Optimization of electrodes in the Patterned Vertical Aligned Liquid Crystal Cell for high optical performance

Optimization of electrodes in the Patterned Vertical Aligned Liquid Crystal Cell for high optical performance.

Wa-Ryong Lee1, Seong-Wook Choi1, Jung-Hee Son1, Kyung-Mi Kim1

Tae-Kyung Huh1, Joun-Ho Lee2,3, and Gi-Dong Lee1

1Divison of Electronics Eng., Dong-A University, Busan, 604-714, Korea

2Department of Electronics Eng., PusanNationalUniversity, Busan, 609-735, Korea

3LG.Philips LCD, 642-3 Jinpyung-dong, Gumi-city, Kyungbuk, 730-350, Korea

Abstract

In this paper, we propose the electrode of the Patterned Vertical Aligned (PVA) LC cell [1] for high transmittance. We use the ‘TechWiz LCD’ for calculation of the director configuration and optical characteristics to ensure the results of the proposed electrode structure. Normally, the transmittance of PVA LC cell is depended on the shape of the electrode and cell gap. In this work, the width of gate line and data line of the improved electrode design is equal to that of conventional PVA. Instead, we modified the shape of the common and pixel electrode on order to decrease the area of the defect. From the results, we show the comparison of the calculated optical transmittance between the PVA conventional LC cell and the PVA LC cell with the proposed electrode structure. We can confirm that the optical loss due to the variation of the retardation of the LC cell around electrode can be definitely decreased by the proposed electrode. Transmittance of improved structureincreases nearly20% higher thanPVA conventional structure.

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Optimization of electrodes in the Patterned Vertical Aligned Liquid Crystal Cell for high optical performance

1. Introduction

In Liquid Crystal displays, a TN mode has been used because of the relatively high transmittance but this mode has narrow viewing angel characteristic. To improve narrow viewing angle characteristic, S-IPS (Super-In plane Switching) mode [2, 3], PVA (Patterned Vertical Alignment) mode, MVA (Multi-domain Vertical Alignment) were proposed. These modes have better than viewing angle of TN mode because these modes have more than two domains. In other words, the electrode of these modes with the excellent viewing angle characteristic has ‘V’ shape to compensate the retardation left and right along with the viewing angle. So the ‘V’ shape is generalized for superior viewing angle characteristic. In this paper, we propose the cell structure of PVA mode with the ‘V’ shape structure. A PVA mode is one of most common modes using a multi-domain for high optical performance LC Cell and high contrast ratio. However, because of the ‘V’ shape, the transmittance reduced more than the TN mode. So, we proposed the PVA improved cell mode with more superior optical performance than the conventional structure. The improved high performance of the proposed structure can obtain as decreasing the dark area.

2. Conventional Structure of PVA mode.

Figure 1 shows PVA mode electrode structure.Pixel and commonelectrode is used indium tin oxide (ITO)and data and gate line isused opaque metal. That is, the cell region except data and gate line is calculation region of transmittance. Because the region of data and gate line is covered by BM(Black matrix).

(a)

Figure1. conventional PVA mode. (a) simulation result (b) directors at dark area(A)

In figure1, we can find the dark area because the direction of the electric field at the edge of a pixelslit differs from the active direction at the pixel center. Itis also important for obtaining good electro-optic characteristicssuch as low driving voltage and high transmittance. And the LC active direction near theedge of the pixel becomes very unstable upon theapplication of a high voltage. To solve this problem, in this study we propose a changed pixel edge structure with good dynamicstability in the PVA mode. The vertical field direction of the activeregion is different from the edge of thepixel slit owing to thepatterned pixel edge shape.So we change the slit angle of the pixel at the dark area (A, B, C) for stability of the edge pixel.

3. Comparison structure conventional and proposed PVA mode.

In figure 2, the width of pixel and common structure of improved PVAmode is equal to that of conventional PVA mode.But, as shownfigure 2, we change the slit angel of the pixel to minimize the dark area (A, B, C). The A and B area make a strong electric field by edge (I, I’) of slit. The edge (I, I’) of slit make unstable directors movement at A, B and C area. Namely, directors at narrowing area (F) become very unstable by influencing two electric fields. Because of the unstable states, the F area makes the dark area (A, B, C). To avoid the unstable states, we change to improved structure (E) from the conventional structure (D). The angle(θ) between E and D is nearly 8°. This angle is selected because of generating the lowest dark area.

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(b)

(c)

Figure 2. The structure of PVA mode : (a) conventional structure, (b) proposed structure, (c)a enlarged photograph of proposed structure (G).

4. Simulation and results

We simulated the two cells in conventional and proposed PVA mode with TechWizLCD. The motion of the LC director iscalculated by a 2x2Jones matrix for optical transmittance calculation.

As shown at figure 3, transmittance of the conventional and the proposed PVA modeis somewhat different. In figure3 (a), the dark regionappears at edge of the pixel but in figure 3(b), the dark region disappears dark area(A, B, C) at edge of the pixel with aninclination of nearly 8°[4]. Namely, as the LC active near theedge of the pixel becomes stable, gradually decrease dark area above pixel and transmittance will increase. In figure 3(b), we can confirm the disappeared dark area at the A, B, C, as changing the slit angle. As moving the slit line to E from D, dark area (A) go to the A’, dark area (B) go to the B’. As disappearing dark area, in shown figure 3, we can obtain the improved transmittance. Transmittance of PVA improved structureincreases nearly20% higher thanconventional PVA about total amount oftransmittance.

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(b)

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Figure 3. Transmittance as changing the slit angle : (a) conventional PVA mode (b) proposed PVA mode. (c) enlarged directors of circle area in figure(a), (d) enlarged directors of circle area in figure(b).

In figure 3 (c), θ1 is nearly the 50°, but θ2 is nearly the 45°. For bright states, directors angle is 45°, but the director angle (θ1) generated by conventional structure make 50°. Namely, directors of circle area in the figure 3(a) make dark states. On the other hand, the director angle (θ2) generated by proposed structure make 45°for bright states.

Figure 4. Transmittance as changing θ.

5. Conclusion and discussion

We studied about the transmittance improvementwith respect to the pixel edge shape in the PVA mode. We proposed animproved PVA cell which does not lose the superior viewing angle characteristic by maintaining ‘V’ shape of conventional PVA mode while it has highertransmittance.And we analyzed and compared electro-opticalcharacteristics of the two cells generated by changing the slit angle. Dark area exists at the edge while there are strong competitive forcesbetween the two fields. The dark area generated near the edge of the pixel slit is strong above boundary of the slit. It is possible to control dark area near the pixel edge by changing pixel shape. From the novel edge structure, we can obtain superior electro-optic characteristics.

6. Acknowledgement

This work was supported in part by the 21stCentury Frontier R&D program funded by the Ministry of Science and Technology of Korea governmentand partly by University IT Research Center Project under supervision of IITA.

7. References

[1]S.S.KIM, SID Symposium Digest, Vol.35 pp.760-763,2004

[2] J. O. Kwang, K. C. Shin, and S. S. Kim, SID, p. 256, 2000

[3] M. Oh-e and K. Kondo, Appl. Phys. Lett., 67, p. 3895,1995

[4] M.S.KIM, Jpn. J. Appl. Phys., Vol. 44, No. 11, 2005