Authors: Adithya Kommini
Source: ScholarWorks@UMassAmherst (University of Massachusetts Amherst) (article, 2021-10-25)
Field: Materials Science — Materials Chemistry

Towards higher power factor in semiconductor thermoelectrics: bandstructure engineering and potential barriers To keep up with the current energy demand and to sustain the growth requires efficient use of existing resources. One of the ways to improve efficiency is by converting waste heat to electricity using thermoelectrics. Thermoelectric devices work on the principle of Seebeck effect, where an applied temperature difference across the material results in a potential difference in the material. The possibility of drastic improvements in the efficiency of thermoelectric (TE) devices using semiconductor nanostructured materials renewed interest in thermoelectrics over the last three decades. Introducing confinement, interfaces, and quantum effects using nanostructures for additional control of charge and phonon transport made it possible to achieve this higher efficiency in thermoelectrics. However, improving TE efficiency by tuning charge transport is not completely understood especially the quantum effects that play a predominant role in nanostructures. This dissertation focuses on understanding the impact of bandstructure engineering, carrier scattering, and potential barriers on charge transport by accurately modeling the charge dynamics. Smart material selection with desired bandstructure properties is explored in this thesis, especially in two-dimensional (2D) materials to maximize TE efficiency. We identify computationally inexpensive material selection rules using properties in 2D materials that can be obtained from material databases. We show that a 2D material having a combination of low effective mass, higher separation in the height of the step-like density of states, and valley splitting, which is the energy difference between the bottom of the conduction band and the satellite valley, equal to 5 kT will lead to a higher TE power factor. Further, we find that inelastic scattering with optical phonons plays a significant role: if inelastic scattering is the dominant mechanism and the energy of the optical phonon equals 5 kT, then the TE power factor is maximized. Introducing these material selection rules in MoS<sub>2</sub> provides a two-order increase in power factor compared to intrinsic values. Potential barriers introduced in materials using nanocomposites, superlattices, as well as single and multiple barrier structures, improve TE performance using energy filtering. Energy filtering is a process of restricting the movement of carriers with kinetic energy smaller than potential barriers (thermionic emission). To study these effects, a comprehensive model is developed that can simulate the classical (thermionic emission) and quantum behavior (tunneling) of carriers by integrating Boltzmann transport equation (BTE) with Wigner approach to include the carrier-potential interactions. Here we study single-layer 2D MoS<sub>2</sub> with lateral potential barriers to introduce either energy filtering or carrier confinement by changing the direction of the electric field, with confinement resulting when the electric field is parallel and energy filtering when the electric field is perpendicular to the potential barriers. A Wigner-Rode model with electronic structure calculated from first principles to simulate the effect of the shape and size of potential barriers on parallel and perpendicular transport is implemented. Our results show that the power factor can be doubled, from 25 mW m<sup>-1</sup> K<sup>2</sup> without barriers to over 50 mW m<sup>-1</sup> K<sup>2</sup> for parallel transport in sharp, narrow potential wells. Generally, modeling carrier transport in materials using semi-classical BTE assumes phonons to be in equilibrium at all temperatures. However, at low temperatures phonons can provide extra momenta to carriers through electron-phonon interactions of non-equilibrium phonons. This phenomenon is called phonon drag that gives a boost to Seebeck coefficient and controlled by the mean free path of phonons. This effect is predominant in low-dimensional materials due to the long mean free path of phonons. To understand the phonon drag contribution in a 2D material, carrier transport is modeled in single-layer MoS<sub>2</sub>. Using accurate phonon distribution to calculate the coupling between carriers and phonons, the phonon drag contribution to Seebeck coefficient is evaluated. Our simulations show phonon drag boosts Seebeck coefficient up to 27% at low temperatures in MoS<sub>2</sub>. Also, TA phonons contribute more towards phonon drag than LA phonons. Recent research on twisted bilayer graphene (TBG) uncovered its unique electronic properties. Experiments showed superconductivity, correlated insulating states, and magnetism at different twist angles. The flat bands in TBG result in a sharp density of states (DOS) that is desirable for superior thermoelectric performance. Using BTE and bandstructure from exact continuum models, the power factor (PF) of TBG at different twist angles is modeled. Our simulations show the power factor in TBG is twice in magnitude at 100 K compared to single-layer graphene (SLG). The peak PF is observed at a twist angle of 1.26 where the bandgap is high enough to improve the Seebeck coefficient compared to SLG by improving the TDF even though the electrical conductivity is lower. We observed an increase in PF of TBG with decreasing temperature, an unique behavior previously observed in superconductors with a high superconducting transition temperature. The strong TE performance along with the ability to fine-tune the behavior using twist angle makes TBG a solid candidate for future TE devices. Our results aid in improving TE power factors and further the development of efficient waste-heat scavenging, flexible 2D TE converters, and Peltier cooling of nanoelectronics. Thermoelectric materials ; Engineering physics ; Semiconductor ; Power (physics) ; Power factor ; Materials science ; Electrical engineering ; Engineering ; Physics ; Electrical resistivity and conductivity ; Thermodynamics Advanced Thermoelectric Materials and Devices ; Advanced Thermodynamics and Statistical Mechanics ; Semiconductor materials and interfaces