Bottom-up Engineering of Thermoelectric Nanomaterials and Devices from Solution-Processed Nanoparticle Building Blocks

Abstract

In the world around us, it is easy to think in different situations in which there are temperature gradients available. These could be converted into a great source of energy if using the proper technology. Thermoelectric devices are environmentally friendly solid-state harvesters able to play this role by converting temperature differences into an electric voltage and vice-versa. These devices, besides being highly reliable since they have no moving parts, if engineered and fabricated in a shape-adaptable manner, are able to fit in countless industrial or domestic applications to improve their efficiency or power low-consumption devices like sensors. If, on top of it, the whole fabrication process is cost-effective and easily scalable, the outcoming thermoelectric devices could potentially reach numerous markets banned to date due to a mix of low efficiencies and high prices of the currently existing solutions. The first milestone towards cost-effective thermoelectric devices relies on the improvement of the thermoelectric conversion efficiency of the constituents materials. However, such improvement cannot be at all costs. New materials with significant improved performance need to be designed and engineered with relatively low production cost. In this framework, solution-processed techniques are an outstanding alternative for the production of thermoelectric materials and devices. In particular, the bottom-up assembly of colloidal nanoparticles, with controlled size, shape, crystal phase and composition, has no competing technology to precisely design functional metamaterials without the need of a high capital equipment or complex procedures, not only for thermoelectrics, but also for a wide range of applications. Nevertheless, some limitations still need to be overcome to exploit the full potential of solution-processed assembly technologies, and two different challenges should be addressed. The first one is regarding materials efficiency enhancement, and the second one to the device development itself. In this work, we undertake a journey from the material development to the engineering of the final device. The thesis is structured in 5 chapters, starting from the Chapter 0 or General Introduction that intends to situate the reader into the broader context of the technology and present the main objectives of the work. Chapter 1 presents a general view of the solution-processed route for the development of bottom-up engineered nanoparticle-based thermoelectric nanomaterials and devices. It is an extended and comprehensive text where main concepts, challenges, advantages and opportunities that the technology offers are exposed. Chapter 2 is built around 3 publications that cover the three different steps of the solution-processed nanomaterials preparation, and how the efficiency can be enhanced within each one. First article is focused on the synthesis stage, and presents the production of core-shell nanoparticles as a way to design

nanocomposites. The second one is related to the purification step, showing how, taking advantage of the nanoparticle surface, an HCl-mediated ligand displacement is able to introduce controlled amounts of dopants in the nanoparticle. Last one, is focused on the final assembly phase, in which by properly assembling two different kinds of nanoparticles, a semiconductor and a metal, the efficiency could be greatly enhanced. In Chapter 3 a step is made towards the production of a ring-shape device, taking PbSe as a model material. These results have been submitted for their publication. Chapter 4, presents the integration of a thermoelectric device together with a nanoparticle-based temperature sensor. This integrated assembly, including an ultra-low-power electronic management, was implemented as an autonomous soil moisture sensor, and shows the great opportunity that both solution-processed techniques and thermoelectrics technology offer for the development of new applications. Finally, some conclusions over the presented project and future work are listed.

Al món que ens envolta és fàcil pensar en situacions en què hi ha gradients de temperatura disponibles. Aquests, es podrien convertir en fonts d’energia molt interessants mitjançant l’ús adequat de la tecnologia. Els dispositius termoelèctrics son conversors d’estat sòlid capaços de jugar aquest important paper, ja que son capaços de transformar diferències de temperatura en energia elèctrica i vice-versa. Poden ser instal·lats a qualsevol emplaçament si son adaptats a l’aplicació en qüestió, ja sigui a escala domèstica o industrial, per millorar la seva eficiència energètica o, per exemple, alimentar altres dispositius de baix cost. Si, a més a més, el conjunt del procés de fabricació és de baix cost i fàcilment escalable per la seva producció en massa, els dispositius termoelèctrics resultants tindran la possibilitat d’entrar dins de nous mercats, fins ara impossibles degut a una barreja fatal d’alts preus i baixes eficiències dels productes comercials disponibles actualment. El primer pas cap a la fabricació de mòduls termoelèctrics més efectius en tots els sentits, és la millora de la seva eficiència a través de la recerca de nous o més efectius materials dels quals estan constituïts. Tanmateix, però, aquesta millora no pot ser a qualsevol cost. És necessari que aquests nous materials mantinguin alhora l’eficiència i baix cost en la seva fabricació. En aquest sentit, les tècniques de processat en solució son una gran alternativa per la producció de materials i dispositius termoelèctrics, i, en particular, la utilització de nanopartícules col·loïdals, amb mida, forma, fase i composició controlada. No hi ha cap altra tecnologia que aconsegueixi el seu nivell de control sobre el disseny de materials funcionals sense la necessitat de costosos equipaments o procediments complexes, no només per termoelèctrics sinó per un ampli ventall d’aplicacions. No obstant això, algunes limitacions encara han de ser superades per tal de poder explotar plenament el potencial que les tècniques de processat en solució ofereixen. Els dos majors reptes als quals la tecnologia s’enfronta son: primer, millorar l’eficiència dels materials, i, segon, en el desenvolupament de nous models de dispositius. En aquest treball, fem un viatge des del desenvolupament del material fins la fabricació d’un dispositiu.