- Email: [email protected]
Fundamentals of liquid crystals and other liquid display technologies
Fundamentals of liquid crystals and other liquid display technologies J. A. CASTELLANO
A review of the fundamental concepts associated with passive displays based on liquid crystals and other liquid media is presented. The operation of dynamic scattering and field-effect type liquid crystal displays is discussed in detail. Electrophoretic, electrochromic, rotatable dipole, and liquid vapour displays are also described.
The purpose of this paper is to review the fundamental concepts underlying the operation of passive displays (not light emitting) based on liquid crystals and other liquid media. Since liquid crystal displays are now in commercial use, much of this discussion will deal with descriptions of the various electro-optic effects involving these interesting materials. However, several of the other liquid display technologies, which have emerged in recent years, will also be discussed in some detail. Liquid crystals Liquid crystals were discovered in 1888 by F. Reinitzer and were first classified into various groups around the turn of the century. In the early days, the materials were primarily a laboratory curiosity having little or no known application. During the mid 1960’s, research at a number of industrial, university, and government laboratories began to focus on applications which exploited the electroand/or magneto-optic characteristics of nematic and cholesteric type liquid crystals. One result of this effort was the development of the dynamic scattering display 1 which demonstrated that liquid crystals could indeed be used in practical electronic display devices. Today liquid crystals are used in digital displays for watches, clocks, calculators, and commercial electronic instrumentation. Liquid crystallinity is a unique state of matter intermediate between a crystalline solid and a normal isotropic liquid. The phenomenon is usually exhibited by long rod-shaped organic molecules which contain dipolar groups.2 The mesomorphic state is exhibited over very specific temperature ranges. Three main types of liquid crystal phases have been recognized, and they have been designated as the nematic, smectic, and cholesteric or twisted nematic phase. For the purpose of this discussion, we will confine ourselves mainly to the nematic and ‘twisted’ nematic phases, since most of the current commercial applications use these phases. The author is with PMS Products Operation, Instrument Corp. Forrestal Road, Princeton, Received 29 July 1975.
OPTICS AND LASER TECHNOLOGY.
Fairchild Camera & New Jersey 08540, USA.
DECEMBER
1975
Dynamic scattering mode
In any discussion of electro-optic effects in liquid crystals, one must include a description of the basic fabrication techniques used in the construction of display devices. The basic cell consists of a parallel plate capacitor with liquid crystalline material acting as the dielectric. The plates are merely two pieces of glass, each having a thin conductive coating such as tin or indium oxide. The thickness of active area, which is generally in the range 6-25 /.un, may be controlled by using appropriate polymeric spacers or glass frit materials. Application of a dc or low-frequency ac signal across the plates changes the material into a milky white liquid. This appearance is due to the creation of scattering centres within the liquid and not to a chemical reaction. When the voltage is removed, the panel returns to its transparent state. Although nematic mesomorphism has been observed in a variety of molecular structures, the large majority of compounds that exhibit the phase are aromatic Schiff base derivatives. The discovery that certain of these compounds exhibit dynamic scattering l prompted studies directed toward the synthesis of materials with low melting points. The first example of a single Schiff base compound to exhibit nematic behaviour at ambient temperatures was A, prepared by Kelker and Scherule.3
I
A -
MBBA
(Nematic
range 22-48’C)
By using mixtures of selected Schiff base compounds it was possible to prepare materials which exhibited, for the first time, stable nematic behaviour at ambient temperatures?
259
the dielectric constant along the long axis of the molecule is much greater than that across the width, resulting in a molecular orientation in which the long axes are parallel to the electric field. There are then two possible ways of constructing the electro-optic device. The first approach involves molecular alignment of the nematic material parallel to both electrodes while in the second method molecular alignment on one electrode surface is at 90” to the alignment on the other electrode surface. In both cases molecular alignment is achieved either by unidirectional rubbing of the plates or by angular evaporation of a dielectric film.
b
a
c
Fig.1
Build up of dynamic scattering in an ac driven nematic:
a - below the instability threshold
threshold;
b - at threshold;
c - above
Dynamic scattering is due to electrohydrodynamic instability. The instability can be produced by dc or ac excitation and it is now believed that the dc and ac instability mechanisms are different. The mechanism proposed by Felici 5 is thought to be dominant with dc while the ac regime appears to follow the Carr-Helfrich model.6 Since most applications today involve the ac regime, let us briefly examine the conduction-induced hydrodynamic instability for this case. The presence of space charge is necessary for instability and in present applications the charge carriers are introduced into the liquid crystal in the form of organic ionic ‘dopants’ (eg a quaternary ammonium salt). Due to the anisotropy of the conductivity, the space charge accumulates at distortion maxima when a field is applied. The electric force acting on the space charges induces hydrodynamic flow which couples to the molecular alignment via viscous friction and tends to increase the amplitude of the initial perturbation. This increases with increasing field and when the applied electric field exceeds a threshold value a turbulent instability results (Fig. 1). Although this turbulence is complex and not well understood, it is believed to be associated with the creation of disclination loops.” In current applications of the dynamic scattering mode, digital displays are made by photoetching the usual sevensegment pattern onto one or both of the indium or tin oxide coated surfaces. Reflective displays have one of the surfaces coated with a metal such as aluminium or a dielectric material such as thorium fluoride. Transmissive displays have etched patterns on both surfaces and in some applications auxiliary lighting is provided. The important features of DSM displays are summarized in Table 1 and an example of a dynamic scattering display in an electronic watch is shown in Fig.2. Field effect
Fig.3 illustrates the action of an electric field on cells constructed using these approaches. When the field adds enough electrical energy to exceed the deformation potential of the molecular system (ie when a threshold is reached) the initial ordering becomes distorted and the molecules align with their long axes in the direction of the field. For the first type of molecular alignment (ie alignment parallel to both electrodes), it was found 8,9 that.this cooperative alignment could be used to orient dichroic dye molecules. The optical absorption spectrum of a dichroic dye molecule is a function of its molecular orientation with respect to the polarization of the incident light. Materials that exhibit dichroism are usually long cylindrically shaped molecules containing chromophoric groups which form part of an extended aromatic system. Thus, if the dichroic molecule is oriented with its long axis parallel to the electric vector of the incident polarized light, absorption of light by the molecule (low energy transition) occurs Table
1. Typical
characteristics
of liquid crystal displays Field effect
DSM Mode of operation
Transmissive reflective
Operating
voltage/
12 V/32
or
Transmissive
or
reflective Hz
3 V/32
Hz
frequency 15:l
4O:l
On time (ms)
20
10
Off time (ms)
200
100
0.1
1 o-3
Contrast
ratio
Power consumption (mW cm-*) Temperature
range (“Q-20
Life expectancy
to +60
> 20 000 h
-10
to +70
> 20 000 h
mode
In discussing the field effect modes of operation, we assume that the device has the same parallel plate geometry as described above. We further assume that the dielectric anisotropy of the nematic material is positive (ie ~11- el > 0) and the zero-field alignment is homogenous. In other words,
Digital watches with liquid crystal displays: a - dynamic Flg.2 scattering; b - field effect
260
OPTICS
AND
LASER
TECHNOLOGY.
DECEMBER
1975
adding dyes and/or selecting polarizer materials. These features are summarized in Table 1 and an example of a field effect display in an electronic watch is shown in Fig.2. Other liquid display media
----___-_----------
II
a
In addition to liquid crystal displays, a number of other liquid display technologies have received attention in recent years, including electrophoretics, electrochromics, rotatable dipoles, and liquid-vapour displays. One mmoxe’of these technologies are expected to b-me: ec.onomicalIy important in the next few years.
--
I-
L
1
I
b
Electrophoretic displays
Electric field ordering of an ~1, > EL nematic liquid: Fig.3 a - below the dielectric reorientation threshold; b - above threshold. Spontaneous zero field ordering is homogeneous
and the characteristic colour of the dye is observed. Conversely, orientation of the molecule with its long axis perpendicular to the electric vector results in little or no absorption by the visible transition, and the incident light is transmitted unchanged. The effect of electric fields on mixtures of these dyes with nematic hosts is illustrated in Fig.4. The very strong permanent dipole moment operating along the long molecular axis enables the molecules to align in the direction of the applied electric field and in turn to orientate the dissolved dye molecules with their long axes perpendicular to the electric vector of the incident polarized light. This produces a large decrease in the optical density and hence in the dissappearance of colour. The second type of zero-field molecular alignment was first reported by Schadt and Helfrich.lO They produced the molecular ordering by rubbing the top and bottom plates in orthogonal directions. This caused the ordering direction to twist 90” in going from one electrode to the other (hence the term, ‘twisted nematic’). Application of an externally applied electric field then resulted in an ‘untwisting’ of the structure and an alignment of the molecules in the direction of the field. This electrical ‘untwisting’ can be used to change the direction of polarization of light passing through the cell and thus to produce one of the most useful lowpower electro-optic light valves known. In Fig.5 a ‘twisted-nematic’ cell is shown with a linear polarizer and analyser crossed with respect to their polarization directions. The polarized light entering the cell is thus rotated 90” and passes through the analyser essentially unchanged (there are very small changes in the absorption of light by the cell). When the ‘twist’ is destroyed by application of an electric field across the liquid crystal cell, however, the linearly polarized light is rotated and becomes absorbed by the analyser. If a seven-segment pattern is photoetched into the electrode surfaces, a digital display with dark numbers on a light background results. The contrast of the display depends upon the efficiency and colour of the polarizer and/or analyser. Numeric displays based on the field effect modes possess several important advantages over other electro-optic effects in liquid crystals. For example. the high resistivity of the material used in the displays enables one to have an extremely low power consumption and low operating voltages. In addition, it is possible to use both a transmissive and reflective mode of operation and to obtain various colours by
OPTICS AND LASER TECHNOLOGY.
DECEMBER
1975
Electrophoretic display d&ices use electrophoresis of charged pigment particles in a suspension.’ 1 In this device the suspension, which consists of the charged pigment particles (such as organic dyes or T1 6) suspended in an organic liquid containing various dyes, is sandwiched between a pair of electrodes, one of which is transparent. A dc electric field across the electrodes moves the particles electrophoretically toward either the cathode or anode depending upon the polarity of the charged particles. The electrophoretic migration of the particles changes the reflective colour of the suspension as viewed through the transparent electrode (Fig.6). We assume that the pigment particles are white in colour and positively charged in a black suspending liquid. When a dc voltage is applied between the pair of electrodes in such a way that the transparent electrode is negative (left side), the white pigment particles move electrophoretically toward the transparent electrode. Consequently, the left
5 ~hao;$c domoms
hv-
Pleochrolc dye (guest)
g :: Q l.!G
A
Tronsporenf conductor
hv-
Fig.4 Schematic diagrams of electro-optic electronic colour switching
cells exhibiting
Tm or lndlum oxide
Llnecr polorlzer, 1’
/
Anolyser
‘++@- Llquld crystal Fig.5
Field-induced
phase change in twisted
nematic
liquids
261
Table 2.
Characteristics
of electrophoretic
suspensions
BW-05 White
I
of particle
Type
Block
BY-17
Encapsulated
Hansa
TiOz
yellow
(white)
Average
size (pm)
3.0
0.5
Particle
concentration
2.5%
2.5%
Positive
Negative
Charge
polarity
Solvent
BrF2CCBrF2
BrF,CCBrF,+ Cl*
Dye concentration
FCCF,Cl
1.5%
1.5%
3.5 cp
8.75 cp
2.0
0.16
(black) Fig.6
Electrophoretic
display cell
Viscosity
of
suspension Current
side of the panel becomes white in reflective colour viewed through the transparent electrode. On the other hand, when the transparent electrode is positive (right side), the reflective colour of the right side changes to black because the white pigment particles move opposite to the transparent electrode and are hidden behind the black suspending liquid. Colour combinations of both the pigment particles and the suspending liquid can be used to achieve a desired colour display. A mixture of the colours of the pigment particles and the liquid can be obtained, for example, by applying an ac voltage to the suspension. A reversal between the colours of the displayed pattern and its background can be obtained by changing the polarities of the applied dc voltage, as shown in Fig.6. The EPD panel has a memory function, because the pigment particles deposited on the surface of the electrode remain on the surface even after removal of an applied voltage, mainly due to van der Waals attractive force between the pigment particles and the electrode. This memory function serves to simplify the driving circuit of the panel and to reduce the electric power dissipation. Two suspensions, BW-05 and BY-l 7, were used in the experimental panels described by OTA. Some characteristics of the two suspensions are shown in Table 2. The suspension BW-05 is composed of titanium dioxide particles encapsulated by polyethylene and a suspending liquid. The suspending liquid is composed of a mixture of solvents, mainly tetrafluorodibromoethane (CBrFa-CBrF,), dyes, and charge control agents. It is black and has a density the same as that of the white encapsulated titanium dioxide particles (average diameter 3 pm). The charge polarity of the white encapsulated particle is positive in the black liquid.
density
(PA cm-? at IOOV)
IO
0
30
20 Trne
Fig.7
50
40
(ms)
Response of EPD to a 20 Hz signal at 100 V
I 50
-
40
-
100
v
-
BW-05
----
BY-17
;” : g
30-
; s 20
-
The suspension BY-1 7 is composed of hansa yellow pigment particles (average diameter 0.5 pm) suspended in a black liquid composed of a mixture of tetrafluorodibromoethane, trichlorotrifluoroethane (CC& F-CCIF,), oil, and dyes. The charge polarity of the yellow pigment particle is negative in the black suspending liquid.
Fig.8
The typical response of the panel to the applied ac voltage is shown in Fig.7. In the rise of the logarithmic response curve, there exists a delay of several milliseconds with respect to the applied voltage. This delay seems to be related to the time taken for the particles to migrate across the black liquid layer. The maximum contrast ratio is 23: 1; the rise and fall times are about 20 ms and 10 ms, respectively.
Fig.8 shows contrast ratio versus thickness for BW-05 and BY-17 at 1 Hz. There is a peak in the curve for a given voltage because the opacity of the suspension layer increases and the field strength decreases with increase in the thickness of the suspension layer. The location of the peak shifts to the right side as the voltage increases. The increase in the concentration of pigment particles and dyes, and in
262
OPTICS
0'
I
0
50 Thickness
Contrast
AND
of
,
,
100
150
suspension
layer
2
(pm)
ratio of EPD versus thickness
LASER
TECHNOLOGY.
DECEMBER
1975
0
the frequency of applied voltage, shifts the location peak in the contrast ratio to the left side.
of the
A clock display was fabricated using EPD and the characteristics are shown in Table 3.
Table 3.
displays
The phenomenon of electrochromism consists of changing the light absorption properties of certain materials by an externally applied electric field. Ordinarily, these materials are completely colourless. However, on applying a moderate electric field, the material develops a colour which remains even after the field has been removed. However, when the polarity of the applied field is reversed, the system returns to its original state. Consequently, this basic phenomenon can be used for display either in the transmissive or in the reflective mode, or a combination thereof. Electrochromism can be exhibited by both organic and inorganic systems. In the organic system l2 an aqueous solution of an organic material, such as heptyl viologen-bromide, is sandwiched between a pair of transparent electrodes (Fig.9). On applying a dc voltage greater than the redox potential of this material, it undergoes an oxidation-reduction reaction and the reduced species is electrodeposited on the cathode surface giving rise to a coloured display (usually purple or blue). On reversing the polarity, the coloured compound is oxidized into its colourless form and is redeposited on the anode. This system has memory because the solid deposit remains on the cathode surface even after the applied voltage is removed. The writing time depends on the voltage across the cell, as plotted in Fig. 10. There is a clear threshold at about 0.2 V. To get an absorption of 20% takes about 2 ms; to get an absorption of 80% takes about 20 ms. The speed depends on the current. A current of 100 mA cms2 of electrode surface is required for a writing time of 20 ms; for a writing time of 200 ms the current is 10 mA cmm2. An absorption of 80% means a contrast of 5 to 1. By transferring more charge a contrast of up to 20 to 1 can be obtained. Because the display has memory, it is only necessary to supply voltage when the information is changed. If this is infrequent then the mean power consumption is, of course, low. The peak power requirement for one write-erase cycle is about 100 mW for a character size of 1 cm2 effective area, and for write and erase times of about 20 ms. With this power consumption and the low switching voltage the display is compatible with bipolar and MOS circuits. Multiplexing is in principle possible because the display is sufficiently fast and there is a threshold. A summary of some typical characteristics of electrochromic displays is shown in Table 4. The inorganic systems operate similarly but make use of oxidation-reduction reactions in transition metal oxides, such as tungsten trioxide or sodium tungstate. l 3d4
l07x59mm
Contrast
4O:l
ratio
dipole
displays
OPTICS
AND
LASER
TECHNOLOGY.
DECEMBER
1975
Hz
Power consumption
7.5 mW
Operating
-
temperature
15” to+50°c
range Demonstrated
> 3000
life
h
Non-uniformity
Mode of failure
of coiour
Oxidotlon Elr- -e E+
A’* + e ~A%lue) Insoluble Soluble
Er
Side reactIons A* + e z$ A
Fig.9 display
Mechanism
of operation
+
AIYellow)
A++ -_)
28”
of organic electrochromic
100
Absorption 00% E =
IO
? = 2
60%
40%
20% I 03
I’ 0
I 06
I 08
1 IO
Writing voltage Fig.10 Writing time versus writing voltage for different of light absorption
Table 4.
Typical
characteristics
values
of organic electrochromic
displays
Contrast
Rotatable dipole displays use iodoquinine sulphate (B) (herapathite) crystals suspended in various organic solvent media.’ 5 These crystals act as dipoles in the presence of an electric field and application of a high-frequency signal enables them to align in the direction of the applied field. Since these crystals are dichroic, the difference in optical density between the random and aligned crystals is quite
75VIl
frequency
Operating Rotatable
of an EPD clock display
Size of characters
Voltage Electrochromic
Characteristics
voltage ratio
l-l.5
v
5 : 1 (20 : 1 possible)
On time (ms)
20
Off time (ms)
1 O-50
Power consumption
150 mW cmq2
(during write-erase) Demonstrated
life
10’
write-erase
cycles
263
-
r
CH,
=CH-,
characteristics
of dipole suspension
displays
cs; \/
Table 5. Typical
/H
Material
N
VARADR
100
(herepathite) .3H,SO,
2HI
I4
Operating
6H,O
voltage frequency
35v
ppf3 kl-lz
HO-&H
Contrast
B-
Quinine
iodosulphate
(hera pathite)
ratio
1
Off time (ms)
50
Power consumption
1 mW cme2
Temperature
-40
range
Life expectancy
Table 6.
large. Thus, the electrically activated portions of the cell will be colourless, while the unactivated portions will be dark blue (Fig.1 1). Displays based on this effect therefore have a very high contrast. The main problems, from a device point of view, appear to be a somewhat high voltage requirement and instability of the suspension. Table 5 illustrates the typical display related parameters for this display system. Liquid-vapour
5:l
On time (ms)
displays
Liquid-vapour displays 16 use low boiling organic liquids as the active element. The display cell is constructed by forming transparent electrodes on roughened or ‘frosted glass surfaces and having the liquid, such as trichloroethylene, sandwiched between the two electrodes.
to +8O”C
Unknown
Typical
characteristics
Material
of liquid-vapour
displays
Trichloroethylene
Operating
voltage
Contrast
50 v
ratio
20 : 1
On time (ms)
2
Off time
10
(ms)
Power consumption
2.5 W cm-’
Temperature
-50
range
Life expectancy
to +85”C
Unlimited
When electrical energy is applied to the transparent electrode, it causes sufficient heating to evaporate the liquid in contact with the electrode. Thus, a combination of vapour film and vapour bubbles forms around the roughened surface. Since the refractive index of most vapours is approximately equal to that of air, ie 1.O, the observer sees the light scattering or translucent appearance of roughened glass. The display element is electrically switchable between a black and white condition. Other colours can be obtained by painting the back plate of the liquid cell an appropriate colour or by using liquids containing various dyes. The display requires higher current than some of the other liquid displays (Table 6).
Table 7. Summary
of display
b
a
Fig.1 1
Operation
of a dipole suspension display
parameters
Dynamic Parameter Operating Contrast
scattering
FED
12
voltage ratio
3
15:l
4O:l
Rotatable
Liquid
EPD
ECD
dipole
vapour
75
1.5
35
50
4O:l
5:l
5:l
2O:l
10-j
10-3
150
10-s
2500
Response time (ms)
20
10
10
20
1
2
Matrix
Possible
Yes
?
2
2
Yes
Power consumption
(mW cme2)
capability
1
Possible Colour Memory Life
264
capability (w/chol)
No
Yes
Yes
Yes
Yes
No
Yes
Yes
>3,000
h
lo7
OPTICS
AND
>20,000
h
>20,000
h
cycles
LASER
Possible
Yes
No
Yes
?
Unlimited
TECHNOLOGY.
DECEMBER
1975
Summary
4
A comparison of the display related parameters for each of the display systems discussed above is presented in Table 7. It will be interesting to see which display technique becomes economically important in the years ahead. In any case, I believe passive displays will capture a larger and larger share of the display market over the next several years.
5 6
References Heilmeier, G. H., Barton, L. A., Zanoni, L. A. Proc IEEE 56 (1968) 1162 Gray, C. W. Molecular structure and the properties of liquid crystals (Academic Press, 1962) Kelker, H.. Scherule, R. Chern. 81 (1964) 903
1 2 3
Castellano, J. A., Goldmacher, J. E. US Patent 3 540 796 (1970)
I 8 9 10 11 12 13 14 15 16
Felici, N. Rev (;erz Electric 78 (1969) 717 Cart, E. F. Mel Cryst 7 (1969) 253 ; Helfrich, W. J Chem Phys 51 (1969) 4092 Janning, J. L. Appl PhysLett 21 (1972) 173 Heilmeier, G. H., Castellano, J. A., Zanoni, L. A. Mel Cryst 8 (1969) 293 White, Taylor. JAppl Phys 45 (1974) 4718 Schadt, M., Helfrich, W. Appl Phys Lett 18 (1971) 271 Ota, I., Ohnishi, J., Yoshiyama, M. Proc IEEE 6 1 (1973) 82 Schoot, C. J., et al J Appl Phys 23 (1973) 64 Deb, S. K. Appl Opt SuppI (1969) 192; Proc Roy Sot A304 (1968) 211 Alburger, J. R Electron Znd (February
1957) 50 Marks, A. M. Appl Opt 8 (1969) 1397; Proc Nerem Mtg (Boston, Massachusetts, November 1973) Taylor, G. W. ProcIEEE61 (1973) 148
-SCIENCE& PUBLIC POLICY the international
journal of the Science Policy Foundation
current awareness for busy people in
government @industry : business education research
l
l
SCIENCE 81PUBLIC POLICY .
provides information on national technology and their effects
policies for science and
.
examines the roles of science and technology in the operations of government (local, national and international), industry and business
.
analyses the social and political environments science and technology operate
.
assesses
.
explores various types of public participation and their influence on national and international policies
appropriate methodologies, and organisational forms
within
information
which
Subscription f 25.00 (865.00) per year (12 issues) including postage by fastest route. A special annual rate of f 10.00 (826.00) is available for the individual who certifies that the copies are for his/her personal use and who is the full-time employee of a current full-price establishment subscriber at the same address.
systems
IPC Business Press Ltd. Oakfield House, Perrymount Haywards Heath, Sussex, England
The journal contains short, pithy news items, concise reports, comment, commissioned reviews, facts in figures, book reviews and extensive bibliographies.
OPTICS AND LASER TECilNOLOGY
The information is gathered through a worldwide network of national correspondents who are intimately involved in researching and applying science and public policy in their country.
. DECEMBER
1975
Road,
Published monthly by IPC Science and Technology Press Ltd: publishers of FUTURES, ENERGY POLICYand RESOURCES POLICY.
A review of the fundamental concepts associated with passive displays based on liquid crystals and other liquid media is presented. The operation of dynamic scattering and field-effect type liquid crystal displays is discussed in detail. Electrophoretic, electrochromic, rotatable dipole, and liquid vapour displays are also described.
The purpose of this paper is to review the fundamental concepts underlying the operation of passive displays (not light emitting) based on liquid crystals and other liquid media. Since liquid crystal displays are now in commercial use, much of this discussion will deal with descriptions of the various electro-optic effects involving these interesting materials. However, several of the other liquid display technologies, which have emerged in recent years, will also be discussed in some detail. Liquid crystals Liquid crystals were discovered in 1888 by F. Reinitzer and were first classified into various groups around the turn of the century. In the early days, the materials were primarily a laboratory curiosity having little or no known application. During the mid 1960’s, research at a number of industrial, university, and government laboratories began to focus on applications which exploited the electroand/or magneto-optic characteristics of nematic and cholesteric type liquid crystals. One result of this effort was the development of the dynamic scattering display 1 which demonstrated that liquid crystals could indeed be used in practical electronic display devices. Today liquid crystals are used in digital displays for watches, clocks, calculators, and commercial electronic instrumentation. Liquid crystallinity is a unique state of matter intermediate between a crystalline solid and a normal isotropic liquid. The phenomenon is usually exhibited by long rod-shaped organic molecules which contain dipolar groups.2 The mesomorphic state is exhibited over very specific temperature ranges. Three main types of liquid crystal phases have been recognized, and they have been designated as the nematic, smectic, and cholesteric or twisted nematic phase. For the purpose of this discussion, we will confine ourselves mainly to the nematic and ‘twisted’ nematic phases, since most of the current commercial applications use these phases. The author is with PMS Products Operation, Instrument Corp. Forrestal Road, Princeton, Received 29 July 1975.
OPTICS AND LASER TECHNOLOGY.
Fairchild Camera & New Jersey 08540, USA.
DECEMBER
1975
Dynamic scattering mode
In any discussion of electro-optic effects in liquid crystals, one must include a description of the basic fabrication techniques used in the construction of display devices. The basic cell consists of a parallel plate capacitor with liquid crystalline material acting as the dielectric. The plates are merely two pieces of glass, each having a thin conductive coating such as tin or indium oxide. The thickness of active area, which is generally in the range 6-25 /.un, may be controlled by using appropriate polymeric spacers or glass frit materials. Application of a dc or low-frequency ac signal across the plates changes the material into a milky white liquid. This appearance is due to the creation of scattering centres within the liquid and not to a chemical reaction. When the voltage is removed, the panel returns to its transparent state. Although nematic mesomorphism has been observed in a variety of molecular structures, the large majority of compounds that exhibit the phase are aromatic Schiff base derivatives. The discovery that certain of these compounds exhibit dynamic scattering l prompted studies directed toward the synthesis of materials with low melting points. The first example of a single Schiff base compound to exhibit nematic behaviour at ambient temperatures was A, prepared by Kelker and Scherule.3
I
A -
MBBA
(Nematic
range 22-48’C)
By using mixtures of selected Schiff base compounds it was possible to prepare materials which exhibited, for the first time, stable nematic behaviour at ambient temperatures?
259
the dielectric constant along the long axis of the molecule is much greater than that across the width, resulting in a molecular orientation in which the long axes are parallel to the electric field. There are then two possible ways of constructing the electro-optic device. The first approach involves molecular alignment of the nematic material parallel to both electrodes while in the second method molecular alignment on one electrode surface is at 90” to the alignment on the other electrode surface. In both cases molecular alignment is achieved either by unidirectional rubbing of the plates or by angular evaporation of a dielectric film.
b
a
c
Fig.1
Build up of dynamic scattering in an ac driven nematic:
a - below the instability threshold
threshold;
b - at threshold;
c - above
Dynamic scattering is due to electrohydrodynamic instability. The instability can be produced by dc or ac excitation and it is now believed that the dc and ac instability mechanisms are different. The mechanism proposed by Felici 5 is thought to be dominant with dc while the ac regime appears to follow the Carr-Helfrich model.6 Since most applications today involve the ac regime, let us briefly examine the conduction-induced hydrodynamic instability for this case. The presence of space charge is necessary for instability and in present applications the charge carriers are introduced into the liquid crystal in the form of organic ionic ‘dopants’ (eg a quaternary ammonium salt). Due to the anisotropy of the conductivity, the space charge accumulates at distortion maxima when a field is applied. The electric force acting on the space charges induces hydrodynamic flow which couples to the molecular alignment via viscous friction and tends to increase the amplitude of the initial perturbation. This increases with increasing field and when the applied electric field exceeds a threshold value a turbulent instability results (Fig. 1). Although this turbulence is complex and not well understood, it is believed to be associated with the creation of disclination loops.” In current applications of the dynamic scattering mode, digital displays are made by photoetching the usual sevensegment pattern onto one or both of the indium or tin oxide coated surfaces. Reflective displays have one of the surfaces coated with a metal such as aluminium or a dielectric material such as thorium fluoride. Transmissive displays have etched patterns on both surfaces and in some applications auxiliary lighting is provided. The important features of DSM displays are summarized in Table 1 and an example of a dynamic scattering display in an electronic watch is shown in Fig.2. Field effect
Fig.3 illustrates the action of an electric field on cells constructed using these approaches. When the field adds enough electrical energy to exceed the deformation potential of the molecular system (ie when a threshold is reached) the initial ordering becomes distorted and the molecules align with their long axes in the direction of the field. For the first type of molecular alignment (ie alignment parallel to both electrodes), it was found 8,9 that.this cooperative alignment could be used to orient dichroic dye molecules. The optical absorption spectrum of a dichroic dye molecule is a function of its molecular orientation with respect to the polarization of the incident light. Materials that exhibit dichroism are usually long cylindrically shaped molecules containing chromophoric groups which form part of an extended aromatic system. Thus, if the dichroic molecule is oriented with its long axis parallel to the electric vector of the incident polarized light, absorption of light by the molecule (low energy transition) occurs Table
1. Typical
characteristics
of liquid crystal displays Field effect
DSM Mode of operation
Transmissive reflective
Operating
voltage/
12 V/32
or
Transmissive
or
reflective Hz
3 V/32
Hz
frequency 15:l
4O:l
On time (ms)
20
10
Off time (ms)
200
100
0.1
1 o-3
Contrast
ratio
Power consumption (mW cm-*) Temperature
range (“Q-20
Life expectancy
to +60
> 20 000 h
-10
to +70
> 20 000 h
mode
In discussing the field effect modes of operation, we assume that the device has the same parallel plate geometry as described above. We further assume that the dielectric anisotropy of the nematic material is positive (ie ~11- el > 0) and the zero-field alignment is homogenous. In other words,
Digital watches with liquid crystal displays: a - dynamic Flg.2 scattering; b - field effect
260
OPTICS
AND
LASER
TECHNOLOGY.
DECEMBER
1975
adding dyes and/or selecting polarizer materials. These features are summarized in Table 1 and an example of a field effect display in an electronic watch is shown in Fig.2. Other liquid display media
----___-_----------
II
a
In addition to liquid crystal displays, a number of other liquid display technologies have received attention in recent years, including electrophoretics, electrochromics, rotatable dipoles, and liquid-vapour displays. One mmoxe’of these technologies are expected to b-me: ec.onomicalIy important in the next few years.
--
I-
L
1
I
b
Electrophoretic displays
Electric field ordering of an ~1, > EL nematic liquid: Fig.3 a - below the dielectric reorientation threshold; b - above threshold. Spontaneous zero field ordering is homogeneous
and the characteristic colour of the dye is observed. Conversely, orientation of the molecule with its long axis perpendicular to the electric vector results in little or no absorption by the visible transition, and the incident light is transmitted unchanged. The effect of electric fields on mixtures of these dyes with nematic hosts is illustrated in Fig.4. The very strong permanent dipole moment operating along the long molecular axis enables the molecules to align in the direction of the applied electric field and in turn to orientate the dissolved dye molecules with their long axes perpendicular to the electric vector of the incident polarized light. This produces a large decrease in the optical density and hence in the dissappearance of colour. The second type of zero-field molecular alignment was first reported by Schadt and Helfrich.lO They produced the molecular ordering by rubbing the top and bottom plates in orthogonal directions. This caused the ordering direction to twist 90” in going from one electrode to the other (hence the term, ‘twisted nematic’). Application of an externally applied electric field then resulted in an ‘untwisting’ of the structure and an alignment of the molecules in the direction of the field. This electrical ‘untwisting’ can be used to change the direction of polarization of light passing through the cell and thus to produce one of the most useful lowpower electro-optic light valves known. In Fig.5 a ‘twisted-nematic’ cell is shown with a linear polarizer and analyser crossed with respect to their polarization directions. The polarized light entering the cell is thus rotated 90” and passes through the analyser essentially unchanged (there are very small changes in the absorption of light by the cell). When the ‘twist’ is destroyed by application of an electric field across the liquid crystal cell, however, the linearly polarized light is rotated and becomes absorbed by the analyser. If a seven-segment pattern is photoetched into the electrode surfaces, a digital display with dark numbers on a light background results. The contrast of the display depends upon the efficiency and colour of the polarizer and/or analyser. Numeric displays based on the field effect modes possess several important advantages over other electro-optic effects in liquid crystals. For example. the high resistivity of the material used in the displays enables one to have an extremely low power consumption and low operating voltages. In addition, it is possible to use both a transmissive and reflective mode of operation and to obtain various colours by
OPTICS AND LASER TECHNOLOGY.
DECEMBER
1975
Electrophoretic display d&ices use electrophoresis of charged pigment particles in a suspension.’ 1 In this device the suspension, which consists of the charged pigment particles (such as organic dyes or T1 6) suspended in an organic liquid containing various dyes, is sandwiched between a pair of electrodes, one of which is transparent. A dc electric field across the electrodes moves the particles electrophoretically toward either the cathode or anode depending upon the polarity of the charged particles. The electrophoretic migration of the particles changes the reflective colour of the suspension as viewed through the transparent electrode (Fig.6). We assume that the pigment particles are white in colour and positively charged in a black suspending liquid. When a dc voltage is applied between the pair of electrodes in such a way that the transparent electrode is negative (left side), the white pigment particles move electrophoretically toward the transparent electrode. Consequently, the left
5 ~hao;$c domoms
hv-
Pleochrolc dye (guest)
g :: Q l.!G
A
Tronsporenf conductor
hv-
Fig.4 Schematic diagrams of electro-optic electronic colour switching
cells exhibiting
Tm or lndlum oxide
Llnecr polorlzer, 1’
/
Anolyser
‘++@- Llquld crystal Fig.5
Field-induced
phase change in twisted
nematic
liquids
261
Table 2.
Characteristics
of electrophoretic
suspensions
BW-05 White
I
of particle
Type
Block
BY-17
Encapsulated
Hansa
TiOz
yellow
(white)
Average
size (pm)
3.0
0.5
Particle
concentration
2.5%
2.5%
Positive
Negative
Charge
polarity
Solvent
BrF2CCBrF2
BrF,CCBrF,+ Cl*
Dye concentration
FCCF,Cl
1.5%
1.5%
3.5 cp
8.75 cp
2.0
0.16
(black) Fig.6
Electrophoretic
display cell
Viscosity
of
suspension Current
side of the panel becomes white in reflective colour viewed through the transparent electrode. On the other hand, when the transparent electrode is positive (right side), the reflective colour of the right side changes to black because the white pigment particles move opposite to the transparent electrode and are hidden behind the black suspending liquid. Colour combinations of both the pigment particles and the suspending liquid can be used to achieve a desired colour display. A mixture of the colours of the pigment particles and the liquid can be obtained, for example, by applying an ac voltage to the suspension. A reversal between the colours of the displayed pattern and its background can be obtained by changing the polarities of the applied dc voltage, as shown in Fig.6. The EPD panel has a memory function, because the pigment particles deposited on the surface of the electrode remain on the surface even after removal of an applied voltage, mainly due to van der Waals attractive force between the pigment particles and the electrode. This memory function serves to simplify the driving circuit of the panel and to reduce the electric power dissipation. Two suspensions, BW-05 and BY-l 7, were used in the experimental panels described by OTA. Some characteristics of the two suspensions are shown in Table 2. The suspension BW-05 is composed of titanium dioxide particles encapsulated by polyethylene and a suspending liquid. The suspending liquid is composed of a mixture of solvents, mainly tetrafluorodibromoethane (CBrFa-CBrF,), dyes, and charge control agents. It is black and has a density the same as that of the white encapsulated titanium dioxide particles (average diameter 3 pm). The charge polarity of the white encapsulated particle is positive in the black liquid.
density
(PA cm-? at IOOV)
IO
0
30
20 Trne
Fig.7
50
40
(ms)
Response of EPD to a 20 Hz signal at 100 V
I 50
-
40
-
100
v
-
BW-05
----
BY-17
;” : g
30-
; s 20
-
The suspension BY-1 7 is composed of hansa yellow pigment particles (average diameter 0.5 pm) suspended in a black liquid composed of a mixture of tetrafluorodibromoethane, trichlorotrifluoroethane (CC& F-CCIF,), oil, and dyes. The charge polarity of the yellow pigment particle is negative in the black suspending liquid.
Fig.8
The typical response of the panel to the applied ac voltage is shown in Fig.7. In the rise of the logarithmic response curve, there exists a delay of several milliseconds with respect to the applied voltage. This delay seems to be related to the time taken for the particles to migrate across the black liquid layer. The maximum contrast ratio is 23: 1; the rise and fall times are about 20 ms and 10 ms, respectively.
Fig.8 shows contrast ratio versus thickness for BW-05 and BY-17 at 1 Hz. There is a peak in the curve for a given voltage because the opacity of the suspension layer increases and the field strength decreases with increase in the thickness of the suspension layer. The location of the peak shifts to the right side as the voltage increases. The increase in the concentration of pigment particles and dyes, and in
262
OPTICS
0'
I
0
50 Thickness
Contrast
AND
of
,
,
100
150
suspension
layer
2
(pm)
ratio of EPD versus thickness
LASER
TECHNOLOGY.
DECEMBER
1975
0
the frequency of applied voltage, shifts the location peak in the contrast ratio to the left side.
of the
A clock display was fabricated using EPD and the characteristics are shown in Table 3.
Table 3.
displays
The phenomenon of electrochromism consists of changing the light absorption properties of certain materials by an externally applied electric field. Ordinarily, these materials are completely colourless. However, on applying a moderate electric field, the material develops a colour which remains even after the field has been removed. However, when the polarity of the applied field is reversed, the system returns to its original state. Consequently, this basic phenomenon can be used for display either in the transmissive or in the reflective mode, or a combination thereof. Electrochromism can be exhibited by both organic and inorganic systems. In the organic system l2 an aqueous solution of an organic material, such as heptyl viologen-bromide, is sandwiched between a pair of transparent electrodes (Fig.9). On applying a dc voltage greater than the redox potential of this material, it undergoes an oxidation-reduction reaction and the reduced species is electrodeposited on the cathode surface giving rise to a coloured display (usually purple or blue). On reversing the polarity, the coloured compound is oxidized into its colourless form and is redeposited on the anode. This system has memory because the solid deposit remains on the cathode surface even after the applied voltage is removed. The writing time depends on the voltage across the cell, as plotted in Fig. 10. There is a clear threshold at about 0.2 V. To get an absorption of 20% takes about 2 ms; to get an absorption of 80% takes about 20 ms. The speed depends on the current. A current of 100 mA cms2 of electrode surface is required for a writing time of 20 ms; for a writing time of 200 ms the current is 10 mA cmm2. An absorption of 80% means a contrast of 5 to 1. By transferring more charge a contrast of up to 20 to 1 can be obtained. Because the display has memory, it is only necessary to supply voltage when the information is changed. If this is infrequent then the mean power consumption is, of course, low. The peak power requirement for one write-erase cycle is about 100 mW for a character size of 1 cm2 effective area, and for write and erase times of about 20 ms. With this power consumption and the low switching voltage the display is compatible with bipolar and MOS circuits. Multiplexing is in principle possible because the display is sufficiently fast and there is a threshold. A summary of some typical characteristics of electrochromic displays is shown in Table 4. The inorganic systems operate similarly but make use of oxidation-reduction reactions in transition metal oxides, such as tungsten trioxide or sodium tungstate. l 3d4
l07x59mm
Contrast
4O:l
ratio
dipole
displays
OPTICS
AND
LASER
TECHNOLOGY.
DECEMBER
1975
Hz
Power consumption
7.5 mW
Operating
-
temperature
15” to+50°c
range Demonstrated
> 3000
life
h
Non-uniformity
Mode of failure
of coiour
Oxidotlon Elr- -e E+
A’* + e ~A%lue) Insoluble Soluble
Er
Side reactIons A* + e z$ A
Fig.9 display
Mechanism
of operation
+
AIYellow)
A++ -_)
28”
of organic electrochromic
100
Absorption 00% E =
IO
? = 2
60%
40%
20% I 03
I’ 0
I 06
I 08
1 IO
Writing voltage Fig.10 Writing time versus writing voltage for different of light absorption
Table 4.
Typical
characteristics
values
of organic electrochromic
displays
Contrast
Rotatable dipole displays use iodoquinine sulphate (B) (herapathite) crystals suspended in various organic solvent media.’ 5 These crystals act as dipoles in the presence of an electric field and application of a high-frequency signal enables them to align in the direction of the applied field. Since these crystals are dichroic, the difference in optical density between the random and aligned crystals is quite
75VIl
frequency
Operating Rotatable
of an EPD clock display
Size of characters
Voltage Electrochromic
Characteristics
voltage ratio
l-l.5
v
5 : 1 (20 : 1 possible)
On time (ms)
20
Off time (ms)
1 O-50
Power consumption
150 mW cmq2
(during write-erase) Demonstrated
life
10’
write-erase
cycles
263
-
r
CH,
=CH-,
characteristics
of dipole suspension
displays
cs; \/
Table 5. Typical
/H
Material
N
VARADR
100
(herepathite) .3H,SO,
2HI
I4
Operating
6H,O
voltage frequency
35v
ppf3 kl-lz
HO-&H
Contrast
B-
Quinine
iodosulphate
(hera pathite)
ratio
1
Off time (ms)
50
Power consumption
1 mW cme2
Temperature
-40
range
Life expectancy
Table 6.
large. Thus, the electrically activated portions of the cell will be colourless, while the unactivated portions will be dark blue (Fig.1 1). Displays based on this effect therefore have a very high contrast. The main problems, from a device point of view, appear to be a somewhat high voltage requirement and instability of the suspension. Table 5 illustrates the typical display related parameters for this display system. Liquid-vapour
5:l
On time (ms)
displays
Liquid-vapour displays 16 use low boiling organic liquids as the active element. The display cell is constructed by forming transparent electrodes on roughened or ‘frosted glass surfaces and having the liquid, such as trichloroethylene, sandwiched between the two electrodes.
to +8O”C
Unknown
Typical
characteristics
Material
of liquid-vapour
displays
Trichloroethylene
Operating
voltage
Contrast
50 v
ratio
20 : 1
On time (ms)
2
Off time
10
(ms)
Power consumption
2.5 W cm-’
Temperature
-50
range
Life expectancy
to +85”C
Unlimited
When electrical energy is applied to the transparent electrode, it causes sufficient heating to evaporate the liquid in contact with the electrode. Thus, a combination of vapour film and vapour bubbles forms around the roughened surface. Since the refractive index of most vapours is approximately equal to that of air, ie 1.O, the observer sees the light scattering or translucent appearance of roughened glass. The display element is electrically switchable between a black and white condition. Other colours can be obtained by painting the back plate of the liquid cell an appropriate colour or by using liquids containing various dyes. The display requires higher current than some of the other liquid displays (Table 6).
Table 7. Summary
of display
b
a
Fig.1 1
Operation
of a dipole suspension display
parameters
Dynamic Parameter Operating Contrast
scattering
FED
12
voltage ratio
3
15:l
4O:l
Rotatable
Liquid
EPD
ECD
dipole
vapour
75
1.5
35
50
4O:l
5:l
5:l
2O:l
10-j
10-3
150
10-s
2500
Response time (ms)
20
10
10
20
1
2
Matrix
Possible
Yes
?
2
2
Yes
Power consumption
(mW cme2)
capability
1
Possible Colour Memory Life
264
capability (w/chol)
No
Yes
Yes
Yes
Yes
No
Yes
Yes
>3,000
h
lo7
OPTICS
AND
>20,000
h
>20,000
h
cycles
LASER
Possible
Yes
No
Yes
?
Unlimited
TECHNOLOGY.
DECEMBER
1975
Summary
4
A comparison of the display related parameters for each of the display systems discussed above is presented in Table 7. It will be interesting to see which display technique becomes economically important in the years ahead. In any case, I believe passive displays will capture a larger and larger share of the display market over the next several years.
5 6
References Heilmeier, G. H., Barton, L. A., Zanoni, L. A. Proc IEEE 56 (1968) 1162 Gray, C. W. Molecular structure and the properties of liquid crystals (Academic Press, 1962) Kelker, H.. Scherule, R. Chern. 81 (1964) 903
1 2 3
Castellano, J. A., Goldmacher, J. E. US Patent 3 540 796 (1970)
I 8 9 10 11 12 13 14 15 16
Felici, N. Rev (;erz Electric 78 (1969) 717 Cart, E. F. Mel Cryst 7 (1969) 253 ; Helfrich, W. J Chem Phys 51 (1969) 4092 Janning, J. L. Appl PhysLett 21 (1972) 173 Heilmeier, G. H., Castellano, J. A., Zanoni, L. A. Mel Cryst 8 (1969) 293 White, Taylor. JAppl Phys 45 (1974) 4718 Schadt, M., Helfrich, W. Appl Phys Lett 18 (1971) 271 Ota, I., Ohnishi, J., Yoshiyama, M. Proc IEEE 6 1 (1973) 82 Schoot, C. J., et al J Appl Phys 23 (1973) 64 Deb, S. K. Appl Opt SuppI (1969) 192; Proc Roy Sot A304 (1968) 211 Alburger, J. R Electron Znd (February
1957) 50 Marks, A. M. Appl Opt 8 (1969) 1397; Proc Nerem Mtg (Boston, Massachusetts, November 1973) Taylor, G. W. ProcIEEE61 (1973) 148
-SCIENCE& PUBLIC POLICY the international
journal of the Science Policy Foundation
current awareness for busy people in
government @industry : business education research
l
l
SCIENCE 81PUBLIC POLICY .
provides information on national technology and their effects
policies for science and
.
examines the roles of science and technology in the operations of government (local, national and international), industry and business
.
analyses the social and political environments science and technology operate
.
assesses
.
explores various types of public participation and their influence on national and international policies
appropriate methodologies, and organisational forms
within
information
which
Subscription f 25.00 (865.00) per year (12 issues) including postage by fastest route. A special annual rate of f 10.00 (826.00) is available for the individual who certifies that the copies are for his/her personal use and who is the full-time employee of a current full-price establishment subscriber at the same address.
systems
IPC Business Press Ltd. Oakfield House, Perrymount Haywards Heath, Sussex, England
The journal contains short, pithy news items, concise reports, comment, commissioned reviews, facts in figures, book reviews and extensive bibliographies.
OPTICS AND LASER TECilNOLOGY
The information is gathered through a worldwide network of national correspondents who are intimately involved in researching and applying science and public policy in their country.
. DECEMBER
1975
Road,
Published monthly by IPC Science and Technology Press Ltd: publishers of FUTURES, ENERGY POLICYand RESOURCES POLICY.