Making a Mobile Display Using Polarizer-Free Reflective LCDs and Ultra-Low-Power Driving Technology

Through refinement of materials and fabrication conditions, a reflective display with reduced flicker and image sticking at low frame rates was produced. The display consumes very little power and works in a wide range of temperatures, making it a potential platform for future mobile devices.

by Kiyoshi Minoura, Yasushi Asaoka, Eiji Satoh, Kazuhiro Deguchi, Takashi Satoh, Ichiro Ihara, Sayuri Fujiwara, Akio Miyata, Yasuhisa Itoh, Seijiro Gyoten, Noboru Matsuda, and Yasushi Kubota

ELECTRONIC DISPLAYS have become commonplace in our daily lives, and low power consumption is more important than ever before. The power consumption of conventional electronic displays is still high enough to limit the amount that it uses or the environments in which it operates effectively. Technologies such as reflective displays and low-frame-rate driving, as well as the ability to "memorize" images, are effective in producing low-power-consumption displays. Imagery can be memorized (held in memory while drawing very little power until the image changes) using electrophoretic¹ or cholesteric liquid-crystal technology,² but displays based on these technologies tend to require relatively high driving voltages and produce slow responses.

A combination of a reflective-type liquid-crystal display (RLCD)³ and a drive scheme such as a low-frame-rate drive⁴ or a pixel-memory circuit^5,6 should be a promising candidate for mobile devices. Because the power consumption of an LCD module generally depends on the driving frequency, the lower the frame rate, the less power the module consumes. And the pixel-memory-circuit technology enables the module to "memorize" images in the pixels with the data driver suspended.

An RLCD using polymer-dispersed liquid crystals (PDLC)⁷ can control the transmission or scattering of incoming light rays without the use of polarizers. Its features of note include high light-utilization efficiency due to a polarizer-free system and a low level of dependence on viewing angle. Although a PDLC usually requires a high driving voltage, a polymer-network LC (PNLC) containing a high LC content ranging from 70 to 90 wt.%, which is higher than that for a conventional PDLC, shows relatively lower driving voltage. Therefore, a PNLC is one solution toward realizing an energy-savings paper-like display. In the meantime, a PNLC holds a couple of challenges for TFT drives. These include slow response and a low voltage-holding ratio. However, recent progress made in PNLC materials has enabled TFT driving of a PNLC by improving the above properties.⁸

Objective

Our objective was to develop an ultra-low-power-consumption RLCD with excellent viewing properties by using a 1-bit pixel-memory technology and a PNLC. This resulting display is composed of a PNLC layer formed between the transparent electrode and mirror-reflective pixel electrodes and 1-bit pixel-memory circuits embedded in the pixel area under the reflective-mirror electrodes. This PNLC mainly utilizes front scattering and a mirror reflection for the displayed imagery; therefore, a 3-μm thickness is enough to achieve high reflectance (50%). And all the circuits of the display are fabricated by low-temperature poly-Si (LTPS).

Pixel-Memory Technology

The system block diagram of our newly developed pixel-memory display is shown in Fig. 1.

The interface of this display is very simple because only five input lines are necessary for operation, including power supplies. A timing generator with a three-line serial-interface circuit, common electrode driver, polar-inversion circuit, and pixel-memory circuits embedded in each pixel are integrated with the scan driver and data driver monolithically on the glass substrate. When displaying still imagery, the image data is stored in the pixel memories, so that it requires no input for refreshing the image data – only the power supply. Moreover, because all the systems are composed of CMOS digital circuits, only 5 V is necessary for operation. Therefore, this display can achieve ultra-low power consumption.

PNLC Technology

The PNLC layer, with a micro-separated structure of liquid crystals and polymer networks, is manufactured by irradiating a mixture of monomers, liquid crystals, and a photo-initiator under UV light. The morphology of the polymer network is shown in the cross-sectional scanning-electron-microscope image shown in Fig. 2.

The droplets are not isolated from each other, but have a sponge-like appearance, and the diameter of a droplet is approximately 1–2 μm. The morphology of the polymer network is critical in determining the electro-optic properties of the PNLC. This morphology is controlled by the kinetics of photo-polymerization. It depends on the concentration of the photo-initiator, the UV intensity, the UV curing temperature, and the surface treatment of the substrates.

The system has no polarizers, so the images are displayed by switching the PNLC layer between the scattering states and the transparent states. In the absence of voltage, the layer scatters due to the variation in the symmetry axis of the LC director in the droplets, which leads to a mismatch in the refractive indices in the different droplets; application of sufficient voltage causes all the LC directors to align vertically and the refractive indices to match in the different droplets, removing the scattering. The scattering PNLC breaks the specular reflection. The observer sees light and recognizes the images as bright (white) states [Fig. 3(a)] at all viewing angles. The transparent PNLC enables mirror reflection. Other than in the specular direction, the observer sees no light and recognizes the images as dark (black) states [Fig. 3(b)].

The PNLC normally exhibits a white state. So, in the white state, 0 V is applied to the PNLC, and in the black state, certain specific voltages are applied to the PNLC. Zero volts or another voltage is kept constant and applied to the pixel electrodes by utilizing the data stored in pixel memory. This drive scheme can operate at a low frame rate without problems, due to a less-than-100% voltage-holding ratio. Low-frame-rate operation using pixel memories could reduce the power consumption drastically, due to the low driving frequency of the data driver.

Fig. 1: A system block diagram for the 1.35-in. prototype display is shown with both black-and-white and RGB SRAMs.

Fig. 2: This cross-sectional scanning-electron-microscope image shows the polymer-network morphology of the PNLC.

 (a)      (b)

Fig. 3: These illustrations show the principle of operation for the prototype. On the left (a) is the bright state. On the right (b) is the dark state.

Power Consumption

Generally, the power consumption of an LCD is related to the driving frequency and the displayed image. The frame-rate dependence on driving power consumption for a 1.35-in. prototype display is shown in Fig. 4.

The power consumption drastically decreases with frame rate. The power consumption when displaying a clock pattern at 1 Hz is 10 μW for the monochrome panel and 25 μmW for the color panel. The power consumption when displaying a black pattern over the entire area at 1 Hz is 15 μW for the monochrome panel and 30 μW for the color panel. These ultra-low-power-consumption panels are realized by a well-controlled process technology that can reduce the leakage current of the TFTs. Each pixel in the color panel has RGB subpixels and a color filter. The power consumption of the panel is related to the number of pixels; hence, the power consumption of the color panel is larger than that of the monochrome panel.

Flicker and Image Sticking

In general, the human eyes are very sensitive to any flicker having a frequency below 60 Hz, but are especially sensitive to frequencies below 30 Hz. Therefore, the most important issue in achieving low-frequency driving below 30 Hz is to decrease the flicker. Image sticking is another big problem in low-frequency driving, due to the movement of impurity ions in the liquid crystal. In order to achieve an ultra-low-power-consumption PNLC display, a reduction in flicker and the inhibition of image sticking are absolutely imperative.

In this system, problems due to the less-than-100% voltage-holding ratio at low-frame-rate driving do not occur. However, when the frame rate is low to the extent of 1 Hz, impurity ions move easily and often form in the electric double layer. The electric double layer produces a reduced applied voltage to the PNLC and becomes a cause of flicker and/or image sticking due to the suppressed relaxation of liquid crystals, with each switching from the dark state (voltage on) to the bright state (voltage off). This flicker and image sticking can be observed in our system using regular PNLC materials. The blue line in Fig. 5 shows the resulting flicker and the image sticking.

The 30/0 reflectance, which is detected in the panel normal direction under parallel light incidence from a 30° polar angle, was measured by using an LCD5200 from Otsuka Electronics. The reflectance at the dark state, during a time period (x-axis) of 1–21 sec, changes periodically and produces a rectangular waveform. This reflectance oscillation is recognized as flicker. The reflectance gradually increases when switching from dark to bright, after a time period (x-axis) of 21 sec. This slow response of the reflectance is recognized as image sticking.

To reduce the flicker and to inhibit image sticking, the PNLC needs to saturate sufficiently at the driving voltage, and the impurities in the PNLC need to be reduced as much as possible. The electro-optic properties of a PNLC are determined not only by its LC properties, such as impurity concentration and saturation voltage, but by the morphology of the polymer network controlled by the kinetics of photo-polymerization.

By modifying the monomer and LC in the PNLC, we decreased the impurity concentration and the saturation voltage without a reduction in reflectance. The level of flicker and the relaxation time of the image sticking also decreased by reducing the amount of the photo-initiator in the PNLC (Table 1).

Here, the flicker value is defined as the value of the standard deviation of the 30/0 reflectance for the dark state. Relaxation time is defined as the time required for the 30/0 reflectance to change from 0% to 97% when the reflectance of the dark state and bright state is set to be 0% and 100%, respectively. This result indicates that the photo-initiator acts as an impurity in the PNLC even after the UV curing. But reducing the photo-initiator in the material caused a reduction in the reflectance and a weak reaction of the polymer networks in PNLC. Therefore, the preparation of the material is very important, as is the need for a sophisticated process for the PNLC, especially when operating at a low frame rate.

Fig. 4: Shown is the frame-rate dependence of power consumption for the 1.35-in. prototype display.

Fig. 5: This diagram shows the optical response of the 1.35-in. prototype display operating at 1 Hz, including flicker and image sticking (blue line).

The electro-optic properties of a PNLC are also determined by the UV cure temperature and the UV intensity that strongly relate to the kinetics of photo-polymerization. The reflectance decreased with a reduction in UV cure temperature and UV intensity. The reflectance strongly depends on the diameter of the droplet size in the PNLC layer; the reflectance decreases with an increase in droplet size. Low UV cure temperature and low UV intensity bring about slow photo-polymerization-induced phase separation (PIPS). The slow PIPS forms large LC droplets due to the long diffusion length of the monomer in the LC and results in low reflectance. In contrast, a high UV cure temperature and high UV intensity form small droplets due to the fast PIPS. The PNLC with small droplets also indicates a high saturation voltage owing to the increase of the surface anchoring effects between the LC and the polymer-network surface. Moreover, both high reflectance and low saturation voltage are achieved only at the mid-temperature range, which is a T_nm (the T_ni before the UV cure) of +3°C and a T_nm of +10°C in our material. The PNLC cured at this temperature range showed no flicker. In the meantime, the flicker value decreased with increasing UV intensity (Fig. 6).

On visual inspection, the flicker is invisible, with less than 0.5% of the flicker value. The PNLC without flicker is achieved by using a high UV intensity. The reduction in flicker by using a high UV intensity is probably due to the increase of polymer walls with a decrease in droplet size, which acts as a barrier against the movement of impurity ions and/or reduces the localization of impurity ions.

Consequently, we have succeeded in eliminating flicker and image sticking for low- frame-rate driving by modifying the material and optimizing the fabrication conditions of the PNLC layer, shown as the red line in Fig. 5.

Performance

The display images operating at a frame rate of 1 Hz are shown in Fig. 7.

The integrated reflectance value d/8 (which is detected at a polar angle of 8° under diffused light) of the monochrome display and the color display are over 50% and 20%, respectively. The d/8 reflectances were measured by using a CM2002 (Konica Minolta Sensing). Standard white (MgO plate) was used as the reference of 100% reflectance. Their contrast ratios are 10:1 and 5:1, respectively. These displays did not show any defects such as flicker or image sticking after low-frame-rate operation (1 Hz) at high temperature (70°C) and low temperature (-20°C) over 500 hours. Moreover, the defects were not generated after the storage test at 80°C, at -30°C, and under sunlight for over 500 hours. These environmental tolerances are almost equal to those of current conventional LCDs for mobile applications.

The specifications of these developed displays are compared with those of commercially available for typical displays in Table 2.

Fig. 6: UV cure intensity affects flicker value. Each sample received an even dose of UV.

 (a)      (b)

Fig. 7: On the left (a) is an example of a monochrome version of the 1.35-in. prototype. On the right (b) is the color version.

The integrated reflectance of the display is twice as high as that of a conventional RLCD with a polarizer and 25% higher than that for an electrophoretic display. In the case of common usage or an adequate switching frequency, the power consumption of our display is reduced to the level of around 1/60 of that of a conventional RLCD with a polarizer, and to the level of around 1/3 of that of an electrophoretic display, even though the power consumption depends on image contents.⁹ Our display has other merits, including moving-image display capability and a wide-operating-temperature limit, whereas an electrophoretic display has difficulty displaying moving-images and an upper operating temperature limit of 50°C.

This reflective display, which has been created by the combination of pixel memory circuits and a new PNLC material has many features that are desirable in mobile devices: good visibility, low power consumption, and the ability to display moving imagery and to adapt to various usage environments. We have also reduced the level of flicker and image sticking at low frame rates by refining the materials and the fabrication conditions.

Acknowledgments

The authors gratefully acknowledge Mr. I. Takahashi, Mr. T. Yamaguchi, Mr. Y. Moriya, and Mr. S. Nishi from Mobile Liquid Crystal Display Group, Sharp Corp., for their great assistance and useful discussions.

References

¹T. Whitesides, et al., "Towards Video-Rate Microencapsulated Dual-Particle Electro-phoretic Display," SID Symposium Digest 35, 133-135 (2004).

²M. Okada, et al., "Reflective Multicolor Display Using Cholesteric Liquid Crystal," SID Symposium Digest 28, 1019-1022 (1997).

³Y. Itoh, et al., "Reflective LCDs with a Single Polarizer," SID Symposium Digest 29, 221-224 (1998).

⁴K. Tsuda, et al., "Ultra low power consumption technologies for mobile TFT-LCD," IDW '02 Digest, 295-298 (2002).

⁵M. Inoue, et al., "Low Power Consumption TFT-LCD with 4-bit Dynamic Memories Embedded in Each Pixel," IDW '01 Digest, 1599-1602 (2001).

⁶Y. Nakajima, et al., "Ultra-Low-Power LTPS TFT-LCD Technology Using a Multi-Bit Pixel."

⁷P. S. Drzaic, Liquid Crystal Dispersions (World Scientific Publishing Co., Pte., Ltd, 1995), pp. 392-399.

⁸Y. Itoh, et al., "Super Reflective Color LCD with PDLC Technology," SID Symposium Digest 38, 1362-1365 (2007).

⁹R. W. Zehner, et al., "Power Consumption of Micro-encapsulated Electrophoretic Displays for Smart Handheld Applications," SID Symposium Digest 33, 1378-1381 (2002). •