Smart Window Applications Research Articles

Towards Room Temperature Phase Transition of W-Doped VO2 Thin Films Deposited by Pulsed Laser Deposition: Thermochromic, Surface, and Structural Analysis.

Vanadium dioxide (VO2) with an insulator-to-metal (IMT) transition (∼68 °C) is considered a very attractive thermochromic material for smart window applications. Indeed, tailoring and understanding the thermochromic and surface properties at lower temperatures can enable room-temperature applications. The effect of W doping on the thermochromic, surface, and nanostructure properties of VO2 thin film was investigated in the present proof. W-doped VO2 thin films with different W contents were deposited by pulsed laser deposition (PLD) using V/W (+O2) and V2O5/W multilayers. Rapid thermal annealing at 400-450 °C under oxygen flow was performed to crystallize the as-deposited films. The thermochromic, surface chemistry, structural, and morphological properties of the thin films obtained were investigated. The results showed that the V5+ was more surface sensitive and W distribution was homogeneous in all samples. Moreover, the V2O5 acted as a W diffusion barrier during the annealing stage, whereas the V+O2 environment favored W surface diffusion. The phase transition temperature gradually decreased with increasing W content with a high efficiency of -26 °C per at. % W. For the highest doping concentration of 1.7 at. %, VO2 showed room-temperature transition (26 °C) with high luminous transmittance (62%), indicating great potential for optical applications.

A bistable cholesteric liquid crystal film stabilized by a liquid-crystalline epoxy/thiol compound-based polymer.

Bistable cholesteric liquid crystals have promising application prospects in various fields, such as smart windows and displays. However, the long-term stability of two individual states is not easy to achieve, hindering their practical use. In this research, the bistable feature was enhanced by constructing a microsphere-type polymer with a liquid-crystalline epoxy/thiol monomer in negative dielectric anisotropic cholesteric liquid crystals. Spectroscopic and optical examinations revealed that either the transparent planar state or the opaque focal conic state can be maintained without the aid of an external field. Moreover, they can be switched to each other by applying a high- or low-frequency electric field. Further, factors such as the chemical structure of thiol compounds, curing temperature and curing time were investigated to explore their influences on the micro morphology of the polymer and thereby the electro-optical properties. In addition, the frequency-dependent driving scheme was analysed. Finally, bistable switching was demonstrated using an optimized sample. This energy-efficient bistable film shines light on future applications in smart windows, photonic paper and other electro-optical devices.

Switchable optics based on guided mode resonance in lithographically patterned vanadium dioxide with integrated heating layer

Vanadium dioxide (VO2) has promising applications in smart windows and active micro-optical devices due to its thermochromic properties. However, the successful fabrication and patterning of VO2 thin films with the correct stoichiometry and phase are challenging. In this study, we investigated lithographically patterned and non-patterned VO2 thin films fabricated by reactive ion beam deposition, using variable angle spectroscopic ellipsometry, Raman spectroscopy, and transmission and reflection measurements. The results show that the refractive index and extinction coefficient exhibit significant changes for near-infrared wavelengths when heated above 68 °C, confirming its thermochromic properties. The Raman spectroscopy results indicate the formation of the monoclinic phase VO2(M) after annealing, which was not changed by reactive ion etching. Lithographically structured VO2-layers were successfully realized demonstrating the potential of VO2 as a material for active micro-optical devices, such as guided mode resonance filters with switchable reflectance. The results suggest that VO2 has great potential as a promising material for actively switched optical elements and micro-optical devices.

Coupling Ti doping with oxygen vacancies in tungsten oxide for high-performance photochromism applications.

A series of Ti-doped W18O49 samples were prepared using a convenient solvothermal route. Due to the synergistic effect of doped Ti and oxygen vacancies, the samples showed excellent visible-light photochromic properties. Their performances as light-printable rewritable paper and smart windows showed great application value and promotion value.

Synthesis of temperature- and humidity-induced dual stimulation film PU-PNIPAm n and its independent film formation as a smart window application.

Dynamic windows, which switch between transparent and opaque states as the temperature changes, can be applied in buildings to reduce building energy consumption. Poly(N-isopropylacrylamide) (PNIPAm) is the most studied thermochromic hydrogel for climate-resilient smart window applications. In addition to its poor mechanical properties and low reaction rate, the PNIPAm hydrogel must be sandwiched between two pieces of glass to form an interlayer in practical applications. Here, durable PU-PNIPAm n copolymers for smart windows were synthesized by reacting the synthesized poly-NIPAm diols with isocyanate (-NCO) monomer, which greatly improved the mechanical properties of the hydrogel and it was able to form a film alone. These temperature-sensitive films can switch between transparent (>80% transmittance) and opaque (<5% transmittance) states in less than 10 minutes, with no degradation in optical contrast, switching speed, or uniformity after at least 100 switching cycles.

An energy-efficient and low-driving-voltage flexible smart window enhanced by POSS and CsxWO3

Smart windows with controllable visible and near-infrared (NIR) light transmittance are highly desirable for energy-saving buildings. Herein, we propose a hybrid polymer dispersed liquid crystal (PDLC) film for smart window applications, which combines dopants of polyhedral oligomeric silsesquioxane (POSS) and cesium tungsten bronze (Cs x WO 3 ). The low surface free energy of POSS and NIR absorption of Cs x WO 3 endow the film with enhanced electro-optical performance (ultra-low driving voltages, high contrast ratio and low optical haze value) and maneuverable light modulation in visible and NIR range. A model house equipped with the proposed film as smart windows reveals its outstanding light shielding and temperature regulating abilities that the indoor temperature rises less than 5 o C after irradiation for 10min under the experimental conditions. The research provides new opportunities for further development of energy-efficient smart windows, especially for cooling energy saving in buildings. • A hybrid PDLC film is proposed that can modulate the light passage in both visible and near-infrared ranges. • Cs x WO 3 is introduced to absorb the near-infrared radiation. • Both Cs x WO 3 and POSS can enhance the electro-optical properties of PDLC. • The as-fabricated film can regulate indoor temperature if used as smart windows.

Growth of cerium oxide nanorods by hydrothermal method and electrochromic properties of CeO2/WO3 hybrid thin films for smart window applications

Innovative electrochromogenic nanomaterials such as composite materials and also hybrid films can improve electrochromic performance, because of their potential application qualities in electronic, low-power screens, automotive anti-reflect mirrors, and smart windows. In this study, we used a hydrothermal method to used grow the CeO2 nanorods both with and without HCl added to the solution. And also, DC magnetron sputtering was used to deposit the tungsten oxide films on the cerium oxide nanorods. The surface plasmon effect changes with the size of CeO2 Nanorods, and this phenomenon influences electrochromic outcomes. The electrochromic characteristics of CeO2/WO3 nanostructures on FTO-coated glass are examined in the visible spectrum to use a 0.5 M concentration of H2SO4 as such electrolyte. At 600 nm, these structures produce significant optical modulation (50 %) and coloring efficiency (11.60 cm2/C at 700 nm).

Hydrothermally prepared ultra-stable multilayer nanoflake NiO-based electrochromic films

Electrochromic materials cycle between colored and bleached states under various applied voltage and have applications in smart windows and displays. The formation of a micro–nano structure in electrochromic thin films using hydrothermal methods can improve their properties. However, the conventional constant-temperature hydrothermal method yields films that easily peel from the conductive substrate during electrochromic cycling, resulting in performance losses. Therefore, we investigated the effect of a gradient-temperature hydrothermal protocol on the electrochromic performance of NiO thin films. First, a layer of small, strongly adhered crystal grains was seeded on an indium tin oxide substrate. Thereafter, NiO nanoflake layers were grown by adjusting the reaction temperature. The prepared film could be operated for 14,000 electrochromic cycles, four-times more than those of the conventionally prepared films, without significant performance loss. Therefore, our proposed method enhances the cyclic stability of micro–nanostructured NiO thin films significantly and will aid the development of high-performance electrochromic thin films having broad applications.

Preparation of Highly Durable Reverse-Mode Polymer-Stabilized Liquid Crystal Films with Polymer Walls.

Reverse-mode polymer-stabilized liquid crystal (PSLC) films have wide applications in smart windows for cars as well as buildings and dimming glasses due to their low haze, low energy consumption, and better safety in case of emergency power off. However, PSLC films usually have poor stability of electro-optical properties due to their low polymer content (ca. 5 wt %), and it still remains a challenging task to improve the stability and processability by increasing the polymer content in PSLC as the driving voltage might dramatically increase. In this work, a reverse-mode PSLC film with polymer walls was prepared, which showed excellent stability of electro-optical properties even after 150 000 cycles. The film was prepared through polymerization with a photomask, in which the monomers concentrated on specific areas to form patterned polymer walls. In this way, the polymer content could be increased dramatically and the anchoring effect would not be too strong, thus avoiding a sharp increase in the driving voltage. As a result, the desired reverse-mode film with high stability, relatively low driving voltage, and high contrast ratio was obtained. The effects of monomer compositions, curing temperature, UV light intensity, and the pattern of the photomask on the microstructures, as well as electro-optical performances of the films were carefully studied. This work provides a new idea for the preparation of reverse-mode electrically switchable light-transmittance controllable films with excellent stability and good electro-optical performance, which would broaden their application in smart cars, building windows, and dimming glasses for light management and potential energy saving.

Facile Synthesis of Island-like ZrO2-VO2 Composite Films with Enhanced Thermochromic Performance for Smart Windows

VO2-based film, as a very promising thermochromic material for smart windows, has attracted extensive attention but has not been widely applied because it is difficult to simultaneously improve in terms of both solar-modulation efficiency (ΔTsol) and visible transmittance (Tlum) when made using the magnetron-sputtering method, and it has poor durability when made using the wet chemical method. Herein, island-like ZrO2-VO2 composite films with improved thermochromic performance (ΔTsol: 12.6%, Tlum: 45.0%) were created using a simple approach combining a dual magnetron-sputtering and acid-solution procedure. The film's ΔTsol and Tlum values were increased initially and subsequently declined as the sputtering power of the ZrO2 target was raised from 30 W to 120 W. ΔTsol achieved its maximum of 12.6% at 60 W, and Tlum reached its maximum of 51.1% at 90 W. This is likely the result of the interaction of two opposing effects: Some VO2 nanocrystals in the composite film were isolated by a few ZrO2 grains, and some pores could utilize their surface-plasmon-resonance effect at high temperature to absorb some near-infrared light for an enhanced ΔTsol and Tlum. More ZrO2 grains means fewer VO2 grains in the composite film and increased film thickness, which also results in a decrease in ΔTsol and Tlum. As a result, this work may offer a facile strategy to prepare VO2-based films with high thermochromic performance and promote their application in smart windows.

Novel system using hydrogel with reduced graphite oxide particles as active layer for potential application in smart window

A suspended particle device (SPD) is a type of smart window that can be switched with an electrical stimulus, and its transmittance varies proportionally with the applied voltage. This work presents a new route to produce SPD, using hydrogels based on conventional polymers with trapped reduced graphite oxide (rGO) micrometric particles as an active layer, to make this product more accessible, as it is not yet cost-effective. The trapped rGO particles were produced from the chemical oxidation of graphite, followed by thermal reduction. The results of X-ray photoelectron spectroscopy (XPS) confirmed the production of reduced graphite oxide particles with a low quantity of oxidized carbon groups. X-ray diffraction (XRD) showed the crystalline graphitic structure of the produced rGO particles and proved their incorporation inside the hydrogel's tridimensional polymeric network. The presence of these particles causes a decrease of 28.3% in the degree of swelling of the hydrogel. The devices were assembled in two different configurations: (i) fluorine-doped tin oxide (cFTO); and (ii) silver paint (cAg) by changing the positions of the electrodes. The cAg device displayed better responses, with an immediate increase of 38.6% in transmittance, due to an applied voltage of 220 V, compared to the switched-off device. The voltage applying time also increased the transmittance of the cAg device, with an increase of 36.6% after 20 min under 220 V, compared to the device immediately turned on.

Investigation of vacuum-integrated switchable polymer dispersed liquid crystal glazing for smart window application for less energy-hungry building

The presence of more than one transparency and controllable intermediate transmission between ON and OFF states make the switchable smart glazing more attractive for less energy-hungry building integration than a static transparent window. However, to enable this switchable glazing for cold climatic buildings, overall heat transfer through the glazing needs to be reduced. In this work, the thermal and optical performance of integrated polymer dispersed liquid crystal and low heat loss transparent vacuum glazing was investigated using indoor characterisation. Two systems were developed where PDLC glazing was sandwiched between a vacuum and one acrylic sheet (Vacuum-PDLCA) and vacuum and glass (Vacuum-PDLCG). The employment of acrylic sheets reduced 21% of the overall weight of the system which made it suitable for retrofit building integration. Use of acrylic reduced 35% of solar transmission of PDLC ON state compared to the use of glass. Overall heat transfer coefficient (U-value) or thermal transmittance was below 1.1 W/m2K for all Vacuum -PDLCG and Vacuum-PDLCA ON and OFF states. Solar heat gain coefficient or solar energy transmittance was highest at 0.45 for the Vacuum-PDLCG ON state and lowest at 0.23 for the Vacuum-PDLCA OFF state.

Construction of a novel self-bleaching photochromic hydrogel embraced within the Zn-MOF@WO3 junction for assembling UV-irradiated smart rewritable device

Photochromic hydrogels, which are dynamic in response to external light stimuli, have aroused increasing attentions due to their potential applications in intelligent devices. However, achieving excellent photochromic activity with fast photoresponse and self-bleaching rates still remains a huge challenge. Herein, a novel photochromic junction of zinc-based metal–organic framework (Zn-MOF) and tungsten oxide (WO3) was established as photochromic unit and embraced within hydrogel of poly(acrylic acid-co-polyethylene glycol) [P(AA/PEG)] via the interaction of ionic and hydrogen bonding (denoted as P(AA/PEG)-Zn-MOF@WO3), following by exploiting as self-bleaching photochromic agent for assembling UV-irradiated smart rewritable device. The photoinduced electrons can be efficiently separated and migrated from Zn-MOF to WO3 in the Zn-MOF@WO3 junction after UV irradiation, thereby increasing the coloration degree. Due to the enhanced fast response to light stimuli, the P(AA/PEG)-Zn-MOF@WO3-3 hydrogel prepared using 200 mg Zn-MOF demonstrated the fast responsive photochromic behavior of 10 s under UV lamp illumination and the self-fading process could be easily performed in room condition (within 20 min). Moreover, the coloration degree of P(AA/PEG)-Zn-MOF@WO3-3 photochromic hydrogel was precisely controlled and repeated by changing the UV irradiation time. Consequently, the fast photochromic and self-fading properties of P(AA/PEG)-Zn-MOF@WO3-3 afforded rapid and easy ‘‘writing’’ and “erasing” abilities. The present work proposes a new way for the construction of the reversibly responsive photochromic hydrogel with smart optical materials, further broadening their applications in intelligence sensors, smart windows, and flexible memory devices.

Electrochromic and optical studies on Nb2O5–NiO mixed oxide films for smart window applications

Coatings of Nickel oxide (NiO), Niobium oxide (Nb2O5) and their mixed oxide (Nb2O5–NiO) were deposited on glass and conducting glass substrates by simple spin coating technique. Structural, morphological, optical and electrochemical properties of the mixed oxide (Nb2O5–NiO) films were analyzed and the data was compared with individual oxide coatings. As-prepared Nb2O5–NiO coatings showed transparent in nature, while Li + intercalated Nb2O5–NiO (termed as Li–Nb2O5–NiO) coatings were black in color. In addition, Nb2O5–NiO coatings showed better electrochromic reversibility and fast switching. All the as-prepared and Li+ intercalated coatings were subjected to IR absorption studies. Interestingly, Li–Nb2O5–NiO coatings showed better Infra-red (IR) absorption whereas bleached Nb2O5–NiO coatings did not show any remarkable IR absorption. The Li–Nb2O5–NiO coating absorbed about 65% of IR radiation falling on it and allowed the remaining 35% radiation to pass through the coating. In order to prevent the remaining 35% of IR radiation from passing through the coating, laser treatment was carried out on the coating using Pulsed Laser Deposition (PLD) unit. This laser treated Li–Nb2O5–NiO coatings showed better IR reflectance compared to normal Li–Nb2O5–NiO due to its improved surface roughness. It is concluded that the collective effect of IR absorption (by modifying the electronic structure of materials by ion intercalation) and IR reflectance (by increasing the surface roughness) played on a laser treated Li– Nb2O5–NiO coating resulted in a better IR blocking window.

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO2) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2 thin films remains a challenge. In this work, high-quality thermochromic VO2 thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO2 thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO2 thin films was studied. The shift of the VO2(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO2 thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO2 films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.

Microstructural Investigations of VO2 Thermochromic Thin Films Grown by Pulsed Laser Deposition for Smart Windows Applications

The structural properties of VO2 thin films, grown on either LSAT or Si substrates by pulsed laser deposition (PLD), are elucidated by means of transmission electron microscopy (TEM) methods. The TEM observations confirmed the successful growth of VO2 by PLD in variable thicknesses, by optimizing the O2 partial pressure and growth temperature. The films adopt a columnar polycrystalline morphology with narrow columns, up to the film thickness height. Four VO2 polymorphs have been detected by electron diffraction and high-resolution TEM (HRTEM) analysis, with M1 being by far the most abundant phase. Post-experimental strain measurements in HRTEM images have revealed that the actual residual strain is minimized due to the columnar morphology of the VO2 grains, as well as intrinsic oxide layers in the VO2/Si epitaxy. The TEM outcomes confirmed the complementary electrical and magnetic measurements in the films, where a transition from a monoclinic M1 to a rutile VO2 R phase has been identified, influenced by the initial percentage of phases in thick VO2 films.

Low-temperature processable transparent liquid crystal light shutter

Tunable light shutters show their great potential as smart windows, especially when energy efficiency is greatly entailed. Polymer-dispersed liquid crystals (PDLCs) are one of the most well-known and promising methods for smart window applications, but there are many obstacles to overcome or to develop further. At first, PDLCs require high voltages not only when to switch from an opaque to a transparent state but also to maintain the transparent state. A transparent mode, which can be produced by polymer-stabilizing homeotropic alignment of LCs, can be one of the ways to reduce energy consumption significantly from this perspective. However, the most efficient material for the induction of the homeotropic alignment, polyimide requires high temperature for the surface treatment process. Here we demonstrate a low-temperature process for plastic substrates, allowed via a homeotropic alignment induction molecule, premixed in LCs. The mixture before polymerization contains LCs, a reactive mesogen, and a homeotropic alignment inducer. The homeotropic alignment inducer plays a role of polymer-stabilization as well. We believe the result clearly suggests a novel approach to resolve the issues in establishing initial structures and energy consumption of PDLCs for smart windows.

Preparation of CoS2/WO3 thin films for the construction of electrochromic devices

In this study, we report CoS2/WO3 (CSW) composites as electrochromic materials for applications in smart windows. A hydrothermal approach was used to obtain CoS2, whereas WO3 was prepared using the sol-gel method. We prepared the CSW composites with various weight ratios of CoS2 (0.2, 0.5, and 2.0 wt%) by ultrasonication dispersion. The physical and chemical properties of the CSW composites (CSW 0.2, CSW 0.5, and CSW 2.0) were examined via X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Additionally, the WO3 and CSW based ECDs were fabricated using indium-tin oxide (ITO) substrate. The CSW 0.5 based ECDs exhibited excellent coloration efficiency of 64.5 cm2C-1 and an optical contrast of 73.31%. The enhanced coloration efficiency and optical contrast may be attributed to synergistic interactions between CoS2 and WO3. The ECDs also exhibited good stability up to 10,000 cycles.

Phase Transition and Optical Properties of VO2 and Al: ZnO/VO2 Thin Films

Thermochromic Vanadium dioxide (VO2) has strong potential for smart window applications but its commercial scale usage is limited due to low visible light transmission. To address this issue, aluminum doped zinc oxide (AZO) anti-reflecting layer is integrated with VO2 layer in the present work. VO2 single layer and AZO/VO2 bilayer thin film samples were deposited by sputtering technique on quartz substrate. The single-phase growth of VO2 and AZO in single layer and bilayer thin film samples is confirmed by X-ray diffraction measurements. Monoclinic M1 phase of VO2 is detected in VO2 and AZO/VO2 thin film samples at room temperature. Monoclinic to rutile structural phase transition (SPT) in these samples is studied by performing temperature-dependent X-ray diffraction measurements. SPT in VO2 thin film samples is close to 68 °C and SPT temperature appears slightly lower in AZO/VO2 sample as compared to VO2 sample. Spectral transmittance measurement at room temperature showed significant improvement in the visible transmittance of AZO bilayer film than that of single layer VO2 thin film. These results demonstrate the possibility of integration of anti-reflecting layers such as AZO with VO2 layer for better visible transmittances suitable for large-scale smart window applications.

Dual-Band Electrochromic Optical Modulation Improved by a Precise Control of Lithium Content in Li4+xTi5O12.

Dual-band electrochromic smart windows that can dynamically and independently control incident solar irradiation and visible light are envisioned as intelligent technology to reduce power consumption of buildings. However, there is still a great challenge to put the dual-band electrochromic technology into practice due to some limits in material systems and preparation techniques. Herein, a new electrochromic material of Li4Ti5O12 is developed to implement the dual-band optical modulation behavior, which could be further improved by a precise control of the lithium content in the active material. It could separately modulate the light and heat based on regulation of the transmittance of visible and near-infrared light. This enables Li4Ti5O12 to operate in three distinct modes of bright, cool, and dark, so as to meet various indoor needs. The optical transmittance contrast reaches over 60% at both visible- and near-infrared-light regions between different modes, and a large range of apparent temperature adjustments (7 °C) could be achieved. The prototype device based on dual-band electrochromic Li4Ti5O12 is further developed into a smart window of a house model, which exhibits good optical and thermal modulation behaviors in response to a high-temperature environment. This work provides a new material system for achieving dual-band electrochromic optical modulation toward smart energy-saving window applications.