Light Absorption Properties of Twisted-Alignment Guest-HostDevices Using Liquid Crystal with Low Refractive Index

Anisotropy

Takaaki Suzuki,1 Hideo Fujikake,2 Takeshi Murashige,1 Hiroto Sato,2 Hiroshi Kikuchi,2 Taiichiro Kurita,2 and Fumio Sato2

1Tokyo University of Science, Tokyo, 162-8610 Japan

2NHK Science and Technical Research Laboratories, Tokyo, 157-8510 Japan

SUMMARY

To reduce the twist angle without satisfying the Mor-gan condition in a twisted-alignment guest-host liquid crys-tal device, the authors fabricated liquid crystal cells havingsmaller optical anisotropy of the liquid crystals and evalu-ated the electrooptic properties. A birefringent phenome-non, not optical rotation, occurred here, and the authorsinvestigated fabrication conditions in which two orthogonalpolarized light components of the incident light were ab-sorbed by a dichroic black dye that was doped into the liquidcrystals. As a result of evaluating the transmittance of theliquid crystal cells, they increased light absorbance andsuppressed the black level according to the selection of aliquid crystal with low refractive index anisotropy and anappropriate twist angle. According to this investigationfrom cell fabrication experiments, the authors clearlyshowed that the twist angle can be reduced, and they foundthat when the twist angle was in the range of up to 180°, aresponse time of 30 ms including both the rise and decaytimes, could be obtained without decreasing the responsespeed. © 2005 Wiley Periodicals, Inc. Electron Comm JpnPt 2, 88(5): 21–28, 2005; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/ecjb.20125

Key words: guest-host liquid crystal; refractive in-dex anisotropy; Morgan condition; optical rotation.

1. Introduction

Liquid crystal displays are increasingly being used asflat panel displays for portable information appliances,computers, and televisions because they are thin, light-weight, and have low power consumption. One of theoperating modes of the liquid crystal cells that constitute adisplay, which does not use polarizers, is the guest-hostmode with a dichroic dye doped within the liquid crystal.In this case, the fact that the light absorption of the dyemolecules (guest) varies with changes in the alignmentdirector of the liquid crystal molecules (host) is used tomodulate the intensity of the transmitted light. As a result,polarizers for lowering the light efficiency below one-halfare unnecessary, and guest-host devices can be expected inreflective displays for advanced portable information appli-ances, which require bright display functions while con-serving electric power. The following two modes are knownas the main guest-host devices that can modulate unpolar-ized incident light including natural light.

One mode takes two dye-doped liquid crystal cellsaligned parallel to the substrate and layers them so that theyare orthogonal to each other [Fig. 1(a)]. This enables two

© 2005 Wiley Periodicals, Inc.

Electronics and Communications in Japan, Part 2, Vol. 88, No. 5, 2005Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J87-C, No. 8, August 2004, pp. 648–654

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incident orthogonal polarized light components to be ab-sorbed. This mode is called the double layer mode [1]because two cells are used. However, this mode has thedisadvantages that two liquid crystal cells are required andthat the manufacturing process for creating panels and thevoltage drive system is complex.

The other mode significantly twists the liquid crystalalignment so that the incident unpolarized light can beabsorbed by using only one liquid crystal cell [Fig. 1(b)].This mode is called the phase-change mode [2] because ituses the phase change from the twisted-alignmentcholesteric phase to the nematic phase when voltage isapplied. However, there are also problems with this modesuch as the relaxation time increases with the twisted align-ment, hysteresis occurs, and grayscale display is difficultdue to bistability. As a result, no practical phase-changemode reflective guest-host devices are currently being used.

The phase-change mode problems described above,which are caused by a large twist angle, occur because theliquid crystal alignment is destabilized. If the twist angle issimply reduced, the twist pitch increases, and the Morgancondition [3], which gives optical rotation for the incidentlight, is satisfied. In this case, since one polarized lightcomponent is transmitted without being absorbed by thedye, the black state transmittance increases. To reduce thetwist angle without increasing the pitch, the liquid crystallayer thickness should be reduced. However, since theabsorption by the dye is reduced if the thickness is reduced,the black state transmittance also increases. This is becausethe thickness of the liquid crystal layer must be greater thanor equal to a certain value to obtain a sufficiently low blacklevel since there is an upper limit to the density of the dyeadded to the liquid crystal.

Using a liquid crystal with low refractive index an-isotropy ∆n so that the Morgan condition is not satisfied isconsidered helpful for overcoming these problems. Also,the effect of refractive index anisotropy on the light absorp-

tion or contrast ratio of guest-host liquid crystals has beeninvestigated up to now according to optical simulations [2].However, it is generally known that if the refractive indexanisotropy is reduced, the alignment order of a liquid crystaldecreases, and the contrast ratio is expected to decrease. Inaddition, the response time of a liquid crystal with low ∆nas well as its transmitting properties have never been inves-tigated. Therefore, the advantages and problems of sup-pressing ∆n must be verified by using actual liquid crystals.

In this research, we fabricated guest-host cells usingseveral liquid crystals having significantly different opticalanisotropy. The results of the liquid crystal materials searchshowed that the contrast ratio could be improved by lower-ing the refractive index anisotropy even if the twist anglewas small [4, 5]. In addition, they clearly showed that noincrease in response time occurred as long as the twist anglewas approximately 180°. In this paper, we describe in detailthe optical absorption theory of the guest-host cells men-tioned above, black state transmittance simulations, andmeasurement results of electrooptic properties.

2. Device Structure and Light AbsorptionTheory

In the cells handled here, a nematic liquid crystaldoped with dichroic black dye is sandwiched betweenalignment layers coated on glass substrates with transparentelectrodes (ITO). The dye that is doped within the liquidcrystal, which has a rodlike molecular structure that issimilar to that of the liquid crystal, is aligned in the samedirection as the liquid crystal. This dye has the property ofabsorbing the polarized light component parallel to the longmajor axis direction of the molecule [6]. This device modu-lates the intensity of transmitted light based on changes inlight absorption due to the dye within the liquid crystal. Inparticular, the twisted alignment is carried out in this caseso that the two incident orthogonal polarized light compo-nents are both absorbed.

Generally, for a twisted-alignment device, if the fol-lowing Morgan condition is satisfied, the two orthogonalpolarized light components that are incident on the deviceare optically rotated along the twisted liquid crystal align-ment

where ∆n, p, and λ are the refractive index anisotropy, twistpitch of the liquid crystal being used, and visible lightregion wavelength, respectively. If the Morgan condition inEq. (1) is not satisfied, the incident orthogonal polarizedlight is not optically rotated and is considerably absorbedsince a polarized light component parallel to the dye mole-cules within the liquid crystal layer arises due to the opticalretardation that accompanies the birefringent effect [Fig.

Fig. 1. Conventional guest-host liquid crystal (LC)devices.

(1)

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2(c)]. Therefore, to cause orthogonal polarized light to beabsorbed, ∆n ⋅ p must be decreased so that Eq. (1) is notsatisfied. Actually, even with the conventional phase-change mode, to make the twisted-alignment pitch psmaller, the alignment is set so that the liquid crystal issignificantly twisted (normally, the twist angle is greaterthan or equal to 360°) and the orthogonal polarized light isabsorbed.

When ∆n ⋅ p is increased so that the Morgan conditionis satisfied, since the polarized light component parallel tothe incident-side liquid crystal molecule among the twoincident polarized light components is optically rotatedalong the liquid crystal twist, it is absorbed by the dye thatis aligned together with the liquid crystal. However, sincethe incident polarized light component that is orthogonal tothe liquid crystal and dye is optically rotated so it remainsorthogonal to the liquid crystal alignment, it penetrates thedevice without being absorbed by the dye [Fig. 2(b)]. Inother words, one of the incident polarized light componentspenetrates without being absorbed in a similar manner asfor an untwisted homogeneous-alignment device [Fig. 2(a)]due to the optical rotary power on orthogonal polarizedlight. As a result, the black display state transmittanceincreases, and the light modulated contrast ratio is consid-erably reduced.

3. Optical Evaluation Method

As the method of evaluating devices, we providedtwo orthogonal polarized light components as incident lightand investigated the intensity of the transmitted light of thedevice since all incident light, including unpolarized naturallight, can be represented as a sum of these two polarizedlight components. In this section, we specifically explainthe effect of the ∆n, cell gap d, and twist angle Φ on thetransmitted light intensity and we minimize the transmittedlight intensity (black level) relative to unpolarized lightaccording to each parameter.

3.1. Optical simulation

First, we obtained the black state transmittance ac-cording to an optical simulation using the Jones vector [7]to check the effect of optical rotation that occurs accompa-nying the Morgan condition. We performed an analysisaccording to the Jones matrix method as follows. Thepolarized state of the incident light is represented by atwo-component vector, and the individual component inwhich that light is propagated is represented by a 2 × 2matrix. In this case, the entire optical system is representedby a matrix multiplication of individual components, and avector representing the polarized light state is obtained bytaking the product of the vector representing the incidentlight and this entire matrix. We assume that this device hasa liquid crystal layer that is twisted by Φ in a thickness ofd. Since this liquid crystal layer can be approximated bystacking uniaxial birefringent plates obtained by dividingthe layer into N parts as shown in Fig. 3 (where N is assumedto be sufficiently large), the entire matrix can be obtainedby multiplying together the Jones matrices of the individualbirefringent plates obtained by shifting the optical axis byθ (= Φ/N) at a time. At this time, the intensity of the lightpenetrating the liquid crystal device is obtained by takinginto consideration the attenuation term due to dye absorp-tion in each individual birefringent plate. The dye attenu-ation constant used in this simulation was obtained fromabsorption measurements of the dye used in later experi-ments.

Figure 4 shows simulation results of transmittance(black level) when the refractive index anisotropy ∆n of theliquid crystal was varied (cell gap: 7 µm; twist angles: 0°,90°, 180°). From this figure, it is apparent that for a devicein which the alignment is not twisted, one polarized lightcomponent is transmitted without being absorbed, and the

Fig. 2. Light absorption mechanism in guest-host liquidcrystal cells with various types of molecular alignments.

Fig. 3. Device model for analysis of light propagationin a twisted liquid crystal layer.

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transmittance is high. It is also apparent that when therefractive index anisotropy ∆n of the liquid crystal is in therange that is normally used (greater than or equal to 0.15),the transmittance of a device that was twisted 90° is ap-proximately the same size as the transmittance of a devicethat is not twisted. This is caused by rotary power since onepolarized light component is transmitted without beingabsorbed. On the other hand, for a liquid crystal with ∆nless than or equal to 0.05, light absorbance increases, opti-cal rotation does not occur, and orthogonal polarized lightis absorbed. In particular, a large absorbance is obtained ina device that was twisted 180°.

Figure 5 shows calculated results of transmittancewhen the liquid crystal layer thickness or, in other words,the cell gap was varied (twist angles: 0°, 90°). The value of∆n was set here to 0.041, which is the same as the value forthe liquid crystal used in the cells fabricated later. From this

figure, it is apparent that when the cell gap was approxi-mately 5 to 7 µm, the transmittance of the twisted-align-ment device twisted 90° and of the homogeneous-alignment device differed significantly, and the low ∆neffect was significant. The balance between the change indye absorption accompanying a change in the cell gap andthe Morgan condition effect is considered to be the maincause for this.

Figure 6 shows the transmittance when the twistangle was varied (cell gap: 7 µm; ∆n: 0.041). From thisfigure it is apparent that absorption increases significantlyfor twist angles up to approximately 180° and that theabsorption changes less for greater twist angles. The or-thogonal polarized light is considered to be absorbed almostuniformly in this kind of saturated region.

3.2. Electrooptic properties

In the liquid crystal cell used to verify the effectsdescribed above, nematic liquid crystals doped with a di-chroic dye (NKX-1366 of HBCL Corporation) weremolecularly aligned according to a rubbed alignment layer(AL-1254 of JSR Corporation; pretilt angle less than orequal to 2°). The dye density was set to 3 wt%, which isclose to the critical concentration. Also, the cell gap was setto 7 µm for which the simulation results showed a largedifference in transmittance between the twisted-alignmentand homogeneous-alignment devices.

The liquid crystals used at this time were JB-1010(Chisso Corporation; ∆n: 0.168), which has a normal ∆nvalue, JB-1027 (Chisso Corporation; ∆n: 0.1), which has arelatively low ∆n value, and JB-1018 (Chisso Corporation;∆n: 0.041), which has an extremely low ∆n value. Whenexamining guest-host mode liquid crystals, the dye align-ment order is an important parameter. Therefore, we meas-

Fig. 4. Effect of the refractive index anisotropy on theblack state transmittance (simulation results).

Fig. 5. Effect of the cell gap on the black statetransmittance (simulation results).

Fig. 6. Effect of the twist angle on the black statetransmittance (simulation results).

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ured the dichroic ratio of each fabricated cell to obtain theiralignment orders [8]. The results indicated that the align-ment orders of JB-1010, JB-1027, and JB-1018 are 0.69,0.77, and 0.56, respectively.

For the fabricated cells, we used unpolarized whitelight (visible light extracted from Xe lamp light by using anoptical filter) as incident light, applied an alternating cur-rent voltage (square wave with 100 Hz frequency), andmeasured the change in transmitted light intensity by usinga photodiode. Figure 7 shows the measurement resultswhen the three types of liquid crystals having different ∆nvalues were used and the twist angle was varied. Note thata chiral dopant (CM-33) was added at 0.7 wt% to the cellsthat were twisted 180° so that no backward twisting oc-curred. Also, 100% in Fig. 7 represents the light intensitywhen no cell is inserted.

For the homogeneous cells shown in Fig. 7(a), theblack level is high because one polarized light componentis transmitted without being absorbed. Also, for the liquidcrystal cell having the lowest ∆n value, the black state whenno voltage is applied or the white state when voltage isapplied (in other words, the bright state when liquid crystaland dye molecules are perpendicular to the substrate) is lowbecause the liquid crystal alignment order is low and thelight absorption anisotropy decreases due to the thermalagitation of the dye molecules.

For the cells twisted 90° in Fig. 7(b), the black levelof the cell decreases when the liquid crystal has the lowest∆n value. However, for the liquid crystals having higher ∆nvalues, practically no change occurs in the black levelbecause one polarized light component is transmitted dueto rotary polarization. For the liquid crystal having thelowest ∆n value, since the ∆n ⋅ p value of the cells twistedby 90° and 180°, which are 1148 and 574 nm, respectively,are approximately the same order of magnitude as thevisible light wavelength (approximately 400 to 800 nm),the Morgan condition is not sufficiently satisfied. There-fore, the black level is reduced more for the cells twisted90° in Fig. 7(b) than for the homogeneous-alignment cellsand orthogonal polarized light absorption of the cells startsto occur.

For the cells twisted 180° in Fig. 7(c), the black levelof the cell is significantly reduced when the liquid crystalhas the lowest ∆n value, and black state absorption in-creases compared with the 90° twisted cell even for theliquid crystal having the intermediate ∆n value. However,the black state absorption barely changes at all for the liquidcrystal having the normal ∆n value. From these facts, itseems that the Morgan condition is not sufficiently satisfiedeven for the liquid crystal having the intermediate ∆n value,and orthogonal polarized light starts to be absorbed.

Figure 8 shows measurement results of the black stateand white state transmittance when the twist angle wasvaried in liquid crystals having the lowest ∆n value and the

normal ∆n value. For the cell having the lowest ∆n value,the black level transmittance was reduced more for greatertwist angles in a similar manner as in the optical simulation.However, for the cell having the normal ∆n value, thechange in the black state transmittance was small evenwhen the twist angle was varied.

For the liquid crystal having the lowest ∆n value, thewhite state transmittance is also low since the alignment

Fig. 7. Changes in electrooptic properties for varioustwist angles and ∆n.

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order is low. However, the black state light absorption wasincreased significantly by twisting the alignment 180°. Asa result, the contrast ratio (white level/black level) increasedto 3.7 compared with the 1.8 for the liquid crystal havingthe normal ∆n value (Fig. 9). Also, for the liquid crystalhaving the intermediate ∆n value, since the alignment orderdoes not decrease, the white level drop is minimized, and ablack level suppression effect is obtained at a twist angle of180°. In other words, increasing the twist increased thecontrast ratio from 1.6 to 2.3. We believe that by introducinga material having a large alignment order and a low ∆nvalue, the contrast improvement can be further increasedwhile preserving the white level.

Figure 10 shows measurement results of the responsetime of each cell using liquid crystal having a low ∆n value.For a device in which the alignment is twisted, the totalresponse time, which includes the rise and decay, is ap-proximately 30 ms. From Fig. 10, it is apparent that whenthis liquid crystal is used and the twist angle is in theexperimental range of up to 180°, no significant change inresponse time occurs. In other words, it is clear that no dropin response time occurs as in conventional supertwistednematic liquid crystal cells.

For a device twisted 180°, liquid crystal domainshaving different twist angles occurred and the liquid crystalalignment was destabilized when the voltage was lowered.Figure 11 shows a polarizing microscope image of this.There exists a domain pattern in which the liquid crystalalignment was twisted 180° (it appears dark due to stronglight absorption) and a domain having homogeneous align-ment with no twisting (it appears bright due to light trans-mittance). Introducing a high pretilt alignment film isknown to be effective for improving this [8].

Fig. 8. Changes in transmittance for various twistangles.

Fig. 9. Changes in contrast ratio for various twistangles.

Fig. 10. Changes in response time for various twistangles.

Fig. 11. Polarizing microscope image of liquid crystaldomains.

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4. Conclusions

We used liquid crystals having low refractive indexanisotropy to fabricate twisted-alignment guest-host liquidcrystal devices. Various liquid crystals were used to verifythat by not satisfying the Morgan condition, light absorp-tion was increased in the black state. As a result, it wasconfirmed that there exist useful liquid crystal materials forthis guest-host mode. We also clearly showed that if thetwist angle is in the experimental range of up to 180°, a goodresponse of 30 ms is obtained without any significantchange.

These investigations enabled us to obtain develop-ment guidelines for liquid crystal materials in twisted-alignment guest-host liquid crystal cells. In other words, aliquid crystal material must be found for which the polari-zation anisotropy in the optical frequency range is small andthe alignment order is high for the molecule itself or, inother words, the dye absorption anisotropy does not de-crease even if ∆n decreases.

In our future work, we will optimize the maximumtransmittance and contrast ratio by varying fabricationconditions such as the cell gap or twist angle of theguest-host liquid crystal device and further reduce thetwist angle and cell gap by applying this mode to reflec-tive devices.

REFERENCES

1. Uchida T, Seki H, Shishido C, Wada M. Bright di-chroic guest-host LCDs without a polarizer. Proc SID1981;22:41–46.

2. White DL, Taylor GN. New absorptive mode reflec-tive liquid-crystal display device. J Appl Phys1974;45:4718–4723.

3. Gooch CH, Tarry HA. The optical properties oftwisted nematic liquid crystal structures with twistedangles < 90°. J Phys D 1975;8:1575–1584.

4. Suzuki T, Fujikake H, Sato H, Kikuchi H, Kurita T,Sato F. Improvements of light absorption propertiesof guest-host liquid crystals with twisted alignment.Extended Abstracts (64th Autumn Meeting, 2004);Japan Society of Applied Physics, 30p-L-12/III,1171, 2003. (in Japanese)

5. Suzuki T, Fujikake H, Murashige T, Sato H, KikuchiH, Kurita T. Guest-host devices with twisted align-ment having low refractive index anisotropy. ITETechnical Report 2003;27:13–16. (in Japanese)

6. Heilmeier GH, Zanoni LA. Guest-host interactions innematic liquid crystals. A new electro-optic effect.Appl Phys Lett 1968;13:91–92.

7. Yoshida M, Ozaki M. Ekisho to display Ouyou NoKiso, 91-94, Koronasha; 1994. p 91–94. (in Japanese)

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AUTHORS (from left to right)

Takaaki Suzuki graduated from Tokyo University of Science (applied physics) in 2003 and is now enrolled in the master’scourse. He is engaged in research on liquid crystal devices.

Hideo Fujikake (member) received his M.E. and Ph.D. degrees from Tohoku University in 1985 and 2003. In 1985, hejoined the Nagano Broadcasting Station of NHK as a broadcasting engineer. Since 1988, he has been with the Science andTechnical Research Laboratories of NHK. He has engaged in research on liquid crystal device and materials, TV programproduction application of liquid crystals, and optical information processing.

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AUTHORS (continued) (from left to right)

Takeshi Murashige graduated from Tokyo University of Science (applied physics) in 2001 and completed the doctoralprogram in 2004. Since then, he has been a postdoctoral fellow at the Science and Technical Research Laboratories of NHK,engaged in research on liquid crystal materials and devices.

Hiroto Sato (member) received his B.E. and M.E. degrees from Chiba University in 1997 and 1999 and joined the JapanBroadcasting Corporation (NHK). He has been with the Science and Technical Research Laboratories of NHK, engaged inresearch on liquid crystal display devices.

Hiroshi Kikuchi (member) received his B.E. and M.E. degrees from Toyohashi University of Technology in 1982 and1984 and joined NHK, where he worked as a broadcasting engineer at the Kobe Broadcasting Station. Since 1987, he has beenwith the Science and Technical Research Laboratories of NHK, engaged in research on electrooptic devices, such as spatiallight modulators, optical bistable devices using liquid crystals and semiconductor lasers, and projection displays.

Taiichiro Kurita completed his M.E. and Ph.D. degrees at Keio Gijuku University in 1980 and 1991. In 1980, he joinedNHK, and worked as a broadcasting engineer at the Nagano Broadcasting Station. In 1982, he began working at the NHKScience and Technical Research Laboratories, performing research on television systems and signal processing of movingpictures (including HDTV, EDTV, etc.) and on display methods and picture quality of PDPs and LCDs. Beginning in 1993, healso held a visiting associate professorship at the University of Electro-Communications until 2000.

Fumio Sato graduated from the Faculty of Engineering, Tohoku University, in 1972 majoring in metal materials,completed the doctoral program in 1977, and joined NHK in 1978. Since then, he has pursued research on semiconductor crystalgrowth, X-ray analysis, process technology, imaging devices, and cold cathode materials.

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