Unique Microstructures and High Thermoelectric Performance in n–type Bi2Te2.7Se0.3 by the Dual Incorporation of Cu and Y

Abstract

n-type thermoelectric systems operating near ambient temperature have been significantly understudied. n-type Bi2Te3-based materials have suffered from lack of efficient performance-enhancing strategies and compositional diversity mainly due to the difficulty in efficient doping and alloying to their structure. Herein, we report new n-type thermoelectric system Bi2-xYxTe2.7Se0.3 (x = 0 – 0.020) and optimized Cu0.01Bi1.985Y0.015Te2.7Se0.3 involving unique yttrium selenide-based microstructures. The incorporated Y and Cu atoms serve multiple favorable roles in improving thermoelectric performance of the title samples. A majority of the introduced Y forms either semiconducting Y2Se3 or metallic Y5-δSe7 depending on its chemical pressure, significantly affecting both thermal and charge transport properties. Changing the Y concentration in the nominal composition dynamically changes the composition of the surrounding matrix, microstructures, and their interfaces. The generation of microstructures and compositional variance driven by the Y and Cu incorporation can be understood and controllable in light of hard-soft acid-base principle and structural chemistry of the constituent elements. Cu atoms are more abundant in the interface than the other areas, and richer inside the microscale precipitate than the surrounding matrix, thereby creating large mass and compositional fluctuation. The Raman spectra verify that the incorporated Y and Cu atoms contribute to scattering and softening phonon modes effectively. All these jointly depress the lattice thermal conductivity of the optimized phase Cu0.01Bi1.985Y0.015Te2.7Se0.3 to ∼0.51 W m-1 K-1 at 300 K. The dual incorporation of Cu and Y atoms induces heavier density of states effective mass, thereby improving a magnitude of Seebeck coefficients. Accordingly, power factor increases especially near room temperature, giving ∼39.1 μW cm-1 K-2 at 300 K. Consequently, the Cu0.01Bi1.985Y0.015Te2.7Se0.3 sample achieves a high thermoelectric figure of merit, ZT, of ∼1.20 at 347 K and an average ZT of ∼1.16 from 300 to 423 K. Its ZT of ∼1.09 at 300 K is comparable to the state-of-the-art n-type polycrystalline Bi2Te3 systems.