Improvement of thermoelectric performance in Sb2Te3/Te composites

Te-impurity-incorporated ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$, i.e., ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}+x\phantom{\rule{0.16em}{0ex}}\mathrm{mol}\phantom{\rule{0.16em}{0ex}}%$ Te ($x=0$, 4, 6, and 9) composites were synthesized by solid-state reaction technique. Analysis of x-ray diffraction indicates not only Te impurity as a second phase but also doping of Te via suppression of inherent Te vacancies in the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ matrix. As a result of this doping and of the change in formation energy of different types of native defects in ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ due to synthesis in a Te-rich condition, carrier concentration $({n}_{\mathrm{H}})$ lower than the pristine sample was observed. Low ${n}_{\mathrm{H}}$ along with gradual convergence of valence bands due to progressive suppression of Te vacancies increases the Seebeck coefficient $(S)$ in Te-incorporated samples. Even though Te impurities increase electrical resistivity (\ensuremath{\rho}), enhanced texturing of lattice planes ensures that charge carrier mobility does not degrade due to Te addition. As a result, a maximum power factor $=17\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ at $T=480$ K for $x=6$ has been achieved. In addition, Te addition strengthens phonon scattering via an increase of phonon-phonon Umklapp scattering and point-defect-induced scattering of phonons. Due to such a strong phonon scattering, thermal conductivity $(\ensuremath{\kappa})$ decreases, and a reduced lattice thermal conductivity $({\ensuremath{\kappa}}_{\mathrm{L}})$, as low as $0.28\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 500 K for $x=6$, has been achieved. As a result of simultaneous increase of $S$ and decrease of \ensuremath{\kappa}, a high $ZT\ensuremath{\sim}0.87$ at 480 K, almost 33% higher than that of the host material, has been achieved.