Thermoelectric Properties of Metal Sulfide Ceramics and Thin Films

Abstract

Sulfides are very promising thermoelectric materials due to their low cost, low toxicity, large earth-abundance and good thermoelectric performance. In this work, tin sulfide (SnS) ceramics, and SnS, copper antimony (Cu12Sb4S13), and copper zinc tin sulfide (Cu2ZnSnS4 - CZTS) thin films were prepared, and their thermoelectric properties enhanced through doping and optimizing preparation parameters. First principles calculations were employed to provide deeper understanding of the relationship between of electronic structure and thermoelectric properties. SnS ceramics were synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). By doping with Ag, the electrical transport properties were improved, and lattice thermal conductivity was reduced, resulting in an enhanced thermoelectric figure of merit zT of 0.22 for 2.78 at% Ag-doped SnS ceramic samples at 800 K. First-principles calculations showed that Ag doping shifted the Fermi level into the valence band and led to band degeneracy. Disruption of the regular bonding network was induced by Ag doping, resulting in a substantial drop in the phonon group velocities. A novel fitting procedure (combining experimental data with theoretical modelling) was used to determine the temperature-dependent carrier concentration n, carrier mobility u, and electron relaxation time. The highest calculated zT of 1.1 was obtained for the 2.78 at% Ag-doped SnS model at 800 K. High quality SnS thin films were deposited on glass substrates using aerosol-assisted chemical vapour deposition (AACVD) by optimizing carrier gas flow rate, precursor solution concentration and deposition temperature. It was found that deposition temperature has a dominant role in charge transport. The power factor was improved to ~0.22 uW cm-1 K-2 at 570 K for SnS films deposited at a flow rate of 150 cm3 min-1, a solution concentration of 0.4 mol L-1, and a deposition temperature of 445 C. The zT for SnS films was evaluated for the first time, yielding a value of ~0.026 at 550 K. Cu12Sb4S13 thin films were deposited on a range of substrates (glass, Sb2O3-coated glass, ITO-coated glass, SiO2-coated Si wafer and mica) via AACVD. A buffer layer of Sb2O3 or ITO with small lattice mismatch to Cu12Sb4S13 improved the quality of thin films and reduced the content of secondary phases. Use of SiO2-coated Si wafers with lower roughness reduced the likelihood of defects in the Cu12Sb4S13 thin film and improved film density and coverage, beneficial to carrier transport. The highest power factor of ~362 uW cm-1 K-2 was obtained at 625 K for thin films deposited on SiO2-coated Si wafers. At 460 K, the peak zT for the Cu12Sb4S13 thin films was ~0.18. Starting with CZTS, the Zn atoms of CZTS were partially substituted by Fe and Cd to prepare CZFCTS thin films. The power factor was improved to ~0.22 uW cm-1 K-2 at 575 K. First-principles calculation results demonstrated that that CZFCTS has a reduced bandgap of ~0.8 eV. Cd and Fe doping forms acceptor levels above the CZTS valence band maximum (VBM) leading to an increase in carrier concentration. Cd and Fe substitution also increased the density of states (DoS) at the top of valence band near the Fermi level, which is advantageous to increasing the Seebeck coefficient. The quality of the CZFCTS thin films was further improved by post-deposition annealing, contributing to an enhanced power factor of ~2.4 uW cm-1 K-2 at 575 K for the CZFCTS thin film annealed for 24 h. The maximum zT values were estimated to be 0.18-0.69 at 550 K.

Date of Award	6 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Freer (Main Supervisor) & David Lewis (Co Supervisor)