The PVA mode was originally developed by
Samsung in 1996. It is characterized by multi-domain structures with
fringe-field effect. The electrodes are designed in a zigzag/cheveron
pattern, as show in the figure 1.
Figure 1. The
working principle of Patterned Vertical Alignment (PVA) mode.
In the off state, the LC molecules are
oriented perpendicularly to the substrate, as result, there is no
phase retardation and an excellent black state is obtained. In the
on state, the LC molecules will align perpendicular to the electric
field because of Δε < 0, consequently a multi-domain structure of LC
director configuration is obtained, which gives excellent viewing
angle. A PVA on-pixel texture under polarized microscope is shown in
figure 2, a retardation plate is inserted to distinguish the
director tilt direction. In the yellowish area, the director tilts
45 degrees upper left, and in the greenish area, the director tilts
45 degrees upper right.
Figure 2.
On-pixels of PVA under polarized microscope. Retardation film is inserted.
Compared with IPS, PVA has higher peak contrast ratio because
of the good black state, however IPS usually has a better off axis
contrast ratio, figure 3 shows a comparison between PVA and S-IPS.
Figure 3. Viewing
angle performance comparison between PVA and IPS LCD.
Recent development of PVA mode has increased
the domain numbers from 4 to 8, so-called S-PVA, which further
increased the viewing angle performance. With Samsung's Dynamic
Capacitance Compensation (DCC) driving scheme, the response time can
be improved from 50ms to 8ms. Figure 4 shows the driving
scheme of the DCC II.
The first low amplitude pulse is to establish the
pretilt angle, and the overshoot speeds up the switching. A
comparison with conventional driving scheme is shown in animation 1.
Animation 1. DCC II
driving scheme compared with conventional driving scheme.
(Right-click to replay the flash).
Now, S-PVA is widely used in large size flat
panel LCD, PC monitors, TV. Samsung used S-PVA to build the world
largest 82-inch TFT-LCD in March 2005 (press
release).
Further Readings
and References:
K.H. Kim, and J. H. Souk et. al, Proceeding of
Asia Display 98, 383 (1998).
K.H. Kim, "Advance of PVA Technology for
Multi-media Applications", SID Digest, 40.4 1208 (2003)
Last
update: April, 2006
Questions? Contact author.
