High Coloration Efficiency Research Articles

Electrochromic and photovoltaic properties of benzothiadiazole-based donor-acceptor conjugated polymers with oligo(ethylene glycol) side chains

Introduced oligo(ethylene glycol) (OEG) chains into high-performance donor-acceptor (D-A) type conjugated polymers is a promising strategy to control the π-π stacking, energy levels, solubility, and electrical properties. In this work, a series of novel PDTBF polymers (PDTBF-DTh, PDTBF-ThSe, and PDTBF-Th) were synthesized based on the typical benzothiadiazole-thiophene (BT-Th)-based polymer substituted with OEG side chains to investigate their electrochromic (EC) and photovoltaic performance. The resultant PDTBF polymers had fast switching speed (i.e., chlorobenzene-prepared PDTBF-Th EC film: Tc0.9 = 1.8 s, Tb0.9 = 1.0 s), and high coloration efficiencies (i.e., chlorobenzene-prepared PDTBF-ThSe EC film: 255.50 cm2 C−1). Moreover, the PDTBF-DTh:Y6-based device shows a power conversion efficiency (PCE) of 3.11%. Due to the induced amphiphilic OEG side chains on the backbone of the conjugated polymer, these materials also exhibited green solvent processing potential. It is worth noting that these PDTBF polymers exhibited much wider optical bandgaps (1.95 eV–2.16 eV) than other reported PDTBF polymers (mostly narrow band gaps: 1.40–1.70 eV), due to the format of linkage between OEG side chains and thiophene units. It is an effective strategy to control the optical properties of the conjugated polymers using OEG side chain engineering investigation.

Electrochemical polymerization of D-A-D type monomers consisting of triphenylamine and benzo[1,2-b:4,5-b′]dipyrazine units for multicolor electrochromism

We developed four donor-acceptor-donor (D-A-D) type monomers (YQ1 ∼ YQ4) with Y-shape configuration consisting of triphenylamine and benzo[1,2-b:4,5-b′]dipyrazine moities, which were further polymerized via electrochemical polymerization for electrochromic applications. The monomers were synthesized facilely from benzene-1,2,4,5-tetraamine tetrahydrochloride by sequential condensation reactions with diketones. The optical properties of these polymers in neutral states can be tuned by variation of the additional substituents on the electron acceptor parts. All these polymers (PYQ1 ∼ PYQ4) exhibited multicolor electrochromism, and appeared similar green color at their second coloration states. Moreover, polymers PYQ1 and PYQ2 with two twisted substituents on the acceptor part showed superior electrochromic performance than the anologues PYQ3 and PYQ4 consisting fused electron acceptor moities with high coplanarity, involving faster response speed, and higher coloration efficiency, which demonstrated that the molecular configuration of monomer was crucial for efficient electrochromic performance.

Robust and swiftly multicolor Zn2+-electrochromic devices based on polyaniline cathode

Multifunctional electrochromic (EC) devices are very important for the current research on smart electronic devices. Herein, we report a robust and swiftly multicolor EC energy storage device design based on polyaniline (PANI) cathode. To achieve this goal, we systematically studied the EC properties of PANI in aqueous and organic Zn 2+ electrolytes for the first time. Compared with Zn 2+ aqueous electrolyte, the PANI film measured in organic Zn 2+ electrolyte delivered a fast-switching time (τ c /τ b = 2.0/2.4 s), high optical contrast (72.2%), high coloration efficiency (205 cm C −1 ), and good cycle stability (optical contrast remains 92.7% after 10,000 cycles). The excellent EC performance could be attributed to the good wettability of the interface between organic Zn 2+ electrolyte and PANI film, which supports rapid charge transfer kinetics. The PANI cathode in the organic Zn 2+ electrolyte also provided a multicolor electrochromism (light yellow, green and dark green) along with a high specific capacitance of 26.4 mAh m −2 and 72.2% spectral modulation range at a current density of 0.1 A m −2 , excellent rate capability of 86.4% capacity retention and 81.3% spectral modulation even at an 8-fold current-density increase. The laboratory prototype device also verified the EC energy storage performance of organic Zn 2+ intercalated/deintercalated PANI, confirming that they are suitable for highly robust multicolor EC energy storage applications. • The EC properties of PANI film in aqueous and organic Zn 2+ electrolytes for the first time were systematically revealed. • The PANI film delivered a fast-switching time, high optical contrast, high coloration efficiency, and excellent cycle stability (optical contrast remains 92.7% after 10,000 cycles). • The laboratory prototype device based on PANI showed a good EC performance as well as energy storage.

Enhanced charge transfer in TiO2 nanoparticles/ MoO3 nanostructures bilayer heterojunction electrode for efficient electrochromism

Electrochromic smart windows play a significant role in energy saving technologies and there is a need to find more efficient as well as cheaper electrode materials suitable for this application. Electrode materials with adequate usage of nanostructuring and novel design strategies can help in improving the charge transfer and achieving better electrochromic performance. Herein, we explored a nanostructured electrode with the bilayer heterojunction design employing environmental friendly & cost effective TiO2 and MoO3 in an electrochromic device. A bilayer electrode with nanoparticulate TiO2 in the bottom layer and randomly oriented MoO3 nanostructures as the top layer has been adopted for the first time. The electrode exemplified a superior performance in terms of high current density and charge storage capacity as well as rate capability as compared to the relevant reports in the literature. A color contrast of 38%, switching response of ~2 s and high coloration efficiency of 72.5 cm2/C were achieved. The device has also shown good cyclic stability and high reversibility. The enhanced performance is ascribed to the nanostructured morphology of the electrode and the built-in potential at the interface, which improved the charge transfer, electrical conductivity and ionic transport in the device. The study presents a new viable approach of electrode design with nanoparticle bottom layer to improve the electrochromic performance capabilities, which can be extended to any other electrochromic materials as well.

MOF‐Derived Carbon Embedded NiO for an Alkaline Zn−NiO Electrochromic Battery

AbstractIn the era of intelligent automation, smart energy storage devices are highly sought after due to their capability to reveal their state of charge by a visual color change. In principle, this is typically obtained by integrating electrochromic materials which demonstrate a change in optical band gap during redox reactions as electrodes in batteries/supercapacitors. Among various electrochromic materials, transition metal oxides have received significant interests because of their natural abundance, good cycling stability and high electrochemical response. In this work, metal organic framework (MOF) derived carbon embedded NiO (NiO@C) demonstrates very fast switching speeds of ∼1.4 s for coloration and ∼3.5 s for bleaching with high coloration efficiency of ∼135.16 cm2/C. These aspects make MOF‐derived NiO@C a potential candidate to be integrated as a smart positive electrode in an alkaline zinc (Zn) battery to construct a rechargeable Zn−NiO electrochromic (EC) battery. The Zn−NiO EC battery delivers an average voltage of ∼1.84 V and demonstrates a specific capacity of ∼85.3 mAh/m2 at 0.1 mA/cm2 current density. It visually displays its energy storage level by changing its color from dark brown (charged state) to colorless (discharged state), making it a potential candidate in smart windows and energy storage displays.

Rational molecular design of electrochromic conjugated polymers: Toward high-performance systems with ultrahigh coloration efficiency

• Three fused aryl amine-based π-conjugated polymers (CPs) are rationally designed. • Electrochromic devices (ECDs) containing synthesized polymers are demonstrated. • The ECDs exhibit fast switching, high coloration efficiency, and stable operation. • The achieved high-performance is explained through kinetics and structural analysis. • Density functional theory reveals the origin of the device stability. Organic electrochromic devices (ECDs) based on π-conjugated polymers (CPs) have notable advantages including relatively fast switching time and abundant colors with wide tunability. However, their practical applications are still limited due to low coloration efficiency and poor long-term stability. Here, we propose a series of reasonably designed CPs for high-performance ECDs. The principle of molecular design includes the introduction of a fused aryl amine (e.g., indolocarbazole or indoloindole) to effectively modify the optical and electrical characteristics of CPs, leading to high redox stability with fast response and high coloration efficiency ( η ). The resulting ECD with the fluorinated indoloindole-based CP exhibits the highest overall performance with rapid coloration and bleaching dynamic responses of ∼5.6 and ∼0.6 s, respectively, a large transmittance contrast of ∼46%, an outstanding η of ∼1563.3 cm 2 /C, and a reliable operation stability with an initial transmittance loss of less than 0.6% even after 6,500 cycles. We elucidate the origin of this superb performance by calculating redox rate constants of each CP via the Nicholson method and determining their thin film crystalline structures using grazing-incidence X-ray diffraction. In addition, the ultrahigh cyclic stability is explained by predicting the degree of molecular distortions of CPs during redox reactions through density functional theory.

1,4-Bis((9H-Carbazol-9-yl)Methyl)Benzene-Containing Electrochromic Polymers as Potential Electrodes for High-Contrast Electrochromic Devices.

Four 1,4-bis((9H-carbazol-9-yl)methyl)benzene-containing polymers (PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF)) were electrosynthesized onto ITO transparent conductive glass and their spectral and electrochromic switching performances were characterized. The PbCmB film displayed four types of color variations (bright gray, dark gray, dark khaki, and dark olive green) from 0.0 to 1.2 V. P(bCmB-co-bTP) displayed a high transmittance variation (∆T = 39.56% at 685 nm) and a satisfactory coloration efficiency (η = 160.5 cm2∙C−1 at 685 nm). Dual-layer organic electrochromic devices (ECDs) were built using four bCmB-containing polycarbazoles and poly(3,4-ethylenedioxythiophene) (PEDOT) as anodes and a cathode, respectively. PbCmB/PEDOT ECD displayed gainsboro, dark gray, and bright slate gray colors at −0.4 V, 1.0 V, and 2.0 V, respectively. The P(bCmB-co-bTP)/PEDOT ECD showed a high ∆T (40.7% at 635 nm) and a high coloration efficiency (η = 428.4 cm2∙C−1 at 635 nm). The polycarbazole/PEDOT ECDs exhibited moderate open circuit memories and electrochemical redox stability. The characterized electrochromic properties depicted that the as-prepared polycarbazoles had a satisfactory application prospect as an electrode for the ECDs.

Ester-functionalized pyrene derivatives: Effects of ester substituents on photophysical, electrochemical, electrochromic, and electrofluorochromic properties

Multifunctional electrochromic materials are widely used in optoelectronic devices. Pyrene is a π-electron-rich group with fluorescence properties. Ester-functionalized pyrene derivatives potentially have interesting multifunctional properties such as electrochromic and electrofluorochromic characteristics. In this work, mono-, di-, tri-, and tetra-ester-substituted pyrene derivatives were synthesized, and the effects of the number of ester substituents on their photophysical, electrochemical, electrochromic, and electrofluorochromic properties were investigated. The number of ester substituents significantly affected these properties. An increase in the number of ester substituents on the pyrene caused red shifts in the absorption and emission spectra, a decrease in the driving voltage, and changes in the electrochromic colored states and electrofluorochromic performance. Mono-, di-, and tri-ester-functionalized pyrene derivatives showed good electrochromic properties, namely good switching stabilities, high optical contrasts, rapid responses, and high coloration efficiencies. Mono- and tri-ester-functionalized pyrenes gave good electrofluorochromic performances with reversible fluorescence quenching–fluorescence restoring between colored and bleached states during the electrochromic process. Ester-functionalized pyrene derivatives were successfully used in two functional digital displays with vivid colors. These results show that ester-functionalized pyrene derivatives with good electrochromic and electrofluorochromic properties are promising candidates for use in various applications, e.g., digital displays, organic electroluminescence displays, electronic shelf labels, energy-saving windows, and electronic paper.

Targeted enhancement of electrochromic memory in Fe(II) based metallo-supramolecular polymer using molybdenum disulfide quantum dots

Bistable electrochromic (EC) display with long EC memory has been a much-anticipated goal owing to its zero-energy consumption when maintaining the colored or bleached state when used in smart windows. However, it is very challenging to engineer the structure of the materials to accomplish a long EC memory along with high optical contrast, durability, switching fastness, and coloration efficiency which are of significance for fabricating the electrochromic devices with essentially power efficiency. Herein, we have judiciously incorporated the charge trapping MoS2 quantum dots (QDs) to prepare an electrochromic nanocomposite (polyFe-QD-10) of an Fe2+-based metallo-supramolecular EC polymer (polyFe) for the targeted enhancement of EC memory than the pristine polyFe polymer. The polyFe-QD-10 based electrochromic device (ECD) exhibits more cycle durability as it produced no loss in ΔT after the measurement of 100 cycle, while pristine polyFe-based ECD produced 14% loss in similar 100 cycle. The prototype ECD with polyFe-QD-10 film is also demonstrated with fast switching time (3.3 s for bleaching and 0.78 s for coloring), and good coloration efficiency (242.2 cm2 C−1). Remarkably, the ECD based on polyFe-QD-10 composite displays higher EC memory in open circuit condition as it takes 32 min for 90% reappearance of the original purple color after complete bleaching, while the pristine polyFe based ECD regains 90% of the transmittance of the color state only in 15 min. The strategy reported here presents the polyFe-MoS2 QD composite thin film an encouraging candidate for developing power efficient electrochromic information displays and smart windows.

Versatile Photo/Electricity Responsive Properties of a Coordination Polymer Based on Extended Viologen Ligands

Responsive chromogenic materials have attracted increasing interest among researchers; however, up until now, few materials have exhibited multifunctional chromogenic properties. The coordination polymers (CPs) provide intriguing platforms to design and construct multifunctional materials. Here, a multifunctional photo/electricity responsive CP named Zn−Oxv, which is based on the “extended viologen” (ExV) ligand, was synthesized. The Zn−Oxv exhibited reversible photochromism, photomodulated fluorescence, electrochromism and electrofluorochromism. Furthermore, we prepared Zn−Oxv thin films and investigated electrochromic (EC) properties of viologen−based CPs for the first time. Zn−Oxv thin films showed excellent EC performance with a rapid switching speed (both coloring and bleaching time within 1 s), high coloration efficiency (102.9 cm2/C) and transmittance change (exceeding 40%). Notably, the Zn−Oxv is by far the fastest CP EC material based on redox−active ligands ever reported, indicating that the viologen−based CPs could open up a new field of materials for EC applications. Therefore, viologen−based CPs are attractive candidates for the design of novel multi−responsive chromogenic materials and EC materials that could promise creative applications in intelligent technology, dynamic displays and smart sensors.

Enhancing the Spectroelectrochemical Performance of WO3 Films by Use of Structure-Directing Agents during Film Growth

Thin, porous films of WO3 were fabricated by solution-based synthesis via spin-coating using polyethylene glycol (PEG), a block copolymer (PIB50-b-PEO45), or a combination of PEG and PIB50-b-PEO45 as structure-directing agents. The influence of the polymers on the composition and porosity of WO3 was investigated by microwave plasma atomic emission spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, X-ray diffraction, and gas sorption analysis. The electrochromic performance of the WO3 thin films was characterized with LiClO4 in propylene carbonate as electrolyte. To analyze the intercalation of the Li+ ions, time-of-flight secondary ion mass spectrometry, and X-ray photoelectron spectroscopy were performed on films in a pristine or reduced state. The use of PEG led to networks of micropores allowing fast reversible electrochromic switching with a high modulation of the optical transmittance and a high coloration efficiency. The use of PIB50-b-PEO45 provided isolated spherical mesopores leading to an electrochromic performance similar to compact WO3, only. Optimum characteristics were obtained in films which had been prepared in the presence of both, PEG and PIB50-b-PEO45, since WO3 films with mesopores were obtained that were interconnected by a microporous network and showed a clear progress in electrochromic switching beyond compact or microporous WO3.

Self-Driven Electrochromic Window System Cu/WOx-Al3+/GR with Dynamic Optical Modulation and Static Graph Display Functions.

Electrochromic devices with unique advantages of electrical/optical bistability are highly desired for energy-saving and information storage applications. Here, we put forward a self-driven Al-ion electrochromic system, which utilizes WOx films, Cu foil, and graphite rod as electrochromic optical modulation and graph display electrodes, coloration potential supplying electrodes, and bleaching potential supplying electrodes, respectively. The inactive Cu electrode can not only realize the effective Al3+ cation intercalation into electrochromic WOx electrodes but also eliminate the problem of metal anode consumption. The electrochromic WOx electrodes cycled in Al3+ aqueous media exhibit a wide potential window (∼1.5 V), high coloration efficiency (36.0 cm2/C), and super-long-term cycle stability (>2000 cycles). The dynamic optical modulation and static graph display function can be achieved independently only by switching the electrode connection mode, thus bringing more features to this electrochromic system. For a large-area electrochromic system (10 × 10 cm2), the absolute transmittance value in its color-neutral state can reach about 41% (27%) at 633 nm (780 nm) by connecting the Cu and WOx electrodes for 140 s. The original transparent state can be readily recovered by replacing the Cu foil with the graphite rod. This work throws light on next-generation electrochromic applications for optical/thermal modulation, privacy protection, and information display.

Three dimensional fluorene-based polyamides facile to transfer ion designed for near-infrared electrochromic application and detection for explosive

Near-infrared electrochromic (NIR EC) functional materials are attracting increasing attention owing to their great potential for applications in smart window. However, obtaining high performance NIR EC material with high optical transmittance contrast, fast redox switching and long-term cyclic stability is still a challenge. Introducing four triphenylamine (TPA) units into one planar fluorene core to build robust three dimensional (3D) framework which not only makes ion and electron move freely but also provides sufficient space and storage sites for hosting ions will meet the requirement. Here, three kinds of new polyamides (PAs) based on conjugated fluorene as the bridge unit were designed and polycondensized by a new diamine N, N'-(9,9-bis(4-(diphenylamino)phenyl)-9H-fluorene-2,7-diyl)-bis(N-(4-methoxyphenyl) benzene-1,4-diamine), with three kinds of dicarboxylic acids to form TPAF-6F, TPAF-CA and TPAF-OA. The obtained new PAs exhibit impressive solution-processability and excellent thermal stability. Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) of the PAs imply strong electronic coupling within molecule. Furthermore, the TPAF-OA film exhibits excellent EC performance with multi-coloured EC behaviour, high optical contrast (92%), high coloration efficiency (203 cm2 C−1) and outstanding cycle stability (optical contrast remains 90% over 1400 cycles) in the NIR range. TPAF-CA also shows impressive fluorescence (PL) quantum efficiency which could be used as sensor for explosive and nonvolatile flash memory properties. The strategy of introducing 3D framework block into polymer will pave a way for fabricating high performance optoelectronic devices.

Bifunctional Application of Viologen-MoS2-CNT/Polythiophene Device as Electrochromic Diode and Half-Wave Rectifier.

A dual purpose solid state electrochromic diode has been fabricated using polythiophene (P3HT) and ethyl Viologen (EV), predoped with multiwalled carbon nanotubes (MWCNTs) and MoS2. The device has been designed by considering two important aspects, first, the complementary redox activity of P3HT and EV and second, the electron holding properties of MoS2 and MWCNTs. The latter is found to enhance the electrochromic performance of the solid state device. On the other hand, the complementary redox nature gives the asymmetric diodic I-V characteristic to the device which has been exploited to use the electrochromic device for rectification application. The MoS2 nanoflower and MWCNTs are synthesized by one-step hydrothermal and pyrolysis techniques and well characterized by scanning electron microscopy (SEM), X-ray analysis (XRD), and Raman spectroscopy. Electrochromic properties of the device have been studied in detail to reveal an improvement in device performance in terms of faster speed and high coloration efficiency and color contrast. In situ bias-dependent Raman spectroscopy has been performed to understand the operation mechanism of the electrochromic diode which reveals (bi-)polaron formation as a result of dynamic doping eventually leading to color change. A half-wave rectifier has been realized from the electrochromic diode which rectifies an AC voltage of frequency 1 Hz or less making it suitable for low frequency operation. The study opens a new possibility to design and fabricate multipurpose frequency selective electrochromic rectifiers.

Novel High-Performance and Low-Cost Electrochromic Prussian White Film.

Prussian white (PW), due to its low cost, easy synthesis, open structure, and fast ion extraction/interaction, is introduced to the electrochromic field. The PW films were successfully grown on indium tin oxide (ITO) glass by a facial hydrothermal method. Impressively, the PW film exhibits excellent electrochemical cycling stability without obvious decay over 10 000 cycles and a high coloration efficiency of 149.3 cm2 C-1. The film also provides the large optical transmittance contrast (over 70%) in a wide wavelength range of 650-800 nm. Furthermore, the PW film shows the rapid coloration and bleaching response. These results suggest that PW is a promising practical candidate of high-performance electrochromic material.

Fast Switching Triphenylamine-Based Electrochromic Polymers with Fluorene Core: Electrochemical Synthesis and Optoelectronic Properties

Most of the triarylamine-based electrochromic polymers are achieved through arylamine-based oxidative coupling reactions where the electrochromic properties of the polymer could only be realized after fabrication of the film on optically transparent electrodes. Electrochemical polymerization, on the other hand, is an in situ polymerization method which offers synthesis of the polymer and fabrication of the films on the electrode surface simultaneously. In this study two new triphenylamine monomers (TPAFLA and TPAFLS) having fluorene core were synthesized and successfully electrochemically polymerized. The electrogenerated polymer films exhibited strong color changes (P(TPAFLA):yellow, orange, green, deep blue, P(TPAFLS): pale yellow, pale blue) and rapid switching responses (P(TPAFLA):1.61 s at 362 nm, P(TPAFLS):0.69 s at 390 nm) and high coloration efficiency (P(TPAFLA):114 cm2C−1, P(TPAFLS):192 cm2C−1) compared to other triphenylamine derivatives. Furthermore, electrochemical copolymerization of TPAFLA with 3,4-ethylenedioxythiophene (EDOT) was successfully achieved for the first time and the copolymer revealed significantly lowered band gap with remarkably enlarged color palette (violet, yellow, green, light blue and blue).

N-(thiophen-2-ylmethylene)aniline- and N-(thiazol-2-ylmethylene)aniline-containing poly(2,5-dithienylpyrrole)s as anodically coloring polymers for high contrast electrochromic devices

Two N-(thiophen-2-ylmethylene)aniline- and N-(thiazol-2-ylmethylene)aniline-containing poly(dithienylpyrrole)s (PTA and PZA) are deposited on indium tin oxide (ITO) glass substrates and their electrochemical, spectral, and electrochromic performances are characterized. PTA and PZA display reversible multicolor chromism in the UV–Vis–NIR region. PTA is ocher, olive, slate grey, and bluish-grey at −0.2, 0.8, 1.0, and 1.2 V, respectively, whereas PZA is terreous, coffee, grey, and deep grey at −0.2, 0.2, 1.0, and 1.4 V, respectively. Kinetic investigations of polymeric films display the transmittance change (ΔΤ) of PTA and PZA between the doped and undoped state is 35.4% at 1286 nm and 31.2% at 1218 nm, respectively. PZA attains a high coloration efficiency (η = 218.8 cm2 C−1 at 1218 nm) in a liquid solution. Bilayer electrochromic devices (ECDs) based on PTA (or PZA) as anodic polymer and PProDOTEt2 (or PProDOTBz2) as cathodic polymer were fabricated. The ΔT of PTA/PProDOTBz2 and PZA/PProDOTBz2 ECDs between the bleached and colored state is 70.2% at 622 nm and 64.7% at 622 nm, respectively. Moreover, four ECDs display short response time and multichromic reversible redox behaviors, indicating that PTA and PZA are promising candidates for future electrochromic and optoelectronic applications.

Electrochromic Behavior Originating from the W6+/W5+ Redox in Aurivillius-type Tungsten-Based Layered Perovskites.

Simple oxide materials, typically, WO3, have been conventionally employed for electrochromic (EC) materials because of their high coloration efficiency; however, it is quite difficult to realize multiple coloration because they involve redox reactions due to single ions. On the other hand, multiple oxides are expected to show various colors when applying different voltages due to the diverse structures and combinations of ions; however, multiple oxide-type EC materials are still in the research stage, and the discovery of further EC materials is necessary. Toward the development of multiple oxide-type EC materials, tungsten-containing layered perovskites have been synthesized, and their optical properties have been evaluated. X-ray diffraction and X-ray absorption fine structure analyses revealed that the discovered tungsten-based layered perovskites Bi2Na0.5La0.5TiWO9 (BNLTW) and Bi2LaTi1.5W0.5O9 (BLTW) have an orthorhombic phase with an Aurivillius-type layered perovskite structure. EC devices fabricated with three kinds of perovskites, including well-known Aurivillius-type Bi2W2O9 (BWO), have no absorption in the visible-light region when no voltage is applied, while they show absorption over the whole visible-light spectrum to black when a voltage of +4.5 V is applied. Furthermore, with an applied voltage of -4.5, the transmittance recovered to the same level as the initial state, meaning the EC function is reversible. In this reaction, only tungsten in the perovskite framework acted as a redox-active species (W6+/W5+ redox) without the redox of the other metal ions. From the electrochemical analysis of the EC materials using cyclic voltammetry, redox peaks could be observed at -0.2 to 0.4 V for reduction and +0.1 to +0.3 V for oxidation. Interestingly, the redox potentials are linearly related to the W content in the perovskite unit, indicating that the redox potentials can be tuned by controlling the chemical formula. The coloration efficiency of the BNLTW EC device was the best at 37.1 cm2/C in the prepared perovskite-based EC device, which is comparable to that of a typical WO3 EC material.

Enhanced electrochromism from non-stoichiometric electrodeposited tungsten oxide thin films

Nanocrystalline structure tungsten oxide (nc-WO3; WO3·2H2O) thin films were deposited on fluorine-doped tin oxide (FTO) substrates via a facile electrochemical deposition (ECD) method. The X-ray diffraction (XRD), Fourier Transform Infrared (FT-IR) spectrum, and scanning electron microscope (SEM) results confirmed the gradual temperature dependent structural transitions from nanocrystalline tungsten oxide (nc-WO3) to amorphous tungsten oxide (a-WO3) at 200 °C. Electrochromic (EC) performances of the films were examined in an aqueous solution of 0.5 M H2SO4 by cyclic voltammetry tests integrated in-situ transmittance measurements. The results showed that the EC properties of WO3 films were closely correlated to its structural arrangements of –OH groups and the structural water molecules present in the WO3 matrix. A fast-switching speed (5 s for coloration and 3 s for bleaching), a significant optical modulation (above 89 % at 632 nm), a high coloration efficiency of 51.1 cm2/C and cycling stability of more than 1000 cycles were achieved for nc-WO3 film. Additionally, an EC device was successfully assembled using NiO film prepared by chemical bath deposition as a complementary electrode and exhibited fascinating performance.

Multicolored and durable electrochromism in water soluble naphthalene and perylene based diimides

Organic small molecules are advantageous in electrochromism as they exhibit high coloration efficiency, fast switching with vivid colors range at low potential range. However, the main problems in this type of electrochromes are film processing and dissolution of the compound during operation to restrict the durability. Herein, we have successfully synthesized two water soluble sulfonate group containing arylene diimides; NDI-cored NTSA and PDI-cored PRSA and comprehensively characterized by different spectroscopic techniques, morphological investigation and electrochemical characterizations, etc. Two diffusion controlled quasi-reversible redox waves are envisaged in both the compounds for stepwise reduction of neutral species to mono-anion radical and mono-anion radical to di-anion radical formation. Both NTSA and PRSA films show multichromic EC behavior in low potential range of +0.5 to −1.5 V with high optical contrast and high coloration efficiencies when measured in solution electrolytes. The durability test unveils the 80% retention of the optical contrast for NTSA, while 82% retention of the same for PRSA even after 800 cycles. The solid state ECD made of PRSA film also divulges the similar multi-chromic EC behavior. The results can provide a viable approach to design the high performance cathodically colored PDI and NDI ECMs as the potential candidate for energy-saving multi-colored smart window. • Synthesis of water soluble redox-active naphthalene and perylene diimides. • Both the compounds (NTSA and PRSA) display multicolored electrochromism. • Durable electrochromism with high optical contrast and coloration efficiency. • Solid state devices also show multicolored electrochromism at low working voltage.