Keyword(s): Abstract:

Electrophoretic liquid crystal displays: How far are we?

Susanne Klein

HP LaboratoriesHPL-2013-23

Liquid crystal; electrophoresis; display

We are in the middle of another revolution in information processing and distribution. After the inventionof script and then print, pictures are taking over again. The number of gadgets with displays is larger thanthe world population. On average every single person on earth owns at least one device with a display on it.These displays are the portals to everything we do: communicating, reading, shopping, learning etc. But arethey up to the job? The most successful display technology to date is still the liquid crystal display. It is awell established technology which can be scaled in size from small to large and, depending on the driveelectronics, has no significant inherent limits to resolution. However, poor light efficiency and datacongestion caused by high pixel densities are still technical challenges in need of good solutions. Couldelectrophoresis in a liquid crystal host be the answer? The fundamental basics and technical challenges ofthis interesting new approach will be discussed.

External Posting Date: March 21, 2013 [Fulltext] Approved for External PublicationInternal Posting Date: March 21, 2013 [Fulltext]Additonal Publication Information: Published in Liquid Crystals Reviews.

Copyright 2013 Liquid Crystals Reviews.

Electrophoretic liquid crystal displays: How far are we? Susanne Klein

HP Labs, Long Down Avenue, Bristol BS34 8QZ, UK

Susanne.Klein@hp.com

We are in the middle of another revolution in information processing and distribution. After the

invention of script and then print, pictures are taking over again. The number of gadgets with

displays is larger than the world population. On average every single person on earth owns at least

one device with a display on it. These displays are the portals to everything we do: communicating,

reading, shopping, learning etc. But are they up to the job? The most successful display technology

to date is still the liquid crystal display. It is a well established technology which can be scaled in size

from small to large and, depending on the drive electronics, has no significant inherent limits to

resolution. However, poor light efficiency and data congestion caused by high pixel densities are still

technical challenges in need of good solutions. Could electrophoresis in a liquid crystal host be the

answer? The fundamental basics and technical challenges of this interesting new approach will be

discussed.

Keywords: Liquid crystal, electrophoresis, display

1. Introduction In the developed world, print on paper has largely become a thing of the past. The main platform for

information distribution has become electronic. The portal is a display. Most information is now

consumed on the move. The requirements for a display on a handheld device are stringent:

lightweight, flat, robust, readable in all lighting conditions and low power consumption. The two

most successful technologies for handheld displays are the backlit, active matrix liquid crystal display

(LCD) and for reading applications the electrophoretic display (EPD). The LCD has the advantage of

having a high (video) optical response speed, full colour, a slim profile and it is relatively cost

efficient since by now it is a mature technology, but like all emissive technologies which are

commercially available, like OLED or Plasma, it is hard to read when used outdoors or in other high

brightness environments. Reflective displays, like electrophoretic displays, can be read in all kinds of

ambient light, from dim to bright sunlight. They do not rely on an inbuilt light source but use the

ambient light to display the content, similar to paper. Electrophoretic displays are the most paper

like displays, but compared to LCDs they are slow to write. Could the combination of electrophoresis

and liquid crystal be the answer to the quest for a flat-panel display readable in all conditions?

Maybe, but there are still major technological challenges to overcome which are addressed in this

paper.

2. Classic liquid crystal display The classic liquid crystal display found in many handheld devices like mobile phones, laptops etc.,

but also used for computer displays and televisions, is based on the ability of a birefringent material,

the liquid crystal, to change the polarization of light. A standard liquid crystal display consists of a

number of layers as can be seen in Figure 1.

Polarisers

Substrates

Colour filters

Alignment layers

Spacer

Liquid CrystalElectrode

Figure 1: The layout of a commercially available liquid crystal display. The polarizers in combination with the

liquid crystal form the light switch. The alignment layers guarantee a uniform orientation of the liquid crystal

and therefore reliable switching. Colour is generated by opening or closing the light valves in front of the

colour filters. Not shown are the pixel borders which would surround the unit shown in the figure.

Its active part is the combination of the polarizers and the liquid crystal which form a light valve.

Light passing the first polariser is polarized, i.e. only one vibration mode is allowed through.

Depending on the orientation of the liquid crystal molecules which react to an applied electric field,

this mode stays untouched or is changed. The second polarizer or analyser blocks or transmits the

light depending on its state of polarization as can be seen in both parts of Figure 2.

Colour is generated by opening or closing the light valve on top of Red, Green, and Blue (RGB) colour

filters. The unit depicted in Figure 1 is called a pixel. The pixel size is dependent upon the application.

The higher the resolution (i.e. the higher the number of pixels), the smaller the pixel size. There is no

real physical constraint on the size of the pixel. The major constraint is the data flow and, coupled

with it, the refresh rate. The more the product of the number of pixels multiplied by the refresh rate,

the more data that has to be channelled to each line in the display. It is easy to visualize how the bit

rate increases considerably for a high resolution display with a 100 Hz refresh rate which is typical of

liquid crystal displays available today.

Unpolarized light

Polarizer

Analyzer

Orientation of the liquid crystal director

2a

Polarizer

Analyzer

Unpolarized light

Orientation of the liquid crystal director

2b

Figure 2: Operational principal of a liquid crystal light valve: whether light is transmitted or not depends on the

orientation of the liquid crystal molecules and the resulting birefringence. The first polarizer transmits only

one out of many possible polarizations which are always perpendicular to the beam direction. In Figure 2a the

liquid crystal molecules are oriented in such a way that incident light experiences a birefringent material and

its polarization direction is rotated until it is parallel to the transmission direction of the second polarizer. In

Figure 2b the liquid crystal is re-oriented by an electric field. It acts now as a single refractive index material

and leaves the light polarization untouched. The untouched polarization is blocked by the second polarizer. No

light is transmitted.

Even though the liquid crystal display is extremely successful, one of its major drawbacks is light

efficiency. Only roughly 12% of the light incident onto the display is transmitted when the display

appears white; that is, when all light valves are open. Since polarizers not only polarize but also

absorb, only 40% of the incident light passes the first polarizer. After the RGB colour filters about

15% of the incident light remains, depending on the bandwidth of the filters. The second polarizer

then absorbs a further 20% of the transmitted light which produces the figure of 12% of the total

transmission. The consequence is that images are only visible when the display is illuminated by a

very strong backlight. It is not possible to use this kind of display in reflective mode. The intensity of

ambient light is too low to show more than a dark grey image (see Figure 3). On the other hand, light

transmitted is so weak that liquid crystal displays perform poorly in outdoor conditions. On a sunny

day, a full colour liquid crystal display will appear dark to the observer.

Figure 3: A classic liquid crystal display in its white state. The display is not lit by a backlight but viewed in

reflection.

A major step to improve the light efficiency is the removal of polarizers and colour filters. In this

case, the transmitted intensity would increase from 12% to at least 70% (taking pixel borders and

losses at the interfaces into account) which, in the reflection mode, amounts to ~ 49%. Compared to

white paper with ~ 40% reflectance [1] it would lead to a paper-like display which could be used with

ambient light as a light source.

3. Electrophoresis When two substances come into contact, the electrical charges arrange themselves at the interface

to minimize free energy, the internal energy of the system. For colloidal particles in solution, the

result is a so-called double-layer which consists of charges on the particles and balancing charges in

the surrounding solution. A schematic diagram of this double layer is shown in Figure 4 which

follows the Stern model [2]. For a more extensive explanation and different models for the double-

layer see [2]. The thickness of the charge ‘cloud’ around the particles is described by the so-called

Debye length [3] given by:

N

i

ii

Br

nze

Tk

1

22

01 (1)

Where:

0 r is the dielectric permittivity of the dispersion medium,

Bk is the Boltzman constant

T is the absolute temperature in Kelvin

N is the number of different ionic species in the system

e is the elementary charge

zi is the charge number or valance of the ith ionic species ni is the mean concentration of charges of species i.

Figure 4: The re-arrangement of charges at the interface between a particle and its surrounding solution leads

to a charge cloud or double-layer at the interface. In the Stern model this cloud is approximated initially by a

tightly bound layer of charges, the so called Stern layer, followed by a diffuse layer. Both the Stern layer and

the diffuse layer are needed to compensate for the charge at the particle’s surface. When the particle is set in

motion the diffuse layer will shear off at the Stern plane. The result is a charged particle.

For solutions with high ionic strength and thus high charge concentration, such as water, the Debye

length is on the scale of nanometres. For non-ionic fluids such as dodecane, having a very low charge

concentrations, this length can be on the order of micrometres because the mean charge

concentration decreases rapidly for low 0 r .

The diffuse layer will shear off at the Stern plane when the particle is set in motion. When

movement is caused by an electric field, the positively charged particle will drift towards the

negative electrode and the negative ions from the diffuse layer will drift towards the positive

electrode. The combination of particle movement and ion movement is called electrophoresis. The

particle velocity v is proportional to the applied field strength E given by:

Ev (2)

Where µ is referred to as the electrophoretic mobility of the particle in

. The equation

describes experimental observations that the particle movement is a function of field direction and

field strength. In the case of a positively charged particle (like in Figure 5) the particle motion is in

the field direction, in the case of a negatively charged particle its motion it against the field direction.

The particle velocity is independent of the sample thickness but directly proportional to the

electrophoretic mobility. In its simplest form electrophoretic mobility µ is given by the Smoluchowski

formula [4] as:

0r

(3)

Where:

0 r is the dielectric permittivity of the dispersion medium,

is the viscosity of the dispersion medium and

is the zeta potential defined as the potential at the shear plane, that is the plane where the liquid

velocity relative to the particle is zero.

Figure 5: Under the influence of an electric field the diffuse layer will shear off the Stern layer and move in the

opposite direction of the particle.

The Smoluchowski formula holds true for particles where the Debye length is small compared to the

particle dimensions, that is, either for suspensions with a high ionic strength, for example water, or

for large particles. In case of a sphere, this can be written as 1a where a is the radius of the

sphere. In the case of very small particles or thick double-layers, that is for 1a , the

electrophoretic mobility is given by the Hückel formula [5]:

3

2 0r

(4)

The implication of this formula is that if the same particle was transferred from a dispersion with

high ionic strength to one with low ionic strength and yet the same field was applied, it could only

reach two-thirds of the velocity reached in the polar environment.

These are the two extreme cases for single particles or highly dilute systems. As always reality is

more complicated – especially when it comes to display applications. To reach any optical contrast,

concentrated suspensions are necessary. Hydrodynamic and electrostatic interactions between

particles cannot be neglected anymore. For detailed information of how the complex behaviour of

clusters of particles in an electric field deviates from single particles see [5-8].

4. Electrophoretic displays A print on paper is colour, either dye or pigment, on a diffuse reflector, felted cellulose. Its

appearance can vary from matt to glossy depending on the surface treatment. Displays based on

electrophoretic technology generate images and text very close to print in appearance and are often

referred to as ‘E-paper’. The technical challenges facing electrophoretic displays and how are they

are tackled in commercially available products are described in the following paragraphs.

4a Charge:

Too little or too much charge will affect motion in an electrophoretic system.

In an organic medium with a low dielectric constant, of around 2, the number of ionic species per

unit volume is low. Not many charges are available per particle. Hsu et al [9] report that in dodecane

they find as few as 290 +/- 30 electrons on PMMA particles with a radius of 780 nm instead of 104 on

the same particle in water. Nevertheless, zeta potentials were also measured and found to be as

high as those in aqueous dispersion media. When the particles are charged they will move. However,

the charging mechanism in non-polar media is complex [10, 11]. Even with added ‘charge directors’,

such as surfactants in many cases, not every particle is charged. Uncharged particles will not react to

any applied field. These particles will deteriorate the contrast of the display.

Considering polar media, the high ionic strength causes another problem. The double-layer around

the particles is compressed to such an extent that some types of particles are no longer charge

stabilized. When two particles come together, the ion cloud around them should inhibit aggregation

by electrostatic repulsion, but this is only the case when the double layer is thicker than the reach of

the attractive van der Waals force [12]. A measure of how close two charges can come together

before Coulombic interaction takes over, or more precisely where the Coulombic interactions are

balanced by the thermal energy TkB , is given by the Bjerrum length λB as follows:

Figure 15: Prototype of the bistable electrophoretically controlled nematic display in reflection. The liquid

crystal is dyed with a blue dichroic dye and the display is viewed in reflection on a white diffuser. No polarizers

are used. The HAN state is the dark state, i.e. the dichroic dye is tilted in such a way by the liquid crystal that

the absorbing transition moment is exposed to the incident and reflected light.

6b Mobile Fine Particle Display

The mobile fine particle display (MFPD) is similar in concept to the classic electrophoretic display.

Optical distinct states are generated by moving particles in and out of the field of view that can

happen by either an up and down or a sideways motion. Electrode design is of great importance

since liquid crystals belong to the group of midpolar solvents and can therefore accommodate a

substantial number of charges. The different electrode designs will not be considered here, instead

we will concentrate on the basics. At first glance the mobile fine particle display and the bistable

electrophoretically controlled nematic display (BECD) seem very similar but the requirements on the

suspension for the MFPD are quite different. Whereas in the case of the BECD a floc of particles is

moved around in the case of the MFPD a compressed floc is formed and re-dispersed again,

especially in the so called curtain mode when the particles are moved sideways (Figure 16).

Figure 16: The curtain mode. In 16a the particles are dispersed across the whole field of view. The observer

sees the colour of the suspended pigments. In b the particles are electrophoretically moved underneath the

top electrode which is covered by a mask. The observer sees now either the colour of the dye doped liquid

crystal or the colour of a bottom reflector.

The curtain mode allows stacking of cyan, magenta and yellow layers to provide full colour, but self -

shielding of the electrodes and field gradients can be a real challenge. A more forgiving set up is the

so called up and down mode (as already shown in figure 6) where the particles are moved towards

and away from the observer. The suspension always contains a scatterer, often a titanium dioxide

particle with a diameter of about 300 nm, and an absorber, which is either a dye or another particle.

To achieve sufficient optical contrast the scattering pigment has to form a compressed layer of

about 2 to 3 µm at the substrate closest to the observer. This has consequences for the cell thickness

and for the particle concentrations in suspension. Calculations and experiments have shown that

cells with cell gaps of less than 20 µm are not feasible since the concentration of the absorber has to

be so high that either, in the case of a dye, the liquid crystal phase is lost due to chemical

‘contamination’ or, in case of a pigment, the total particle concentration reaches values which lead

to a situation where a particle network with liquid crystal pockets fills the cell gap and not a liquid

crystal with suspended particles. In an electric field the network will deform but the two particle

species will not move past each other. Cells gaps of 100 µm and more require containment

structures. The liquid crystal is not kept between the substrate by capillary forces anymore. For

research purposes and for the ease of cell assembly cell gaps of 50 µm were chosen. Mixtures with

an acceptable contrast consisted of about 10wt% scatterer and of absorber at about 2wt%. The

particle content is still 5 to 6 times higher than in the BECD. We experimented with different types of

liquid crystals to determine whether different Δε would influence the electrophoretic mobility of the

particles, but we could not observe any dependency on Δε. To optimize our chances to find the right

surface treatment for the particles and therefore to achieve a stable suspension we decided to use

the commercially available mixture ZLI 2293 from Merck for all further experiments and for the

prototypes. The particles have to be stabilized sufficiently to hinder permanent aggregation. Even

then defect formation around the particle resulted in a dramatic increase of the viscosity of the

suspension. The cells could not be filled by capillary action anymore. A drop of the suspension was

applied to the bottom substrate and then covered with the top substrate. The cell gap was

controlled with Mylar film and the cell was held together with UV curable clue. To discourage

particle sticking at the substrates the insulating dielectric film on top of the electrodes was covered

with a low surface energy homeotropic treatment. Because of the high particle concentration the

surface treatment of the substrates does not influence the bulk behaviour of the suspension. It is

determined by the interaction between liquid crystal and particles and hence their surface

treatment. We could not see any difference in switching behaviour or optical appearance whether

the substrates were covered with a planar or a homeotropic alignment layer, but we observed

increased adhesion of the particles to the substrates when planar alignment layers were used. Figure

17 shows three types of up and down display prototypes. Figure 17a clearly shows problems with

aggregation. All three samples are disconnected from any power supply.

Figure 17: Three versions of the up and down display. a) fully pixelated on plastic. Dyed liquid crystal with

white scatterers. b) Gray scale sample of a dual particle system. The white particle is titanium dioxide, the

black particle copper oxide. c) Both pigments are absorbers. Magenta and Cyan are toner pigments provided

by Clariant.

The prototypes are bistable and show a relatively soft threshold. They can be switched with a DC

biased AC voltage as well, but the increased viscosity of the suspensions led to high voltages. The

lowest voltage where particle movement was observed under the microscope was 30V DC across 50

µm, but optical switching, that is a colour change, was only observed at 80V DC and above.

Electrophoretic mobilities of m2/Vs were measured. To observe the change of optical contrast

as a function of applied field we used low frequency AC voltages with frequencies between 0.5 and

10 Hz. In some cases strong electroconvection set in and the result was irreversible aggregation.

Uneven cell gaps favoured dielectrophoresis which again had aggregation as a consequence. When

pigments with some conductivity, for example carbon black, were used aggregation which bridged

the cell gap, often triggered by electroconvection, led to a short in the cell. When samples were

opened after several switching cycles, sticking of particles to the substrates was observed. This

sticking appears to occur within the first 5 to 10 switching cycles when a rapid deterioration of the

contrast and then a plateauing was observed. The contrast was always affected by immobile

particles. Even though liquid crystals are midpolar solvents not all particles seem to charge. Neither

with dyes nor with absorbing pigments could a true white be reached. In the case of dyes the

particles will always be surrounded by dyed liquid crystal. Absorbing pigments could never be

completely removed from the scattering layer because of the caging effect. It came out that one of

our best mixtures, a white titanium dioxide particle in a green dyed liquid crystal was indeed a dual

particle system. The green dyed had crystalized into a needle network which filled the cell gap. The

white scatterers were moved through the gaps when a field was applied.

7. Scientific, engineering challenges and conclusions The major scientific challenge for display applications of electrophoresis in liquid crystal is the design

of the right surface treatment for the particles. The surface treatment should mimic the liquid

crystalline solvent to reduce defect formation on one hand but on the other should not supress it

completely since defects are needed to make the system bistable. The surface treatment should

provide enough steric hindrance to prevent aggregation when the particles are compressed either in

the hidden state when driven in curtain mode or when close to the substrates. Again the right

molecule size is important. Molecules too big would lead to increased viscosity, molecules too small

to aggregation. The ideal situation would be the chemical design of the liquid crystalline solvent in

combination with the design of the surface treated particle. This is only possible when all

components are known, i.e. engineering for commercially available liquid crystal mixtures of

undisclosed composition is not possible.

What about the engineering problem of encapsulation to avoid contact with electrodes and particle

drift? Confinement structures have to be designed with upmost care. Liquid crystals interact with

any surface they come into contact with and align to it in one way or the other. Low energy surfaces,

like air for example, align the liquid crystal homeotropic. High energy surfaces, like rubbed polyimide

will align the liquid crystal planar. When this happens in a sphere with 40 µm diameter, the liquid

crystal will try to align across the whole volume but with different alignment directions coming from

different sides as defect formation cannot be avoided. Any containment in this size range has to be

designed carefully to avoid the introduction of additional defect lines and therefore causes an

increase in viscosity of the suspension.

The additional features of mesomorphic solvents seem to bring electrophoretic displays closer to a

full colour e-paper display. Prototypes built so far are promising, but highlight the immense

difficulties which need to be overcome to reach this goal. A central problem is the chemical design of

the combination of liquid crystal host and matching particle. The particle surface does not only need

to be treated with some molecules which provide steric hindrance and inhibit irreversible

aggregation, the same surface treatment must allow charging and must be compatible with the

liquid crystalline host. Ideally it should be a liquid crystal itself and make the particle ‘invisible’ for

the surrounding nematic matrix. This is not an easy task. It took twenty years between the first

patents for the electrophoretic e-reader and the first commercial product and it will probably take

the same time for a liquid crystalline technology to become more mainstream.

Acknowledgement

I would like to thank my work colleagues Fraser Dickin, John Rudin and Steve Simske for many

helpful discussions. The majority of the work on electrophoretic displays in liquid crystals within HP

was performed by the late David Sikharulidze.

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