Arrays of Planar Vacancies in Superior Thermoelectric Ge1−x−yCdxBiyTe with Band Convergence

The multivalence bands in GeTe provide an additional handle to manipulate the thermoelectric performance. Herein, the density‐functional‐theory calculation indicates that Cd doping enables the convergence of these multivalence bands. Plus, the additional Bi dopant serving as the electron donors optimizes the carrier concentration, leading to an enhanced power‐factor in Ge1−x−yCdxBiyTe. Moreover, comprehensive electron microscopy characterizations demonstrate the array of high‐density planar vacancies in Ge1−x−yCdxBiyTe stemming from the absence of {111} Ge atomic planes, which is driven by the reduced formation energy in the scenario of Cd/Bi codoping. Simulations of phonon transport confirm the significant role of planar vacancies in scattering mid‐frequency phonons. Such high‐density planar vacancies, in tandem with grain boundaries and point defects, lead to a lattice thermal conductivity of 0.4 W m−1 K−1 in Ge1−x−yCdxBiyTe, reaching the amorphous limit. Ultimately, a peak zT of 2.2 is realized, which promotes GeTe into the first echelon of cutting‐edge thermoelectric materials. The strategy of combining band convergence and planar vacancies opens an avenue to develop Pb‐free derivatives with superhigh thermoelectric efficiency. Band convergence and high‐density planar vacancies render a figure‐of‐merit of 2.2 in Ge0.9Cd0.05Bi0.05Te. Doping with Cd reduces the energy separation between the multivalence bands in GeTe, which enhances the power‐factor, provided the optimized carrier concentration by auxiliary Bi doping. Driven by the decreased formation energy, high‐dense planar vacancies are formed in Cd/Bi codoped GeTe, leading to an ultralow thermal conductivity.