Full colour e-posters could reach 10,000 dpi
Designing a flexible sheet of e-paper pixels as a proof-of-concept, the researchers claim the technology could cut today’s electrophoretic e-papers power consumption more than ten folds while allowing very high resolutions on large scale formats, for posters or foldable e-readers less than a micrometre thin.
Publishing their findings in the Advanced Materials journal in a paper titled “Plasmonic Metasurfaces with Conjugated Polymers for Flexible Electronic Paper in Color“, Andreas Dahlin and his PhD student Kunli Xiong report a seemingly simple reflective display architecture.
Using parallel lithographic processing steps compatible with large areas and plastic films, they created plasmonic metasurfaces made up of: a base reflection layer of silver, 150nm thin; a spacer layer made of alumina whose thickness determined the reflective colour obtained by Fabry–Pérot interference; and last a 20nm thin layer of gold patterned with nanoholes 150nm apart. The reflective colour of the base plasmonic metasurface can be varied across the whole colour spectrum through the alumina deposition process, choosing a thickness from 40 to 95nm (the primary colours red, green, and blue corresponding to an alumina thickness of 48, 93, and 83nm respectively).
“The nanohole array in the thin gold film enhances the coloration since it enables coupling to surface plasmons and provides strong resonant scattering”, the paper explains.
In order to control reflectivity and be able to turn the reflective colour “on” and “off”, the researchers topped the plasmonic metasurface with films of doped polypyrrole, forming conjugated conductive polymers whose optical absorption could be modulated electrically. A liquid electrolyte completed the “pixel circuit”, turning it “on” (reflective) under a -1V bias or “off” (dark state as the metasurfaces absorb light).
modulating the optical absorption of a layer of
Polypyrrole on top of the gold nanohole array.
In their experiments, the researchers characterized samples of plasmonic metasurfaces forming 50μm pixels of the primary colours red–green–blue, achieving the same reflectivity and contrast as ink produced by an ordinary printer, with a viewing angle up to 60° for correct colour appearance. Offering good contrast, with a polarization-independent resonant reflection over 90%, the e-paper had response times in hundreds of milliseconds and operated with less than 0.5mW per square centimetre.
Because of the low voltages and the thin polymer films, the paper reveals that power density is more than one order of magnitude lower compared to emissive displays and also much less than that for existing electrophoretic displays. Also, the lithographic process steps that were used could easily scale the pixels down to a few microns, leading to maximum display resolutions over 104 dpi.
Although the display is not exactly bi-stable and requires a voltage potential to maintain the “on” state, leakage current density is very low, under 400 microA/cm2 at 1.0V, wrote us CUT Associate Professor Andreas B. Dahlin. “However, the polymer can maintain its oxidation state even with the voltage off for 5 to 10 seconds, and this can probably be improved with other electrolytes” he added.
“We are currently looking into the possibility to use cheaper materials for the solid state parts, other polymers for even better contrast and different electrolytes” Dahlin concluded. For a possible industrialization, transparent flexible conductors such as graphene or indium-tin-oxide on plastic could be used as counter electrode opposite to the metasurface and encapsulating a thin layer of electrolyte.
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