Mechanism behind the high thermoelectric power factor of SrTiO3 by calculating the transport coefficients

Abstract

The thermoelectric power factor of SrTiO3 is unusually high with respect to its mobility and band gap. Good thermoelectrics usually have high mobility and a narrow band gap, but such properties are not found in SrTiO3. We have determined the mechanism behind the high power factor by calculating the transport coefficients. The key to understanding the power factor is that different effective masses contribute to different transport phenomena. The discrepancy between the effective mass for the conductivity and the thermoelectric power showed that the conductivity and thermoelectric power are conveyed by electrons with different effective masses in the Brillouin zone. Light electrons were responsible for the high conductivity, whereas heavy electrons were responsible for the high thermoelectric power. The high carrier concentrations of more than 1020 cm−3 did not reduce the thermoelectric power of SrTiO3 above the classical limit. This indicates that the electrons carrying the thermoelectric power were not degenerate. This is achieved by a decrease in the Fermi energy and the contribution of the heavy electrons to the Seebeck coefficient. The strong dielectric screening also contributed to the high power factor. The Coulomb scattering by ionized impurities, which would usually reduce the carrier mobility, was effectively screened. These results clarify the mechanism behind the contribution of different types of electrons, and show that high thermoelectric power does not necessarily reduce conductivity. Our findings provide a new direction for the band engineering of thermoelectric materials.

Phonon-glass electron-crystal behaviour by A site disorder in n-type thermoelectric oxides

“…However, by calculating the electronic structure of STO for different tilt angles (ESI †) we found that the coexistence of dispersive and dispersion-less bands at the bottom of the conduction band is retained for the range of tilt angles observed in the Nb-doped LNTO series, and the ratio of effective electronic mass for the two bands remains 5 for all tilt angles examined, which is similar to the values reported for cubic STO previously. 22 Tauc plots from diffuse reflectance data show that the parent and doped compositions of LNTO have band gaps of 3.2 eV, which is similar to STO (Fig. S8, Table S9, ESI †).…”

“…20,21 Both p-and n-type semiconductors are needed for device fabrication, and it has been more difficult to achieve comparable performance in n-type oxides. By exploiting the band structure of the perovskite oxide SrTiO3 (STO), which has contributions from both light and heavy electrons at the Fermi level, 22 it is possible to achieve large power factors of 28-36 μWK -2 cm -1 at 300 K in n-type Sr1-xLaxTiO3-δ (x = 0.015-0.1) single crystals, however, the performance is negated by the large κ of ≈ 12 Wm -1 K -1 . 23 The nanostructuring approaches used to reduce κ in classical compound semiconductors and intermetallics by increasing grain boundary scattering are inefficient in titanate perovskites as the phonon MFP of SrTiO3 is 2-3 nm, requiring nanometer-sized grains for any noticeable effect on κ.…”

Thermoelectric properties of n-type SrTiO3

“…Moreover, m * plays different roles in σ and S. For thermoelectric performance it is important to have both high S and high σ, which as discussed previously 59 have opposite dependences on both carrier concentration and effective mass. This conundrum can be resolved by certain complex band structures that exploit the different transport integrals that enter σ and S. [60][61][62][63][64][65][66] STO has a particular band shape arising from the degeneracy of the t 2g levels in an octahedral crystal field that gives effectively lower dimensional behaviour in transport even though the material is cubic, analogous to the case of PbTe. 65 The sharp onset of the DOS at the CBM reflects the low-dimensional nature of the electronic structure.…”

Impact of Ferroelectric Distortion on Thermopower in BaTiO3

“…13 Recently, Shirai et al casted another interpretation on the large thermopower in SrTiO 3 as following. 14 It is often said that the t 2g components of the d electrons form dispersionless and three-fold degenerate bands. However, the lift of the degeneracy as leaving from the Γ point should not be overlooked.…”