Phase Diagrams and Defect Thermodynamics to Devise Doping Strategies in Lead Chalcogenide Thermoelectric Materials and its Alloys

Abstract

This thesis discusses the application of phase diagrams and the associated thermodynamics to semiconductor materials through theoretical computational calculations. The majority of work is focused on thermoelectric semiconducting materials that enable direct inter-conversion between electrical and thermal energy. First, one of the most efficient thermoelectric material, PbTe, is picked to demonstrate the assessment of unknown phase diagrams by combining two methods - DFT and CALPHAD. Since there had been no previous investigations of defect stability in this material using computations, DFT is used to deduce the stability of various intrinsic point defects, and in turn attribute origins of n- and p-type conductivity to the most stable defects. Then, the calculated defect formation energies are used in the Pb-Te thermodynamic model built using the CALPHAD method to compare the estimated solubility lines and non-stoichiometric range of the PbTe phase with experimental data. Next, another lead chalcogenide, PbSe, is picked to explore the phase stability of the PbSe phase upon the addition of dopants (Br, Cl, I, Na, Sb, Bi, In), which is a common strategy to make thermoelectric materials and devices more efficient. The range of efficiencies and thermoelectric properties as functions of composition and temperature that can be achieved depends on the amount of dopant that can be added without precipitating secondary phases. Also, depending on the system and its phase diagram, there can be more than one way of doping a material. To help detail which method(s) of doping into PbSe will result in maximum dopant solubility, a procedure similar to the above for PbTe is followed by using DFT in combination with Boltzmann statistics to map solvus boundaries of the PbSe phase, but now in the ternary phase space of composition and temperature. This method also helps predict electrical conductivity, n- or p-type, in each region of the phase diagrams that represent different doping methods. Lastly, the role of surface energy contributions in changing phase stability at nano-dimensions is explored. The CALPHAD approach is employed to investigate these changes in three systems by calculating their phase diagrams at nano dimensions and comparing them with their bulk counterparts.