Electrochromism denotes the characteristic colour change of a material associated with the material’s reduction-oxidation state. The electrochromic polymer that we have investigated most extensively is poly(3,4-ethylenedioxythiophene) (PEDOT) with poly(styrene sulfonate) (PSS) providing the counter ion (PEDOT:PSS).
Electrochemical switching of electrochromic materials results in different optical absorption spectra. Oxidation of PEDOT:PSS results in an absorption spectra in the near infra red region due to more free charge carriers, i.e. bi-polarons, and the initial sky-blue coloured polymer turns to a more transparent state. Upon reduction, on the other hand, PEDOT:PSS absorbs strongly in the red/orange wavelength region and appears dark blue to the human eye. The colour shift in PEDOT:PSS can be utilized in electrochromic display cells.
An EC display element consists of at least two conductors, an electrochromic material, and an electrolyte combined on a carrying substrate. The schematic drawing shows an EC paper display made entirely of organic polymers (the electroactive materials) and an organic electrolyte on ordinary double-coated fine paper. The display element is updated by applying a voltage of typically 0.6 to 0.9 V. Switching the colour takes about 1 second while the associated memory time is on the order of 10 minutes to several hours. The energy required for one switch cycle (1cm2 display area) is less than 1mJ. Colour contrasts up to 20 (CIE l.a.b. colour coordinates) have been demonstrated in our EC paper displays.
EC displays are manufactured as follows:
First, a thin film of a conducting polymer is coated onto fine paper. The coated conducting polymer is then patterned, using offset- or screen-printing, and a high contrast electrochromic polymer colorant is deposited on top. A polymer electrolyte layer is printed on the conducting polymer patterns, forming the display elements. As the final step a protective coating is added to seal the display.
The resulting paper display is fully flexible and the printed devices are less than 100 micrometer thick. Today, the EC displays can be manufactured in separate printing tools in a handcraft fashion. Design of the electronics can be achieved using traditional graphical design tools, such as Adobe Illustrator software.
In the EC paper project we are developing an all-in-line reel-to-reel printing process for organic electrochromic (EC) paper displays.
Examples of displays are shown in our demonstrator photo exhibition.