Improvement of thermoelectric performance in ${\mathrm{Sb}}_2{\mathrm{Te}}_3\text{/}\mathrm{Te}$ composites

Te-impurity-incorporated ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, i.e., ${\mathrm{Sb}}_2{\mathrm{Te}}_3+x\mathrm{mol}\%$ 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 $\left(n_{\mathrm{H}}\right)$ 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 $\left(S\right)$ in Te-incorporated samples. Even though Te impurities increase electrical resistivity (ρ), enhanced texturing of lattice planes ensures that charge carrier mobility does not degrade due to Te addition. As a result, a maximum power factor $=17\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ 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 $\left(\kappa \right)$ decreases, and a reduced lattice thermal conductivity $\left({\kappa }_{\mathrm{L}}\right)$, as low as $0.28\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 500 K for $x=6$, has been achieved. As a result of simultaneous increase of $S$ and decrease of κ, a high $ZT\sim 0.87$ at 480 K, almost 33% higher than that of the host material, has been achieved.

Figure 1

Room temperature x-ray diffraction (XRD) patterns for ${\mathrm{Sb}}_2{\mathrm{Te}}_3+x\mathrm{mol}\%$ Te ($x=0$, 4, 6, and 9) composite samples. XRD peaks which appeared due to the presence of elemental Te in $x=4$, 6, and 9 are marked with asterisks (*). Inset: Variation of lattice parameters (i.e., $a$ and $c$) of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ with increasing Te incorporation. Estimated errors in the refined lattice parameters are within the size of the data points and are provided in Fig. SM1 in the Supplemental Material [36]. The lines joining the data points are guidelines for the eye.

Figure 2

(a) Electron diffraction (ED) patterns collected along main zone axis indexed based on rhombohedral ($R\overline{3}m$) ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ structure. (b) Low-magnification high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image and simultaneously acquired energy-dispersive x-ray (EDX)-STEM elemental mapping for Te L, Sb L, and overlaid color image. (c) Middle-magnification [100] HAADF-STEM image of single ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ crystallite.

Figure 3

(a) Left panel: High-resolution [100] high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ structure. The magnified experimental image with overlaid simulated image and structural model is given as the inset, where Sb is red and Te yellow. Right panel: The magnified high-resolution HAADF-STEM image and corresponding intensity line scan profile along the $c$ axis. (b) Left panel: High-resolution [100] HAADF-STEM image of ${\mathrm{Sb}}_2{\mathrm{Te}}_{3+\delta }$ structure. The white arrows depict the atomic layers with an enhanced contrast. Right panel: The magnified high-resolution HAADF-STEM image and corresponding intensity line scan profile along the $c$ axis. The maximum intensity peaks (marked with black arrows) correspond to Te-rich atomic layers.

Figure 4

Thermal variation of carrier concentration $\left(n_{\mathrm{H}}\right)$ for different mole percentage ($x$) of Te in ${\mathrm{Sb}}_2{\mathrm{Te}}_3/\mathrm{Te}$ samples. Lines connecting the data points are provided as a guide to the eye.

Figure 5

Thermal variation of the Seebeck coefficient $S$ for ${\mathrm{Sb}}_2{\mathrm{Te}}_3/\mathrm{Te}$ samples. The lines joining the data points are guidelines for the eye.

Figure 6

Temperature dependent (a) electrical resistivity ρ and (b) power factor (PF) of all Te-impurity-incorporated ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ samples. Lines connecting the data points are provided as guides to the eye.

Figure 7

Temperature dependence of (a) thermal conductivity κ and (b) electronic part of thermal conductivity ${\kappa }_e$ for different mole percentage ($x$) of Te in ${\mathrm{Sb}}_2{\mathrm{Te}}_3/\mathrm{Te}$ samples. Inset: Thermal variation of Lorenz number $L$ for all samples.

Figure 8

(a) Temperature-dependent lattice thermal conductivity ${\kappa }_{\mathrm{L}}$ for ${\mathrm{Sb}}_2{\mathrm{Te}}_3+x\mathrm{mol}\%$ Te samples. Solid red lines are the best fits to Eq. (3) for $x=0$, 4, 6, and 9. (b) Contribution of different types of phonon scattering toward lattice thermal conductivity ${\kappa }_{\mathrm{L}}$ for a typical sample $x=6$ considering the frequency (ω)-dependent terms successively for phonon relaxation time: ${\omega }^{-2}$ for phonon-phonon (solid black line), ${\omega }^{-4}$ for point defect (solid blue line), and ${\omega }^{-1}+{\omega }^{-3}$ for dislocation scattering [solid red line in (b)].

Figure 9

Thermal variation of weighted mobility ${\mu }_W$ for ${\mathrm{Sb}}_2{\mathrm{Te}}_3/\mathrm{Te}$ composite systems. The solid red line shows roughly the temperature dependence of ${\mu }_W(\sim T^{-3/2})$.

Figure 10

Experimental $S$ vs $n_{\mathrm{H}}$ data at 300 K of ${\mathrm{Sb}}_2{\mathrm{Te}}_3+x\mathrm{mol}\%$ Te ($x=0$, 4, and 6) with the theoretically calculated Pisarenko curve by varying energy gap between light and heavy hole valence bands of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$.

Figure 11

Thermal variation of (a) quality factor $B$ and (b) thermoelectric figure of merit $ZT$ for ${\mathrm{Sb}}_2{\mathrm{Te}}_3/\mathrm{Te}$ samples with different mole percentage ($x$) of Te. The unit of $B$ is in $\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}/{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$. Lines connecting the data points are provided as guides to the eye.