Enhanced Thermoelectric Performance of Cu_1.95−yIn_yS_1−xSe_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 Se²⁻ 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. In³⁺ 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_1.94In_0.01S_0.3Se_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.