Rhombohedral GeSe Thermoelectric Breakthrough by Strategic Pb Alloying

Abstract Thermoelectric materials demand simultaneous optimization of electronic and thermal transport, a daunting task owing to their intricate interplay. Orthorhombic GeSe holds promise for thermoelectric applications, yet optimizing its carrier concentration is difficult because of its rigid crystal structure. Substituting Te for Se stabilizes a single rhombohedral phase in GeSe _0.6 Te _0.4 , although its thermoelectric performance requires further improvement. This study explores the influence of Pb alloying at Ge sites in rhombohedral GeSe _0.6 Te _0.4 . Pb alloying modifies both electronic structure and phonon properties. First‐principles calculations indicate a non‐rigid band effect upon Pb incorporation. A low Pb content widens the bandgap, suppressing bipolar thermal diffusion, whereas an excessive amount narrows it because of spin‐orbit coupling. Pb also promotes multi‐valence band convergence, effectively boosting the Seebeck coefficient. Furthermore, Pb alloying triggers point‐defect scattering and lattice softening, notably reducing the lattice thermal conductivity, which is confirmed by the Callaway–Klemens model and Raman spectroscopy. Simultaneously, Ge _0.85 Pb _0.15 Se _0.6 Te _0.4 attains a record‐high figure of merit ( ZT ) of ≈1.5 at 621 K and an average ZT of ≈0.8 (300–621 K). The single‐leg device demonstrates a record‐high conversion efficiency of 7.0% under the temperature difference of 374 K. These findings underscore the potential of rhombohedral GeSe‐based systems for mid‐temperature thermoelectric applications.