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
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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.
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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.
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RGBW Panel Using Two-Stacked White OLED and Color
Filter for Large-Sized Display Applications", SID Digest,
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[2] W. Y. So, M. S. Weaver, J. J. Brown, "Power Efficient
RGBW AMOLED Displays Incorporating Color-Down-
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[4] N. Morosawa, M. Nishiyama, Y. Ohshima, A. Sato, Y.
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[5] P. G. Engeldrum, "Image Quality Models: Where are we?"
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[6] J. K. Yoon, E. M. Park, J. S. Son, H. W. Shin, H. E. Kim,
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[7] H.J. Peng, M. Wong, H. S. Kowk, "Design and
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[8] International Committee for Display Metrology (ICDM),
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