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Metal nanowire-based transparent electrodes for smart window applications.


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Huang, Shengyun (2020). Metal nanowire-based transparent electrodes for smart window applications. Thèse. Québec, Doctorat en sciences de l'énergie et des matériaux, Université du Québec, Institut national de la recherche scientifique, 147 p.

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Nowadays, energy crisis has emerged as one of the serious issues that, if not addressed properly, can have dramatic consequences on our planet in the near future. Among the several factors that caused this crisis are rapid industrial and economic development, dramatic increase in the world’s population, and heavy reliance on energy-based appliances. Consequently, research and development on new sustainable energy, energy conversion and storage technologies, and wise use of energy have attracted a great deal of interest. In particular, managing buildings’ environment in a smart way emerged as one of the ways to address the energy issue as the construction sector uses as much as 30%-40% of the primary energy in the world. In a typical building, windows could be the major source of energy loss, or gain, depending on their design. Smart windows with tunable transmittance levels can reflect or block sunlight on scorching days and lower energy consumption by air conditioning appliances. Meanwhile, they can also be put in a transparent state to improve light harvesting in a low lighting condition, or enhance heat capture in cold weather. In addition, depending on personal preference, smart windows can control the transmission of solar radiation into buildings so as to tune visibility between indoors and outdoors for privacy and comfort. Although the efficiency depends on a variety of factors, their overall performance strongly relies on the transparent conductive electrodes used. Indium tin oxide (ITO) based transparent conductive electrodes have been the industrial standard for a long time. However, the use of ITO as a transparent electrode material has certain limitations, such as ever increasing cost due to indium scarcity, complicated processing requirements, sensitivity to acidic and basic environments, and high surface roughness. Moreover, ITO is brittle and can easily crack when used in applications where bending cannot be absolutely avoided. To overcome these issues, intense research efforts should be devoted to develop flexible, cheap, and solution-processable transparent conductive electrodes by exploring other materials for next generation smart windows. In this thesis, our work is mainly focused on developing nanowires (NWs) based network films and their practical applications as transparent conductive electrodes in high-performance suspended particle devices (SPDs) and electrochromic (EC) devices smart windows. In the first part of the thesis, we developed a Cu-reduced graphene oxide core-shell NWs network film by vacuum filtration and subsequent thermal annealing. Moreover, a new SPD, holding a great potential for smart windows application, was built upon Cu-reduced graphene oxide core shell NWs film as a transparent conductive electrode for the first time. As we know, Cu is 1000 times more abundant and 100 times cheaper than indium or silver. In addition, its electrical resistivity (16.8 nΩ m) is almost as low as that of silver, which is known to have the lowest resistivity (15.9 nΩ m) among all the materials. Based on these fundamental advantages, Cu NW based transparent conductive electrodes have attracted growing interest as a potential alternative to ITO glass. However, several intrinsic drawbacks, such as low oxidation resistance, weak adhesion to substrates, and poor stability in harsh environments, have severely limited their widespread adaptation. With the wrapping of reduced graphene oxide, the Cu NW electrodes demonstrate both high optical transparency and electrical conductivity, as well as significantly improved stability under various testing conditions. The novel sandwich-structured SPDs, based on these electrodes, show large change in their optical transmittance (42 %) between “on” and “off” states, impressively fast switching time (20 s and 40 s for color bleaching and recovery, respectively) and superior stability. These high performances are comparable to those of the SPDs based on commercially available ITO electrodes. These promising results pave the way of our electrodes to be an integral part of a variety of optoelectronic devices, including energy-friendly and flexible electronics. The Cu-reduced graphene oxide core-shell NWs network transparent conductive electrodes developed in Part I shows high optoelectronic performance and good stability. However, the area of such Cu-reduced graphene oxide core-shell NWs network film is too small due to the vacuum filtration method used during the film fabrication process, which is not suitable for large-area device application. Moreover, the high temperature thermal reduction process of the Cu-graphene oxide NWs, imperative during the electrode post-treatment, limits their application on flexible substrates. Ag, as another very promising candidate for transparent conductive electrodes, has the highest electrical conductivity among all the metals and better oxidation/corrosion resistance than Cu, which provides it with huge market opportunities. Therefore, in the Part II, we developed a free-standing, ultra-flexible and high-performance Ag NW transparent conductive electrodes with large size of 19 cm × 9 cm using a facile and high-throughput automatic blade coating system for the first time, which is quite homogeneous and transparent over the entire area. Conventional solution-based electrode fabrication processes have several drawbacks for transparent conductive electrode applications, such as non-uniform morphology, difficulty of scale-up, high junction resistance between NWs, poor adhesion to the substrate and low stability. The blade coating method exhibits more advantages in terms of higher film uniformity and easier operation as well as more convenient process scale-up. However, this automatic technique has so far rarely been applied in the fabrication of large-area NW transparent conductive electrode, highly likely due to the relatively rough surface of the NW film processed from this technique. In our case, with the smart use of a flexible and transparent polymer, the transparent conductive electrodes showed smoother surface, better conductivity, superior mechanical flexibility as well as strong structural integrity. The polymer played multiple roles: passivation (protection), and performance and structure strengthening. Our Ag NW transparent conductive electrodes showed excellent flexibility, which can be repeatedly bent for 10,000 cycles without any performance degradation, significantly better than commercial ITO-based transparent conductive electrodes. To explore the high potential of these transparent conductive electrodes, as a proof of concept demonstration we fabricated foldable SPDs for smart window applications, using Ag NW network-based film as transparent electrodes for the first time. Our devices showed large change in their optical transmittance (optical modulation 60.2 %) and fast switching time (21 s), as well as excellent stability. Significantly better than ITO-polyethylene terephthalate transparent conductive electrode based electro-optic devices, the Ag NW electrode-based SPDs showed excellent mechanical flexibility, which can be folded by 180° for more than 200 cycles without obvious degradation of switching performance. The present method for fabricating large-area and ultra-flexible Ag NW transparent conductive electrode can be extended to fabricate a variety of NW-based nonplanar or curved electronic and optoelectronic devices in the future. Although, the developed foldable Ag NW transparent conductive electrode-based SPDs in part II exhibited high optical modulation, fast switching time as well as excellent stability. The current SPDs technology requires a relatively high alternating current voltage of 110 V to control light transmission, which is not safe in use, especially in wearable devices. Moreover, nanoscale Ag NWs easily get oxidized and thus inevitably stop working in a long run as irreparable oxidation continues. Especially, when the Ag NW transparent conductive electrode was used as the anode in the EC device, the NWs lost their conductivity due to the oxidation during the charge carrier exchanging process with electrolyte. To address these issues, we developed a solid state, flexible EC device in the last part of the thesis. Such a device requires the delicate design of every component to meet the stringent requirements for transparency, flexibility and deformation stability. However, the electrode technology in flexible EC devices remains stagnant, wherein inflexible ITO and fluorine-doped tin oxide are the main materials being used. Meanwhile, the brittle metal oxide usually used in an active layer and the leakage issue of liquid electrolyte during transformation further negatively affect device performance and lifetime. Therefore, we developed a novel and fully ITO-free flexible organic EC device by using Ag-Au core-shell NW (Ag-Au NW) network, EC polymer and LiBF4/propylene carbonate/poly(methyl methacrylate) as electrodes, active layer, and solid electrolyte. The Ag-Au NW electrode integrated with a conjugated EC polymer together displayed excellent stability in harsh environments due to the tight encapsulation of the Ag NWs by the Au shell, and high area capacitance of 3.0 mF/cm² and specific capacitance of 23.2 F/g at current density of 0.5 mA/cm2 . The EC device showed high EC performance with reversible transmittance modulation in the visible region (40.2 % at wavelength of 550 nm) and near-infrared region (-68.2 % at wavelength of 1600 nm). Moreover, the device presented excellent flexibility and fast switching time. The strategies developed and demonstrated here for flexible EC devices may also serve as a platform technology for futuristic deformable electronics and optoelectronics.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ma, Dongling
Mots-clés libres: énergie; fenêtre intelligente;
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 21 avr. 2021 15:06
Dernière modification: 21 avr. 2021 15:06
URI: https://espace.inrs.ca/id/eprint/11507

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