Information-Providing Flexible and Transparent Smart Window Display

Kim K., Yun T.Y., You S., Tang X., Lee J., Seo Y., Kim Y., Kim S.H., Moon H.C., Kim J.K.

Mesoporous metal oxides consisting of fully interconnected network structures with small pores (20–50 nm) have high surface areas and decreased ion intercalation distances, making them ideal for use in high-performance electrochromic supercapacitors (ECSs). Evaporation-induced self-assembly (EISA), which combines sol–gel chemistry and molecular self-assembly, is a powerful method for the fabrication of mesoporous metal oxides through a solution phase synthesis. Herein, we introduce ultrafast sub-1 s ECSs based on an amorphous mesoporous tungsten trioxide (WO3) that is prepared by EISA. Compared to that of a compact-WO3 film-based device, the performances of an ECS with mesoporous WO3 exhibits a large optical modulation (76% at 700 nm), ultrafast switching speeds (0.8 s for coloration and 0.4 s for bleaching), and a high areal capacitance (2.57 mF/cm2), even at a high current density (1.0 mA/cm2). In addition, the excellent device stability during the coloration/bleaching and charging/discharging cycles is observed under fast response conditions. Moreover, we fabricated a patterned mesoporous WO3 for ECS displays (ECSDs) via printing-assisted EISA (PEISA). The resulting ECSDs can be used as portable energy-storage devices, and their electrochromic reflective displays change color according to their stored energy level. The ECSDs in this work have enormous potential for use in next-generation smart windows for buildings and as portable energy storage displays. Networks of tiny holes improve the energy storage properties of materials that can also be used for smart windows. In electrochromic materials, the fraction of light passing through the material can be controlled using an electrical voltage. This is useful for smart windows which electrically switch from being transparent to opaque. This change is associated with the storage or release of energy, so the same materials are being investigated for energy storage. Keon-Woo Kim from Pohang University of Science and Technology, South Korea, and co-workers have developed an improved electrochromic supercapacitor made from tungsten trioxide. They used an evaporation-induced self-assembly process to deposit a film of tungsten trioxide containing pores approximately 30 nanometers across. This porous structure increased the material’s switching speed and capacitance compared to a conventional tungsten trioxide thin film. Ultra-fast electrochromic supercapacitors (ECSs) are demonstrated based on mesoporous WO3 prepared by evaporation-induced self-assembly (EISA). Mesoporous WO3 based ECSs show excellent electrochromic and supercapacitor performances under fast operating condition. Furthermore, printing assisted EISA is introduced to produce patterned mesoporous WO3 for ECS displays (ECSDs). The resulting ECSDs have great potential as next-generation smart electrochemical components.

Scheffold F.

Due to their controlled size, sensitivity to external stimuli, and ease-of-use, microgel colloids are unique building blocks for soft materials made by crosslinking polymers on the micrometer scale. Despite the plethora of work published, many questions about their internal structure, interactions, and phase behavior are still open. The reasons for this lack of understanding are the challenges arising from the small size of the microgel particles, complex pairwise interactions, and their solvent permeability. Here we describe pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization, such as super-resolution microscopy, scattering methods, and modeling. Despite their widespread use, many fundamental questions about the internal structure of microgels are still open. Here the authors describe several pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization.

Inoue M., Hayashi T., Hikiri S., Ikeguchi M., Kinoshita M.

In water, poly(N-isopropylacrylamide) (PNIPAM) is in a soluble coil state below the lower critical soluble temperature (LCST) but in an insoluble globule state above LCST. Namely, as the temperature decreases, PNIPAM exhibits a globule-to-coil transition at LCST~305 K. We generate structural ensembles of coil and globule states by all-atom molecular dynamics simulations conducted at 273 and 323 K, respectively. We then calculate a variety of energetic and entropic components of thermodynamic quantities of the two states at the two temperatures using our recently developed, accurate statistical-mechanical method for solute hydration where molecular models are employed for water and the PNIPAM structure is taken into account at the atomic level. We identify the physical factors driving or opposing the transition and evaluate their relative magnitudes and temperature dependences. The presence of PNIPAM generates an excluded volume (EV) which is inaccessible to the centers of water molecules in the entire system. The presence of a water molecule also generates an EV for the other water molecules with the result that all of the water molecules are entropically correlated, causing water crowding. The globule state, where the EV is smaller and water crowding is less significant, is more favored in terms of the translational, configurational entropy of water. This effect always opposes the globule-to-coil transition. At low temperatures, however, this effect becomes significantly weaker, yielding to the factors driving it. The mechanism of the transition is physically the same as that of cold denaturation of a protein.

Cho H., Kwon J., Ha I., Jung J., Rho Y., Lee H., Han S., Hong S., Grigoropoulos C.P., Ko S.H.

We demonstrated a reversible transparency-changing smart glass that can be operated by mechanical impacts and heat.

Zhao Y., Ju X., Zhang L., Wang W., Faraj Y., Zou L., Xie R., Liu Z., Chu L.

Transparent thermo-responsive poly(N-isopropylacrylamide)-l-poly(ethylene glycol)acrylamide conetwork hydrogels with rapid deswelling response are developed with multi-arm star poly(ethylene glycol)acrylamide as a cross-linker.

Milster S., Chudoba R., Kanduč M., Dzubiella J.

The selective solute partitioning within a polymeric network is of key importance to applications in which controlled release or uptake of solutes in a responsive hydrogel is required. In this work we investigate the impact of cross-links on solute adsorption in a swollen polymer network by means of all-atom, explicit-water molecular dynamics simulations. We focus on a representative network subunit consisting of poly($N$-isopropylacrylamide) (PNIPAM) and $N$,$N'$-methylenebisacrylamide (BIS/MBA) cross-linker types. Our studied system consists of one BIS-linker with four atactic PNIPAM chains attached in a tetrahedral geometry. The adsorption of several representative solutes of different polarity in the low concentration limit at the linker region is examined. We subdivide the solute adsorption regions and distinguish between contributions stemming from polymer chains and cross-link parts. In comparison to a single polymer chain, we observe that the adsorption of the solutes to the cross-link region can significantly differ, with details depending on the specific compounds' size and polarity. In particular, for solutes that have already a relatively large affinity to PNIPAM chains the dense cross-link region (where many-body attractions are at play) amplifies the local adsorption by an order of magnitude. We also find that the cross-link region can serve as a seed for the aggregation of mutually attractive solutes at higher solute concentrations. Utilizing the microscopic adsorption coefficients in a mean-field model of an idealized macroscopic polymer network, we extrapolate these results to the global solute partitioning in a swollen hydrogel and predict that these adsorption features may lead to non-monotonic partition ratios as a function of the cross-link density.

Hippler M., Blasco E., Qu J., Tanaka M., Barner-Kowollik C., Wegener M., Bastmeyer M.

Stimuli-responsive microstructures are critical to create adaptable systems in soft robotics and biosciences. For such applications, the materials must be compatible with aqueous environments and enable the manufacturing of three-dimensional structures. Poly(N-isopropylacrylamide) (pNIPAM) is a well-established polymer, exhibiting a substantial response to changes in temperature close to its lower critical solution temperature. To create complex actuation patterns, materials that react differently with respect to a stimulus are required. Here, we introduce functional three-dimensional hetero-microstructures based on pNIPAM. By variation of the local exposure dose in three-dimensional laser lithography, we demonstrate that the material parameters can be altered on demand in a single resist formulation. We explore this concept for sophisticated three-dimensional architectures with large-amplitude and complex responses. The experimental results are consistent with numerical calculations, able to predict the actuation response. Furthermore, a spatially controlled response is achieved by inducing a local temperature increase by two-photon absorption of focused light. Stimuli-responsive microstructures are important in soft robotics and biosciences, but water compatible materials to fabricate 3D structures are scarce. Here, the authors demonstrate pNIPAM based hetero-microstructures with substantially different material parameters by variation of the local exposure dose in 3D laser lithography.

Cheng W., He J., Dettelbach K.E., Johnson N.J., Sherbo R.S., Berlinguette C.P.

Summary Electrochromic windows dynamically control solar irradiation to buildings by using transition-metal oxide films that change color in response to an electrical current. Electrochromic glass companies currently manufacture these films by physical deposition methods. We report here a solution-based protocol that forms layers of electrochromically active metal oxides (e.g., WO 3 , Nb 2 O 5 , MoO 3 , and V 2 O 5 ) on a timescale that rivals current industry practice. Indeed, the high-porosity WO 3 thin films prepared in this study displayed electrochromic performance parameters—including an optical modulation of 70% at 700 nm, colored and bleached interchange times on the order of seconds, and a coloration efficiency > 130 cm 2 /C—that are commensurate with the state of the art. This photodeposition method is scalable and therefore offers a potentially significant advancement for the large-scale deployment of electrochromic windows.

Wu T., Zrimsek A.B., Bykov S.V., Jakubek R.S., Asher S.A.

The best-known examples of smart, responsive hydrogels derive from poly( N-isopropylacrylamide) (PNIPAM) cross-linked polymer networks. These hydrogels undergo volume phase transitions (VPTs) triggered by temperature, chemical, and/or environmental changes. PNIPAM hydrogels can undergo more than 50-fold volume changes within ∼1 μs intervals. Studies have tried to elucidate the molecular mechanism of these extraordinarily large responses. Nevertheless, the molecular reaction coordinates that drive the VPT remain unclear. Using visible nonresonance Raman temperature-jump spectroscopy, we determined the molecular ordering of this VPT. The PNIPAM hydrophobic isopropyl and methylene groups dehydrate with time constants of 109 ± 64 and 104 ± 44 ns, initiating the volume collapse of PNIPAM. The subsequent dehydration of the PNIPAM amide groups is significantly slower, as our group previously discovered (360 ± 85 ns). This determination of the ordering of the molecular reaction coordinate of the PNIPAM VPT enables the development of the next generation of super-responsive materials.

Cao D., Xu C., Lu W., Qin C., Cheng S.

Davy N.C., Sezen-Edmonds M., Gao J., Lin X., Liu A., Yao N., Kahn A., Loo Y.

Current smart window technologies offer dynamic control of the optical transmission of the visible and near-infrared portions of the solar spectrum to reduce lighting, heating and cooling needs in buildings and to improve occupant comfort. Solar cells harvesting near-ultraviolet photons could satisfy the unmet need of powering such smart windows over the same spatial footprint without competing for visible or infrared photons, and without the same aesthetic and design constraints. Here, we report organic single-junction solar cells that selectively harvest near-ultraviolet photons, produce open-circuit voltages eclipsing 1.6 V and exhibit scalability in power generation, with active layers (10 cm2) substantially larger than those typical of demonstration organic solar cells (0.04–0.2 cm2). Integration of these solar cells with a low-cost, polymer-based electrochromic window enables intelligent management of the solar spectrum, with near-ultraviolet photons powering the regulation of visible and near-infrared photons for natural lighting and heating purposes. Smart windows are used to regulate the amount of visible and near-infrared light entering buildings or cars. Here, Davy et al. develop near-UV harvesting organic solar cells, scalable up to 10 cm2, for powering electrochromic windows without competing for photons in the visible or near-infrared.

Su G., Zhou T., Liu X., Ma Y.

The micro-dynamics mechanism of the volume phase transition of PNIPAM-co-HEMA hydrogels was established using temperature-dependent FTIR spectroscopy, PCMW2D, and 2DCOS analysis.

Kim D., Kim H., Lee E., Jin K.S., Yoon J.

In order to achieve a hydrogel capable of programmable volume change, poly(N-isopropylacrylamide)-graft-methylcellulose hydrogel (PNIPAm-g-MC) was prepared through the grafting of PNIPAm onto a MC backbone and simultaneous cross-linking of the chains. PNIPAm-g-MC exhibited large thermal hysteresis in its volume change, which results from the stable hydrophobic junctions between the MC strands formed during heating. By combining photothermal magnetite nanoparticles as a heat transducer with the prepared hydrogel, programmable volume phase transition between the shrunken and swollen state could be triggered by visible light irradiation and excessive cooling, respectively. Based on this programmable feature, a bilayer actuator capable of static bending was fabricated. The developed programmable hydrogels are expected to provide a platform for the next generation of origami, microvalves, and drug delivery systems.

Ke Y., Balin I., Wang N., Lu Q., Tok A.I., White T.J., Magdassi S., Abdulhalim I., Long Y.

Two-dimensional (2D) photonic structures, widely used for generating photonic band gaps (PBG) in a variety of materials, are for the first time integrated with the temperature-dependent phase change of vanadium dioxide (VO2). VO2 possesses thermochromic properties, whose potential remains unrealized due to an undesirable yellow-brown color. Here, a SiO2/VO2 core/shell 2D photonic crystal is demonstrated to exhibit static visible light tunability and dynamic near-infrared (NIR) modulation. Three-dimensional (3D) finite difference time domain (FDTD) simulations predict that the transmittance can be tuned across the visible spectrum, while maintaining good solar regulation efficiency (ΔTsol = 11.0%) and high solar transmittance (Tlum = 49.6%). Experiments show that the color changes of VO2 films are accompanied by NIR modulation. This work presents a novel way to manipulate VO2 photonic structures to modulate light transmission as a function of wavelength at different temperatures.

Liu T., Liu B., Wang J., Yang L., Ma X., Li H., Zhang Y., Yin S., Sato T., Sekino T., Wang Y.

A series of smart window coated multifunctional NIR shielding-photocatalytic films were fabricated successfully through KxWO3 and F-TiO2 in a low-cost and environmentally friendly process. Based on the synergistic effect of KxWO3 and F-TiO2, the optimal proportion of KxWO3 to F-TiO2 was investigated and the FT/2KWO nanocomposite film exhibited strong near-infrared, ultraviolet light shielding ability, good visible light transmittance, high photocatalytic activity and excellent hydrophilic capacity. This film exhibited better thermal insulation capacity than ITO and higher photocatalytic activity than P25. Meanwhile, the excellent stability of this film was examined by the cycle photocatalytic degradation and thermal insulation experiments. Overall, this work is expected to provide a possibility in integrating KxWO3 with F-TiO2, so as to obtain a multifunctional NIR shielding-photocatalytic nanocomposite film in helping solve the energy crisis and deteriorating environmental issues.

Yang S., Tang S., Cai Y., Ye Z., Zou X., Yuan X., Song Y., Li B., Tang D., Liu M.

AbstractPassive thermal‐regulation strategies have become increasingly important due to the strain alleviated on power grids for temperature management. Designing a system capable of automatically switching between cooling and heating modes in response to changing ambient conditions presents several specific challenges that engineers and researchers are actively addressing. In this research, a CaCl2 incorporated PNIPAM coated fluorinated poly(aryl ether) (FPAE) porous film with tunable reflectance is developed. The aim is to mitigate the reliance on active cooling systems, which consume significant amounts of energy. The moisture‐temperature dual sensitive film exhibits a tunable reflectance range between 91.1% and 39.1% via phase change of the PNIPAM layer. Coupled with an infrared emissivity of 96.0%, a daytime cooling of 10 °C compared to the control experiment is achieved. Coating the film with a photothermal layer results in an adaptive Janus film that is capable of autonomous switching between heating and cooling, and demonstrates a heating of 22.5 °C in a cold environment. The facile preparation method, excellent cyclic stability, mechanical properties, and UL‐94 V‐0 rating enable promising applications of the smart film under diverse living environments.

Mai S., Liu X., Zhao J., Yu Z., Jin J., Shi L., Zhou T.

ABSTRACTElectrophoretic displays (EPDs) are commonly employed in applications like e‐books and electronic price tags due to their benefits of minimal power consumption, excellent contrast, and broad viewing angles. This article establishes a dynamic model for nanoparticles after the removal of the applied electric field. The model combines the Poisson equation, the Navier–Stokes equation, and the Nernst–Planck equation. The Arbitrary Lagrangian–Eulerian method is applied to simulate nanoparticle diffusion motion under varying conditions, such as solution viscosity, particle radius, and reverse micelle radius, after the electric field is removed. The results indicate that after the electric field is removed, high‐viscosity solutions exert a stronger hindrance on the particles, resulting in a shorter displacement over the same time period. With equal charge, smaller particle radius exhibits higher surface charge density, allowing them to travel further within the same time frame. Additionally, a smaller reverse micelle radius facilitates the rapid neutralization of surface charge on the particles, thereby limiting their diffusion distance. These findings provide theoretical support for a deeper understanding of the operating mechanism of EPDs.

Yu C., Luo S.

Chen W., Kang J., Zhang J., Zhang Y., Zhou X., Yan Q., Kim H., Guo T., Wu C., Kim T.W.

Bai Y., Tang T., He Y.

Smart windows, capable of dynamically regulating indoor heat, offer a promising avenue for effectively reducing energy consumption. Hydrogel-based smart windows are excellent at thermal modulation and daylighting, but they are difficult to commercialize globally due to problems like winter ice formation, which can affect thermal insulation, daylighting, and structural integrity, as well as an impractical cloud point temperature (tcp). To solve these issues, a ternary anti-freezing system consisting of ethylene glycol (EG), glycerol, and water is proposed. With the tcp regulated at 31.6°C, the system strikes a balance between outstanding daylighting (91.89%) and solar modulation ability (78.32%). Furthermore, the system shows resilience even below −30.0°C and long-term stability, which qualifies it for use in densely populated regions even with severely cold weather. To further illustrate the distinct impacts of EG and glycerol, the optical characteristics and tcp of binary systems containing EG and water as well as glycerol and water were examined. The durability test includes severely cold temperature of −30.0°C and solar exposure temperature of 60.0°C. This work would offer insights to advance the field’s understanding of anti-freezing capability modification in smart windows and advance the development of environmentally and energy-efficiently designed buildings.

Yu H., Kim D., Lim D., Choi S.

Chen H., Chang G., Lee T.H., Min S., Nam S., Cho D., Ko K., Bae G., Lee Y., Feng J., Zhang H., Kim J., Shin J., Hong J., Jeon S.

AbstractSmart windows, capable of tailoring light transmission, can significantly reduce energy consumption in building services. While mechano-responsive windows activated by strains are promising candidates, they face long-lasting challenges in which the space for the light scatterer’s operation has to be enlarged along with the window size, undermining the practicality. Recent attempts to tackle this challenge inevitably generate side effects with compromised performance in light modulation. Here, we introduce a cuttlefish-inspired design to enable the closing and opening of pores within the 3D porous structure by through-thickness compression, offering opacity and transparency upon release and compression. By changing the activation mode from the conventional in-plane to through-thickness direction, the space requirement is intrinsically decoupled from the lateral size of the scatterer. Central to our design is the asymmetry of pore orientation in the 3D porous structure. These inclined pores against the normal direction increase the opaqueness upon release and improve light modulation sensitivity to compression, enabling transmittance regulation upon compression by an infinitesimal displacement of 50 μm. This work establishes a milestone for smart window technologies and will drive advancements in the development of opto-electric devices.

Yu G., Hong W., Ran L., Du Q., Wang H., Wang Z., Guo S., Li C.

Sudhakaran N., Abraham M., Parvathy P.A., Das S., Sahoo S.K.

Albalawi M.A.

A simple strategy was introduced to develop fluorescent wood with the ability to alter its color when exposed to both visible and ultraviolet lights. Injecting a combination of europium and dysprosium doped aluminate (EDA; 7-12 nm) nanoparticles and polyester resin (PET) into a lignin-modified wood (LMW) produced a translucent smart wooden window with fluorescence and afterglow emission properties. In order to prevent formation of aggregates and improve the preparation process of transparent wood, EDA must be properly disseminated in the polyester matrix. We analyzed the fluorescent wood samples using a variety of spectroscopic and microscopic methods, including energy-dispersive X-ray (EDX), scanning electron microscopy (SEM), photoluminescence spectra, and hardness tests. We found that the photoluminescent woods had an excitation band at 365 nm and emission peaks at 437 nm and 517 nm. The translucent luminous wood showed rapid and reversible emission response to ultraviolet light. Fluorescence emission was detected for samples with lower EDA content, and afterglow emission was detected for wood samples with higher EDA content. Increases in EDA content were associated with improvements in water resistance and ultraviolet radiation protection in the EDA@PET-infiltrated wood.

Salih M.M., Ahmed W.S., Hamdoun S.H., Shalenko V.

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

Lee J., Ma J., An C., Lee G., Oh S.

AbstractA novel ultra‐broadband polarization rotator with advanced angular adjustability is proposed for functional devices such as displays and smart windows. The new solution offers dynamic control of light polarization across a broad range of wavelengths, encompassing the complete visible spectrum, ultraviolet and near‐infrared. Moreover, it boasts a smaller footprint, faster response times, and lower dispersion compared to conventional rotators. The findings are remarkable in that they show that as the viewing angle increases, the hybrid alignment takes on a twist‐like configuration, with the polarization rotation angle determined by the spatial variation in the twist angle. This intriguing behavior leads to an improved range of angular adjustability, as the effective polarization rotation depth is extended. The improved angular adjustability of reconfigurable smart devices surpasses the limitations of traditional polarization rotators, unlocking new innovative possibilities. For example, the rotator plays a crucial role in display technologies, allowing for effective control of viewing angles and minimizing reflection from disturbing external light. Similarly, in smart windows, it optimizes energy conservation by regulating direct sunlight transmission while ensuring clear visibility in normal conditions. It is believed that the proposed advanced ultra‐broadband polarization rotator is a significant step forward in the development of reconfigurable smart devices.

Wang M., Liang S., Zhao S., Gao W., Li Z.

Liu G., Liu J., Fan Q., Liu Y., Zeng Z., Li Z., Wu X., Yang M., Yang B.

Smart windows can dynamically control the optical transmittance of natural light and efficiently reduce the energy consumption of buildings. Currently, the practical application of smart windows is limited by high power consumption, low transmittance, and complicated preparation process. Developing smart windows with fast response and high transmittance is challenging. Herein, we fabricated a smart window based on electrophoretic display (EPD) technology, which can switch between white and transparent states by changing the stacking states of the electrophoretic particles on electrodes. To further increase the transmittance of the smart window, the polyvinyl alcohol (PVA) hydrophilic layer was spin-coated on the electrode to reduce the adhesion of particles. Compared with other EPD smart window technologies, our device exhibits the fastest response time of up to 375 ms at 30 V, high transmittance of up to 78% at 632 nm, and a high contrast ratio of up to 89. In addition, to reduce the particle’s lateral diffusion and improve our device's stability, the microcup array was aligned on the interdigital electrode. The particles can be driven and stacked onto the side walls of the microcup to achieve a transparent state. This work demonstrated that the EPD device would provide a promising future for practical smart windows with high transmittance and fast response time.