2-1-1THE STRUCTURE OF OCB CELL
In Fig. 2-A, OCB cell‘s structure and five configurations are shown [25-27]. For an OCB cell, it usually consists of two ITO glasses as the upper and bottom substrates. The alignment layer on top of the substrate helps liquid crystals inside to align, and the pretilt angle is around 6 to 14 degree, such pretilt angle makes the LC molecules align in splay state in a plane without distortion. The normal state for OCB as a result will be splay state. When applied a voltage above the threshold voltage, the splay state would turn into symmetric splay state (also called Hs state) or asymmetric splay state (also called Ha state). When a voltage above the critical voltage applied, the cell would have a nucleation and change to bend state, that‘s where the liquid crystals were operated for OCB cell. After the applied voltage released, the cell would become 180° twist state first before it goes back to splay state.
Splayed
Fig. 2-A: OCB cell‘s structure and five configurations.
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-Hs state and Ha state is a transient state, when the tilt angle on both substrates is the identical, the cell would turn into Hs state, otherwise, the cell would turn into Ha state. Vc1 in the figure usually would be 1Vrms, and Vc2 is around 2Vrms for most materials. The bend state is topologically different from splay state, that‘s the reason for a nucleation process and usually takes few seconds to minutes to complete the transition. The twist state to splay state process for conventional twisted nematic device usually takes longer than OCB cell because of the backflow-induced alignment phenomenon [28]. OCB cell has symmetry pretilt angle on each surface and causes the flow which produces no backwards torque on the director near the center of the cell, and eliminates the adverse effects of flow alignment. This phenomenon can be explained by Van Doorn [29] using the numerical solution of the Ericksen-Leslie equations [30]. Fig.2-B shows the torque which induces by the flow accelerates the relaxation.
Fig. 2-B: Schematic figure of the dynamics in the pi-cell. The flow induces the torque to accelerate to relax. [31]
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-2-1-2 FAST RESPONSE
One of the famous advantages of OCB cell is fast response [32-33].OCB cell operates in bend mode as Fig. 2-C (b) and homeotropic state as Fig.2-C (c) show. Since the alignment between these two states is alike, the response time is fast. Plus the no backward torque as mentioned earlier when removed the voltage, the response time is about 1 to 10ms, faster than TN mode (50 ms) and the response of human eyes (20 ms).
Fig.2-C (a) Fig.2-C (b) Fig.2-C (c)
Fig. 2-C: (a) splay state, (b) Voff (Vcr), bend mode, and OCB cell is in the bright state. (c)Von, homeotropic state, and OCB cell is in the dark state.
2-1-3WIDE VIEWING ANGLE
Another advantage of OCB cell is wild viewing angle. From Fig. 2-D, compare TN mode with OCB mode, the transmittance is 50%, the curve of TN mode has asymmetric distribution, while the curve of OCB cell has symmetric distribution. TN mode LCD is difficult to compensate the birefringence because of the complicated alignment structure. From the cross-section view of OCB cell in Fig. 2-E, the alignment of LC molecules is symmetric in the vertical direction and experience the same optical path along the symmetric direction in this plane. There is self-compensated effect in this direction. Therefore, the horizontal viewing angle of π cell is wider and symmetric than TN mode.
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H o r i z o n t a l V e r t i c a l
O C B
TN
Fig. 2-D: Experimental results of the viewing-angle-dependent transmittance for TN cell and OCB cell [11].
Fig. 2-E: cross-section view of OCB cell, it has symmetric viewing angle.
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-2-1-4 NUCLEATION AND PHASE TRANSITION
As mentioned in earlier section, when applying a high voltage to the OCB cell, the splay state would transit to bend state. It is found that the transition is divided into two stages [34].
Fig. 2-F explains the process. Because of the topologically difference between these states, the transition starts with breaking the anchoring symmetry at the cell surfaces and forming a bend domain with a disclination loop around it. It clearly demonstrates it in the simulation by P. J. Bos et al. [35]. The velocity of the disclination lines motion can be calculated from the energy difference of the two states. The energy difference of the two states can be obtained in a simple formulation, called Boundary-Layer Model [36-37]. The declination lines finally annihilate with each other when the transition is completed.
Fig. 2-F: Mechanism of bend transition [34].
Bend transition cores can be generated by defects and structures such as spacers [38].
When applied voltage, anisotropic structures around the spacers by adsorption are able to nucleate the bend state.
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-Fig. 2-G: The pretilt angle versus energy distribution of splay state and bend state.
In Fig.2-G, the relation between Elastic energy and pretilt angle are presented. From this point, when the pretilt angle is larger, the elastic energy of splay state and bend state will be closer [39-40] and become easier to transit or don‘t even need a transition. When the pretilt angle is around 45 to 50 degree, the energy of these two states is almost the same. Since the pretilt angle of conventional OCB cell is around 6 to 14 degree, the elastic energy is smaller than bend‘s. Applied voltage will change the original energy distribution like Fig. 2-H shows.
Fig. 2-H: Gibbs‘s free energy of bend and splay states as a function of applying voltage [34].
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-Therefore, we usually need to applied a voltage larger than critical voltage to the OCB cell for a while and wait the cell to transit to the higher energy bend state. It usually takes seconds to minutes to fulfill the process. The larger the voltage, the fast the process, however, too large voltage will not be suitable for TFT-drivers [41-42].
After the nucleation, 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, OCB cell usually operates between these two voltage.
Some methods have been brought out to solve the phase transition issue, in addition to those mentioned in the introduction, there are still some other examples like generate bend core or chiral dopant [43-44] or multi-dimensional alignment [45] and offer a twist electric field[46]…etc. These methods may shorten the transition time but sacrifice some innate advantages of OCB cell.
2-1-5 LIGHT LEAKAGE IN DARK STATE
The pretilt angle of OCB is usually around 6 to 14 degree. Compare to VA mode displays, the pretilt angle is 90 degree. When applied a voltage higher than critical voltage and make OCB cell become homeotropic or dark state, the liquid crystals near the substrates suffer huge constrained strength from the PI layer, and couldn‘t stand up like the VA mode displays. Fig.
2-I could explain the arrangement of VA and OCB mode. As a result, there will be a light leakage in the dark state. The contrast ratio will be lower than the VA mode, even with an additional compensation film. This phenomenon causes a weakness of OCB cell in the dark state.
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Fig. 2-I (a) Fig. 2-I (b)
Fig. 2-I: The LC molecules alignment of (a) VA mode, and (b) dark state of OCB cell.
2-2SUMMARY
In this chapter, some of the details of OCB cell were introduced. OCB cell maintains many advantages, such as fast response time and wild viewing angle. Unfortunately, the fast response means operate in ‗bend mode‘, and needs a phase transition to become bend mode.
The transition time could be long if the driving voltage is low, but high voltage is not suitable for TFT-driving devices. On the other hand, the contrast ratio is poor than VA mode displays, and light leakage could cause power consumption waste. Based on the good side of OCB cell, it is worth to improve the OCB cell and make it more applicable for the liquid crystal display.
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