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Advanced constructive materials, such as electrochromic smart windows (ECSWs) and building integrated photovoltaics modules (BIPV), can improve the energy efficiency in buildings. A good optical and thermal characterization of these elements is necessary to assess and compare their performance. The existing testing procedures for glass in buildings are applied to both types of elements, and it is considered that while the optical procedures are suitable and allow good comparison of the two technologies, the indoor thermal testing procedures are not valid for BIPV nor ECSWs, because temperature of these absorbing elements strongly depend on the irradiance, something not considered in the current standards. To show and characterize this dependence, simultaneously monitoring of different photovoltaics (PV) modules and electrochromic windows has been performed outdoors under solar irradiance. A relationship between the surface temperature, the irradiance, and the ambient temperature has been obtained for each sample to compare both technologies.

Thermochromic smart windows technology can intelligently regulate indoor solar radiation by changing indoor light transmittance in response to thermal stimulation, thus reducing energy consumption of the building. In recent years, with the development of new energy-saving materials and the combination with practical technology, energy-saving smart windows technology has received more and more attention from scientific research. Based on the summary of thermochromic smart windows by Yi Long research groups, this review described the applications of thermal responsive organic materials in smart windows, including poly(N-isopropylacrylamide) (PNIPAm) hydrogels, hydroxypropyl cellulose (HPC) hydrogels, ionic liquids and liquid crystals. Besides, the mechanism of various organic materials and the properties of functional materials were also introduced. Finally, opportunities and challenges relating to thermochromic smart windows and prospects for future development are discussed.

A smart window is an optical dimming device with intelligent functions that can control its relevant performances through external stimuli, achieving functions such as privacy protection and temperature regulation. Light is an ideal stimulus for regulating smart windows, which is noninvasive and allows self‐adaptable manipulation of materials. This review highlights recent significant achievements in smart windows constructed by photo‐responsive liquid crystals (LCs) systems that can undergo the transition between different phases. The smart windows based on photo‐responsive LCs are used in a plethora of areas, including privacy protection, absorption glass, building decoration, energy saving, and climate modulation applications. The review concludes with a brief perspective on some significant challenges and opportunities for the future development of photo‐responsive smart windows, which is crucial for expanding the applications of smart windows and improving their performances. Smart windows based on photo‐responsive LC system doped with photoswitches: Photoisomerization of photoswitches leads to the phase transition of the LC system because of the change of the LC orientation. As a result, smart windows based on the photo‐responsive LC system appear in different optical states at various LC phases. Here, we review the working principles and applications of smart windows based on photo‐responsive LCs featuring phase transition.

Perovskite‐based thermochromic smart windows that can change color have attracted much interest. However, the high transition temperature (>45 °C in air) hinders their practical application. Herein, a near‐infrared (NIR) activated thermochromic perovskite window that enables reversible transition cycles at room temperature is proposed. Under natural sunlight (>700 W m−2), it efficiently harvests 78% NIR light to trigger the thermochromism of perovskites, blocking the heat gain from both the visible and NIR light. Meanwhile, it also exhibits a low mid‐infrared emissivity of <0.3, suppressing thermal radiation to the indoor environment. A field test demonstrates that this smart window can reduce the indoor temperature by 8 °C compared to a normal glass window at noon. The near‐room‐temperature color change, multispectral thermal management, outstanding energy‐saving ability, and climate adaptability, and solution‐based process of this window make it unique and promising for real applications. A self‐activated thermochromic perovskite smart window (T‐PCL window) is developed. This window can smartly harvest the near‐IR part of sunlight to trigger the thermochromism at room temperature. Simultaneously, the T‐PCL window demonstrates room‐temperature solar regulation ability for visible light, shielding effect for near‐IR, and low thermal radiation for mid‐infrared, showing a promising future for application on energy‐efficient buildings.

Human–computer interfaces, smart glasses, touch screens, and some electronic skins require highly transparent and flexible pressure‐sensing elements. Flexible pressure sensors often apply a microstructured or porous active material to improve their sensitivity and response speed. However, the microstructures or small pores will result in high haze and low transparency of the device, and thus it is challenging to balance the sensitivity and transparency simultaneously in flexible pressure sensors or electronic skins. Here, for a capacitive‐type sensor that consists of a porous polyvinylidene fluoride (PVDF) film sandwiched between two transparent electrodes, the challenge is addressed by filling the pores with ionic liquid that has the same refractive index with PVDF, and the transmittance of the film dramatically boosts from 0 to 94.8% in the visible range. Apart from optical matching, the ionic liquid also significantly improves the signal intensity as well as the sensitivity due to the formation of an electric double layer at the dielectric‐electrode interfaces, and improves the toughness and stretchability of the active material benefiting from a plasticization effect. Such transparent and flexible sensors will be useful in smart windows, invisible bands, and so forth. An opaque to transparent transition happens when a microporous dielectric is filled with ionic liquid with a close refractive index. For a capacitive‐type pressure sensor with a porous dielectric, the filling of ionic liquid can significantly improve its transparency to 90%, and enhance its sensitivity by introducing electric double layers, thus enabling wide applications.

Energy consumption of buildings during heating and cooling accounts for about 15% of global total energy consumption. Advanced dynamic switchable windows that enable independent control of solar heat will contribute to optimal energy efficiency in heating, cooling, and artificial lighting systems throughout buildings. Recently, energy‐efficient plasmonic electrochromic smart windows (PESWs) based on metal oxide nanocrystals (NCs) have been gaining increasing attention due to their effective and controllable regulation over the near‐infrared region of the solar spectrum without affecting the dynamic visible transmittance of the smart windows. Herein, the current state‐of‐the‐art results of colloidal metal oxide NCs for PESWs are highlighted, along with their design strategies and working principles. The recent research status of PESWs in typical colloidal metal oxide NCs is reviewed in detail, and the challenges and corresponding countermeasures in this field are discussed. Furthermore, an outlook into novel opportunities in PESW‐related academic research and how to accelerate the pace of real‐world applications is presented. Plasmonic electrochromic smart windows (PESWs) based on metal oxide nanocrystals are promising to reduce building energy consumption via dynamically and selectively regulating the near‐infrared light of sunshine without affecting the visible transmittance tunability of the smart windows. Herein, the recent research status, challenges and perspectives of metal oxide nanocrystal based PESWs are summarized and discussed.

HighlightsTransition metal phosphates were first developed for the electrochromic application.The obtained NiHPO4·3H2O film achieves an ultra-large optical modulation of 90.8%, and the electrochromic mechanism is systematically elucidated using in situ and ex situ techniques.A large-area electrochromic smart window with 100 cm2 is constructed, which displays superior performances in regulating natural lighting and storing electrical charges.Exploring materials with high electrochemical activity is of keen interest for electrochemistry-controlled optical and energy storage devices. However, it remains a great challenge for transition metal oxides to meet this feature due to their low electron conductivity and insufficient reaction sites. Here, we propose a type of transition metal phosphate (NiHPO4·3H2O, NHP) by a facile and scalable electrodeposition method, which can achieve the capability of efficient ion accommodation and injection/extraction for electrochromic energy storage applications. Specifically, the NHP film with an ultra-high transmittance (approach to 100%) achieves a large optical modulation (90.8% at 500 nm), high coloration efficiency (75.4 cm2 C−1 at 500 nm), and a high specific capacity of 47.8 mAh g−1 at 0.4 A g−1. Furthermore, the transformation mechanism of NHP upon electrochemical reaction is systematically elucidated using in situ and ex situ techniques. Ultimately, a large-area electrochromic smart window with 100 cm2 is constructed based on the NHP electrode, displaying superior electrochromic energy storage performance in regulating natural light and storing electrical charges. Our findings may open up new strategies for developing advanced electrochromic energy storage materials and smart windows.

HighlightsA reversible Zn2+ insertion in anatase TiO2 nanocrystals is reported for the first time.This is the first report regarding TiO2 for zinc-anode-based electrochromic devices, which will subsequently broaden its applications to zinc-ion electrochemical cells.A prototype device based on the TiO2 nanocrystals delivers a high optical modulation, fast response times, and robust electrochemical stability.Zinc-anode-based electrochromic devices (ZECDs) are emerging as the next-generation energy-efficient transparent electronics. We report anatase W-doped TiO2 nanocrystals (NCs) as a Zn2+ active electrochromic material. It demonstrates that the W doping in TiO2 highly reduces the Zn2+ intercalation energy, thus triggering the electrochromism. The prototype ZECDs based on W-doped TiO2 NCs deliver a high optical modulation (66% at 550 nm), fast spectral response times (9/2.7 s at 550 nm for coloration/bleaching), and good electrochemical stability (8.2% optical modulation loss after 1000 cycles).

Though thermoresponsive lower critical solution temperature (LCST) hydrogels have attracted intense attention to be applied in smart windows, less efforts are paid on the LCST effect on the heat transfer process. Herein, the research mainly focuses on heat transfer process of thermoresponsive PNIPAm hydrogels. It is the first time reported that LCST behaviors can decrease thermal conductivity upon heating process. To be utilized as a smart window, thermal conduction flux is investigated yet the thermal conduction energy occupies little to manage the thermal transfer process in a model house. It is found that radiation thermal transfer predominates the heat transfer process for PNIPAm‐based smart windows. These results are meaningful to provide basic data for energy transfer in using thermoresponsive hydrogels. The research mainly focused on the heat transfer process of thermoresponsive PNIPAm hydrogels. Lower critical solution temperature behaviors decreased thermal conductivity upon the heating process.In a model house, thermal conduction flux through the PNIPAm hydrogel window occupied little in the thermal transfer process, while radiation thermal transfer predominated the heat transfer process.

High-performance glazing technologies are essential for achieving the occupant comfort and building energy efficiency required in contemporary and future buildings. In real-world applications, glazing façades are selected from a steadily increasing number of glazing technologies. However, the authors could not identify a systematic and comprehensive review and classification of glazing technologies in the literature. This creates a barrier when comparing typologically different glazing technologies and combining multiple technologies in a glazing unit. This paper provides a systematic review and classification of established and emerging glazing technologies based on publications from 2001–2022 which were interpreted following the PRISMA methodology. This study reveals that the majority of high-performance glazing systems used in practice are in multi-layer glazing configurations and that the glazing system performance can focus on including additional and multiple functionalities, which aim at improving overall building performance. It was also found that there is a large potential for improvement of multilayer, evacuated, aerogels, electrochromic, and solar cell glazing by incorporating other technologies or innovative materials in multi-layer glazing units for either improving existing technologies or for the development of new ones. However, their longevity, robustness, and cost affordability should be ensured.