Enhanced Thermoelectric Performance of Cu 1.95− y In y S ₁− x Se x via a Three‐Tier Hierarchical Strategy

Copper‐based chalcogenides Cu 2− x (S, Se) offer a more cost‐effective alternative to commercial tellurium‐based thermoelectric materials. However, enhancing their ZT values is constrained by intrinsically complex phase structures. The present study proposes a three‐tier hierarchical strategy that integrates phase regulation, chemical bonding engineering, and band engineering to decouple electronic and thermal transport. Phase regulation via Se 2− alloying increases configurational entropy and stabilizes a high‐symmetry hexagonal/cubic multiphase coexistence, thereby suppressing detrimental low‐temperature phases and inducing beneficial valence‐band flattening. In 3+ doping weakens Cu─S/Se bonding by enhancing antibonding states, inducing lattice softening and strengthening superionic behavior, while introducing impurity levels that increase the density‐of‐states effective mass to 2.5 m e and boost the Seebeck coefficient to 131 µV K −1 at 323 K. Further integration of multiscale defects, such as point defects, dislocations, interfaces, and nanopores, enables Cu 1.94 In 0.01 S 0.3 Se 0.7 to achieve a power factor of 11.2 µW cm −1 K −2 and an ultralow lattice thermal conductivity of 0.27 W m −1 K −1 , yielding a peak ZT of 1.45 at 773 K. Notably, the ZT values in the 523–773 K range exceed those of most previously reported Cu‐based chalcogenides. The three‐tier hierarchical “phase‐bond‐band” framework provides a useful guideline for designing high‐performance chalcogenide thermoelectric materials with complex phase structures.