Electronic paper in color by electrochromic materials and plasmonics

Doctoral thesis, 2023

The most common display today is emissive. It produces its own light and emits it to the viewer's eye. A reflective display, also known as electronic paper, uses ambient light and reflects it to the viewer, just like a newspaper. Electronic paper has some advantages over emissive displays such as: lower power consumption and readability in sunlight. Today, electronic papers in color lack a desirable color gamut - the colors look bad. The purpose of this thesis is to investigate how structural colors, plasmonics and electrochromics can be used to increase the optical performance of electronic paper in color.
By using structural colors (metal-insulator-metal) and plasmonics, we could create highly reflective color pixels. The pixels could be made to turn ON and OFF using electrochromic materials. In this thesis, conjugated polymers (PProDOT-Me2 and PProDOP) or tungsten oxide were employed. The reflection difference between the ON and OFF states was 60%. This was better than previously reported values for other electrochromic materials.
If the electrochromic material instead was incorporated into the nanostructure (metal-electrochromics-metal), applying a voltage would then alter the color of the pixel. If tungsten oxide was used inside the structure, the color of one pixel could change, but it would not be able to span the whole visible spectra. If, instead, the conjugated polymer (PT34bT) was used inside the structure, the whole visible spectra could be accessed with one pixel.
To create a real display, it is not enough to have one pixel that can change color. Millions of pixels in a grid are necessary. This poses a problem since each pixel needs to be individually contacted. This can be overcome by using a matrix configuration such as a passive matrix (PM) or an active matrix (AM). This thesis investigates both these configurations. PM requires the color change to be strongly non-linear with the applied voltage. It must have memory such as hysteresis. This effect can be incorporated by utilizing an indium-tin-oxide electrode as a counter electrode to a metal working electrode coated with a conjugated polymer as electrochromic material. To avoid crosstalk between pixels, a photo patterned electrolyte was used.
Commercial thin-film transistor arrays were used for AM configuration. The red, green, and blue nanostructures were deposited on the array. The conjugated polymer PProDOT-Me2 is synthesized directly on individual pixels and switched without crosstalk.