Boosting the power factor with resonant states: A model study

Abstract

A particularly promising pathway to enhance the efficiency of thermoelectric materials lies in the use of resonant states, as suggested by experimentalists and theorists alike. In this paper, we go over the mechanisms used in the literature to explain how resonant levels affect the thermoelectric properties, and we suggest that the effects of hybridization are crucial yet ill-understood. In order to get a good grasp of the physical picture and to draw guidelines for thermoelectric enhancement, we use a tight-binding model containing a conduction band hybridized with a flat band. We find that the conductivity is suppressed in a wide energy range near the resonance, but that the Seebeck coefficient can be boosted for strong enough hybridization, thus allowing for a significant increase of the power factor. The Seebeck coefficient can also display a sign change as the Fermi level crosses the resonance. Our results suggest that in order to boost the power factor, the hybridization strength must not be too low, the resonant level must not be too close to the conduction (or valence) band edge, and the Fermi level must be located around, but not inside, the resonant peak.

Ab initio investigation of the role of vanadium impurity states in SrTiO3 for thermoelectricity

“…The major effect occurs for the Seebeck coefficient which exhibits a sign inversion close to the resonant states. This point, as we have already pointed out in a previous work [26], is opening new perspectives for obtaining both n and p-type TE materials using the same dopant. Moreover, for a concentration of 3.7%, the amplitude of the Seebeck coefficient (at the maximum of the PF) is of the same amplitude that pristine STO.…”

“…In contrast to this scenario, it has recently been proposed that impurity states actually suppress the conductivity although with a strong variations of the transport distribution function leading to a boost in the Seebeck coefficient which overcompensates the conductivity reduction [26]. Herein, we report on the theoretical basis of ab initio calculations the use of vanadium to dope STO which advantageously exhibits impurity states whithin the gap which promotes a boost of the Seebeck coefficient.…”

Optimum in the thermoelectric efficiency of nanostructured Nb-doped TiO2ceramics: from polarons to Nb–Nb dimers

“…18,19 These states are responsible for singularities in the electronic density which can be used to boost the thermoelectric efficiency. 27 Among the different possibilities of these states, the most studied are achieved either by the substitutional replacement of Ti 4+ by a cation with a higher d-electron count, such as group VB cations, or by oxygen vacancy. Interesting TE properties have been discovered in oxygen-deficient rutile, known as Magneli phase TiO x .…”

Ultrahigh figure‐of‐merit of Cu2Se incorporated with carbon coated boron nanoparticles

“…As an evaluation for energy conversion efficiency, figure‐of‐merit ( zT ) is the decisive factor to screen good TE materials, namely zT = S 2 σT / κ , where S , σ , T , and κ are the Seebeck coefficient, electrical conductivity, absolute temperature in Kelvin, and thermal conductivity, respectively . In the past decades, many novel concepts and effective experimental approaches aiming to enhance zT have emerged, such as band convergence, phonon‐glass electron‐crystal, multiscale phonon scattering, resonant states, anharmonicity, quantum confinement, and grain boundary engineering, and so on. Consequently, these methods have been adopted to successfully raise up the zT s in a wide range of TE materials, including SnSe, PbTe, GeTe, CoSb 3 , Bi‐Te, Sb‐Te, and so on, promoting TE modules applied in converting waste heat to electricity with competitive energy conversion efficiency …”

“…effective experimental approaches aiming to enhance zT have emerged, such as band convergence, 8 phonon-glass electroncrystal, 9 multiscale phonon scattering, 10 resonant states, 11 anharmonicity, 12 quantum confinement, 13 and grain boundary engineering, 8 and so on. Consequently, these methods have been adopted to successfully raise up the zTs in a wide range of TE materials, including SnSe, PbTe, GeTe, CoSb 3 , Bi-Te, Sb-Te, and so on, promoting TE modules applied in converting waste heat to electricity with competitive energy conversion efficiency.…”