Electrochromic Films Research Articles

Enhancement of electrochemical reversibility in Poly 3-hexylthiophene layers for flexible dynamic glass windows applications

Thin films of Poly 3-hexylthiophene (P3HT) were deposited onto Indium Tin Oxide (ITO) coated glass substrates using a spin coating method and the films were vacuum annealed at different temperatures to study the effect of annealing on the electrochemical reversibility. From the absorption spectra, the variation of the optical band gap with annealing temperature was studied for different annealing temperatures. The optical band gap obtained for P3HT film deposited at room temperature (RT) was around 1.94 eV and 1.92 eV for the sample annealed at 100 °C. A cyclic voltammetry (CV) analysis showed that at certain applied redox potentials, P3HT thin films changed the colour from magenta to transparent and then back to magenta in the reverse potential. P3HT films annealed at 100 °C showed the best electrochemical reversibility among all the annealed samples. These electrically switchable electrochromic P3HT thin films can be used in flexible dynamic windows (FDWs) as smart windows for controlling the temperature and light in buildings, displays and mirror light modulators.

Correlation of Structural Changes and Hydrogen Diffusion in Polycrystalline WO3 Thin Films by Combining In Situ Transmission Measurements and Raman Spectroscopy

AbstractDiffusion in polycrystalline tungsten trioxide (WO3) thin films is studied in a lateral geometry to better understand the impact of hydrogen‐induced structural phase transitions on the diffusion. WO3 thin films are coated with polymethylmethacrylate layer (PMMA). The latter is microstructured in such a way that a narrow stripe‐like gap occurs in the PMMA layer exposing the surface of the WO3 thin film. This stripe serves as the contact to the electrolyte in the intercalation experiment with hydrogen. After intercalation, the lateral diffusion of hydrogen inside WO3 below the PMMA layer can be observed, increasing the analyzable path and time scale by several orders of magnitude compared to the film thickness, thus, significantly improving spatial and temporal resolution of in situ transmission and Raman measurements. Spatially resolved transmission measurements in the wavelength range of 633±55 nm show that the diffusion process is dependent on hydrogen concentration and exhibits two regimes describable by different diffusion coefficients. Time‐resolved Raman spectroscopic measurements at different distances from the electrolyte contact area show that the switching between the two diffusion coefficients occurs at the phase transition from the orthorhombic to the tetragonal phase. The results are further supported by a simulation. The measurement approach is universally applicable for electrochromic films or multilayers.

Thermal Cross-Linking preparation of Aqueous-Based cathodic films for enhanced stability in organic electrochromic devices

Four derivatives of naphthalene diimide (NDI) small molecules with different polar functional side chains were designed and synthesized. The corresponding polymers, PNDI-5C/8C/DC/TC, were further prepared by thermal cross-linking method and utilized to fabricate organic electrochromic films. All of the PNDI-5C/8C/DC/TC films exhibit solvent resistance characteristics in commonly used organic solvents and water, and they possess varied surface energies, reduction voltages and electrochemical stabilities in aqueous electrolytes. The PNDI-TC film, which has the highest hydrophilicity, has a high optical contrast of 43.3 % with a short coloring time of 1.9 s and bleaching time of 0.6 s. Most importantly, the performance of the organic electrochromic device based on PNDI-TC can maintain almost 100 % contrast after 1000 cycles, showing great potential in the application of aqueous-based organic electrochromic device.

Significantly enhanced oxygen vacancies in W18O49 nanowires for electrochromic films by annealing in argon

Owing to the energy-efficient attributes of electrochromic technology, there has been a growing emphasis on the fabrication of electrochromic devices with high-contrast and stable performance. As an electron donor, the deliberate incorporation of oxygen vacancies into an electrochromic material is poised to significantly elevate the film's electrochromic characteristics. In this study, we successfully synthesized one-dimensional W18O49 nanowires thin films with enhanced oxygen vacancies using a straightforward combination of solvothermal reactions and annealing in Ar. In comparison to films annealed in air, those treated in argon (Ar) exhibited a higher concentration of oxygen vacancies, resulting in improved light absorption characteristics. The optimal annealing conditions were determined to be 400 °C for 40 min. Under these conditions, the thin film demonstrates enhanced electrochromic performance, exhibiting a notable 71.6% modulation in transmittance, a 27.2% increase compared to the air condition. It displays a rapid response time (Tb = 3.3 s, Tc = 3 s), high coloration efficiency (CE = 55.9 cm2 C−1), and excellent cycling stability (with a ΔT retention rate of 93.9% after 750 cycles).

Capturing ion trapping and detrapping dynamics in electrochromic thin films

Ion trapping has been found to be responsible for the performance degradation in electrochromic oxide thin films, and a detrapping procedure was proved to be effective to rejuvenate the degraded films. Despite of the studies on ion trapping and detrapping, its dynamics remain largely unknown. Moreover, coloration mechanisms of electrochromic oxides are also far from clear, limiting the development of superior devices. Here, we visualize ion trapping and detrapping dynamics in a model electrochromic material, amorphous WO3. Specifically, formation of orthorhombic Li2WO4 during long-term cycling accounts for the origin of shallow traps. Deep traps are multiple-step-determined, composed of mixed W4+-Li2WO4, amorphous Li2WO4 and W4+-Li2O. The non-decomposable W4+-Li2WO4 couple is the origin of the irreversible traps. Furthermore, we demonstrate that, besides the typical small polaron hopping between W5+ ↔ W6+ sites, bipolaron hopping between W4+ ↔ W6+ sites gives rise to optical absorption in the short-wavelength region. Overall, we provide a general picture of electrochromism based on polaron hopping. Ion trapping and detrapping were demonstrated to also prevail in other cathodic electrochromic oxides. This work not only provides the ion trapping and detrapping dynamics of WO3, but also open avenues to study other cathodic electrochromic oxides and develop superior electrochromic devices with great durability.

Electropolymerization of an EDOT-Quinoxaline Monomer for Green Electrochromic Thin Films and Devices.

In this study, we present a 5,8-bis(3,4-ethylenedioxythiophene)quinoxaline monomer with two 4-(octyloxy)phenyl side chains (EDOTPQ) that can be electropolymerized on ITO glass in standard electrolytes containing lithium salts and propylene carbonate as solvent. The electrochemically deposited PEDOTPQ layers show very good adhesion and homogeneity on ITO. The green-colored polymer thin films exhibit promising electrochromic (EC) properties and are interesting for applications such as adaptive camouflage, as well as smart displays, labels, and sensors. Novel organic-inorganic (hybrid) EC cell configurations were realized with Prussian blue (PB) or titanium-vanadium oxide (TiVOx) as ion storage electrodes, showing a highly reversible and fast color change from green to light yellow.

Structure and electrochromic properties of polypyrrole films synthesized by AC electrochemical impedance spectroscopy under different amplitude

Alternating current (AC) electrochemical impedance spectroscopy (EIS) was employed to synthesize polypyrrole (PPy) electrochromic films on fluorine-doped tin oxide (FTO) transparent conductive glasses. When depositing the PPy films, a fixed initial voltage was 700 mV and AC of different amplitudes was applied. The influence of AC voltage amplitude on the structure and electrochromic performance of PPy films was investigated. Scanning electron microscope (SEM) observation reveals that the PPy films prepared under AC are composed of uniform-distributed nanospheres when the AC amplitude is below 400 mV, which is different from the submicron structure displayed by PPy films prepared under direct current (DC). This kind of nanospherical structure is helpful to ion transport, resulting in improved electrochromic properties of the PPy films prepared under AC compared with the PPy film prepared under DC. In particular, the PPy film prepared under 100 mV amplitude AC shows ideal electrochromic properties, with an ion diffusion coefficient of 8.13 × 10−10 cm2·s−1, a coloring efficiency of 137.4 cm2·C−1, quick coloration and bleaching switching times of 5.0 s and 6.5 s, respectively, and retains 65.7% of its optical modulation after 100 cycles.

Multi-color electrochromic copolymer films based on 2,2′-thiophene and 3, 4-ethylenedioxythiophen

In this work, the copolymers were successfully synthesized by electrochemical polymerization of two monomers, bithiophene (BT) and 3,4-ethylenedioxythiophene (EDOT), in a lithium perchlorate/propylene carbonate (LiClO4/PC) solution. The as-prepared were characterized with the aid of cyclic voltammetry, colorimetric analysis, Fourier transform infrared spectroscopy, and Ultraviolet-visible spectroscopy. The electrochromic and colorimetric properties of copolymers poly(2,2′-bithiophene-co-3,4-ethylenedioxythiophene) (P(BT-co-EDOT)) and homopolymer poly(2,2′-bithiophene) (PBT) in 0.1 M LiClO4/PC solution were investigated, and the coloring process based on the redox reaction of the film was observed. When an anodic potential is applied, the copolymer P(BT-co-EDOT) films show a color change from blue-purple to blue-gray changes, and the homopolymer PBT shows a color change from orange to brown. In addition, spectroelectrochemistry studies indicate that copolymer P(BT-co-EDOT) has a lower optical band gap (1.73–1.87 eV) than homopolymer PBT. The copolymers P(BT-co-EDOT) show a high optical contrast (36.65–42.62%) and coloration efficiency (215.8–296.6 cm2 C−1). The copolymer P(BT1-co-EDOT1) demonstrates excellent redox cycling stability, with 90% of the original optical contrast maintained after 500 cycles, making it an ideal candidate for copolymers.

Silver Nanowire-Based Transparent Electrodes for V2O5 Thin Films with Electrochromic Properties.

The development of electrochromic systems, known for the modulation of their optical properties under an applied voltage, depends on the replacement of the state-of-the-art ITO (In2O3:Sn) transparent electrode (TE) as well as the improvement of electrochromic films. This study presents an innovative ITO-free electrochromic film architecture utilizing oxide-coated silver nanowire (AgNW) networks as a TE and V2O5 as an electrochromic oxide layer. The TE was prepared by simple spray deposition of AgNWs that allowed for tuning different densities of the network and hence the resistance and transparency of the film. The conformal oxide coating (SnO2 or ZnO) on AgNWs was deposited by atmospheric-pressure spatial atomic layer deposition, an open-air fast and scalable process yielding a highly stable electrode. V2O5 thin films were then deposited by radio frequency magnetron sputtering on the AgNW-based TE. Independent of the oxide's nature, a 20 nm protective layer thickness was insufficient to prevent the deterioration of the AgNW network during V2O5 deposition. On the contrary, crystalline V2O5 films were grown on 30 nm thick ZnO or SnO2-coated AgNWs, exhibiting a typical orange color. Electrochromic characterization demonstrated that only V2O5 films deposited on 30 nm thick SnO2-coated AgNW showed characteristic oxidation-reduction peaks in the Li+-based liquid electrolyte associated with a reversible orange-to-blue color switch for at least 500 cycles. The electrochromic key properties of AgNW/SnO2 (30 nm)/V2O5 films are discussed in terms of structural and morphological changes due to the AgNW network and the nature and thickness of the two protective oxide coatings.

Electrochemical synthesis and properties of multicolor electrochromic triphenylamine-based polymer films

Electrochemical synthesis and properties of multicolor electrochromic triphenylamine-based polymer films

Solvothermal Synthesis and Growth of Covalent Organic Framework Electrochromic Film with Triphenylamine Active Unit

Solvothermal Synthesis and Growth of Covalent Organic Framework Electrochromic Film with Triphenylamine Active Unit

Molybdenum induced defective WO3 multifunctional nanostructure as an electrochromic energy storage device: Novel assembled photovoltaic-electrochromic Mo–WO3 film

In this work, a self-powered electrochromic device incorporating molybdenum-doped tungsten oxide (WO3) is developed for enhanced performances, offering a potential solution for energy efficient technologies. Defective nanostructure of WO3, enabled with molybdenum doping, is achieved through an electrochemical co-deposition method. A film of Mo–WO3 is investigated for multifunctional purposes, acting as both an electrochromic (EC) and energy storage device (ESD). As an EC film, it exhibits an optimal optical contrast of 84.5 %, minimum coloration time of 0.4 s, high coloration efficiency of 90.4 cm2/C, and excellent optical stability. In ESD applications, Mo–WO3 displays charge-transfer resistance of 7.6 Ω, a commendable capacitance of 39.1 mF/cm2, and excellent capacitance retention of 80.4 %. The integration of Mo ions into the WO3 nanostructure results in the formation of oxygen defect sites, which enhances the mobility of Li+ ions over the amorphous and deagglomerated morphology of Mo–WO3. Furthermore, the EC activity of Mo–WO3 film does not require an external power source as the film is powered by an integrated quantum dot-sensitized solar cell (QDSSCs). The open circuit voltage of 0.62 V, resulting from the QDSSCs power source, successfully facilitates the switching operation of the Mo–WO3 EC film.

Improved cyclic durability and chemical stability of WO3 thin films by incorporation of Nb2O5

To achieve commercial smart windows operating in environments exposed to air and moisture with a hot weather, there has been a steady demand for large-area electrochromic thin films with high cyclic durability and chemical stability. In this article, we discuss the change in the electrochromic performance of WO3 thin films through incorporation of Nb2O5, which is stable against an acid electrolyte and heated water. With the increase in the content of Nb2O5 achieved by increasing the sputtering power, the charge density and transmittance modulation gradually decreased, while the cyclic durability was noticeably improved. When the as-deposited WO3 thin films were soaked in water at 70 °C, their flat surface and electrochromic properties degraded significantly. Conversely, the deterioration of the surface morphology of the Nb2O5-incorporated WO3 thin films after etching in the heated water was alleviated, which led to electrochromic performances on par with those of the unetched films. This is an effective approach for manufacturing of electrochromic devices with an enhanced stability to prevent structural degradation under hot and humid conditions.

Amorphous bismuth and GO co-doped WO3 electrochromic film with fast-switching time and long-term stability.

The sol-gel process for fabricating electrochromic thin films is straightforward, offering advantages such as low cost and ease of compositional control. Herein we prepared GO-Bi-WO3 films with improved electrochromic performance using a simple sol-gel spin-coating method. The sample shows a fast-switching time (1.8 s for coloring and 1.8 s for bleaching), large optical modulation (85% at 630 nm), excellent stability (86.4% retention after 10 200 cycles), and high coloration efficiency (65.9 cm2 C-1). This work indicates the electrochromic performance of WO3-based films can be enhanced by incorporating GO, which provides an effective strategy for the rapid, safe, and efficient fabrication of electrochromic thin films.

Viologen-based solution-processable ionic porous polymers for electrochromic applications.

Electrochromic porous thin films are promising for applications in smart windows and energy-efficient optical displays. However, their generally poor processing ability and excessive processing times remain grand challenges. Herein, we report the design and convenient synthesis of core-altered N-arylated viologens with aldehyde groups (πV-CHO) as new building blocks to prepare soluble, viologen-embedded ionic porous polymers. We also demonstrate that these polymers can be easily solution-processed by drop-coating to fabricate high-quality electrochromic films with tunable optoelectronic properties in a cost-effective fashion. The prepared films exhibit excellent electrochromic performance, including a low driving voltage (1.2-1.4 V), fast switching times (0.8-1.7 s), great coloration efficiency (73-268 cm2 C-1), remarkably high optical contrast up to 95.6%, long cycling stability, and tunable oxidation and reduction colors. This work sheds important light on a new molecular engineering approach to produce redox-active polymers with combined properties of intrinsic porosity, reversible and tunable redox activity, and solution processability. This provides the materials with an inherently broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

Insight into the generation of near infra-red (NIR) absorbing species in electrochromic surface-anchored metal-organic frameworks.

Due to their versatility and easy processing, Surface-Anchored Metal-Organic Frameworks (SurMOFs) have gained interest in recent times as promising electrochromic thin films. Herein a step forward in their use and characterization was achieved thanks to the integration of {Zn2(PDICl4)2} SurMOFs in a multi-layer electrochromic device (ECD), based on a membrane-like electrolyte. The optical and electrochemical properties of the ECD were fully characterized, revealing a two-step reduction process localized on the organic ligand and involving subsequent near infra-red (NIR) and cyan absorbing states, leading to optical modulation of the films. The species responsible for this absorption were isolated and identified in the reduced states. In parallel to experimental characterization, quantum chemistry was successfully used to investigate the structure-property relationship of the SurMOF, revealing additional information regarding the structure and the local environment of the electrochromic ligand.

Nickel oxide electrochromic films: mechanisms, preparation methods, and modification strategies–a review

Research progress in NiO based electrochromic materials and devices.

Effect of Co-doping on the electrochromic performance of hexagonal phase WO3 nanorods.

In this study, cobalt-doped hexagonal phase WO3 nanorods were prepared by a template-free hydrothermal method. The effects of varying the cobalt doping concentration on the microscopic morphology and electrochromic properties of hexagonal phase WO3 films were investigated. Films synthesized with the optimal cobalt element doping concentration demonstrate a notable improvement in their electrochromic properties compared to the pure hexagonal phase WO3 films. The film doped with 1.5% Co exhibited excellent cycling stability, retaining 98.55% of its original value after 500 cycles. The introduction of cobalt results in the formation of a nanorod structure with a high specific surface area within the film. This structure provides additional reaction sites for the electrochromic reaction process, thereby enhancing the optical modulation and coloration efficiency of WO3. The resulting films with excellent electrochromic properties provide a convenient and effective means for ion-doped modification of WO3-based electrochromic films.

Non-contact computer vision enables analysis of the dynamic performance of naphthalene diimide electrochromic films

Non-contact multi modal analysis of naphthalene diimide chromic films using Kineticolor computer vision software.

Transparent-to-Brown-Black Patterned Electrochromic Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) exhibit promising electrochromic (EC) performance owing to their porous structure, regular channel, and tunable component characteristics. However, few reports focus on MOF materials with the EC performance of a transparent to brown-black (neutral colored state) change that is more suitable for smart windows. In this work, we proposed a strategy for synthesizing MOF (named Ni-BPY) EC materials and corresponding films fabricated via a low-cost electrostatic spray deposition technique. The obtained film exhibits excellent EC performance with a neutral color change from transparent to brown-black, a large optical modulation of 70% at 430 nm, and a fast response within 10 s. Benefiting from good electrical and chemical stability, the Ni-BPY film can be cycled over 500 times. Notably, the Ni-BPY MOF film also delivers a stepwise-controlled process during the bleached state due to its porous characteristics. In addition, the unique color variation of the Ni-BPY film derives from the redox reaction of the Ni metal node between Ni2+ and Ni3+, which is verified by the in situ potential-dependent Raman and X-ray photoelectron spectroscopy (XPS) measurement. As a proof of application, the patterned Ni-BPY EC films and devices are additionally constructed to demonstrate their potential application in electronic tags and logo displays.