Progress in Aerodynamically Focused Color Tunable Patterns

Due to the global extreme heat wave, the time spent indoor increases as the energy consumption for air-cooling rapidly rises. For this reason, the need for an electrochromic smart window which can control the transmittance of light and heat energy upon application of voltage, is on the rise. Properties of the electrochromic devices (ECDs) can be controlled by the types of the electrochromic materials as well as the surface property of the electrochromic layer. Electrochromic technology can enable optical modulation in visible and infrared ranges through low energy consumption and high controllability. These technologies can expand the range of various substrates including wearable electronic devices. Previously we have fabricated various electrochromic layers using nanoparticle deposition system (NPDS) which is one of the dry deposition methods. In NPDS, various ceramic/metal powders are supersonically sprayed to the substrates through the pressure difference using the air-compressor and the vacuum chamber, inevitably making film rough. Since thin films are fabricated using low cost carrier gas in room temperature and low-vacuum atmosphere, the process cost is relatively cheap. NPDS can easily control the physical properties of thin films such as surface roughness or thickness by adjusting its size or phase of powders.
In this study, we have developed a new approach called the Aerodynamically Focused Nanoparticle (AFN) system to make electrochromic patterns. AFN was developed as a solvent-free patterning system that utilizes aerodynamic control to direct nanoparticle flow.
The novelty of this system lies in its ability to fabricate fine micro-width lines, narrower than the nozzle diameter, by adjusting the air pressure in a two-step excitation process. AFN system enables the creation of electrochromic patterns with narrow linewidths on the flexible substrates through dry deposition, thus eliminating the processes associated with solvent evaporation and mask removal. Consequently, this is especially useful to make devices that are flexible and bendable since this process is done at room temperature.
The AFN system would be a suitable technology electrochromic patterns on various substrates including flexible substrates, employing tungsten trioxide (WO₃), a representative electrochromic material. These patterns can reversibly change color from clear to blue, operable at low voltage, potentially enhancing new types of smart clothing and flexible screens. The width and thickness of WO₃ patterns were optimized by changing the nozzle size and nozzle scan speed. Electrochromic pattern devices with these WO₃ patterns successfully developed and tested their ability to change colors using a UV-Vis spectroscopy. The shape of the pattern deposited through AFN was studied through a Scanning Electron Microscope (SEM), while the performance as an electrochromic device is evaluated through a transmittance, cycle test and a bending test.
Therefore, our research discusses the potentials of AFN system to replace conventionally patterning technology such as inket printing and lithography patterning, in further advancing electrochromic device technology, specifically electrochromic patterns.