Osaka University-led researchers developed technology to combine “a transparent paper” with high transparency (90% of visible-light transmittance of paper made from cellulose nanofibers) and a conventional “white paper” made from cellulose pulp fibers, fabricating a highly transparent electrode and a white electrolyte with high visibility. Through the combination of the electrode and electrolyte, they produced paper-based electrochromic (EC) displays. Their research results were published in ACS Applied Materials & Interfaces.
In EC devices, when voltage is applied to a transparent EC electrode, ions or electrons move into the EC layer in the electrolyte (ionic liquid), resulting in coloration or decolzoration. However, conventional EC devices had problems: sealing was necessary for preventing leakage of the electrolyte, making thin films was difficult, and EC performance was compromised due to evaporation of the electrolyte.
A group of researchers led by Hirotaka Koga succeeded in preparing a paper electrolyte by supporting a non-volatile electrolyte (1-butyl-3-methylimidazolium tetrafluoroborate [bmim]BF4) on the surface of cellulose pulp fibers through hydrogen bonding. Furthermore, they evenly coated conducting polymers with EC function, poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS), onto the entire surface of the transparent paper made from cellulose nanofibers, fabricating a transparent EC paper electrode.
By sandwiching the as-prepared LiClO4/[bmim]BF4@paper electrolyte between EC conductive PEDOT:PSS-coated transparent cellulose nanofiber papers (denoted as PEDOT:PSS@nanopapers) as a transparent EC electrode, this group fabricated an EC paper device.
This EC device not only resolves the above-mentioned problems, but is also flexible and easily bent because the whole device is paper-based. In addition, a white paper electrolyte with high optical reflectance enhances the visibility of the EC displays.
The researchers have created a new application for paper, which has traditionally acted as a medium on which to display information by writing and printing, as a display using electricity. They have succeeded in developing various paper-based electronic devices, such as memory, transistors, antennas, and supercapacitors. If these technologies are integrated, it will become possible to produce paper-based electronic books as well.