Design and Fabrication of Tetradymites Based Vertically-Oriented Thermoelectric Generators (TEGs)

As the world governments and lawmakers shift to concerted environmentally-friendly policies to achieve net zero in carbon emissions by the year 2050, much interest and discourse are being displayed in scientific fora on how research in renewable energy materials and devices can pave the way to a more sustainable planet. In light of that, thermoelectric materials and devices based on Bi₂Te₃ and its alloys have received renewed attention from industry and academia due to their renewability, large scalability, and commercial viability in producing electricity from waste heat for niche applications in micro refrigeration and small power generation. These materials exhibit the dimensionless figure of merit ZT values around 1, at room temperature and can be utilized to design and fabricate novel, vertical, flexible, wearable, and portable thermoelectric devices. However, for thermoelectric devices to be competitive with fluid-based and other energy-related devices, ZT ≥ 2 is usually sought. The complexities of achieving high ZT reside in the fact that the dimensionless figure of merit presents a number of conflicting parameters. One cannot change one parameter without the detrimental effect of impacting the others. How then do we navigate the delicate and intricate relationship between the Seebeck coefficient or thermopower S, the electrical conductivity α, and the thermal conductivity Κ in order to achieve high ZT? From band engineering to grain boundary scatterings, to multilayer superlattice (SL) structures, thermoelectric researchers worldwide are investigating a plethora of options combined with state-of-the-art techniques to achieve just that.<br/>In this work, we designed vertical and flexible thin-film thermoelectric generators (TEGs) based on n-type Bi₂Te₃, p-type Sb₂Te_3, and their ternary chalcogenide n-type Bi₂Te_2.83Se_0.17 and p-type Bi_0.4Sb_1.6Te₃ in the form of multilayer superlattice nanostructures using RF magnetron sputtering deposition method. The inclusion of interstitial atoms (Sb and Se) in these nanostructured ternary chalcogenides provides a two-fold benefit as it can greatly reduce the thermal conductivity of the material, due to scattering events of a wide range of phonons carrying heat but also improve the power factor through modification of the electronic structure of the material. In addition, if the layer thickness is in close proximity to the phonon's mean free path (MFP), heat-carrying phonons of different wavelengths (short, medium, or long) can be scattered more effectively at the superlattice interfaces, thereby resulting in the decrease of the lattice thermal conductivity of the material and subsequently the enhancement of the thermoelectric figure of merit. Furthermore, we investigated the optimal growth parameters for each material that will render high thermopower, S, and suitable electrical conductivity, α.