Download Mode switching of liquid crystals improves display performance

10.1117/2.1200802.0971
Mode switching of liquid crystals
improves display performance
Tae-Hoon Yoon, Gak Seok Lee, Jeong Hyun Lee,
Dong Han Song, Jae Chang Kim, Dae Lim Park,
Seong Soo Hwang, Dae Hyun Kim, and Sung II Park
A novel LCD technology yields both high reflection and transmission.
Mobile display devices such as portable media players, satellite
navigators, and laptop computers demand good readability in
both indoor and outdoor settings. These stringent constraints
make transflective LCDs (displays that are readable under both
bright sunlight and low-light situations) a promising technology. In transflective LCDs, each pixel is divided into two parts,
one transmissive and the other reflective. The limited suitability
of LCDs due to their narrow viewing angles can be overcome
by introducing in-plane (IPS) and fringe-field horizontal switching (FFS) modes. However, manufacturing horizontal-modeswitching LCDs requires complicated and expensive fabrication
processes.1, 2
Transflective LCDs operating in a single mode and manufactured with a so-called single-cell-gap structure are very desirable
to reduce production costs. But they require in-cell retardation
layers to prevent image inversion between the reflective and
transmissive modes.1, 2 Moreover, these retarders, too, have a
number of significant problems, including increased operating
voltage and unreliability of the materials used for their fabrication. A new optical configuration for a transflective IPS LCD was
recently reported,3 which employs a twisted-nematic liquidcrystal (TNLC) cell with a single-cell-gap structure and without
in-cell retarders. Although the transmissive-mode viewing angle
performance is significantly improved, the resulting reflectivemode display capability is poor, and both modes exhibit different electro-optic characteristics.
Here, we propose an optical TNLC cell design for a single-cellgap transflective display in FFS mode (see Figure 1). A systematic manufacturing procedure simultaneously achieves optimal
display performance of both reflection and transmission. The
wavelength dispersion of the TNLC cell is suppressed effectively
by introducing a half-wave plate (HWP), the best conditions
Figure 1. Optical configuration of the proposed transflective LCD.
HWP: Half-wave plate. QWP: Quarter-wave plate. LC: Liquid Crystal. TN: Twisted nematic.
Figure 2. Color prototype transflective FFS panel (∼5×5cm).
for which are found using the Muller-matrix method. High reflectance and transmittance can both be achieved by applying
bidirectional electric fields for either mode. The configuration
can be used to produce a high-contrast transflective display with
a single-cell gap and without in-cell retardation layers
The combination of the front polarizer, an HWP, and a TNLC
layer plays the role of a circular polarizer in the reflective mode
Continued on next page
10.1117/2.1200802.0971 Page 2/2
(see Figure 1). The addition of an orthogonal circular polarizer
behind the mirror can provide cross-polarization in the transmissive mode, resulting in the dark state. To obtain the bright state,
a horizontal electric field is applied to align the LCs to −30◦ in
transmission and to +15◦ in reflection.
Employing different electric-field directions in either mode,
both high reflection and transmission, and single gamma can
be realized simultaneously. We have developed a working color
prototype panel employing the proposed design (see Figure 2).
Efforts in this area are still ongoing. Application of the technology will require solving a number of practical problems. In
particular, the reflective component of the device will require a
bumpy reflector and a hole-type color filter for wide viewing angle, high reflectance, and color reproduction.
This research was supported by LG. Philips LCD and by grant
F0004130-2007-23 from the Information Display Research and Development Center, one of the 21st Century Frontier R&D Programs
funded by the Korean Ministry of Commerce, Industry, and Energy.
Author Information
Tae-Hoon Yoon, Gak Seok Lee, Jeong Hyun Lee,
Dong Han Song, and Jae Chang Kim
Pusan National University
Busan, Korea
Dae Lim Park, Seong Soo Hwang, Dae Hyun Kim, and
Sung II Park
LG. Philips LCD
Kumi City, Korea
References
1. K.-H. Park, J. C. Kim, and T.-H. Yoon, Horizontal switching of half-wave liquid crystal cell for transflective display, Jpn. J. Appl. Phys. 44, p. 210, 2005.
2. G. S. Lee, J. C. Kim, and T.-H. Yoon, Horizontal switching double-cellgap liquid crystal cell for superachromatic transflective display, Jpn. J. Appl. Phys. 45, p. 8769, 2006.
3. O. Itou, S. Hirota, Y. Sekiguchi, S. Komura, M. Morimoto, J. Tanno, T. Ochiai, H.
Imayama, T. Nagata, and T. Miyazawa, A wide viewing angle transflective IPS LCD
applying new optical design, 2006 Soc. Inf. Display Int’l Symp., pp. 832–835, 2006.
c 2007 SPIE