Unique Semi‐Coherent Nanostructure Advancing Thermoelectrics of N‐Type PbSe

Abstract Introducing nanostructures into bulk thermoelectric materials is an effective strategy for reducing lattice thermal conductivity (κ_lat). However, large numbers of hanging bonds emerge on account of lattice mismatch usually at the interfaces between nanostructures and matrix, thereby significantly deteriorating carrier mobility (µ_H) and seriously limiting thermoelectric performance. Here, utilizing the cointroduction of Gd‐Cu₂Te into n‐type PbSe as a paradigm, the zT could be greatly improved. The conduction band flattening caused by Gd doping can successfully compensate for the small Seebeck coefficient in n‐type PbSe, thus achieving extraordinary electrical performance. Further advanced electron microscopy and X‐ray absorption fine structure spectra directly demonstrate that most of the Cu forms nanoscale Cu₂Se precipitates, especially a unique semi‐coherent interface between the PbSe matrix and the Cu₂Se nanoprecipitate is observed for the first time. Such nanoprecipitate accompanied by the semi‐coherent interface not only ensures higher µ_H as an excellent charge transfer mediator but also collectively scatters the heat‐carrying phonons over a wide frequency range to obtain the ultralow κ_lat, resulting in the prominent improvement of the ratio µ_H/κ_lat. The present finding represents an important step forward in electron–phonon decoupling via constructing semi‐coherent nanostructure and interface engineering, which should be applicable for other thermoelectric materials.