Growth Control and Study of Ultrathin Silver Films for Energy-Saving Coatings

Les couches minces fonctionnelles jouent un rôle prépondérant dans la plupart des secteurs industriels actuels. Ils peuvent aussi bien être une partie intégrante d’un dispositif (cellule solaire, diode électroluminescente, photodétecteur, Laser, capteur thermosolaire, cellule thermoélectrique et bien d’autres), ou bien y amener de nouvelles fonctionnalités (revêtements résistants à la corrosion, l’usure et l’érosion, revêtement antireflet). La montée rapide de cette science est à l’origine d’un développement tout aussi rapide des techniques de dépôt et de synthèse de couches minces. Aujourd’hui, la croissance d’une couche mince avec une précision au nanomètre peut être effectuée par un simple couchage à lame au sein d’un laboratoire de recherche aussi bien que par des techniques d’évaporation dans des chambres à vide industrielles à grande échelle. La facilité d’accès aux techniques de dépôt ainsi que l’envergure des applications scientifiques et technologiques font des couches minces une solution potentielle pour beaucoup d’enjeux technologiques et de sociétés. Certainement, un des plus grands enjeux actuels est le problème de la consommation énergétique à travers le monde et qui peut seulement qu’empirer si aucune solution convenable n’est adoptée. Une approche afin de contrer cette consommation énergétique est de modifier les vitrages architecturaux dans les bâtiments commerciaux et résidentiels en revêtant une fenêtre avec une couche réfléchissante la chaleur afin de réduire de façon drastique les charges de chauffages et de refroidissement. Le recouvrement des fenêtres par de fines couches optiques se fait par des chambres de dépôts montées en ligne, souvent jumelées avec la production du verre flotté. Bien que le maintien et l’installation de ces systèmes de dépôt est d’un grand intérêt et pose de nombreux défis, le travail de recherche présenté dans cette thèse se penche sur le mécanisme de croissance des couches minces d’argent déposé en phase vapeur par assistance plasma pour les filtres à basse émissivité. Le projet est mené en collaboration avec Guardian Industries dans le cadre des vitrages à économies d’énergie. Les couches minces d’argent possèdent des propriétés physiques changeantes dépendamment de leur mécanisme de croissance ainsi que de leur épaisseur. Elles ont tendance à croître en îlots en dessous d’une épaisseur critique et convergent vers une couche réfléchissante la radiation infrarouges. Cette épaisseur critique se nomme le seuil de percolation et dépend fortement de la couche sous-jacente.----------Abstract Functional thin films play a key role in almost all industries today. They can either form an integral part of a device (as heat-reflectors, solar cells, light-emitting diodes, photodetectors, lasers, thermal collectors, thermoelectric cells and many more) or bring additional coating functionalities (such as corrosion, wear and erosion resistance and antireflective coating). The rapid development of thin film science has led to the equally fast growth of thin film deposition techniques. The coating of a surface with precisions in the nanometers can be conducted by simple blade coating in a laboratory setting or large-scale vacuum chambers in heavy industrial environments. Moreover, the rapid rise of thin film science can also be attributed to progresses in characterization techniques. The accessibility of thin film deposition techniques and their wide-ranging scientific and technological applications make thin film science appear as an attractive answer to many industrial and societal challenges. Probably the greatest of these challenges is the energy consumption problem present in large parts of the world and which can only amplify in time if no suitable solutions are adopted. One approach to decrease this energy consumption is to alter glazing units in commercial and residential buildings by coating one side of the window pane with a heat-reflecting layer in order to drastically reduce heating and cooling loads. These glass panes are manufactured by large, in-line vacuum coaters that can be found on the glass production site. Even though the configuration and maintenance of such systems is of great interest and brings important challenges, the research work conducted throughout this thesis is focused in the growth mechanism of very thin silver films inside a low-emissivity stack deposited by plasma-assisted physical vapour deposition, with collaboration with Guardian Industries in the context of energy-saving glazing. Silver thin films have unique varying physical properties attributed to their distinct growth mechanism. They tend to progressively grow as light-absorbing agglomerated clusters below a certain thickness to infrared heat-reflective, continuous films. The main challenge in the context of silver film growth is the inability to obtain heat-reflecting properties below a certain thickness. This thickness is determined by the surface properties of the underlying layer, limiting the possible options available for silver film coating

Time-resolved imaging of non-diffusive carrier transport in long-lifetime halide perovskite thin films

Owing to their exceptional semiconducting properties, hybrid inorganic-organic perovskites show great promise as photovoltaic absorbers. In these materials, long-range diffusion of charge carriers allows for most of the photogenerated carriers to contribute to the photovoltaic efficiency. Here, time-resolved photoluminescence (PL) microscopy is used to directly probe ambipolar carrier diffusion and recombination kinetics in hybrid perovskites. This technique is applied to thin films of methylammonium lead tri-iodide MAPbI$_3$ obtained with two different fabrication routes, methylammonium lead tribromide (MAPbBr$_3$), and an alloy of formamidinium lead tri-iodide (FAPbI$_3$) and methylammonium lead bromide FA$_{0.85}$MA$_{0.15}$Pb(I$_{0.85}$Br_${0.15}$)$_3$. Average diffusion coefficients in the films leading to the highest device efficiencies and longest lifetimes, i.e., in FA$_{0.85}$MA$_{0.15}$Pb(I$_{0.85}$Br$_{0.15}$)$_3$ and acetonitrile-processed MAPbI$_3$, are found to be several orders of magnitude lower than in the other films. Further examination of the time-dependence shows strong evidence for non-diffusive transport. In particular, acetonitrile-processed MAPbI$_3$ shows distinct diffusion regimes on short and long timescales with an effective diffusion constant varying over 2 orders of magnitude. Our results also highlight the fact that increases in carrier lifetime in this class of materials are not necessarily concomitant with increased diffusion lengths and that the PL quantum efficiency under solar cell operating conditions is a greater indication of material, and ultimately device, quality

Alkali Metal Halide Salts as Interface Additives to Fabricate Hysteresis-Free Hybrid Perovskite-Based Photovoltaic Devices

A new method was developed for doping and fabricating hysteresis-free hybrid perovskite-based photovoltaic devices by using alkali metal halide salts as interface layer additives. Such salt layers introduced at the perovskite interface can provide excessive halide ions to fill vacancies formed during the deposition and annealing process. A range of solution-processed halide salts were investigated. The highest performance of methylammonium lead mixed-halide perovskite device was achieved with a NaI interlayer and showed a power conversion efficiency of 12.6% and a hysteresis of less than 2%. This represents a 90% improvement compared to control devices without this salt layer. Through depth-resolved mass spectrometry, optical modeling, and photoluminescence spectroscopy, this enhancement is attributed to the reduction of iodide vacancies, passivation of grain boundaries, and improved hole extraction. Our approach ultimately provides an alternative and facile route to high-performance and hysteresis-free perovskite solar cells

Interacting polariton fluids in a monolayer of tungsten disulfide

Atomically thin transition metal dichalcogenides (TMDs) possess a number of properties that make them attractive for realizing room-temperature polariton devices. An ideal platform for manipulating polariton fluids within monolayer TMDs is that of Bloch surface waves, which confine the electric field to a small volume near the surface of a dielectric mirror. Here we demonstrate that monolayer tungsten disulfide ($\text{WS}_2$) can sustain Bloch surface wave polaritons (BSWPs) with a Rabi splitting of 43 meV and propagation constants reaching 33 $\mu$m. In addition, we evidence strong polariton-polariton nonlinearities within BSWPs, which manifest themselves as a reversible blueshift of the lower polariton resonance by up to 12.9$\pm$0.5 meV. Such nonlinearities are at the heart of polariton devices and have not yet been demonstrated in TMD polaritons. As a proof of concept, we use the nonlinearity to implement a nonlinear polariton source. Our results demonstrate that BSWPs using TMDs can support long-range propagation combined with strong nonlinearities, enabling potential applications in integrated optical processing and polaritonic circuits.Comment: 7 pages, 4 figure