Development of Oxide-TFT OLED-TV Technologies
hat怎么读语音Tsung-Hsiang Shih, Hung-Che Ting, Pei-Ling Lin, Chih-Lei Chen, Lun Tsai, Chia-Yu Chen, Li-Fong Lin, Chun-Hsin Liu, Chih-Cheng Chen, Hong-Shen Lin, Lee-Hsun Chang, Yu-Hsin Lin, Hong-Jye Hong
OLED Technology Center, AU Optronics Corporation, Taiwan
ABSTRACT
We reported amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) TFT OLED TV panel development in AUO. The Generation 6 (G6) threshold voltage uniformity can be lower than 0.8V. The RGB side-by-side OLED patterning was realized by fine metal mask. By the application of a source follower connection pixel compensation circuit, the panels driven by AUO engine show excellent characteristics. For future product performance requirement, amorphous Indium-Tin-Zinc-Oxide (a-ITZO) TFT was developing, high mobility as 33.2 cm2/Vs was achieved. Metal-organic chemical vapor deposition (MOCVD) system was also ud to improve the mobility of IGZO TFT. To improve manufacturing efficiency of OLED process, inkjet printing technology is also under development in AUO.
Author keywords: Oxide TFTs; High mobility; IJP; OLED TV
1. INTRODUCTION
High performance flat plane display using Oxide TFTs have actively developed for the application in the next generation display industry. Oxide TFTs have gained special attention as an attractive alternative to poly-Si TFTs, becau they provide better large area uniformity, and high field effect mobility can easily be obtained [1-4]. The solution process by inkjet printing (IJP) has many advantages such as simple structure, inexpensive process, easier to scale up to large panel sizes, and it does not require any FMM (fine metal mask) to produce a full color panel. Due to such advantages, the IJP-bad OLED process has become a remarkable candidate to bring down the cost of OLED panel manufacturing. AUO has demonstrated the 32”, 37”, 65” large size a-IGZO TFT LCD TV and 14”, 32”, 65” a-IGZO TFT OLED TV. For a-IGZO TFT, its mobility is ~10 cm2/Vs [5]. ITZO TFT was reported with a higher mobility of >30 cm2/Vs, which is a good candidate for the next-generation high performance displays [6]. AUO has also reported a 56” a-ITZO AMOLED TV [7]. For IJP technology, AUO had been demonstrated 6” and 14” panels. Recently, a 65” IJP OLED panel was also developed [10]. In this paper, we summarize the Oxide TFT OLED TV development in AUO. 2. FABRICATION
Array Structure
Figure 1 shows the schematic cross-ction of oxide TFTs, including coplanar, IS, and BCE type structures. AUO demonstrated the worldwide first LCD and OLED TV panels using a-IGZO TFT backplane, which are with coplanar and IS type, respectively [8-9]. For the fabrication of IS type TFTs for OLED application, cleaned G6 (1850*1500mm) glass was ud as the substrate. SiOx/SiNx was ud as the gate insulator (GI) layer. Oxide miconductor active layer was deposited by a sputtering or metal-organic chemical vapor deposition (MOCVD) system. The MOCVD technology can achieve and balance the mobility and the reliability due to the composition can be easily adjusted. SiOx/SiNx IS layer and N2O plasma pre-treatment were ud. The Al-ba or Cu-ba metal were ud as electrodes. SiOx/SiNx was ud as passivation layer (BP). The transfer and output characteristics were measured at room temperature with an Agilent E5270B precision miconductor parameter analyzer. The stress characteristics of the oxide TFTs were measured at 60 o C for 14.4Kc in an air/dark box. Gate voltage biad at -15V and Drain voltage biad at +10V for negative bias temperature stress (NBTS) test.
(a) (b) (b)
Figure 1. Schematic cross-ction of (a) coplanar, (b) IS
失物招领怎么写and (c) BCE type TFTs
OLED Structure
Fig.2 shows OLED device structure we adopted, which is a stack of hole injection layer (HIL), double hole transporting layer (HTL), emitting layer (EML), electron transporting layer (ETL), and a highly reflective cathode structure. Double HTLs are necessary for a good carrier injection into EML, it further helps to confine the excitons in the emitting layer and extend the EML lifetime.
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Figure 2. OLED structure
Full-color OLED Manufacturing Process
Regarding OLED device structure, there are HIL, HTL, EML, ETL, electron-injection layer (EIL), and top electrode (cathode) to be included. Each layer was deposited by planar, line and point sources. Generally speaking, there are mainly three kinds of methods for OLED full-color patterning evaporation, included 1) RGB lective evaporation, or RGB side-by-side (RGB SBS) deposition; 2) White OLED with color filter; 3) blue OLED with color change medium (as shown in Fig.3). RGB SBS is one of the most important technologies. RGB colors are formed parately by FMM (as shown in Fig.4).
Figure 3. OLED full-color patterning methods
Typical BE -AMOLED
Cap Glass EL Element
Figure 4. RGB-SBS OLED evaporation by FMM
Panel Specification
The high space and color resolution have greatly enhanced the TV ability of showing details and made TV color clor to the real world. The specification of the panel is shown in Table 1.
Display type a-IGZO AMOLED
Resolution qHD~FHD Gray scale RGB 10-bit
Brightness 200 cd/m 2 Contrast ratio >100,000:1
Table 1. Oxide AMOLED TV panel specifications
In Fig. 5~8, AUO has been demonstrated the 14”, 32”, 65” a-IGZO and 56” a-ITZO TFT OLED TV . The panels achieve good image quality, such as high contrast ratio, wide viewing angle, and fast respon time, and thin module thickness.
Figure 5. 14” qHD MOCVD-IGZO AMOLED TV panel
Figure 6. 32” FHD a-IGZO TFT OLED TV panel
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Figure 7. 65” FHD a-IGZO TFT OLED TV panel
Figure 8. 56” a-ITZO TFT OLED TV panel
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3. RESULTS AND DISCUSSION
Array Large Size Uniformity Improvement
英中互译After fine tune the Oxide TFT process, the long range Vth range within a panel was improved from 1.40V (32”, FPDI 2011/SID 2012) to 0.34V (65”, SID 2013). The sub-threshold swing (SS) are also improved from 0.50 to 0.38 (V/decade). The initial Vth is controlled to ~0V , which can be adjusted to positive by additional process or treatment. The G6 Vth range can lower than 0.8V . The delta Vth of hysteresis is around 0.033V .
High Mobility Material Development
Figure 9 shows the normalized transfer and output characteristics (int) of IGZO (black) and ITZO (red) TFTs. The IGZO TFT initial Vth and SS are ~0V and 0.39 (V/decade), respectively, The ITZO TFT initial Vth and SS are ~0V and 0.35 (V/decade), respectively. The I DS of ITZO TFT is higher than IGZO TFT. The mobility of ITZO TFT can reach 33.2 cm 2/VS. It’s ~3.3 times of a-IGZO TFT and better to drive the OLED TV .
The Vth shift of ITZO and IGZO TFTs under NBTS can be improved ~44.4% and ~31.8% after the usage of N 2O plasma pre-treatment, respectively. The result implied that the power of N 2O plasma passivation effect on back channel. However, even with N 2O plasma pre-treatment, the Vth shift of IGZO TFT under NBTS is still ~43.8% better than ITZO TFT. This maybe attributed from the high mobility ITZO TFT induced higher electric stress field under the same NBTS condition.
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Figure 9. Transfer characteristics of ITZO and IGZO
TFTs
For IGZO grown by MOCVD system, the mobility can reach ~23.1 cm 2/VS and SS keep ~0.2V/decade. The mobility higher than 50 cm 2/VS is also possible.
Panel Design
A source follower connection pixel compensation circuit is applied for the panel design. The pixel circuit can compensate the Vth of the driving transistor and also keep the constant current from the voltage variation of an OLED device. Moreover, the compensation pixel circuit can reduce the influence of the shifts in
the TFT and OLED characteristics due to temperature and stress time.
Panel Driver
Figure 10 shows AUO OLED driving block diagram. In this block, we can get the signal RGB data and AC power from system side. Power block deals with the AC source and supply to TCON bock, then TCON block calculate the RGB data and convert it to AUO OLED engine. In the AUO OLED engine, we specially process the data for OLED application. For example: OLED Engine(1) – lifetime extension. A pre-defined look up table in the engine will be ud to control the adaptive brightness bad on white ratio of input RGB data, while still keep good display performance. Figure 11 shows the lifetime extension block diagram, the Fig.12 shows the result.
Figure 10. AUO OLED driving block
Figure 11. Lifetime extension block diagram
Figure 12. The origin (left) and after extension (after)
Another example, OLED Engine(2) – Optical compensation. For the non-uniformity of brightness, we can detect the brighter and darker area through the CCD detector, compress and store the data in the look-up table (LUT). Finally OLED engine decompress the table, and make the non-uniformity disappears. We can easily check the image quality after applying optical compensation. The performance is still good even at low gray scale.
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Panel Performance
OLED device with weak-cavity structures were adopted into a-IGZO TFTs OLED panel. For example, 65” panel, the color characteristics of red, green, and blue were listed in Table2: (0.661, 0.337) for red, and (0.254, 0.650) for green and (0.140, 0.09) for blue emission spectra. Color gamut of OLED panel is about 83% in CIE 1931 coordinate. In order to meet the TV specification, the OLED devices can be modified into suitable color space and met the sRGB space for the commercia
l TV product. In HDTV category, sRGB is usually lected in the standardization, so that all display platforms have the capability to reproduce it in sRGB space if it is probably calibrated.
Table 2. RGB CIE performance table
L u m i n a n c e R a t i o
Viewing Angle
Figure. 13 LCD/OLED luminance ratio comparison of
different viewing angle
OLED TV has another merit – wider viewing angle compared with LCD TV . As shown in Fig. 13, we show the LCD TV & OLED TV luminance ratio in the same scale. From the measurement data, OLED TV has 80% luminance within ±60 degree and 50% luminance within ±80 degree. A normal LCD TV has 50% luminance within only ±30 degree. Different from LCD controls the light by liquid crystal, OLED is lf-emitting. This characteristic makes urs can watch in larger angle with less brightness drop. This means that urs can watch OLED TV comfortably at every angle.
IJP OLED Development
To improve OLED manufacturing efficiency, IJP OLED technology is also developing. We had demonstrated 6” and 14” panels in which the HIL, HTL, and EML (RGB) layers are fully printed by IJP machine. Recently, a 65” IJP OLED TV panel was also developed.
4. CONCLUSIONS
We reported large size a-IGZO TFTs OLED TV panel development in AUO. The G6 Vth uniformity can be lower than 0.8V and hysteresis is ~0.033V . The a-ITZO TFTs show a higher mobility of 33.2
cm 2/VS. MOCVD system was also ud to improve mobility of a-IGZO TFT. The RGB SBS OLED device was realized by FMM. IJP OLED is also under development to improve OLED manufacturing efficiency. By the application of a source follower connection pixel compensation circuit, the panels driven by AUO engine show excellent characteristics. 5. REFERENCES
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