Understanding the Effects of Dilute Sulfur Additions, and Metallization, on the Thermoelectric Properties of Pnictogen Chalcogenides and their Interfaces

Realizing materials with high thermoelectric figure-of-merit ZT is an exacting challenge because it entails simultaneously obtaining a high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity, while these properties are usually unfavorably coupled. This thesis demonstrates multifold enhancements in the power factor in sulfur-doped binary and ternary pnictogen chalcogenide nanocrystals and assemblies, and describes the property enhancement mechanisms. The correlations between interfacial thermal and electronic transport, and interfacial diffusion and phase formation in metallized n- and p-type pnictogen chalcogenide structures are also revealed. We show that 400 ppm to 2 at.% sulfur doping can increase both Seebeck coefficient and electrical conductivity, while maintaining low thermal conductivity. Our results show that sulfur-induced property enhancements in Bi2Te 2Se are underpinned by increased density of states effective mass, unlike the mechanism of diminished bipolar charge carrier transport prevalent in sulfur-doped Bi2Te3. Exploiting such effects is anticipated to be attractive for realizing higher ZT nanomaterials. We also show that electrical contact conductivity in metallized pnictogen chalcogenide interfaces is sensitive to metal diffusion and telluride formation. In particular, Ni contacts yield the highest electrical contact conductivity and Cu the lowest, correlating with extent of metal diffusion and p-type metal-telluride formation. We finally show that pnictogen chalcogenides metallized with Sn-Ag-Cu/Ni solder-barrier bilayers exhibit ten-fold higher interfacial thermal conductance than that obtained with In/Ni bilayer metallization. Decreased interdiffusion and diminution of interfacial SnTe formation due to Ni layer correlates with the higher interfacial thermal conductance. Our findings should facilitate the design and development of pnictogen chalcogenide-based thermoelectric materials and devices.