Desktop inkjet printer as a tool to print conducting polymers

Synthetic Metals 156 (2006) 779–783

Desktop inkjet printer as a tool to print conducting polymers Yuka Yoshioka, Ghassan E. Jabbour ∗ Flexible Display Center & Department of Chemical and Materials Engineering, Arizona State University, 7700 S. River Pkwy, Tempe, AZ 85284, USA Received 21 November 2005; received in revised form 22 February 2006; accepted 6 March 2006 Available online 9 June 2006

Abstract In order to exploit mechanical flexibility of organic-based electronic devices, conducting polymer anodes, such as polyaniline or poly(3,4ethylenedioxy)-thiophene-poly(styrene sulfonate) (PEDOT-PSS), have been extensively studied. Along with the use of solution based processing techniques, conducting polymers can simplify the device fabrication procedure and yield themselves easily to printing techniques. In this paper, we present the results of utilizing desktop inkjet printer as a tool for direct printing and patterning of conducting polymer. Design of printable patterns and adjustment of printing parameters can be performed using any software such as Power Point. PEDOT-PSS suspension can be loaded into an inkjet cartridge and deposited on a given substrate in any designed pattern. The gray-scale color scheme can be employed to control the layer thickness and sheet resistivity of the inkjet printed layers. These layers are then used as anodes in organic light-emitting devices (OLEDs). © 2006 Elsevier B.V. All rights reserved. Keywords: Conducting polymers; Inkjet printing; Organic light-emitting diodes; Morphology; PEDOT-PSS

1. Introduction One of the most successful conducting polymers to be developed and studied is PEDOT-PSS [1,2]. PEDOT has been prepared by standard oxidative chemical or electrochemical polymerization methods [3,4]. PEDOT, itself, is found to be highly conductive (400–600 S cm−1 ), highly transparent [5], electrochemically stable [6], and thermally stable (up to 230 ◦ C) [7] in thin, oxidized films (doped with PF6 − , BF4 − , or CF3 SO3 − ). However, the processability of PEDOT is very poor because it is an insoluble polymer. This drawback can be mitigated by polymerizing it in combination with a water-soluble polyelectrolyte, PSS [8]. The resultant PEDOT-PSS is dark blue, electrochemically stable in its p-doped form, moderately transparent with high electrical conductivity (1–10 S cm−1 ) [9], and has excellent capabilities to form films. Although PEDOT-PSS has a higher sheet resistance compared to the state-of-the-art transparent conductor ITO (indium tin oxide), it is possible to replace ITO by a conducting polymer in some applications, especially in low information content flexible display applications such as signage. Moreover, PEDOT-PSS

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lends itself readily to patterning and printing techniques. Printing methods such as inkjet [10], gravure [11], screen printing [12], and imprinting [13], have been used to deposit and/or pattern PEDOT-PSS. Inkjet printing is one of the most promising direct pattering techniques. It has the advantages of being fast and simple with high throughput. It has been used in a variety of fields including ceramics, metals, organic semiconductors, and biopolymers [14–16]. A desktop computer and a standard inkjet printer can control the delivery of picoliter volumes of liquids in precise patterns. Therefore, inkjet printing does not consume significant amount of material, as is the case with spin coating. It promises a low cost, mask-less, and non-contact patterning approach. Inks which are based on PEDOT-PSS have been extensively used to fabricate microelectro-mechanical/chemical structures and devices with conducting-polymer based electronic parts [17–25]. In building device structures by inkjet printing, there are a number of approaches of forming individual dots, lines, or thick layers. Generally, by arranging the printing conditions, one can connect individual dots to form a line. By reducing the offset of line alignment, one can connect lines to form a layer. Furthermore, in office type inkjet printer, about 50–100 ␮m diameter dots are dispersed over the area of the printed image and the amount of dot overlap increases as the original density

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(gray scale) of dots increases. The shape, thickness, and surface morphology of droplets and layers are greatly influenced by energetics of the substrate surface and ink, as well as the process of evaporation. One of the most extensive application of inkjet printing is pixel printing for OLED applications [19–21]. Many companies including Cambridge Display Technology, Seiko-Epson, DuPont Displays, and Toshiba have demonstrated display prototypes ranging in size from several to 40 in., in which the polymeric color pixels and base (hole injection) layers were deposited. In these displays, inkjet printer was used to dispense polymers in a photolithographyically pre-defined pixel pattern. In this paper, we discuss a method of direct patterning of conducting polymers by using desktop inkjet printer. Well-

Fig. 1. (a) Examples of the gray-scale, (b) HSL values used to program ink-loads. S = 0 and H was not defined, (c) measured film thickness for various luminosity values, L = 0–50. Film became discontinuous for L ≥ 70, and (d) measured sheet resistivity of inkjet printed films with various thickness.

established desk jet printing technology allows us to simplify the modification of printing parameters. In comparison with abovementioned methods, our approach requires no pre-patterning or pre-coating of the substrate. 2. Experimental part This study was carried out using a modified Hewlett Packard (HP 5550) thermal desktop inkjet printer. A pattern was designed using Microsoft Power Point software. A PEDOT-PSS based ink was formed by mixing 86 vol.% of Baytron P, 4.5 vol.% of glycerol, 0.045 vol.% of triton X-100 (surfactant), and about 9.5 vol.% of water. Glycerol was added to enhance the conductivity of formed films and acts as a humectant to avoid nozzle clogging during the printing process. The surfactant was added to enhance the wetting property of ink to form a continuous layer on the substrate.

Fig. 2. Surface topography of printed films of the PEDOT-PSS: (a) inkjet printed and dried at 110 ◦ C and (b) spin-coated and dried at 110 ◦ C. Each layer was dried on the hot plate in atmospheric conditions for 2 h.

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The thickness of the inkjet printing films was controlled through the HSL (hue, saturation, luminosity) function using the existing Power Point values. In this case, the ‘saturation’ (S), which controls the respective quantity of RGB (red, green, and blue) color, was held at 0. The ‘hue’ (H), which identifies the composition of the color, was also held constant. Only ‘luminosity’ (L), which dictates the darkness of the color itself (and thus the amount of printed PEDOT-PSS ink), was changed over six different values as shown in Fig. 1a and b. The density of droplets, defined by the gray scale of the image, depends upon the ink surface tension. The PEDOT-PSS ink described above has a low surface energy, and can therefore form a fully interconnected film.

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3. Results and discussion Continuous conductive films were printed within a certain range of thickness. The continuity of films was visually inspected after the PEDOT-PSS ink was printed. After printing, substrates were immediately placed on a hot plate at 110 ◦ C for 2 h in ambient atmosphere to dry the residues of glycerol and water. The film thickness, sheet resistivity and surface roughness average were then measured. As shown in Fig. 1c, a systematic decrease of film thickness is observed with decreasing ink load as the luminosity, L, varies from 0 to 50. Films were discontinuous below L = 70. Sheet resistivities of inkjet printed films were measured using the four-point probe method and results

Fig. 3. (a) Current density, (b) forward light output, and (c) external quantum efficiency vs. bias voltage of devices having various PEDOT-PSS thicknesses.

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are shown in Fig. 1d. As expected, a distinct increase in sheet resistivity is observed with decreasing film thickness. The formulated ink was spin-coated on a glass substrate to result in 220 nm thick PEDOT-PSS film. For comparison, film with similar thickness was inkjet printed on a glass substrate. Surface morphology of films was measured by the optical interferogram (Wyko) over 60 ␮m × 45.5 ␮m area. In both cases, many randomly positioned voids have been observed on the dried PEDOT-PSS surface. Fig. 2a shows the surface of the inkjet printed film when the substrate was heated at 110 ◦ C. The pit diameter was 2–3 ␮m and the depth was approximately 25–35 nm [Ra (roughness average) = 3.89 nm]. The spin-coated PEDOT-PSS film showed smaller pits than that of the inkjet printed film. The depth of voids was about 15 nm with 1–2 ␮m width and Ra of 4.29 nm (Fig. 2b). Since the miscibility of PEDOT-PSS and glycerol is poor, as water evaporates, remaining glycerol coalesces onto the PEDOTPSS surface and leaves behind voids in the PEDOT-PSS layer upon evaporation. Smaller pits were observed as the layer thickness decreases since there is less glycerol on the PEDOT-PSS surface. Compared to inkjet printing, the centrifugal effect in spin-coating shortens the drying time. It reduces the evaporation time gap between the two solvents resulting in layers with smaller pits. PEDOT-PSS deposited by inkjet were used as an anode in an OLED. On top of the inkjet printed anode, a 60–70 nm of hole transport layer (HTL) was obtained by spin-coating a solution of (TPD [N ,N -bis(3-methylphenyl)-N ,N -dimethyl benzidine] 67.6 wt.%, PC [polycarbonate] 29.0 wt.%, rubrene 3.4 wt.%, 10.35 mg/ml chloroform) at 1000 rpm for 1 min in a class 100 clean room. A 60 nm layer of Alq3 [tris-(8-hydroxyquinoline)aluminum] was then thermally deposited under high vacuum ˚ s−1 . A 300 nm layer of Mg:Ag (magneat the rate of 0.7 A sium:silver) was thermally co-evaporated at the ratio of 10:1 on the top of electron transport layer (ETL) layer. A second device where the thickness of the spin-coated layer was matched to that of the inkjet printed (L = 0) was fabricated. Fig. 3a shows the current density (mA cm−2 ) versus bias voltage (V) for the various devices made with varying L values. As expected, a decrease in current density was observed with decreasing thickness (increasing L) and increasing sheet resistivity of the PEDOT-PSS layer. The forward light output (Fig. 3b) from the devices followed the same trends. Lower light output at a given voltage can be seen with decreasing thickness (increasing sheet resistivity). At any given voltage, the device with the spin-coated layer had a higher current and brightness compared to an OLED fabricated with inkjet printed anode layer. This is primarily due to the slight difference in sheet resistivity of spin coated (1680
/sq) and inkjet printed layers (1850
/sq). For comparison, Fig. 3 show results of OLED characteristics with the same layer configuration except that ITO was used as the anode layer. The peak external quantum efficiencies (ηext = 1.2–1.3%) of devices containing PEDOT-PSS anodes were close to those of devices with an ITO anode. The results also show that ηext values of OLEDs using inkjet printed PEDOT-PSS anode remain almost constant at higher voltages, unlike the ηext of

Fig. 4. Photograph of an OLED on glass substrate where the anode was deposited and patterned via inkjet.

devices with ITO anode, which rapidly dropped off after the peak ηext is achieved due to Joule heating effects. The constant ηext behavior was presumably due to the low current density of PEDOT-PSS anodes and the topography of the printed surface. However, more detailed studies will be necessary to explore the relation between the printed PEDOT:PSS surface morphology and OLED performance. Fig. 4 shows a patterned OLED logo in which PEDOT-PSS was inkjet printed and patterned as described in this paper. 4. Conclusion In summary, the strategy of controlling ink loads using HSL color function and a black cartridge of desktop inkjet printer has been briefly described. The thickness and sheet resistivity of the anode can be controlled simply by using HSL color function. Surface roughness and conductivity of our inkjetted films are comparable with those made by spin coating. However, PEDOTPSS layer morphology and thickness can be greatly influenced by the processing conditions, additives and solute concentrations. This technique can be easily extended to print conducting polymers onto more delicate surfaces, such as textiles or paper based substrates. Acknowledgement We would like to thank Prof. Paul Calvert for the fruitful discussions. References [1] L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481. [2] S. Kirchmeyer, K. Reuter, J. Mater. Chem. 15 (2005) 2077. [3] G. Heywang, F. Jonas, Adv. Mater. 4 (1992) 1163.

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