1. A Novel Vertically-aligned In-plane-switching LCD Mode with Great
Picture Quality
Sau-Wen Tsao, Cho-Yan Chen, Tien-Lun Ting, Yen-Ying Kung, Mei-Ju Lu, Wen-Hao Hsu,
and Jenn-Jia Su
AU Optronics Corporation, Hsinchu Science Park, Hsinchu 30078, Taiwan, R.O.C.
Abstract
In this work, an LCD mode with vertical alignment and in-plane
switching electrodes (VA-IPS) is proposed to improve optical
performance. This mode can control tilt angles of LC molecules
via combinations of various distances of the in-plane-switching
electrodes, and thus improve color washout. Also, a new driving
scheme combining T-T type driving method with charge-shared
(CS) structure is presented to further improve the picture quality at
oblique viewing angle by not only adjusting spacing of in-plane-
switching electrode pairs but also creating an extra voltage
difference by CS structure in one pixel. After simulation and
optimization, this mode possesses very small oblique gamma
distortion and TRDI as low as 0.13.
Author Keywords
VA-IPS; wide viewing angle; high contrast ratio; picture quality;
TRDI; color washout; charge-shared.
1. Introduction
Vertical-alignment (VA) liquid crystal display mode such as
polymer sustained alignment (PSA) has many advantages
including high contrast ratio and wide viewing angle [1][2].
However, the picture quality at oblique viewing angle of VA mode
has always been an issue, and lots of efforts were made on this
topic [3-5].
In this paper, a new vertical-alignment mode with in-plane-
switching electrodes (VA-IPS) is proposed. To a certain extent,
the VA-IPS mode combines the concept of the VA and the IPS
mode; thus it can provide an excellent performance in color
washout while keeping the advantages of VA mode such as high
contrast ratio and rubbing-free process. Besides, we use a
vertically-aligned in-plane-switching (VA-IPS) mode combined
with dual data (T-T type) [6][7] driving to enhance the driving
voltage, and then further apply the charge-shared (CS) structure[8]
to modulate the voltage of pixel electrode and create extra
domains. Thus the picture quality can be greatly improved at
oblique viewing angle by creating a voltage difference among the
different-spacing electrode pairs. We use the tone rendering
distortion index (TRDI) [9] to judge the performance of picture
quality at oblique viewing angle and attempt to find the optimum
spacing design and the corresponding voltage relation of the pixel
electrode pairs.
2. Experimental Results
Fig. 1 is a schematic cross sectional view of VA-IPS mode. The
VA-IPS mode uses liquid crystal with positive dielectric
anisotropy and vertical alignment layer, and it has the interdigital
pixel electrodes with different voltages alternately disposed in the
same plane just like IPS. As shown in Fig. 1(a), at the off state, the
liquid crystal molecules are vertically aligned. When the voltage
difference applied upon the in-plane-switching electrodes is large
enough such as Fig. 1(b), the liquid crystal molecules incline
along the direction of the electric field.
Comparing to the general vertical-alignment mode with vertical
electric field applied to LC of negative electrical anisotropy, the
VA-IPS mode is capable of improving picture quality at oblique
viewing angle by adjusting spacing of electrode pairs. As shown in
Fig. 1(b), the liquid crystal molecules above the first spacing, S1,
tilt differently from those above the second spacing, S2, because
of different electric field caused by the inequality between S1 and
S2. In this experiment, we fabricate several cells filled with tested
LC. Each of these cells has single-spacing ITO pairs ranging from
4µm to 16µm, and can be measured to retrieve on-axis and
oblique-view V-T curves. By linear combination, we can simulate
the V-T curves of multi-spacing electrode pairs with a certain
voltage relation, and then we can derive the oblique viewing
gamma curves of our proposed structures. Fig.2 shows on-axis V-
T curves of VA-IPS cell for (a) Spacing 4um; (b) Spacing 14um;
(c) Combination of spacing 4um and 14um. We can see that the
difference of threshold voltages of (a) and (b) is almost 2V. This
property gives the opportunity to reduce color washout by multi-
threshold effect, which is widely used in current VA-category
LCDs.
Glass
Glass
Polarizer
Analyzer
S2S1
Electrode
LC (∆ε >0)
Glass
Glass
Polarizer
Analyzer
S2S1
Electrode
LC (∆ε >0)
(a) OFF State
S2S1
+- -
Glass
Glass
Polarizer
Analyzer
Electrode
LC (∆ε >0)
S2S1
+- -
Glass
Glass
Polarizer
Analyzer
Electrode
LC (∆ε >0)
(b) ON State
Figure 1. Schematic diagram of the VA-IPS cell for (a)
OFF, (b) ON state
ISSN 0097-966X/13/4401-0342-$1.00 © 2013 SID
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2. 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10 12 14 16
NormalizedTransmittance(100%)
Voltage
(a) Spacing= 4
(b) Spacing= 14
(c) Spacing= 4/14
Figure 2. V-T curves of VA-IPS cell for (a) Spacing 4um,
(b) Spacing 14um, (c) Combination of spacing 4um and
14um
To drive the liquid crystal molecules with larger electrode spacing,
say 14um, we must ensure that the torque applied to liquid crystal
molecules to be large enough. Eq. (1) is the net torque applied to
LC molecules.
θεετ 2sin
2
1 2
0 ∆= Enet
r , (1)
where ε0 is the vacuum permittivity, E is the electric field applied
to LC, ∆ε is the electrical anisotropy of LC and θ is the angle
between electric field and the induced dipole of LC molecules.
Hence, the torque can be enhanced by increasing electric field or
electrical anisotropy. Based on our test, we believe that the
electrical anisotropy must be larger than 12; meanwhile the
voltage applied to LC must be larger than 14V.
To achieve the requirement above, a dual-data driving scheme is
adopted to enlarge applied voltage. Fig.3 shows the equivalent
circuit diagram of the T-T type driving structure; the voltage
across the LC is equal to the voltage difference between two data
signals. The largest voltage we can now apply is around 15V
without altering the current data driver.
The microscope images of the VA-IPS pixel in Fig.4 shows how
different electrode distances within a pixel lead to the multi-
threshold characteristic and multiple tilting angles of the LC. It is
obvious that small-spacing areas light up at low grey levels while
large-spacing areas don’t until mid-high grey levels. The multi-
threshold property of VA-IPS pixel is critical to the improvement
of color washout. The gamma curves at normal and oblique
direction and the local gamma curves are shown in Fig.5 and
Fig.6, respectively. The result shows the high similarity between
simulation and measurement. The tone rendering distortion index
(TRDI) is used to evaluate the color washout performance as listed
in Table 1. We can see from the table that the T-T type VA-IPS is
much better than current main-stream 8-Domain PSA.
CST1 CST2
CLC
Pixel 1 Pixel 2
Gate
Data1 Data2
CST1 CST2
CLC
Pixel 1 Pixel 2
Gate
Data1 Data2
Figure 3. Equivalent circuit diagram of dual-data (T-T
type) driving VA-IPS mode
Figure 4. The microscope images of the T-T type VA-IPS
pixel at various gray levels
0
0.2
0.4
0.6
0.8
1
0 32 64 96 128 160 192 224 256
N
o
rm
a
lize
d
L
u
m
in
a
n
c
e
(a
.u
.)
Gray Level
Normal
Oblique, T-T type VA-IPS, Simulated
Oblique, T-T type VA-IPS, Measured
Oblique, T-T type with CS VA-IPS, Simulated
Oblique, 8-Domain PSA
Figure 5. Normal and oblique gamma curves of various
driving VA-IPS mode and 8-Domain PSA
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3. 0
0.5
1
1.5
2
2.5
3
3.5
0 32 64 96 128 160 192 224 256
Gray Level
L
o
c
a
lG
a
m
m
a
Normal
Oblique, T-T type VA-IPS, Simulated
Oblique, T-T type VA-IPS, Measured
Oblique, T-T type with CS VA-IPS, Simulated
Oblique, 8-Domain PSA
Figure 6. Local gamma curves of various driving VA-IPS
and 8-Domain PSA
Table 1. TRDI and transmittance of VA-IPS mode with
various driving scheme
TRDI
D D
+
D-
Tr%
T-T type driving,
simulated
0.165 0.015 0.149 100%
T-T type driving,
experiment
0.146 0.020 0.126
T-T type driving with
CS structure,
simulation
0.129 0.017 0.112 92%
8-Domain PSA 0.257 0.028 0.229
Figure 7. Iso-contrast contour of the VA-IPS prototype
Fig.7 shows the iso-contrast contour. The compensation film used
in the experiment is similar to those used in MVA or PSA LCD
because the alignment of LC molecules of the dark state of VA-
IPS is completely the same. Hence VA-IPS shares the same
benefits of VA-type LCD, including low light leakage in dark
state, high contrast and wide viewing angle. Fig.8 gives the plot of
normalized contrast ratio versus polar angle. Comparing to PSA
mode, VA-IPS has no necessity of photo-curing process, which
causes polymerized clusters on the surface of alignment layer, and
thus results in higher normalized contrast ratio with increasing
polar angle.
Normalized CR versus polar angle
0%
20%
40%
60%
80%
100%
120%
-80 -60 -40 -20 0 20 40 60 80
VAIPS
PSA
Figure 8. Normalized contrast ratio versus polar angle
of proposed VA-IPS prototype
3. Combining T-T Type Driving VA-IPS Mode with
Charged-shared Structure
Furthermore, we combine the T-T type driving VA-IPS mode with
the charge-shared structure and greatly improve the picture quality
at oblique viewing angle of VA-IPS mode. Fig. 9 is the equivalent
circuit diagram of the combined structure we propose. Gn is the nth
row of the gate lines in the panel, while Gn+1 is the (n+1)th
row,
which is turned high right after Gn is turned low. D1 and D2 are the
data lines acting the same as the previous statement. Without
losing generality, let’s assume that D1 transmits high voltage and
D2 transmits low voltage during a certain time frame. We focus on
D1 side first to understand how this circuit functions. When Gn
goes high and turns on the TFTs connected to it, the point S1 and
the point P1 are both charged to the same high voltage transmitted
from D1 from the low voltage of the previous frame. The voltage
of the point Q1 is lower than S1 and P1 because of the capacitance
divider composed of C1A and C1B. When Gn is turned low and Gn+1
goes high, S1 and Q1 are connected through the TFT turned on by
Gn+1 and thus share each other’s charge until there’s no voltage
difference between them. The voltage of Q1 is going higher than it
was because that of S1 was higher than Q1. This upward change
along with the C1A causes the voltage of P1 to be pushed higher as
well, while the voltage of S1 is drawn to a lower voltage. On the
other hand, the D2 side is completely contrary to the D1 side. This
means the voltage of P2 and S2 are going down and up,
respectively. To create a fully complementary structure, the
capacitances of the D2 side (C2A, C2B, CSUB2, and CST2) have the
same values as the corresponding ones of the D1 side (C1A, C1B,
CSUB1, and CST1). The voltage change effect as described makes
CLC1 perceive a larger voltage while CLC2 perceive a smaller one
than the original voltage transmitted by D1 and D2. To simplify the
simulation, the final voltages applied to CLC1 and CLC2 are
assumed to have a certain linear ratio which is determined by the
all capacitances of the D1 side and the D2 side. With this structure,
we can find out more possibilities of the picture quality at oblique
viewing angle and identify the optimum design.
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4. CST1
C1A
C1B
CSUB1
CLC1
P1 P2
Q1
S1
D1 D2
Gn
Gn+1
CLC2
S2CST2
C2B
C2A CSUB2
Q2
CST1
C1A
C1B
CSUB1
CLC1
P1 P2
Q1
S1
D1 D2
Gn
Gn+1
CLC2
S2CST2
C2B
C2A CSUB2
Q2
Figure 9. Equivalent circuit diagram of T-T type driving
VA-IPS mode with charge-shared structure
Based on our simulation, a pure T-T type driving VA-IPS mode
with 3 kinds of electrode spacing has an optimum TRDI of about
0.17 at oblique direction. However, the T-T typed driving with CS
structure can furthermore improve TRDI to about 0.13 with 3
kinds of spacing and a voltage ratio of 0.7 between CLC1 and CLC2,
which is much better than current main-stream 8-Domain PSA[10]
as listed in Table 1. We can find in Fig. 5 that at low gray levels
the proposed structure has oblique gamma curve closer to on-axis
gamma than that of the pure T-T driving structure. The benefit of
the proposed structure can be seen more clearly from Fig. 6 which
shows the oblique local gamma curves. At the most of the region
from gray level 32 to gray level 224, the proposed structure
possesses oblique local gamma curve closer to 2.2 than the
original design. Because of the change of spacing and voltage, the
transmittance is changed as well. With each one’s optimum
design, the transmittance of the proposed structure is about 92% of
the original design.
4. Conclusion
In this paper, we demonstrate the great optical performance of the
T-T type driving VA-IPS mode. The dark state and the viewing
angle is the same or even better than MVA or PSA modes because
no pre-tilt angle of LC molecules is necessary. With proper
combination of distances of pixel electrodes, color washout can be
much better than 8-Domain PSA. Also, a new driving scheme by
combining T-T type driving with charge-shared structure can
further enhance the picture quality at oblique viewing angle by not
just adjusting ITO spacing but also modulating the ratio of
voltages of CLC1 and CLC2. According to simulation, with 3 kinds
of ITO spacing design and the voltage ratio of 0.7, the optimum
TRDI can be lowered to 0.13, which is very difficult for a VA-
type LCD to achieve. The compensation film of VA-IPS is
identical to that of the typical VA-type LCD. And more
importantly, the array glass is completely compatible to the
current process. No investment or capacity loss will occur for
switching. Thus, a display with high performance and low
investment can be realized by VA-IPS mode. This novel structure
can be considered as the next generation of LCD.
5. References
[1] Te-Sheng Chen, et al., SID Symposium Digest of Technical
Papers, pp. 776-779, 2009
[2] Chia-Hsuan Pai, et al., Journal of SID, pp. 960-967, 2010
[3] Yi-Pai Huang, et al., SID Symposium Digest of Technical
Papers, pp. 1010-1013, 2007
[4] Seung Beom Park, et al., SID Symposium Digest of Technical
Papers, pp. 1252-1254, 2007
[5] Bongim Park, et al., SID Symposium Digest of Technical
Papers, pp. 204-207, 2008
[6] S.S. Kim, SID’05 Symposium Digest, pp.1842-1847, 2005.
[7] C.Y. Chen et al., SID’08 Symposium Digest, pp. 1120-1122,
2008.
[8] C.H. Lin et al., US patent US7286192.
[9] J.K. Song et al., J. Display Technol. vol. 7, no. 7, pp. 365-372,
2011.
[10] Kun-Cheng Tien et al., SID’12 Symposium Digest, pp. 371-
374, 2012
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