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Inorganic semiconductor Ag2S with excellent plasticity is highly desired in flexible and wearable thermoelectrics. However, the compromise between plasticity and thermoelectric performance limits the advances in Ag2S‐based thermoelectric materials and their devices. Here, a 0.5 mol.% Ag2Te‐alloyed Ag2S0.5Se0.5 bulk material is designed, which has a competitively high near‐room‐temperature figure of merit of ≈0.43 at 323 K and an ultra‐high bending strain of ≈32.5% without cracks. Introducing Ag2Te can optimize the carrier concentration and mobility of the Ag2S0.5Se0.5 matrix due to its metal‐like conducting features, leading to a maximum power factor of ≈6 µW cm−1 K−2. Simultaneously, Ag2Te induces Ag‐poor amorphous phase boundaries, serving as buffer layers to enhance the overall plasticity. Moreover, such amorphous phase boundaries combined with multiscale phonon scattering sources can significantly suppress the lattice thermal conductivity to ≈0.28 W m−1 K−1 at 323 K, leading to a high figure of merit. This study demonstrates an innovative route to simultaneously boost the thermoelectric performance and plasticity of ductile semiconductors. [ABSTRACT FROM AUTHOR]

According to the Mott's relation, the figure‐of‐merit of a thermoelectric material depends on the charge carrier concentration and carrier mobility. This explains the observation that low thermoelectric properties of GeTe‐based materials suffer from the degraded carrier mobility, on account of the fluctuation of electronegativity and ionicity of various elements. Here, high‐performance CuBiSe2 alloyed GeTe with high carrier mobility due to the small electronegativity difference between Cu and Ge atoms and the weak ionicity of CuTe and BiTe bonds, is developed. Density functional theory calculations indicate that CuBiSe2 alloying increases the formation energy of Ge vacancies and correspondingly reduces the amount of Ge vacancies, leading to an optimized carrier concentration and a high power factor of ≈37.4 µW cm−1 K−2 at 723 K. Moreover, CuBiSe2 alloying induces dense point defects and triggers ubiquitous lattice distortions, leading to a reduced lattice thermal conductivity of 0.39 W m−1 K−1 at 723 K. These synergistic effects result in an optimization of the carrier mobility, the carrier concentration, and the lattice thermal conductivity, which favors an enhanced peak figure‐of‐merit of ≈2.2 at 723 K in (GeTe)0.94(CuBiSe2)0.06. This study provides guidance for the screening of GeTe‐based thermoelectric materials with high carrier mobility. [ABSTRACT FROM AUTHOR]