Operating characterization of a smectic A LCD

LCD D F ALIEV, V L ARISTOV*, V V MITROKHIN* A N D V P S E V O S T Y A N O V

The properties of smectic A phase liquid crystals used in displays for a wide range of applications have been discussed. Conditions for two optic stable states of smectic A phase material, i.e. focal-conic scattering texture as a result of s o m e electrohydrodynamic instability, and a

homeotropically aligned texture produced during focal-conic to homeotropic transition have been analysed. Electrooptic and dynamic characteristics of LCDs concerning writing and erasing processes h a v e b e e n d e v e l o p e d . R e s e a r c h results have been realized in matrix liquid crystal dhq)]ay developments. Depending on the method of control the trallsparent-dJspersed state transition (or vice venm) can be set s o as to fix the ~ with varying voltage, or to fix the vo]a,ge with varying freq.ancy, within the sound frequency range.

Keywords: smecfic IL'Ds, electrooptic and dynar~dc characteristics

Currently, nematic LCDs should be considered as the most improved high information content display from the point of view of theoretical aspects and applications. Because of relatively cheap manufacturing, and compatibility with LC drivers' ergonomics independence on illumination, this type of device can be used in an ever-widening range of applications. However, further increasing of nematic LCD information content is limited by substantial degradation of ergonomics with a raise in multiplexing level L2 The liquid-crystalline composition physiochemical properties have been elaborated and investigated; a relation between their molecular structure has been studied. All this justifies work to elaborate highinformation matrix display smectic A on their base 3'4. Since bistable optic states may be created in optic uniaxial smectic A liquid crystals by means of external electric fields, it is possible to provide LCD means with long term memory, hemispherical viewing angle, unlimited levels of muitiplexibility, etc 3"4.

Baku State University, 370145 Baku, P Lumumba Str. 23, USSR *Saratov State University, 410026 Astrakhanskaya Str. 83, USSR 86

This paper discusses the main electrooptic and dynamic characteristics of reversible memory of smectic A phase LCs with anisotropic permittivity and conductivity. Ae > 0, Aor=0, respectively, which is a basic design element of matrix LCDs of such a class, used in dialogue video devices and block dynamic information panels. WRITING AND ERASING PROCESS Two stable states, when applying a suitable electric field along the normal to SA layers, are due to different physical mechanisms. The scattering focal-conic texture is formed if the disturbing field has been removed, as a result of onset of electrohydrodynamic instability (erasure). Transmissive homeotropic texture is realized during focal-conic to homeotropic transition in the dielectric mode (writing). The threshold voltage (U~at/U~a"S)and critical frequency fcr restricted both the processes above. For the practical 10 mm smectic A liquid crystal layer (Ae = el] - e] = 12, or ~ 2 x 10-8ohm -1 cm-1, orl/orll = 1.7) the l rscat = 60 V and ~.Jth 1 Itrans = 35 V. thresholds are ~.-~th According to the current respresentation 5 if f > fer, space charge distribution will become constant in time and electrostatic rotation of the director will dominate,

0141-9382/91/020086-05 (~) 1991 Butterworth-HeinemannLtd

DISPLAYS, APRIL 1991

which defines collective process of smectic A liquid crystal molecular reorientation. As shown below, LCD electrooptics depends essentially on competition, as well as relation between threshold and control voltages driving modes of electrohydrodynamic and focal-conic process. Furthermore, the profile of a substrate surface, contracting with SA layers, plays an important role. Regular steplike microrelief formation (the system of electrical conductive square elements) in the experimental samples 8 × 10-Smm area, approximately, and (5-10) x 10-Smm thick (two orders of magnitude higher than SA layers thickness) surely stimulated the onset of some electrohydrodynamic instability. This appeared to be due to SA layer deformation and the onset of some electrohydrodynamic instability centres in the form of defects (edge dislocations) on the substrates-electrode interface. To cover the electrodes by transparent conductive substance (SnO2, INO2) small defects emerge on the surface. This defect causes the formation of an electrodynamic instability. Some investigations have been carried out in this direction 7. The measured rate of some electrohydrodynamic instability spreading (V) versus the applied voltage (U) is expressed by the following approximation: scat n V = A(U-Uth )

where A and n depend on the applied field frequency. To determine n and A, a dependence In v on the In scat (U-Uth) has been drawn up. If f = 40Hz has been obtained, n = 4.3. It should be noted that the similar dependence in the narrow range ( U - U ~ a t ) has been reported earlier 6.

This fact is associated with nonreversibly 'smoothing' of the smectic layer deformation as centres assuring space charge separation, which is necessary for some electrohydrodynamic instability to onset locally. In practice, it means that high voltages used for writing make subsequent erasing and rewriting difficult. In addition, response time decreases with increasing erasure voltage, and it was defined that contrast ratio decreases according to the following relation: K

=

Itrans/Iscat

where Itra, s and Iscat is a laser ray intensity (h = 0.63) measured by photoelectric multiplier and passing through the homeotropic and focal-conic textures, respectively. Figure 1 shows a typical curve. According to the figure at U < 2U, applied voltage does not affect the contrast ratio, but the latter decreases if the former increases. Moreover, when the dynamic scattering state converts the focal-conic scattering texture after the applied voltage has been removed, optical density in the region ol rscat decreases. We believe that LCD becomes of U > z+~Jth clear due to competition of different instability mechanisms. On the other hand, the dielectric moment (Ae x E2) produces molecular homeotropic orientation. On the other hand, Atr enables a space charge to be formed, and thus, destabilizes the homeotropic texture, which is exhibiting at sufficiently high voltage. In order to obtain the image of maximum contrast, there is a possibility to choose a driving voltage from the following range: u I ; a"~ < u < u .

The drastic increase of the U ~ at (1.5-2.0 times) and the formation of metastable scattering textures with lifetime of 1 s, which relax spontaneously into the homeotropic texture after voltage has been removed, were observed at the fixed frequencies, f < fcr, for the substrates specially treated to produce homogeneous surface to eliminate defects formation capability as far as possible. To determine a contrast the light intensity is measured by multiplier disposed behind the display.

CONTROL VOLTAGE FOR WRITING AND ERASING trans If the control voltage for writing !ncreases (U > Uth ), LCD response time decreases, which seems to stimulate the trend to raise U to breakdown fields. However, in expectation of constraints associated with LCD switching, one should consider threshold voltage change of some electrohydrodynamic instability at the high control voltages applied. '3! l t r a n s , the Thus we have found out that at U < .~.ath threshold voltage of some electrohydrodynamic instability practically remains constant, but if U---~Ut~a"s. Uth increases by 2-5 V. The identical situation is observed under the field applied during the period longer than the time the line is being written.

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A P R I L 1991

where a voltage is presented by Ucr, at which a contrast diminishes. Ucr can be changed to vary Atr and A~ according to general opinion Ucr ~ A~r/Ae. According to the general considerations, critical clear voltage Ucr ~ A~r/Ae and results given here specify the possibility of control voltage optimization in the regions U ~ Ucr. "~! l s c a t , t h e optical Next, it was concluded that at U < ~,-~th state density is unchangeable after erasure voltage has been removed. However, at U > 2 U t hscat, optical density of the state into which system relaxes after removing the voltage (focal-conic texture) decreases resulting to further contrast ratio degradation (Figure 1A). For the frequencies of one order with fcr, this characteristic has been seen for the values of U > 1. . qX?scat . . th •

DRIVE M O D E F R E Q U E N C Y CHARACTERISTIC Figure 2 shows threshold voltages of some electrohydrodynamic instabilities and a focal-conic to homeotropic transition versus frequencies. Critical frequency 87

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Figure 1A. Contrast ratio K as function of the applied voltage U 1. f = 160 Hz; 2. f = 80 Hz; 3. f = 40 Hz. Solid lines are for dynamic scattering state, dot lines are for the focal-conic scattering state. Insertion: Contrast ratio ranging during line erasing-writing process by bipolar pulses of the specified frequency and amplitude; A K is a difference of optical density between dynamic scattering texture and focal-conic one.

2s0

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Frequency [Hz)

Figure2A. Effectoffrequencyonthedynamicscattering (1) and focal-conic to homeotropic transition (2) threshold voltages. Insertion: Contrast ratio (K) as a function of the frequency for U < 2~,at: 1. U = 140 V; 2. U = 120 V. Solid lines are for the dynamic scattering state; dot lines are for the focal-conic state.

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Figure lB. The dependence of the passing light intensity on the voltage impressed to the display and increasing and decreasing. A primary texture is homeotropic, a final one is focal-conic. for a given LC composition is specified as f ~- 500 Hz, and at frequencies f > f~r some electrohydrodynamic instability (erasure) does not occur. Writing is performed in the specified frequency region 500 Hz < f < 20 Hz. It is possible to use two methods for addressing LCDs. Writing and erasing at the same frequency make the switching system simpler, but for several reasons, different modes of frequency rewriting are preferred. 88

Figure 2 B. A schema showing a principle of the display separate element: 1. the glasses of current-carrying coats; 2. the smectic layers; 3. the liquid crystalline molecules; 4. the planar texture. 5. the focal-conic texture. The dispersion of light occurs on the separate domain boundaries.

Figure 2 shows that the writing threshold for the frequencies f > f c r in practice does not depend on the frequency and increases slightly at f > for- Moreover, joint microscopic and photometric analysis of homeotropic orientation in smectic LC layers illustrates that at the same value of writing voltage (U Uth) for the frequencies f < for, the realized homeotropic texture does not provide maximum light transmission. In addition, it is necessary to bear in mind that the competition problems concerning hydrodynamic and D I S P L A Y S , A P R I L 1991

dielectric modes become more complicated at frequencies f < fc~, and that the problems can not be solved only by means of LCD design features. Therefore, one should write data at frequency f < fcrThe upper frequency limit, f = 20Hz, is due to substantial increasing of LCD temperature under the prolonged influence of a radio-frequency field which significantly affected its optical characteristics. As for erasure, it may be performed at 0 < f < f¢~, as seen in Figure 2. It should be noted that direct-current voltage also forms some electrodynamic instability and that there is nonuniform contrast of different matrix elements both at direct current and relatively high frequency voltages. The more homogeneous scattering texture for the optical contrast to be provided was observed the frequency f ~ 100 Hz. (Figure 2a). The above frequency functions confirm that the focal-conic texture density decreases in the process of its formation out of some electrohydrodynamic instability, and this effect increases with increasing frequency. Figure 3 shows density variation of light flux passing through a focal-conic and homeotropic textures of smectic A phase LC depending on frequency of the applied field. Effectively writing is not likely to be performed at frequencies f < fc~, which agrees with previous data. Thus, optimal conditions for K are specified by different driving modes for the maximum value of Itrans and the minimum value of Iscat to be provided, respectively. It is feasible to use any driving frequencies and control voltage in the said range according to the results above.

3

1;

LCD DYNAMIC ~ C T E ~ C S The problem of preferred control field characteristics of high information content LCD, in addition to optimal contrast, is closely associated with response time, and should be taken into account when analysing the functions above. In developed LCDs voltagecontrast transconductance (~- 1) for a dielectric process and long-term memory limitation associated with fast scan-multiplexing are practically absent 4"6, and response time is defined by the value of threshold and control voltages for writing and erasing processes. Figure 4a shows response time dependence upon Ttrans voltages during writing. Line address time at U > l~th is less than 50ms; at frequencies f > fcr maximum response times ( ~ 10 ms) were recorded at f >2fcr and U ~ "~uthAlTtrans-- response time dependence upon frequency was not observed in this range. Erasing time dependence upon the voltage at different frequencies is expressed by a family of curves of the same type (Figure 4), with this dependence changed in different ranges of the effective voltage. Keeping in mind frequency dependence of threshold voltage of electrohydrodynamic instability, erasing time for U < 2U~cat is adequately described by the following law "r (U - U~at) -m. The value of m has been obtained from the dependence In a- = - m + In (U - U~cat) in the whole region of the frequencies examined7, while 2 < o! lscat, which is in agreement with m < 5 for U > "-Uth previous data. 8 Figure 4 shows that from the point of view of OlTtrans for response time, the voltage range U > ,.Uth

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DISPLAYS,APRIL1991

~

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Figure 4. The response time versus applied voltage: 1. f = 4 0 H z ; 2. f = 60V; 3. f = 8 0 H z ; 5. f = 200Hz. Insertion: Writing time versus applied voltage: 1. f = 200 Hz; 2. f = lO00 Hz; 3. f = 200 Hz.

89

Table 1. Matrix smectic A LCD performance Display size Matrix Pixel pitch Contrast ratio Viewing angle Writing time/pixel Response time/screen Memory

60 x 130 mm 64 x 128 lines (pixels) 0.4 x 0.4 >20:1 Virtually hemispherical 12ms (120V, 2kHz) 100ms (150V, 40Hz) Several months

image of the high magnitudes of the impressed voltage etc).

Figure 5. Smectic A liquid crystal matrix display sample - the active area fragment.

erasing are of considerable interest. In addition, the acceptable frequency regions are restricted by f > f= and f ,~ fcr conditions, respectively. The synthesis of special smeetie A LC compounds having the threshold voltages',of/eleetrohydrodynamie instability and ifoealconic to homeotropic transition as small as possible is likely to be necessary to decrease the control voltage.

With regard to theoretical interpretation, we must unfortunately confine ourselves to qualitative reasoning, which is due to the absence of theories for predicting a smectic A liquid crystal behaviour at voltages higher than the threshold ones. Thus, in order to compare quantitatively an experience with the theory, we should construct a model to take into account, firstly, the solid boundary conditions of liquid crystals, and secondly, nonlinear terms of the hydrodynamic equation of the liquid crystals.

REFERENCES If these results are compared with those above, it will be seen that the regions of acceptable frequencies and control voltages in which optimal conditions for the contrast and response time are being simultaneously satisfied, are rather small. However, different alternatives depending on optical and ergonomic requirements are possible due to selection of driving mode in consideration of switching limitations. Thus, established regularities enable the optimal driving mode to be defined depending on the functional problems of LCDs being developed. Figure 5 demonstrates the active area fragment of the smectic A LCD sample. Table 1 shows its characteristics; the matrix displays of 300 x 300mm have been elaborated, and the liquid-crystalline composition allowing the control voltage to be reduced has been produced.

CONCLUSIONS An effect used in similar displays has been discovered by the authors almost at the same time as Japanese workers. The principal properties of the effects has been investigated in References 10 and 11. The dependence of the electrodynamic instability threshold voltage and of focal-conic and homeotropic transitions on the cell thickness of U(th) d and on the liquid crystal substance parameters, as U(th), (Ae), Uth (tr) and (e) and U (Ae) satisfies the theory of Guerst-Goussens 5 (to increase a recording threshold voltage with the increasing .of the magnitude and of the longitude of the erasing voltage (or vice versa), to decrease the contrast

90

1 Duchene, J 'Multiplexed liquid crystal matrix displays' Displays Tech. & Applic. (Jan 1986) pp 3-11 2 Shimuzu, K et al. 'Large dot matrix liquid crystal display module' Nat. Tech. Rep. Vol 31 No 2 (Apr 1985) pp 56-63 (in Japanese) 3 Aliev, D F, Bayramov, G M and Mlrlmgirova, G M 'On the physico-chemical properties of the liquidcrystalline composition exhibiting the smectic A phase' Mol. Cryst. Liq. Cryst. Vol 151 (1987) p 385 4

Aiiev, D F and Abbassov, Kh F 'The smectic A liquid crystal electroconductivity anisotropy and its connection to A molecular structure' Mol. Cryst. Liq. Cryst. Vol 151 (1987) p 345

5 Guerst, D and Goussens, W J 'Electro-dynamic instability in smectic A phase of liquid crystals' Appl. Phys. Lett. Vol 41a (1972) p 369 6 Coates, D, Crossland, W A, Morissy, J H and Needarn, B 'Electrically induced scattering in smectic A phase and their electrical reversion' J. Phys. D.: Appl. Phys. Vol 11 (1988) p 208 7 Chirkov, V N; Aliev, D F, Radjabov, G M and Zeinally, A Kh 'Stimulation of the electrohydrodynamic instability in the smectic A phase' Mol. Cryst. Liq. Cryst. Vol 49 (1977) p 293 8 Crossland, W A and Contor, L 'Large flat panel displays using smectic memory LCDs' Electronics Eng. Vol 57 (1985) p 35 Rogerson, S 'The new liquid-crystalline indicator of the STS having the high resolving capability' Electronics Vol 60 No 9 (Apr 1987) p 39

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