Electrochromic nanostructures based on tungsten trioxide for reflective displays

Doctoral thesis, 2022

Our lives are dominated by the usage of displays. Every day we spend hours looking at displays and our eyes are surely affected by them. Reflective displays have gained a lot of attention in recent years due to their lower impact on our eyes' health, very low power consumption, and high performance in sunlight. In this work structural coloration and electrochromic materials have been explored to enhance the performance of this kind of display. We fabricated colored surfaces based on Fabry-Perót and plasmonic coloration. The nanostructures consist of a stack of three thin films in a metal-insulator-metal manner. By tuning the insulator thickness we can achieve red, green, and blue vibrant colors. Electrochromic materials, and in particular tungsten trioxide (WO3), are implemented on top of the structures to act as on/off shutters. The optical properties of WO3 can indeed be electrochemically tuned, so that the reflectivity of the colored structures can be controlled with high contrast. Moreover, we have fabricated a similar kind of structural color, where WO3 replaces the insulator middle layer in the cavity. In this configuration, the cavity is not shut on and off, but its resonance (the color seen) is shifted when WO3 changes its optical properties. Hence, the surfaces can display several colors and a combination of two of them could generate all the colors necessary for a display. Lastly, we have made a step forward towards a functioning display by succeeding in the fabrication, using several lithography steps, of the colored structures on thin film transistor (TFT) arrays.