Fieldinduced deformation of hybridaligned nematic liquid crystals: New multicolorliquid crystal displayShoichi Matsumoto, Masahiro Kawamoto, and Kiyoshi Mizunoya Citation: Journal of Applied Physics 47, 3842 (1976); doi: 10.1063/1.323245 View online: http://dx.doi.org/10.1063/1.323245 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/47/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Viewing-angle controllable liquid crystal display using a fringe- and vertical-field driven hybrid aligned nematicliquid crystal Appl. Phys. Lett. 92, 261102 (2008); 10.1063/1.2953456 Application of nanoparticle-induced vertical alignment in hybrid-aligned nematic liquid crystal cell Appl. Phys. Lett. 91, 141103 (2007); 10.1063/1.2794007 Light propagation and transmission in hybrid-aligned nematic liquid crystal cells: Geometrical optics calculations Appl. Phys. Lett. 89, 091912 (2006); 10.1063/1.2345042 Dual-frequency addressed hybrid-aligned nematic liquid crystal Appl. Phys. Lett. 85, 3354 (2004); 10.1063/1.1809282 New multicolor liquid crystal displays that use a twisted nematic electrooptical cell J. Appl. Phys. 44, 4799 (1973); 10.1063/1.1662046

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Field-induced deformation of hybrid-aligned nematic liquid crystals: New multicolor liquid crystal display

Shoichi Matsumoto, Masahiro Kawamoto, and Kiyoshi Mizunoya

Toshiba Research and Development Center. Tokyo Shibaura Electric Company. Limited. Komukai Toshiba- cho. Saiwai-ku. Kawasaki-city 210. Japan (Received 12 April 1976)

Electric field-induced deformation is studied in a unique hybrid-aligned nematic (HAN) cell, in which the director of a nematic liquid crystal (LC) with either positive or negative dielectric anisotropy is varied continuously. lying perpendicular to one substrate and parallel to the other. The experimental results show the absence of threshold field in the' birefringence-vs-applied voltage relationship as well as the slow variation of birefringence with voltage; these phenomena are analyzed in terms of the Frank-Oseen continuum theory. Also, the above characteristics of HAN are demonstrated to have such advantages in multicolor LC display applications as low operating voltage and fairly good color separation together with uniform and bright color generation.

PACS numbers: 81.55. + x, 85.60.Pg, 78.20.Fm

Not only basic but also practical studiesl-

7 have been extensively made on the so-called electrically tunable birefringence of uniformly aligned nematic liquid crys­tals (LC's), which is caused by electric field-induced deformation of the LC's due to the dielectric anisotropy. The effect has aroused a great deal of interest from the viewpoint of multicolor LC display applications.

For such variable color displays, mostly perpendic­ularl

-3 and planar6

,7 nematic cells have been used; in the former cell the director of a negative dielectric­anisotropic LC is aligned uniformly perpendicular to the substrates, whereas in the latter the director of a posi­tive dielectric-anisotropic LC is aligned uniformly parallel to the substrates. These perpendicular and planar cells, however, have certain limitations restrict­ing the practical device applications. i. e .. relatively high operating voltage, compared with the twisted ne­matic devices currently being used, poor color separa­tion, and lack of generated color uniformity and brightness.

The cell dealt with in the present paper is a unique hybrid-aligned nematic (HAN) cell. Arranging the cell between polarizers, the electric field-induced deforma­tion is studied and, from the experimental and theoret­ical results, the electro-optical effect of HAN is shown to be much more applicable to multicolor LC display than those of the above-mentioned perpendicular and planar cases. The term HAN is used here to describe such an aligned nematic LC whose director varies con­tinuously, lying perpendicular to one substrate and par­allel to the other. A few recent articles8

,9 have alluded only briefly to such aligned LC's, but have discussed neither field-induced deformation of HAN nor multi­color display based on the deformation. 10

The HAN cell used in the experiments was fabricated as follows: Two types of nematic LC's with positive and negative dielectric anisotropy were, respectively, sand­wiched in a thin layer between two parallel indium oxide-coated glass plates with a 5 x 3 cm2 area, sepa­rated by 12- and 17-f.,I.m-thick spacers; one of the sub­strates was treated with a monocarboxylatochromium complex [tetrachloro- j..l-hydroxo- f.,I.-myristatodichromi­um (lIn] to produce a perpendicular boundary surface, 11

3842 Journal of Applied Physics, Vol. 47. No.9, September 1976

while the other was unidirectionally rubbed with a cot­ton swab to produce a parallel boundary surface. 12

Some other monocarboxylatochromium complexes11 and the rubbing of dicarboxylatochromium complex-coated13

and carbon-evaporated14 surfaces were also available for these orienting treatments, respectively. Typically, a mixture of 20 wt% P-butoxybenzylidene-p-cyanoani­line (BBCA) in p-methoxybenzylidene-p-butylaniline (MBBA) and MBBA were employed as the dielectrical­ly positive- and negative-type LC's, respectively. The HAN alignment achieved and the uniformity over all of the cell area were confirmed with a polarizing micro­scope orthoscopically a s well as conoscopically.

A collimated He-Ne laser beam (6328 A) with a 2-mm diameter was normally incident upon the sample cell located between crossed linear polarizers. whose polar­ization axes were fixed at 45° relative to the direction of rubbing. The transmitted light with electric-field excitation was detected with a photomultiplier on the axis of incidence, whose output was fed to an X - Y re­corder. All measurements were performed with ac sine­wave (500 Hz) application and at 25 DC.

2 c ::>

" .0 " I1I1 0 'II,

:1:: f- :::\ z 1111 0 t,'1 I (f) I111 I (f) ,II I

~ 1111

(f) I" I z J\:: I <t - --- I I 0:: I I f- I

I I

" • 0 2

I I

I I I

I I I

I , I

'J

I I

I

I I

I I I

I

4 6

VOLTAGE V (V,msl

8 10

FIG. 1. Transmission of 632. 8-nm light vs applied voltage for hybrid-aligned nematic cell (solid curve) and planar-aligned nematic cell (dashed curve)_ Liquid crystal: BBCA (20 wt%) in MBBA. Cell thickness: 121-'m. Temperature: 25°C. voltage: ac sine wave at 500 Hz. Crossed polarizers.

Copyright © 1976 American Institute of Physics 3842

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VOLTAGE V {V,msl

3000°,-,--,4_,-_8r--,~12_,-_1,6_r-~20

::t.. E

2000 cr z 0

f-<! 0 cr <! f-w 1000 cr

, + : ,

VOLTAGE V (V,msl

FIG. 2, Retardation for 632.S-nm light vs applied voltage for hybrid-aligned nematic cells of dielectrically positive-type (solid curve; open circles) and negative-type (solid curve; solid circles) liquid crystals (LC's) and planar-aligned (dashed curve; open circles, the same positive-type LC used) and per­pendicular-aligned (dashed curve; solid circles, the same neg­ative-type LC used) nematic cells, Positive-type LC: BBCA (20 wt%) in MBBA. Negative-type LC: MBBA. Cell thickness: 12 !-lm for the hybrid cell of positive-type LC and the planar cell; 17 !-lm for the hybrid cell of negative-type LC and the perpendicular cell. Temperature: 25°C. Voltage: ac sine wave at 500 Hz.

Figure 1 presents the transmission variation with applied voltage for the HAN cell of a positive-type LC (solid curve), together with that for the planar cell employing the same positive LC and also with the same cell thickness (dashed curve). For the planar cell, there exists a definite threshold at about 1. 3 V rms below which the LC alignment is unaffected. However, for the HAN cell, there no longer exists such a threshold field. In addition to this difference, the transmission curve for the HAN cell runs through fewer maxima and minima, compared with that for the planar cell. There­fore, for the HAN cell, the voltage difference between neighboring maxima and minima becomes much larger. Qualitatively, the same results were obtained with the HAN cell of a negative-type LC, comparing the trans­mission-vs-voltage curves for the HAN cell and the per­pendicular cell consisting of the same negative LC and with the same cell thickness.

To describe more clearly the aforementioned elec­tro-optical characteristics of HAN based on the field­induced birefringence change, Fig. 2 shows the retarda­tion R-vs-applied voltage relationship for the HAN cells of both positive- and negative-type LC's (solid curves), together with that for the planar and perpen­dicular cells' (dashed curves), where R = tl.nd and tl.n and d are the birefringence of the LC and the cell thick­ness, respectively. The plotted values of R in Fig. 2

3843 J. Appl. Phys., Vol. 47, No.9, September 1976

were determined from the transmission-vs-voltage re lation described in the preceding section (see Fig. 1), using the relation Io::sin2 (1TR/>"), where>.. and I are the wavelength of incident light (6328 A) and the transmit­ted intensity. Figure 2 indicates definitely that, in the case of HAN, there exists no threshold in the electric field-induced birefringence change, regardless of the sign of dielectric anisotropy of the LC and that the variation of birefringence with voltage is much slower than for the planar and perpendicular cases. It should also be noted that the values of birefringence of the HAN cells at zero field are nearly half that of the un­excited planar cell.

The above resulting phenomena of HAN may be an­alyzed in terms of the Frank-Oseen continuum theory of LC's. 15 The HAN cell has independently parallel and perpendicular boundary conditions at the substrates; taking the z axis to be perpendicular to the substrates and defining the angle e between the nematic director a and this axis, a varies continuously, lying at e = 0 on one substrate and at e = t1T to the other, as shown schematically in Fig, 3. The equilibrium alignment in zero field [Fig, 3 (a)] can be determined by minimizing the elastic free energy Fo with respect to 80 under the boundary conditions that 80 = 0 and t1T at z = 0 and d, respectively, where

Fo=tld(kl1sin280+k33cOS2eo)(aaezoY di, (1)

eo=e in zero field, d is the cell thickness, and kl1 and k33 are the Frank splay and bend elastic constants. This leads to

(2)

where E(K) and E(e 0' K) are the complete and incomplete second-kind elliptic integrals, respectively, and K = 1

Np Nn ---r--- Z = 0

E E

---===---z = d

Ib) (a) Ie)

FIG. 3. Schematic representation of the deformation 6 in the hybrid-aligned nematic liquid crystal (LC) cell having perpen­dicular and parallel boundary conditions at z = 0 and z = d (d is cell thickness), respectively, the z axis being taken normal to the substrates. The rod lines show the alignment of the direc­tor a at various points in the cell. 60 and 1> indicate the elastic and dielectric deformations. An electric field E is applied along z. (a) Quiescent initial state (zero field) in both cases of dielectrically positive I.N~ and negative I.Nn) LC's; (b) field-on state in the case of Np ; and (c) field-on state in the case of Nn •

Matsumoto, Kawamoto, and Mizunoya 3843

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on B c " .ci 0

f-

Z 0 (fJ (fJ

~ (fJ z « 0:: f-

0

R G

2 3

VOLTAGE V (V,ms)

4 5

FIG. 4. Three-color characteristic transmission of the hy­br.id-align~d ne.matic cell as a function of applied voltage; ob­tamed by filtenng the transmitted light of incident white light independently through red (R), green (G), and blue (B) color filters. Liquid cyrstal: BBCA (20 wt%) in MBBA. Cell thick­ness: 8 /lm. Temperature: 25°C. Voltage: ac sine wave at 500 Hz. Dominant wavelength of filters: A(R) = 632.7 nm, A(G) = 554. 6 nm, A(B) =452.0 nm. Crossed polarizers.

- k ll /k33 • Equation (2) indicates that the HAN alignment in zero field depends upon only the ratio of the elastic constants. Here, making the simplification kll = k33' which is not unreasonable, 16 Eq. (2) gives the relation

(3)

When an electric field E along the z axis is applied to the HAN cell, the dielectric deformation </>, in addition to the initial elastic deformation eo, may be so induced that the directors tend to rotate such that ailE and alE in accordance with positive (Np) and negative (Nn ) LC cases, as shown schematically in Figs. 3(b) and 3(c), respectively. Therefore, the total free energy F in the presence of E may be written

F=~ Iad[k(~; + 2~r - (EII-E1)E2

COS2(1>+ ;~)JdZ.

(4)

where the Simplification kll = k33 (= k) has again been made, the relation given by Eq. (3) has been used, and Ell and El are the dielectric constants parallel and per­pendicular to the director. The dielectric deformation can be determined from Eq. (4), using the prinCiple of energy minimization. Here, if a small deformation with E is assumed for simplicity, then the deformation is given by

(5)

Equation (5) implies that there is no threshold field because </> a:. E2. This theoretical prediction is in agree­ment with the experimental results summarized in Fig. 2 that, in the HAN cases, there exists no threshold in the electric field-induced birefringence change.

The retardation R of nematic LC cells is generally expressed by17

R- f d n.nodz - 0 (n~cos2e +n~sin2e)172 -nod,

(6)

3844 J. Appl. Phys., Vol. 47, No.9, September 1976

where no and no are the extraordinary and ordinary refractive indexes, e is the local angle between the director and the propagating direction of incident light (the z axis), and d is the cell thickness. Therefore, R

is (ne - lZo)d for the unexcited planar cell in which the director is aligned uniformly normal to the z axis, while. substituting Eq. (3) into Eq. (6), R of the HAN cell in zero field [Fig. 3(a)1 becomes approximately

R = nod[2K( 1 - n~/ n~)/ 'IT - 11. (7)

where K(1 - n~ n;) is the complete first-kind elliptic in­tegral. Thus, the ratio of the R values for the HAN cell to the planar cell in zero field is estimated to be 0.481, using the refractive ovalues for MBBA (n.= 1. 743; no = 1. 563 at A = 6328 A).18 This is consistent with the observed results (see Fig. 2) that the HAN cells at zero voltage have nearly half the birefringence of the unex­cited planar cell, which definitely indicates that the HAN cells fabricated for the present experiments have the director alignment shown schematically in Fig. 3(a). In addition. this, together with the absence of a thres­hold field, may explain the slower variation of birefrin­gence with voltage observed for the HAN cells (see Fig. 2).

The foregOing unique electro-optical characteristics of HAN may bring forth some very favorable features for multicolor LC display device applications. To dem­onstrate this typically, Fig. 4 presents a three-color characteristic transmission of the HAN cell as a func­tion of applied voltage. These data were taken by placing a 8-iJ,m-thick HAN cell employing the positive-type LC (20 wt% BBCA in MBBA) betwen crossed polarizers and filtering the transmitted light of normally incident white light independently through red, green, and blue color filters with the dominant wavelengths of 632.7, 554.6, and 452.0 nm, respectively. As seen from Fig. 4. the remarkably low voltage required to transmit the various colors is one of the practically favorable features and the second favorable feature is the re lative ly large volt­age difference between the transmission peaks of dif­ferent colors, which is in practice important for the various colors to be well separated.

With the same cell setup as used in the experiment of Fig. 4 but without color filter, we could actually ob­serve that the generated colors with voltage were green at 0.4 V rm,' blue at 0.8 Vr'll" red at 1. 2 Vrm" and yellow at 1. 5 V rm,' Also, these observed colors and their brightness were highly uniform over all of the cell area (5 x 3 cm2

). This uniformity can be attributed to a spatially unidirectional director rotation with voltage provided by the HAN configuration. Qualitatively, the same results were obtained with different cell thick­nesses, other positive-type LC's, and also negative­type LC's.

In summary, electric field-induced deformation has been both experimentally and theoretically studied in a unique hybrid-aligned nematic (HAN) liquid crystal (LC) cell, which has independently perpendicular and parallel boundary conditions at the substrates. The HAN cell, whether the used LC has positive or negative dielectric anisotropy, exhibits no threshold in the field­induced birefringence change, the slow variation of

Matsumoto, Kawamoto, and Mizunoya 3844

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birefringence with voltage, and the spatially unidirec­tional director rotation with voltage. These unique electro-optical characteristics of HAN bring forth such favorable features for multicolor LC display device ap­plications as remarkably low operating voltage, fairly good color separation, and highly uniform and bright color generation, compared to the previously described perpendicular- and planar-aligned nematic LC cases.

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3845 J. Appl. Phys .• Vol. 47, No.9, September 1976

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Matsumoto, Kawamoto, and Mizunoya 3845

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