Low-Temperature Thermally Annealed Niobium Oxide Thin Films as a Minimally Color Changing Ion Storage Layer in Solution-Processed Polymer Electrochromic Devices

Amorphous Mixed-Vanadium-Tungsten Oxide Films as Optically Passive Ion Storage Materials for Solid-State Near-Infrared Electrochromic Devices

Near infrared (NIR) electrochromic (EC) devices that selectively modulate the NIR light without affecting the daylight represent a promising window technology for saving energy consumption of buildings. Current research efforts have been focused on developing NIR-EC materials, while little attention has been directed to the optically passive ion storage materials that are crucial for balancing charges in a full NIR-EC device. Herein, we report that amorphous phase mixed-vanadium-tungsten oxide films exhibit minimum optical change with high ion storage capacity, which enables the usage of the mixed-metal oxides as optically passive counter electrode materials for NIR-EC devices. The mixed-vanadium-tungsten oxide films are synthesized by a room-temperature solution-based photodeposition method that allows us to precisely engineer the metal compositions and thicknesses of the mixed-metal oxide films, thus optimizing their optical inertness and ion storage capability. A solid-state NIR-EC device assembled with the mixed-vanadium-tungsten oxide film as an ion storage layer and the amorphous tungsten oxide hydrate as the NIR-EC layer shows fast response speed with cycling stability up to 10,000 cycles, proving the outstanding charge balancing capability of mixed-metal oxide. Our work provides an efficient strategy for developing optically passive ion storage films with high ion storage capability for high-performance EC devices.

A Review on Mechanochemistry: Approaching Advanced Energy Materials with Greener Force

Mechanochemistry with solvent-free and environmentally friendly characteristics is one of the most promising alternatives to traditional liquid-phase-based reactions, demonstrating epoch-making significance in the realization of different types of chemistry. Mechanochemistry utilizes mechanical energy to promote physical and chemical transformations to design complex molecules and nanostructured materials, encourage dispersion and recombination of multiphase components, and accelerate reaction rates and efficiencies via highly reactive surfaces. In particular, mechanochemistry deserves special attention because it is capable of endowing energy materials with unique characteristics and properties. Herein, the latest advances and progress in mechanochemistry for the preparation and modification of energy materials are reviewed. An outline of the basic knowledge, methods, and characteristics of different mechanochemical strategies is presented, distinguishing this review from most mechanochemistry reviews that only focus on ball-milling. Next, this outline is followed by a detailed and insightful discussion of mechanochemistry-involved energy conversion and storage applications. The discussion comprehensively covers aspects of energy transformations from mechanical/optical/chemical energy to electrical energy. Finally, next-generation advanced energy materials are proposed. This review is intended to bring mechanochemistry to the frontline and guide this burgeoning field of interdisciplinary research for developing advanced energy materials with greener mechanical force.

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

Macroporous Vanadium Oxide Ion Storage Films Enable Fast Switching Speed and High Cycling Stability of Electrochromic Devices

Compared to the significant effort dedicated toward developing efficient electrochromic materials for the working electrodes of electrochromic (EC) devices, the attention paid to developing ion storage counter electrode materials for EC devices has been trivial. Herein, we report that a macroporous crystalline V₂O₅ film as an ion storage layer paired with a WO₃ working electrode results in an EC device with high performance. The macroporous vanadium oxide films are prepared by a simple template-free photodeposition method that allows us to tune the thickness and crystallinity of the film, thus giving access to a full EC device with optimal EC performance: short response time of about 2 s, high electrochromic cycling stability up to 10,000 times, long memory effect over 24 h, and an exceedingly high coloration efficiency of 189 cm²/C that are superior to the state-of-the-art performance of solution-processed vanadium oxide based EC devices. The extraordinary EC performance can be attributed to the macroporous structure, high crystallinity, and optimized thickness of the vanadium oxide films that boost the charge-balancing capability of the films. The easy and controllable preparation and the efficient charge-balancing capability of the macroporous vanadium oxide film make it a promising ion storage material for developing high-performance EC devices.

Bioinspired Dynamic Camouflage from Colloidal Nanocrystals Embedded Electrochromics

Camouflage is often seen in animals, and it presents in both passive and active forms. For instance, the wings of Closterocerus coffeellae exhibit distinct appearances against different backgrounds, while the chameleon actively changes its skin colors to morph into the environment. Herein, we report an artificial skin-like optoelectronic device that enables actively changing appearances and passively morphing into the environment by manipulating light-matter interactions with electrochromic polymers and photonic colloid nanocrystals. To construct the new electrochromic device, highly reflective, yet transmissive photonic nanocrystals are introduced into the gel electrolyte and sandwiched between the layers of electrochromic polymers and ion storage materials. Through voltage-controlled color switching of electrochromic polymers from colored state to bleached state, the degree of light absorbance, transmittance, and reflectance can be finely balanced and precisely modulated with the device. A broad synthesized color gamut and angle-dependent visual effects can be realized on this electronic skin-like device.

Dual-Cation Electrolytes Crosslinked with MXene for High-Performance Electrochromic Devices

MXene, a 2D material, is used as a filler to manufacture polymer electrolytes with high ionic conductivity because of its unique sheet shape, large specific surface area and high aspect ratio. Because MXene has numerous -OH groups on its surface, it can cause dehydration and condensation reactions with poly(4-styrenesulfonic acid) (PSSA) and consequently create pathways for the conduction of cations. The movement of Grotthuss-type hydrogen ions along the cation-conduction pathway is promoted and a high ionic conductivity can be obtained. In addition, when electrolytes composed of a conventional acid or metal salt alone is applied to an electrochromic device (ECD), it does not bring out fast response time, high coloration efficiency and transmittance contrast simultaneously. Therefore, dual-cation electrolytes are designed for high-performance ECDs. Bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSI) was used as a source of lithium ions and PSSA crosslinked with MXene was used as a source of protons. Dual-Cation electrolytes crosslinked with MXene was applied to an indium tin oxide-free, all-solution-processable ECD. The effect of applying the electrolyte to the device was verified in terms of response time, coloration efficiency and transmittance contrast. The ECD with a size of 5 × 5 cm2 showed a high transmittance contrast of 66.7%, fast response time (8 s/15 s) and high coloration efficiency of 340.6 cm2/C.

Stabilizing Hybrid Electrochromic Devices through Pairing Electrochromic Polymers with Minimally Color-Changing Ion-Storage Materials Having Closely Matched Electroactive Voltage Windows

Conjugated electrochromic polymers hold great promises for next-generation color-changing windows and displays. One of the major roadblocks in their solid-state electrochromic devices is the relatively poor cycling stability. Finding ion-storage materials as a charge-balancing component is critical in improving their electrochemical cycling stability. A key criterion for the selection of ion-storage materials is to match their electroactive voltage windows with the paired electrochromic polymers. Thus, we developed a nanostructured, amorphous vanadium oxide (VO_x) ion-storage material that exhibits high transmissivity, minimal color interference, and excellent charge-balancing capability in a closely matched electroactive window with the paired electrochromic polymers. High-performance magenta-to-transmissive hybrid electrochromic devices constructed from pairing the polymer with the VO_x can be reversibly switched for 50 000 cycles with an optical loss less than 5%.

Design and Synthesis of Annulated Benzothiadiazoles via Dithiolate Formation for Ambipolar Organic Semiconductors

Substituted 2,1,3-benzothiadiazole (BTD) is a widely used electron acceptor unit for functional organic semiconductors. Difluorination or annulation on the 5,6-position of the benzene ring is among the most adapted chemical modifications to tune the electronic properties, though each sees its own limitations in regulating the frontier orbital levels. Herein, a hitherto unreported 5,6-annulated BTD acceptor, denoted as ssBTD, is designed and synthesized by incorporating an electron-withdrawing 2-(1,3-dithiol-2-ylidene)malononitrile moiety via aromatic nucleophilic substitution of the 5,6-difluoroBTD (ffBTD) precursor. Unlike the other reported BTD annulation strategies, this modification leads to the simultaneous decrease in both frontier orbital energies, a welcoming feature for photovoltaic applications. Incorporation of ssBTD into conjugated polymers results in materials boasting broad light absorption, dramatic solvatochromic and thermochromic responses (>100 nm shift and a band gap difference of ∼0.28 eV), and improved crystallinity in the solid state. Such physical properties are in accordance with the combined electron-withdrawing effect and significantly increased polarity associated with the ssBTD unit, as revealed by detailed theoretical studies. Furthermore, the thiolated ssBTD imbues the polymer with ambipolar charge transport property, in contrast to the ffBTD-based polymer, which transports holes only. While the low mobilities (10^-4 to 10^-5 cm² V^-1 s^-1) could be further optimized, detailed studies validate that the thioannulated BTD is a versatile electron-accepting unit for the design of functional stimuli-responsive optoelectronic materials.

Mechanical breathing in organic electrochromics

The repetitive size change of the electrode over cycles, termed as mechanical breathing, is a crucial issue limiting the quality and lifetime of organic electrochromic devices. The mechanical deformation originates from the electron transport and ion intercalation in the redox active material. The dynamics of the state of charge induces drastic changes of the microstructure and properties of the host, and ultimately leads to structural disintegration at the interfaces. We quantify the breathing strain and the evolution of the mechanical properties of poly(3,4-propylenedioxythiophene) thin films in-situ using customized environmental nanoindentation. Upon oxidation, the film expands nearly 30% in volume, and the elastic modulus and hardness decrease by a factor of two. We perform theoretical modeling to understand thin film delamination from an indium tin oxide (ITO) current collector under cyclic load. We show that toughening the interface with roughened or silica-nanoparticle coated ITO surface significantly improves the cyclic performance.

Advances in nanomaterials for electrochromic devices

This review article systematically highlights the recent advances regarding the design, preparation, performance and application of new and unique nanomaterials for electrochromic devices.

Dual-Band Electrochromic Devices with a Transparent Conductive Capacitive Charge-Balancing Anode

Dual-band electrochromic devices (DBEDs), which can selectively modulate near-infrared (NIR) and visible (VIS) light transmittance through electrochromism, have gained increasing interest as a building energy saving technology. The technology is strongly dependent on the progress in electrochromic materials. Most current research has focused on the dual-band properties of the cathode materials, leaving the charge-balancing anode materials under-explored by comparison. This is a report of our study on the suitability of tin-doped indium oxide (ITO) nanocrystals (NCs) as a capacitive anode material for DBEDs. The ITO NCs are electrically conductive and VIS light transparent throughout the device operating range. As a result, they would not affect the NIR-selective modulation of the electrochromic device like most other anode materials do. The high surface area and good conductivity of the ITO NCs facilitate the adsorption/desorption of anions; thereby increasing their effectiveness as an ion storage thin film on the anode to balance the cathode charge. The best DBED prototype assembled from an ITO NC anode and a WO_3-x cathode showed effective and independent control of VIS light and NIR transmittance with high optical modulation (71.1% at 633 nm, 58.1% at 1200 nm), high coloration efficiency (95 cm² C^-1 at 633 nm, 220 cm² C^-1 at 1200 nm), fast switching speed, good bistability, and cycle stability.