The adverse effect of fossil fuels on the environment is driving research to explore alternative energy sources, with the focus being on renewable energy. Efficient renewable energy sources, such as solar photovoltaic, solar fuels, biomass, hydro (thermal) energy, wind turbine, and hydrogenation, have been explored over the last few decades to provide a solution to the environmental degradation to the impact of the use fossil fuel. These renewable technologies have shown great potential in power generation and contributed to lessening the environment-related issues due to their clean energy harvesting. However, the high cost, bulkiness, effect on aquatic life, and weather-dependent nature of these renewable energy sources make their application not very attractive. Thus, thermoelectric (TE) technology emerges as an ideal choice due to its lightweight, scalability, cost-effectiveness, no moving part and can be integrated into devices (micro/nano) for power generation (heating/cooling applications). Thermoelectric materials are characterised by the dimensionless figure of merit (ZT; which is directly propositional to Seebeck coefficient and electrical conductivity and inversely proportional to thermal conductivity). The challenge with state of the art thermoelectric materials research is the conflicting property of the Seebeck coefficient (thermopower) and the electrical conductivity resulting in low power factor and hence lower figure of merit. Over the years, researchers have shown various techniques to enhance the figure of merit in metal chalcogenides TE materials. Among these, only Bi₂Te₃ has seen the light of commercialisation, due to its high-power factor. The undesirable setback of Bi₂Te₃ is the high thermal conductivity value. Therefore, further improvement in the thermoelectric performance of metal chalcogenides requires decoupling the Seebeck coefficient and electrical conductivity. This will yield further improvement in the power factor. In this thesis; we adopt a novel doping technique for simultaneous enhancement of thermopower and electrical conductivity as well as a substantial reduction of the total thermal conductivity of metal chalcogenide thermoelectric materials.

I have shown from the Landauer Boltzmann formalism, the fundamentals of thermoelectric properties and their interdependency. Thus, I propose (1) an isovalent substitution as a practical approach to simultaneously enhance the Seebeck coefficient and electrical conductivity (2) dual isovalent doping as phonon glass modulator for a drastic decrease of the total thermal conductivity. Previous research on anionic site substitutions in metal chalcogenides based thermoelectric materials has proven useful in TE improvement. Similarly, our studies have elucidated the potential of cationic site substitution as another means of ensuring simultaneous enhancement in both Seebeck coefficient and electrical conductivity. This will serve as a direction for current and future research to enhance the thermoelectric performance and its device application.

Rare earth (RE) isovalent substitution at bismuth site in Bi₂Se₃ is introduced. With this approach, I can achieve a simultaneous increase of both electrical conductivity and Seebeck coefficient of the material by tuning the content of the dopants, due to the formation of neutral impurities and consequently improving the carrier mobility. Through this, the power factor is improved without compromising the thermal conductivity. Our theoretical calculation reveals a downward shift of the valence band with cerium concentration, which influences the thermoelectric enhancement in the synthesised materials. The RE-Substitution in Bi₂Se₃ not only promotes the simultaneous increase of Seebeck coefficient and electrical conductivity but also decreases the thermal conductivity via an enhancement in phonon scattering. I numerically show that both cerium (Ce) and erbium (Er) substitution alters the Fermi energy of the Bi₂Se₃ materials, thereby enhancing the effective mass. Through Raman and XPS characterisation, I also elucidate that both Ce and Er substitution does not change the chemical structure and bonding of the pristine material appreciably but improve the Seebeck coefficient and electrical conductivity. Finally, an order of magnitude enhancement in the figure of merit is attained due to isovalent substitution, thus paving a new avenue for enhancing the thermoelectric performance of materials.

Further, I have used dual isovalent doping (indium (In) and antimony (Sb)) to reduce the thermal conductivity of bismuth chalcogenide (Bi₂Ch₃, Ch = Se, Te). Through the experimental and theoretical approach, I present a possible manifold decrease in thermal conductivity in metal chalcogenides while controlling the carrier density. The insertion of large atoms in the layered Bi₂Se₃ and Bi₂Te₃ structures distorts the crystal lattice and hence contribute significantly to phonon scattering. This is essential for ensuring total thermal conductivity (K_T) reduction. The ultralow thermal conductivity (K_T = 0.35 Wm^-1K^-1 at 473K for Bi_1.9In_0.1Sb_0.067Te₃ and KT = 0.25 Wm^-1K^-1 at 300K for Bi_1.8In_0.2Sb_0.067Se₃) compensates for the low power factor and hence show enhancement in the TE performance of both BISS and BIST samples. Our DFT calculation results reveal the formation of deep defects states in the valence band. The creation of deep states influences the electronic transport properties of the system. The dual dopants (In and Sb) thus show a coupled effect of improvement in the density of state near the Fermi level and reduction in the conduction band minimum. I also establish that the dual doping favours acoustic phonon scattering where the carrier mobility deterioration is attributed to the enhanced electron-phonon scattering due to the structural complexity of the doped and Bi₂Se₃ and Bi₂Te₃ structures. The dual doping strategy creates an interference pattern for phonon transport and thus drastically reduce the lattice thermal conductivity for improving thermoelectric material efficiency.