Optimizing the thermoelectric properties of transition metal doped Sb2Te3 mediated by carrier effective mass

Charge carrier and phonon dynamics tuning are promising approaches for transport property modulation. Improvement of thermoelectric (TE) parameters via atomic substitution and band engineering is prevalent. Besides, carrier effective mass (m*) is a crucial factor that has a noteworthy influence on TE properties. Transition metals (TMs), possessing outstanding valence electronic properties and distinctive electronic state distributions, have recently been used as potential candidates for enhancing TE performance. Here, we report the structural, electronic, and TE characteristics of p-type Sb2Te3 by TM (=Fe, Co, Ni) doping. Consequently, the synergistic amelioration of electrical and thermal transport properties is elucidated. Structural and phonon vibrational properties are characterized by synchrotron powder x-ray diffraction and Raman spectroscopic (RS) measurements. Raman peak position and full-width at half-maximum provide insight into electron-phonon interactions and phonon anharmonicity. Anharmonic phonon-phonon interaction is illustrated via a four-phonon decay model. Furthermore, optothermal RS measurement is used to estimate the thermal conductivity kappa of the samples. A reduction in the lattice thermal conductivity, kappa(L), is observed after TM doping. Experimentally measured transport parameters, viz., S(T), rho(T), and n(H)(T) are simulated via the Boltzmann transport equation (BTE), and reasonable quantitative agreement between the experimental and simulated data is obtained. The role of m* and valence band convergence, as estimated from BTE analysis, is highlighted. Weighted mobility and m* are found to be increased, significantly enhancing the power factor in the Co-doped sample. As compared to its pristine counterpart, around three times the improvement of the ZT value in Sb1.97Co0.03Te3 is reported.