Studies on the Thermoelectric Performance of Lead and Tin Chalcogenides
With the growing interest of the world’s energy demand, sustainable and green energy is becoming a global challenge. Thermoelectric power generation, that directly converts waste heat into electricity, is considered as one of the potent alternatives to solve this challenge. IV-VI alloys, especially lead chalcogenides are one of the most studied thermoelectric materials due to their decent thermoelectric figure of merit, ZT, from 573 K-873 K. On the contrary, tin chalcogenides, especially tin-selenides are rarely studied as thermoelectric materials due to their larger band gaps. This dissertation, addresses the systematic study of the thermoelectric performance of lead chalcogenides (PbTe, PbSe, PbS and their alloys) and tin chalcogenides (n and p-type SnSe) prepared by melting and hot pressing. We found that, Cr doping increases the Seebeck coefficient and power factor of n-type PbSe and PbTe1-ySey alloys near room-temperature. The higher Seebeck coefficient and power factor enhancements are attributed to the higher Hall mobilities of ~1000-1120 cm2 V-1s-1 at lower carrier concentrations. The high power factor combined with lower thermal conductivity due to large Grüneisen parameter in PbSe, and alloy scattering in PbTe1-ySey, lead to a room-temperature ZT enhancement of ~0.4 in Cr0.005Pb0.995Se, ~0.5 in selenium-rich Cr0.01Pb0.99Te0.25Se0.75, and ~0.6 in tellurium-rich Cr0.015Pb0.985Te0.75Se0.25 respectively. Consequently, the average ZT was improved from 300 K-873 K and lead to thermal to electrical conversion efficiency of up to ~12 % in Cr0.005Pb0.995Se, ~10 % in tellurium-rich Cr0.015Pb0.985Te0.75Se0.25, and ~12 % in selenium-rich Cr0.01Pb0.99Te0.25Se0.75, withcold-side temperature of 300 K and hot-side temperature of 773 K. In another study of PbTe1-ySy alloy, by combination of indium doping and thermal conductivity reduction via alloy scattering and spinodal decomposition, the room-temperature and average ZT was significantly improved leading to efficiency of ~12 % in In0.02Pb0.98Te0.8S0.2 with cold-side temperature of 323 K and hot-side temperature of 773 K. In addition to lead chalcogenides, we also investigated the thermoelectric performance of polycrystalline SnSe. We achieved a peak ZT of ~1 and ~0.8 at ~773 K in n-type SnSe0.87S0.1I0.03 and p-type Na0.015Sn0.985Se respectively, by combination of optimized doping and lower thermal conductivity due to mainly anharmonic and anisotropic bonding.