WO3 for reflective displays in color

Licentiate thesis, 2021

Reflective displays, also known as electronic paper, are very attractive due to their low power consumption and great visibility under bright ambient light conditions. However, reflective displays in color are still limited on the market for two main reasons. First, their low absolute reflectivity requires the integration of a strong backlight which has the consequence of a higher power consumption and the loss of the paper-like appearance. Second, their color quality (chromaticity) is poor. Both reasons are the result of how the red, green and blue (RGB) subpixels are generated: color filters are applied on top of white surfaces. The 3 subpixels design itself, limits the reflectivity to 33% for each color, moreover the addition of color filters makes it drop to less than 30%.

Plasmonic structural colors based on metal-insulator-metal Fabry-Pérot cavities, on the other hand, have been proven to show great reflectivity (>90%) and chromaticity and, combined with electrochromic materials, could be used as pixels in reflective displays. In this thesis 2 types of these metasurfaces are fabricated, combined with the inorganic electrochromic material tungsten trioxide (WO3) to generate dynamic structures.

The first type includes an aluminum mirror, an alumina spacer layer, and a top gold film with a nanoholes array. The thickness of the alumina spacer layer is tuned to select the reflected wavelengths via thin film interference to fabricate RGB surfaces. The nanoholes on the top layer, moreover, contribute to the chromaticity via plasmonic resonance. On top of the Fabry-Pérot cavities, we deposit a layer of WO3. This electrochromic material, once placed in an electrochemical cell with an electrolyte containing lithium ions, undergo an optical change (it turns deep blue) when a voltage is applied, hence when the ions are intercalated or de-intercalated in the film. As a result, the RGB pixels can be turned on and off, completely or partially, to display all the other colors.

The second type is also based on a Fabry-Pérot cavity, with a platinum mirror and a top gold layer. The difference relies in the spacer layer, which is constituted by the electrochromic WO3. Here are presented 2 designs based on this, one with nanoholes in the top gold film and one with nanoholes in the bottom platinum mirror. In both designs the holes only serve as passages through the metal to the WO3 film and do not show any plasmonic resonance in the visible part of the spectra. The result is dynamic structures that can reflect several different colors due to the change in the optical constants of the spacer layer. With this design it could then be possible to move away from a 3 subpixels configuration towards a 2 or 1 subpixels configuration with higher reflectivity.