Average Atomic Size Mismatch Engineering Achieving High Sulfur Solubility in Bi ₂ Te ₃ for Thermoelectric Power Generation

Doping and alloying are pivotal methods for tailoring the properties of materials. Yet, the Hume‐Rothery rule indicates that a significant atomic radius difference between the dopant and matrix atoms leads to low solubility, which limits the enhancement of material performance. Here, we propose a strategy to reduce the average atomic size mismatch (), thereby enhancing the solubility of insoluble elements. This strategy is demonstrated in sulfur‐alloyed Bi 2 Te 3 , where co‐alloying with Sb and Se reduces , boosting the solubility of S from 3% to over 16% and effectively widening the bandgap. Meanwhile, configurational entropy enhancement via alloying induces lattice distortion, which significantly strengthens phonon scattering. Directional solidification is employed to form a near‐single‐crystal texture that boosts carrier mobility. As a result, an ultralow lattice thermal conductivity of 0.49 W m −1 K −1 at 423 K and a remarkable ZT of 0.92 at 450 K are achieved in n‐type (Bi 2 Te 3 ) 0.8 (Sb 2 Se 2 S) 0.2 . A segmented module realizes an exceptional conversion efficiency of 8.27% at a temperature difference of 290 K, surpassing that of the state‐of‐the‐art Bi 2 Te 3 ‐based power generators. This work provides an effective strategy for enhancing the solubility of dopants, offering a pathway for the design of advanced material properties.