Electrochromic Smart Windows Research Articles

Influence of cycling temperature on the electrochromic properties of WO3//NiO devices built with various thicknesses

Electrochromic smart windows, which are able to vary their optical transmittance by the application of a voltage, improve the indoor comfort in buildings by controlling light and temperature. Among electrochromic devices, ECDs, WO3 and NiO are often associated leading to neutral color. This work focuses on the influence of the thickness of WO3 and NiO single layers on the ECD electrochemical and optical properties. Initial characterizations in lithium-based liquid electrolyte, show a non-linear evolution of the electrochemical capacity for each oxides vs. thickness while for similar thickness, WO3 thin films exhibit larger EC properties than NiO ones. The combinations of various WO3 and NiO thicknesses lead to ECDs of which capacity appears limited by the one of NiO, while the switch to a neutral color always remain. Besides, the influence of each oxide on the ECD cyclic voltammogram shape is discussed. Finally, the study of the cycling temperature, in the range −40°C to 80°C on the electrochromic properties of ITO/650nm-WO3/LiClO4-PC +PMMA/360nm-NiO/ITO shows an increase in performance with temperature nevertheless associated with signs of degradation above 60°C. At low temperature, the decrease in capacity is interestingly associated with a remaining significant optical contrast and the capacity recovery if the ECD is further cycled at RT.

Galvanostatic Rejuvenation of Electrochromic WO3 Thin Films: Ion Trapping and Detrapping Observed by Optical Measurements and by Time-of-Flight Secondary Ion Mass Spectrometry.

Electrochromic (EC) smart windows are able to decrease our energy footprint while enhancing indoor comfort and convenience. However, the limited durability of these windows, as well as their cost, result in hampered market introduction. Here, we investigate thin films of the most widely studied EC material, WO3. Specifically, we combine optical measurements (using spectrophotometry in conjunction with variable-angle spectroscopic ellipsometry) with time-of-flight secondary ion mass spectrometry and atomic force microscopy. Data were taken on films in their as-deposited state, after immersion in a Li-ion-conducting electrolyte, after severe degradation by harsh voltammetric cycling and after galvanostatic rejuvenation to regain the original EC performance. Unambiguous evidence was found for the trapping and detrapping of Li ions in the films, along with a thickness increase or decrease during degradation and rejuvenation, respectively. It was discovered that (i) the trapped ions exhibited a depth gradient; (ii) following the rejuvenation procedure, a small fraction of the Li ions remained trapped in the film and gave rise to a weak short-wavelength residual absorption; and (iii) the surface roughness of the film was larger in the degraded state than in its virgin and rejuvenated states. These data provide important insights into the degradation mechanisms of EC devices and into means of achieving improved durability.

(Invited) Inkjet Printed Multifunctional Smart Windows

Electrochromic smart windows that are able to dynamically control the light and heat transmission into a building have attracted intense interests in the field of green building. However, currently external energy consumption is required for electrochromic smart windows to change their optical properties such as transmittance, reflectance and absorption. How to recycle the energy expended and finely integrate the multifunctional smart window are still challenging at present. With our earlier established work on the various of electrochromic nanomaterials such as NiO, WO3 on transparent conductors such as ITO, FTO, silver grid/PEDOT substrates, the importance and progresses of multifunctional smart window have been highlighted.1-4 We extended our efforts on large size multifunctional smart windows (more than 300 cm2) fabricated by inkjet printing technique which is one of the low cost and solution processible methods. The assembled electrochromic windows exhibit outstanding performance including large optical modulation, high coloration efficiency, fast switching and excellent bistability. The designed multifunctional smart window can be used as typical electrochromic window as well as energy-storage device. The smart window is able to change its color to dynamically control the light and heat into a building and store the energy in the day time, and then releases the stored energy to power other electronic devices at night. Moreover, we can monitor the level of stored energy in the window via the visually detectable reversible color variation. The reported multifunctional smart window represents a leap towards the realization of fascinating and energy-efficient smart green building. References (1) Cai, G.; Wang, X.; Cui, M.; Darmawan, P.; Wang, J.; Eh, A. L.-S.; Lee, P. S. Nano Energy 2015, 12, 258. (2) Cai, G.; Wang, J.; Lee, P. S. Acc. Chem. Res. 2016, 49, 1469. (3) Cai, G.; Darmawan, P.; Cui, M.; Chen, J.; Wang, X.; Eh, A. L.-S.; Magdassi, S.; Lee, P. S. Nanoscale 2016, 8, 348. (4) Cai, G.; Darmawan, P.; Cui, M.; Wang, J.; Chen, J.; Magdassi, S.; Lee, P. S. Adv. Energy Mater. 2016, 6, 1501882.

Influence of the Temperature on the Electrochromic Properties of Complementary and Symmetrical WO3//NiO Devices

Electrochromic smart windows benefits include among others energy efficiency and indoor comfort [1]. However, on a daily use, the temperature is a key factor that may affect the device durability. Among materials, WO3 and NiO thin films are often combined for the realization of electrochromic devices with high color efficiency, short switching time, low cost and good stability [2]. Nevertheless, in the literature, the influence of the temperature is scarcely addressed [3]. In this study, the influence of temperature between 80 °C and -40 °C on the EC properties of WO3 and NiO based devices either in complementary WO3/electrolyte/NiO or in symmetrical structure WO3/electrolyte/WO3 and NiO/electrolyte/NiO, is discussed. WO3 and NiO thin films were deposited on ITO coated glasses (~25Ω/□) by reactive direct current (DC) magnetron sputtering. Laminated devices were prepared using lithium based composite polymer gel. The electrochromic properties of complementary devices and symmetrical devices were characterized in temperatures, both after storage for few hours and upon cycling. As a general trend, the device capacity decreases in low temperature and increase in high temperature, while intermediate cycling at room temperature shows slight degradation above 45 °C, and good recovery after cycling at low temperature (T < 0 °C). Interestingly, symmetrical devices behave differently in respect of NiO or WO3nature. In this presentation, further investigations on the evolution of the EC behavior with temperature will be discussed.

Electrochemical Charging of CdSe Quantum Dots: Effects of Adsorption Versus Intercalation

Effects of electrochemical charging of quantum dots (QDs) have been reported in the past, wherein optical and electrical properties could be modulated through cation adsorption and electron injection into the quantum-confined 1Se states. In this presentation, we report two different modes of electrochemical double-layer charging in CdSe QDs and their effects on the electronic and optical properties. We show that the charging mechanism at the interface involves cation intercalation for smaller ions, such as Li+, Na+, or K+, and cation adsorption for larger bulky ions, such as tetrabutylammonium ions, where steric hindrance precludes intercalation. As a result, it was observed that while cation adsorption leads to an increase in the absorbance in the mid-infrared spectral range, cation intercalation into the CdSe core results in an absorbance increase from the visible to infrared spectral range, an enhancement in radiative lifetime of e−, an increase of 158% in the intensity of band-edge photoluminescence, and strong emission in the near-infrared spectral range as a result of the formation of Se vacancies. In this talk, the nature of charging mechanisms will be discussed using the results from combined photoluminescence studies, radiative lifetime studies, and X-ray photoemission studies. The cation-coupled electronic and optical modulation reported here in CdSe QDs have important implications for electrochromic smart windows, photovoltaics and other devices.

Preparation and characterization of protonic solid electrolyte applied to a smart window device with high optical modulation

Solid electrolytes based on gelatin, cross-linked with formaldehyde and containing different quantities of glycerol, acetic acid and/or hydrochloric acid, with preparation routine modification have been conducted and characterized using X-ray diffraction, mechanical test, impedance spectroscopy and UV–vis spectrophotometry.The x-ray investigation revealed that all the prepared layers are predominantly amorphous and are influenced noticeably by the addition of different quantities of plasticizer as well acids. The tensile, elongation at break and modulus of the prepared membranes are evaluated and discussed. Using impedance spectroscopy, it was found that, the ionic conductivity acquired a maximum value of 1.28×10−5S/cm at 26wt% acetic acid contents, then decreases at higher concentration due to ion pair formation. The optical investigation revealed transmission ≥98% at wavelengths >580nm. Optimized membrane that has both high conductivity, high transparency, mechanical stability and show good adhesion to both counter and working electrodes was successfully applied to an electrochromic smart window device including tungsten and nickel oxides as electrochromic layers. Using cyclic voltammetry (CV), chronoamperometry (CA), and the device performance has been characterized at different working voltages. The device showed reversible blue coloration, high lifetime cyclic stability, and long-term open circuit memory. The coloration efficiency η600 is enhanced reaching a value 38.1cm2/C. Such results indicate that protonic gelatin-based solid electrolyte is a good candidate as ionic conductor layer in EC devices and of technological and commercial interest.

Smart Window Technologies: Electrochromics and Nanocellulose thin film Membranes and Devices

Strategies for incorporating energy-efficiency requirements into building standards have been implemented by governments in developed countries in order to introduce the concept of green nanotechnology. Substituting regular glass windows in residential/commercial buildings with smart windows is the objective. The smart window is to function as electric dimming glass that is to be built with innovative nanomaterial based membranes/ coatings. Currently, the regular windows are made of glass and curtains/blinds are used to block sun light; eliminating such elements is of importance due to its limited functionality. Though these curtains/blinds blocks UV and prevents sun light illumination inside the rooms, but there are additional issues such as health hazards (e.g. dust and germ collection especially in hospitals) relating to asthma problems, disposal/recycling issues, regular cost, and maintenance which are some of the possible glitches. The smart window is expected to block harmful UV light and provide controlled privacy while also eliminating the problems related to curtains/blinds. Electrochromic (EC) smart windows are already in use that varies the throughput of visible light by electrical voltage or by solar energy application and are able to provide energy efficiency and indoor comfort. These smart windows comprises of electrochromic materials such as LixWO2.89 and HxNiO2 as cathodic and anodic oxide films, respectively, and other complex polymers, which are complicated to create, expensive and some are hazardous in nature. Nanocellulose (achieved from wood/pulp product) is already being used in flexible electronics, so nanomaterial membrane based suspended particle device (SPD) technology for smart window is a probable alternative to EC devices, but still under research. This paper reviews, introduces and discusses the present situation of both (EC and SPD) options for the state-of-the-art smart windows and its applications while also provide ample references to current literature of particular relevance and thereby, hopefully, an easy entrance to the research field. Also the paper presents an outlook for an innovative technology for smart-windows with SPD-EC technology to work as dimming glass on voltage application while also induce the capability of solar applications in smart windows leading to green nanotechnology in buildings.

Electrochemical Charging of CdSe Quantum Dots: Effects of Adsorption versus Intercalation.

Effects of electrochemical charging of quantum dots (QDs) have been reported previously, wherein optical and electrical properties could be modulated through cation adsorption and electron injection into the quantum-confined 1Se states. In this work, we report two different modes of electrochemical double-layer charging in CdSe QDs and their effects on the electronic and optical properties. We show that the charging mechanism at the interface involves cation intercalation for smaller ions, such as Li+, Na+, or K+, and cation adsorption for larger bulky ions, such as tetrabutylammonium ions, where steric hindrance precludes intercalation. As a result, while cation adsorption leads to an increase in the absorbance in the mid-infrared spectral range, cation intercalation into the CdSe core results in an absorbance increase from the visible to infrared spectral range, an enhancement in radiative lifetime of e-, an increase of 158% in the intensity of band-edge photoluminescence, and strong emission in the near-infrared spectral range as a result of the formation of Se vacancies. The nature of charging mechanisms is discussed using the results of combined photoluminescence, radiative lifetime, and X-ray photoemission studies. The cation-coupled electronic and optical modulation reported here in CdSe QDs have important implications for electrochromic smart windows, photovoltaics, and other devices.

Dual-Functional WO3 Nanocolumns with Broadband Antireflective and High-Performance Flexible Electrochromic Properties.

The three-dimensional, high-porous, and oriented WO3 nanocolumn film with broadband antireflective and high-performance flexible electrochromic dual-functionalities is achieved by utilizing a simple, one-step, room-temperature glancing angle deposition without any catalysts and templates. It is found that the WO3 nanocolumn film is effective in increasing the optical transparency in the visible range, enhancing the color-switching response time as well as improving the mechanical flexibility and electrochemical cycling stability in comparison to dense WO3 film. The further optical, morphological, and electrode reaction kinetics analyses reveal that these improvements can be attributed to its unique porous nanocolumn arrays, which reduce the refractive index, facilitate the interfacial charge-transfer and ion-penetration, and alleviate the internal stress of the film under the bending treatment. These results would provide a simple and effective guidance to design and construct low-cost, robust, flexible, stable, and transparent electrochromic smart windows.

Poly(aryl ether) bearing electroactive tetraaniline pendants and allyl groups: Synthesis, photo-crosslinking and electrochemical properties

We describe the synthesis of a novel electroactive photo-crosslinkable poly(aryl ether), containing tetraaniline pendants and allyl groups (EPPAE). The polymer structure and spectral properties are studied in detail. By introducing the electroactive tetraaniline pandents, EPPAE reveals reversible electrochemical activity, expected electrochromic behavior and good anticorrosive performance. After crosslinking the allyl groups with UV exposure, the resultant polymer (c-EPPAE) shows drastic change in electrochemical properties. The c-EPPAE/ITO electrode exhibits declining electroactivity but excellent electrochemical stability, which are associated with densification of the electroactive layer. Furthermore, in the corrosive protection measurements, c-EPPAE reveals an outstanding protection efficiency (99.92%) for stainless steel substrates. A comprehensive study of electrochromic properties and corrosive protection of EPPAE before and after UV exposure will provide an insightful tool for the developing electrochromic smart windows, electrochromic displays, and anticorrosive paint. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2321–2330

Crystallized TiO2 (A)–VO2 (M/R) nanocomposite films with electrochromism–thermochromism dual-response properties

This work offers a method to combine the advantages of thermochromic (TC) and electrochromic (EC) smart windows by synthesizing TC–EC dual-response TiO2–VO2 nanocomposite films.

Gasochromic smart window: optical and thermal properties, energy simulation and feasibility analysis

In this paper, first of all, the configuration, optical and thermal properties of WO3-based gasochromic (GC) smart window are briefly reviewed. Subsequently, using the eQUEST building simulation program, the effect of GC smart window on the energy consumption for a commercial office building has been calculated and compared with various glazing systems currently available on the market, including SAGE@ electrochromic (EC) smart window. Simulation results indicate that the hot summer and cold winter regions are the most suitable testing grounds employing smart windows, and the decrease of HVAC loads in shanghai is 28.4% and 11.5% when using GC smart window to replace the single clear float glass and the colored absorbing double glass unit, respectively. Despite the limitation of coloration depth, GC smart window consumes less energy in Harbin than EC does. Also, according to the simulation data, there is no need to change the window’s state hourly unless the occurrence of extreme weather condition.

Performance requirements for electrochromic smart window

To accomplish specific energetic and environmental tasks in buildings large area electrochromic windows must exhibit acceptable levels in specific performance indicators. These parameters concern a number of electrical, thermal and optical properties which depend on the structural composition and configuration of the electrochromic device. In this paper a comprehensive and systematic analysis of the optimal performance requirements of electrochromic windows from the perspective of building energy efficiency and indoor comfort has been carried out. A comparison with the performance of a home-made fully solid-state electrochromic device tested in laboratory controlled conditions and of large-area electrochromic glazing currently available in the market is also made. The study points out the actual potential of the electrochromic technology for smart window applications and identifies some desirable performance improvements for optimizing building integration.

Investigation of Optical Response of Gasochromic Thin Film Structures through Modelling of Their Transmission Spectra under Presence of Organic Vapor

Properties of chromogenics materials have been of great interest for more than 50 years till now. Some examples of their practical application are photochromic lenses, electrochromic smart windows or even some examples of sensors devices based on gasochromic thin lms have already been commercially available on the market. However, recognition of di erent physical and chemical processes that in uence the optical response of such materials under changes in surrounding atmosphere is still an open subject for discussion. This work presents results of experimental and theoretical investigations of optical response of the two selected gasochromic (Ti V Ta W)Ox and (Ti V Ta Nb)Ox oxide thin lms under ethanol vapor stimulations. Based on the measured experimental transmission spectra, the complex refraction index characteristics were plotted using optical models elaborated for the VIS-NIR spectral range. The models were further used for the prediction of optical responses of optical gas sensing structures with observed gasochromic behavior.

Innovative and Energy Efficient Smart Window Based on Nanomaterial Technologies

Strategies for incorporating energy-efficiency requirements into building standards have been implemented by governments in developed countries in order to introduce the concept of green nanotechnology. Substituting regular glass windows in residential/commercial buildings with smart windows is the objective. This paper describes the development of innovative nanomaterial based membranes/coatings for smart windows that would work as electric dimming glass. Currently, curtains and blinds function to block sun light; eliminating such elements is of importance due to its limited functionality (e.g. blocking UV prevents sun light illumination), health (e.g. dust and germ collection especially in hospitals), disposal/recycling issues, cost, and maintenance. The proposed smart window is expected to block harmful UV light and provide a controlled privacy. Electrochromic smart windows are already in use and are able to vary their throughput of visible light and solar energy by application of electrical voltage and are able to provide energy efficiency and indoor comfort in buildings. These smart windows comprises of electrochromic materials such as LixWO2.89 and HxNiO2 as cathodic and anodic oxide films, respectively, and other complex polymers, which are complicated to create, expensive and some are hazardous in nature. Nanocellulose (achieved from wood/pulp product) is already being used in flexible electronics, so nanomaterial membrane for smart window is a probable alternative. This paper presents an innovative technology for smart-windows, utilizing nanocellulose fiber (abundantly available) doped with conductive nanoparticles (work as dimming glass on voltage application), mixed with minute amount of electrochromic material form thin film membranes.

Syntheses and Applications of Inorganic Electrochromic Materials and Devices

Tungsten oxide (WO3) nanorods were synthesized by a hydrothermal process. Transparent film was fabricated by an aggregation-deposition process of WO3 nanorods on transparent conductive glass. The as-synthesized WO3 nanorods exhibit good electrochromic properties. WO3 nanopaltes were directly grown on transparent conductive glasses by a crystal-assisted hydrothermal process, showing enhanced response and coloration efficiency. These properties of WO3 nanostructures endow their promising application in large-area smart windows. Prussian blue films were fabricated and used in color-changeable betteries. A bi-functional Prussian blue/Aluminium device has been demonstrated for self-powered electrochromic window and self-rechargeable transparent battery. Without applying any external bias, Prussian blue can be bleached to Prussian white by connecting the Prussian blue and Al electrodes. After bleaching, the blue colour of the device can be spontaneously recovered. In this regard, the designed device can be used as an energy-saving electrochromic smart window which requires no external power source. The as-prepared device can also be used as a battery with a high output voltage. Most importantly, after discharging, such a battery can automatically self-recharge. The as-prepared device exhibits promising applications in broad fields, for example, as a self-powered smart window to perform indoor light and heat management, and as a self-rechargeable transparent battery to power some delicate devices.

Room Temperature Processed Electrochromic Smart Windows with Flexible Film

Electrochromic (EC) devices, which dynamically change color under applied potential, are widely studied for use in energy-efficient smart windows. To improve the viability of smart windows, many researchers are utilizing nanomaterial, which can provide electrochromic devices with improved coloration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. We report the synthesis of nanomaterials for transparent conducting electrodes (TCE). The smart window characteristics were measured and studied. For fabrication of smart window, WO3 ink was spin-coated on ITO/glass and electrolyte of ethylene carbonate, dimethyl carbonate, and lithium perchlorate was used. The counter TCE was fabricated by bar coating method with silver nanowires (Ag NW) on PET film, which has 25 ohm/sq of sheet resistance and 90% of transmittance. EC device with Ag NW TCE was assembled and sealed with epoxy. The performances of smart window were characterized with the performance of Ag NW TCE. The potential range for coloring was 1.5 – 2.5 V and the transmittance was 87 %. The bleaching was observed at -2.5 V and the transmittance was below 40 %. The flexible TCE can be applied flexible and bendable smart window with low process cost.

Electrochromic Devices Incorporating Conducting Polymer or Novel Viologens

Organic electrochromes such as conducting polymers and viologens (di-substituted-di-pyridinium salts) are attractive for electrochromic smart window applications due to high optical contrast, possibility of switching between multiple colors, ease of preparation and good write-erase efficiency. While conducting polymers are electrode bound species and undergo a color change via redox reactions at the electrode, viologens are typically colorless solution phase compounds, which form a reversible insoluble deeply colored film on the conducting electrode, upon application of an external bias (Figure 1). In this talk, the development of new viologen derivatives like the poly(cyclotriphosphazene-4,4′-bipyridinium) chloride, the effect of substituents on the electro-optical response, issues related to electrolytes and their compatibility with the electrochromic materials will be discussed. In the realm of conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT), is a dual function material: it can serve as a transparent conducting electrode and a cathodically coloring electrochrome. In our lab, large area PEDOT films have been fabricated over plastic substrates and their ability to concurrently work as flexible conducting substrates while switching between the pale blue and dark blue hues is demonstrated. Concerns pertaining to scaling up of conducting polymer coatings and viologen based devices will also be discussed. Figure 1

Spray deposited CeO2–TiO2 counter electrode for electrochromic devices

Optically passive thin films of CeO2–TiO2 mixed oxides with molar ratio of Ce/Ti of 0.05 were deposited by the spray pyrolysis technique (SPT) on a glass and fluorine-doped tin oxide (FTO)-coated glass substrates. Precursor solution containing cerium nitrate hexahydrate (Ce(NO3)2⋅6H2O) and titanium tetraiso-propoxide (Ti(OiPr)4) having different volumetric proportions (0–5 vol% of Ti) in methanol were used. These films were characterized for structural, morphological, molecular, optical, electrochromic and colourimetric analysis. CeO2–TiO2 films deposited at 400 ∘ C were found to be polycrystalline with cubic fluorite crystal structure. Transformation from polycrystalline to amorphous phase was observed with increasing TiO2 content. The band centred at 539 cm−1 is assigned to Ce–O stretching vibration and the two medium intensity bands assigned to (Ti–O) and (Ti–O–Ti) stretching modes at 798 and 451 cm−1, which confirms the mixed CeO2 and TiO2 phases. The band gap energy decreases (E g) from 3.45 eV for pristine CeO2 to 2.98–3.09 eV for CeO2–TiO2 films. The ion storage capacity (ISC) of CeO2–TiO2 thin film with 3 vol% Ti (Ce–Ti3 sample) was found to be 26 mC cm−2 and electrochemical stability up to 30,000 cycles in 0.5 M LiClO4-PC electrolyte. The optically passive behaviour of CeO2–TiO2 thin film is confirmed by its negligible transmission modulation (Δ T ∼ 2.5%) upon Li+ ion insertion/extraction, irrespective of the extent of Li+ ion intercalation. The optical modulation of sputter deposited electrochromic WO 3 thin film was found to be enhanced from 56 to 61% with rapid increase in colouration efficiency (CE) from 42 to 231 cm2 C−1 when CeO2–TiO2 is coupled as a counter electrode with WO3 in an electrochromic device (ECD). On reduction of WO3 thin film with CeO2–TiO2 as counter electrode, the CIELAB 1931 2 ∘ colour space coordinates show the transition from colourless to the deep blue state (L ∗ = 88.07, a ∗ = −2.37, b ∗ = 24.59 and L ∗ = 40.32, a ∗ = −1.16, b ∗ = −5.65) with steady decrease in relative lightness. Yxy and L ∗ a ∗ b ∗ coordinates signify CeO2–TiO2 films and it also exhibits the application as counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion/extraction.

A bi-functional device for self-powered electrochromic window and self-rechargeable transparent battery applications

Electrochromic smart windows are regarded as a good choice for green buildings. However, conventional devices need external biases to operate, which causes additional energy consumption. Here we report a self-powered electrochromic window, which can be used as a self-rechargeable battery. We use aluminium to reduce Prussian blue (PB, blue in colour) to Prussian white (PW, colourless) in potassium chloride electrolyte, realizing a device capable of self-bleaching. Interestingly, the device can be self-recovered (gaining blue appearance again) by simply disconnecting the aluminium and PB electrodes, which is due to the spontaneous oxidation of PW to PB by the dissolved oxygen in aqueous solution. The self-operated bleaching and colouration suggest another important function of the device: a self-rechargeable transparent battery. Thus the PB/aluminium device we report here is bifunctional, that is, it is a self-powered electrochromic window as well as a self-rechargeable transparent battery.