Grain Boundary and Interface Engineering Enables Thermally Stable PbS Nanocomposites for High‐Performance Thermoelectric Devices

ABSTRACT Lead sulfide (PbS) is an earth‐abundant thermoelectric (TE) material. Cu, despite being an effective dopant, becomes thermally mobile at elevated temperatures, causing interfacial reactions and compositional inhomogeneity that undermine structural and electronic stability. In this study, we report a composite route that integrates colloidal PbS nanocrystals (NCs) with aqueous‐synthesized Cu‐doped PbS NCs to form PbS‐based nanocomposites. The approach integrates high Cu‐doping efficiency in aqueous synthesized NCs and structural robustness of colloidal NCs, thereby suppressing Cu migration at elevated temperature and improving thermal stability and mechanical robustness. The microstructure yields dense grain boundaries and heterointerfaces, where traces amount of Cu 2 S nanodomains are present, thereby modulating carrier concentration while enhancing phonon scattering. The nanocomposites exhibit 37% increase in mechanical strength, superior thermal stability over consecutive heating‐cooling cycles and prolonged isothermal test, and reduced lattice thermal conductivity of 0.63 W m −1 K −1 , resulting in a peak zT of ∼1.1 at 790 K. Furthermore, a single‐leg TE device fabricated from the optimized composite achieves a conversion efficiency of 6.3% at ΔT = 449 K, exceeding that of most PbS‐based counterparts operating at similar temperatures. This study demonstrates a microstructure and defect‐engineering that balances efficiency with long‐term service reliability for PbS‐based TEs.