Superior room-temperature power factor in GeTe systems via multiple valence band convergence to a narrow energy range

GeTe systems are excellent p-type thermoelectric materials at medium temperatures (500–800 K). Recently, their electronic properties were enhanced by band convergence of L and Σ valence bands, which increased the power factor at 500–800 K. However, there are only a few thermoelectric materials suitable for lower-temperature waste-heat recovery applications, and their demand is rapidly increasing. Herein, we demonstrated that the well-known L and Σ bands of GeTe and the additional Δ band converge at a desirable energy level to a very narrow energy range as a result of fine-tuning the crystal symmetry and composition by forming a GeTe-Sb2Te3 alloy. Alloying additionally facilitates a decrease in the bandgap, thereby increasing the near-room-temperature power factor. The GeTe-Sb2Te3 alloy achieved very high room-temperature and maximum power factors of 2.6 mW m−1 K−2 at 300 K and 3.3 mW m−1 K−2 at 423 K, respectively, despite GeTe being a medium-temperature thermoelectric material. Thus, we confirmed the applicability of GeTe as a thermoelectric material at room temperature. This study facilitates the development of high-performance near-room-temperature thermoelectric materials based on existing medium-temperature systems.