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Improving the contrast of all-printed electrochromic polymer
on paper displays
Payman Tehrani,a^ Lars-Olov Hennerdal,c^ Aubrey L. Dyer,b^ John R. Reynoldsb^ and Magnus Berggren*a
Received 19th November 2008, Accepted 3rd February 2009 First published as an Advance Article on the web 16th February 2009 DOI: 10.1039/b820677e
PEDOT:PSS-based electrochromic displays have been explored for manufacture on flexible paper substrates in roll-to-roll printing presses at high volumes and low costs. Here, we report the improvement of the optical contrast of such devices by adding an extra layer of a dihexyl-substituted poly(3,4-propylenediox- ythiophene) (PProDOT-Hx 2 ) to complement the optical absorption spectrum of PEDOT:PSS. The oxidized state of PProDOT-Hx 2 is highly transparent and is an intense magenta color while in the reduced state. By adding a layer of PProDOT-Hx 2 directly on top of PEDOT:PSS, we were able to improve the optical contrast by nearly a factor of two. In this report, we present optical and elec- trochemical data of PProDOT-Hx 2 /PEDOT:PSS-based electro- chromic paper displays and compare their performance with PEDOT:PSS-only equivalents.
Over the centuries, paper has served our society as the main carrier for written information and documentation. Associated with paper, a vast array of printing tools, coating techniques and other conver- sion techniques have been developed offering the ability to manu- facture paper-based products at high volumes and low costs. This has led to a vast improvement in the accessibility of information in our modern society. Over the last 50 years, the electronics revolution has made a tremendous impact on daily life with respect to processing, transfer and displaying of information. The next natural leap would be to merge the two worlds of paper and electronics, creating combined features that cannot be achieved in either of the two formats alone. Several attempts have been carried out towards achieving various kinds of e-paper technologies, in part pioneered by consumer prod- ucts companies such as Philips, and Sony Inc, among others, and by technology providers such as E-ink. In these technologies, electro- phoretic inks,^1 Gyricon balls^2 or electro-wetting^3 are used as the fundamental electronic colorant system on large area planar substrates. This technology allows for high resolution and good performance, but at a considerably higher cost when compared to ordinary printed paper. On the other hand, electrochromic (EC) displays4,5^ can be fabricated using a much simpler and robust device architecture when compared to the above-mentioned display tech- niques presently being considered for e-paper technologies. The development of organic EC materials that can merge with the well
established technology of paper substrates promises for high-volume manufacturing of paper displays. Among the organic EC systems, poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS),6,7^ has received much attention due to its high conductivity and chemical stability. By electrochemical means, the doping level of PEDOT:PSS films can be reversibly controlled, resulting in switching of the bulk conductivity, as well as the color,^8 of the film. The redox control of conductivity and color can be utilized in electrochemical transistors^9 and EC displays,^10 respectively. EC switching of PEDOT is relatively fast and fully reversible but higher contrast must be developed for applications in monochromatic displays and electronic indicators. In its oxidized state PEDOT has a light blue, close to transparent, color while in its reduced state it is dark blue in color with an absorption centered around 650 nm as shown in Fig. 1. Careful examination of these results shows the, as expected, low absorptivity of the oxidized form from 350 to 850 nm. The colored state is evident upon reduction with growth of 650 nm absorption. Interestingly, having PEDOT:PSS printed directly on the paper substrate (as opposed to using a metallic contact electrode) inhibits full reduction of the polymer to its elec- trically insulating form. As such, the reduced form in Fig. 1 retains absorption at 800–900 nm due to polaronic charge carriers, and the remnant conductivity allows the material to be repeatedly switched as an electrochrome. In previous studies, an anodically coloring polymer has been utilized on the counter electrode in a vertical structure in order to increase the optical contrast.11,12^ However, in this study we increase the optical contrast by applying an additional layer of an EC polymer with complementary color characteristics to PEDOT in a lateral design, further creating a simplified device from a manufacturing
Fig. 1 Absorption spectra of reduced and oxidized PEDOT:PSS films.
a (^) Organic Electronics, Department of Science and Technology (ITN) Linkoping University, SE-601 74 Norrk€ oping, Sweden€ b (^) The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, FL 32611, USA cAcreo AB, Bredgatan 34, SE-601 21 Norrkoping, Sweden€
This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19 , 1799–1802 | 1799
COMMUNICATION www.rsc.org/materials | Journal of Materials Chemistry
point of view. This complementary color property requires a highly transmissive oxidized state, along with a deeply colored reduced state having a different color (shifted lmax) relative to PEDOT:PSS. Previously, we explored various polythiophene derivatives to enhance the contrast of PEDOT:PSS-based displays^13 with the contrast values calculated using the CIE Lab color system from measurements of the transmission spectra. The previous study highlighted several factors of importance while choosing an EC polymer for use in combination with PEDOT, such as the need for the material to have a low oxidation potential, high ionic conduc- tivity, and spectral absorption complementary to PEDOT. In this current study, we report a bi-layer display structure that includes PProDOT-Hx 2 coated on top of PEDOT:PSS films, where the repeat unit structures of these two components are given in Fig. 2, and the optical switch contrast characteristics and its associated dynamic electrical switch parameters are measured. Indium tin oxide (ITO) coated glass substrates were rinsed in isopropanol, acetone and deionized water before use. PProDOT-Hx 2 was dissolved in chloroform at a concentration of 10 mg ml^1 and the solution was spin-coated on the substrate at 1000 rpm. PEDOT:PSS (400 nm thick film) coated on paper substrates were supplied by Agfa-Gevaert (see Fig. 3a). For electrochemistry measurements, the liquid electrolyte was an aqueous solution of 0.1 M LiCF 3 SO 3. A gelled electrolyte was utilized for the displays, consisting of 7% hydroxyethyl cellulose, 64% water, 21% glycerol, 7% sodium citrate and 1% LiCF 3 SO 3. The PEDOT:PSS-coated paper substrates were patterned using the previously detailed subtractive patterning method by using a Nilpeter Rotolabel FA3300/5 label printer^14 modified with a rotary screen printer, as shown in Fig. 3b. A UV-curable electrolyte, consisting of a commercial UV-lacquer mixed with salt (1% NaCl), was patterned through the rotary screen mesh with a conducting squeegee. The squeegee was biased as the cathode while the PEDOT:PSS film was anodically biased. A 150 V potential is applied between the PEDOT:PSS film and the squeegee, and where the PEDOT:PSS is in contact with the electrolyte, it becomes irreversibly over-oxidized and permanently loses its electronic conductivity. The electrolyte was then exposed to a UV source and cured, substantially reducing its conductivity. The resulting resistance between the electrodes in the display was typically greater than 1 MU. This process allows for the PEDOT:PSS electrodes to be patterned at manufacturing speeds from 5 to 10 m min^1.
The PProDOT-Hx 2 films were then spin-coated from chloroform solutions at various concentrations at 1000 rpm onto the PEDOT electrodes (resulting in different thicknesses from 5 nm to 160 nm). A 200 mm thick plastic film, containing holes for the electrolyte, was then laminated onto the films to serve as a gasket for the gelled electrolyte. The holes were then filled with the aqueous electrolyte and heated at 50 C for 2 min to cure the gelled electrolyte. An adhesive tape was used to encapsulate the device for both mechanical protection and to prevent drying out of the electrolyte. Electrical and optical characterizations were performed for the EC displays using a test pixel configuration (see Fig. 2a), which includes adjacent PEDOT:PSS electrodes patterned via electrochemical over-oxidation. Three-electrode electrochemical cell measurements were performed with platinized titanium as the counter electrode and Ag/AgCl as the reference electrode with a 0.1 M aqueous solution of LiCF 3 SO 3 as the electrolyte. Absorption spectroelectrochemical measurements of the PProDOT-Hx 2 films coated on ITO were measured with a Lambda 900 UV-Vis–NIR spectrometer with varying potentials applied using a 283 potentiostat (EG&G Princeton Applied Research). To achieve electrochromic switching of the PEDOT:PSS/PPro- DOT-Hx 2 display, a potential difference of 1.5 V was applied to the electrodes and 1.0 V was applied for the PEDOT:PSS-only displays. The color coordinates, of the displays, were measured with a hand- held Datacolor Microflash spectrophotometer, which yields the L, a and b* coordinates of colored samples. The measurements were conducted in the gloss-trap mode, which eliminated spectral reflection occurring along the plastic encapsulation film. The color contrast of the display cells, while switched between the reduced and oxidized state, respectively, was calculated from eqn (1).
DE* ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDLÞ^2 þðDaÞ^2 þðDb*Þ^2
q (1)
A more in-depth description of this measurement and analysis method has been published previously.13,15^ Images of the displays were obtained with a commercial flatbed scanner, CanoScan 9900F from Canon. The switching speed measurements were performed in a reflective setup where the incident light was directed at an angle of 45 ^ to the display surface, and the reflected light was collected in the normal direction. The reflected light was collected in real time using an Andor Shamrock spectrograph (SR303i) including an Andor Newton CCD detector (DU940N). Spectroelectrochemical measurements of PProDOT-Hx 2 on ITO (Fig. 4) show switching of the absorption spectrum at positive potentials. The absorption peak at 550 nm disappears completely at
Fig. 2 (a) The device architecture of the PEDOT:PSS/PProDOT-Hx 2 paper display. The repeat unit structure of (b) PEDOT:PSS and (c) PProDOT-Hx 2.
Fig. 3 (a) PEDOT:PSS-coatings on photo paper were supplied by AGFA-Gevaert N.V. and were used in the roll-to-roll printing press. (b) The rotary screen printing unit is slightly modified to allow for electro- chemical over-oxidation of the PEDOT:PSS film in the roll-to-roll printing press.
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(550 nm), which switches nearly as fast as the PEDOT in Fig. 7a, but is rather attributed to a slow switching of the PEDOT:PSS film ( nm) underneath. We attribute this to the relatively poor ionic conductivity of the PProDOT-Hx 2 in comparison to the PEDOT:PSS film and therefore it functions as a barrier to ion diffusion from the electrolyte to the PEDOT:PSS film underneath. One method to overcome this issue and increase the switching speed of the device would be to improve the ionic conductivity of the PProDOT-Hx 2 phase or create a more open morphology to allow ion diffusion to occur to the underlying PEDOT:PSS layer. Additionally, for the ON-switch (see Fig. 7b), the PEDOT:PSS layer (640 nm) switches faster than the PProDOT-Hx 2 layer (550 nm) and on close inspection the oxidation of the PProDOT-Hx 2 film occurs after the PEDOT:PSS film has been fully oxidized. This is due to the fact that the reduced PEDOT:PSS film has a low electronic conductivity and acts as a poor electrode for switching the ProDOT- Hx 2 layer to its reduced state as we have illustrated in the past for conducting polymer bilayers.16–18^ For the ON-switch, the PEDOT:PSS film is first oxidized, increasing in conductivity, allow- ing the supply of electronic charges, transported laterally, to the PProDOT-Hx 2 film coating on top. Another main contributor to the switching speed of a PEDOT:PSS-based display is the ionic conductivity/diffusion characteristics of the electrolyte In this study the electrolyte utilized was one that allowed for stable switching of the devices for comparative purposes and was adaptable for the manufacturing processes employed for this specific device fabrication. Further work will focus on the screening of additional electrolytes that still allow for ease of fabrication and long device lifetime while increasing electro- chemical switching of the device. The manufacturing of PEDOT:PSS-based EC displays is relatively simple and involves only three printing steps: Patterning PEDOT:PSS, electrolyte deposition and device encapsulation. Because of this simple manufacturing process, the EC displays are promising as low-cost displays and indicators in various applications. In this paper we have improved the optical contrast of the PEDOT:PSS paper display by the addition of an added layer of PProDOT-Hx 2 in a bi-layer structure. As the synthesis of this poly- mer can be scaled up to manufacture large quantities and the polymer is solution processable, it can be incorporated in the PEDOT-based display devices using the roll-to-roll manufacturing method. We found that the switching speed of the contrast-enhanced displays was slower than the PEDOT:PSS-only displays and attribute this to the poor ionic conductivity of the PProDOT-Hx 2 film in the aqueous electrolyte employed. From previous studies,^13 it has been
shown that oligoethylene oxide side groups can improve the ionic conductivity of the polymer film. Other routes to increasing switching speed are to induce a more open morphology in the PProDOT-Hx 2 film, use two polymers that are optimized for the same electrolyte and possibly blend the two films; all of which will be investigated in future studies.
Acknowledgements
The University of Florida acknowledges the AFOSR (FA9550-06-1-
- for funding. JRR appreciates the efforts of the researchers who developed and carried out the synthesis of the PProDOT-Hx 2 employed in this work. The Linkoping University acknowledge€ financial support from the Swedish Foundation for Strategic Research, the Swedish Research Council, VINNOVA, The Royal Academy of Sciences and Knut and Alice Wallenberg Foundation.
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Fig. 7 Comparison of the switching speed without (a) and with (b) the added layer of PProDOT-Hx 2 (80 nm) of the PEDOT-based paper displays. The numbers indicate the switching speed at respective wavelengths.
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