Guobo Dong

In situ electrochromic efficiency of a nickel oxide thin film: origin of electrochemical process and electrochromic degradation

Electrochromic nickel oxide (NiOx) thin films are one of the most promising anodic colored materials. However, there is lack of accurate description of their electrochemical process and degradation mechanism. In this study, a novel approach involving in situ electrochromic efficiency is proposed to reveal the electrochemical origin of an electrochromic NiOx thin film cycled in a Li+-ion electrolyte. The results indicate that the coloring process of the NiOx thin film refers to the oxidation reactions of Ni2+ to Ni3+ and Ni2+ to Ni4+ (in two forms of Ni3O4 and Ni2O3), and the bleaching process is associated with the reduction reactions of Ni4+ to Ni2+, Ni4+ to Ni3+, and Ni3+ to Ni2+. The irreversible reduction of Ni4+ to Ni3+ plays a dominant role in the activation procedure of NiOx. It is deduced that the Li+-ion trapping in the bleaching process along with the reduction reactions of Ni4+ to Ni3+ and Ni3+ to Ni2+ causes the degradation of the electrochromic properties. This study provides a further insight into the electrochromic mechanism and is conducive to the improvement of the long-term cyclic durability for Li+-based electrochromic NiOx. Moreover, the study significantly establishes a direct connection between an electrochemical process and a variation in the optical absorbance of materials.

Improved performance of co-sputtered Ni–Ti oxide films for all-solid-state electrochromic devices

Thin films of Ni–Ti oxide were co-sputtered by reactive direct current magnetron sputtering and their structures, morphologies, and compositions were investigated by X-ray diffraction, atomic force microscopy and X-ray photo-electron spectroscopy. Their electrochromic (EC) performances were studied using cyclic voltammetry, alternating current impedance and optical transmittance measurements. The proper addition of Ti helps the films achieve excellent EC behavior, including the stable coloration–bleaching cycles, high optical transmittance modulation (∼78%), great coloration efficiency (96 cm2 C−1) and low alternating current impedance. Finally, the multiple-layer stacks ITO/NiOx:Ti/PVB(Li+)/WO3/ITO were laminated based on the optimization of NiOx:Ti single layers. A large optical contrast (∼60%), a fast response time (3.2 and 4.4 s) and a good durability were obtained for the all-solid-state full EC device.

Optical, electrical, and electrochemical properties of indium tin oxide thin films studied in different layer-structures and their corresponding inorganic all-thin-film solid-state electrochromic devices

In this investigation, the indium tin oxide (ITO) film layers were prepared by reactive direct current magnetron sputtering at room temperature with different deposition parameters on six different substrates: glass, polyethylene terephthalate (PET), NiOx/ITO/glass, NiOx/ITO/PET, WO3/ITO/glass, and WO3/ITO/PET. The ITO deposited on WO3/ITO/glass showed great advantage than others having a resistance as low as 9.2 Ω/◻ and a high average optical transmittance of 77.1% and reaching the balance between the optical and electrical properties of ITO thin films. Meanwhile, the WO3 film as electrochromism layer on ITO (under different deposited parameters)/glass shows nearly perfect reversibility, charge density of 20 mC/cm2, and fast response time of about 2–6 s. With the outer ITO thin film layer optimized, the electrochromic devices (ECDs) with typical five-layer structure were deposited. ECDs show the reversible dark-blue-coloration and bleaching cycles, large optical contrast, fast response time, and good sta...

Preparation and properties of all-solid-state inorganic thin film glass/ITO/WO3/LiNbO3/NiOx/ITO electrochromic device

The all-thin-film inorganic electrochromic device (ECD) with LiNbO3 as the ion conductor layer was prepared. The ECD was fabricated monolithically in a same vacuum chamber layer by layer using DC reactive sputtering for WO3, NiOx and ITO, and radio frequency (RF) sputtering for LiNbO3. The properties and performance of WO3 thin film and the ECD were studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet-visible spectrometry. WO3 thin film has more than 60% optical modulation with porous amorphous structure. The visible transmittance modulation of the ECD is more than 65%, and the response time of coloring and bleaching are 45 s and 25 s, respectively.

Electro-optical performance of inorganic monolithic electrochromic device with a pulsed DC sputtered Li x Mg y N ion conductor

Lithium magnesium nitride (LixMgyN) thin films were deposited by pulsed DC reactive magnetron sputtering from a LiMg alloy target in the mixture gas of Ar and N2. The as-prepared LixMgyN films were amorphous. A monolithic inorganic electrochromic device (ECD) based on WO3/NiO complementary structure was fabricated using the LixMgyN as the ion conductor layer. The addition of a 150-nm thick Si3N4 buffer layer between LixMgyN and NiO made coloration and bleaching reversible and stable. Electrochemical and optical characterizations were conducted to evaluate the performance of the ECD. Electro-optical data were recorded for both 1000 chronoamperometric cycles and 1000 voltammetric cycles. Activation and degradation of the electro-optical properties of the ECD were observed.

Lithium trapping as a degradation mechanism of the electrochromic properties of all-solid-state WO3//NiO devices

There has been keen interest for years in the research of all-solid-state transmittance-type electrochromic (EC) devices due to their various applications especially in ‘‘smart windows’’. A step forward has been taken in the successful preparation of full multilayered devices with enlarged optical contrast and fast switching response. However, limited durability remains a severe issue. Upon cycling, EC devices suffer from decline of charge capacity as well as optical modulation while the detailed degradation mechanisms remain poorly understood. Here, we demonstrate unambiguous ion-trapping evidence to interpret the charge density decay of the EC device induced using various voltammetric cycling protocols, namely long-term cycling and accelerated cycling. Pronounced comparable ion trapping occurs in cathodically colored WO3 films whatever the cycling procedure is, suggesting the existence of the trapping ‘‘saturation’’ phenomenon. From second-ion-mass-spectroscopy analysis, the 7Li+/184W+ ratio in the degraded WO3 films is more than 100 while it is almost zero in the as-prepared films. In contrast, for anodically colored NiO, a larger number of trapped cations is determined in the long-term cycled films than in the accelerated ones. In combination with X-ray-photoelectron-spectroscopy, variable bonding energies indicate that the ions are trapped at different types of sites, depending on the cycling procedure, and they can reside in the structural channels or break the network chains to form new chemical bondings, thus resulting in a significant color difference. In addition, a clear upward trend in the trapped Li concentration along with depth is observed. All our findings provide a deep insight into the degradation phenomenon taking place in electrochromic films as well as in full devices and offer valuable information for the understanding of micro mechanisms.