Theoretical Investigation of Thermoelectric Properties in Bulk and 2D Materials

Abstract

Considerable research has been done on sustainable energy sources due to the severe environmental problems and the growing energy needs. The thermoelectric (TE) technology enables the extraction of clean energy from waste heat by converting thermal energy into electrical energy and vice versa. The efficiency of a thermoelectric device is determined by the figure of merit (ZT), which depends on both electrical and thermal transport. Finding highly efficient and cost-effective thermoelectric materials is crucial for designing a thermoelectric device with good performance. Our work mainly focuses on the theoretical understanding of the electronic and lattice vibrational properties of bulk and low-dimensional materials for thermoelectric applications. Density functional theory (DFT) is used to study the electronic structures and a semiclassical model based on Boltzmann transport theory is used to investigate the electron and phonon transport properties of different classes of materials. We have first studied the thermoelectric properties of bulk materials. Although a large number of p type Zintl compounds have shown high thermoelectric efficiency, very few n-type compounds are reported. Here, we investigated the thermoelectric properties of two n-type Zintl compounds, potassium antimonide (KSb) and potassium bismutide (KBi). The electron-phonon scattering rate is calculated using a deformation potential-based model and compared with the mode-dependent scattering rates using Born approximation. The maximum value of ZT 1 at 600 K is obtained by ∼ tuning the carrier concentration and temperature, revealing these two n-type materials as efficient thermoelectric materials. Recently, oxide materials have attracted more attention than traditional TE materials for photo-thermoelectric applications due to their non-toxicity and low ecological concern. Studies show that strontium titanate (STO) can have very high photoconductivity under UV light. We theoretically found an 182 times enhancement in the TE power factor after UV illumination on STO. The combination of two different perovskite oxides forms a heterojunction interface that tailors the charge transfer mechanism. The two-dimensional electron gas (2DEG) at the interface of two perovskite oxides (LVO/KTO) is found and further investigated for thermoelectric applications. The LVO/KTO heterostructure exhibits high electrical conductivity and power factor due to the n-type conducting interface and highly dispersive degenerate conduction bands. Also, the creation of heterojunctions between 2D materials leads to charge transfer and redistribution. The Phosporus doped graphene and Janus MoSSe heterostructure reduces the band gap of molybdenum dichalcogenide and results in high electrical conductivity. Also, the heterostructures of Janus MoSSe and graphene have a negative correlation between electrical and thermal conductivity. It is also possible to design both p-type and n-type legs by using a pure and phosphorus-doped heterostructure for a highly efficient 2D thermoelectric device. Twisted bilayer graphene (tblg) is an intriguing low-dimensional material due to the possible modulation of electronic and thermal properties. We have studied the electrical and thermal properties of twisted bilayer graphene and found a significant enhancement in thermoelectric propertiesof 20° twisted blg is obtained compared to untwisted bilayer graphene. A strong effect of boundary scattering on thermal transport reduced its thermal conductivity and made it a suitable material for thermoelectric devices. Further, we have obtained 48% reduction in the thermal conductivity for 21.8° tblg compared to blg. Boron nitrides considers as an excellent dielectric material for graphene to develop various electronic devices; here, we investigate theoretically the structural, electronic, and thermoelectric properties of the 21.8° tblg and 21.8° tBN, and 21.8° twisted Gr/BN heterostructure. An enhancement in the thermoelectric power factor and reduction in thermal conductivity is obtained for twisted Gr/BN heterostructure compared to its parent materials. Overall, the bulk and 2D materials from different classes investigated in this thesis are very promising thermoelectric materials that encourage their experimental studies for thermoelectric power generators and relevant applications