- Email: [email protected]
A printed OTFT-backplane for AMOLED display
Organic Electronics 14 (2013) 1218–1224
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
A printed OTFT-backplane for AMOLED display Gi Seong Ryu a, Jae Seon Kim b, Seung Hyeon Jeong b, Chung Kun Song b,⇑ a b
Media Device Lab., Dong-A University, Busan, Republic of Korea Dept. of Electronics Eng., Dong-A University, Busan, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 4 March 2013 Keywords: OTFT OLED Printing electronics TIPS-pentacene Inkjet Printing
a b s t r a c t An AMOLED panel driven by an OTFT-backplane is an attractive display because OTFTs and OLEDs use organic materials with unique characteristics such as low temperature and solution processing ability, and thus are able to implement the key features of future displays. In this study we applied some printing technologies to fabricate an OTFT-backplane for AMOLEDs. Screen printing combined with photolithography with Ag ink was used for the gate electrodes and scan bus lines and contact pads. Ag metal lines with a width of 20 lm and thickness of 60 nm and resistivity of 3.0 105 X cm were achieved. Inkjet printing was applied to deposit TIPS-pentacene as an organic semiconductor. The OTFTbackplane using the Ag gate electrodes and TIPS-pentacene exhibited uniform performance over 17,500 pixels on a 7 in. panel. The mobility was 0.31 ± 0.05 cm2/V s with a deviation of 17%. The AMOLED panel successfully demonstrated its ability to display patterns. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently the annual growth of the display market is rapidly decreasing, resulting in gradual saturation of the entire market [1]. A major reason is that the flat panel display market has not continuously expanded due to shrinkage of TV demand caused by economic recession and the reduction of panel prices resulted from strong competition among display manufacturers. In order to break this stagnation, new displays are required to create new markets. Many expect that the flexible display will be one of the new future displays because flexibility can meet the various form requirements of future displays that can be installed on any shaped objects, such as rounded domed ceilings, cylindrical columns, large windows and even walls [2–4]. In addition the demand to access information anywhere and anytime in future will require many displays everywhere, and thus the price of display panels should be inexpensive. In conclusion, the key features of future displays can be summarized by flexibility and low price. ⇑ Corresponding author. Tel.: +82 51 200 7711; fax: +82 51 200 6965. E-mail address: [email protected] (C.K. Song). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.02.006
Flexible displays manufactured by printing technology can implement these key features [5–7]. From the point of view of device, the organic thin film transistor (OTFT) and organic light emitting diode (OLED) is the most promising combination as the driving transistor and display mode, respectively, to achieve a flexible display in terms of compatibility with a flexible substrate originating from a low temperature process and of solution processing capability for printing. Several articles have reported about AMOLED panel employing OTFT [8–18]. However, we have not found articles on using printing technology for AMOLEDs. In this paper we applied printing technologies to fabricate AMOLED panels that used OTFTs as the driving devices. In particular, screen printing was used for the gate electrodes of the OTFTs and the scan bus lines and pads of the panels, which occupied most of the area of the substrate and thus needed to be achieved by a simple and cheap process such as screen printing. Additionally, inkjet printing was also used for deposition of the organic semiconductor (OSC) layer to conserve the expensive OSC material because inkjet can jet a tiny amount of OSC solution on a specific channel area only. We investigated the
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
feasibility of printing technologies for the OTFTs-backplane of AMOLED panels.
2. Design and fabrication of AMOLED panel A pixel consists of 2 OTFTs and 1 capacitor and 1 OLED. The pixel pitch was determined to be 1000 lm 1000 lm by considering the limitation of the printing technology used in this study. The channel length of the switching (SW) and driving (DR) OTFT was 20 lm and the width was 1040 lm and 1980 lm, respectively. The inter-digitated structure for the source and drain (S/D) electrodes could implement a wide channel in a small area and thus supply sufficient current to the OLED. The area of the OLED was 730 lm 360 lm, resulting in an aperture ratio of 26.3%. The diagonal length of the panel was 7.4 in. in which 17,500 pixels (=140 125) were contained. The schematic diagram of the fabrication processes is depicted in Fig. 1. First, an indium zinc oxide (IZO) layer, which was pre-deposited on a glass substrate for the anode electrode of the OLED, was patterned by photolithography and an etching process. Then a silver (Ag) layer for the gate electrodes of the OTFTs and the scan bus lines and the contact pads of the panel were coated by screen printing with Ag ink over the whole area of the substrate and then patterned by photolithography. The process related to screen printing will be described in detail in next subsection. Next, the gate insulator layer using a photo-patternable organic polymer was spin coated and then patterned by photolithography. In this step since the gate dielectric polymer
1219
itself was photo-patternable, the several steps for patterning gate dielectric such as photoresist coating and photolithography and etching of gate dielectric, which would be carried out if the conventional non-patternable material were used, could be eliminated. Thus, we could simplify the process steps and avoid the performance degradation of the OTFTs associated with the effects of the photolithography process with a photoresist on the channels [19]. Subsequently, Au was thermally evaporated for the S/D electrodes and data bus lines, which was followed by inkjet printing of TIPS-pentacene for an organic semiconductor layer. Finally, the layers for the OLED and Al for the cathode electrode were continuously deposited by thermal evaporation through a shadow mask. Totally five masks including one shadow mask were used. In this process two printing technologies were employed screen printing for the gate electrodes and ink jet printing for the organic semiconductor. We will describe the printing processes in detail in the following section. 2.1. Screen printing of the gate electrodes and scan bus lines In an AMOLED panel, the gate electrode of the SW OTFT is directly connected to a long scan line as shown in Fig. 2 and the requirements are very restrictive in terms of geometric dimensions and resistivity. The small resistance of the gate electrode and scan line is very important to reducing the signal delay along the long scan line. The resistance L is expressed by R ¼ q Wd where q is the resistivity, L the length, W the width, and d the thickness of scan line. The length is determined by the panel size, and thus, a large
Fig. 1. Schematic diagram of fabrication process of AMOLED panel.
1220
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
in a pixel was for the space between the gate electrode of the DR OTFT and the scan line as shown in Fig. 3, which was 20 lm. Such a narrow space cannot be produced by any present printing technologies alone. Therefore, in order to implement the above geometric and electrical requirements simultaneously, we adopted photolithography together with screen printing. First, an Ag thin film layer was deposited over the entire area of the substrate by screen printing. In this step it is very important to make the thickness less than 0.1 lm, which was achieved by reducing the viscosity of Ag ink to 80– 120 cps. Subsequently, the Ag film was cured at 200 °C for 30 min. Finally, photolithography and the etching process of the Ag film were carried out to obtain the final patterns. Fig. 2. The layout of gate electrode of switching and driving OTFT and scan bus line of a pixel.
2.2. Inkjet printing of TIPS-pentacene for OTFTs panel can be a cause of high resistance. The width is also limited by resolution. Additionally, the thickness should be thin for good step coverage of the gate dielectric on the gate electrode. Therefore, these limitations comprise the key factor of reducing resistivity as much as possible for a high performance panel. In this study the AMOLED panel operating at 60 Hz of frame frequency required a resistivity of less than 8 104 X cm for an allowable delay time of 1.3 ls. One of objectives of this study was to achieve a long, narrow, thin, and low resistive gate electrode and scan bus lines with printing, which occupied most of the area of a pixel as shown in Fig. 2. We selected a screen printing process with nano-Ag ink because of its simplicity and economy. The above geometric and electrical properties of Ag film are strongly dependent on the viscosity of ink and are determined by Ag content of ink as shown in Fig. 3. As the viscosity increased, the sheet resistance decreased and the deviation of width also decreased while the thickness increased. Moreover, the intersecting point of two oppositely dependent curves could not provide the appropriate viscosity to satisfy the requirements simultaneously. Additionally, the minimum feature size
Another printing process used in this study was inkjet printing. Ink jet is an appropriate printing process for OSCs because the area of the OSC in an AMOLED is only a small channel region of the OTFT and inkjet can efficiently conserve the expensive OSC by dropping OSC ink on the specific small area. However, the droplet of OSC usually produces the so-called ‘coffee ring’ where the most of molecules accumulate on the contact line of the droplet as shown in Fig. 4b. In that case the molecules are rare in the middle channel area so that, in general, it is difficult to make high performance OTFTs with ink jet printing. Thus, it is important to obtain a uniformly thick OSC layer without a coffee ring. We determined the mechanism of coffee ring formation and found a way to remove it. To identify the mechanism of coffee ring formation, we took serial pictures of the motion of the particles in a droplet over time. As shown in Fig. 4a the particles continuously moved from the central bottom to the top and then to the contact line along the surface due to the convective flow of the solvent. Furthermore, the contact line was fixed during drying so that the particles continuously accumulated on the contact line because the contact line played the role of a thermally stable
Fig. 3. The dependence of line width and thickness and resistivity on the viscosity of Ag ink.
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
1221
Fig. 4. Model of coffee ring formation.
nucleate site, resulting in generating the coffee ring after completely dried. Based on this mechanism a simple process was designed to eliminate the coffee ring and to obtain uniform performance of the OTFTs over a large area. This process is described in the next section. The number of droplets on a single channel also affected the performance of the OTFTs. The performance variation was examined with respect to the number of droplets. The number of droplets changed the final size of the TIPS-pentacene layer, which was restricted by the pixel pitch. Thus, the appropriate number of droplets was determined to satisfy the required performance and the limited resolution as well. 3. Results and discussion
Fig. 5. The variation of mobility and the off state current of OTFTs according to Ag content of gate electrode.
3.1. Screen-printed gate electrodes and scan bus lines The content of Ag particles in the Ag ink is an important factor in this screen printing. It determined the viscosity of the ink and thus affected the thickness and resistivity of the gate electrode as well as the performance of the OTFTs. As shown in Fig. 5, the OTFTs using the screen printed gate electrodes exhibited Ag content dependence of mobility and off state current. The mobility reached the maximum value of 0.3 cm2/V s at 20 wt% of Ag content. Above that, the mobility was slightly decreased. However, the off state current rapidly increased above 30 wt%. The reason can be found in the large leakage current through the gate dielectric at the edge of the gate electrode caused by the poor step coverage of the gate dielectric on the thick gate electrode which was formed by a high Ag content of 30 wt%. Therefore, 20 wt% of Ag content was selected for screen printing of gate electrodes. The corresponding viscosity of the Ag ink was about 100 cps. With a viscosity of 100 cps, the thickness and resistivity of the gate electrode were 60 nm and 3.0 105 X cm,
respectively. Two requirements of the gate electrode – a thickness of less than 1000 nm and a resistivity of less than 8.0 104 X cm, which exhibited the opposite dependence on viscosity to each other as shown in Fig. 3, were satisfied simultaneously with a viscosity of 100 cps. The last requirement of the gate electrode, that is, a narrow line width, was achieved by photolithography. In Fig. 6, the final printed gate electrodes and scan bus lines are shown. The figure successfully exhibits the clear patterns of the gate electrodes and scan bus lines and contact pads. In particular, the space between the DR OTFT gate electrode and scan bus line, which is the minimum feature size of a pixel, is clearly shown. The line edge was very clear compared to that with thermal evaporation. It originated from the nano-sized Ag particles. Although this study was not for the sole usage of screen printing, the significance of this study can be found in replacing the conventional thermal evaporation process with the cheap screen printing for deposition of the metal
1222
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
Fig. 7. The variation of mobility and diameter of droplet according to the number of droplets and the inset is transfer curves of forty OTFTs selected from AMOLED panel. Fig. 6. The picture of gate electrodes and scan bus lines for AMOLED panel printed by screen printing combining with photolithography.
layer over the entire area. In future, the photolithography can be replaced by other printing processes, such as reverse off-set. However, that process needs alignment equipment on the printer because gate electrodes should be aligned with the IZO anode of the OLED. It will be a hard task to align patterns during printing, especially for large area printing. 3.2. OTFTs with ink-jetted TIPS-pentacene In order to eliminate the coffee ring and obtain a uniformly thick layer, the substrate was heated during jetting and drying. With heating, the contact line moved toward the central region during drying instead of being fixed as shown in Fig. 4c because the solvent at the contact line evaporated faster than at the center. Since the contact line was a thermally stable nucleate site, the moving contact line grew crystal grains from the edge to the center, generating a uniform layer. However, the speed of the contact line movement should be properly matched to that of crystal formation; otherwise, the thickness varied. Additionally the contact line movement also depended upon the surface condition of the channel and the solvent type of droplet. Therefore, a suitable temperature must be determined for each specific case. For the TIPS-pentacene, which was mixed in anisole and jetted on a PVP gate dielectric, an appropriate temperature was 46 °C. This temperature was also applied for the photo-patternable gate dielectric used in this study. The details can be found in the Refs. [20,21]. The attractiveness of this heating process is its simplicity and efficiency, which is able to produce uniform performance over a large area just by keeping the whole area of the substrate at the specific temperature. Forty OTFTs randomly selected from the panel exhibited concentrated transfer curves within a small range as shown in the inset of Fig. 7. The mobility extracted from the 40 OTFTs was 0.31 ± 0.05 cm2/V s. The deviation of 17% seems to be very small considering a panel area of 12 in.2. The threshold voltage ranged from 0 V to +1.5 V, and the off state current in the range of 3.6–
9.6 104 pA/lm. The performance parameters are summarized in Table 1. By the number of droplets on a single channel the mobility also changed as shown in Fig. 7. At four drops the mobility reached the maximum value of about 0.5 cm2/V s. However, the diameter of the droplet increased to about 400 lm, which was over the limit of the OTFT pitch 380 lm. Therefore, three drops were chosen by compromising the mobility with the pitch limit. For this case the mobility was about 0.3 cm2/V s and the diameter 350 lm. 3.3. OLED In this study, a green OLED was employed to investigate the feasibility of the OTFTs for the driving capability in AMOLED panel. The structure of the OLED is shown in the inset of Fig. 8. All the layers were deposited by thermal evaporation through shadow mask. It is very important for the shadow mask to be strongly attached to the substrate in order to prevent the defects from being induced by the shadow effect. The OLEDs emitted green light with the wavelength of 514 nm. The luminescence intensity linearly varied with the current density as shown in Fig. 8 so that the grey level could be implemented. At a current density of 0.7 mA/cm2 the luminescence intensity was 100 cd/m2 which was brightly visible with bare eyes. 3.4. AMOLED panel The inset of Fig. 9 shows the fabricated pixel array. The droplet on the SW OTFT seems larger than channel and overlaps the data bus line resulting in an electrical short. However, there was a gate dielectric layer between them so that the electrical short did not occur. From the pixel array the waveforms of the scan bus line and the voltage across the OLED were measured and are depicted in Fig. 9. The signal at each node of the pixel circuit could not be measured in a pixel because it was difficult to pick a node with a probe in the small area. Thus, we examined the voltage across OLED, VOLED, by connecting a resistor across the OLED outside of the pixel. Thus, we could examine the operation of SW and DR OTFT, and also the charging
1223
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224 Table 1 Summary of performance parameters of OTFT array in AMOLED panel employing screen printed Ag gate electrode and inkjet printed TIPS-pentacene.
l (cm2/V s)
Ion/Ioff
VTH (V)
SS (V/dec)
Ioff (pA/lm)
0.31 ± 0.05
3.96 ± 1.45 107
0.64 ± 0.85
0.47 ± 0.17
6.61 ± 3.08 104
Fig. 8. The structure and characteristics of fabricated OLED.
Fig. 10. Images of AMOLED panel driven by the printed OTFT-backplane.
of the OLED and generation of dark spots in the OLED due to operation stress. 4. Conclusion
Fig. 9. The waveforms of scan bus line and the voltage across OLED in a pixel.
across storage capacitor Cst in a pixel with the variation of VOLED. As shown in Fig. 9 when a voltage of 20 V was applied to the scan bus line for 50 ms, VOLED increased, reflecting the SW OTFT turning on, Cst being charged, and DR OTFT supplying current to the OLED. As the scan voltage returned to +20 V again, VOLED decreased down by 30% from the peak, representing kick-back voltage, and gradually decreased down to 50% of the peak during the off state of the SW OTFT. Based on the waveforms we confirmed that the operation of the pixel circuit was successful. Patterns were successfully displayed in the AMOLED panel as shown in Fig. 10. However, several kinds of defects were found. The defects related to the fabrication process were generated by dropping of TIPS-pentacene droplets on the wrong position and misalignment of the OLED pattern due to drooping of metal shadow mask and also to contaminations. The defects caused by degradation of the devices were associated with burning of the Al cathode
In this paper printing technologies were used to fabricate OTFT-backplane for AMOLED panels. Screen printing combined with photolithography was used to make gate electrodes and scan bus lines and contact pads. The opposite requirements of gate electrodes, thin and low resistance, were achieved by producing a thickness of 60 nm and resistivity of 3.0 105 X cm with Ag ink. TIPS-pentacene was used for organic semiconductors and printed by inkjet printing. A simple process to eliminate coffee ring was developed by applying heat to the substrate during jetting and drying of droplets. The heating process was efficient to produce uniform performance of the OTFTs over a large area substrate as far as it was kept at a specific temperature over entire area, which was 46 °C in this study. In addition, a suitable number of droplets was also determined to be three droplets. With these process conditions, the OTFTs exhibited a mobility of 0.31 ± 0.05 cm2/V s with a deviation of 17% over a 7 in. size panel. The OLEDs were fabricated by an evaporation process and produced green light with a wavelength of 514 nm. The intensity linearly varied with the current density, resulting in implementation of a grey level. The final AMOLED panel successfully displayed patterns. Acknowledgements This work was supported by the IT R&D Program of MKE/KEIT [10041957, Design and Development of fiberbased flexible display] and partly by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Knowledge Economy (No. 20114030200030).
1224
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
References [1] F.M. Tseng, A.C. Cheng, Y.N. Peng, Technol. Forecast. Soc. 76 (2009) 897–909. [2] K.J. Allen, Proc. IEEE 93 (2005) 1394–1399. [3] M.C. Choi, Y.K. Kim, C.S. Ha, Polym. Sci. 33 (2008) 581–630. [4] G.P. Crawford, Flexible Flat Panel Display Technology, John Wiley & Sons, Ltd., 2005. [5] A. Nathan, B.R. Chalamala, Proc. IEEE 93 (2005) 1391–1393. [6] R.A. Street, W.S. Wong, S.E. Ready, M.L. Chabinyc, A.C. Arias, S. Limb, A. Salleo, R. Lujan, Mater. Today 9 (2006) 32–37. [7] M. Young Lee, M.W. Lee, J.E. Park, J.S. Park, C.K. Song, Microelectron. Eng. 87 (2010) 1922–1926. [8] M. Mizukami, N. Hirohata, T. Iseki, K. Ohtawara, T. Tada, S. Yagyu, T. Abe, T. Suzuki, Y. Fujisaki, Y. Inoue, S. Tokito, T. Kurita, IEEE Electron. Dev. Lett. 27 (2006) 249–251. [9] L. Zhou, A. Wanga, S.C. Wu, J. Sun, S.K. Park, T.N. Jackson, Appl. Phys. Lett. 88 (2006) 83502–83504. [10] M.C. Suh, J.H. Jeong, T. Ahn, J.S. Park, S.Y. Kim, Y.J. Kim, T.J. Kim, H.J. Lee, S.M. Lee, Y.W. Park, Y.G. Mo, H.K. Chung, B.W. Koo, S.Y. Kim, S.Y. Lee, SID 06 Dig. (2006) 116–118. [11] I. Yagi, N. Hirai, M. Noda, A. Imaoka, Y. Miyamoto, N. Yoneya, K. Nomoto, J. Kasahara, A. Yumoto, T. Urabe, SID 07 Dig. (2007) 1753– 1756.
[12] S.H. Han, Y.R. Son, S.H. Lee, K.J. Lee, G.S. Cho, W.S. Kim, T.J. Park, W.S. Jeon, D.J. Choo, J. Jang, SID 07 Dig. (2007) 1757–1760. [13] I. Yagi, N. Hirai, Y. Miyamoto, M. Noda, A. Imaoka, N. Yoneya, K. Nomoto, J. Kasahara, A. Yumoto, T. Urabe, J. Soc. Inf. Display 16 (2008) 15–20. [14] J.Y. Yan, H.L. Chen, S.T. Yeh, J.L. Liao, Y.Y. Wu, M.R. Lin, K.P. Liao, C.Y. Hung, T.W. Lee, K.Y. Cheng, Y.Y. Lee, J.C. Ho, K.Y. Ho, H.C. Lin, C.H. Cheng, P.H. Wang, H.S. Dai, Y.S. Huang, SID 09 Dig. (2009) 986–987. [15] K. Nomoto, SID 10 Dig. (2010) 1155–1158. [16] M. Noda, N. Kobayashi, M. Katsuhara, A. Yumoto, S. Ushikura, R. Yasuda, N. Hirai, G. Yukawa, I. Yagi, K. Nomoto, T. Urabe, J. Soc. Inf. Display 19 (2011) 316–322. [17] T.C. Huang, K. Ishida, T. Sekitani, M. Takamiya, T. Someya, T. Sakurai, SID 11 Dig. (2011) 149–152. [18] T. Sekitani, T. Yokota, Takao Someya, SID 12 Dig. (2012) 426–429. [19] G.S. Ryu, S.H. Jeong, D.H. Kwon, J.S. Kim, C.K. Song, IMID 2012 Dig. (2012). [20] M.J. Kim, H.W. Heo, Y.K. Suh, C.K. Song, Org. Electron. 12 (2011) 1170–1176. [21] M.W. Lee, G.S. Ryu, Y.U. Lee, C. Pearson, M.C. Petty, C.K. Song, Microelectron. Eng. 95 (2012) 1–4.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
A printed OTFT-backplane for AMOLED display Gi Seong Ryu a, Jae Seon Kim b, Seung Hyeon Jeong b, Chung Kun Song b,⇑ a b
Media Device Lab., Dong-A University, Busan, Republic of Korea Dept. of Electronics Eng., Dong-A University, Busan, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 4 March 2013 Keywords: OTFT OLED Printing electronics TIPS-pentacene Inkjet Printing
a b s t r a c t An AMOLED panel driven by an OTFT-backplane is an attractive display because OTFTs and OLEDs use organic materials with unique characteristics such as low temperature and solution processing ability, and thus are able to implement the key features of future displays. In this study we applied some printing technologies to fabricate an OTFT-backplane for AMOLEDs. Screen printing combined with photolithography with Ag ink was used for the gate electrodes and scan bus lines and contact pads. Ag metal lines with a width of 20 lm and thickness of 60 nm and resistivity of 3.0 105 X cm were achieved. Inkjet printing was applied to deposit TIPS-pentacene as an organic semiconductor. The OTFTbackplane using the Ag gate electrodes and TIPS-pentacene exhibited uniform performance over 17,500 pixels on a 7 in. panel. The mobility was 0.31 ± 0.05 cm2/V s with a deviation of 17%. The AMOLED panel successfully demonstrated its ability to display patterns. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently the annual growth of the display market is rapidly decreasing, resulting in gradual saturation of the entire market [1]. A major reason is that the flat panel display market has not continuously expanded due to shrinkage of TV demand caused by economic recession and the reduction of panel prices resulted from strong competition among display manufacturers. In order to break this stagnation, new displays are required to create new markets. Many expect that the flexible display will be one of the new future displays because flexibility can meet the various form requirements of future displays that can be installed on any shaped objects, such as rounded domed ceilings, cylindrical columns, large windows and even walls [2–4]. In addition the demand to access information anywhere and anytime in future will require many displays everywhere, and thus the price of display panels should be inexpensive. In conclusion, the key features of future displays can be summarized by flexibility and low price. ⇑ Corresponding author. Tel.: +82 51 200 7711; fax: +82 51 200 6965. E-mail address: [email protected] (C.K. Song). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.02.006
Flexible displays manufactured by printing technology can implement these key features [5–7]. From the point of view of device, the organic thin film transistor (OTFT) and organic light emitting diode (OLED) is the most promising combination as the driving transistor and display mode, respectively, to achieve a flexible display in terms of compatibility with a flexible substrate originating from a low temperature process and of solution processing capability for printing. Several articles have reported about AMOLED panel employing OTFT [8–18]. However, we have not found articles on using printing technology for AMOLEDs. In this paper we applied printing technologies to fabricate AMOLED panels that used OTFTs as the driving devices. In particular, screen printing was used for the gate electrodes of the OTFTs and the scan bus lines and pads of the panels, which occupied most of the area of the substrate and thus needed to be achieved by a simple and cheap process such as screen printing. Additionally, inkjet printing was also used for deposition of the organic semiconductor (OSC) layer to conserve the expensive OSC material because inkjet can jet a tiny amount of OSC solution on a specific channel area only. We investigated the
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
feasibility of printing technologies for the OTFTs-backplane of AMOLED panels.
2. Design and fabrication of AMOLED panel A pixel consists of 2 OTFTs and 1 capacitor and 1 OLED. The pixel pitch was determined to be 1000 lm 1000 lm by considering the limitation of the printing technology used in this study. The channel length of the switching (SW) and driving (DR) OTFT was 20 lm and the width was 1040 lm and 1980 lm, respectively. The inter-digitated structure for the source and drain (S/D) electrodes could implement a wide channel in a small area and thus supply sufficient current to the OLED. The area of the OLED was 730 lm 360 lm, resulting in an aperture ratio of 26.3%. The diagonal length of the panel was 7.4 in. in which 17,500 pixels (=140 125) were contained. The schematic diagram of the fabrication processes is depicted in Fig. 1. First, an indium zinc oxide (IZO) layer, which was pre-deposited on a glass substrate for the anode electrode of the OLED, was patterned by photolithography and an etching process. Then a silver (Ag) layer for the gate electrodes of the OTFTs and the scan bus lines and the contact pads of the panel were coated by screen printing with Ag ink over the whole area of the substrate and then patterned by photolithography. The process related to screen printing will be described in detail in next subsection. Next, the gate insulator layer using a photo-patternable organic polymer was spin coated and then patterned by photolithography. In this step since the gate dielectric polymer
1219
itself was photo-patternable, the several steps for patterning gate dielectric such as photoresist coating and photolithography and etching of gate dielectric, which would be carried out if the conventional non-patternable material were used, could be eliminated. Thus, we could simplify the process steps and avoid the performance degradation of the OTFTs associated with the effects of the photolithography process with a photoresist on the channels [19]. Subsequently, Au was thermally evaporated for the S/D electrodes and data bus lines, which was followed by inkjet printing of TIPS-pentacene for an organic semiconductor layer. Finally, the layers for the OLED and Al for the cathode electrode were continuously deposited by thermal evaporation through a shadow mask. Totally five masks including one shadow mask were used. In this process two printing technologies were employed screen printing for the gate electrodes and ink jet printing for the organic semiconductor. We will describe the printing processes in detail in the following section. 2.1. Screen printing of the gate electrodes and scan bus lines In an AMOLED panel, the gate electrode of the SW OTFT is directly connected to a long scan line as shown in Fig. 2 and the requirements are very restrictive in terms of geometric dimensions and resistivity. The small resistance of the gate electrode and scan line is very important to reducing the signal delay along the long scan line. The resistance L is expressed by R ¼ q Wd where q is the resistivity, L the length, W the width, and d the thickness of scan line. The length is determined by the panel size, and thus, a large
Fig. 1. Schematic diagram of fabrication process of AMOLED panel.
1220
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
in a pixel was for the space between the gate electrode of the DR OTFT and the scan line as shown in Fig. 3, which was 20 lm. Such a narrow space cannot be produced by any present printing technologies alone. Therefore, in order to implement the above geometric and electrical requirements simultaneously, we adopted photolithography together with screen printing. First, an Ag thin film layer was deposited over the entire area of the substrate by screen printing. In this step it is very important to make the thickness less than 0.1 lm, which was achieved by reducing the viscosity of Ag ink to 80– 120 cps. Subsequently, the Ag film was cured at 200 °C for 30 min. Finally, photolithography and the etching process of the Ag film were carried out to obtain the final patterns. Fig. 2. The layout of gate electrode of switching and driving OTFT and scan bus line of a pixel.
2.2. Inkjet printing of TIPS-pentacene for OTFTs panel can be a cause of high resistance. The width is also limited by resolution. Additionally, the thickness should be thin for good step coverage of the gate dielectric on the gate electrode. Therefore, these limitations comprise the key factor of reducing resistivity as much as possible for a high performance panel. In this study the AMOLED panel operating at 60 Hz of frame frequency required a resistivity of less than 8 104 X cm for an allowable delay time of 1.3 ls. One of objectives of this study was to achieve a long, narrow, thin, and low resistive gate electrode and scan bus lines with printing, which occupied most of the area of a pixel as shown in Fig. 2. We selected a screen printing process with nano-Ag ink because of its simplicity and economy. The above geometric and electrical properties of Ag film are strongly dependent on the viscosity of ink and are determined by Ag content of ink as shown in Fig. 3. As the viscosity increased, the sheet resistance decreased and the deviation of width also decreased while the thickness increased. Moreover, the intersecting point of two oppositely dependent curves could not provide the appropriate viscosity to satisfy the requirements simultaneously. Additionally, the minimum feature size
Another printing process used in this study was inkjet printing. Ink jet is an appropriate printing process for OSCs because the area of the OSC in an AMOLED is only a small channel region of the OTFT and inkjet can efficiently conserve the expensive OSC by dropping OSC ink on the specific small area. However, the droplet of OSC usually produces the so-called ‘coffee ring’ where the most of molecules accumulate on the contact line of the droplet as shown in Fig. 4b. In that case the molecules are rare in the middle channel area so that, in general, it is difficult to make high performance OTFTs with ink jet printing. Thus, it is important to obtain a uniformly thick OSC layer without a coffee ring. We determined the mechanism of coffee ring formation and found a way to remove it. To identify the mechanism of coffee ring formation, we took serial pictures of the motion of the particles in a droplet over time. As shown in Fig. 4a the particles continuously moved from the central bottom to the top and then to the contact line along the surface due to the convective flow of the solvent. Furthermore, the contact line was fixed during drying so that the particles continuously accumulated on the contact line because the contact line played the role of a thermally stable
Fig. 3. The dependence of line width and thickness and resistivity on the viscosity of Ag ink.
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
1221
Fig. 4. Model of coffee ring formation.
nucleate site, resulting in generating the coffee ring after completely dried. Based on this mechanism a simple process was designed to eliminate the coffee ring and to obtain uniform performance of the OTFTs over a large area. This process is described in the next section. The number of droplets on a single channel also affected the performance of the OTFTs. The performance variation was examined with respect to the number of droplets. The number of droplets changed the final size of the TIPS-pentacene layer, which was restricted by the pixel pitch. Thus, the appropriate number of droplets was determined to satisfy the required performance and the limited resolution as well. 3. Results and discussion
Fig. 5. The variation of mobility and the off state current of OTFTs according to Ag content of gate electrode.
3.1. Screen-printed gate electrodes and scan bus lines The content of Ag particles in the Ag ink is an important factor in this screen printing. It determined the viscosity of the ink and thus affected the thickness and resistivity of the gate electrode as well as the performance of the OTFTs. As shown in Fig. 5, the OTFTs using the screen printed gate electrodes exhibited Ag content dependence of mobility and off state current. The mobility reached the maximum value of 0.3 cm2/V s at 20 wt% of Ag content. Above that, the mobility was slightly decreased. However, the off state current rapidly increased above 30 wt%. The reason can be found in the large leakage current through the gate dielectric at the edge of the gate electrode caused by the poor step coverage of the gate dielectric on the thick gate electrode which was formed by a high Ag content of 30 wt%. Therefore, 20 wt% of Ag content was selected for screen printing of gate electrodes. The corresponding viscosity of the Ag ink was about 100 cps. With a viscosity of 100 cps, the thickness and resistivity of the gate electrode were 60 nm and 3.0 105 X cm,
respectively. Two requirements of the gate electrode – a thickness of less than 1000 nm and a resistivity of less than 8.0 104 X cm, which exhibited the opposite dependence on viscosity to each other as shown in Fig. 3, were satisfied simultaneously with a viscosity of 100 cps. The last requirement of the gate electrode, that is, a narrow line width, was achieved by photolithography. In Fig. 6, the final printed gate electrodes and scan bus lines are shown. The figure successfully exhibits the clear patterns of the gate electrodes and scan bus lines and contact pads. In particular, the space between the DR OTFT gate electrode and scan bus line, which is the minimum feature size of a pixel, is clearly shown. The line edge was very clear compared to that with thermal evaporation. It originated from the nano-sized Ag particles. Although this study was not for the sole usage of screen printing, the significance of this study can be found in replacing the conventional thermal evaporation process with the cheap screen printing for deposition of the metal
1222
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
Fig. 7. The variation of mobility and diameter of droplet according to the number of droplets and the inset is transfer curves of forty OTFTs selected from AMOLED panel. Fig. 6. The picture of gate electrodes and scan bus lines for AMOLED panel printed by screen printing combining with photolithography.
layer over the entire area. In future, the photolithography can be replaced by other printing processes, such as reverse off-set. However, that process needs alignment equipment on the printer because gate electrodes should be aligned with the IZO anode of the OLED. It will be a hard task to align patterns during printing, especially for large area printing. 3.2. OTFTs with ink-jetted TIPS-pentacene In order to eliminate the coffee ring and obtain a uniformly thick layer, the substrate was heated during jetting and drying. With heating, the contact line moved toward the central region during drying instead of being fixed as shown in Fig. 4c because the solvent at the contact line evaporated faster than at the center. Since the contact line was a thermally stable nucleate site, the moving contact line grew crystal grains from the edge to the center, generating a uniform layer. However, the speed of the contact line movement should be properly matched to that of crystal formation; otherwise, the thickness varied. Additionally the contact line movement also depended upon the surface condition of the channel and the solvent type of droplet. Therefore, a suitable temperature must be determined for each specific case. For the TIPS-pentacene, which was mixed in anisole and jetted on a PVP gate dielectric, an appropriate temperature was 46 °C. This temperature was also applied for the photo-patternable gate dielectric used in this study. The details can be found in the Refs. [20,21]. The attractiveness of this heating process is its simplicity and efficiency, which is able to produce uniform performance over a large area just by keeping the whole area of the substrate at the specific temperature. Forty OTFTs randomly selected from the panel exhibited concentrated transfer curves within a small range as shown in the inset of Fig. 7. The mobility extracted from the 40 OTFTs was 0.31 ± 0.05 cm2/V s. The deviation of 17% seems to be very small considering a panel area of 12 in.2. The threshold voltage ranged from 0 V to +1.5 V, and the off state current in the range of 3.6–
9.6 104 pA/lm. The performance parameters are summarized in Table 1. By the number of droplets on a single channel the mobility also changed as shown in Fig. 7. At four drops the mobility reached the maximum value of about 0.5 cm2/V s. However, the diameter of the droplet increased to about 400 lm, which was over the limit of the OTFT pitch 380 lm. Therefore, three drops were chosen by compromising the mobility with the pitch limit. For this case the mobility was about 0.3 cm2/V s and the diameter 350 lm. 3.3. OLED In this study, a green OLED was employed to investigate the feasibility of the OTFTs for the driving capability in AMOLED panel. The structure of the OLED is shown in the inset of Fig. 8. All the layers were deposited by thermal evaporation through shadow mask. It is very important for the shadow mask to be strongly attached to the substrate in order to prevent the defects from being induced by the shadow effect. The OLEDs emitted green light with the wavelength of 514 nm. The luminescence intensity linearly varied with the current density as shown in Fig. 8 so that the grey level could be implemented. At a current density of 0.7 mA/cm2 the luminescence intensity was 100 cd/m2 which was brightly visible with bare eyes. 3.4. AMOLED panel The inset of Fig. 9 shows the fabricated pixel array. The droplet on the SW OTFT seems larger than channel and overlaps the data bus line resulting in an electrical short. However, there was a gate dielectric layer between them so that the electrical short did not occur. From the pixel array the waveforms of the scan bus line and the voltage across the OLED were measured and are depicted in Fig. 9. The signal at each node of the pixel circuit could not be measured in a pixel because it was difficult to pick a node with a probe in the small area. Thus, we examined the voltage across OLED, VOLED, by connecting a resistor across the OLED outside of the pixel. Thus, we could examine the operation of SW and DR OTFT, and also the charging
1223
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224 Table 1 Summary of performance parameters of OTFT array in AMOLED panel employing screen printed Ag gate electrode and inkjet printed TIPS-pentacene.
l (cm2/V s)
Ion/Ioff
VTH (V)
SS (V/dec)
Ioff (pA/lm)
0.31 ± 0.05
3.96 ± 1.45 107
0.64 ± 0.85
0.47 ± 0.17
6.61 ± 3.08 104
Fig. 8. The structure and characteristics of fabricated OLED.
Fig. 10. Images of AMOLED panel driven by the printed OTFT-backplane.
of the OLED and generation of dark spots in the OLED due to operation stress. 4. Conclusion
Fig. 9. The waveforms of scan bus line and the voltage across OLED in a pixel.
across storage capacitor Cst in a pixel with the variation of VOLED. As shown in Fig. 9 when a voltage of 20 V was applied to the scan bus line for 50 ms, VOLED increased, reflecting the SW OTFT turning on, Cst being charged, and DR OTFT supplying current to the OLED. As the scan voltage returned to +20 V again, VOLED decreased down by 30% from the peak, representing kick-back voltage, and gradually decreased down to 50% of the peak during the off state of the SW OTFT. Based on the waveforms we confirmed that the operation of the pixel circuit was successful. Patterns were successfully displayed in the AMOLED panel as shown in Fig. 10. However, several kinds of defects were found. The defects related to the fabrication process were generated by dropping of TIPS-pentacene droplets on the wrong position and misalignment of the OLED pattern due to drooping of metal shadow mask and also to contaminations. The defects caused by degradation of the devices were associated with burning of the Al cathode
In this paper printing technologies were used to fabricate OTFT-backplane for AMOLED panels. Screen printing combined with photolithography was used to make gate electrodes and scan bus lines and contact pads. The opposite requirements of gate electrodes, thin and low resistance, were achieved by producing a thickness of 60 nm and resistivity of 3.0 105 X cm with Ag ink. TIPS-pentacene was used for organic semiconductors and printed by inkjet printing. A simple process to eliminate coffee ring was developed by applying heat to the substrate during jetting and drying of droplets. The heating process was efficient to produce uniform performance of the OTFTs over a large area substrate as far as it was kept at a specific temperature over entire area, which was 46 °C in this study. In addition, a suitable number of droplets was also determined to be three droplets. With these process conditions, the OTFTs exhibited a mobility of 0.31 ± 0.05 cm2/V s with a deviation of 17% over a 7 in. size panel. The OLEDs were fabricated by an evaporation process and produced green light with a wavelength of 514 nm. The intensity linearly varied with the current density, resulting in implementation of a grey level. The final AMOLED panel successfully displayed patterns. Acknowledgements This work was supported by the IT R&D Program of MKE/KEIT [10041957, Design and Development of fiberbased flexible display] and partly by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Knowledge Economy (No. 20114030200030).
1224
G.S. Ryu et al. / Organic Electronics 14 (2013) 1218–1224
References [1] F.M. Tseng, A.C. Cheng, Y.N. Peng, Technol. Forecast. Soc. 76 (2009) 897–909. [2] K.J. Allen, Proc. IEEE 93 (2005) 1394–1399. [3] M.C. Choi, Y.K. Kim, C.S. Ha, Polym. Sci. 33 (2008) 581–630. [4] G.P. Crawford, Flexible Flat Panel Display Technology, John Wiley & Sons, Ltd., 2005. [5] A. Nathan, B.R. Chalamala, Proc. IEEE 93 (2005) 1391–1393. [6] R.A. Street, W.S. Wong, S.E. Ready, M.L. Chabinyc, A.C. Arias, S. Limb, A. Salleo, R. Lujan, Mater. Today 9 (2006) 32–37. [7] M. Young Lee, M.W. Lee, J.E. Park, J.S. Park, C.K. Song, Microelectron. Eng. 87 (2010) 1922–1926. [8] M. Mizukami, N. Hirohata, T. Iseki, K. Ohtawara, T. Tada, S. Yagyu, T. Abe, T. Suzuki, Y. Fujisaki, Y. Inoue, S. Tokito, T. Kurita, IEEE Electron. Dev. Lett. 27 (2006) 249–251. [9] L. Zhou, A. Wanga, S.C. Wu, J. Sun, S.K. Park, T.N. Jackson, Appl. Phys. Lett. 88 (2006) 83502–83504. [10] M.C. Suh, J.H. Jeong, T. Ahn, J.S. Park, S.Y. Kim, Y.J. Kim, T.J. Kim, H.J. Lee, S.M. Lee, Y.W. Park, Y.G. Mo, H.K. Chung, B.W. Koo, S.Y. Kim, S.Y. Lee, SID 06 Dig. (2006) 116–118. [11] I. Yagi, N. Hirai, M. Noda, A. Imaoka, Y. Miyamoto, N. Yoneya, K. Nomoto, J. Kasahara, A. Yumoto, T. Urabe, SID 07 Dig. (2007) 1753– 1756.
[12] S.H. Han, Y.R. Son, S.H. Lee, K.J. Lee, G.S. Cho, W.S. Kim, T.J. Park, W.S. Jeon, D.J. Choo, J. Jang, SID 07 Dig. (2007) 1757–1760. [13] I. Yagi, N. Hirai, Y. Miyamoto, M. Noda, A. Imaoka, N. Yoneya, K. Nomoto, J. Kasahara, A. Yumoto, T. Urabe, J. Soc. Inf. Display 16 (2008) 15–20. [14] J.Y. Yan, H.L. Chen, S.T. Yeh, J.L. Liao, Y.Y. Wu, M.R. Lin, K.P. Liao, C.Y. Hung, T.W. Lee, K.Y. Cheng, Y.Y. Lee, J.C. Ho, K.Y. Ho, H.C. Lin, C.H. Cheng, P.H. Wang, H.S. Dai, Y.S. Huang, SID 09 Dig. (2009) 986–987. [15] K. Nomoto, SID 10 Dig. (2010) 1155–1158. [16] M. Noda, N. Kobayashi, M. Katsuhara, A. Yumoto, S. Ushikura, R. Yasuda, N. Hirai, G. Yukawa, I. Yagi, K. Nomoto, T. Urabe, J. Soc. Inf. Display 19 (2011) 316–322. [17] T.C. Huang, K. Ishida, T. Sekitani, M. Takamiya, T. Someya, T. Sakurai, SID 11 Dig. (2011) 149–152. [18] T. Sekitani, T. Yokota, Takao Someya, SID 12 Dig. (2012) 426–429. [19] G.S. Ryu, S.H. Jeong, D.H. Kwon, J.S. Kim, C.K. Song, IMID 2012 Dig. (2012). [20] M.J. Kim, H.W. Heo, Y.K. Suh, C.K. Song, Org. Electron. 12 (2011) 1170–1176. [21] M.W. Lee, G.S. Ryu, Y.U. Lee, C. Pearson, M.C. Petty, C.K. Song, Microelectron. Eng. 95 (2012) 1–4.