Operando Raman and UV-Vis spectroscopic investigation of the coloring and bleaching mechanism of self-powered photochromic devices for smart windows

Sarwar S., Park S., Dao T.T., Lee M., Ullah A., Hong S., Han C.

A scalable photoelectrochromic (PEC) glass which can be darkened on exposure to sunlight and return to its original clear state in the absence of sunlight without any expensive materials like ruthenium dye or transparent conductive oxide was fabricated. The PEC glass is composed of light absorbing layer of ligand attached titanium oxide, color changing layer of tungsten oxide and the electrolyte containing redox mediator. This work reports, for the first time, on an application of salicylic acid attached TiO2 photoactive layer for PEC devices. The applicability of various salicylic acid derivatives attached TiO2 films in PEC device is individually investigated in dye solar cells. Among various salicylic acid derivatives studied, 5-methylsalicylic acid showed the best performance in terms of charge generation for coloration of WO3. By using 5-methylsalicylic acid instead of ruthenium dye, lower cost, higher stability, and simpler electrolyte composition have been realized. The changes in transmittance for coloration (ΔTC) depend on the thickness of WO3 and TiO2 in this device. Thus, we changed the thickness of WO3 and TiO2 to get the highest variation of transmittance for coloration. The preliminary optimization of PEC device exhibits ΔTC of 40.8% at 550 nm, the wavelength most sensitive to the human eye, and of 64.3% at 700 nm.

Huang Y., Lu R., Wang M., Sakamoto J., Poudeu P.F.

Nanocomposites consisting of hexagonal WO3 nanorods embedded into nitrogen, sulfur co-doped reduced graphene oxide nanosheets (h-WO3/NSG) have been synthesized through a combination of ultrasonic processing and hydrothermal reactions. The nanocomposite containing 54 ​wt% of (N, S)-doped reduced graphene oxide shows an excellent discharge capacity of 1030 mAh g-1 at the first cycle and high rate capacities ranging from 513 to 150 mAh g-1 at the current density ranging from 100 to 2000 ​mA ​g-1, respectively. In addition, the synthesized WO3/NSG nanocomposite also exhibits remarkable cycle stability with a high discharge capacity of 196 mAh g-1 at the current density of 1500 ​mA ​g-1 for 200 cycles. The excellent electrochemical performance of h-WO3/NSG nanocomposite can be attributed to synergistic contributions from (1) the small size of WO3 nanorods, which increases the contact surface area with the electrolyte, effectively shortening the diffusion length of Li+ ions leading to high Li+ incorporation within h-WO3; (2) the structural support provided by the NSG matrix, which alleviates the large volume change of WO3 nanorods during Li+ insertion/extraction; and (3) the large numbers of surface defects provided by nitrogen and sulfur co-dopants, which serve as channels to improve the Li+ diffusion. Furthermore, electrochemical impedance spectroscopy (EIS) measurement demonstrates that nitrogen, sulfur co-doped reduced graphene oxide can reduce the electrochemical impedance of the electrode, further improving the electrochemical performance of the anode. This study provides important strategies for the design of future high performance anode materials for lithium ion battery.

Tang K., Zhang Y., Shi Y., Cui J., Shu X., Wang Y., Qin Y., Liu J., Tan H.H., Wu Y.

Unique WO3/TiO2 core-shell nanowire heterostructures are rationally constructed based on a solvothermal method combining magnetron sputtering. High resolution transmission electron microscopy (HRTEM) analysis evidences the existence of a semi-coherent interface between a hexagonal phase WO3 core and anatase TiO2 shell. X-ray photoelectron spectroscopy and optical absorption spectra suggest an interfacial interaction between the WO3 core and TiO2 shell. The WO3/TiO2 core-shell nanowires exhibit superior electrochromic performance in both visible and near infrared regions with large optical modulation (87.0% at 1500 nm and 85.3% at 633 nm), short bleaching–coloration times (1.1 s and 3.0 s), decent coloring efficiency (102.1 cm2 C−1) and excellent cycling stability (95.6% after 3000 cycles). The obtained WO3/TiO2 core-shell nanowire heterostructures may find great potential in producing electrochromic nanodevices and other energy-efficient applications.

Fulton G., Lunev A.

Tungsten, a plasma-facing material for future fusion reactors, may be exposed to air during abnormal operation or accidents. Only limited information is available on the evolution of related oxide phases. This work addresses the effect of substrate orientation on structural variations of tungsten oxides. Annealing experiments in an argon-oxygen atmosphere have been conducted at T = 400 {\deg}C under varying oxygen partial pressure and oxidation time. A combination of EBSD, Raman spectroscopy and confocal microscopy shows preferential oxidation initially on {111} base material planes. The oxide scale changes its phase composition dynamically, influencing the kinetics of its growth.

Estrella L.L., Kim D.H.

New porphyrin-based dyes were designed by modifying the acceptor and donor groups of the champion SM315 dye. The modified acceptor units of the new dyes rendered stronger electron withdrawing ability which resulted in better charge transfer and light harvesting properties with reference to SM315. The acceptor design of K-04 dye, composed of benzothiadiazole and 4-(cyanomethyl)benzoic acid, is the most successful in improving the charge transfer property, evidenced by the highest amount of transferred charge (qCT), charge transfer distance (DCT), and variation of dipole moment (μCT), and the most bathochromically shifted broad Q-band with enhanced molar extinction coefficient. For quantitative estimation of the light harvesting ability of the dyes, the LHE curve and Jscmax were calculated revealing that K-04 dye is the best light harvester among the dyes with varied acceptor unit. Moreover, the highest dipole moment of the dye-TiO2 complex ( μ d y e - T i O 2 ) and largest spatial separation distance (r) between the TiO2 surface and hole centroid of K-04 dye estimates its highest Voc within the group. With respect to K-04, changing the donor group to 9,9-dimethyl-10-phenyl-9,10-dihydroacridine, a semi-rigid triphenylamine (tPA) donor named as D1, resulted in a significant bathochromic shift of the Q-band with a remarkably high molar extinction coefficient. Among the derivatives of D1, the D4 donor group functionalized with diphenylamine enforced the push-pull character resulting in the most favorable CT properties and LHE of K-44 dye. These properties, along with the highest Jscmax, and largest μ d y e - T i O 2 and r, made K-44 dye the most promising candidate in this series.

Estrella L.L., Lee S.H., Kim D.H.

Four novel dyes featuring new semi-rigid triphenylamine donor groups have been synthesized and successfully employed as sensitizers in dye-sensitized solar cells (DSSCs). Opting for the new semi-rigid triphenylamine (tPA) electron-donating unit yielded a better Voc and Jsc resulting to a power conversion efficiency (PCE) which is 16% higher compared to a reference dye whose electron-donating group is a conventional tPA unit. Density functional theory (DFT) and time-dependent DFT calculations revealed the more favorable charge-transport properties of the dye based on the new semi-rigid donor unit. Among the synthesized dyes, the DSSC device based on Dhkx-4 dye in conjunction with iodine-based electrolyte achieved a PCE of 6.23%, with Voc of 0.661 V, Jsc of 13.31 mAcm−2, and FF of 0.71 under simulated AM 1.5 G condition. Therefore, the novel semi-rigid tPA donor unit presented in this work is a promising donor group for a sensitizer in DSSCs and systematic structural engineering of the presented designs could offer valuable contributions for the development of the photovoltaic devices.

Prabhu S., Cindrella L., Kwon O.J., Mohanraju K.

The synthesis of rGO-TiO2 WO3 composites by simple solvothermal method followed by calcination process is reported. The prepared composite samples were characterized by various techniques such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy and UV–vis diffuse reflectance spectroscopy. The photoelectrochemical (PEC), photocatalytic and photochromic properties of the prepared composites were investigated. The higher photon-to-hydrogen conversion efficiency of 6.5% was achieved by the rGO-TiO2 WO3 composite. The prepared composites show efficient removal of MB in aqueous solution by adsorption and visible light photocatalytic processes. Under solar light irradiation, the photochromic property of the transparent rGO-TiO2 WO3 composite coatings was high. The enhanced efficiency of the rGO-TiO2 WO3 composites is attributed to the presence of reduced graphene oxide (rGO). The mechanism of the PEC, photocatalytic and photochromic properties is also illustrated. This study shows that rGO-TiO2 WO3 composites can be used for indoor and outdoor photovoltaic as well as smart window applications.

Hočevar M., Opara Krašovec U.

The transmittance of a photochromic (PC) window is reduced in sunlight but returns to its original value in the dark similar to photo-sensitive sun glasses. A 30 cm × 30 cm double glazed window containing a PC multi-layer coating on a single glass pane has been developed. A single PC glass pane can be used to build a window with a low heat transfer coefficient, e.g. double or triple glazed unit, using existing technology. To develop the PC glass with a multi-layered coating that combines chromogenic and photoactive materials we employed sol-gel chemistry approaches and up-scaled the synthesis. Double glazed windows have been assembled using PC glass with two different layer configurations. The transmittance (Tvis) was reduced from 76% to 35% under outdoor solar radiation (750 W/m2) within 15 min.

Estrella L.L., Balanay M.P., Kim D.H.

This paper reports new D-D-π-A dyes based on N-annulated perylene, emphasizing the enhanced dye-to-semiconductor charge-transfer mechanism. A series of DFT calculations for new tPA-perylene-based dyes was conducted, starting from the systematic selection of DFT methods by reproducing the experimentally obtained properties of known perylene-based sensitizers. Accordingly, using the LC-ωPBE xc functional with 6-31+G(d) basis set for the time-dependent calculations of the excitation energies, a damping parameter of ω = 0.150 Bohr-1 was found to be most appropriate for dyes having spatial orbital overlap value of 0.21 ≤ ΛHL ≤ 0.38, while ω = 0.175 Bohr-1 is suitable for analogues with 0.43 ≤ ΛHL ≤ 0.57. Moreover, the mPWHandHPW91/6-31G(d) method gave high accuracy in GSOP calculations. The comparison between the properties of tPA-based donor groups has revealed that the semirigid tPA-based D4 unit is an effective donor group for perylene-based dye. Initial screening of the acceptor designs resulted in PLz4 dye with promising charge-transfer mechanism and highly favorable dye-TiO2 interaction based on the calculated dipole moment of the dye and dye-TiO2 complex. The attachment of the substituted-hydroacridine donor unit (D4) to PLz4 afforded a bathochromically shifted absorbance and improved molar absorptivity signifying its effective electron-donating ability. Among the D-D-π-A dyes, DP46 is expected to render a relatively high Voc and Jsc supported by the calculated optical properties, oxidation potentials, ionization potential, and electron affinity values.

A. Yazid S., Rosli Z.M., Juoi J.M., Johari N.D.

TiO2 coating films were prepared by sol-gel dip coating method using TTiP precursors with and without ethanol as a medium to hydrolyze the HCl catalyst. The prepared samples of titanium dioxide (TiO2) coating were dried at 110°C for 30 minutes then heated at 500°C and 600°C for 1h and 3h respectively to study effect of annealing temperature and soaking time. The crystallinity of the TiO2 coating were identified by using X-ray Diffraction technique and crystallite size was calculated. Raman Spectroscopy also used as the confirmation of phases formation of TiO2 coating films. Crystallite size of TiO2 films were obtained at 12.35 nm, 17.29 nm for anatase, 21.71 nm, 28.95 nm for rutile and 2.19 nm for brookite. Thus, the establishment of the desired phases, crystallinity and grain size of TiO2 thin films sol-gel can be controlled and encouraging to explore as an effort toward producing a sustainable photocatalytic.

Hočevar M., Bogati S., Georg A., Opara Krašovec U.

Different approaches have been studied to evaluate the efficiency of the photoactive layer in a photochromic (PC) device. The studied device combines materials typically used in dye-sensitized solar cell (DSSC) and an electrochromic (EC) layer of WO3. First, we examine if the dye could be attached directly to the EC layer forming an efficient photoactive layer to colour the PC device. Further, different TiO2 layers have been coated on the EC layer to enhance dye loading and two different sol-gel TiO2 layers and the post-treatment of the EC layer with a titanium(IV) chloride tetrahydrofuran complex have been studied. The dye loading of different photoactive layers and their efficiencies in DSSC have been evaluated and discussed with regard to the performance of the PC devices. The results confirmed that the dye could be attached to the WO3 layer, but that the voltage of the DSSC is too low to colour the device. To realize a functioning PC device, the addition of TiO2 is necessary. The best performance (deep and fast colouration) is obtained for a PC device with a TiO2 sol-gel layer; the solar transmittance of the PC device decreases from 57% to 7.5% in under three minutes. According to our knowledge, this is the first study of the impact of the photoactive layer on the photochromic glazing's response.

Bogati S., Basnet R., Graf W., Georg A.

A photochromic (PC) device comprising an electrochromic film (WO 3 ) together with a photoactive film (usually, dye sensitized TiO 2 ) coated on glass and a redox electrolyte (e.g., iodide/triiodide together with lithium salt) reduces the optical transmission under solar illumination and bleaches, again, in dark or at low intensity of light. This work reports, for first time, on an application of sputter coated TiO 2 and WO 3 films for this type of PC devices. The applicability of TiO 2 and WO 3 films in PC device is individually investigated in dye solar cells and electrochromic cells, respectively. A dye solar cell with 180 nm TiO 2 was shown to be sufficient for the charge generation for electrochromic coloration of the WO 3 . The compatibility of WO 3 and redox electrolyte was studied in electrochromic devices by considering their redox potentials. The bleaching of the PC device has to be adjusted by the so called “loss current”. To address this, a thin catalytic layer of platinum (Pt) is deposited in between the photoactive and electrochromic layers. The optimized PC device exhibits a visual (solar) transmission modulation from 74 (63) down to 30% (20%) in 40 min under 1.5 A.M. illumination, and the bleaching is regained in 45 min in dark.

Cao X., Dai X., Liu J.

Energy consumption has dramatically increased in buildings over the past decade due to population growth, more time spent indoors, increased demand for building functions and indoor environmental quality, and global climate change. Building energy use currently accounts for over 40% of total primary energy consumption in the U.S. and E.U. Nevertheless, significant energy savings can be achieved in buildings if they are properly designed, constructed and operated. For this reason, building energy efficiency can provide key solutions to energy shortages, carbon emissions and their serious threat to our living environment. This paper offers a brief overview of building energy-consumption situations, relevant energy-saving approaches, and the influence of global climate change. Building energy-consumption situations based on data derived from international energy reports are initially compared between the U.S., China and the E.U. Both similarities and differences are found in aspects of building energy end-uses and final energy fuel-types among these top three building energy consumers. We then introduce the current concept of the zero-energy building (ZEB). State-of-the-art approaches for ZEB technologies are summarized in three categories: passive energy-saving technologies, energy-efficient building service systems and renewable energy production technologies. The feasibility of these technologies is reviewed. In addition, we briefly discuss the influence of global climate change on the evolution of building energy use in the future. We find that climate change significantly impacts building energy performance, particularly in space heating and cooling. Improvements on building envelope and ventilation can play an important role in reducing space heating and cooling consumption levels. We also provide some suggestions for further developing ZEBs.

Hočevar M., Opara Krašovec U.

We report on the development of WO3 layers with increased symmetry induced by the incorporation of titanium into a crystalline WO3 matrix. The atomic ratio between W and Ti was varied stepwise from 100:1 to 100:20 with pure WO3 layers, made using peroxo sol–gel synthesis, as a reference. Layers were then formed by dip-coating from the corresponding sols. IR spectroscopy and XRD analysis of layers and powders heat-treated at 450 °C proved that the presence of titanium ions induces ordering and increases the symmetry of WO6 octahedra in the WO3 crystalline matrix leading to the transformation of the cell symmetry from monoclinic through tetragonal up to the cubic. Cyclic voltammetry confirmed that this increased symmetry leads to an up-shift in redox potential, which favors the coloration of the layers in photochromic devices. According to our knowledge, this is the first report of colorless cubic WO3 layers, which have potential applications in optoelectronic devices and sensors.

Gu X., Wu F., Lei B., Wang J., Chen Z., Xie K., Song Y., Sun D., Sun L., Zhou H., Fang F.

Bamboo-like WO3 nanorods were anchored on three-dimensional nitrogen-doped graphene frameworks (r-WO3/3DNGF) by a facile one-step hydrothermal synthesis plus heating processes. There is a strong dependence of the obtained r-WO3/3DNGF nanostructures on the content of 3DNGF. The composite with 20 wt% 3DNGF content shows the most favorable structure where bamboo-like WO3 nanorods lie flat on the surface of fungus-like 3DNGF, and exhibits a high discharge capacity of 828 mAh g−1 over 100 cycles at 80 mA g−1 with the largest capacity retention of 73.9% for WO3 and excellent rate capacities of 719, 665, 573, 453 and 313 mAh g−1 at 80, 160, 400, 800 and 1600 mA g−1, respectively. The electrochemical performance is better than most of reported WO3-based carbonaceous composites, which can be attributed to the synergistic effects of the following actions: i) WO3 nanorods effectively shorten the diffusion path of Li+; ii) mechanically strong 3DNGF alleviates the huge volume change of WO3 upon Li+ intercalation/extraction; and iii) nitrogen-doping in 3D graphene frameworks improves electronic conductivity and provides large numbers of lithium ion diffusion channels.

Shen K., Zhang R., Jin Y., Li Y., Hu Y.

Semiconductor oxides are widely used in the field of optical materials due to their broad spectral response, tunable structure, low cost, high chemical and thermal stability, among other advantages. The design and fabrication of photochromic materials in semiconductor oxides have shown tremendous potential in various fields such as optoelectronic displays, optical storage, and light-stimulated responses. In this work, we have designed and fabricated a novel inorganic photochromic material by doping Sm3+ ions into the semiconductor oxide CaBiNb2O9 (CBN). The introduction of Sm3+ ions induces the formation of oxygen vacancies and cationic vacancies in CBN, greatly enhancing the photochromic effect of the matrix. CaBiNb2O9: Sm3+ (CBN: Sm) can achieve reversible photochromic response through 254 nm light irradiation and 350 ℃ thermal stimulation, and exhibit excellent stability and fatigue resistance. The photochromic process and performance of the CBN: Sm materials were characterized, and the potential mechanisms were analyzed and discussed. The conducted research and insights will provide support for other researchers in the design and fabrication of photochromic materials based on semiconductor oxides.

Huang J., Yuan L., Liao J., Liu Y., Li D., Wang Y., Lin H., Ji C., Ma X., Huang C., Luo X.

AbstractWindows offer the most promising avenue for mitigating energy consumption and reducing greenhouse gas emissions. However, the balance between comfortable natural lighting and all‐season energy savings is often neglected in extensive explorations of energy‐efficient windows. Herein, a Janus glazing is proposed that enables the switch of passive radiative cooling and heating under the precondition of conveying sufficient natural light. Measurement results indicate that the Janus window maintains a visible transmittance of 0.47, while possesses a near‐infrared (NIR) reflectivity/absorptivity of 0.75/0.71 and a mid‐infrared (MIR) emissivity of 0.94/0.13 for the cooling and heating modes, respectively. As demonstrated by the outdoor test, the Janus window realizes a reduction of 7.1 °C for room cooling and an increase of 0.4 °C for room heating compared with commercial low‐e window, potentially conserving 13%–53% of the total building energy consumption across China. Meanwhile, attributed to the photothermal effect, the Janus window can elevate the surface temperature by 6.1 °C compared with the low‐e window, which can effectively reduce fogging occurrences on the window surface for ensuring sunlight entrance in the cold‐weather conditions. This strategy offers novel prospects for enhancing energy efficiency in diverse applications, including architectural windows, greenhouse cultivation, photovoltaic generation, etc.

Teixeira H., Gomes M.G., Moret Rodrigues A., Aelenei D.

The application of films on conventional glazing aims at increasing the glazing performance by reducing energy needs and increasing indoor comfort. Photochromic films in particular can alter their optical properties due to a chromatic change in response to solar radiation, allowing for reduced glare levels and solar heat gains. However, despite the potential of application, this refurbishment solution is still poorly explored. Therefore, the main purpose of this work was to experimentally evaluate the thermal and luminous performance of a double glazing with and without a photochromic film installed, using two office rooms in Lisbon as case study. An extended field experimental campaign was conducted simultaneously in both offices, where temperature, solar radiation and illuminance levels were collected. The key contribution and novelty of this research lies on the experimental assessment of the thermal and visual comfort conditions with the photochromic film under real-occupancy. Even though the photochromic film significantly increased the surface temperatures of the glazing, the indoor air temperature was not negatively affected, with an increase up to 14% of working hours with comfortable temperature during the heating period. Illuminance levels on vertical/horizontal plane were reduced by 24/36% in the presence of the photochromic film, when compared to the clear glazing without film, resulting in an increase of 7% of working hours with useful illuminance during the heating period, and large areas in the office room with imperceptible daylight glare levels when facing the glazing system. The impact of the film was less noticeable during the cooling period.

Dong X., Dang Y., Wu Z., Tong Y., Liu X., Lu Y.

Because of their good chemical stability and excellent optical properties, MoO3, WO3, and Bi2WO6 are important in photochromism. Their light-to-color conversion is highly dependent on the electronic band structure and charge transfer, and they obey the mechanism of electron accumulation in semiconductors when excited within the bandgap. Pure semiconductors face limitations in practical applications due to insufficient light absorption, charge carrier recombination, and low charge capacity. Diverse forms of photochromic hybrids (nanopowders, films, hydrogels, and multilayer structures) with rapid change, repeatability, and reversibility are possible via nanocustomization, surface/interface engineering, heterojunction fabrication, and complexing organic ligands. Manipulating the function of photochromic systems through light stimulation is becoming an attractive paradigm, divided into two branches: light-color complementarity and photoconductivity. This review examines the widely accepted photoresponsive principles and the still controversial energy transfer models. We emphasize the correlation between material properties and performance enhancement to inspire the rational structure design. The bottlenecks in current development are identified by analyzing application-specific innovation concepts, fabrication processes, and performance metrics. In addition, we present several perspectives to encourage meaningful multidisciplinary collaboration.

Hu X., Cheng Y., Wei Z., Zhan Y., Zhang R., Xia H., Jiang X.

Thermochromic smart windows with adjustable electromagnetic interference (EMI) shielding capability are integral elements in modernized buildings, especially in military applications, due to their energy-saving benefits and ability to mitigate electromagnetic pollution. However, its application is limited by the relatively poor light transmission of conductive substances and the high demand for light transmission of thermochromic smart windows. Maintaining the thermochromic performance of smart windows while adjusting EMI shielding effectiveness (SE) poses a significant challenge. Here, we propose a straightforward approach to create a smart EMI shielding material with adaptable properties, including light transmission, shape, and EMI SE. This method involves a layer-by-layer polymerisation process followed by solvent substitution. The resulting hydrogel exhibits tunable optical properties, making it suitable for use as a smart window that responds to external conditions and offers local tunability. Noteworthy characteristics include high light transmittance (80.87 %), ultra-fast response time (16 s), multiple shape adjustments, adjustable EMI SE ranging from 0 to 27.64 dB, and strong adhesion, among other features. These findings suggest that the obtained hydrogels have significant potential to provide a substantial advantage in developing next-generation soft and intelligent smart EMI shielding windows.

Akoth Okwako J., Han Song S., Park S., Van Tran H., Aduda B.O., Waita S., Hong Y., Hong S., Han C.

In this work, we conduct a detailed study to understand the photoelectrochromic properties of Al-Pt co-doped tungsten oxide (WO3). Similarly, we support the photoelectrochromic findings with material characterization, providing additional understanding of the photoelectrochromic performance exhibited by the prepared samples. Additionally, we discuss the effects of Al and Pt doping on photoelectrochromic performance in detail, with a focus on X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) combined with EDS, X-ray photoelectron spectroscopy (XPS), and optical transmittance. XRD peaks confirm the presence of hexagonally-structured WO3 in the prepared samples with a polycrystalline nature, and a similar observation is confirmed via TEM analysis. Surface characterizations conducted through SEM and EDS analyses unveil the morphologies and particle sizes. Optical studies show a bandgap of 1.88 eV and 1.95 eV for Al doped and Al-Pt co-doped WO3, indicating bandgap shrinkage in comparison to the optical bandgap of WO3 of 2.36 eV. The Al-Pt co-doping results in a significant enhancement in photoelectrochromic performance, attaining an optical modulation of 43.61 % at 550 nm and consequently 85 % of the initial transmittance recovered after 2 h of bleaching in the dark. This is an improvement in comparison to the Al doped sample which attains a coloration depth of 43.15 % at 550 nm with only 75 % of the original transmittance being recovered after 2 h under dark conditions.

Sun A., Nan F., Wei Q., Wang L., Yu W.W.

Organic-inorganic hybrid photochromic materials have been considered one of the most promising photochromic candidates for light detection and information anti-counterfeiting. However, challenges still exist, such as single-color variation, long decoloration time, and lack of in-depth understanding of the photochromic mechanism. Here, a three-component photochromic composite system was designed, composed of two polymer matrices (electron donors) and phosphotungstic acid. By adjusting the ratio of one electron donor, the composite can initiate color change from transparent to deep blue, near-colorless, light mauve, mauve, and grey mauve, respectively, upon UV light irradiation, which has never been reported. These composites also feature flexibility, mechanical robustness, high transparency, faster photochromic reversibility, as well as dispensing printing. Importantly, the underlying multi-color photochromic mechanism is elucidated by leveraging the theoretical simulation and characterization techniques. A UV detection bracelet for real-time UV monitoring and a proton conductor for a wearable biomechanical energy harvester were fabricated using the photochromic composites, indicating application potentials for smart wearable electronics. This work introduces a new type of organic-inorganic photochromic composite and explores the photochromic mechanism, which may guide the design of new photochromic systems for future wearable technologies.

Yong W., Chen N., Xiong T., Fu G.

In recent years, there has been a growing focus on reducing energy consumption, particularly in buildings. One effective method proposed by scientists is the use of intelligent photoelectric color-changing glass. While previous studies in this field have mainly concentrated on the rapid chromic performance, this study aims to enhance the shading rate by developing a photo-electrochromic bright window using the Mo doped WO3 sol-gel method. The study investigates the impact of different levels of MoO3 doping on the photo-electrochromic performance. The results demonstrate that increasing the MoO3 content leads to a wider coloring range and faster color change response, with transmittance values dropping below 25 % compared to the original transmittance of approximately 80 %. Particularly, there is a significant 55 % (ΔTC) change in transmittance for coloration at the wavelength of 550 nm, which is the most responsive to the human eye, compared to previous research. Furthermore, the smart windows exhibit stable switching for transmittance modulation, maintaining consistency over 20 repeated coloration and bleaching cycles. Overall, the photo-electrochromic film prepared using the doped sol-gel method exhibits excellent performance in terms of high-quality photo-electrochromic layers, short response time, high coloration efficiency, and long service life. This research shows the potential of smart windows in various applications, including energy-saving, smart displays, supercapacitor devices, and anti-counterfeiting.

Wei D., Zhang Y., Xue W., Dou Y., Liu F., Yan M., Wang W.

Zhang H., Zhang L., Zheng Y., Liu Z.

Improving the thermal performance of windows is important for building energy efficiency. Filling windows with variable transparency shape-stabilized phase change materials (VTSS-PCM) improves the thermal inertia of windows while avoiding the leakage of PCM. In this paper, a new type of VTSS-PCM window was proposed, tested, simulated and optimized in hot summer and cold winter climate zone. A numerical model of the VTSS-PCM window was built, and the model was validated using experimental tests. On this basis, three key parameters of VTSS-PCM were investigated and optimized. Finally, the energy and economic performance of the optimized VTSS-PCM window were compared with a typical hollow glass window. The results showed that the total annual unfavourable heat transfer (TAHT) and the annual investment cost (AIC) of the VTSS-PCM window obtained from the optimization were 118.16 kWh/m2 and 8.53 CNY/m2, respectively. Compared with the hollow glass window, the VTSS-PCM window reduced the TAHT by 30.14% and the total annual cost by 28.39%. The VTSS-PCM window produced a better development potential in terms of energy performance and economic performance. This study provided a reference for the application of the VTSS-PCM window in hot summer and cold winter regions of China.

Meng W., Kragt A.J., Hu X., van der Burgt J.S., Schenning A.P., Yue Y., Zhou G., Li L., Wei P., Zhao W., Li Y., Wang J., Jiang L.

AbstractPhotochromic smart windows have drawn increasing attention as an approach to improve building energy efficiency and enhance indoor daylight comfort. However, existing photochromic smart windows still block sunlight from entering the room on sunny winter days, causing additional energy consumption for heating. Herein, a dual‐mode smart window is designed with decoupled photo and thermal functions by combining colorless Fe‐doped WO3 photochromic film with window rotation. Based on this, selective heating and cooling of the room between winter and summer is achieved while maintaining the daylight comfort benefits during all seasons. As a proof of concept, the smart window reduces the temperature of a model house by up to 7.9 °C in summer mode, while in winter mode the temperature is only reduced by 0.7 °C. The proposed seasonally adaptive dual‐mode smart window obtains by window rotation overcomes the limitations of conventional photochromic smart windows, which not only achieves better energy efficiency but also retains improved daylight comfort. Furthermore, it demonstrates that the heat absorbed by the smart window can be harnessed to produce electricity through the integration of thermoelectric modules within the glazing, which enhances its impact on reducing energy consumption.

Zhang C., Yang R., Lu Y., Arıcı M., Ma Y., Yang X., Qi Z., Li D.

In conventional glazing windows filled with phase change material (PCM), the type of PCM is usually chosen as a solid-liquid PCM, resulting the potential leakage problems in its implementation which hinder its engineering applications. Replacement of solid-liquid PCM with solid-solid PCM (SSPCM) in PCM glazing window is proven as an effective way. For the sake of providing guidance for the further development and preparation of the SSPCM adopted in glazing windows, a parametric study is conducted that focuses on the thermal performance of the SSPCM glazing window for various PCM thermal and optical properties, over 24 h in the rigorous cold weather of Anda City. A heat transfer model of the multilayer glazing window with SSPCM and silica aerogel integration is developed, and the obtained results show that both the melting temperature and latent heat of PCM have significant impacts on the thermal performance of the glazing window, while the absorption coefficient and refractive index have slight effects on. Additionally, the optimal melting temperature, latent heat, absorption index, and refractive index of SSPCM varying in the selected property ranges are found as 18 °C, 120 kJ/kg, 60 m−1, and 1.3, respectively.

Chen T., Xu B., Han J., Zhu M., Zhang J., Li Z.

Gadgil C., Ghosh A., Bhattacharjee A., Praveen P.L.

Energy consumption within buildings, predominantly driven by non-renewable sources, remains a substantial contributor to greenhouse gas emissions. This is primarily attributed to the demand for occupant comfort, encompassing air-conditioning, lighting, and electrical usage. In response to this pressing challenge, switchable smart windows have emerged as a highly promising solution applicable to both residential and commercial structures. By effectively modulating light and heat, these windows offer a multifaceted approach to energy conservation, encompassing reduced heat loss, diminished reliance on artificial lighting, and consequential cost savings. This research paper critically evaluates the latest advancements in electrically actuated smart windows, with a specific focus on AC-powered variants such as Suspended Particles, Liquid Crystal, and DC-powered Electrochromic windows. The study meticulously delves into the operational principles, technical parameters, advantages, limitations, prospects, applications, energy-saving potential, and market penetration of these intelligent window technologies. Notably, the investigation extends to key thermal metrics like overall heat transfer coefficient and solar heat gain coefficient, alongside optical attributes including correlated colour temperature (CCT) and colour rendering index (CRI). Furthermore, the report delves into the intricate challenges associated with integrating smart windows into building infrastructure, presenting viable solutions and perspectives to address these concerns. These challenges encompass the absence of standardized regulations within the UK, elevated costs, technical intricacies, limited research and development, and uncharted compatibility with both new constructions and retrofit designs. Through a comprehensive analysis, this paper endeavours to shed light on potential avenues to surmount these obstacles, ultimately unlocking the full potential of smart windows in establishing energy-efficient built environments.

Shang J., Zhang Y., Zhang J., Zhang X., An Q.