Anodic Electrochromism for Energy‐Efficient Windows: Cation/Anion‐Based Surface Processes and Effects of Crystal Facets in Nickel Oxide Thin Films

Anodic electrochromic (EC) oxides are of major interest as counter electrodes for smart window applications owing to their unique optical properties upon charge insertion and extraction. However, performance optimization of such oxides has been hampered by limited understanding of their EC mechanism, particularly in Li⁺‐conducting electrolytes. This paper reports on NiO_x films with 1.16 ≤ x ≤ 1.32, prepared by sputter deposition. These films are immersed in an electrolyte of lithium perchlorate in propylene carbonate, and EC properties are studied by cyclic voltammetry and in situ optical transmittance measurements. The electrochromism is significantly enhanced at large values of x. It has been found that charge exchange in Ni oxide is mainly due to surface processes and involves both cations and anions from the electrolyte, which is different from the case of cathodic EC materials such as WO₃. Whereas previous studies of Ni oxide have focused on cation intercalation, the cation/anion‐based mechanism presented here offers a new paradigm for designing and developing EC devices such as smart windows for energy efficient buildings.     

Bio‐Inspired Photonic Materials: Prototypes and Structural Effect Designs for Applications in Solar Energy Manipulation

Natural creatures have evolved elaborate photonic nanostructures on multiple scales and dimensions in a hierarchical, organized way to realize controllable absorption, reflection, or transmitting the desired wavelength of the solar spectrum. A bio‐inspired strategy is a powerful and promising way for solar energy manipulation. This feature article presents the state‐of‐the‐art progress on bio‐inspired photonic materials on this particular application. The article first briefly recalls the physical origins of natural photonic effects and catalogues the typical natural photonic prototypes including light harvesting, broadband reflection, selective reflection, and UV/IR response. Next, typical applications are categorized into two primary areas: solar energy utilization and reflection. Recent advances including solar‐to‐electricity, solar‐to‐fuels, solar‐thermal (e.g., photothermal converters, infrared detectors, thermoelectric materials, smart windows, and solar steam generation) are highlighted in the first part. Meanwhile, solar energy reflection involving infrared stealth, radiative cooling, and micromirrors are also addressed. In particular, this article focuses on bioinspired design principles, structural effects on functions, and future trends. Finally, the main challenges and prospects for the next generation of bioinspired photonic materials are discussed, including new design concepts, emerging ideas, and possible strategies.     

Thermochromic Lead‐Free Halide Double Perovskites

Lead‐free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature‐derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs₂AgBiBr₆ double perovskites is demonstrated. Along with the thermochromism, temperature‐dependent changes in the bond lengths of Ag? Br (R_{Ag? Br}) and Bi? Br (R_{Bi? Br}) are observed. The first‐principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the R_{Ag? Br} and R_{Bi? Br} at high temperatures. The synergy of anharmonic fluctuations and associated electron–phonon coupling, and the peculiar spin–orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs₂AgBiBr₆ shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead‐free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation‐induced bandgap narrowing of Cs₂AgBiBr₆ provide new insights for the further development of optoelectronic devices based on the lead‐free halide double perovskites.     

Responsive Smart Windows from Nanoparticle–Polymer Composites

Windows play significant roles in commercial and residential buildings and automobiles, which direct and control light illumination, thermal insulation, natural ventilation, and aesthetics. Various approaches are attempted to make windows “smart” by tailoring their transparency and thermal insulation in response to environmental changes. Hence, there has been much effort to develop smart windows that can dynamically modulate the transmission and reflectance of the visible light and solar radiance into buildings according to weather conditions or personal preferences. Development of smart window materials is also beneficial to applications including wearable sensors, energy harvesting and storage, and medical devices. By carefully matching the refractive indices of nanoparticle (NPs) and polymer matrix, surface chemistry, and their mechanical properties, particle‐embedded polymer composites can exhibit synergistic effects with improved chemical and mechanical stability, enhanced dispersion of NPs, and optimized and stimuli‐responsive optical properties. Here, an overview of recent progresses in the development of smart windows based on electro‐, thermo‐, and mechanoactuations is provided. Additional functionalities, e.g., flexibility, stretchability, and mechanical/chemical stability, can also be achieved by careful choices of NPs and polymers.     

Smart windows switch on the light

Optical switching devices for regulating incident solar energy that can replace traditional windows in buildings, vehicles, and aircraft are discussed. The chromogenic material in these smart windows exhibits a large change in optical properties with variation in applied electrical field, charge, light intensity, spectral composition, or temperature. The optical change transforms the material from a highly transmitting state to a partly reflecting (or absorbing) state over all or part of the visible and solar spectra. The optical switching can be activated electrically or nonelectrically. Electrically activated types, including electrochromic, liquid-crystal, and dispersed-particle (electrophoretic) devices, as well as devices based on reversible electrodeposition, and nonelectrically activated types, including devices based on photochromic and thermochromic materials, are described     

Semitransparent Energy‐Storing Functional Photovoltaics Monolithically Integrated with Electrochromic Supercapacitors

Energy‐storing functional photovoltaics, which can simultaneously harvest and store solar energy, are proposed as promising next‐generation multifunction energy systems. For the extension of conventional organic photovoltaics (OPVs), electrochromic supercapacitors (ECSs) are monolithically integrated with semitransparent (ST) quaternary blend‐based OPVs (ST Q‐OPVs) to achieve compact, energy‐efficient storage with great aesthetic appeal. In particular, ST Q‐OPVs with low‐power‐consumption ECSs allow full operation, even under low‐intensity irradiance, including artificial indoor light circumstances, and thereby exhibit potential for all‐day operating energy suppliers. The prepared ST energy‐storing functional photovoltaics also serve as a backup power source for external electronic equipment (e.g., light‐emitting diodes, and sensor nodes for Internet of Things) by consuming charged power. In addition to features that include unrestricted operation under any circumstances, color tunability, feasibility of designs with various shapes, rapid charging/discharging, and real‐time indication of stored energy levels, ST energy‐storing functional photovoltaics could potentially be applied in electronic devices such as advanced smart windows or portable smart electronics.     

Li4Ti5O12: A Visible‐to‐Infrared Broadband Electrochromic Material for Optical and Thermal Management

Broadband electrochromism from visible to infrared wavelengths is attractive for applications like smart windows, thermal camouflage, and temperature control. In this work, the broadband electrochromic properties of Li₄Ti₅O₁₂ (LTO) and its suitability for infrared camouflage and thermoregulation are investigated. Upon Li⁺ intercalation, LTO changes from a wide bandgap semiconductor to a metal, causing LTO nanoparticles on metal to transition from a super‐broadband optical reflector to a solar absorber and thermal emitter. Large tunabilities of 0.74, 0.68, and 0.30 are observed for the solar reflectance, mid‐wave infrared (MWIR) emittance, and long‐wave infrared (LWIR) emittance, respectively, with a tunability of 0.43 observed for a wavelength of 10 µm. The values exceed, or are comparable to notable performances in the literature. A promising cycling stability is also observed. MWIR and LWIR thermography reveal that the emittance of LTO‐based electrodes can be electrochemically tuned to conceal them amidst their environment. Moreover, under different sky conditions, LTO shows promising solar heating and subambient radiative cooling capabilities depending on the degree of lithiation and device design. The demonstrated capabilities of LTO make electrochromic devices based on LTO highly promising for infrared‐camouflage applications in the defense sector, and for thermoregulation in space and terrestrial environments.     

Windows of opportunity [smart windows for light/heat blocking]

Though the technology is still far from commonplace, the glass and window industry has steadily been introducing smart product lines because the market is, potentially, enormous. This article presents the different technologies being developed for smart windows. It details polymer-dispersed liquid-crystal privacy glass, suspended particle display (SPD) technology using electrophoresis, electrochromic layers, and thermochromic materials. For the future, being able to switch heat and light separately seems to be one of the main issues remaining to make smart windows really successful.     

Rear‐Illuminated Perovskite Photorechargeable Lithium Battery

Photovoltaic power‐conversion systems can harvest energy from sunlight almost perpetually whenever sunrays are accessible. Meanwhile, as indispensable energy storage units used in advanced technologies such as portable electronics, electric vehicles, and renewable/smart grids, batteries are energy‐limited closed systems and require constant recharging. Fusing these two essential technologies into a single device would create a sustainable power source. Here, it is demonstrated that such an integrated device can be realized by fusing a rear‐illuminated single‐junction perovskite solar cell with Li₄Ti₅O₁₂‐LiCoO₂ Li‐ion batteries, whose photocharging is enabled by an electronic converter via voltage matching. This design facilitates a straightforward monolithic stacking of the battery on the solar cell using a common metal substrate, which provides a robust mechanical isolation between the two systems while simultaneously providing an efficient electrical interconnection. This system delivers a high overall photoelectric conversion‐storage efficiency of 7.3%, outperforming previous efforts on stackable integrated architectures with organic–inorganic photovoltaics. Furthermore, converter electronics facilitates system control with battery management and maximum power point tracking, which are inevitable for efficient, safe, and reliable operation of practical loads. This work presents a significant advancement toward integrated photorechargeable energy storage systems as next‐generation power sources.     

Recent progress in thin-film cadmium telluride solar cells

Cadmium telluride (CdTe) with a room-temperature bandgap energy of 1.45 eV has been shown to be the most promising low-cost, thin-film photovoltaic material for terrestrial applications. Significant progress has been made during the past several years, and thin-film CdTe solar cells of > 1 cm² area with conversion efficiencies higher than 12% have been prepared by several techniques. Thin-film CdTe photovoltaic modules with 10% efficiency have also been produced. They are of the heterojuntion configuration using a transparent conducting semiconductor (TCS) as the window and p-CdTe as the absorber. In this paper, the potential window materials for thin-film CdTe solar cells are discussed. Thus far, cadmium sulphide (CdS) with a bandgap energy of 2.42 eV at room temperature has been found to be best suited for efficient CdTe solar cells. the deposition techniques for p-CdTe films capable of producing efficient solar cells, including close-spaced sublimation (CSS), electrodeposition, screen printing and spraying, are briefly reviewed, and the characteristics of the resulting solar cells are discussed. It is concluded that the efficiency of thin-film CdTe solar cells can be increased to 18-19% in the near-term, leading to 15-16.5% efficient modules.     

Solution‐Processed Bulk‐Heterojunction Solar Cells Based on Monodisperse Dendritic Oligothiophenes

A novel family of soluble conjugated dendritic oligothiophenes (DOTs) as monodisperse 3D macromolecular architectures was characterized with respect to optical and redox properties in solution and in solid films. Band gaps of 2.5–2.2 eV, typical for organic semiconductors, were determined as well as HOMO/LUMO energy levels ideal for efficient electron transfer to acceptors such as [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) identifying them as suitable materials for solar cell applications. Solution‐processed bulk‐heterojunction solar cells using DOTs as electron donor and PCBM as acceptor were prepared and investigated. High open‐circuit voltages V_OC of 1.0 V and power‐conversion efficiencies up to 1.72% were obtained for the DOT‐based devices. The higher generations DOTs provide the highest efficiencies. Based on the monodispersity of the DOTs, an analysis of the molar ratio between donor and acceptor in the blended film was possible leading to an optimal value of five to six thiophene units per PCBM.     

Fully Solution‐Processed Small Molecule Semitransparent Solar Cells: Optimization of Transparent Cathode Architecture and Four Absorbing Layers

Semitransparent solar cells (SSCs) can open photovoltaic applications in many commercial areas, such as power‐generating windows and building integrated photovoltaics. This study successfully demonstrates solution‐processed small molecule SSCs with a conventional configuration for the presently tested material systems, namely BDTT‐S‐TR:PC₇₀BM, N(Ph‐2T‐DCN‐Et)₃:PC₇₀BM, SMPV1:PC₇₀BM, and UU07:PC₆₀BM. The top transparent cathode coated through solution processes employs a highly transparent silver nanowire as electrode together with a combination interface bilayer of zinc oxide nanoparticles (ZnO) and a perylene diimide derivative (PDINO). This ZnO/PDINO bilayer not only serves as an effective cathode buffer layer but also acts as a protective film on top of the active layer. With this integrated contribution, this study achieves a power conversion efficiency (PCE) of 3.62% for fully solution‐processed SSCs based on BDTT‐S‐TR system. Furthermore, the other three systems with various colors exhibited the PCEs close to 3% as expected from simulations, demonstrate the practicality and versatility of this printed semitransparent device architecture for small mole­cule systems. This work amplifies the potential of small molecule solar cells for window integration.     

Room‐Temperature Dielectric Switchable Nanocomposites

Room‐temperature switchable dielectric materials are of interest for many applications, including solar energy storage, smart switches, automatic filters, and next‐generation sensors. Here, a temperature‐triggered dielectric switchable nanocomposite by dispersing octadecylamine‐grafted multiwalled carbon nanotubes (ODA‐MWCNTs, for short) into hexadecane is reported. The composite has low permittivity at molten state and high permittivity at frozen state, and the permittivity switch is triggered around 18 °C. The highest permittivity contrast ratio reaches 106.4 at 2.0% CNT volume fraction. The composite shows frequency‐sensitive and temperature‐ramping‐rate‐sensitive properties. Further investigation indicates that the permittivity switch is caused by the change of the ODA‐MWCNT percolating networks during phase transition.     

Pyrite‐Based Bi‐Functional Layer for Long‐Term Stability and High‐Performance of Organo‐Lead Halide Perovskite Solar Cells

Organo‐lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine‐capped pyrite nanoparticles (ODA‐FeS₂ NPs) as a bi‐functional layer (charge extraction layer and moisture‐proof layer) for organo‐lead halide PSCs. FeS₂ is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi‐functional layer based on ODA‐FeS₂ NPs shows excellent hole transport ability and moisture‐proof performance. Through this approach, the best‐performing device with ODA‐FeS₂ NPs‐based bi‐functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.     

A Single‐Step Dual Stabilization of Smart Window by the Formation of Liquid Crystal Physical Gels and the Construction of Liquid Crystal Chambers

Smart windows are very attractive because they not only provide comfortable indoor conditions for cars and buildings, but also protect privacy. However, current smart windows have problems such as high energy consumption, slow response time, and poor stability. To solve these problems, a single‐step dual stabilization (SSDS) is newly proposed for the fabrication of robust liquid crystal (LC) smart windows switching fast at low voltage. Upon irradiating ultraviolet light on the selected area of the nematic (N) LC optical cell with photoisomerizable macrogelators (B3AZ) and photopolymerizable monomers, NLC physical gels (LCPGs) and partition walls are simultaneously constructed. LCPGs play a role of light shutter under a low electric field (9.6 V_pp) which is ten times lower than that of the conventional polymer‐stabilized LC‐based smart windows. Partition walls constructed by the selected area photopolymerization significantly enhance the mechanical stabilities. Based on the experimental results, it is realized that the NLC layer generated near the partition walls makes the LCPGs respond to a low voltage. Robust SSDS smart windows could open new doors for the development of high‐performance smart windows.     

Polyfluorene Derivatives are High‐Performance Organic Hole‐Transporting Materials for Inorganic−Organic Hybrid Perovskite Solar Cells

Photovoltaics based on organic?inorganic perovskites offer new promise to address the contemporary energy and environmental issues. These solar cells have so far largely relied on small‐molecule hole transport materials such as spiro‐OMeTAD, which commonly suffer from high cost and low mobility. In principle, polyfluorene copolymers can be an ideal alternative to spiro‐OMeTAD, given their low price, high hole mobility and good processability, but this potential has not been explored. Herein, polyfluorene derived polymers‐TFB and PFB, which contain fluorine and arylamine groups, are demonstrated and can indeed rival or even outperform spiro‐OMeTAD as efficient hole‐conducting materials for perovskite solar cells. In particular, under the one‐step perovskite deposition condition, TFB achieves a 10.92% power conversion efficiency that is considerably higher than that with spiro‐OMeTAD (9.78%), while using the two‐step perovskite deposition method, about 13% efficient solar cells with TFB (12.80%) and spiro‐OMeTAD (13.58%) are delivered. Photo­luminescence reveals the efficient hole extraction and diffusion at the interface between CH₃NH₃PbI₃ and the hole conducting polymer. Impedance spectroscopy uncovers the higher electrical conductivity and lower series resistance than spiro‐OMeTAD, accounting for the significantly higher fill factor, photocurrent and open‐circuit voltage of the TFB‐derived cells than with spiro‐MeOTAD.     

Sliding‐window forward error correction using Reed‐Solomon code and unequal error protection for real‐time streaming video

Forward error correction (FEC) techniques are widely used to recover packet losses over unreliable networks in real‐time video streaming applications. Traditional frame‐level FEC encodes 1 video frame in each FEC coding window. By contrast, in the expanding‐window FEC scheme, high‐priority frames are included in the FEC processing of the following frames, so as to construct a larger coding window. In general, expanding‐window FEC improves the recovery performance of FEC, because the high‐priority frame can be protected by multiple windows and the use of a larger coding window increases the efficiency. However, the larger window size also increases the complexity of the coding and the memory space requirements. Consequently, expanding‐window FEC is limited in terms of practical applications. Sliding‐window FEC adopts a fixed window size in order to approximate the performance of the expanding‐window FEC method, but with a reduced complexity. Previous studies on sliding‐window FEC have generally adopted an equal error protection (EEP) mechanism to simplify the analysis. This paper considers the more practical case of an unequal error protection (UEP) strategy. An analytical model is derived for estimating the playable frame rate (PFR) of the proposed sliding‐window FEC scheme with a Reed‐Solomon erasure code for real‐time non‐scalable streaming applications. The analytical model is used to determine the optimal FEC configuration which maximizes the PFR value under given transmission rate constraints. The simulation results show that the proposed sliding‐window scheme achieves almost the same performance as the expanding‐window scheme, but with a significantly lower computational complexity.     

Diving–Surfacing Smart Locomotion Driven by a CO2‐Forming Reaction,with Applications to Minigenerators

Harvesting energy from environment has attracted increasing attention for its potential applications in fabricating minigenerator. However, most studies in the fabrication of mini‐ or nanogenerators are based on the concept of piezoelectricity or triboelectrification while few of the reports paid attention to the classical theory of Faraday's law. Herein, a pH responsive smart surface is combined with the reaction between CaCO₃ and HCl to develop a new minigenerator, which can convert mechanical energy generated from the chemical reaction into electrical energy through cutting magnetic lines with moving conductive lines. The conductive lines are connected with a smart device consisting of a pH‐responsive cube, a hydrophobic cube, and a quartz cell window; the device can perform diving‐surfacing cycled motions with an intelligent initiation through the adjustment of the solution. The device can surface through gathering CO₂ bubbles from the reaction between CaCO₃ and HCl and dive by releasing the bubbles on the water/air interface. Moreover, the results demonstrate that the inert CO₂ was nonhazardous to the smart surfaces, which is meaningful for durable electricity generation.     

Functional Free‐Standing Graphene Honeycomb Films

Fabricating free‐standing, three‐dimensional (3D) ordered porous graphene structure can service a wide range of functional materials such as environmentally friendly materials for antibacterial medical applications and efficient solar harvesting devices. A scalable solution processable strategy is developed to create such free‐standing hierarchical porous structures composed of functionalized graphene sheets via an “on water spreading” method. The free‐standing film shows a large area uniform honeycomb structure and can be transferred onto any substrate of interest. The graphene‐based free‐standing honeycomb films exhibit superior broad spectrum antibacterial activity as confirmed using green fluorescent protein labeled Pseudomonas aeruginosa PAO1 and Escherichia coli as model pathogens. Functional nanoparticles such as titanium dioxide (TiO₂) nanoparticles can be easily introduced into conductive graphene‐based scaffolds by premixing. The formed composite honeycomb film electrode shows a fast, stable, and completely reversible photocurrent response accompanying each switch‐on and switch‐off event. The graphene‐based honeycomb scaffold enhances the light‐harvesting efficiency and improves the photoelectric conversion behavior; the photocurrent of the composite film is about two times as high as that of the pure TiO₂ film electrode. Such composite porous films combining remarkably good electrochemical performance of graphene, a large electrode/electrolyte contact area, and excellent stability during the photo‐conversion process hold promise for further applications in water treatment and solar energy conversion.     

Single‐Thread‐Based Wearable and Highly Stretchable Triboelectric Nanogenerators and Their Applications in Cloth‐Based Self‐Powered Human‐Interactive and Biomedical Sensing

The development of wearable and large‐area fabric energy harvester and sensor has received great attention due to their promising applications in next‐generation autonomous and wearable healthcare technologies. Here, a new type of “single” thread‐based triboelectric nanogenerator (TENG) and its uses in elastically textile‐based energy harvesting and sensing have been demonstrated. The energy‐harvesting thread composed by one silicone‐rubber‐coated stainless‐steel thread can extract energy during contact with skin. With sewing the energy‐harvesting thread into a serpentine shape on an elastic textile, a highly stretchable and scalable TENG textile is realized to scavenge various kinds of human‐motion energy. The collected energy is capable to sustainably power a commercial smart watch. Moreover, the simplified single triboelectric thread can be applied in a wide range of thread‐based self‐powered and active sensing uses, including gesture sensing, human‐interactive interfaces, and human physiological signal monitoring. After integration with microcontrollers, more complicated systems, such as wireless wearable keyboards and smart beds, are demonstrated. These results show that the newly designed single‐thread‐based TENG, with the advantage of interactive, responsive, sewable, and conformal features, can meet application needs of a vast variety of fields, ranging from wearable and stretchable energy harvesters to smart cloth‐based articles.