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[Pi-Cell (Optically Compensated Bend - OCB)  mode]

The idea of Pi-cell was first proposed by P. Bos in 1984, it utilizes the backflow to speed up the switching process. It is also known as Optically Compensated Bend.

In the conventional Freedericksz ECB cells have a uniform alignment of molecules with a pretilt angle in the opposite direction on two substrates, also called anti-parallel alignment of substrates, as shown in figure 1, after the electric field is switched on and then off, the molecules start to relax back to the original state, causing a flow. The molecules in the mid-layer feel a torque, which causing a back-flow of the material and trying to rotate them through a large angle to the original state. However this slow down the switching speed.

Figure 1. Comparison between conventional ECB cell and Pi-cell.

In the Pi-cell, the pretilt angles on both substrates are in the same direction, also called parallel alignment of substrates. When the field is switched off, the molecules in the mid-layer no longer feel the torque during relaxing to the off state. The angle to tilt back is small, caused a faster switching speed.

Figure 2. Flow direction comparison between conventional TN cell and Pi-cell.

The back-flow can cause the optical bounce in the electro-optical response to a rectangular waveform pulses; this bounce increases the off time. In stead in the Pi-cell, no such bounce exists, hence a faster switching off is obtained.

The Pi-cell cell has an intrinsic wide viewing angle because of the self-compensated structure, as shown in figure 3, light ray 1 and 2 passing the two oblique incidence directions experience the same retardation.

Figure 3. Wide viewing angle from the self-compensation nature of the Pi-cell.

However this self-compensation nature is not valid for incidence out of the the director plane. Moreover, the on-axis contrast ratio (CR) is low because of the residual birefringence even at high voltage; therefore optical compensation films are required for high CR and wide viewing angle. P. Bos et al. proposed a method  of compensation for the cell using a negative birefringence film with the optic axis in the normal direction (negative C-plate) in 1993. In the same year, using biaxial films for optical compensation was proposed by Yamaguchi et al.

To obtain the bend structure of the cell, an electric field with amplitude above the splay to bend transition voltage must be applied. However, the transition from splay to bend structure takes a long time, typically, in the order of tens of seconds. Another issue is that splay domains appear at the regions with weak electric field in a complicated electrode pattern. These problems associated with the bend alignment stabilization were solved after the efforts of LCD researchers and manufacturers. The first commercial Pi-cell (OCB) panel is now available in the market.

Further Readings and References:

P. J. Bos and K. R. Koehler-Beran, Mol. Cryst. Liq. Cryst. 113, 329 (1984).

Y. Yamaguchi, T. Miyashita and T. Uchida, "Wide-viewing-angle display mode for the active-matrix LCD Using bend-alignment liquid-crystal cell", SID'93 Dig., pp. 273-276 (1993).

P. J. Bos and J. A. Rahman, ¡°An optically ¡¯self-compensating¡¯ electro-optical effect with wide angle of view",,¡± in SID¡¯93 Dig., vol. 273, (1993).


Last update: April, 2006
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