2-2-1 The structure of π cell
Fig. 2-1 is the structure of the π cell. The two polarizer above and under the cell are crossed, and the alignment layers on the two substrate have the same rubbing direction, and make an angle of 45 degree with the two polarizer. In general, the pretilt angle of the alignment layer used in π cell is about in the range of 6~14 degree, such pretilt angle makes the LC molecules align in splay state in a plane without distortion
Fig. 2-1: The structure of the π cell.
2-2-2 Fast response
π cell is a fast liquid crystal display mode[17,18,19], this is mainly because the π cell operates between bend state (Fig. 2-2A) and homeotropic state (Fig. 2-2B), the difference of LC alignment between these two state is not much, and the LC molecules align bent in π cell, the LC molecules will rotate in the same direction when switch, we called that flow effect (Fig. 2-3), this is not like TN mode, which must overcome the delay caused by the backflow when changing the distorted alignment in operation process, especially in the relaxation when removing the voltage. So in the operation of π cell, the response time is about 1 ~ 10 ms, faster than TN mode (50 ms) and the response of human eyes (20 ms)
Polarizer substrate Glass
ITO layer
Alignment
crystal Liquid
Polarizersubstrate Glass
ITO layer
Alignment
(A) (B)
Fig. 2-2: (A) Voff (Vcr), π cell is in the bright state. (B)Von, π cell is in the dark state, the LC alignment between these two state is much like, so the π cell has fast response.
Fig. 2-3: Schematic figure of the dynamics in the π cell. The flow induces the torque to accelerate to relax.
2-2-3 Wide viewing angle
From Fig. 2-4, we can see that when the transmittance is 50%, the curve of TN mode has asymmetric distribution, while the curve of π cell has symmetric distribution. This phenomenon is related to the alignment of LC molecules. Because in the plane of rubbing direction, the alignment of LC molecules is symmetric in the vertical direction, so we will experience the same optical path along the symmetric direction in this plane (Fig. 2-5). There is self-compensated effect in this direction. Therefore, the horizontal viewing angle of π cell is
Torque Flow
wider and symmetric than TN mode.
(A) (B)
Fig. 2-4: The relation between the horizontal viewing angle and the transmittance of (A)TN mode(B) π cell[11]. TN mode shows asymmetric transmittance, but π cell shows symmetric transmittance.
Fig. 2-5: π cell has the symmetric viewing angle in the plane of rubbing direction.
2-2-4 Light leakage in dark state
The alignment layer of π cell usually provides the pretilt angle of 6~14 degree, when applying electric field, the LC molecules near the alignment layer suffer larger constraining strength from the alignment layer, these molecules don’t stand up exactly, leads to more severe dark state light leakage (Fig. 2-6). Compare to the dark state of VA mode, the LC
Flow
molecules can stand up exactly, so the contrast ratio of π cell is usually less than VA mode in the normal direction. Accordingly, Uchida proposed a biaxial negative type compensation film to improve the dark state of π cell, but the contrast ratio is still not as good as the VA mode.
(A) (B)
Fig. 2-6: The LC molecules alignment of (A)VA mode(B) π cell in dark state. The major cause of dark state light leakage is the molecules near the substrate in π cell can not stand up exactly.
2-2-5 Phase transition
Fig. 2-7: The V-T curve of a general π cell.
The V-T curve of π cell usually looks like Fig. 2-7. The voltage corresponding to the
100% transmittance is called critical voltage (Vcr), and the voltage corresponding to the 0%
transmittance is called dark state voltage, π cell usually operates between these two voltage.
Fig. 2-8: The pretilt angle versus energy distribution of splay state and bend state.
Fig. 2-9: The energy distribution of splay and bend state when applying voltage.
From the energy point of view, when the pretilt angle is low, splay state has lower energy (Fig. 2-8). When the pretilt angle increases, the energy of bend state and splay state becomes closer[20,21]. When the pretilt angle is about 45~50 degree, the energy of these two state almost the same. The pretilt angle of a general π cell is about 6~14 degree, so the beginning state of a general π cell is stable in splay state. Applying voltage on the π cell will change the original energy distribution (Fig. 2-9). Therefore, we usually need to offer a
voltage larger than critical voltage to the π cell, for a long period of time, to enable the π cell transit to bend state, then operate it. This is a problem need to be solved in application.
To improve the problem of phase transition, the common method likes to increase pretilt angle[22,23], generate bend core or chiral dopant[24,25] ,multi-dimensional alignment[26], offer a twist electric field[27], polymer stabilized[28,29]…etc. Although these method can shorten the transition time from splay state to bend state, but they usually affect other optical properties (ex. worse the dark state), or the fabrication process, driving method become complicated, make the π cell lose its predominance.
2-3 Summary
In this chapter, we introduced the fast response and wide viewing angle properties of π cell, and brought up the problem in dark state and the drawback of needing phase transition of π cell. In the chapter afterward in this thesis, we proposed a new cell structure which can improve the dark state and phase transition problem of a π cell in the same time, and easy to fabricate, make π cell have better performances and advantages.