Defect Driven Electronic Structure Reconfiguration and Hierarchical Phonon Suppression in Ag ₂ Se Thermoelectrics

ABSTRACT Ag ₂ Se is a promising n‐type thermoelectric for near‐room‐temperature applications. Nevertheless, further enhancement of its figure of merit ( ZT ) demands a delicate balance between electronic and phononic transport. Here, we develop a facile and controllable wet‐chemical strategy under ambient conditions to synthesize orthorhombic Ag ₂ Se nanocrystals. Indium substitution at the Ag site modulates the electronic structure, leading to a Fermi‐level upshift, band‐gap narrowing, and increased carrier concentration, thereby enhancing electrical transport. Meanwhile, In's incorporation introduces local dislocations and anisotropic strain fields that induce lattice softening and reduce the sound velocity, resulting in enhanced phonon scattering and suppressed lattice thermal conductivity. First‐principles calculations further reveal that In incorporation weakens Ag─Se antibonding interactions and suppresses Ag s‐d hybridization in favor of In s‐p hybridization, thereby mitigating potential fluctuations and enhancing carrier mobility. Benefiting from this synergistic phonon‐electron optimization, Ag _1.9287 In _0.0713 Se achieves a high power factor of ∼3100 µW m ^{− 1} K ^{− 2} and a peak ZT of ∼1.2 at 398 K. Prototype modules exhibit consistently higher output voltage and power density, while hardness mapping confirms improved lattice resilience. This work highlights the potential of coupling defect chemistry with electronic structure engineering as an effective strategy to advance high‐performance and mechanically robust Ag ₂ Se ‐based thermoelectrics.