53.2: invited paper : advanced technologies for large-sized oled tv

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Advanced Technologies for Large-sized OLED TV Chang-Wook Han, Jung-Soo Park, Young-Hoon Shin, Moo-Jong Lim, Bong-Chul Kim, Yoon-Heung Tak and Byung-Chul Ahn LG Display Co., Ltd., Paju-si, Gyeonggi-do, Korea Abstract Large-sized OLED displays require advanced technologies to realize mass production. The competitiveness of OLED TV comes from the combination of oxide TFT and WRGB OLED technology. Those technologies including image quality of OLED TV, oxide TFT, WOLEDs, and solid phase encapsulation enable panel size scalability as well as mass production with lifetime reliability. We will introduce technological progress for commercializing large-sized OLED TV. Author Keywords OLED TV; Oxide TFT; WOLED; Solid Phase Encapsulation 1. Introduction Organic Light Emitting Diodes (OLEDs) are the most interesting and promising new display technology. OLEDs provide a number of major technology enhancements for display and TV such as high contrast ratio, wide viewing angle, and very fast response time. Moreover, OLEDs could be used to make paper-thin display, transparent display, and flexible display. In 2009, LG has commercialized 15-inch OLED TV set which was based on fine metal mask (FMM) technology and low temperature poly silicon (LTPS) thin film transistor (TFT). However, producing a large-sized OLED TV is considerably more difficult for many technical issues. FMM is applied to mass production of small sized OLED display to form RGB sub- pixel. However, this method is not proper for large-sized OLED display because of defects and color mixing which are caused by sagging and mis-alignment of FMM and glass. In addition, there still remain important issues for extending the TFT backplane technology to the large-sized substrate. It is necessary to further improve long-term stability and improve production yield. Therefore, both fine color patterning and uniform TFT backplane process are major obstacles to realize large-sized panel manufacturing. In recent years, there have been some breakthroughs in large- sized OLED TV. One way is the use of White OLEDs (WOLEDs) with a combination of color-layered RGB patterning. WOLEDs have been suggested to provide large size manufacturing [1-2]. This technology does not require a sophisticated FMM as well as mis-align margin in pixel design and has excellent scalability over Gen. 8 glass substrate with high productivity and process yield. Another way is the use of oxide TFT which has been intensively developed to meet the requirement of large-sized back plane for OLED TV [3-4]. Oxide TFT has the advantages in terms of scalability (>Gen. 8) and can compatibility with amorphous silicon (a-Si) TFT infrastructures. The combination of WOEDs and oxide TFT is able to realize the mass production of large-sized OLED TV which has higher yield and lower cost. We have commercialized large-sized OLED displays including 55-inch full-high definition (FHD) flat and curved OLED TV in early 2013. After the first commercialization of OLED TV, we have demonstrated 77-inch ultra-high definition (UHD) curved OLED TV at IFA 2013. In this paper, we will introduce OLED TV and its advanced technologies including image quality of OLED TV, oxide TFT, WOLEDs, and Solid Phase Encapsulation (SPE) technologies. 2. Image Quality of OLED TV There are various ways to define the image quality of TV, and one of the most famous ways to define the image quality is to use the image quality circle which suggested by Engeldrum as shown in Figure 1 [5]. Image quality circle is a robust model that is connected using 4 steps of process such as customer quality preference, technology variables of the specific imaging systems, physical image parameter, and customer perception. At the technology variables process, manufacturer can control various design factors to enhance physical image parameters which can be measured. Furthermore, the customer perception process can be enhanced by visual algorithm based on the physical image parameter to reach the customer quality preference of the product. In order to discuss of the image quality of OLED technologies, physical image parameter step is only considered in this paper. Figure 1. Conceptual diagram of Image Quality Circle Generally, OLED has a great deal of merits for TV application i.e., wide viewing angle, real black comes with infinite contrast ratio, and fast response time, and so forth. For several decades, contrast ratio values have been used for evaluation of viewing angle performance. The contrast values for viewing angle of OLED technologies, however, has no meaning to measure since they are almost infinite values at viewing directions even though their luminance and color values are changed. The new concepts of viewing angle measurement are, therefore, required. In this section, viewing angle performance is evaluated using luminance values instead of using contrast ratio values. Recently we analyzed the picture quality of WRGB OLED TV 53.2 / C.-W. Han 770 SID 2014 DIGEST ISSN 0097-966X/14/4502-0770-$1.00 © 2014 SID Invited Paper

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Page 1: 53.2:               Invited Paper               : Advanced Technologies for Large-sized OLED TV

Advanced Technologies for Large-sized OLED TV

Chang-Wook Han, Jung-Soo Park, Young-Hoon Shin, Moo-Jong Lim, Bong-Chul Kim,

Yoon-Heung Tak and Byung-Chul Ahn LG Display Co., Ltd., Paju-si, Gyeonggi-do, Korea

Abstract Large-sized OLED displays require advanced technologies to

realize mass production. The competitiveness of OLED TV

comes from the combination of oxide TFT and WRGB OLED

technology. Those technologies including image quality of

OLED TV, oxide TFT, WOLEDs, and solid phase encapsulation

enable panel size scalability as well as mass production with

lifetime reliability. We will introduce technological progress for

commercializing large-sized OLED TV.

Author Keywords OLED TV; Oxide TFT; WOLED; Solid Phase Encapsulation

1. Introduction Organic Light Emitting Diodes (OLEDs) are the most

interesting and promising new display technology. OLEDs

provide a number of major technology enhancements for display

and TV such as high contrast ratio, wide viewing angle, and

very fast response time. Moreover, OLEDs could be used to

make paper-thin display, transparent display, and flexible

display.

In 2009, LG has commercialized 15-inch OLED TV set which

was based on fine metal mask (FMM) technology and low

temperature poly silicon (LTPS) thin film transistor (TFT).

However, producing a large-sized OLED TV is considerably

more difficult for many technical issues. FMM is applied to

mass production of small sized OLED display to form RGB sub-

pixel. However, this method is not proper for large-sized OLED

display because of defects and color mixing which are caused by

sagging and mis-alignment of FMM and glass. In addition, there

still remain important issues for extending the TFT backplane

technology to the large-sized substrate. It is necessary to further

improve long-term stability and improve production yield.

Therefore, both fine color patterning and uniform TFT

backplane process are major obstacles to realize large-sized

panel manufacturing.

In recent years, there have been some breakthroughs in large-

sized OLED TV. One way is the use of White OLEDs

(WOLEDs) with a combination of color-layered RGB

patterning. WOLEDs have been suggested to provide large size

manufacturing [1-2]. This technology does not require a

sophisticated FMM as well as mis-align margin in pixel design

and has excellent scalability over Gen. 8 glass substrate with

high productivity and process yield. Another way is the use of

oxide TFT which has been intensively developed to meet the

requirement of large-sized back plane for OLED TV [3-4].

Oxide TFT has the advantages in terms of scalability (>Gen. 8)

and can compatibility with amorphous silicon (a-Si) TFT

infrastructures. The combination of WOEDs and oxide TFT is

able to realize the mass production of large-sized OLED TV

which has higher yield and lower cost.

We have commercialized large-sized OLED displays including

55-inch full-high definition (FHD) flat and curved OLED TV in

early 2013. After the first commercialization of OLED TV, we

have demonstrated 77-inch ultra-high definition (UHD) curved

OLED TV at IFA 2013. In this paper, we will introduce OLED

TV and its advanced technologies including image quality of

OLED TV, oxide TFT, WOLEDs, and Solid Phase

Encapsulation (SPE) technologies.

2. Image Quality of OLED TV There are various ways to define the image quality of TV, and

one of the most famous ways to define the image quality is to

use the image quality circle which suggested by Engeldrum as

shown in Figure 1 [5]. Image quality circle is a robust model

that is connected using 4 steps of process such as customer

quality preference, technology variables of the specific imaging

systems, physical image parameter, and customer perception. At

the technology variables process, manufacturer can control

various design factors to enhance physical image parameters

which can be measured. Furthermore, the customer perception

process can be enhanced by visual algorithm based on the

physical image parameter to reach the customer quality

preference of the product. In order to discuss of the image

quality of OLED technologies, physical image parameter step is

only considered in this paper.

Figure 1. Conceptual diagram of Image Quality Circle

Generally, OLED has a great deal of merits for TV application

i.e., wide viewing angle, real black comes with infinite contrast

ratio, and fast response time, and so forth.

For several decades, contrast ratio values have been used for

evaluation of viewing angle performance. The contrast values

for viewing angle of OLED technologies, however, has no

meaning to measure since they are almost infinite values at

viewing directions even though their luminance and color values

are changed. The new concepts of viewing angle measurement

are, therefore, required. In this section, viewing angle

performance is evaluated using luminance values instead of

using contrast ratio values.

Recently we analyzed the picture quality of WRGB OLED TV

53.2 / C.-W. Han

770 • SID 2014 DIGEST ISSN 0097-966X/14/4502-0770-$1.00 © 2014 SID

Invited Paper

Page 2: 53.2:               Invited Paper               : Advanced Technologies for Large-sized OLED TV

compared to LCD TV [6]. From the luminance values depending

on different viewing directions, half luminance angle can be

retrieved to be compared. The half luminance angle of WRGB

OLED TV is approximately 80 degree while that of LCD TV is

around 50 degree. LCD TV needs liquid crystal distortions for

gray expression, whereas WRGB OLED TV does not need those

and viewing angle is widened with less luminance change. This

results show that users can watch WRGB OLED TV without

difficulty in brightness at any position.

In case of OLED technology, micro cavity effect is an important

factor to be applied to increase the efficiency of each pixel. This

strong micro-cavity effect [7], however, can lead to reduce the

luminance values and to change the color coordinate values

when it is watched at different viewing directions. One of the

benefits of the WRGB OLED technology is that employs white

sub-pixels, red, green, and blue in a vertical configuration, and

the additional white sub-pixel helps to increase overall

efficiency of the display panel. Therefore, the strong micro-

cavity effect is not strictly required to be applied on the panel.

As it is mentioned, viewing angle is one of the essential physical

image parameters to explain of customer perception items such

as brightness, naturalness, and gradation on the image quality

circle. From a different standpoint, artifact of the display

technologies should be also considered to account for image

quality of TV, since it can be also affected to the customer

perception items, such as sharpness and colorfulness. In this

paper, color washing artifact is evaluated for example. Figure

2(a) shows a test pattern referenced from IDMS [8] to check the

color washing artifact and it is composed of various lines and

dots. If a display has any color washing artifacts for some

reasons, the color of the test pattern can be changed. We propose

a novel sharpening method for WRGB OLED TV to enhance the

image quality using only white sub-pixels (advanced WRGB

OLED). Figure 2(b) shows the comparison of the color washing

phenomena when the test pattern is shown on each tested TV.

The benefit of advanced WRGB OLED TV for this artifact is

that there is a potential to reduce those artifact using white sub-

pixels.

Figure 2. Color washing test pattern and comparisons

between samples

Table 1. Comparison of color shift caused by color

washing effect

Color shift(Δu’v’) White Red Green Blue

WRGB OLED 0.028 0.093 0.064 0.005

Advanced WRGB OLED 0.014 0.005 0.002 0.007

Table 1 summarizes the calculation results of color difference

between reference and color washed test patch for each

measured TV sample.

Consequently, WRGB OLED is a promising future technology,

especially for TV application. It outperforms the other TV

technologies such as LCD and the other conventional OLED

technologies in terms of performance as well as artifact factors.

3. Oxide TFTs The a-IGZO TFTs with an etch stop layer are fabricated on Gen.

8 glass substrate. Copper is used as a gate material and silicon

oxide (SiO2) as a gate insulator is fabricated using plasma

enhanced chemical vapor deposition (PECVD). The a-IGZO

layer is deposited by AC sputtering at room temperature. The

etch stop layer is fabricated by PECVD. Copper is also used as a

source and drain material. The SiO2 layer fabricated by PECVD

is employed for passivation layer. In order to improve the photo

reliability of a-IGZO TFTs, red color resist deposited on TFTs

channel region during color-layered RGB patterning. After

completing TFT fabrication process, the a-IGZO TFTs are

annealing to improve device performance and stability.

There are two issues to overcome when employing oxide TFTs

for OLED TV, those are how we achieve VTH uniformity and

reliability of oxide TFTs on the entire area of Gen.8 glass

substrate. Those characteristics are very closely related to the

image quality of OLED TV. Figure 3(a) shows transfer

characteristics of a-IGZO TFTs. The channel width (W) and

length (L) of TFTs are 140 and 12.3 µm, respectively. The

transfer characteristics are measured at the 50 points of transfer

properties over the entire Gen. 8 glass substrate. The measured

results show good long-range VTH uniformity. The a-IGZO

TFTs represent VTH of 0.60 V, S-Factor of 0.12 V/decade, and

μsat of 11.62 cm2/Vs. Excellent S-Factor of 0.12 V/decade might

be attributed to the reduction of interface defects between the

active and gate insulator. Output characteristics in Figure 3(b)

show that the drain currents over the saturation drain current was

0.58 A at VGS = 1 V.

Figure 3. (a) Transfer and (b) output characteristics of the

a-IGZO TFTs

It is known that positive bias temperature stress (PBTS) and

negative bias temperature illumination stress (NBTIS)

characteristics are associated with the long-term stability of a-

IGZO TFTs. In order to evaluate the reliability characteristics of

a-IGZO TFTs, PBTS and NBTIS test were performed under gate

voltage (Vg) of ± 30 V and drain voltage (Vd) of 0 V. The

samples temperature during the PBTS and NBTIS test was fixed

at 60 °C, and the VTH shifts (∆VTH) were calculated using the

transfer curve of TFTs before and after BTS whose Vd was 10

V. The VTH shift after PBTS is 1.08 V at 10 hours. The change

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of VTH under NBTIS at 4,500 nit after 1 hour is -0.23 V. Finally,

it is confirmed whether oxide TFTs can be stable during the

guaranteed lifetime of OLED TV.

We commercialized oxide TFTs being used for OLED TV for

the first time. It was confirmed that oxide TFTs have good long

range VTH uniformity and long-term reliability.

Figure 4. Threshold voltage shift for a-IGZO TFTs (a)

under PBTS and (b) NBTIS

4. White OLEDs Nowadays, the trend of large-sized TV requires higher

resolution definition display such as UHD TV. However, as

displays increase in resolution, the pixel size decreases due to

the increase of pixel number on the display. The aperture ratio of

bottom-emissive OLED pixel is serious problem for high-

resolution displays. The aperture ratio would be sacrificed about

30 % according to change FHD from to UHD. Small aperture

ratio means a rather large current density through OLED. The

large current density would reduce the lifetime of OLED,

directly deteriorating reliability of displays. So, the problem of

reduced aperture ratio can be overcome by enhancing the

lifetime of OLEDs.

In order to improve efficiency and lifetime of OLEDs, the

degree of exciton confinement is necessary. It is well-known

that mixed-host structure blurs the HTL/EML interface which

broadens the emission zone and hence elongates the operation

lifetime [9]. We present high efficiency and long-lifetime

WOLED which was developed with a mixed-host structure of a

hole transport-type host (host A, B) and an electron transport-

type host (host C) as an yellowish green (YG) emissive layer.

Host B was obtained by slightly modifying host A to improve

the balance of holes and electrons in the EML.

Figure 5. The structure of hybrid tandem WOLED and device characteristics of WOLED @ 10 mA/cm

2 (Table 2)

We developed high efficiency and long-lifetime WOLED on the

basic of the evaluation of the distribution of excitons. Figure 5

shows the hybrid tandem WOLED structure which is comprised

of a fluorescent blue stack, a charge generation layer, and a

phosphorescent YG stack in series. The performances of the

fabricated WOLED are presented in table 2. The external

quantum efficiency (EQE) is 32.5 % and the luminous efficiency

is 78.7 cd/A at the current density of 10 mA/cm2.

To test the shift of recombination zone on different type of host

combination in YG EML, we have analyzed the distribution of

excitons. A method to estimate the distribution of excitons was

applied by doping a small amount of phosphorescent red dopant

in EML with varied positions. The phosphorescent red dopant

was chosen as the sensing layer because it emits a red color

around 630 nm which is easily distinguishable from YG

emission. We fabricated YG OLED with host A: host C and host

B: host C as the mixed-host for device A and device B,

respectively. Devices were fabricated with the same structure

except that a small amount of phosphorescent red dopant was

doped in the YG EML of devices with variation of the doping

position, as illustrated in figure 6.

Figure 6. Conceptual device with red sensing layer in YG

EML

Figure 7. (a) Distribution of excitons in EMLs, (b) lifetime

of YG device for different host

By examining the evolution of spectra from various probe

positions, the intensity ratio of red-to-YG component can be

obtained. As shown in figure 7(a), the highest red emission

occurred at 20 nm away from the HTL/EML interface (Device

A). We found that excitons were formed mainly in the regions

near the HTL/EML interface. On the other hand, a device B

resulted in a recombination zone shift from the HTL/EML

interface to the center of the EML as introduction of host B

content in the EML. Thus, the recombination zone was

distributed over the entire EML and the distribution of excitons

in the device B was flatter than that of device A. As a result, the

lifetime of device B improved around 4 times longer than that of

device A, as shown in figure 7(b). It is expected that carriers are

effectively dispersed from the HTL/EML interface to the center

of EML, thus effectively preventing the damage by excessive

charge accumulation.

53.2 / C.-W. Han

772 • SID 2014 DIGEST

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The major effects of the introduction of the mixed-host are

described as follows. Thanks to ideal matching between hole

transport-type host and an electron transport-type host, carriers

are effectively dispersed in the center of EML. Accordingly, the

flat distribution of excitons over the entire EML could prevent

the local degradation of the EML by releasing the stress induced

by heat or charge molecules, thus increasing the efficiency and

lifetime of WOLED.

5. Solid Phase Encapsulation (SPE) We have developed solid phase encapsulation technology for

large-sized OLED TV which has longer environment stability

and mechanical stability.

First, SPE adopted a Si-based barrier layer on the OLED devices

deposited by PECVD. The barrier layer prevents OLED devices

from being degraded by humidity and mechanical damages

(Figure 8). Most outstanding aspect of barrier layer is its

thickness. It is only 0.5 µm thickness with single-mono layer

that can accomplish expected performances, especially with

respect to shelf lifetime. It is possible by carefully optimized

synthesis of the barrier at extremely low process temperature

(<100 °C) as Si-based PECVD process. This high quality

thinner barrier layer leads to reduce the quantity of deposition

facilities, resulting in lower cost.

Figure 8. A cross sectional diagram of SPE structure of

OLED TV

Recently, we substituted metal foil for glass as the encapsulation

lid material. Thin (0.1 mm of thickness) metal foils enable to

reduce the material cost with additional several performance

benefits. Metal foils are quite durable, impermeable to water

vapor, and flexible enough for curved, bendable OLED

applications. Those characteristics of metal foils contribute to

enhancement of the mechanical stability of curved OLED TVs.

Metal foil is laminated with a specific water-proof ‘adhesive

sheet’ after the barrier layer is deposited on the TFT glass. The

final structure of SPE is metal foil/adhesive/TFT glass without

cavity, i.e., full-solid structure. Because two sheets of substrates

are laminated whole surface with adhesive sheet, OLED panel

has uniform mechanical reliability without any vulnerable area.

This full-solid structure feature of SPE also makes itself as a

promising encapsulation scheme for future advanced

applications of OLEDs such as curved display, flexible display,

and so on.

In conclusion, SPE is a mature encapsulation technology for

large-sized OLED TV commercialization in terms of simple

structure, longer environment stability and cost-effectiveness.

6. Conclusion We have developed large-sized OLED TV by the combination

of oxide TFT and WRGB OLED technology. We introduced

advanced technologies including image quality of OLED TV,

oxide TFT, WOLEDs, and solid phase encapsulation

technologies to realize the commercialized large-sized OLED

displays. It is expected that recent technological progress for

commercializing large-sized OLED TV enable panel size

scalability as well as mass production with cost competitiveness

for OLED TV.

7. References [1] C. W. Han, S.H. Pieh, H.S. Pang, J.M. Lee, H.S. Choi, S.K,

Hong, B.S. Kim, Y.H. Tak, N.Y. Lee, B.C. Ahn, "15-inch

RGBW Panel Using Two-Stacked White OLED and Color

Filter for Large-Sized Display Applications", SID Digest,

136-139 (2010).

[2] W. Y. So, M. S. Weaver, J. J. Brown, "Power Efficient

RGBW AMOLED Displays Incorporating Color-Down-

Conversion Layers", SID Digest, 282-285 (2012).

[3] T. H. Shih, T. T. Tsai, K. C. Chen, Y. C. Lee, S. W. Fang,

J. Y. Lee, W. J. Hsieh, S. H. Tseng, Y. M. Chiang, W. H.

Wu, S. C. Wang, H. H. Lu, L. H. Chang, L. Tsai, C. Y.

Chen, Y. H. Lin, "A 32-inch Active-Matrix Organic Light-

Emitting Diode Television Panel Driving by Amorphous

Indium–Gallium–Zinc Oxide Thin-Film Transistors", SID

Digest, 92-94 (2012).

[4] N. Morosawa, M. Nishiyama, Y. Ohshima, A. Sato, Y.

Terai, K. Tokunaga, J. Iwasaki, K. Akamatsu, Y. Kanitani,

S. Tanaka, T. Arai, K. Nomoto, "High Mobility Self-

Aligned Top-Gate Oxide TFT for High-Resolution AM-

OLED", SID Digest, 85-88 (2013).

[5] P. G. Engeldrum, "Image Quality Models: Where are we?"

Final program and proceedings image processing image

quality image capture systems (PICS) conference, IS&T,

251 (1999).

[6] J. K. Yoon, E. M. Park, J. S. Son, H. W. Shin, H. E. Kim,

M. Yee, H. G. Kim, C. H. Oh, B. C. Ahn, "The Study of

Picture Quality of OLED TV with WRGB OLEDs

Structure", SID Digest, 326-329 (2013).

[7] H.J. Peng, M. Wong, H. S. Kowk, "Design and

characterization of OLED with Micro-cavity structure",

SID Digest, 516-519 (2003).

[8] International Committee for Display Metrology (ICDM),

"International Display Measurement Standard (IDMS) 1.0"

(2011).

[9] H. M. Aziz, Z. D. Popovic, N.-X. Hu, A. M. Hor,"Organic

light emitting devices including mixed region", US

6392339.

53.2 / C.-W. Han

SID 2014 DIGEST • 773

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