Conjugated polymers on plasmonic metasurfaces for high contrast electronic paper in color

Licentiatavhandling, 2021

Electronic paper is the collective name of displays that, instead of emitting light, reflect the ambient
light. These displays are known to have good readability in daylight and very low power consumption.
However, there is a lack of electronic paper in color with high contrast and the ability to achieve video
speed. This work presents that by utilizing a plasmonic metasurface, highly reflective subpixels in red,
green, and blue can be achieved. Electrochromic materials are used to modulate the reflectivity,
turning the pixels ON and OFF.

Conjugated polymers, such as polypyrrole, polyethylenedioxythiopene (PEDOT), and poly(dimethylpropylenedioxythiophene)
(PProDOT-Me2), are known for their electrochromic properties and are
evaluated for their optical performance in organic solvent on a thin gold film. The optical contrast of
a film depends on both the polymer and the thickness, and it is found that by measuring the optical
extinction in both the bleached (transmissive) state and the colored (absorbing) state of an arbitrary
thickness, the maximum contrast of the polymer can be extracted by using the ratio of the optical
extinction values.

PProDOT-Me2 is shown to have the highest maximum contrast and is electropolymerized on top of
the red, green, and blue plasmonic metasurfaces with an excellent optical contrast of 50%-60%.
Tungsten oxide is evaluated as an inorganic electrochromic option and compared with PProDOTMe2—
Its optical contrast is around 60% for all samples. The bistability is in favor of tungsten oxide,
which showed little to no change after 1000 s, while PProDOT-Me2 has a slight change after 100 s. On
the contrary, PProDOT-Me2 had a switching time of less than two seconds, while tungsten oxide had
ten times higher.

For PProDOT-Me2, the distance between the electrodes with the polymer and the counter electrode
influences the switching time. Reducing this distance below 1 mm and using a suitable solvent resulted
in a time between 10-50 ms, i.e. in the video speed regime. By producing a positive curvature on the
electrode and deposit the polymer, it is also possible to keep the contrast high with video speed.
Using fast pulses also facilitated to switch the polymer from bleached to colored more than 10 million
times with low degradation using an ionic liquid. The previously reported number of switches is one
order of magnitude less.