Generally, for transflective liquid crystal displays with different modes and different cell gaps between the refractive and transmissive parts, precise process control to pattern the electrode and match the cell gaps may reduce the yield and thus, require high cost. This paper proposes a simple transflective liquid crystal display with a single-mode and single-cell thickness without a patterned electrode to achieve better productivity. The proposed transflective liquid crystal display consists of three half-wave retardation films, two quarter-wave retardation films, and an LC layer, whose optical performance was confirmed by both simulation and experiment. The optimal optical configuration to obtain an excellent dark state in the visible range was determined by the Mueller matrices calculus, which was applied to each optical component. The calculated and experimental results showed that the proposed transflective LC structure has excellent electro-optical properties and is expected to have many liquid crystal display applications.
© 2016 Optical Society of America
Currently, small displays, such as digital cameras, smart phones and smart watches are divided into two major displays: organic light emitting diodes (OLEDs) [1–5] and liquid crystal display (LCD) [6–39].
OLED displays have many merits over LCD, e.g., fast response speed, wide angle and low power consumption. On the other hand, it cannot be adopted to reflective-type displays because it is an emissive-type display that only shows excellent image quality in indoor environments with feeble light. Most people spend much of their time outdoors, where they use electronic devices. Hence, emissive-type displays or transmissive-type displays do not provide clear images due to reflections generated by the intense ambient light. Therefore, a reflective-type LC display has been proposed as an alternative of outdoor display [20–26]. On the other hand, such displays are unsuitable for indoor environments with weak light. Therefore, transflective LCDs that can be used in both environments have attracted considerable attention, and optical designs have been suggested to adapt them to industry [27–39]. Despite this, almost transflective LCDs have not been applied to industry due to two optical modes and two cell gaps, which require a more difficult fabrication process in addition to the poor productivity caused by the small cell gap even though Apple iPhone 3 used transflective LCD. Furthermore, complicated electrode patterns that also give rise to poor productivity are needed to match the electrooptical property of the reflective part to that of the transmissive part.
This paper proposes a novel optical design for a single-mode and single-cell gap transflective LCD with a simple conventional electrode structure to mitigate the productivity issue and then apply it more easily to industry.
2. Optical structure of the proposed transflective LC cell
The proposed transflective LC cell consists of three half-wave retardation films, two quarter-wave retardation films, an LC cell, reflector, and a polarizer, as shown in Fig. 1. In the optical configuration of the transflective LC cell, the optic axes of optical components of the reflective part were determined using the 4 × 4 Mueller matrices and a small band approximation based on the light polarization theory [24, 25]. The resulting values for optics axes of the optical components were θH3 = ± 15°, θQ2 = ± 15°, and θL = ± 75°. Here, θH3, θQ2 and θL are the angles between the optic axis of the half-wave retardation film (HF3) and the transmission axis of the top polarizer, between the optic axis of the quarter-wave retardation film (QF2) and the transmission axis of the top polarizer, and between the optic axis of the LC layer and the transmission axis of the top polarizer, respectively. In the optical design of this reflective part, the total retardation of the LC cell is π (λ/2). Therefore, a larger cell thickness can be used to achieve better productivity. Although the cell thickness is doubled from the typically reflective modes, this reflective type LC cell can generate an excellent dark state that enhances the contrast ratio. More detailed optical description was presented in reference .
First, to obtain an initially dark state, the transmissive part uses crossed polarizer, as shown in Fig. 1. The optic axes of the optical components included in transmissive part were also determined. The 4 × 4 Mueller matrices were used to find the optic axes of the LC layer and the retardation films. The optical components of the Mueller matrix in parallel-aligned molecular retardation layers can be expressed as follows:Eq. (2) and calculating and rearranging Eq. (2), the θH1, θQ1, and θH2 can finally be determined to be 105°, 75°, and 165°, respectively. Under this condition, the top polarizer will perfectly block the incident light through all visible regions. Under the bright state condition, the angles of the optical axes of each optical component were fixed to θH1 = 105°, θQ1 = 75°, θH2 = 165°, θL = 75°, θQ2 = −15°, and θH3 = 15°, which is identical to the condition of the dark state. The retardation of the half-wave LC layer was changed only by the vertical electric field and the maximum brightness was achieved when the retardation was almost zero. Eventually, the optimized optical configuration achieved excellent electrooptic characteristics by a single mode and single cell gap.
3. Optical principle
The Poincare sphere combined with the Stokes vectors was used to explain how the polarization of light when input light passes through the each optical component changes and better understand why the dark and bright states are generated in the proposed optical configurations.
First, for the reflective part, we have explained the optical principle by using Poincare sphere shown in Figs. 2(a) and 2(b) . Consequently, in the proposed optical configuration, the reflective part appears as obvious dark and bright states because the series of procedures for the polarization path described before is by a wide band optical design considering polarization path-compensation.
In the case of the transmissive part, Figs. 2(c) and 2(d) show the polarization path of the light propagating through the optical components. Figure 2(c) shows the polarization path of an input light for producing a dark state with the field-off. The linearly polarized light of the 90° direction produced by the bottom polarizer is changed to linear polarization of 120° after passing through the half-wave retardation film (HF1) with an optical axis of 105°, as shown in path(1) of Fig. 2(c). The linear polarized light of 120° appears as right-handed circular polarization, the south pole of the Poincare sphere after propagating through the quarter-wave retardation film (QF1) with an optical axis of 75°, as depicted in path(2) in Fig. 2(c). After passing through the half-wave retardation film (HF2) with an optical axis of 165°, the right-handed circularly-polarized light becomes left-handed circular polarization, the north pole of the Poincare sphere, as indicated in the path(3) of Fig. 2(c). The light after passing through the LC layer with the optical axis of 75° comes to the right-handed circular polarization again, as illustrated in path(4) in Fig. 2(c). The right-handed circularly polarized light turns into the linear polarization of 120° after propagating through the quarter-wave retardation film (QF2) with an optical axis of −15°, as sketched in path(5) in Fig. 2(c). After propagating through the half-wave retardation film (HF3) with an optical axis of 15°, the linearly polarized light of 120° appears as the linear polarization of 90°, as shown in the path(6) in Fig. 2(c). Consequently, the input lights are perfectly blocked by the top polarizer and the transmissive part appears as a perfect dark state for all wavelengths because the series of procedures for the polarization path described before is in a perfect polarization path-compensation for all wavelengths.
Figure 2(d) shows the polarization path of the input light for generating a bright state with field-on. The polarization paths depicted in Fig. 2(d) can also be described in similar a manner to the description of the dark state above. The linearly polarized light of the 90° direction produced by the bottom polarizer is changed to the left-handed circular polarization after passing through the half-wave retardation film (HF1), the quarter-wave retardation film (QF1), and the half-wave retardation film (HF2) in the same manner to the dark state. The light, after passing through the LC layer with zero retardation generated by an electric field, remains intact as the left-handed circular polarization without any polarization change. The left-handed circularly polarized light turns to linear polarization of 30° after propagating through the quarter-wave retardation film (QF2), as sketched in path(4) in Fig. 2(d). Finally, after propagating through the half-wave retardation film (HF3), the linearly polarized light of 30° appears as the linear polarization of 0°, as shown in path(5) in Fig. 2(d). Eventually, the input light is transmitted and an excellent bright state is also obtained in the optical configuration of the transmissive part.
The optical calculation for the proposed transflective LC cell was performed to determine if the proposed optical concept was correct. A commercial 3D-LCD simulator, TechWiz LCD,was used for an exact optical calculation in the proposed optical configuration. The physical properties of ZLI-1557, LC provided from Merck Co., which was applied to this experiment were used in this calculation. The optical anisotropy, birefringence, was Δn(0°) = 0.1159 (ne = 1.6182 and no = 1.5023) at a wavelength of 560 nm at room temperature and the dielectric constant was Δε = 4.2. The pretilt angle was adjusted to 3° because the LC alignment layer used in this experiment is AL2001, a polyimide supplied from Nissan Chem., which produces a pretilt of 3° after the conventional rubbing process. Therefore, the actual birefringence, Δn(θ), of the LC was 0.1144 according to an equation expressing the relationship between the pretilt angle, θ, and birefringence, Δn, which is Therefore, the cellgap, d, should be approximately 2.45 μm to fit Δnd to λ/2 = 280 nm (λ = 560 nm as a central wavelength).
To confirm the calculation, an experiment was carried out using the general method. An LC and alignment layer identical to those used for the calculation were adopted for a more exact comparison. Liquid crystal cells were prepared for the experiments in the following manner. No patterned indium-tin-oxide (ITO) for the transmissive LC cell and no patterned aluminum (Al) for the reflective LC cell was used as the electrodes to lead to a simple fabrication process that enables a device to be applied to industry. Al was also used as a reflector in the reflective LC cell. The polyimide, AL2001 (Nissan Chemical Co. Ltd.), was prebaked at 80 °C for 10 min and cured at 180 °C for 1 hour. The top and bottom substrates with the polyimide were rubbed with velvet and assembled for anti-parallel LC alignment. The λ/4 and λ/2 films, and the polarizers were attached well by considering their optical axes on the fabricated LC cell. To obtain the voltage-reflectance and voltage-transmittance curves, a He-Ne laser emitting a wavelength of 632.8 nm was used as the light source and a voltage with a 1 kHz square wave with varying amplitude was applied to the electrodes. In the experiment, the light source was exposed to 12° with respect to the normal direction and the angle of the detector was −12°. A Cary 5000 UV-VIS-NIR spectrophotometer (Agilent-Korea) was used as the light source and detector to observe the dispersion properties at the grey level.
5. Calculated and experimental results
First, the dispersion characteristic of the proposed transflective LC mode is shown because the dispersion characteristics of the input wavelengths at the dark state have an absolute effect on the contrast ratio. Figure 3 shows the calculated results for the optical dispersion characteristics of the reflective part of Fig. 3(a) and transmissive part of Fig. 3(b) in the proposed transflective LC mode. As expected, they exhibited excellent dispersion properties in both the dark and bright states. In particular, the dark state showed perfect optical properties over all wavelengths for the transmissive part and a wider band property except for a portion of the visible region for the reflective part.
The electro-optical properties of the fabricated LC cells were measured to confirm the calculated results for the dispersion characteristics of the proposed transflective LC mode. Figure 4 shows the experimental results for the optical dispersion characteristics of the reflective of Fig. 4(a) and transmissive part of Fig. 4(b) in the proposed transflective LC mode. The experimental results agree well with the calculated results, even though it makes a slight difference due to some retardation mismatch between the LC layer and the retardation films.
The insets in Fig. 4 show photographs taken from the fabricated reflective and transmissive LC cells. The cells exhibited a uniform and clear grey level according to the applied voltages.
Figure 5 shows the calculated and experimental results of the reflectance and transmittance as a function of the applied voltage in the reflective and transmissive LC cells. As expected, it showed obvious dark and bright states in both the simulated and experimental results. In the case of the reflective part, however, the threshold voltage obtained from experiment was 0.4 V lower than that of the calculation, which may be due to the difference in anchoring strength in the used LC cell and dielectric constant of the used LC, compared to the conditions of the calculation. In the case of the transmissive part, both the threshold and saturation voltages were matched but there was some disagreement at the grey level, which may be due to other physical or chemical properties of the used LC. The LC cell showed the maximum reflectance near 2.5 V, which retarded the LC layer to π/2 for the reflective part and the maximum transmittance near 5 V, which retarded the LC layer to zero for the transmissive part. As a result, the proposed transflective LC cell can have a very low operation voltage, ~2.5 V for the reflective part and ~5 V for the transflective part. The contrast ratios of the reflective and transmissive parts were 15:1 and 153:1 in the visible range. Accordingly, as a result, the electro-optical characteristics of the proposed LC cell were verified.
Consequently, the proposed transflective LC mode can have LCD applications.
This study examined a simple transflective LCD designed optically for a single-mode and single-cell gap to lead to better productivity. The proposed transflective LCD consisted of three half-wave retardation films, two quarter-wave retardation films and a LC layer. Using theoretical calculations, the resulting optical axes of the each optical component, i.e., the first half-wave retardation films (HF1), the first quarter-wave retardation films (QF1), the second half-wave retardation film (HF2), the LC layer, the second quarter-wave retardation film (QF2), the third half-wave retardation film (HF3), were 105°, 75°, 165°, 75°, −15°, and 15°, respectively, in reference to the transmission axis of the top polarizer. The calculated and experimental results obtained by adopting the optical configuration confirmed that the proposed transflective LC has excellent electro-optical properties and is expected to find applications to LCD industries.
This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2013R1A1A1A05006783).
References and links
1. C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes,” Appl. Phys. Lett. 51(12), 913–915 (1987). [CrossRef]
2. S. R. Forrest, M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, and M. E. Thompson, “Highly efficient phosphorescent emission from organic electroluminescent devices,” Nature 395(6698), 151–154 (1998). [CrossRef]
3. J. Y. Lee and S. T. Lee, “Layer-induced thermal imaging of polymer light-emitting materials on poly(3,1-ethylenedioxythiophene):silane hole-transport layer,” Adv. Mater. 16(1), 51–54 (2004). [CrossRef]
4. J. C. Scott, J. H. Kaufman, P. J. Brock, R. DiPietro, J. Salem, and J. A. Goitia, “Degradation and failure of MEH‐PPV light‐emitting diodes,” J. Appl. Phys. 79(5), 2745–2751 (1996). [CrossRef]
5. M. P. de Jong, L. J. van IJzendoorn, and M. J. A. de Voigt, “Stability of the interface between indium-tin-oxide and poly (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) in polymer light-emitting diode,” Appl. Phys. Lett. 77(14), 2255–2257 (2000). [CrossRef]
6. S. H. Lee, S. L. Lee, and H. Y. Kim, “Electro-optic characteristics and switching principle of a nematic liquid crystal cell controlled by fringe field switching,” Appl. Phys. Lett. 73(20), 2881–2883 (1998). [CrossRef]
7. T. Miyashita, Y. Yamaguchi, and T. Uchida, “Wide-viewing angle display mode using bend-alignment liquid crystal cell,” J. Appl. Phys. 34(Part 2, No. 2A), L177–L179 (1995). [CrossRef]
8. C. J. Jhun and J. S. Gwag, “Improved vertically-aligned nematic mode for high performance displays,” J. Opt. Soc. Korea 18(6), 783–787 (2014). [CrossRef]
9. S. G. Kim, S. M. Kim, Y. S. Kim, H. K. Lee, S. H. Lee, G.-D. Lee, J.-J. Lyu, and K. H. Kim, “Stabilization of the liquid crystal director in the patterned vertical alignment mode through formation of pretilt angle by reactive mesogen,” Appl. Phys. Lett. 90(26), 261910 (2007). [CrossRef]
10. S.-T. Wu and C.-S. Wu, “Mixed-mode twisted nematic liquid crystal cells for reflective displays,” Appl. Phys. Lett. 68(11), 1455–1457 (1996). [CrossRef]
11. P. J. Bos and K. R. Beran, “Fast liquid-crystal optical-switching device,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 113(1), 329–339 (1984). [CrossRef]
12. J. S. Gwag, K.-H. Park, J. L. Lee, J. C. Kim, and T.-H. Yoon, “Two domain hybrid-aligned nematic cell fabricated by ion beam treatment of vertical alignment layer,” Jpn. J. Appl. Phys. 44(4A), 1875–1878 (2005). [CrossRef]
13. S.-H. Chen and C.-L. Yang, “Dynamics of twisted nematic liquid crystal pi-cells,” Appl. Phys. Lett. 80(20), 3721–3723 (2002). [CrossRef]
14. J. S. Gwag, “Advanced patterned vertical aligned nematic mode to elevate transmittance,” J. Opt. Soc. Korea 18(1), 78–81 (2014). [CrossRef]
15. J. S. Gwag, Y.-J. Lee, J.-H. Kim, H. J. Lee, and M. H. Yi, “Single-mode transflective liquid crystal display tailored for wide-band single gamma curve,” Opt. Express 16(22), 18102–18108 (2008). [CrossRef] [PubMed]
16. F. S. Yeung, Y. W. Li, and H.-S. Kwok, “Pi-cell liquid crystal displays at arbitrary pretilt angles,” Appl. Phys. Lett. 88(4), 041108 (2006). [CrossRef]
17. J. S. Gwag, J. Fukuda, M. Yoneya, and H. Yokoyama, “In-plane bistable nematic liquid crystal devices based on nanoimprinted surface relief,” Appl. Phys. Lett. 91(7), 073504 (2007). [CrossRef]
19. P. K. Son, S. H. Yu, J. Yi, J. H. Kwon, and J. S. Gwag, “Electro-optical characteristics of two domain normally black-electrically controlled birefringence mode,” J. Appl. Phys. 114(6), 064506 (2013). [CrossRef]
20. S. T. Tang, F. H. Yu, J. Chen, M. Wong, H. C. Huang, and H. S. Kwok, “Reflective twisted nematic liquid crystal displays,” J. Appl. Phys. 81(9), 5924–5929 (1997). [CrossRef]
21. S. T. Wu and C. S. Wu, “Mixed‐mode twisted nematic liquid crystal cells for reflective displays,” Appl. Phys. Lett. 68(11), 1455–1457 (1996). [CrossRef]
22. G. D. Lee, G. H. Kim, T.-H. Yoon, and J. C. Kim, “Configuration optimization of a reflective bistable-twisted-nematic cell for high-contrast operation,” Jpn. J. Appl. Phys. 39(Part 1, No. 5A), 2716–2720 (2000). [CrossRef]
25. G. J. Choi and J. S. Gwag, “Optical design for reflective liguid crystal display,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 613(1), 103–109 (2015). [CrossRef]
26. K. C. Heo, J. Yi, J. H. Kwon, and J. S. Gwag, “High contrast reflective liquid crystal display using a thermochromic reflector,” J. Opt. 17(2), 025401 (2015). [CrossRef]
27. P. K. Son, J. Yi, J. H. Kwon, and J. S. Gwag, “Single-cell gap-transflective liquid crystal display using two optical modes of a bistable liquid crystal,” Appl. Opt. 50(10), 1333–1337 (2011). [CrossRef] [PubMed]
28. J. S. Gwag, Y.-J. Lee, J.-H. Kim, H. J. Lee, and M. H. Yi, “Single-mode transflective liquid crystal display tailored for wide-band single gamma curve,” Opt. Express 16(22), 18102–18108 (2008). [CrossRef] [PubMed]
29. Y. J. Lim, J. H. Song, and S. H. Lee, “Transflective liquid crystal display with single cell gap and single gamma curve,” Jpn. J. Appl. Phys. 44(5A), 3080–3081 (2005). [CrossRef]
30. S. Pancharatnam, “Achromatic combinations of birefringent plates,” Proc. Ind. Acad. Sci. A 41(71), 130–144 (1956).
31. J. H. Lee, X. Zhu, Y. H. Lin, W. Choi, T. C. Lin, S. C. Hsu, H. Y. Lin, and S. T. Wu, “High ambient-contrast-ratio display using tandem reflective liquid crystal display and organic light-emitting device,” Opt. Express 13(23), 9431–9438 (2005). [CrossRef] [PubMed]
32. Z. Ge, X. Zhu, R. Lu, T. X. Wu, and S. T. Wu, “Transflective liquid crystal display using commonly biased reflectors,” Appl. Phys. Lett. 90(22), 221111 (2007). [CrossRef]
33. H. C. Cheng and S. T. Wu, “Sunlight readable transmissive LCDs,” J. Display Technol. 8(4), 183–185 (2012). [CrossRef]
34. Z. Ge and S. T. Wu, Transflective liquid crystal display (Wiley, 2010).
35. X. Zhu, Z. Ge, T. X. Wu, and S. T. Wu, “transflective liquid crystal displays,” J. Display Technol. 1(1), 15–29 (2005). [CrossRef]
36. K. H. Liu, C. Y. Cheng, Y. R. Shen, C. M. Lai, C. R. Sheu, Y. Y. Fan, C. C. Chen, and I. J. Lin, “A novel double gamma driving transflective TFT LCD,”in Proceedings of the International Display Manufacturers Conference (2003), pp. 215–218.
37. S. Hirota, S. Oka, and O. Itou, “Transflective LCD combining transmissive IPS and reflective in-cell retarder ECB,” SID Symp., 1661–1664 (2007).
38. Y.-C. Yang, J. Y. Choi, J. Kim, M. Han, J. Chang, J. Bae, D.-J. Park, S. I. Kim, N.-S. Roh, Y.-J. Kim, M. Hong, and K. Chung, “Single cell gap transflective mode for vertically aligned negative nematic liquid crystals,” SID Symp., 829–831 (2006). [CrossRef]
39. H. L. Baek, Y. B. Kim, K. S. Ha, D. G. Kim, and S. B. Kwon, “New design of transflective LCD with single retardation film,” in Proceedings of the International Display Workshops (2000), pp. 41–44.