Solar Transmittance Modulation Research Articles

Free‐Standing Bacterial Cellulose‐Templated Radiative Cooling Liquid Crystal Films with Self‐Adaptive Solar Transmittance Modulation

AbstractPassive radiative coolingmaterials can efficiently reflect solar radiation and spontaneously dissipate heat through the long‐wave infrared (LWIR) transmission window to the coldouter space. They are highly attractive for application in walls and roofs of sustainable energy‐efficient buildings, but it is a challenging task to develop switchable radiative coolers for transparent windows. Herein, we report free‐standing bacterial cellulose‐templated radiative cooling liquid crystal films with high LWIR emissivity and self‐adaptive solar transmittance. Biosynthetic silanized bacterial cellulose with 3D interconnected porous nanostructure is harnessed asa robust template for confining novel liquid crystals with smectic A to chiral nematic phase transition, thereby endowing the film with high solar transmittance modulation ability. The resulting film is found to not only exhibit a remarkable LWIR emissivity, but also adaptively change its solar transmittance between a transparent state and an opaque state according to environmental temperature variation. Outdoor radiative cooling experiments were performed, and the energy‐saving performance was evaluated through a simulation of a 12‐story reference office building with the films applied as radiative cooling windows. This research can shine light on the development of advanced radiative cooling materials with switchable transmittance and their widespread applications in buildings, vehicles, and transparent photovoltaics.

Impact of Zn and W Doping Levels on Properties of Thermochromic VO2-Based Thin Films

DC magnetron sputtering at a substrate temperature of 425 °C was used to successfully deposit W/Zn-doped VO2 thin films on soda lime glass (SLG) substrates. The aim was to investigate the influence of Zn doping levels on the transition temperature ( ), luminous transmittance (Tlum) and solar transmittance modulation (ΔTsol) of VO2-based thin films. UV/VIS/NIR spectrometer, X-ray Diffraction (XRD), Atomic Force Microscope (AFM), and Rutherford Backscattering Spectroscopy (RBS) were used to characterise the thin films. It was revealed that W/Zn co-doped VO2 thin films with ~ 10.8 at.% Zn showed a luminous transmittance of ~ 40.4%, with excellent solar transmittance modulation of 10.2%. Furthermore, the transition temperature obtained for the Zn and W co-doped VO2 films with ~ 10.8 at.% Zn was lower at 23.1 °C compared to 25.6 °C and 26.8 °C for thin films with ~ 3.9 at.% Zn and ~ 2.8 at.% Zn, respectively. It was not possible to deposit the films with Zn doping level above 10.8 at.% due to some technical limitations. These findings indicate that thin films with a controlled proportion of Zn in the W/Zn co-doped VO2-based thin films have the potential to be employed for applications such as smart windows.

Ideal thermochromic smart window in a south-facing office room of China considering daylighting and energy performance

ABSTRACT Thermochromic (TC) smart window which allows modulating the transmission of solar heat energy and sunlight in response to environmental conditions has great potential for improving the building energy and daylighting performance simultaneously. In this study, the multi-objective optimization method is used to determine the ideal optical properties of TC windows. Parametric assessment is firstly conducted to investigate the influence of several crucial parameters. Multi-objective optimization is then carried out by applying Non-dominated Sort Genetic Algorithm-III, maximizing daylight availability and building energy efficiency. Finally, the decision-making of the Pareto frontier is performed to achieve the ideal TC windows in 32 representative cities in different climatic zones of China. Results demonstrate that increasing the luminous transmittance at low-temperature and decreasing luminous transmittance modulation ability are beneficial to daylighting and energy performance. Lower solar transmittance at low-temperature contributes to building energy savings in Beijing, Shanghai, Guangzhou and Kunming, except for Harbin. Higher solar transmittance modulation ability is desired for building energy efficiency. The optimum value for the transition temperature is in the range of 23.67–38.85°C. The building energy consumption can be reduced by 13.01%, and the desired daylighting hours can be improved by 8.90%, relative to the clear double-glazed windows.

Active and passive modulation of solar light transmittance in a uniquely multifunctional dual-band single molecule for smart window applications

Functional materials may change color by heat and electricity separately or simultaneously in smart windows. These materials have not only demonstrated remarkable potential in the modulation of solar radiation but are also leading to the development of indoor environments that are more comfortable and conducive to improving individuals' quality of life. Unfortunately, dual-responsive materials have not received ample research attention due to economic and technological challenges. As a consequence, the broader utilization of smart windows faces hindrances. To address this new generational multi-stimulus responsive chromic materials, our group has adopted a developmental strategy to create a poly(NIPAM)n-HV as a switchable material by anchoring active viologen (HV) onto a phase-changing poly(NIPAM)n-based smart material for better utility and activity. These constructed smart windows facilitate individualistic reversible switching, from a highly transparent state to an opaque state (thermochromic) and a red state (electrochromic), as well as facilitate a simultaneous dual-stimuli response reversible switching from a clear transparent state to a fully opaque (thermochromic) and orange (electrochromic) states. Absolute privacy can be attained in smart windows designed for exclusive settings by achieving zero transmittance. Each unique chromic mode operates independently and modulates visible and near-infrared (NIR) light in a distinct manner. Hence, these smart windows with thermal and electric dual-stimuli responsiveness demonstrate remarkable heat regulation capabilities, rendering them highly attractive for applications in building facades, energy harvesting, privacy protection, and color display.

Enhancing thermochromic properties of VO2 amorphous films on glass substrates by Sn-W co-doping

Co-doping is an effective method for VO2 films to meet the requirements of smart window applications. In this research, we prepared pure VO2, Sn-doped, and Sn-W co-doped amorphous films on glass substrates. Compared with pure and Sn-doped VO2 films, Sn-W co-doped samples showed better thermochromic properties. For the 4.8 % Sn-2.1 % W co-doped sample, the luminous transmittance (Tlum) reached 76 %, the metal-insulator transition (MIT) temperature (Tc) decreased to 46.7 °C, the solar transmittance modulation capacity (ΔTsol) was maintained at around 5.2 %, and the near-infrared modulation capacity (ΔTIR) was up to 13.5 %. These results show that co-doping Sn and W into VO2 is feasible for smart window applications.

Thermochromic smart window utilizing passive radiative cooling for Self-Adaptive thermoregulation

Being a major impediment, the development of advanced smart windows with exceptional thermoregulation in both the visible-near infrared (Vis-NIR) and the long-wave infrared remains a significant aspiration to scholars. Herein, an innovative strategy was implemented to achieve self-adaptive smart window that seamlessly integrates the thermochromic and radiative cooling technologies (TRSW), and holds great potential in effectively addressing the intricate thermal radiation requirements arising from unpredictable climatic conditions. The TRSW demonstrated an impressive capacity to regulate solar energy through automatic modulation of emissivity (ε) at both high (εLWIR-H = 0.68) and low (εLWIR-L = 0.35) temperatures, while effectively maintaining Vis-NIR regulation (20.12%). Moreover, the results of TRSW’s outdoor performance exhibited its efficiency in implementing dual solar energy transmission modes, effectively inhibiting the Vis-NIR transmission to mitigate solar overheating in high-temperature conditions, while diminishing heat dissipation and promoting radiative cooling in low-temperature situations. Compared to conventional cooling materials, the dynamic emissivity modulation of TRSW provided an effective approach to prevent overcooling at lower temperatures. Consequently, TRSW fostered an efficient synergy between the solar transmittance modulation (ΔTsol) and Δε, which led to heightened modulation of the entire spectrum and facilitated exceptional thermal management. This work paves an intriguing prospect of developing advanced smart windows with remarkable thermoregulation and promises a wide range of thermal regulation applications, such as windows, roofs, walls, and advanced textiles.

Multistimuli-responsive smart windows based on paraffin-polymer composites

Despite their potential to reduce energy consumption in buildings and increase user's comfort, the general application of smart windows is currently hampered by economic and technological factors. To overcome these limitations, herein we report a new strategy for the fabrication of thermoresponsive smart windows based on polymer films loaded with paraffin nanoparticles. Proper selection of these components allows refractive index matching when the paraffin nanoparticles are in their solid state - i.e., high visible and near-infrared light transparency -, which is lost after melting, thereby resulting in pronounced light scattering - i.e., opacity. As a result, these films show ample modulation of sunlight transmittance upon heating, while additionally exhibiting other advantageous features: facile preparation from low-cost materials; high photostability, flexibility and scalability; and fine tunability of their thermal response. Moreover, by deposition onto transparent electrodes or incorporation of nanostructured photothermal agents, the solar transmittance modulation of our paraffin-polymer composites can also be triggered with low voltages - i.e., to warrant user's control - or sunlight absorption - i.e., to improve self-adaptation to ambient conditions. Actually, films with combined thermal and photothermal activity allow high sunlight transmission in cold weather while efficiently reducing solar heat gain indoors in sunny hot days.

A solar/radiative cooling dual-regulation smart window based on shape-morphing kirigami structures.

The energy efficiency of buildings has become a critical issue due to their substantial contribution to global energy consumption. Windows, in particular, are often the least efficient component of the building envelope, and conventional smart windows focus solely on regulating solar transmittance while overlooking radiative cooling. Although several recent designs achieved dual-control of solar and radiative cooling, these windows still face limitations in terms of durability, limited modulation ability and energy-saving performance. To address these challenges, we propose a novel dual-control smart window design consisting of a reconfigurable kirigami structure and polydimethylsiloxane-laminated thermochromic hydrogel coated with silver nanowires. In summer, the thermochromic hydrogel turns translucent to suppress the solar heat gain, while the high emissivity kirigami structure covers the exterior surface of the window, promoting radiative cooling. In winter, the hydrogel becomes transparent to allow for solar transmission. Additionally, the kirigami structure undergoes an out-of-plane structural change, opening towards the outside environment to expose the underlying low-emissivity silver nanowires and suppress heat radiation. Our design achieves a promising solar transmittance modulation ability of ∼24% and a good long-wave infrared emissivity regulation ability of 0.5. Furthermore, it exhibits significantly improved durability, which is nine times longer than the lifespan of conventional smart hydrogels. Our novel approach offers a promising solution for constructing energy-efficient and durable smart windows and outperforms existing state-of-the-art solar/radiative cooling dual-regulation smart windows in the literature.

Role of the double-glow plasma pre-sputtering in the growth mechanisms and metal–insulator transition of VO2 film

High phase transition temperature and low utilization rate of sunlight of vanadium dioxide (VO2) have been an important obstacle to expand its application in smart semiconductor materials. In this paper, we propose a novel method by the double-glow plasma pre-sputtering to induce to generate a lot of surface vacancy defects/residual stresses, which can promote the high-quality growth of VO2 thin films on quartz substrates. The resultsshowed that the plasma pretreatment and annealing temperature affected the stability of phase structureto cause the formation of surface residual stresses of VO2films. The optical-electrical properties of the treated VO2 film indicated that the incident light at 2.5 μm has a great transmittance contrast of 63.25% and the solar transmittance modulation efficiency (ΔTsol) of 14.9 %before and after the phase transition. It is found that the metal–insulator phase transition temperature of VO2 after plasma treatment is 39.7℃, which is 20.4℃ lower than that of conventional VO2 film, greatly increases its application in intelligent windows and photoelectric fields. Our research provides an effective and low-cost approach to prepared high-quality VO2 thin films with low phase transition temperature and large-phase-transition amplitude on quartz substrate.

Design and reactive magnetron sputtering of thermochromic coatings

Thermochromic coatings based on vanadium dioxide exhibit great potential in various fields, including smart energy-saving windows with temperature-dependent transmittance in the infrared at preserved transmittance in the visible. However, these promises come with challenges concerning the low-temperature preparation of high-quality crystalline VO2-based films by industry-friendly techniques and the simultaneous optimization of all coating characteristics, such as thermochromic transition temperature, luminous transmittance, and modulation of solar energy transmittance. This Perspective outlines these challenges, highlights the recent progress in the field of design and reactive magnetron sputtering of thermochromic coatings, explains the physics that allowed this progress, and provides ideas and recommendations for future research. A combination of the advantages of controlled high-power impulse magnetron sputtering with the not yet fully experimentally utilized multilayered designs constitutes the main reason why further progress is anticipated in the future.

Achieving transmissive-to-black chromism via engineered dual electro-thermoresponsive single-molecule for full modulation of solar transmittance

Pursuit of complete and null transmissive optical material in smart windows for full control of the intensity and spectrum of incoming solar light has led to the advancement of transmissive-to-black chromism. While most reported transmissive-to-black devices are electrically stimulated, we report here an alternative approach based on dual stimulation of a new dual responsive molecule. By integrating asymmetric viologen onto a newly engineered poly(2-isopropyl-2-oxazoline)-based molecule, one can achieve transmissive-to-black chromism. The synergistic effect of its thermochromic and electrochromic response when stimulated simultaneously shows full absorption of the entire visible solar spectrum resulting to a truly black state that is ideal for building facades requiring absolute privacy. Autonomous chromic switchability function is also possible by independently stimulating the material either electrically or thermally where it displays unique colored state of vibrant magenta and a hazy state, respectively. Tunability of the material’s overall performance in terms of optical contrast, switching kinetics, coloration efficiency and cyclic stability based on the lattice water concentration was elucidated in this work. Furthermore, the fabricated device displays self-bleaching property and has a short-term “memory” effect that may broaden its potential for other applications.

Embedding Quaternary V1–x–ySrxWyO2 into Multilayer Systems to Enhance Its Thermochromic Properties for Smart Glass Applications

Smart window glazing technology possesses a great potential for reducing the overall energy demand in the building sector. Thin films based on thermochromic vanadium dioxide (VO2) are ideally suited as functional layers of thermochromic smart windows. VO2 itself exhibits a significant change in optical transparency and reflectivity in the infrared spectrum. However, the optimization of VO2-based thin films for application as intelligent window glazing is still ongoing. The improvement is rather tedious because the key parameters set by the application such as transmittance modulation ΔT2500, luminous transmittance Tlum, solar transmittance modulation ΔTsol, and transition temperature ϑc affect each other. Codoping of VO2 with strontium (Sr) and tungsten (W) drives the parameters close to the desired range. However, the requirements set for the use as smart window coating cannot be fully met. Here, we show by a systematic comparison of simulations and experiments that embedding a layer of thermochromic V1–x–ySrxWyO2 between an optimized TiO2 buffer and a TiO2 antireflection layer already adds additional degrees of freedom to simultaneously fulfill all smart window requirements for the optical parameters ΔT2500, Tsol, Tlum, and ΔTsol. In particular, the luminous transmittance Tlum is improved by not less than ≈10% in such a trilayer structure.

Engineered Ionene/PNIPAM Hybrid Dual-Response Material Generating Tunable and Unique Optical Modes for Adaptive Solar Transmittance Modulation.

A hybrid smart window exhibiting dual chromic response properties based on an ionene/polymer material is successfully engineered. Thermochromic poly(N-isopropylacrylamide) is integrated with an electrochromic viologen-tethered ionene, also acting as an electrolyte, to produce a smart window that can adaptively control solar visible light transmittance in response to multiple stimuli. This new blend allows the formation of unique reversible optical states, namely, "clear", "amber", "cloudy", and "grainy" states, which are passively triggered by environmental temperature and actively induced by external potential or simultaneously by both. This hybrid material shows tunability in terms of its electrochemical and optical properties, switching kinetics, and coloration efficiency and can also achieve a nearly absolute zero-transmissive state. With the material's excellent solubility and film-forming ability, the smart device can be fabricated with much flexibility and ease. Finally, this device has an all-in-one layer configuration, creating a more compact and simplified design. With all these properties combined, the development of a next-generation multifunctional smart window device, which can efficiently control incoming solar light for energy-saving in buildings and also provide visual comfort, is possible.

Multi-objective optimization of thermochromic glazing based on daylight and energy performance evaluation

Many efforts have been detected to investigate thermochromic (TC) glazing for improving building energy saving, while only a few approaches for daylight performance analysis. In this study, the performance of TC glazing is investigated based on multi-objective optimization for minimizing energy demand while maximizing daylight availability. The effects of five parameters including transition temperature, solar transmittance in clear state, solar transmittance modulation ability, luminous transmittance in clear state, and luminous modulation ability on the building energy consumption and useful daylighting illuminance (UDI300–3000) are examined. Linear Programming Technique for Multi-dimensional Analysis of Preference (LINMAP) is used for the decision-making of Pareto frontier. This research aims to explore the ideal thermochromic glazing by considering the daylight and energy performance of a typical office room, taking the weather condition of Xiamen, China as an example. Although it is impossible to achieve both optimal values of energy consumption and UDI300–3000 simultaneously, the proposed multi-objective optimization method could still provide low energy consumption with sufficient daylight. The non-dominated sorting of Pareto optimal solution (POS) demonstrated that the optimum building energy consumption and UDI300–3000 for single glazed windows are 46.64 kWh/m2 and 70.92%, respectively, while the values for double glazed windows are 44.40 kWh/m2 and 71.88%, respectively. The selected hypothetical TC windows can improve the building energy and daylighting performance simultaneously when compared with traditional clear single and double glazed windows. The presented framework provides a multi-objective optimization method to determine the most suitable TC glazing technologies for designers and architects during the design and retrofit procedure.

Achieving Transmissive-to-Black Chromism Via Engineered Dual Electro-Thermoresponsive Single-Molecule for Full Modulation of Solar Transmittance

Achieving Transmissive-to-Black Chromism Via Engineered Dual Electro-Thermoresponsive Single-Molecule for Full Modulation of Solar Transmittance

Sputtered VO2 coatings on commercial glass substrates for smart glazing applications

VO2 is a well-known thermochromic material that can be potentially applied as a coating on “smart” glazing systems. In this work VO2 thermochromic films, were grown by the rf sputtering technique on two different types of commercial glass substrates, namely fused silica and float glass, at the low deposition temperature of 300 °C. The optimum deposition conditions, in order to succeed the thermochromic monoclinic phase of VO2(M), were investigated and found to be subject to substrate and thickness. It was observed that on the float glass substrate, only films with thicknesses below 50 nm grown with 3% O2 in plasma exhibited a thermochromic behavior, displaying a critical transition temperature of TC = 46.1 °C, width of transmittance hysteresis loop at 2000 nm ΔTC = 9.1 °C, IR-switching ΔTrIR = 14.6%, integrated solar transmittance modulation ΔTrsol = 1.3% and integrated luminous transmittance at 25 °C equal to Trlum = 36.6%. In contrast, films deposited on fused silica glass demonstrated a thermochromic phase, for all thicknesses, when grown with 1% O2 in the plasma. These thermochromic films were characterized by much lower TC(<40 °C) and ΔTC<10 °C, along with significant thickness variations of the ΔTrIR, ΔTrsol and Trlum parameters. Finally, rapid thermal annealing (RTA) was used and found to enhance the thermochromic performance of films deposited on float glass, with thickness greater than 50 nm. These differences might be attributed to the different composition of each kind of glass, as well as to diffusion phenomena between film and substrate.

A Phase-Changing Polymer Film for Broadband Smart Window Applications.

Smart windows (SWs) with tunable opacity are sought to regulate solar-irradiation and privacy protection. A new smart window material based on a phase-changing polymer that can be reversibly switched between a semicrystalline, opaque state and an amorphous, transparent state is introduced. The polymer film is a network of the phase-changing poly(stearyl acrylate) crosslinked with a poly(ethylene oxide) oligomer. The two constituent polymers show strong phase separation. The transmission switching of the resulting copolymer film is resulted from the combination of three different mechanisms: reversible phase changing of the poly(stearyl acrylate) component, phase separation between the two distinct constituent polymers, and a large change of refractive index of the phase-changing polymer during the amorphous-to-semicrystalline transition. The opaqueness switching can be reversed and repeated for more than 500 cycles of heating and cooling. A silver nanowire (AgNW)-based transparent heater is combined with the SW film to control the semicrystalline-to-amorphous phase transition. The resulting smart window exhibits a high infrared transmittance modulation (ΔTIR ) of 80.4% and solar transmittance modulation (ΔTsolar ) of 70.2%, which significantly outperform existing thermochromic smart windows.

High-Performance Planar Thin Film Thermochromic Window via Dynamic Optical Impedance Matching.

Window coatings with dynamic solar transmittance represent an excellent opportunity to reduce building heating and cooling loads, which account for >40% of energy consumed by the built environment. In particular, inorganic vanadium dioxide-based thermochromic coatings offer long lifetimes (>30 years) and can be passively integrated into a window system without additional electronics or power requirements. However, their limited solar modulation depth and wide phase-change hysteresis have traditionally restricted their ability to adapt to changing weather conditions. Here, we derive an optical performance limit for thin film vanadium dioxide coatings, which we find to be far beyond the current literature. Furthermore, we experimentally demonstrate a solution-processed multilayer thin film coating that uses temperature-dependent optical impedance matching to approach the optical performance limit. The thin film coating demonstrated has a record solar transmittance modulation of 21.8% while maintaining a high level of visible transparency (∼50%) and minimal hysteresis (∼10 °C). This work represents a step-change in thin film thermochromic window coatings and, as a result, establishes planar thin film vanadium dioxide as the most viable morphology for high-performance thermochromic windows.

Optical switching and nanothermochromic studies of VO2(M) nanoparticles prepared by mild thermolysis method

Vanadium dioxide (VO2) has been widely studied due to its promising thermochromic property. VO2 nanoparticles film exhibits a unique nanothermochromic advantage with higher light transmittance (Tlum) and larger solar light transmittance modulation (∆Tsol) than continuous film. In this paper, a mild thermolysis method was developed to prepare VO2 nanoparticles with controlled size and morphology, which was then used to build the nanothermochromic films. By using vanadyl ethylene glycol (VEG) nanorods as a precursor, VO2 nanoparticles can be obtained at a thermolysis temperature as low as 250 °C within 5 min under static argon atmosphere. The VEG nanorods with uniform size and excellent dispersity were prepared from amorphous vanadium oxide nanopowders synthesized by electrochemical anodic-oxidation of vanadium target. The influence of the thermolysis conditions on the microstructure, phase transition and thermochromic properties of VO2 nanoparticles were investigated and discussed. By combining with transparent conductive substrate ITO, the VO2 nanoparticles films demonstrate an active optical switching performance, and the optimal thermochromic properties with Tlum of 48% and ∆Tsol of 11% can be achieved.

High quality thermochromic VO2 films prepared by magnetron sputtering using V2O5 target with in situ annealing

Vanadium dioxide (VO2), a well-known Mott insulator and a prominent thermochromic material, exhibits an intelligent optical response from the infrared-transparent state to the infrared-blocking state below and above its transition temperature, respectively. Despite there have been numerous studies on its synthesis, a few could readily obtain high quality VO2 thin films except on single crystal epitaxial substrates. Its lack of versatility to form onto various substrates while retaining the specific function also needs to be addressed. Here, we present a sputtering method using V2O5 target with in situ annealing to prepare high-quality VO2 thin films on Si and quartz substrates. The results show that pure monoclinic VO2 could be obtained in a narrow range of oxygen flow ratio (from 0 to 0.74%) in Ar/O2 working gas during sputter deposition. The structural evolution was verified by Raman spectroscopy. At the transition temperature, the remarkably large optical transmittance contrasts of ~74% and ~77% were obtained at the incident wavelength of 2.5 and 3 μm, respectively. The luminous transmittance values at room temperature and high temperature were 36.63 and 38.87%, respectively. The measured solar transmittance modulation efficiency (∆Tsol) was 5.87%, which is comparable to or even better than the reported values of VO2 films prepared using sputtering. We also demonstrated the spontaneous response of electrical performance with applying potential and varying temperature. Our results revealed the facile preparation of large area and high-performance thermochromic VO2 thin film on non-epitaxial substrates of Si and quartz which greatly augment its applications in solar-heat managements and optoelectronics.