and provide a versatile strategy for propelling skutterudites toward high‐efficiency, device‐level applications.