Electrochromic Thin Films Research Articles

Silver doping effect on the electrochromic performance of the WO3 thin films produced by thermal evaporation in vacuum

Ag doped WO3 thin films are successfully fabricated by thermal evaporation method in vacuum as precursor, and then annealed at 450 °C in nitrogen gases atmosphere. Cyclic voltammetry, repeating chronoamperometry, chronocolumetry and optical transmittance are recorded in 1 M LiClO4/propylene carbonate electrolyte. The color of the WO3 thin films changes from transparent to the blue under the applied potential of -1.8 and +1.9 V. XRD studies show that all the films have a polycrystalline structure of WO3. The molecular formation is also confirmed by Raman and X-Ray photoelectron spectroscopy (XPS). The EDS elemental mappings clearly confirm the homogeneous distribution of Ag, W and O elements into the WO3 network. From the optical studies, indirect band gap energy of the WO3 decreases as Ag doping level increases. The best coloration and bleaching times (2.33 and 1.25 s) are obtained for the Ag(2 %):WO3 thin films, compared to the pure WO3 (3.79 and 1.96 s). The produced WO3 films exhibit high reversibility (39.6/69.1 %), high coloration efficiency values (26.3/141.2 cm2/C), and good cycling stability. From the EIS measurements, the charge-transfer resistance is Ag(2 %):WO3 < the pure WO3 < Ag(5 %):WO3 < Ag(8 %):WO3, which means that Ag(2 %):WO3 has the biggest electrochemically active surface area. Electronic energy levels of the pure WO3 and the Ag:WO3 thin films are also given. From the Mott-Schottky studies, it is determined that the donor concentration of the materials is of the order of 1014–1015 cm−3. In the literature, there are very limited studies related to electrochromic thin films by thermal evaporation in vacuum. In this sense, the reported results in this study are promising for the electrochromic applications.

Optical and Structural Properties of V2O5 Electrochromic Thin Films

The increase in global temperature has led to a significant surge in energy consumption within the air conditioning industry, resulting in environmental deterioration. Electrochromic (EC) windows have emerged as a promising solution to address these challenges. Vanadium pentoxide (V2O5) stands out among all metal oxide materials due to its remarkable EC properties, including substantial Li+ ion insertion capacity and multicolor capabilities. Despite the potential of V2O5, there remains a lack of comprehensive research on the structural and optical properties of V2O5 films with varying thicknesses. Therefore, this study aims to investigate the structural and optical properties of V2O5 thin films with thicknesses ranging from 46 to 344 nm. By employing the sol-gel spin coating method, V2O5 thin films were fabricated and analyzed using X-ray diffraction (XRD) spectroscopy and ultraviolet-visible (UV-Vis) spectrophotometry. The fabricated V2O5 thin films with thicknesses of 46-274 nm demonstrated an average film transparency of 83 %. XRD analysis further revealed that the V2O5 thin films reached their peak crystallinity at a thickness of 344 nm. Moreover, CV analysis revealed that the V2O5 device, with a thickness of 274 nm, exhibited a cathodic peak current of -1.63 mA, indicating its excellent ability to facilitate Li+ ion diffusion. Additionally, CA measurements displayed a high optical modulation of 37.78 %. Ultimately, this research contributes to the development of energy-efficient solutions for sustainable environmental practices.

Flexible automated system for laser modified layer by layer assembly.

An open-source automated system for laser modified layer by layer assembly is described. This flexible system, the first designed to be used with this process, can be used to fabricate a range of laser patterned, layer by layer thin films. The Arduino microcontroller-based system features a stepper motor-controlled turntable that holds solutions and water rinses for dipping. The substrate can be moved vertically to be dipped into each of the solutions throughout the process. A semiconductor laser is used to modify the thickness of the thin film during the chosen dipping cycles. Several aspects of the robotic system are easily controlled via software, including the average laser power, irradiation time, horizontal laser position, and vertical substrate position. The system is fully automated and, once started, does not require any user interaction. To demonstrate the capability of the automated system for patterning, electrochromic thin film devices using 50-bilayer laser patterned films using the polymers poly(allylamine hydrochloride) and sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] are presented. One device is patterned with the shape of a large "C," created by irradiating the sample (55 mW average power, 405nm) while the substrate was moved vertically up and down or the laser was moved horizontally. The laser irradiates the sample during only the dipping in the polycation polymer solution. A second electrochromic thin film device is based on a sample with five parallel laser patterned lines, where each line is fabricated with different average laser powers and, hence, different thicknesses. The thicknesses of the lines vary by about 30nm.

Constructing an Al3+/Zn2+-Based Solid Electrolyte Interphase to Enable Extraordinarily Stable Al3+-Based Electrochromic Devices.

The interface between the electrochromic (EC) electrode and ionic conductor is crucial for high-performance and extraordinarily stable EC devices (ECDs). Herein, the effect of the ALD-AZO interfacial layer on the performance of the WO3 thin film was examined, revealing that an introduction of the ALD-AZO interfacial layer to the Al3+-based complementary ECDs can lead to improved EC performance and stability, such as an extraordinary cyclability of more than 20,000 cycles, an outstanding coloration efficiency of 109.69 cm2 C-1, and a maximum transmittance modulation of 63.44%@633 nm. The probable explanation is that the introduced ALD-AZO interfacial layer can effectively regulate the band gap of WO3, promote the electron transport process, and induce the formation of a robust solid electrolyte interphase to protect the electrode during cycling. These findings offer valuable insights for enhancing the EC performance of the EC thin films and new space for the construction of advanced multivalent Al3+-based ECDs.

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.

Recent progress in transmissive and reflective electrochromic devices for multi-color modulation

Multi-color electrochromic devices (M-ECDs), which dynamically and reversibly regulate the visible optical properties of the objects, have received significant interest in various fields such as displays, flexible intelligent electronics, smart windows and military camouflage. This review provides comprehensive summaries of the latest advances in inorganic electrochromic materials and M-ECDs. We discuss the basic principles for achieving M-ECDs, from the intrinsic multicolor characteristics of materials to a combination with optical structures. The progress on the construction of multicolor electrochromic thin-films and associated devices are unfolded in detail based on their transmissive or reflective working scenarios. Moreover, the limitations and challenges are also outlined for consideration for the future development of M-ECDs.

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.

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.

Film Deposition of Electrochromic Metal Oxides through Spray Coating: A Descriptive Review

Electrochromism induces reversible changes of coloration in specific organic and inorganic materials through electrical charge/discharge reactions. When processed into thin films, electrochromic metal oxides can be integrated into glazing applications such as displays, rearview mirrors, goggles and, most notably, smart windows in energy-efficient buildings. Over the years, the use of spray coating as a liquid-based approach has been acknowledged for its cost-efficient, high-throughput samples production with a low volume consumption. It represents an interesting alternative to vacuum processes and to other wet methods, suitably responding to the current limitations of electrochromic thin films production by offering improved control over deposition parameters and capacities of up-scaling, together with lowered energetic and economic costs. The present review summarizes the main theoretical and practical aspects of spray coating, notably distinguishing room-temperature methodologies from pyrolysis-based, under heating protocols. The main families of functional electrochromic metal oxides are then screened and discussed, establishing how spray processing can challengingly lead to higher levels of optical contrast, commutation kinetics, coloration efficiency and cycling durability, and how low-toxic and environment-friendly precursors can be favored while sustaining large deposition areas.

Rapid post-annealing effect on the TiO2-based electrochromic films

The electrochromic (EC) window is a promising energy-saving smart window that can vary between three states: transparent, translucent, and opaque by applying low voltages. One of the potential EC materials is known as titanium dioxide (TiO2) due to its unique chemical and physical properties. Conventional post-annealing has been commonly employed for phase transition in EC thin films, but it suffers from drawbacks such as long treatment time and high energy consumption. Hence, in this study, TiO2 thin films were deposited on indium-doped tin oxide (ITO) and fluoride-doped tin oxide (FTO) glass substrates using a sol-gel spin coating technique, followed by a rapid post-annealing interval of 5 min at temperatures ranging from 300 to 450 °C. The optical, morphological, structural, and EC properties of TiO2 thin films were investigated. The measurement depicted that rapid post-annealing was a cost-effective and time-saving method for realising high-quality TiO2 EC thin films. Hence, this approach reduces the overall fabrication time and minimises energy consumption, rendering it an environmentally friendly process. The successful deployment of TiO2 in EC applications paves the way for integrating EC and photovoltaic technologies in energy-saving window glass applications, considering TiO2 is a crucial element in photovoltaic devices.

Rapid and low-temperature preparation of tungsten oxide electrochromic thin films by oxygen plasma treatment

Electrochromic materials have attracted considerable attention due to their ability to change color reversibly and rapidly under an external electrical stimulus, making them highly promising for use in various applications such as smart windows, displays, and optical memory devices. In particular, tungsten trioxide (WO3) thin films have shown excellent electrochromic properties, leading to their wide application in electrochromic devices. In this study, oxygen plasma treatment was used to convert WCl6 thin film to WO3 electrochromic thin film on ITO glass substrate. Surface morphology, element composition, phase, optical property, and electrochromic characteristics of WO3 film are analyzed. It has been proved that the conversion of precursor films is rapid and efficient, the mean roughness and root mean square of amorphous WO3 thin film are lower than 1.8 nm and 2.2 nm respectively, and the film exhibits good electrochromic performance parameters. The optical modulation range and coloration efficiency of the film can reach 58.5% and 80.2 cm2/C before cycle and 56.3% and 84.0 cm2/C after 100 cycles, coloration time and bleaching time are within 10 s. The method of plasma treatment avoids annealing and prepare WO3 film with good and stable performance. This method has the potential to expand to other electrochromic materials, and provides a new solution for the large area preparation and flexible application of electrochromic materials.

Nb2O5 passivation of WO3 thin films to prevent electrochromic performance degradation in heated water

Electrochromic thin films must be highly stable against heat and moisture for their application to devices operating in air atmosphere. In this study, we discuss the degradation of the surface morphology of sputtered WO3 thin films in heated water. When the water temperature and soaking times increase, the flat surface of film undergoes a pronounced transition and becomes rough causing partial etching off the film. Although the surface area available for ion intercalation increases by etching, the electrochromic performance of films degrades. We propose stacking a passivation layer of Nb2O5 to prevent the etching off the WO3 thin films and changes in their surface morphology. The surface morphology of the bilayer thin films soaked in water at 70 °C for 30 and 180 s does not change noticeably when the thicknesses of the Nb2O5 layers are 25 and 100 nm, respectively. Both the thin films exhibit transmittance modulations and coloration efficiencies comparable to those of the WO3 thin film without etching, suggesting effective passivation. Moreover, the stacks are stable during cyclic tests up to 700 cycles in an H2SO4 solution. In contrast, the soaked WO3 thin films lose their electrochromic switching capability after the 35th cycle. In conclusion, Nb2O5 passivation of WO3 thin films can effectively prevent their degradation under hot and moist conditions.

Investigation of P3HT:WO3 hybrid electrochromic thin films prepared by solution blending doping

Although poly(3-hexylthiophene) (P3HT) is a promising p-type conjugating polymer to optimize optical and electrical properties, it is known to be chemically unstable. To overcome this unstability, WO3 incorporated P3HT hybrid thin films has been synthesized by very simple solution blending doping method. Different amounts of WO3 were added into the P3HT solution and the incorporation of WO3 particles was confirmed by scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy. The electrochemical reactions in 1 M lithium perchlorate (LiClO4)/propylene carbonate (PC) were studied by cyclic voltammetry and by electrochemical impedance spectroscopy. With the increase amount of WO3 in P3HT, the electrochromic efficiency increases first and then decreases. The optimum concentration was found as 30 wt% of WO3. The efficient interaction and well distribution between WO3 and P3HT improves the capacitive properties and electrochromic performance resulting a 110% increase in coloration efficiency (from 220 cm2/C to 464 cm2/C). Moreover, WO3 doped hybrid films shows long-term cyclability than undoped P3HT films. Structural and electrochemical investigations suggest that the optimum amount of WO3 doping is an alternative way to obtain high performance and environmentally stable electrochromic devices.

Mathematical characterization of performance and durability for the WO3 – Nb2O5 electrochromic thin films using finite experimental data

A comparison is made among the peak current density values of 1000 cycles of experimental chronoamperometry for WO3-Nb2O5 films with the simulated values. Quadratic and linear regression models are developed by using MATLAB standard function ‘fitlm’ to get least MSE and ‘predict’ method to extrapolate the current density values. Initial 400, 500, and 600 experimental cycles are used as starting references for predictions. Predicted model having −0.09 percent change with the experimental curve is followed to predict 10,000 cyclic values with −4.73 % difference. Binding energy values for W6+ (35.9, 38.05 eV), W5+ (36.8, 34.65 eV) and Nb5+ (210.35, 207.55 eV) are measured.

Visible and Near‐Infrared Dynamic Electrochromic Modulation With the Plasmon‐Enhanced Two‐Dimensional Ordered Macroporous Au/PProDot‐Me2 Thin Film

AbstractLocalized surface plasmon resonance (LSPR) is attractive to fabricate visible (VIS) and near‐infrared (NIR) dual‐band electrochromic materials. However, the introduction of the LSPR effect in the available dual‐band electrochromic materials can only be manifested to the spectral enhancement in the NIR region, and difficult issues of electrochromic materials such as low coloration efficiency and unsatisfactory long‐term cycling durability still remain. Here, a dual‐band electrochromic thin film consisting of a 2D ordered macroporous (2DOM) Au LSPR layer and a poly(3,4‐(2,2‐dimethylpropylenedioxy)thiophene) (PProDot‐Me2) electrochromic layer is presented, where LSPR and electrochromism interact in a complementary fashion. The 2DOM Au exhibits LSPR effect at wavelengths of 550 and 1700 nm, leading to a unique electrochromic performance improvement of the film in both of the VIS and NIR spectral regions. Large optical modulation, rapid switching speed, high coloration efficiency, and long‐term durability are achieved due to the collective effect of the near‐field enhancement of the 2DOM Au and the reduced bandgap of PProDot‐Me2. These advantages can be well retained in the dual‐band electrochromic device using the 2DOM Au/PProDot‐Me2 films as the electrochromic layers. This work provides new ideas for designing LSPR‐based devices, especially dual‐band electrochromic materials and devices, both experimentally and theoretically.

Oriented Surface‐Anchored Metal–Organic Frameworks (SurMOFs) as Electrochromic Thin Films

AbstractNovel electrochromic hybrid thin films are synthesized following a liquid‐phase epitaxy in a layer‐by‐layer fashion, yielding highly oriented surface‐anchored metal–organic frameworks (SurMOFs). The initial orange color of this {Zn2(PDICl4)2} crystalline thin films is reversibly switched by electrochemical reduction. In a lithium‐based electrolyte, a dark blue color is obtained in few seconds with a color efficiency of 104.8 cm2 C−1 (λ = 645 nm). Interestingly, the color of the reduced state is easily modulated by changing the electrolyte composition. Clearly, two different mechanisms are implicated and the reduced species responsible of the color are identified. Taking advantage of this unusual property, electrochromic devices with a novel configuration are designed leading to an original prototype capable of reversibly switching from a single initial orange color to two different colors simultaneously, namely dark blue and cyan.

Hydrothermally prepared ultra-stable multilayer nanoflake NiO-based electrochromic films

Electrochromic materials cycle between colored and bleached states under various applied voltage and have applications in smart windows and displays. The formation of a micro–nano structure in electrochromic thin films using hydrothermal methods can improve their properties. However, the conventional constant-temperature hydrothermal method yields films that easily peel from the conductive substrate during electrochromic cycling, resulting in performance losses. Therefore, we investigated the effect of a gradient-temperature hydrothermal protocol on the electrochromic performance of NiO thin films. First, a layer of small, strongly adhered crystal grains was seeded on an indium tin oxide substrate. Thereafter, NiO nanoflake layers were grown by adjusting the reaction temperature. The prepared film could be operated for 14,000 electrochromic cycles, four-times more than those of the conventionally prepared films, without significant performance loss. Therefore, our proposed method enhances the cyclic stability of micro–nanostructured NiO thin films significantly and will aid the development of high-performance electrochromic thin films having broad applications.

Preparation of electrochromic thin films by humidity-controlled spin coating

• Development of humidity-controlled spin-coating system using N 2 gas. • Variation of the film thickness in presence or absence of N 2 gas. • Doubled film thickness obtained in high humidity compared to low humidity. Spin coating has been used for various applications, such as the coating of resist solutions in semiconductor processes and the fabrication of functional thin films. By rotating at a high speed, thin films can be formed by centrifugal force. Since the film thickness can be controlled by the number of rotations and time, spin coating is often used for simple experiments at the laboratory. However, in a normal laboratory environment, i.e. not a clean room, temperature and humidity can vary depending on the season, time of day, and weather. In this study, we attempted to fabricate electrochromic WO 3 thin films using a spin coater, which we set up independently, that can control humidity. Compared to the WO 3 thin film deposited directly during the rainy season in Japan, the film thickness was doubled by controlling the humidity, even when deposited under the same spin-coating conditions. The WO 3 thin films prepared by this humidity-controlled process exhibited favorable electrochromic properties. Because, especially in systems containing volatile solvents, it was possible to control the film thickness and function during deposition by controlling the humidity, and the results of this research provide a reference to develop a convenient method for obtaining functional thin films regardless of the weather or season.

Pseudocapacitive Charge Storage in Electrochromic Transition-Metal Oxide Thin Films

Electrochromic pseudocapacitive transition-metal oxide materials, such as tungsten oxide, which combine fast response, high energy density, and optical effects, can play a significant role as energy storage materials. Here we investigate the electrochemical kinetics of thin films of tungsten oxide, which turn transparent to sky-blue color in the lithiated state due to the reduction of W6+ to W5+. We investigated the charge density, charge transfer, ion diffusion, and interfacial behavior upon Li+ insertion/de-insertion in WO3. The electrochromic thin film’s pseudocapacitive and electrical double layer mechanism was differentiated based on the power-law. Faradaic diffusion-controlled process dominates over the surface capacitive behavior at scan rates below 40 mV s−1. These films exhibit an areal charge density of around 100 mC cm−2 and a capacitance of 80 mF cm−2, superior to most comparable electrochromic materials and supercapacitors. This work combines electrochromics and energy storage properties and provides a fundamental understanding of pseudocapacitive and electrochromic mechanisms in WO3.