Supertwisted nematic liquid crystal displays

Supertwisted nematic liquid crystal displays E P RAYNES AND C M WATERS*

The rapid development of supertwisted nematic liquid crystal displays since their invention in 1 M has opened up a wide range of potential applications for complex high-informationcontent liquid crystal displays. Monochrome, A4 sized, 640 x 400 pixel supertwistod nematic displays are now avanahle commercially with a performance superior to twisted nematic displays. Development of the supertwisted nematic display is reviewed and the physical principles involved in its operation are described.

geymorcls: display devices, liquid crystal displays, supertwisted nernatic

Complex liquid crystal displays (LCDs) with a high information content have many potential applications in a wide range of information technology products, such as personal computers, where their combination of low power, low voltage and large area makes them potentially very attractive. However, such complex displays require multiplex drive to reduce the number of connections, and this severely reduces the optical appearance of the twisted nematic (TN) LCD. This poor performance has limited the development and acceptance of innovative products based on high information content TN LCDs. Although attempts at improvements have been made, mainly through developments in LC materials, the performance of current TN displays with multiplex ratios of 100:1 still falls far short of the specifications of contrast and viewing angle required. As the scope for further improvements in LC materials for TN displays has decreased, novel displays based on the recently discovered supertwisted nematic (STN) device1 have been developed. These STN LCDs use currently available LC materials together with established LCD fabrication and electronic drive technologies to better advantage in complex displays, and are a direct substitute for complex TN LCDs but with a much improved performance.

Royal Signals and Radar Establishment, Malvern, Worcestershire WR14 3PS, UK *Thorn-EMI Central Research Laboratories, Dawley Road, Hayes, Middlesex UB3 1HH, UK DISPLAYS,APRIL 1987

DEVELOPMENT OF SUPERTWISTED N E M A T I C DISPLAYS The reasons for the limited performance of TN LCDs lie in the root mean square (r.m.s.) multiplexing scheme necessary for addressing complex TN LCDs. Each row in a matrix is selected sequentially while appropriate data waveforms are applied to the columns, and the slow response times of LCDs (~100 ms) means that each display pixel responds to the r.m.s, of the resulting waveforms. As the number of lines (n) in the matrix increases, the fraction (l/n) of the total time for which the selected pixels see the full select pulse decreases, thereby reducing the ratio of the r.m.s, voltages seen by the selected (on) and unselected (off) pixels. Alt and Pleshko2 showed that the maximum ratio of select to unselect voltages is given by von.,

1

and for a multiplexing ratio of 100:1, the effective select voltage is only 11% higher than the voltage on unselected pixels. The multiplexing ratio achievable (the value of n) in an LCD is therefore determined by the steepness of the voltage dependence of the transmission of the device, with any angular dependence of the contrast degrading the effective performance even further. The importance of both the voltage and angular dependence of contrast can be combined by considering the voltage dependence of Om, the mid-plane tilt angle of the LC director, shown in Figure 1. The LC director

0141--9382/87/020059--05 $03.00© 1987Butterworth& Co (Publishers)Ltd

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that the STN device could be operated in either a guesthost dyed mode or in a double polarizer mode. The STN device was first announced publicly in 19833 using a dyed display operating in the guest-host mode. In 1984, detailed calculations4 were presented on the influence of the various device and material parameters on the director response, which showed how it was possible to vary these parameters to produce the infinitely steep voltage dependence of 0m desirable for r.m.s. multiplexing. These calculations also showed that an infinitely steep response could still be achieved for STN LCDs with twist angles as low as 180", provided the material parameters were chosen carefully.

m

hOm

f

l Figure 1. Liquid crystal layer showing Or,, the mid-plane director tilt angle responds to an applied voltage most easily in the middle of the layer, and 0m represents the maximum value of the director tilt angle. A small change of 0, with voltage results in poor multiplexing performance with a very restricted viewing cone; conversely a large change of 0m with voltage will produce a high contrast over a wide viewing cone. TN LCDs, with their twist angle of 90", show a change of 0m with voltage that is only quite gradual (Figure 2), resulting in their poor optical contrast and limited angle of view when driven using multiplexing ratios up to 100:1. In 1982, the situation was changed dramatically by the invention of the supertwisted nematic (STN) deviceI. It was shown that by changing the twist angle (tO) of the LC layer, the steepness of the voltage dependence of 0m could be increased significantly and made infinite for tO,~ 270* (Figure 2), thereby producing the optimum condition for r.m.s, multiplexing. It was also recognized 90

o~ "13

30 180 ° 225 °

90 °

1.0

/ f_/S 1.5

2.0

270 °

315 °

i

J

2.5

3.0

Voltage, V

Figure 2. Voltage dependence of Orein TN and STN layers with various twist angles 60

Since 1984, many LCD manufacturers have developed displays using the supertwisted nematic device. These displays are referred to by a number of names and abbreviations: STN, SBE, 3n/2 and HBE (highly twisted birefringence effect)6. In the following section we examine the physics underlying supertwisted nematic displays.

PHYSICS OF SUPERTWISTED NEMATIC DISPLAYS We can conveniently divide the physics of supertwisted nematic displays into two parts: the director response and the optics.

Director r e s p o n s e in S T N d i s p l a y s

60

0

Shortly afterwards, a 270" STN LCD operating in the double polarizer mode, called the supertwisted birefringence effect (SBE), was demonstrated5. Contrast was optimized by using particular combinations of LC birefringence (An), device thickness (d) and polarizer orientations.

The director response can be calculated by minimizing the free energy of the system in the presence of an applied voltage. As discussed above it is usually quite adequate to consider only the voltage dependence of 0m, the midplane tilt angle of the director. Using the numerical modelling techniques pioneered by Berreman7, a full solution of the voltage dependence of either the total director response or just 0. can be computed; Figure 2 is an example of the use of this technique. However, the physics underlying the director response can be more easily understood from an analytical solution developed recently by Rayness, who extended the equations describing the voltage dependence of 0m just above the threshold voltage Vo of the TN LCD with tO = 90* to STN LCDs with 180* < tO < 360*. These show that V~

+"" DISPLAYS, APRIL 1987

Iqevim.u where A is a complex combination of material and device parameters 8 and only the first-order term is important just above Vc. Using typical material parameters to calculate A we find that: 2 + 3(9/It)2[I._V_s_~/ O~ -

4

7 -

4(q,/n)~jt

vo J

+""

For the small twist angles (9 = 90*) found in TN LCDs, there is a finite slope of the voltage dependence of 0m; this slope can be increased slightly by a suitable choice of material parameters. However, as the twist angle 9 is increased above 90", the coefficient of the first-order term increases rapidly, becoming infinite and eventually negative for twist angles approaching 9 ,~270". An infinite slope corresponds to the optimum condition for r.m.s, multiplexing. Figure 2 shows the exact computation of the voltage dependence of 0m with the same feature of increasing slope for larger twist angles. For a multiplex ratio of 100:1 (corresponding to a voltage select ratio of 1.11), the change of 0m for the select and unselect voltages for the TN layer (9 = 90*) is always small, whereas for the STN layer with q~ = 270* there is a large change of 0m from ~ 5* to ,~ 75*. The precise value of tO for infinite slope depends on the magnitude of the various material parameters, but for most known LC materials it lies in the range 180° < 9 < 360°. For the curves in Figure 2, the material parameters were chosen to give infinite slope for an STN device with cp ~ 270°; the details of this optimization procedure can be found in References 4 and 8. The portion of the curve with negative slope for the layer with 9 = 315° in Figure 2 is not observed in practice; instead, the unstable re-entrant region is replaced by a hysteresis loop and the device becomes bistable. These higher-twist, bistable devices formed the basis of earlier work by Heffner and Berreman 9 and require novel multiplex drive schemes 1° which are incompatible with the r.m.s, drive systems developed for multiplexed TN LCDs. Neither the bistable displays nor the drive schemes have yet been developed commercially.

Optics of S T N

displays

The discontinuous change in 0 m with voltage in S T N devices can be exploited opticallyby operating the device in either a guest-host mode with a dye or in a double polarizer mode. Optimization of the guest-host mode has been discussed by Waters ct al.4 and requires the combination of high-order-parameter dyes with a suitable range of L C birefringence (An) for use with either a single polarizer or no polarizer in the White-Taylor mode. Although potentiaUy of great interest, the dye modes have been largelysuperseded by the two-polarizer SBE mode, which has, at present, the advantages of higher contrast ratio and fasterresponse times, the latter DISPLAYS, A P R I L 1987

being a consequence of the lower viscosity of the materials available for this mode. As with the TN LCD, the two polarizers in an STN LCD can be arranged to give either a positive contrast mode with a dark 'on' state and a bright 'off" state, or a negative contrast mode obtained by rotating one of the polarizers through 90*. The larger twist angle of STN displays introduces a wavelength dependence, and imparts a characteristic colour to the display. The positive contrast mode has black pixels written on a bright yellow background and is often referred to as the 'yellow mode'; the negative contrast mode has white pixels written on a blue background and is known as the 'blue mode'. Full modelling of the optical properties of the 'on' and the 'off' states of STN devices has been carried out by Scheffer and Nehring 1' using the principles reviewed by Berreman 7, and these computations show how the contrast may be optimized by a suitable choice of the orientations of the polarizer and analyser. They also show how the optical properties of the 'off' state may be optimized by a careful choice of (An.d), the product of birefringence and LC layer spacing. This optimization of the 'off" state can most easily be understood from analytical solutions of the transmission of twisted layers derived recently by Raynes 12. When the polarizer is oriented at an angle of + 45* to the input director and the analyser at an angle of +45* to the exit director, then the transmission (7) of a twisted LC layer is given by'2: T = cos2• 4(9/n)2 + (An.d/~.) 2 and maximum transmission (T = 1) occurs when (An.d/~) = 4 p 2 - (9/n) 2

where p is an integer. This expression for maximum transmission is identical to that found by Gooch and Tarry '3 for the case of guiding through twisted LC layers in TN LCDs. The optimum values of An.d and the transmission spectra and characteristic colour of STN displays are readily derived from these equations '2. LC materials for STN LCDs are now available which optimize both the director response and the optical properties. Figure 3 shows the electro-optic response j4 of a mixture which has been optimized for 270" STN LCDs and shows the ideal case of small hysteresis together with a conveniently low threshold voltage.

SUPERTW~TED DEVICES

NEMATIC

DISPI~Y

The physics outlined above shows that, in principle, the large discontinuity in em found in S T N L C D s can allow 61

I

I

I

A

=>

I I I I

Vunsel

4.,

I

C 0

I I P I-

I

0 0

J 1

2

3

4

Voltage, V

Figure 3. Electro-optic response of an S T N display operating in the yellow, two-polarizer mode; the LC used is mixture 14954 from BDH Ltd, and the two voltages indicated correspond to a mutiplexing ratio of 100:1; Vsd/V,,,~el = 1.11

virtually infinite r.m.s, multiplexing of these displays; however several practical limitations apply. The dependence of the electro-optic response on layer spacing and temperature3'4 restricts the multiplexing ratio that can be achieved. Also, STN LCDs become progressively slower as the multiplexing ratio is increased and the select and unselect voltages approach the device threshold voltage ~s. The other manufacturing limitation is related to the twist angle 9. STN devices with lower twist angles in the range 180" < ¢ < 225 ° are more compatible with existing TN production technology, but show a marginally reduced performance; however, new production techniques are currently under development which should allow the superior performance 270" devices to be manufactured in the near future. All these practical limitations have restricted the multiplexing ratio in available STN LCDs to 100:1 or, in a few cases, to 200:1, although manufacturers are optimistic about achieving a further factor-of-two improvement in the multiplexing ratio ~6. Several recent publications ~7have described the development of complex STN LCDs, and A4 sized STN LCDs with 640 x 400 pixels (a multiplexing ratio of 200:1) are already becoming commercially available. Figure 4 shows an example of a complex STN display.

Figure 4. Example of a complex STN display using HBE technology (Courtesy Hitachi Electronic Components (UK) Ltd) 62

DISPLAYS, APRIL 1987

I=levteuJ CONCLUSIONS The rapid development of STN LCDs since their invention in 1982 has already reached the stage where they appear set to replace the TN LCD in complex, highly multiplexed applications. Although STN displays may not be suitable for more demanding applications such as colour TV, they do seem likely to become estabfished as a major, monochrome, A4 sized, 640 x 400 pixel fiat screen display technology over the next few years. Their superior performance looks certain to increase significantly the acceptance of complex highinformation-content LCDs for a wide range of applications.

REFEREHC'ZS 1 Waters, C M and Rayaes, E P Liquid crystal devices UK Patent GB 2123163 B (1982) 2 AIt, P M and Pleshko, P 'Scanning limitations of liquid crystal displays' IEEE Trans. Electron Devices Vol ED-21 (1974) p 146 3 Waters, C M, BrimmeH, V and Raynes, E P 'Highly multiplexed dyed liquid crystal displays' Proc. 3rd Int. Display Res. Conf. Kobe, Japan (1983) p 39.6; also 'Highly multiplexable dyed LCDs' Proc. SID Vol 25/4 (1984) p 261

4 Waters, C M, Raynes, E P and BrimmeH,V 'Material design for highly multiplexed liquid crystal dye displays' Proc. lOth Int. Liquid Crystal Conf. York, UK (1984) p G10; also 'Design of highly multiplexed liquid crystal dye displays' Mol. Cryst. Liq. Cryst. Vol 123 (1985) p 303 5 Scheffer, T J and Ndhring, J 'A new, highly multiplexable liquid crystal display' Appl. Phys. Lett. Vol 45 (1984) p 1021

6 Kinugawa, K, Kando, Y, Kanasaki, M, Kawakami, H and Kaneko, E '640 x 400 pixel LCD using highly

DISPLAYS,APRIL 1987

twisted birefringence effect with low pretilt angle' SID Digest (1986) p 122 7 Berremaa, D W 'Numerical modelling of twisted nematic devices' Phil. Trans. Roy. Soc. Vol A309 (1983) p 203 8 Raynes, E P 'The theory of supertwist transitions' MoL Cryst. Liq. Cryst. Lett. Vol 4 (1986) p 1 9 Heffner, W R and Berreman, D W 'Switching characteristics of a bistable cholesteric twist cell' J. Appl. Phys. Vol 53 (1982) p 8599 10 Seheffer, T J 'Liquid crystal display with high multiplex rate and wide viewing angle' Proc. 3rd Int. Display Res. Conf. Kobe, Japan (1983) p 400 11 Seheffer, T J and Nehring, J 'Investigation of the electro-optic properties of 270" chiral nematic layers in the birefringence mode' J. Appl. Phys. Vol 58 (1985) p 3022 12 Raymes, E P 'The optical properties of supertwisted liquid crystal layers' Mol. Cryst. Liq. Cryst. Lett. (In press) 13 Gooch, C H and Tarry, H A 'The optical properties of twisted nematic liquid crystal structures with twist angles ~< 90 °, J. Phys. D: AppL Phys. Vol 8 (1975) p 1575 14 Gass, P A Private communication 15 Seheffer, T J, Nehring, J, Kaufmnn, M, Amstutz, H, Heimgartner, D and Eglin, P '24 x 80 character LCD panel using the supertwisted birefringence effect' SID 85 Digest (1985) p 120 16 Kaneko, E 'Directly addressed matrix liquid crystal display panel with high information content' Mol. Cryst. Liq. Cryst. Vol 139 (1986) p 81 17 Proc. 6th Int. Display Res. Conf. Tokyo, Japan (1986) pp 384-403; also Saito, S, Kamihara, M and Kobayashi, S 'Influence on the hysteresis effect of various parameters in supertwisted nematic liquid crystals' Mol. Cryst. Liq. Cryst. Vol 139 p 171

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