Band Engineering of Copper-Based Chalcogenides for Thermoelectric Power Generation

Abstract

The global energy crisis and the massive waste of thermal energy across industrial, transportation, and consumer systems have renewed urgency around the development of efficient thermoelectric materials — solid-state devices capable of converting heat directly into electricity. Central to this challenge is a longstanding materials science problem: the interdependence of the Seebeck coefficient, electrical conductivity, and thermal conductivity makes it extraordinarily difficult to optimize all three parameters simultaneously, as improving one tends to degrade the others. The dimensionless figure of merit, zT, encapsulates this tension and remains the benchmark against which all thermoelectric materials are measured. This thesis investigates copper antimony selenide, CuSbSe₂ — an earth-abundant, lead-free chalcogenide with an orthorhombic Pribramite crystal structure — as a candidate p-type thermoelectric material for the mid-temperature operating range of 500–700 K. While CuSbSe₂ possesses several intrinsically favorable properties, including a naturally anharmonic lattice driven by the stereochemically active lone-pair electrons on Sb³⁺, and flat valence bands arising from Cu 3d–Se 4p orbital hybridization that yield high Seebeck coefficients, it suffers from critically low intrinsic carrier concentration and poor electrical conductivity in its undoped form. These deficiencies have limited reported zT values to around 0.1 for pristine polycrystalline samples. The central problem explored in this thesis is whether systematic substitution of selenium with tellurium — a strategy known as isoelectronic chalcogen alloying across the series CuSbSe₂₋ₓTeₓ (x = 0, 0.5, 1.0, 1.5, 2.0) — can serve as an effective band engineering lever to simultaneously narrow the optical bandgap, enhance electrical transport, and suppress lattice thermal conductivity through enhanced point-defect phonon scattering, thereby improving the overall thermoelectric performance. A suite of characterization techniques was employed, including powder and pellet X-ray diffraction with Rietveld refinement, Raman spectroscopy, UV-Vis-NIR optical measurements, X-ray photoelectron spectroscopy, scanning electron microscopy with energy-dispersive spectroscopy, laser flash thermal diffusivity analysis, and Seebeck/electrical resistivity measurements. An additional and rarely reported contribution is the nanoindentation mechanical study across the series, providing insight into thermomechanical stability — a critical consideration for device longevity. The thesis uncovers a fundamental boundary to this substitution strategy: at full tellurium content, the target CuSbTe₂ ternary phase does not form; instead, the system phase-separates into a eutectic mixture of Cu₁.₈Te and Sb₂Te₃. This thermodynamic finding reframes the scope of chalcogen alloying in this system. Nevertheless, intermediate compositions prove thermally stable up to at least 700 K, and the combined effect of point-defect scattering and improved carrier transport yields a peak zT of approximately 0.3 at 630 K — a threefold improvement over the pristine compound — establishing both the promise and the limits of this band engineering approach.