Printed passive matrix addressed electrochromic displays
Graphical abstract
Introduction
Major efforts are currently being devoted to explore and develop new devices and systems for printed electronics applications [1], [2], [3], [4], [5], [6], [7]. The theme of this effort aims at providing novel electronic functionalities on flexible substrates such as plastic foils or papers. Flexible display devices are one of the crucial cornerstones of this research and development effort, and a large variety of different flexible display devices and systems have successfully been demonstrated during the last decade, e.g. electrochromic displays, organic light emitting diode displays and electrophoretic displays [8], [9], [10], [11], [12]. Since such devices and display systems most often are manufactured in a complex production flow containing one or several vacuum processing steps, there is still a huge technical barrier to carry out an entire manufacturing process using standard sheet-based or roll-to-roll printing, coating and lamination techniques.
Electrochromic (EC) active matrix addressed displays (AMAD) have been explored for various low-end display applications, in particular for areas in which the requirements on e.g. switching time and pixel resolution are a bit relaxed. EC-AMADs, which are built up from pixels including a display element and an addressing transistor, have been demonstrated on either paper or plastic substrates [12], [13], [14]. Even though the very same materials have successfully been utilized as the active material in both display elements and transistors, EC-AMADs still require a fairly large number of printed layers, thus ruling out simple manufacturing and low cost.
In passive matrix addressed displays (PMAD) addressing transistors are not utilized to provide addressability for the display element. In PMAD configuration, the system complexity is then much lower which enables a relatively much simpler manufacturing scheme. However, one particular drawback of PMADs is that they typically suffer from cross-talk effects, i.e. the applied voltage aiming to update one specific pixel, at the intersection of a certain row and column, also affects and causes coloration of neighbouring non-addressed pixels [15]. There are various ways to circumvent such cross-talk effects in PMADs, for example by combining each pixel with a diode exhibiting rectifying and strong non-linear current vs. voltage (I–V) characteristics [16], but yet again, additional components implies an increased number of processing steps.
Here, we have developed EC pixels exhibiting non-linear coloration vs. addressing voltage characteristics with a threshold voltage at around ±1 V. The EC pixel exhibits good bi-stability as the addressing voltage is decoupled. The electrode and electrolyte materials were deposited by screen printing and wire bar coating, respectively, on top of flexible plastic substrates and the number of included layers was kept at a minimum; an electrically conducting carbon paste, an ionically conducting polyelectrolyte and an electronically conducting electrochromic polymer served as the counter electrodes, the electrolyte and the pixel electrodes, respectively. This very simple three-layer device architecture was enough to define an entire EC-PMAD containing up to 7 × 128 EC pixels. This EC-PMAD, based on the robust and simple device architecture shown in Fig. 1, is promising as the display interface in all-printed electronic systems in applications such as distributed diagnosis, home healthcare or sensor platforms monitoring the status of perishable goods in logistic chains.
Section snippets
Materials
Transparent polyethylene terephthalate (PET, Polifoil Bias) film was purchased from Policrom Screen and was used as the solid substrate carrying the display components. As a top electrode, PET film with pre-coated PEDOT:PSS (Orgacon EL-350) was purchased from AGFA-Gevaert. This product was also used as the counter electrodes for the reference samples. Conducting carbon paste (7102) for screen printing was purchased from DuPont and printed onto the PET substrate by using a 120-34 nylon screen
I–V characteristics of EC pixel devices in ambient atmosphere
Carbon-based counter electrodes have previously been used in various applications related to iconic electrochromic displays and smart windows [18], [19]. Carbon is considered to be an advantageous counter electrode material in electrochemical devices for several other reasons; its durability and stability has been proven excellent in commercial electrolyte capacitors, its resulting electric double layer have high charge storage capacity independently of the polarity of the charges, and counter
Conclusions
Passive matrix addressed electrochromic displays manufactured by printing techniques is demonstrated. The addressability of unique electrochromic pixels in the resulting display is obtained by the introduction of a threshold voltage that is generated by combining a carbon-based counter electrode with a PEDOT:PSS-based pixel electrode that together sandwiches a polyelectrolyte. The threshold voltage that enables control of pixel charging, and thus also coloration, is governed by electrolysis.
Acknowledgements
Prime funding for this project was provided by Lintec Corporation. Also, the authors wish to thank the Centerprise project at VINNOVA (research and innovation for sustainable growth in Sweden).
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