Reflective–emissive photoluminescent cholesteric liquid crystal display Jang-Kyum Kim,1,2 Suk-Hwan Joo,1 and Jang-Kun Song1,* 1
School of Electronic & Electrical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea 2
Display Laboratory, Samsung Institute of Technology, Youngin 446-711, South Korea *Corresponding author: [email protected]
Received 5 September 2013; revised 3 November 2013; accepted 5 November 2013; posted 6 November 2013 (Doc. ID 197201); published 25 November 2013
We fabricated a photoluminescent cholesteric liquid crystal (PL-CLC) cell for a display application that can be used to display high-quality moving pictures under all ambient conditions including dark and sunlit conditions. The PL-CLC cell is switchable between the reflective mode under bright conditions and the emissive mode in the dark. The effective reflectance of the PL-CLC is higher than that of a conventional CLC device by more than 30%, and the contrast ratios were approximately 10 and 7 in the reflective and emissive modes, respectively. We directly compared the proposed PL-CLC cell with conventional LCD and CLC cells under sunlit, office, and dark environments and confirmed that the PL-CLC cell exhibited superior visibility under all ambient conditions. © 2013 Optical Society of America OCIS codes: (120.2040) Displays; (160.3710) Liquid crystals; (250.5230) Photoluminescence. http://dx.doi.org/10.1364/AO.52.008280
1. Introduction
Displaying high-quality static and dynamic images in all ambient environments, including both dark and sunlit conditions, is one of the most challenging issues in recent display technologies [1]. Liquid crystal displays (LCDs) and organic LED displays actively emit light with vivid colors exhibiting excellent indoor image quality, but they suffer from poor outdoor image quality due to the reflection of ambient light and have a relatively high power consumption. [2,3] Alternatively, reflective displays such as electrophoretic [4] and cholesteric liquid crystal (CLC) displays [5–7] make passive use of external light to provide paper-like quality outdoor images. However, these technologies have poor indoor image quality and a color gamut [8,9]. The transflective LCD, in which a part of a pixel is used for transmitting backlight and another for 1559-128X/13/348280-07$15.00/0 © 2013 Optical Society of America 8280
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reflecting external light, was developed to simultaneously accommodate these two requirements [10–13]. However, the reflectance efficiency is gained only at the expense of the transmittance. As a result, neither the indoor nor the outdoor image quality is completely satisfactory. Thus, no display technology has simultaneously satisfied all of the image quality requirements for both indoor and outdoor environments, despite increasing demands due to the rapid growth of portable electronic devices, which are used in various ambient environments. Meanwhile, it is well known that a CLC device can display dynamic images with vivid color by using the selective reflection of its periodic photonic crystal structure without the use of a polarizer or color filter while demonstrating low power consumption. However, the reflectance of CLC devices is still lower than that of white paper. Therefore, reflectance has been a shortcoming for CLCs that has been intensively investigated [14,15]. Another weakness of CLC devices arises from the slow helical axis reorientation, which is on the order of several seconds, during
the transition from homeotropic to planar alignment, causing slow color changes while displaying dynamic images [16,17]. A CLC device also has a limited viewing angle property due to the strong viewing angle dependence of the structure color of the onedimensional photonic crystal of a CLC [18]. Recently, we reported that the complementary relationship between the photoluminance and selective reflection in a photoluminescent CLC (PL-CLC) cell can compensate the slow response time caused by the helix reorientation and the narrow viewing angle property that arises from the high sensitivity of the wavelength of light reflecting in the viewing direction [19]. In this respect, the PL-CLC may be a candidate material for a reflective–emissive display that can solve the intrinsic problems of the CLC display including its limited reflectance, its viewing angle dependency, and the necessity of a light source in dark conditions. In this study, we demonstrated PL-CLC cells with a well-designed UV blacklight unit (BLU) for displaying both high-quality static and dynamic images in all ambient environments by combining PL materials and one-dimensional photonic crystalline CLCs. The electro-optical properties of a PL-CLC cell were investigated from the viewpoint of display applications. In particular, the visibility of a PL-CLC cell was compared with an LCD and a conventional CLC cell under all ambient environments. 2. Experiments
A positive nematic liquid crystal (LC) and a chiral dopant (R-811, Merck) were mixed to produce three kinds of CLC mixtures that reflect red, green, and blue colors. Three PL materials, coumarin 6 (C6), 2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophen (BBOT) and 4-(dicyanomethylene)-2-methyl-6[19,20], (p-dimethylaminostyryl)-4H-pyran (DCM) [21], which emit green, blue, and red light, respectively, as shown in Fig. 1(a), were mixed with the CLC
mixtures at concentrations of 0.1–2.0 wt. % depending on the experiment. BBOT was added to the blue CLC mixture, C6 and BBOT were added to the green CLC mixture, and DCM and BBOT were added to the red CLC mixture. BBOT in the green and red cells was used as an energy transfer dopant, which harvests a wider range of light energy. The green PL-CLC cell depicted in Figs. 1(b) and 1(c) was prepared by using two patterned indium-tin oxide (ITO) substrates coated with vertical alignment layers. The bottom substrate had four ITO electrodes that were able to be controlled independently, and the top substrate had a single common electrode that covered the entire area. The adoption of a vertical alignment layer prevents the specular reflection on the surface of the CLC layer. Another reason to adopt a vertical alignment layer rather than a planar alignment layer is to improve the contrast ratio (CR) in the emissive mode, which will be discussed in the next section. The UV BLU, which was used for the emissive mode, had a UV power of 60 mW∕cm2 with a peak wavelength of 365 nm. A UV pass filter was placed between the PL-CLC cell and the UV-BLU, and it absorbed the incident visible light from outside, while the UV light from the BLU could pass through the UV pass filter. A UV filter on the cell absorbed the UV light that passed through the PL-CLC layer so that an observer is not exposed to the UV light from the UV BLU. A green PL-CLC mixture containing C6 and BBOT dopants was injected into the cell. The effective reflectance shown in Fig. 2(a) was measured using a CM-2600d (Konica-Minolta Company, Japan). The CM-2600d is a reflective-type spectrophotometer with an integrating sphere and an internal light source. Since it supports the Video Electronics Standards Association standard diffuse reflectance measurement, it is suitable to use a CM-2600d for the diffuse reflectance of a reflective display device. However, the CM-2600d does not
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Wavelength(nm) Fig. 2. (a) Relative reflectance of green PL-CLC cells measured using a CM-2600d compared to a pure green CLC cell as a function of the concentration of C6 and BBOT. The blue circle data set represents the PL-CLC cell containing the same concentrations of C6 and BBOT. UV BLU was turned off. (b) Spectra of the fluorescence of the cells with C6 and C6+BBOT. The BBOT-containing PL-CLC cell shows enhanced effective reflectance and luminance intensity, which were measured using an SR-3A, because of the Förster energy transfer.
measure luminance in the emissive mode, and the luminance in the emissive mode [Figs. 2(b) and Fig. 3] was measured by an SR-3A (Topcon Company, Japan). The SR-3A is a camera-type spectrophotometer that can measure both luminance and reflectance. For the reflectance measurement using the SR-3A [Fig. 3(a)], an external D65 light source that was placed 23° from normal to the samples used. Each spectrophotometer typically yields different reflectance values for the same sample due to different illumination conditions. The dynamic response was measured using a luminance colorimeter, the BM-7 (Topcon Company, Japan). The BM-7 has a fast mode suitable for measuring dynamic response and is widely used for measuring dynamic response in a flat panel display. 3. Results and Discussion
The effective reflectance of the planar state of green PL-CLC cells [Fig. 1(b)] as a function of the C6 and BBOT concentrations were measured using a CM2600d and are plotted in Fig. 2(a). The effective reflectance was determined by the ratio of the reflected luminance over the incident luminance, which is the standard reflectance measurement method for reflective display devices. Here, the 8282
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terminology “effective reflectance” is used because the fluorescence from the PL material improves the effective reflectance. With an increasing C6 concentration, the effective reflectance (red solid line) was enhanced up to 30% compared to that of a pure green CLC cell. Additional improvement due to the BBOT (blue dotted line) was observed with intermediate dopant concentrations. The C6 molecules absorb the photon energy in the blue and a limited part of the UV region and emit green light. Hence, the fluorescent efficiency is not sufficiently high under UV irradiation. Meanwhile, BBOT absorbs UV light and emits the blue near 430 nm, which roughly coincides with the absorption spectrum of C6, as shown in Fig. 1(a). Thus, the BBOT molecules play a role of transferring UV energy to C6, which enhances the green light emission. However, the saturated values were similar in the two dopant systems, which may be due to the low UV density of the CM-2600d illumination. In order to verify the Förster energy transfer [22] of BBOT, we measured the fluorescence spectra of two PL-CLC cells with and without BBOT, and the results are shown in Fig. 2(b). The addition of BBOT increased the fluorescence peak in the green region but not in the blue region, which is an indication that the blue light emitted by BBOT is entirely absorbed by C6 by the Förster energy transfer. Due to the use of a UV filter, only a tail of the UV light source was detected, as indicated in Fig. 2(b), where the UV source tail of the cell with BBOT was weaker than that of the other cell, indicating increased absorption of UV light by BBOT. The electro-optical responses of the PL-CLC cells are shown as a function of the applied voltage in Figs. 3(a) and 3(b), measured using the SR-3A in the reflective mode under external D65 light and in the emissive mode without external light, respectively. In Fig. 3(a), the optical reflectance in the reflective mode exhibited three regions depending on the applied voltage, corresponding to the planar, focal conic, and homeotropic LC alignment states [23]. The planar state had the highest effective reflectance due to the combination of selective CLC reflection and PL dopant fluorescence, as illustrated in Fig. 1(b). The focal conic state, which has a randomly distributed helical structure, scattered incident light. Here, the majority of the scattered light was absorbed by the UV pass filter, and the remainder was rescattered or re-emitted from the cell, resulting in a low effective reflectance. In the homeotropic alignment, most of the incident light was absorbed by the UV pass filter after passing through the vertically aligned LC layer. The luminance in the planar state increased dramatically with increasing dopant concentration, while only a small increase was detected in the homeotropic state. Interestingly, the reflectance improvement of the planar state was measured to be approximately 50% by the SR-3A using a single light source measurement, as indicated in Fig. 3(a). The reflectance improvement
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was maximized when the incident angle from the light source was small, which is the reason why the single light source system measured a better luminance improvement than the diffuse reflectance measurement. In the emissive mode [Fig. 3(b)], in which the UV lamp in the backlight was turned on, the PL molecules emit green light after absorbing UV light. The fluorescent curve exhibited a shape similar to the reflectance curve. The two modes are thus interchangeable or can be used simultaneously. In the emissive mode, the energy transfer from BBOT was clearly observed. The addition of BBOT improved the luminance of the cell significantly compared to the cell containing only C6, as shown in the inset of Fig. 3(b). The CRs between the dark and bright states in the reflective mode were determined to be approximately 10 in the effective reflectance measured by the CM-2600d [Fig. 4(a)] and about 50 in the data measured by the SR-3A (data not shown here). The difference results from the different illumination, where illumination from close to the top viewing
direction maximizes the CR. The contrast of a typical newspaper was measured to be approximately 6 to 7 by a diffuse reflectance measurement, confirming that the CR of the PL-CLC cell is reasonably high. The CR in the reflective mode was not highly sensitive to the type of alignment layer used in the cell, but the insertion of index-matching oil between the UV pass filter and the cell increased the CR by 20%, from 42 to 51, in the SR-3A measurement. The CR in the emissive mode was approximately 7, as shown in Fig. 4(b). Unlike the reflective mode, the CR in the emissive mode was highly sensitive to the type of alignment layer. The PL-CLC cell with a vertical alignment layer had approximately twice the CR of the cell with a planar alignment layer, as shown in the blue diamond data point in Fig. 4(b). In a cell with a planar alignment layer, PL molecules within LCs near the surface align parallel to the surface due to strong anchoring even under a high electric field, and the PL molecules can easily emit fluorescent light after absorbing UV light in the dark state, which deteriorates the CR in a cell with a planar alignment layer. The CR of the PL-CLC cell was slightly decreased compared to that of the CLC cell, as indicated in Fig. 4(a), where the first data point corresponds to the CR of a pure CLC cell. CR is obtained by dividing the bright luminance (LB ) by the dark luminance (LD ). In a PL-CLC cell, the total luminance is given by the summation of the CLC reflection (LB R or LD R ) and the PL fluorescence (LB F or LD F ). Hence, the CR can be expressed as LB F R LD F LB R LB R LB F LB × LD R LD F LD F LD
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Fig. 4. CR of the green PL-CLC cell from the top view as a function of the concentration of C6 and BBOT (UV BLU off), (a) in the reflective mode measured by the CM-2600d and (b) in the emissive mode measured by the SR-3A. 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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(2)
Here, CR0 LB R ∕LD F . In our sample, CREM is lower than CRCLC , as shown in Figs. 4(a) and 4(b), and the CR of the PL-CLC cell was lower than that of a pure CLC cell. When the concentration of C6 is very low, CR0 is very large and Eq. (2) approaches CRCLC . However, when the C6 concentration increases, CR0 decreases and Eq. (2) approaches CREM . Therefore, in order to achieve a high CR in the PL-CLC device, the CR in the emissive mode should be improved. In order to improve CREM further, the beam profile of the UV BLU and the order parameter of LCs and PL molecules should be considered. An inclined UV emission from the BLU and disordered PL molecules increase LD F and decrease REM . As we reported in a previous paper, the PL-CLC cell exhibited much less color shift with the viewing angle, which enhances the viewing angle property of PL-CLC cells [19]. The improvement of the viewing angle property arises from the complementary relationship between the selective reflection and the emission from PL molecules, as described in detail in the previous paper. The response times were measured to be 7.0 and 12.2 ms for the on and off times, respectively, in a cell of 0.3 wt. % of C6 and BBOT, as shown in Fig. 5, where the on and off times represent the transitions from the planar to the homeotropic transition and vice versa. During the switching-off time, the addition of PL material into the CLC significantly reduced the response time, as shown in Fig. 5(a), where the pure CLC cell exhibits a slowly increasing luminance with a response time of 31.6 ms that arises from the helical reorientation. Although the off response time is slower than for the usual transmissive-type LCDs [24], both the on and off response times are within one frame time of 60 Hz display devices. Hence, the PL-CLC display is capable of displaying moving picture images. Note that electrophoretic displays, which are typical reflective-type displays, are too slow to display moving picture images [4]. The PL-CLC cell exhibited excellent visibility under the various ambient circumstances, as indicated in Fig. 6, where three display types, LCD, conventional CLC, and PL-CLC cells, are compared. The dark areas in the pattern of the CLC and PL-CLC cells are the switching-on state (homeotropic state), and the bright green areas are the switching-off state (planar state). The active LCD has poor visibility under sunlight, as shown in Fig. 6(a), where the ambient luminance is 8284
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Fig. 5. Electro-optical dynamic response during switching (a) off and (b) on. The response times during switching off and on were approximately 12.2 and 7.0 ms, respectively. The dynamic response was recorded using the BM-7.
about 4000 Lux, which is a typical outdoor ambient light level. On the other hand, the conventional CLC cell is not visible at all in the dark room, as shown in Fig. 6(c), but the PL-CLC cell exhibits good visibility regardless of the ambient conditions. The PL-CLC cell image in the dark room was taken with the UV backlight turned on. The visibility of the PL-CLC cell was quite good even under normal office lighting conditions without using the UV backlight, allowing the emissive mode to be used as a supplementary function for working in dark environments. Considering that approximately 70% of the power used in LCDs is consumed by the backlight, the PL-CLC cell can provide an extremely low-power display technology. The visibility of a PL-CLC cell in office conditions can be further improved by turning the UV BLU on, as shown in Figs. 6(d) and 6(e). Moreover, the brightness and color purity can be optimized depending on the ambient illumination condition by controlling the intensity of the UV backlight, which is a highly demanded feature in the usual reflectivetype displays. A newspaper was used as the background to allow comparison of the visibilities, and good CRs of the PL-CLC cells are demonstrated in Fig. 6. Finally, we tested two additional PL-CLC cells with red and blue colors, as shown in Fig. 7. The DCM and BBOT were added to the red CLC and BBOT was added to the blue CLC. The concentration of each PL dye was 0.6 wt. % for both red and blue PL-CLC cells. The images of the red and blue pure CLC and PL-CLC cells taken under bright and dark condition are depicted in Fig. 7. In the reflective mode [Fig. 7(a)], the effective reflectance
Fig. 6. (a)–(c) Comparison of conventional LCD, CLC, and PL-CLC with 0.3 wt. % (C6+BBOT) in various ambient circumstances: (a) under sunlight (∼4000 Lux), (b) in an office (∼300 Lux), and (c) in a dark room with the UV BLU turned on. (d), (e) Comparison of a CLC cell and a PL-CLC cell with 1.0 wt. % (C6+BBOT) (d) with and (e) without UV backlight in an office (∼300 Lux). In the CLC and PL-CLC cells, 14 V was applied to the dark area and 0 V was applied to the green area. The newspaper background allows comparison of the visibility.
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Fig. 7. Comparison of red and blue PL-CLC cells with conventional CLC cells at different modes: (a) reflective mode and (b) emissive mode. The reflectance of the PL-CLC cells was increased by around 30%. (The color of PL-CLC cells in the emissive mode looks very different from that in the reflective mode, which may be partially due to the mismatch between CLC reflection and PL fluorescent spectra and partially due to overexposure in the photos taken in a dark room.)
improvements for the red and blue cells were measured to be approximately 31% and 29%, respectively. Under dark conditions, the PL-CLC cells exhibited blue and red colors with the aid of the UV BLU. Thus, the proposed PL-CLC cell can provide a solution for full-color devices that are able to display high-quality images in all ambient light level environments, by combing with existing multicolor reproduction technologies in CLC displays [6]. 4. Conclusion
We demonstrated a PL-CLC cell that exhibited superior visibility to other types of displays by comparing the PL-CLC cell with an LCD and a conventional CLC cell under all ambient conditions. This is an important and challenging issue, especially for portable appliances. PL molecules harvested the energy of external illuminant light in a wide wavelength range and improved the effective reflectance by more than 30%, which varied depending on the illumination conditions. The response time was within one frame time of a 60 Hz display device, which ensures
the capability of displaying quality moving picture images. The PL-CLC cell exhibited excellent visibility, even in low-light environments without using a UV backlight, which enables extremely low power consumption of the PL-CLC device. By demonstrating each of the three colored cells, we showed that a full colored PL-CLC display device can be achievable, although further development should be pursued, particularly in multicolor pixilation, color adjustment of fluorescence, reliable PL materials, and driving schemes. We thank Merck Co. for providing materials. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012R1A1A1012167) and by the Technology Innovation Program (No. 10041596) funded by the Ministry of Knowledge Economy (MKE, Korea). References 1. K.-H. Kim and J.-K. Song, “Technical evolution of liquid crystal displays,” NPG Asia Mater. 1, 29–36 (2009). 2. B. Geffroy, P. Le Roy, and C. Prat, “Organic light-emitting diode (OLED) technology: materials, devices and display technologies,” Polym. Int. 55, 572–582 (2006). 3. M. Katayama, “TFT-LCD technology,” Thin Solid Films 341, 140–147 (1999). 4. P. Kazlas, J. Au, K. Geramita, H. Gates, M. Steiner, C. Honeyman, P. Drzaic, K. Schleupen, B. Wisnieff, and R. Horton, “ 12.1: 12.1" SVGA microencapsulated electrophoretic active matrix display for information appliances,” SID Int. Symp. Dig. Tech. Papers 32, 152–155 (2001). 5. H.-K. Lin, C.-H. Li, and S.-H. Liu, “Patterning electrode for cholesteric liquid crystal display by pulsed laser ablation,” Opt. Lasers Eng. 48, 1008–1011 (2010). 6. B.-Y. Lee and J.-H. Lee, “Printable flexible cholesteric capsule display with a fine resolution of RGB subpixels,” Curr. Appl. Phys. 11, 1389–1393 (2011). 7. K.-H. Kim, B.-H. Yu, S.-W. Choi, S.-W. Oh, and T.-H. Yoon, “Dual mode switching of cholesteric liquid crystal device with three-terminal electrode structure,” Opt. Express 20, 24376–24381 (2012). 8. W. D. St. John, Z. J. Lu, and J. W. Doane, “Characterization of reflective cholesteric liquid-crystal displays,” J. Appl. Phys. 78, 5253–5265 (1995). 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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School of Electronic & Electrical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea 2
Display Laboratory, Samsung Institute of Technology, Youngin 446-711, South Korea *Corresponding author: [email protected]
Received 5 September 2013; revised 3 November 2013; accepted 5 November 2013; posted 6 November 2013 (Doc. ID 197201); published 25 November 2013
We fabricated a photoluminescent cholesteric liquid crystal (PL-CLC) cell for a display application that can be used to display high-quality moving pictures under all ambient conditions including dark and sunlit conditions. The PL-CLC cell is switchable between the reflective mode under bright conditions and the emissive mode in the dark. The effective reflectance of the PL-CLC is higher than that of a conventional CLC device by more than 30%, and the contrast ratios were approximately 10 and 7 in the reflective and emissive modes, respectively. We directly compared the proposed PL-CLC cell with conventional LCD and CLC cells under sunlit, office, and dark environments and confirmed that the PL-CLC cell exhibited superior visibility under all ambient conditions. © 2013 Optical Society of America OCIS codes: (120.2040) Displays; (160.3710) Liquid crystals; (250.5230) Photoluminescence. http://dx.doi.org/10.1364/AO.52.008280
1. Introduction
Displaying high-quality static and dynamic images in all ambient environments, including both dark and sunlit conditions, is one of the most challenging issues in recent display technologies [1]. Liquid crystal displays (LCDs) and organic LED displays actively emit light with vivid colors exhibiting excellent indoor image quality, but they suffer from poor outdoor image quality due to the reflection of ambient light and have a relatively high power consumption. [2,3] Alternatively, reflective displays such as electrophoretic [4] and cholesteric liquid crystal (CLC) displays [5–7] make passive use of external light to provide paper-like quality outdoor images. However, these technologies have poor indoor image quality and a color gamut [8,9]. The transflective LCD, in which a part of a pixel is used for transmitting backlight and another for 1559-128X/13/348280-07$15.00/0 © 2013 Optical Society of America 8280
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reflecting external light, was developed to simultaneously accommodate these two requirements [10–13]. However, the reflectance efficiency is gained only at the expense of the transmittance. As a result, neither the indoor nor the outdoor image quality is completely satisfactory. Thus, no display technology has simultaneously satisfied all of the image quality requirements for both indoor and outdoor environments, despite increasing demands due to the rapid growth of portable electronic devices, which are used in various ambient environments. Meanwhile, it is well known that a CLC device can display dynamic images with vivid color by using the selective reflection of its periodic photonic crystal structure without the use of a polarizer or color filter while demonstrating low power consumption. However, the reflectance of CLC devices is still lower than that of white paper. Therefore, reflectance has been a shortcoming for CLCs that has been intensively investigated [14,15]. Another weakness of CLC devices arises from the slow helical axis reorientation, which is on the order of several seconds, during
the transition from homeotropic to planar alignment, causing slow color changes while displaying dynamic images [16,17]. A CLC device also has a limited viewing angle property due to the strong viewing angle dependence of the structure color of the onedimensional photonic crystal of a CLC [18]. Recently, we reported that the complementary relationship between the photoluminance and selective reflection in a photoluminescent CLC (PL-CLC) cell can compensate the slow response time caused by the helix reorientation and the narrow viewing angle property that arises from the high sensitivity of the wavelength of light reflecting in the viewing direction [19]. In this respect, the PL-CLC may be a candidate material for a reflective–emissive display that can solve the intrinsic problems of the CLC display including its limited reflectance, its viewing angle dependency, and the necessity of a light source in dark conditions. In this study, we demonstrated PL-CLC cells with a well-designed UV blacklight unit (BLU) for displaying both high-quality static and dynamic images in all ambient environments by combining PL materials and one-dimensional photonic crystalline CLCs. The electro-optical properties of a PL-CLC cell were investigated from the viewpoint of display applications. In particular, the visibility of a PL-CLC cell was compared with an LCD and a conventional CLC cell under all ambient environments. 2. Experiments
A positive nematic liquid crystal (LC) and a chiral dopant (R-811, Merck) were mixed to produce three kinds of CLC mixtures that reflect red, green, and blue colors. Three PL materials, coumarin 6 (C6), 2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophen (BBOT) and 4-(dicyanomethylene)-2-methyl-6[19,20], (p-dimethylaminostyryl)-4H-pyran (DCM) [21], which emit green, blue, and red light, respectively, as shown in Fig. 1(a), were mixed with the CLC
mixtures at concentrations of 0.1–2.0 wt. % depending on the experiment. BBOT was added to the blue CLC mixture, C6 and BBOT were added to the green CLC mixture, and DCM and BBOT were added to the red CLC mixture. BBOT in the green and red cells was used as an energy transfer dopant, which harvests a wider range of light energy. The green PL-CLC cell depicted in Figs. 1(b) and 1(c) was prepared by using two patterned indium-tin oxide (ITO) substrates coated with vertical alignment layers. The bottom substrate had four ITO electrodes that were able to be controlled independently, and the top substrate had a single common electrode that covered the entire area. The adoption of a vertical alignment layer prevents the specular reflection on the surface of the CLC layer. Another reason to adopt a vertical alignment layer rather than a planar alignment layer is to improve the contrast ratio (CR) in the emissive mode, which will be discussed in the next section. The UV BLU, which was used for the emissive mode, had a UV power of 60 mW∕cm2 with a peak wavelength of 365 nm. A UV pass filter was placed between the PL-CLC cell and the UV-BLU, and it absorbed the incident visible light from outside, while the UV light from the BLU could pass through the UV pass filter. A UV filter on the cell absorbed the UV light that passed through the PL-CLC layer so that an observer is not exposed to the UV light from the UV BLU. A green PL-CLC mixture containing C6 and BBOT dopants was injected into the cell. The effective reflectance shown in Fig. 2(a) was measured using a CM-2600d (Konica-Minolta Company, Japan). The CM-2600d is a reflective-type spectrophotometer with an integrating sphere and an internal light source. Since it supports the Video Electronics Standards Association standard diffuse reflectance measurement, it is suitable to use a CM-2600d for the diffuse reflectance of a reflective display device. However, the CM-2600d does not
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Wavelength(nm) Fig. 2. (a) Relative reflectance of green PL-CLC cells measured using a CM-2600d compared to a pure green CLC cell as a function of the concentration of C6 and BBOT. The blue circle data set represents the PL-CLC cell containing the same concentrations of C6 and BBOT. UV BLU was turned off. (b) Spectra of the fluorescence of the cells with C6 and C6+BBOT. The BBOT-containing PL-CLC cell shows enhanced effective reflectance and luminance intensity, which were measured using an SR-3A, because of the Förster energy transfer.
measure luminance in the emissive mode, and the luminance in the emissive mode [Figs. 2(b) and Fig. 3] was measured by an SR-3A (Topcon Company, Japan). The SR-3A is a camera-type spectrophotometer that can measure both luminance and reflectance. For the reflectance measurement using the SR-3A [Fig. 3(a)], an external D65 light source that was placed 23° from normal to the samples used. Each spectrophotometer typically yields different reflectance values for the same sample due to different illumination conditions. The dynamic response was measured using a luminance colorimeter, the BM-7 (Topcon Company, Japan). The BM-7 has a fast mode suitable for measuring dynamic response and is widely used for measuring dynamic response in a flat panel display. 3. Results and Discussion
The effective reflectance of the planar state of green PL-CLC cells [Fig. 1(b)] as a function of the C6 and BBOT concentrations were measured using a CM2600d and are plotted in Fig. 2(a). The effective reflectance was determined by the ratio of the reflected luminance over the incident luminance, which is the standard reflectance measurement method for reflective display devices. Here, the 8282
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terminology “effective reflectance” is used because the fluorescence from the PL material improves the effective reflectance. With an increasing C6 concentration, the effective reflectance (red solid line) was enhanced up to 30% compared to that of a pure green CLC cell. Additional improvement due to the BBOT (blue dotted line) was observed with intermediate dopant concentrations. The C6 molecules absorb the photon energy in the blue and a limited part of the UV region and emit green light. Hence, the fluorescent efficiency is not sufficiently high under UV irradiation. Meanwhile, BBOT absorbs UV light and emits the blue near 430 nm, which roughly coincides with the absorption spectrum of C6, as shown in Fig. 1(a). Thus, the BBOT molecules play a role of transferring UV energy to C6, which enhances the green light emission. However, the saturated values were similar in the two dopant systems, which may be due to the low UV density of the CM-2600d illumination. In order to verify the Förster energy transfer [22] of BBOT, we measured the fluorescence spectra of two PL-CLC cells with and without BBOT, and the results are shown in Fig. 2(b). The addition of BBOT increased the fluorescence peak in the green region but not in the blue region, which is an indication that the blue light emitted by BBOT is entirely absorbed by C6 by the Förster energy transfer. Due to the use of a UV filter, only a tail of the UV light source was detected, as indicated in Fig. 2(b), where the UV source tail of the cell with BBOT was weaker than that of the other cell, indicating increased absorption of UV light by BBOT. The electro-optical responses of the PL-CLC cells are shown as a function of the applied voltage in Figs. 3(a) and 3(b), measured using the SR-3A in the reflective mode under external D65 light and in the emissive mode without external light, respectively. In Fig. 3(a), the optical reflectance in the reflective mode exhibited three regions depending on the applied voltage, corresponding to the planar, focal conic, and homeotropic LC alignment states [23]. The planar state had the highest effective reflectance due to the combination of selective CLC reflection and PL dopant fluorescence, as illustrated in Fig. 1(b). The focal conic state, which has a randomly distributed helical structure, scattered incident light. Here, the majority of the scattered light was absorbed by the UV pass filter, and the remainder was rescattered or re-emitted from the cell, resulting in a low effective reflectance. In the homeotropic alignment, most of the incident light was absorbed by the UV pass filter after passing through the vertically aligned LC layer. The luminance in the planar state increased dramatically with increasing dopant concentration, while only a small increase was detected in the homeotropic state. Interestingly, the reflectance improvement of the planar state was measured to be approximately 50% by the SR-3A using a single light source measurement, as indicated in Fig. 3(a). The reflectance improvement
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was maximized when the incident angle from the light source was small, which is the reason why the single light source system measured a better luminance improvement than the diffuse reflectance measurement. In the emissive mode [Fig. 3(b)], in which the UV lamp in the backlight was turned on, the PL molecules emit green light after absorbing UV light. The fluorescent curve exhibited a shape similar to the reflectance curve. The two modes are thus interchangeable or can be used simultaneously. In the emissive mode, the energy transfer from BBOT was clearly observed. The addition of BBOT improved the luminance of the cell significantly compared to the cell containing only C6, as shown in the inset of Fig. 3(b). The CRs between the dark and bright states in the reflective mode were determined to be approximately 10 in the effective reflectance measured by the CM-2600d [Fig. 4(a)] and about 50 in the data measured by the SR-3A (data not shown here). The difference results from the different illumination, where illumination from close to the top viewing
direction maximizes the CR. The contrast of a typical newspaper was measured to be approximately 6 to 7 by a diffuse reflectance measurement, confirming that the CR of the PL-CLC cell is reasonably high. The CR in the reflective mode was not highly sensitive to the type of alignment layer used in the cell, but the insertion of index-matching oil between the UV pass filter and the cell increased the CR by 20%, from 42 to 51, in the SR-3A measurement. The CR in the emissive mode was approximately 7, as shown in Fig. 4(b). Unlike the reflective mode, the CR in the emissive mode was highly sensitive to the type of alignment layer. The PL-CLC cell with a vertical alignment layer had approximately twice the CR of the cell with a planar alignment layer, as shown in the blue diamond data point in Fig. 4(b). In a cell with a planar alignment layer, PL molecules within LCs near the surface align parallel to the surface due to strong anchoring even under a high electric field, and the PL molecules can easily emit fluorescent light after absorbing UV light in the dark state, which deteriorates the CR in a cell with a planar alignment layer. The CR of the PL-CLC cell was slightly decreased compared to that of the CLC cell, as indicated in Fig. 4(a), where the first data point corresponds to the CR of a pure CLC cell. CR is obtained by dividing the bright luminance (LB ) by the dark luminance (LD ). In a PL-CLC cell, the total luminance is given by the summation of the CLC reflection (LB R or LD R ) and the PL fluorescence (LB F or LD F ). Hence, the CR can be expressed as LB F R LD F LB R LB R LB F LB × LD R LD F LD F LD
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Fig. 4. CR of the green PL-CLC cell from the top view as a function of the concentration of C6 and BBOT (UV BLU off), (a) in the reflective mode measured by the CM-2600d and (b) in the emissive mode measured by the SR-3A. 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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(2)
Here, CR0 LB R ∕LD F . In our sample, CREM is lower than CRCLC , as shown in Figs. 4(a) and 4(b), and the CR of the PL-CLC cell was lower than that of a pure CLC cell. When the concentration of C6 is very low, CR0 is very large and Eq. (2) approaches CRCLC . However, when the C6 concentration increases, CR0 decreases and Eq. (2) approaches CREM . Therefore, in order to achieve a high CR in the PL-CLC device, the CR in the emissive mode should be improved. In order to improve CREM further, the beam profile of the UV BLU and the order parameter of LCs and PL molecules should be considered. An inclined UV emission from the BLU and disordered PL molecules increase LD F and decrease REM . As we reported in a previous paper, the PL-CLC cell exhibited much less color shift with the viewing angle, which enhances the viewing angle property of PL-CLC cells [19]. The improvement of the viewing angle property arises from the complementary relationship between the selective reflection and the emission from PL molecules, as described in detail in the previous paper. The response times were measured to be 7.0 and 12.2 ms for the on and off times, respectively, in a cell of 0.3 wt. % of C6 and BBOT, as shown in Fig. 5, where the on and off times represent the transitions from the planar to the homeotropic transition and vice versa. During the switching-off time, the addition of PL material into the CLC significantly reduced the response time, as shown in Fig. 5(a), where the pure CLC cell exhibits a slowly increasing luminance with a response time of 31.6 ms that arises from the helical reorientation. Although the off response time is slower than for the usual transmissive-type LCDs [24], both the on and off response times are within one frame time of 60 Hz display devices. Hence, the PL-CLC display is capable of displaying moving picture images. Note that electrophoretic displays, which are typical reflective-type displays, are too slow to display moving picture images [4]. The PL-CLC cell exhibited excellent visibility under the various ambient circumstances, as indicated in Fig. 6, where three display types, LCD, conventional CLC, and PL-CLC cells, are compared. The dark areas in the pattern of the CLC and PL-CLC cells are the switching-on state (homeotropic state), and the bright green areas are the switching-off state (planar state). The active LCD has poor visibility under sunlight, as shown in Fig. 6(a), where the ambient luminance is 8284
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Fig. 5. Electro-optical dynamic response during switching (a) off and (b) on. The response times during switching off and on were approximately 12.2 and 7.0 ms, respectively. The dynamic response was recorded using the BM-7.
about 4000 Lux, which is a typical outdoor ambient light level. On the other hand, the conventional CLC cell is not visible at all in the dark room, as shown in Fig. 6(c), but the PL-CLC cell exhibits good visibility regardless of the ambient conditions. The PL-CLC cell image in the dark room was taken with the UV backlight turned on. The visibility of the PL-CLC cell was quite good even under normal office lighting conditions without using the UV backlight, allowing the emissive mode to be used as a supplementary function for working in dark environments. Considering that approximately 70% of the power used in LCDs is consumed by the backlight, the PL-CLC cell can provide an extremely low-power display technology. The visibility of a PL-CLC cell in office conditions can be further improved by turning the UV BLU on, as shown in Figs. 6(d) and 6(e). Moreover, the brightness and color purity can be optimized depending on the ambient illumination condition by controlling the intensity of the UV backlight, which is a highly demanded feature in the usual reflectivetype displays. A newspaper was used as the background to allow comparison of the visibilities, and good CRs of the PL-CLC cells are demonstrated in Fig. 6. Finally, we tested two additional PL-CLC cells with red and blue colors, as shown in Fig. 7. The DCM and BBOT were added to the red CLC and BBOT was added to the blue CLC. The concentration of each PL dye was 0.6 wt. % for both red and blue PL-CLC cells. The images of the red and blue pure CLC and PL-CLC cells taken under bright and dark condition are depicted in Fig. 7. In the reflective mode [Fig. 7(a)], the effective reflectance
Fig. 6. (a)–(c) Comparison of conventional LCD, CLC, and PL-CLC with 0.3 wt. % (C6+BBOT) in various ambient circumstances: (a) under sunlight (∼4000 Lux), (b) in an office (∼300 Lux), and (c) in a dark room with the UV BLU turned on. (d), (e) Comparison of a CLC cell and a PL-CLC cell with 1.0 wt. % (C6+BBOT) (d) with and (e) without UV backlight in an office (∼300 Lux). In the CLC and PL-CLC cells, 14 V was applied to the dark area and 0 V was applied to the green area. The newspaper background allows comparison of the visibility.
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Fig. 7. Comparison of red and blue PL-CLC cells with conventional CLC cells at different modes: (a) reflective mode and (b) emissive mode. The reflectance of the PL-CLC cells was increased by around 30%. (The color of PL-CLC cells in the emissive mode looks very different from that in the reflective mode, which may be partially due to the mismatch between CLC reflection and PL fluorescent spectra and partially due to overexposure in the photos taken in a dark room.)
improvements for the red and blue cells were measured to be approximately 31% and 29%, respectively. Under dark conditions, the PL-CLC cells exhibited blue and red colors with the aid of the UV BLU. Thus, the proposed PL-CLC cell can provide a solution for full-color devices that are able to display high-quality images in all ambient light level environments, by combing with existing multicolor reproduction technologies in CLC displays [6]. 4. Conclusion
We demonstrated a PL-CLC cell that exhibited superior visibility to other types of displays by comparing the PL-CLC cell with an LCD and a conventional CLC cell under all ambient conditions. This is an important and challenging issue, especially for portable appliances. PL molecules harvested the energy of external illuminant light in a wide wavelength range and improved the effective reflectance by more than 30%, which varied depending on the illumination conditions. The response time was within one frame time of a 60 Hz display device, which ensures
the capability of displaying quality moving picture images. The PL-CLC cell exhibited excellent visibility, even in low-light environments without using a UV backlight, which enables extremely low power consumption of the PL-CLC device. By demonstrating each of the three colored cells, we showed that a full colored PL-CLC display device can be achievable, although further development should be pursued, particularly in multicolor pixilation, color adjustment of fluorescence, reliable PL materials, and driving schemes. We thank Merck Co. for providing materials. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012R1A1A1012167) and by the Technology Innovation Program (No. 10041596) funded by the Ministry of Knowledge Economy (MKE, Korea). References 1. K.-H. Kim and J.-K. Song, “Technical evolution of liquid crystal displays,” NPG Asia Mater. 1, 29–36 (2009). 2. B. Geffroy, P. Le Roy, and C. Prat, “Organic light-emitting diode (OLED) technology: materials, devices and display technologies,” Polym. Int. 55, 572–582 (2006). 3. M. Katayama, “TFT-LCD technology,” Thin Solid Films 341, 140–147 (1999). 4. P. Kazlas, J. Au, K. Geramita, H. Gates, M. Steiner, C. Honeyman, P. Drzaic, K. Schleupen, B. Wisnieff, and R. Horton, “ 12.1: 12.1" SVGA microencapsulated electrophoretic active matrix display for information appliances,” SID Int. Symp. Dig. Tech. Papers 32, 152–155 (2001). 5. H.-K. Lin, C.-H. Li, and S.-H. Liu, “Patterning electrode for cholesteric liquid crystal display by pulsed laser ablation,” Opt. Lasers Eng. 48, 1008–1011 (2010). 6. B.-Y. Lee and J.-H. Lee, “Printable flexible cholesteric capsule display with a fine resolution of RGB subpixels,” Curr. Appl. Phys. 11, 1389–1393 (2011). 7. K.-H. Kim, B.-H. Yu, S.-W. Choi, S.-W. Oh, and T.-H. Yoon, “Dual mode switching of cholesteric liquid crystal device with three-terminal electrode structure,” Opt. Express 20, 24376–24381 (2012). 8. W. D. St. John, Z. J. Lu, and J. W. Doane, “Characterization of reflective cholesteric liquid-crystal displays,” J. Appl. Phys. 78, 5253–5265 (1995). 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS
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