Highly Flexible Electrochromic Devices Enabled By Electroplated Transparent Nickel Grid Electrodes

Electrochromism refers to the change of molecular structure caused by the chemical reaction and the directional movement of electrons and ions of the color-changing material under the action of an applied electric field,and the change of transmittance reflectance absorption rate of the device caused by the incident light.Electrochromic devices are widely used in electronic devices,such as display,intelligent discoloration window,anti-glare rearview mirror for their low power consumption,rich color change,high contrast and no visual angle difference.The traditional electrochromic devices are composed of five layers:upper and lower transparent conductive layer,electrolyte layer,electrochromic material layer and ion storage layer.However,the multilayered structure increases the structure complexity of the device,hinders the ion transport between layers.Indium tin oxide(ITO)transparent conductive film is widely used in electronic devices,,the inherent brittleness of the ITO limited its application in the flexible electronic devices.Carbon nanotubes and graphene have excellent optoelectronic properties,but it is difficult to be produce large areas transparent electrode based on them.Silver nanowires have excellent optoelectronic performance,but poor electrochemical stability.And meanwhile they are prone to oxidation and dissolution in electrolytes.In view of the complexity of the multilayered structure of the traditional electrochromic devices and the poor performance of the transparent conductive films,we proposed and fabricated a flexible metal nickel grid electrode by photoetching mask and electrochemical deposition method.Additionally,the introduction of hydrogel simplify the complicated structure of the original electrochromic device,reduced from five layers to three layers,which greatly reduces the process complexity and improves the overall stability and flexibility of the device.The main research contents and results are as follows(1)Preparation and study of metal nickel mesh transparent electrode with high transmittance,high flexibility and ultra-low resistivity:Laser direct writing lithography technique is used to fabricate high precision target image on a mask.Then,the target structure was obtained by mask exposure and development on ITO substrate coated with photoresist,exposing the patterned conductive micro-grooves.Nickel particles were deposited by electrochemistry at constant current,and then transferred to the flexible substrate by UV curing process.The metal nickel mesh transparent electrode prepared exhibits excellent structural stability and electrochemical performance,with transmittance as high as 90%,sheet resistance lower than 0.5?/sq,and good mechanical flexibility.,which can be used as an ideal flexible conductive substrate for future flexible electronic devices.Flexible nickel grid electrodes with arbitrary distribution and complex structure can be prepared by high-resolution laser direct writing technique,electrochemical deposition and other techniques based on software-based graphic structure design,showing huge commercial and scientific prospects(2)Preparation and research of three-ayered flexible electrochromic devices based on hydrogel:The liquid electrolyte has good ion mobility,which affects the response time and contrast of the device,however,the liquid electrolyte is difficult to package and electrolyte leakage;Solid electrolyte can solve the problem of difficult packaging,but the decrease of ion mobility makes the response time longer,which affects the performance of the device.The gel electrolyte is located between the liquid electrolyte and the solid electrolyte,which can avoid the problems of difficult encapsulation of the liquid electrolyte and low ion mobility of the solid electrolyte,and can maintain good ion mobility without electrolyte leakage;The preparation method of gel electrolyte is simple,and it can be well combined with electrochromic layer and transparent conductive layer of flexible nickel mesh electrode.The transmittance of hydrogel electrolyte is higher than 95%,which greatly improves the initial transmittance of electrochromic devices.The hydrogel layer can act as the electrolyte layer and the transparent conductive layer at the same time,which reduces the five-layer structure to three layers,greatly reduces the structure complexity of the device,and avoids the degradation of ion migration performance caused by the multi-layer structure.At the same time,hydrogel has excellent bending and stretchable properties,which further improve the bending performance of electrochromic devices and are expected to be used in emerging product fields such as flexible capacitor and:flexible battery for flexible electronic displays.