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Fabrication of OTFT-backplane with solution process for Electrophoretic Display panel

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35-2 / M. W. Lee

• IMID 2009 DIGEST

Abstract

We fabricated flexible OTFT-backplanes with combining printing technique and conventional photolithography process for the electrophoretic display(EPD). The active area size of backplane was 6” in diagonal direction and consisted of 192 x 150 pixels, containing 1 OTFT employed bottom contact structure and 1 capacitance in each pixel.

1. Introduction

Electrophoretic display(EPD) is expected as medium of the next generation display for electronic books because of its paper-like appearance, flexibility and low power consumption.1)-3) However, EPD does

not have threshold voltage for rotating microcapsule, it should be controlled by a switch with threshold voltage such as TFTs in order to implement a display panel. Among the various TFT, organic TFT have drawn much attention due to their low production cost and processing flexibility, Recently, there have been numerous reports about OTFTs based on solution process using various printing techniques, such as screen, micro-contact, gravure, and inkjet printing.4)-6)

According to its merit, we could choose printing technique for OTFT characteristics.

The gate electrodes of OTFTs requires to have a high conductivity(< 10 Ω/cm) and a good adhesion with substrate as well as gate dielectric layer, and also a thin layer thickness and smooth surface to prevent a poor step coverage of dielectric layer inducing an electrical short. The source drain electrodes requires to have high conductivity and the lower height of the charge injection barrier into the active layer.

In this work, in order to achieve requirement above mention, Nano Ag-ink, poly(3,4-ethylenedioxythio phene): poly(styrene sulfonate)(PEDOT/PSS) was patterned by printing technology combining screen-printing with wet-etching process for gate electrodes and inkjet-printing with O2 plasma process for source

drain electrodes, respectively. We also presented results of enhancement conductivity of PEDOT/PSS film treated with organic compound for Source/Drain electrodes. It led to high performance OTFT. Finally we fabricated solution processable OTFT-backplanes on flexible substrate. It worked successfully and demonstrated to display some patterns

2. Experimental

We used polycarbonate(PC) substrate with thickness of 125um, plastic substrate was purchased from DuPont Teijin. Particle contamination was minimized by cleaning the substrate before depositing the materials in class 1000 clean room. The plastic substrate was also pre-annealed for reduction of polymer shrinkage because it causes serious problem in adjusting alignment for fabrication.

The OTFT-backplane were employed bottom contact structure. The active area size of backplane was 6” in diagonal direction and consisted of 192 x 150 pixels, containing 1 OTFT and 1 capacitance in each pixel. The fabrication process of OTFT-backplane included the following :

At first scan-line and gate electrode consists of Ag were patterned onto plastic substrate using screen printing with wet etching process. PVP was used for gate dielectric made by spin coating. To achieve sufficient field effect mobility, surface roughness of gate insulator should be smooth enough. The PEDOT/PSS was used for Source/Drain electrodes made by ink-jet printing combining plasma treatment. With this printing technique, We have fabricated devices with channel length down to 10 um, limited by the resolution of our photolithography setup. Next, a 45 nm thick layer of pentacene was deposited at a vacuum level of 2×10−8 torr at a deposition rate of

0.3 Å/s, The substrate was held at room temperature during deposition. Then, we used PVA/Acryl double

Fabrication of OTFT-backplane with solution process for Electrophoretic Display panel

Myung Won Lee1, Mi Young Lee1, Jong Seung Park2 and Chung Kun Song1

1Dept. of Electronics Eng., Media Device Lab., Dong-A University, Busan 604-714, Korea 2Dept. of Textile Engineering, Dong-A University, Busan 604-714, Korea

TEL:82-51-200-7366, e-mail: mwlee@dau.ac.kr

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35-2 / M. W. Lee

IMID 2009 DIGEST • layers for passivation of backplane as well as for

interlayer dielectric(ILD) between backplane and EPD panel. And also we formulated Ag ink, in order to use Ag-paste ink for pixel electrode of backplane with screen printing. Finally, we fabricated active matrix OTFT-EPD panel as laminating font EPD.

3. Results and discussion

To obtain solution process with high resolution and conductivity, we used a practical printing technology for the gate electrode of OTFTs by combining screen-printing with wet-etching process using nano-silver(Ag) ink as a conducting material.(fig. 1.) The screen-printed Ag electrode exhibited a minimum line width of ~5 um, the thickness of ~70 nm, and a resistivity of ~10-6 Ω·cm, producing good geometrical

and electrical characteristics for gate electrode.

Fig. 1. The fabrication process of the gate electrodes combining the screen-printing with the wet-etching process

In order to use source/drain(S/D) electrodes instead of Au in OTFT, We chose PEDOT/PSS and tried to enhance conductivity. The conductivity enhancement of the PEDOT/PSS films is strongly dependent on the properties and structure of the organic compounds.7)

Among the various organic compounds, we chose the glycerol which acts as a plasticizer and increases the chain mobility of polymer.

The addition of Glycerol helped not only improving conductivity and surface wetting properties but also preventing nozzle clogging problem and improving the ink jet printing process reliability due to high boiling point solvent. 8)9)

Here we applied different concentrations of glycerol into PEDOT/PSS to optimize property for inkjet printing. The evolution of sheet resistance(measured by four-point probe techniques) versus the percentage in weight of glycerol added to the PEDOT/PSS is reported in Fig. 2. The results showed that, while the sheet resistance of pure PEDOT/PSS is about 2 MΩ, that of modified PEDOT/PSS is found to be decreased

by four orders of magnitudes, reaching minimum value at a glycerol concentration of 10 wt%. This result showed that electrical performance of OTFT used modified PEDOT/PSS was superior to OTFT used PEDOT/PSS as S/D electrodes. Fig. 3 compared OTFT characteristics using PEDOT/PSS with/ without glycerol for S/D electrodes.

In order to have pattern with high resolution for OTFT-backplane, some modification is necessary to accommodate the wetting characteristics between ink and PVP surface. We could change the surface energy using O2 plasma treatment. It was distinguished by

hydrophilic and hydrophobic region. It was contributed to confine the PEDOT/PSS keeping the suppressed droplet from overflowing.

Fig. 2. Sheet resistance according to the concentration of glycerol

Fig. 3. Electrical transfer characteristics of bottom contact pentacene OTFT with different source drain electrodes(Au, pristine and modified PEDOT/PSS).

Although modified PEDOT/PSS increased performance of OTFT, it was not suitable for data line of backplane because of low electrical conductivity. Therefore we made data line of hybrid type which was applied Ag ink with ink jet printing technique to compensate conductivity of PEDOT/PSS in backplane. In order to control EPD, OTFTs have to interlayer dielectric(ILD) layer between backplane and EPD panel. We used PVA/Acryl double layers for ILD as well as passivation of backplane. It is important to

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35-2 / M. W. Lee

• IMID 2009 DIGEST

make a good ILD because ILD is required not to give a serious damage to the lower OTFT-backplanes and also protect it from upper EPD panel on lamination process. And also ILD was photo-sensitive so that conventional photolithography can be used to make a pattern. At the lastly, The pixel electrode pads, which were directly contacted to EPD panel and applied voltage to pixel, were connected to drain electrode of OTFT through via holes formed in ILD. Pixel electrode pads were deposited by screen printing.

Ag-paste for pixel electrode was made from acryl resin because of normal Ag-paste dissolved ILD and required high cure temperature. Therefore we designed to low curing temperature(120 oC) process to

minimizing damage below OTFT. In fig. 4 schematic depiction of Ag-paste formation from acryl resin and its incorporation as pixel electrode. Finally, we fabricated OTFT-EPD panel which worked successfully and demonstrated to display some patterns which showed in fig. 5

(a)

(b) (c)

Fig. 4. (a)schematic depiction of Ag-paste formation from arcryl resin (b) image of pixel electrode using screen printing (c) the cross-section of a pixel consisting of OTFT and EPD.

Fig. 5. The image displayed on flexible OTFT-EPD panel

4. Summary

In summary, in order to fabricate flexible OTFT- backplane for the EPD on plastic substrate, We developed a solution process with various printing method. Nano Ag, PEDOT/PSS ink was patterned by printing technology combining screen-printing with wet-etching process for gate electrodes and inkjet-printing with O2 plasma process for source drain

electrodes, respectively. Moreover, to overcome the resolution and accuracy limit of inkjet printing, PEDOT/PSS aqueous dispersion has been deposited onto surface treated insulator and OTFT with a channel length of 10um have been achieved. This result showed that it is possible to use PEDOT/PSS instead of Au for OTFT in very simple and efficient way. Furthermore, We shows a result of driving flexible EPD with printed AMOTFT backplane. They indicate that the OTFT solution processing with various printing method potentially has impact on the organic electronics including flexible display.

5. References

1. B. Chomiskey, J. D. Albert, H. Yoshizawa and J. Jacob-son, Nature 394, 253 (1998).

2. A. Henzen, J. van de Kamer, T. Nakamura, T. Tsuji.,M. Yasui and M. Pitt, SID'03 Digest, 176 (2003).

3. S. Maeda, S. Hayashi, K. Ichikawa, K. Tanaka, R.Ishikawa and M. Omodan, IDW'03 Digest, 1617 (2003).

4. D. A. Pardo, G. E. Jabbour and N. Peyghambarian,

Adv. Mater. 12 1249 (2000).

5. H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran, W. Wu and E. P. Woo,

Science 290 2123 (2000).

6. K. M. Choi and J. A. Rogers, J. Am. Chem. Soc. 125 4060 (2003).

7. J. Ouyang, Q. Xu, C. W. Chu, Y. Yang, G. Li, J. Shinar Polymer 45 8443-8450 (2004).

8. J. A. Lim, J. H. Cho, Y. D. Park, D. H. Kim, M. Hwang, and K. Cho, Appl. Phys. Lett. 88, 082102 (2006).

9. J. F. Chang, B. Sun, D.W. Breiby, M. M. Nielsen, T. I. Solling, M. Giles, I. McCulloch, H. Sirringhaus,

수치

Fig. 2. Sheet resistance according to the concentration  of glycerol
Fig. 4. (a)schematic depiction of Ag-paste formation  from arcryl resin (b) image of pixel electrode  using screen printing    (c) the cross-section of  a pixel consisting of    OTFT and EPD

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