POLYMER STABILIZED CHOLESTERIC DICHROIC DYE DISPLAYS

Transcription

1 POLYMER STABILIZED CHOLESTERIC DICHROIC DYE DISPLAYS Fang Zhang, Julie Kim, Mary Neubert and Deng-Ke Yang Chemical Physics Program and Liquid Crystal Institute Kent State University, Kent, OH 22 Abstract We used polymer stabilization to develop two cholesteric dichroic dye displays. In the first display, with dispersed polymer networks (cured in the homeotropic texture) perpendicular to cell surface, the hysteresis is enhanced in the focal conic-homeotropic transition; the material is bistable under a bias voltage in the hysteresis loop. In the second display, with random polymer network (cured in the isotropic phase), the hysteresis in the focal conic-homeotropic transition is reduced; and the material can be operated in gray scales. We systematically studied the electro-optical performance of the displays which are suitable for direct view display applications. Introduction Guest-host displays make use of the dichroic absorption of dichroic dyes: light with polarization along the absorption transition dipole is absorb while light with polarization perpendicular to the dipole is not absorbed. When a small amount of dye is dissolved into a liquid crystal, the dye molecules can be aligned by liquid crystal molecules; therefore, the orientation and absorption of dye molecules can be changed along with the orientation of liquid crystal molecules by applied fields. There have been many works done on dichroic dye displays 1. The traditional ones are TN dichroic dye display and phase change effect dichroic dye display. The TN dichroic dye display is operated between the absorbing homogeneous state at zero field and the transparent field-induced homeotropic state. It has gray scale but the on-state transmittance is low because it needs a polarizer. The phase change effect dichroic dye display uses a cholesteric liquid crystal and is operated between the absorbing planar texture at zero field and the transparent field-induced homeotropic texture. It has high on-state transmittance but has no gray scales because of the hysteresis in the planar-homeotropic transition. A relatively new dichroic dye technology is the dichroic dye doped polymer dispersed liquid crystal display. It is operated between the absorbing randomly aligned state at zero field and the transparent field-induced uniformly aligned state. It has high on-state transmittance and gray scales but low contrast because the dye dissolved in the polymer is not switchable. We developed polymer stabilized cholesteric dichroic dye displays. In these displays, we used cholesteric liquid crystals with positive dielectric anisotropies and pleochroic (or positive dichroic ) dyes, and we dispersed a small amount of polymer network in the display materials. The pleochroic dye will absorb the light with E vector along the long molecular axis of the dye. Due to the dispersed polymer, the focal conic state is stabilized at zero field, and the material has a random orientation throughout the cell; therefore cell is absorbing.. When a voltage is applied, the liquid crystal directors tends to align with the field and so do the dye molecules, thus absorption becomes lower under applied field and transmittance

2 increases. When a sufficiently high field is applied, the material is switched to the homeotropic texture and cell becomes transparent. 2- There is intrinsic hysteresis in the focal conic homeotropic transition. If the polymer network is perpendicular to the cell surface by polymerization in the field-induced homeotropic texture, the hysteresis in the focal conic-homeotrpic transition is greatly enhanced. The material is bistable under a bias voltage and therefore a bistable dichroic dye display is achieved. If the polymer network is randomly oriented by polymerization in the isotropic phase at elevated temperature, the focal conichomeotropic transition becomes smooth and the hysteresis is much suppressed; therefore, a gray scale dichroic dye display is achieved. Because the concentration of the polymer network is low, the fraction of the dichroic dye dissolved in the polymer network is low. These displays have good s and low driving voltages. Results Our dichroic dye displays were made by mixing nematic liquid crystal ZLI803, chiral dopant CB1, monomer BMBA-6, photoinitiator BME and dichroic dye 1,-bis-(-hexyloxy-phenylamino)- anthraquinone. The pitch was adjusted by the concentration of chiral dopant. The mixture was in cholesteric phase at room temperature. The mixture was filled into the display cells in a vacuum chamber. There was no alignment for these cells. Then the filled cells were cured for 30 minutes under UV light to photo-polymerize the monomer. During the curing process, for the bistable mode, the mixture was in the homeotropic state in the presence of an strong applied electric field; while for the hysteresis-less mode, the mixture was in the isotropic state at an elevated temperature. We tried to optimize the performance of the displays by varying polymer concentration, dye concentration, cholesteric pitch and curing conditions. The displays were developed for direct view display applications, instead of projection display applications. In measurement of the display materials, the light source was a green He-Ne laser at 3. nm. The applied voltage was 1 khz square wave. The collection angle of the detector was 3 o. With the large collection angle, nearly all the scattering light was collected. Therefore, the final contrast shows the effect due to dye absorption only, not the scattering effect. For the bistable polymer stabilized cholesteric dichroic dye material, a typical transmittancevoltage curve is shown in Fig. 1. The cell thickness was 1 µm. At 0 V, the material was in the focal conic texture. The dye absorption was high and transmittance was low. As voltage was increased, the focal conic domain size became larger, and dye molecules were more aligned the applied field, so dye absorption was lowered and the transmittance was increased. The material was switched to the homeotropic texture when the voltage was increased to 17 V. The transmittance of the homeotropic texture was around 7%. The light loss was mainly caused by dye absorption as well as the reflection from the two glass-air interfaces. When the voltage was decreased, the material remained in the homeotropic state until the voltage was decreased below 10V. Then it relaxed back into the focal conic texture.. In switching the liquid crystal from the focal conic texture to the homeotropic state, the polymer network did not affected the transition. When the material was in the homeotropic texture, the polymer network exerted an aligning force on it, which tended to remain it in the homeotropic texture. Hence, the hysteresis was enhanced and display can be operated under a bias voltage. The hysteresis increased with increasing polymer

3 concentration. The voltage to switch the material from the focal conic texture to the homeotropic texture was higher than the voltage at which it relaxed back from the homeotropic texture to the focal conic texture. V up was defined as the voltage at which the transmittance was increased to the middle transmittance [ 1 / 2 (minimum transmittance+maximum transmittance)], and V down was defined as the voltage at which the transmittance was decreased to the middle transmittance. The hysteresis was characterized as V = V up - V down. When a bias voltage V b in the hysteresis loop was applied to the material, the material was bistable and its transmittance depended on the history. If the material was initially in the focal conic texture and the voltage was increased to V b, it remained in the focal conic texture and the transmittance was low (T off ). If the material was initially in the homeotropic texture and the voltage was decreased to V b, it remained in the homeotropic texture and the transmittance was high (T on ). The material was operated between the bistable states under a bias voltage. The contrast was defined as C = T on /T off. C max Transmittance Vb applied voltage bias voltage Fig.1. The voltage-transmittance curve for the bistable cholesteric dichroic dye display. The vs. bias voltage. The collection angle was 3 o. 0. cell F Z99 h0 3a, colle ction angle deg transmittance applied voltage (V) bias voltage (V) Fig.2. The voltage-transmittance curve for the bistable cholesteric dichroic dye display with scattering effect. The vs. bias voltage. The collection angle was o.

4 In measuring the contrast at a bias voltage, we used the scheme described below. T on was measured 30 seconds after the applied voltage was switched to V b from a higher voltage which switched the material to the homeotropic texture; T off was measured 30 seconds after the applied voltage was switched to V b from 0 V which let the material relax to the focal conic texture. The contrast versus bias voltage is shown in Fig. 1. When the bias voltage was low, it was not able to hold the material in the homeotropic texture. Both T on and T off were low, and therefore the contrast was low. When the bias voltage was in the hysteresis loop, T on was high and T off was low, so the contrast was high. When the bias voltage was high, the material was switched to the homeotropic texture and both T on and T off were high, thus the contrast was low. The maximum contrast was indicated by C max, the bias voltage at which C max was obtained was defined as the drive voltage V d. It was desirable that C max be large and the width of the peak be wide so that the display has more tolerance on the cell thickness uniformity and the driver precision. For our optimized formula (Fig. 1), the hysteresis was about 6. V and the maximum was.0 at the drive voltage of 11. V. This result shows the effect due to dye absorption only. If we included the scattering effect, in addition to dye absorption, the contrast would be 6 at drive voltage of 11. V as shown in Fig. 2 where the collection angle was o. With the smaller collection angle, the contrast was much higher. Fig. 3 Transmittance voltage curve at different polymer concentration. The cell 0.8 po lymer 2.% po lymer 3.6% po lymer 3.9% 0.6 transmittance appliced voltage (V) thickness was 10 µm. We have systematically studied the factors determining the display performance. The polymer was one of the important factors, which has a profound influence on the hysteresis, as shown in Fig. 3, where other factors were fixed. When polymer concentration was increased first, V up decreased slightly, while V down decreased much more. The hysteresis V increased with increasing polymer concentration. The different behavior of V up and V down may be explained by the aligning effect of polymer network, which was perpendicular to cell surface and was in favor of the homeotropic texture. The aligning effect of the polymer network increased with the polymer concentration. In the focal conic texture, the liquid crystal was in a random poly-domain structure. Only the liquid crystal near the polymer network felt the aligning effect of polymer network. However, in the homeotropic texture, the liquid crystal was uniformly aligned with the applied field. The aligning effect of the polymer network penetrated throughout the whole sample.

5 Therefore, V down decreased much more as compared to V up, and hysteresis increased with the increasing polymer concentration. The polymer concentration also affected the optical properties of the material. When the polymer concentration was too low, focal conic domain size was too big, it did not scatter light strongly, so light optical path inside cell was short, which reduced the dye absorption. When the polymer concentration was increased, the focal conic domain size became smaller and the scattering became stronger, which enhanced dye absorption in the off state and reduce T off. Thus, contrast increased. However, beyond certain value, increased polymer concentration decreased the hysteresis and contrast, but did not change much in drive voltage, as shown in Fig.. This can be explained as the following: when increased polymer concentration too much, polymer network was so strong that it tended to stabilize the homeotropic texture. Therefore, V up also decreased as much as V down ; Thus, hysteresis decreased (Fig. ). Furthermore, due to smaller focal conic domain size, the scattering became weaker; light optical path inside cell decreased, resulting less light absorption. Hence, T off became bigger and decreased, as shown in Fig. (c). There existed an optimized focal conic domain size in the big hysteresis loop, which resulted low T off and high contrast. The effect of the polymer concentration on the drive voltage is shown in Fig.. Drive V oltage (V) hysteresis (V) polymer concentration (%) (c) Fig. Effect of polymer concentration on drive voltage, hysteresis and contrast. The cell thickness was 10 µm. Another important factor was the cholesteric pitch which had a strong effect on the focal conic domain size and changed the optic path length of light inside cell, and thus changed dye absorption. At low chiral concentration, the focal conic domain size was large; the material was not very scattering for visible light and the light absorption was low, and hence contrast was low. As the chiral concentration was increased, the pitch became shorter and domain size decreased, thus light absorption increased; so did the, as shown in Fig.. When the chiral concentration was increased further, the domain size became too small and the material became less scattering, the decreased. The drawback of increasing the chiral concentration was that the driving voltage was almost linearly increased, which was

6 what we expected (Fig. ). () The chiral concentration had an insignificant effect on the hysteresis as shown in Fig. (c). Drive Voltage (v) hysteresis (V) (c ) chiral concentration (%) Fig. Effect of chiral concentration on display performance. The cell thickness was 1 µm. Another important factor was dye concentration. As shown in Fig. 6, when dye concentration was low, the absorption was small, thus T off was high and contrast was low. As dye concentration was increased, the absorption increased and T off decreased; contrast increases. However, when dye concentration was increased further, T on was greatly reduced as well as T off, their ratio (contrast) decreased. Dye concentration approximately had no effect on hysteresis, as shown in Fig. 6(c) because dye did not change polymer network. The driving voltage somehow became higher as dye concentration increased, as shown in Fig. 6. driving voltage (V) hysteresis (V) dye concentration (%) (c) Fig. 6 Effect of dye concentration on display performance. The cell thickness was 1 µm.

7 UV intensity was another factor affecting the display performance. When UV intensity was too low, laterally large but less dense polymer network was formed. The domain size was too big, scattering was weak and contrast was low; when UV intensity was too high, the polymer network became thinner and denser, the domain size was too small and scattering was also weak, contrast was low and drive voltage was high. Because UV intensity affected the polymer network size and density, it also affected the alignment effect of polymer network; therefore hysteresis increased, and then decreased as UV intensity increased, as shown in Fig. 7. dr ive voltag e Contrast Ratio hysteresis (V) UV intensity (arbitrary unit) (c) Fig. 7 Effect of UV intensity on drive voltage, contrast and hysteresis. The cell thickness was 1 µm. Due to the intrinsic hysteresis in the focal conic to homeotropic transition, it is impossible for pure cholesteric material to achieve gray scale performance. However, by using random polymer network formed in the isotropic phase at elevated temperature, we were able to reduce the hysteresis in the focal conic-homeotropic transition and made a hysteresis less display. The voltage-transmittance curve of the display was shown in Fig. 8. Although there was some small hysteresis, gray scale operation became possible transmittance applied voltage Fig. 8 Transmittance vs. applied voltage for hysteresis-less cholesteric dichroic dye display.

8 Conclusion We studied polymer stabilized cholesteric dichroic dye materials for direct view display. We have developed the bistable display and the hysteresis-less display. The bistable material can be used to make high multiplexed dichroic dye displays on passive matrix. The hysteresis-less material can be used to make gray scale dichroic dye displays. The displays have the merits of low driving voltage, wide viewing angle and no polarizer, and can be used for direct view display applications. Acknowledgement This research was supported by NSF under ALCOM grant number DMR Reference 1. B. Bahadur, Dichroic Liquid Crystal Displays in the book Liquid Crystals: Applications and Uses, Vol. 3, ed. by B. Bahadur, (1992) 2. D.K. Yang, L.-C. Chien and J.W.Doane, Cholesteric liquid crystal/polymer dispersion for haze-free light shutters, App. Phys. Lett., 60(2), (1992). 3. Y.K. Fung, D.-K. Yang, Y. Sun, L. C. Chien, S. Zumer and J. W. Doane, "Polymer networks formed in liquid crystals," Liq. Cryst., 19, (199).. R. Su, W. Jang and D.-K. Yang, "Optimization of polymer stabilized cholesteric texture materials for high-brightness projection displays," SID Intl Symp Digest Tech. Papers, XXX,62-6 (1999).. D.-K. Yang, L.-C. Chien and Y.K. Fung, "Polymer stabilized cholesteric textures: materials and application," in Liquid crystals in Complex Geometries Formed by Polymer and Porous Networks, ed. G.P. Crawford and S. Zumer Taylor & Francis, London (1996), Chapter,