01 reading guide


As a promising color display technology, electrochromic display technology has been widely studied. At present, the most advanced inorganic polychromatic electrochromic displays use nanostructures that sacrifice transparency, but also limit the diversity of their applications.

Recently, the research team led by Professor abdulhakem y. elezzabi and Dr. haizing li of the ultrafast optics and nanophotonics Laboratory of the University of Alberta, Canada, published a paper on light: Science & applications. The team proposed a new concept of inorganic multi-color display integrating high transparency and energy efficiency for the first time, A transparent inorganic polychromatic display based on zinc based electrochromic devices is developed. This device enables the top and bottom electrochromic electrodes to operate independently, thus providing additional configuration flexibility for the device by utilizing dual electrochromic layers in the same or different color states.


The zinc based electrochromic display material prepared in this study is the most promising key to achieve high energy efficiency, transparent, inorganic multi-color display.




02 background


In recent years, electrochromic devices have attracted more and more attention in various application fields, including smart windows, displays and color tunable optical elements, due to their zero energy consumption and optical transparency or color. In particular, multicolor electrochromic displays are one of the most common applications because they can maintain their color state without power supply. Although electrochromic displays based on organic molecules, polymers and metal organic frameworks show color characteristics, the thermal and chemical stability of these materials are lower than that of inorganic electrochromic materials. These shortcomings seriously hinder their application and potential commercialization in the real world.


Recently, inorganic multi-color electrochromic displays have realized the multi-color function of monochrome WO3 thin films by combining photon Fabry Perot nano cavity or plasma chromium metal insulator nano hole cavity, so that these devices can support multi-color processing in reflection mode. In order to achieve a wide range of applications, a device configuration with both high optical transparency and color state is urgently needed.


It should be noted that in terms of energy efficiency, the above reflection mode device consumes electric energy and still needs external voltage to trigger the dyeing / bleaching process. Although the field is still in its infancy, the power recovery of multi-color electrochromic displays will make the devices have high energy efficiency, especially in large-area displays.


In addition to nano based inorganic multi-color displays, the color superposition strategy is also a simpler method. The color palette of an inorganic electrochromic device is enlarged by superimposing different color layers (E. G., color superimposition). Vanadium oxide (V2O5) is considered as the most promising inorganic material for multicolor electrochromic displays. As far as we know, vanadium oxide can only achieve three colors (yellow ⇄ green ⇄ blue) by using the configuration of traditional electrochromic equipment. When the traditional electrochromic equipment is running, the simultaneous coloring of the counter layer limits the effect of color superposition.

Since the V2O5 electrode can be used as both an electrochromic layer and a counter layer, the configuration shown in Fig. 1A can be configured to eliminate the simultaneous coloring effect of the counter V2O5 electrode. However, because the top and bottom of V2O5 electrode can only be colored in the anti redox state, only three colors can be realized in different redox states when the equipment is running. This study shows a zinc based electrochromic device consisting of a thin zinc foil anode sandwiched between two electrochromic electrodes (e.g. WO3, Pb) (Fig. 1b). Due to the monochromaticity of WO3 and Pb films, these devices show great potential in the field of smart windows.


In this study, we propose a new concept, transparent inorganic polychromatic electrochromic display, using sodium ion stabilized vanadium oxide (SVO) nanorods as electrochromic materials. Although there are many studies on the effect of sodium ion doping on the conductivity of SVO in zinc ion batteries, so far, there is no report on the application of SVO in electrochromic devices. At present, the most advanced vanadium based electrochromic display research focuses on the development of nanostructured vanadium oxide (such as V2O3, v3o7, V2O5), but does not pay much attention to potential electrochromic materials, such as SVO. When mixed with cellulose, SVO nanorods are compatible with a simple rod coating method for manufacturing electrochromic films. Due to the oxidation property of SVO, the added cellulose can be fully decomposed at a low temperature of 200 ℃, avoiding its influence on conductivity.


Zinc sodium vanadium oxide (Zn SVO) electrochromic displays are assembled by clamping Zn between two SVO electrodes. They can be reversibly switched between a variety of colors (orange, amber, yellow, brown, yellow green) while maintaining high optical transparency. These Zn SVO electrochromic displays represent the most colorful transparent inorganic material based electrochromic displays so far. In addition, the Zn SVO electrochromic display has an open circuit potential (OCP) of 1.56 V, which makes it have self color rendering behavior and remarkable energy recovery function. For the first time, transparent inorganic polychromatic displays have more than three colors. Polychromatic characteristics and energy recovery function are expected to become an important catalyst to accelerate the development of energy-efficient electrochromic displays in the future.


03 innovation research


SVO electrode is prepared by applying a well-designed environmental friendly svo/ cellulose slurry by rod coating method. The electrochromic electrodes at the top and bottom can operate independently in the same or different redox states, so that the color superposition effect can greatly broaden the palette range. In addition, compared with the operation of traditional electrochromic devices, which require external voltage to trigger the coloring / fading process, it is worth noting that this new zinc based electrochromic device structure can realize self color development through its built-in battery power supply without external energy input, so that it can partially recover the power consumed in the fading process.

Fig. 2 characterization of SVO nanorods


Source: light SCI appl 9, 121 (2020) （Fig.2）


Specifically, SVO electrode realizes reversible color switching (orange ⇄ yellow ⇄ green) with zn2+ insertion (self coloring / discharge) and extraction (fading / charging), and has high optical transparency. Three inherent orange, yellow and green are used as basic colors to develop polychromatic Zn SVO electrochromism, which is realized through the color superposition effect of two SVO electrodes. The constructed electrochromic display can switch between a variety of colors (orange, amber, yellow, brown, yellow green) and recover part of the consumed energy.

Fig. 3 design and performance of color palette widened Zn SVO electrochromic display


Source: light SCI appl 9, 121 (2020) （Fig.5）



04 conclusion and Prospect


The key feature of this research is the significant improvement of the existing electrochromic displays, which makes Zn SVO electrochromic displays very promising in switchable filters, electrochromic tunable micro optics and transparent displays. The realized devices represent a new example of electrochromic displays, and may promote the development of a new generation of electrochromic displays.