Solution Processed Chalcogenide Nanomaterials for Thermoelectric Application

Abstract

The bottom-up engineering of nanomaterials using solution-processing strategies is of particular interest for reducing cost and optimizing the performance of TE materials and devices. This thesis focuses on the development of scalable methods for the production of TE nanomaterials with optimized performance. The thesis is divided into 5 chapters. Chapter 1 introduces solution-based approaches for producing functional nanomaterials and the general state of the art in the field of thermoelectricity. Chapter 2 and chapter 3 present a fast and simple molecular ink-based method to produce low cost and crystallographically textured SnSe2 and SnSe nanomaterials. Molecular ink printing techniques could offer a scalable approach to fabricate TE devices on flexible substrates. In these chapters, I proved that cost-effective p-type SnSe NPLs could be produced by a molecular ink-based strategy that allowed introducing controlled amounts of Te to achieve unprecedentedly high TE figure of merit. On the other hand, n-type SnSe2 nanomaterials were also intentionally produced from the same strategy to complement an all Sn-Se based device. Both of the bulk nanomaterials displayed significant crystallographic texture after hot pressing, resulting in highly anisotropic charge and heat transport properties. Different approaches were applied to optimize their TE performance: SnSe2 NPLs were blended with metal NPs to produce a metal-semiconductor NC. The electrical conductivities of the blends were significantly improved with respect to bare SnSe2 bulk nanomaterial and a three-fold increase in the TE figure of merit was obtained, reaching unprecedented values up to ZT = 0.65 for SnSe2 material. For SnSe nanomaterials, I demonstrate that the introduction of small amounts of tellurium in the precursor ink allowed reducing the band gap, increasing both charge carrier concentration and mobility, especially cross plane, with a minimal decrease of the Seebeck coefficient. This strategy translated into record out of plane ZT values at 800 K, ZT=1.05 Chapter 4 and chapter 5 describe two different strategies to produce Bi2Te3-Cu2-xTe NCs based on the consolidation of nanostructured building blocks. I first detail a two-step solution-based process to produce the Bi2Te3-Cu2-xTe heteronanostructures, based on the growth of Cu2-xTe nanocrystals on the surface of Bi2Te3 nanowires. The transport properties of the NCs are investigated as a function of the amount of Cu introduced, which reveal that the presence of Cu decreases the material thermal conductivity through promotion of phonon scattering, modulates the charge carrier concentration through electron spillover, and increases the Seebeck coefficient through filtering of charge carriers at energy barriers. These effects result in an improvement of over 50% of the TE figure of merit of Bi2Te3. As comparison, I produced Bi2Te3-Cu2-xTe NCs by directly mixing proper ratio of individual Bi2Te3 nanowires with Cu2-xTe nanocubes and consolidating the resulting NP mixture by hot-press. A significant difference of transport properties was detected when compared with NCs fabricated by hot-pressing heterostructured Bi2Te3-Cu2-xTe nanowires. On the contrary to the composite obtained from hetero- nanostructures, the presence of Cu2-xTe nanodomains did not lead to a significant reduction of the lattice thermal conductivity of the reference Bi2Te3, which is already very low here, but it resulted in a nearly threefold increase of its power factor. Additionally, the presence of Cu2-xTe resulted in a strong increase of the Seebeck coefficient. This increase is related to the energy filtering of charge carriers at energy barriers within Bi2Te3 domains created by the accumulation of electrons in the regions nearby Cu2-xTe/Bi2Te3 junctions. Overall, a significant improvement of figure of merit, up to a 250%, was obtained with the suitable combination of Cu2-xTe NPs and Bi2Te3 nanowires. Finally, the main conclusions of this thesis and some perspectives for future work are presented.

La ingeniería de nanomateriales a partir del procesado en solución es de particular interés para optimizar el rendimiento de los materiales y dispositivos termoeléctricos. . Esta tesis estáse centra en el diseño y el ensamblaje racional de nanomateriales termoeléctricos de alto rendimiento a través de procesado en solución. La tesis se divide en 5 capítulos. El Capítulo 1 aborda la introducción fundamental del enfoque sintético para producir nanomateriales funcionales. Los capítulos 2 y 3 presentan un método rápido y simple basado en soluciones para producir nanomateriales SnSe2 y SnSe con textura cristalográfica. Dado que los calcogenuros de estaño son materiales especialmente interesantes para la conversión de energía termoeléctrica, se sintetizaron nanoplacas SnSe y SnSe2 controlables por forma mediante una estrategia basada en tinta molecular para lograr una figura de mérito termoeléctrica sin precedentes por dopaje con Te/Cu. Ambos nanomateriales mostraron una textura cristalográfica significativa después del prensado en caliente, lo que dio como resultado unas propiedades de transporte de carga calor altamente anisotrópicas. Los capítulos 4 y 5 describen dos estrategias diferentes para producir nanocompuestos Bi2Te3-Cu2-xTe basados en la consolidación de nanoestructuras. La presencia de Cu2-xTe da como resultado un fuerte aumento del coeficiente de Seebeck. Este aumento está relacionado con el filtrado de los portadores de carga en función de su energía en las barreras de energía dentro de los dominios Bi2Te3 creados por la acumulación de electrones en las regiones cercanas a las uniones Cu2-xTe / Bi2Te3. En general, se obtiene una mejora significativa de la figura de mérito con nanocompuestos Bi2Te3-Cu2-xTe. Finalmente, en el último capítulo se presentan las principales conclusiones de esta tesis y algunas perspectivas para trabajos futuros.