Investigation of Electrochemical Relationship between Kinetically Sprayed Film and Electrolyte for Electrochromic Device

Abstract

The electrochromic device (EC) based on consumption of low external voltage, can be used to change its optical transmittance in accordance with user’s demands; it is known to be the next generation for energy saving technology. The device is composed of an EC film, ion storage layer and an electrolyte layer. In the case of EC device, its improvement in EC performance greatly depends on electrochemical properties within EC film, the ion storage layer, and the electrolyte. Conventionally, the functional film for the EC device, can be fabricated using solution based processes, such as sol-gel and spray pyrolysis, which required the post-processing, the cracks on the surface of the thick film are easily generated due to its rapid evaporation of solvent through the annealing step, resulting in the degradation of the EC device. Moreover, the sputtering deposition method requires high vacuum condition, increasing fabrication cost of the EC device. Therefore, we have used particle printing technology in order to overcome these drawbacks to deposit thin films in this study. The particle printing technology used in this study is kinetically sprayed process, which uses nano- to micro-sized particles to deposit thin or thick layer on various substrate at supersonic speed, resulting in good adhesion between film and substrate. Since the powder forms a thin film on the substrate with the mechanical impact, the surface roughness is a problem compared to the sputtering based thin film. However, in the case of the thin films contacting the electrolyte layer, the EC performance can be enhanced by relatively rough porous thin film reacting better with ions in the electrolyte due to its increase in the activated surface area. Moreover, the electron transport path can be increased by controlling the shape of the porous structure of thin film using various size of particles as well as decreasing the crystalline size of the deposited thin film. Therefore, the EC films were fabricated via particle printing technology using various size as well as materials of powders. The effect of each parameter for the EC device is evaluated through the electrochemical analyses in this study. First, a transparent conductive electrode (TCE) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) was developed using a particle printing technology for the application of an EC device. To improve its electrical conductivity and stable EC performance, Ag NW and TiO2 nanoparticles were included, resulting in hybrid film showing sheet resistance of 23 /sq. We have found that the film could be used for the EC device. The transmittance change at the colored states, was 23% at 630 nm upon the voltages of ±2.0 V as well as cyclic transmittance with a switching voltage for 3 h showed stable transmittance of 31%. Moreover, two types of ion storage layers were fabricated on ITO-glass substrates; a) similar microstructure with different materials, such as NiO and ATO, b) various morphology using different size of NiO powders. A porous film was formed using nano-sized NiO and antimony tin oxide (ATO) powders. Electrochemical analyses revealed that the nano-porous NiO layer had a high charge capacity with a low charge transfer resistance. Moreover, an EC device using a NiO film with nano-sized pores had an optical transmittance difference of 42% and a stable cyclic transmittance for 1 h at its wavelength of 630 nm. In addition, we have evaluated that the EC device was fabricated with a NiO ion storage layer having a high charge capacity. As a result, the ion storage layer formed from smaller-sized crystalline NiO particles, had improved initial transmittance as well as enhanced electrochemical reaction; this was attributed to its low internal resistance and the high diffusion rate of ions in the electrolyte. Finally, we have developed low voltage modulated inorganic the EC device using solid polymer electrolyte (SPE), which is composed of UV-cured electrolyte with ferrocene (Fc). As a result, the EC device using the 0.1 M Fc, a high transmittance changes of more than 60 % under the low potential change of 2.5 V, was shown due to its role of because Fc into the electrolyte, assisting the ion transfer to the thin films. Moreover, this EC device with excellent self-bleaching effect, resulted in improved transmittance by 37 % for 2,000 s. Therefore, we have proven that the relationship between the thin film and an electrolyte, was evaluated by controlling the parameters for the device, such as an EC film, an ion storage layer and an electrolyte layer, resulting in demonstration of enhancing EC performance; An energy level design for the device can be improved for its fast charge transfer and effective separation of e-hole pair, resulting in high transmittance change with a stable switching. In addition, a stable and high EC performance can be achieved through improving the electrochemical properties in ion storage layers, which has a high charge capacity, low charge transfer resistance and high diffusion rate. Finally, the EC device with high transmittance change and stable switching performance up to 1,000 cycles, was successfully demonstrated using 0.1 M Fc based solid polymer electrolyte through reducing its internal resistance of the EC device.