liquid crystal institute john west kent state university april 22, 2004

Liquid Crystal Institute

John WestKent State University

April 22, 2004


• Why Flexible

• Printed Flexible Cholesteric Displays– Printing techniques– Polymer walls

• Stressed Liquid Crystals

• Flexible Optical and Electronic Device Manufacturing Facility


Anatoliy Glushenko

Guoqiang(Matt) Zhang

Ke Zhang

Ebru Aylin BuyuktanirToshihio Aoki

Greg R. Novotny

David Heineman

Mike Fisch


RuggedLightweight

Cheap

Roll-to-RollManufacturing

Conformable


• Conventional LCD’s and OLED/PLEDS require expensive substrates and development of organic TFT’s – Conventional LCD’s

• Use polarized light (non birefringent substrates)• Active matrix (organic TFT’s)• Surface alignment (high temperature and solvent stability)

– OLED/PLEDS• Oxygen sensitive (barrier layers)• Active matrix (organic TFT’s)

• Unconventional LC Conventional SubstrateApproaches – Polymer Dispersed Liquid Crystals– Bistable Chosterics– Bistable Smectics– Dichroic Dye LCDs


Manufactured in a continuous roll-to-roll process

Conventional ITO coated polyester substrates (no barrier coatings, no alignment layers)

Single pixel.


Bistable Cholesterics

1. Bright, high contrast images

2. High resolution with passive matrix

3. No polarizers required (can use birefringent substrates)

4. Polymers added for mechanical stability


Reflective Cholesteric Displays

planar focal conic

Switch between reflecting planar texture and weakly scattering focal conic texture


Ele

ctri

c F

ield

planar focal conic

focal conic

transient planar

homeotropic

slow

fastfa

st

Switching Mechanism


Flexible Plastic Bistable Cholesteric Display

Figure 1: A four inch square, 320 by 320 pixel bistable cholesteric display made using flexible polyester substrates

• 4 inch square• 320 x 320 pixel• bistable cholesteric with polymer• flexible polyester substrate

West Rouberol, Francl, Ji, Doane and Pfeiffer

Asia Display, 1995


Problems

1. Photolithography makes roll-to-roll processing difficult (expensive)

2. High polymer content formulation produces light scattering

a) Reduces reflection in planar state

b) Increases back scatter in focal conic state

c) Lowers brightness and contrast


Preliminary Solution

1. Print resist for etching of electrodes:(roll-to-roll processing)

2. Segregate polymer into the inter-pixel region: Polymer Walls (bright display)


Wax Transfer Printing of Resist

Tektronix Phaser 240 Wax Transfer Printer

Thermal Print Head

Wax Transfer Sheet

ITO Coated Polyester Film


A replica of the wax resist pattern.

Close-up of the wax pattern printed onto the ITO coated Mylar. The dark lines are the wax

pattern.

Resist Pattern30 pixels/inch


Etching and Stripping

1. Standard etch bath of nitric-sulfuric acid

2. Strip using warmed (50 C) tetrahydrofuran or toluene


Cell AssemblyTop

Substrate

Bottom Substrate

Cholesteric Polymer Mixture

BL 094 92%

NOA 65 8%


Form Polymer Walls

• Improve contrast

• Provide rugged displays

• Improve the pressure resistance of displays -- PM-STN-LCDs by Sharp*

• Make possible large area, flexible plastic displays -- adhere the top and bottom substrates -- maintain uniform thickness

* T. Shinomiya, K. Fujimori, S. Yamagishi, K. Nishiguchi, S. Kohzaki, Y. Ishii,

F. Funada, and K. Awane, Asia Display, 255 (1995).


Polymer Wall Formation Methods• Photo-mask -- patterned UV exposure of homogeneous mixtures of UV-curable monomers and liquid crystals

• N. Yamada, S. Kohzaki, F. Funada, and K. Awane, SID Digest of Technical Papers, 575 (1995).

• T. Shinomiya, K. Fujimori, S. Yamagishi, K. Nishiguchi, S. Kohzaki, Y. Ishii, F. Funada, and K. Awane, Asia Display, 255 (1995).

• Y. Ji, J. Francl, W. J. Fritz, P. J. Bos, and J. L. West, SID Digest of Technical Papers, 611 (1996).

• Patterned Electric Field -- blanket UV exposure after phase separation


Field Formed Polymer Walls1) Apply an electric field while solution is warmed above

clearing/phase separation temperature

2) Cool to RT with field applied to induce polymer segregation

3) UV expose to form polymer walls

Kim, Francl, Taheri, West,Appl. Phys. Lett., 72, 2253 (1998).


Field Induced Phase Separation

• A patterned electric field is applied to a mixture of liquid crystal and UV curable monomer

• Due to a larger dielectric constant, liquid crystal migrates to the high field pixel region while the monomer moves to the low field inter-pixel region

• When phase separation is complete, exposure to UV light polymerizes the monomer locking in the wall structure


Electric Field Distribution

SEM Image of Polymer Walls


AL AL AL

Detector

UV Source

Glass Substrate Aluminum Electrodes

Liquid Crystal& Monomer

AT-720Barrier Layer

ITO

h

Measure Rate of Field Induced Phase Separation

• One side of cell has aluminum electrodes which block incoming UV light.

• Only light passing through inter-pixel makes it to the detector

• This allows study of change in concentration of E44 in inter-pixel over time

Diagram of Test Cell


Absorbance vs TimeMixture of E44 and Trimethylolpropane tris(3-

mercaptopropionate)at 40 °C

Absorbance vs Time: Varying Voltages @ 50 C Normalized

0.96

0.965

0.97

0.975

0.98

0.985

0.99

0.995

1

1.005

0 20 40 60 80 100 120 140

Time (sec)

Ab

sorb

ance

5 V

10 V

20 V

30 V

40 V


How T and V Affect the Rate

• With the same voltage applied, increasing temperature decreases change observed in absorbance between field on and field off states

Absorbance vs Time: Varying Temperature

0. 72

0. 74

0. 76

0. 78

0. 8

0. 82

0. 84

0. 86

0. 88

0. 9

0 20 40 60 80 100 120 140

Time (sec)

Ab

sorb

ance T (C)=30

T (C)=40

T (C)=50

T (C)=60

T (C)=70

T (C)=80


Rate of Phase Separation

1. Occurs in several seconds.

2. Increasing temperature decreases the magnitude of the effectbut has little effect on the rate.

3. Increasing the voltage increases the rate and extent of phase separation


Flexible Plastic Display

1. Compatible with roll-to-roll processing2. Uses commercially available materials.


Stressed Liquid Crystals

• Developed for beam steering applications– Decouple thickness and speed– Eliminate alignment layers

• Fastest nematic devices


Unidirectionally Oriented Micro-Domains of Liquid Crystal Separated by Polymer Network

our results

LC

F

polymer

5

10

15

20

AFM image

SEM image

• Middle range of concentration of the polymer: between those for traditional polymer network structures and PDLC.

• Well-developed interpenetrating structure of polymer chains and connected liquid crystal domains.

• The active area may be of any size

• Application of shearing deformation in order to orient the liquid crystal domains


Effect of Shearing

1000 1500 2000 25000

20

40

60

80

100

Before shearing After shearing

Wavelength, nm

Tra

nsm

ittan

ce, %

1.Reduces the relaxation time of the material.

2.Decrease the scattering in visible region of spectrum.

3.The liquid crystal domains become oriented in the direction of shearing

4.By adjusting the degree of shearing one can control the total phase shift.


0 50 100 150 200 2500

2

4

6

8

10

Voltage, V

Inte

nsity

, arb

. un.

Phase retardation shift vs an applied electric field

0 50 100 150 200 250

0.0

0.5

1.0

1.5

2.0

2.5

3.0

= /2

= 9

= 7

= 5

= 3

=

Voltage, V

Pha

se r

etar

datio

n,

m

For a 22 m film almost all change of the phase retardation occurs below 130V.

The change of phase retardation depends linearly on the applied voltage – simple driving devices


0 1 2 3 4 50

2

4

6

8

10

Time, ms

Inte

nsity

, arb

. un.

0.00 0.04 0.08 0.12 0.16 0.200

2

4

6

8

10 nd = 0.63 m

nd = 0.31 m

nd = 0.15 m

Time, ms

Inte

nsity

, arb

. un.

A phase retardation shift of 2 m occurs within 1 ms.

Phase retardation of 0.15 m occurred just in 40 microseconds

Dynamics of the Relaxation


Possible applications: basic design of an OPA device

• Beam steering: a tilted LC director will yield an index of refraction gradient

l=l0/nHl=l0/nL

millimeters

mm

• Industrial application (laser cutting of metals or glass)

• Free space communications

• Fiber-optics connectors

• Military applications

• Laser displays

• Imaging applications


SLCs for display applications

• 30 volt offset• 10 volt pulse• 20 sec turn on time• 40 sec turn off time


Conclusion

• Stressed LC films produce ferroelectric speeds in a nematic film

• No alignment layer

• No light scattering

• No hysteresis

• Linear response


The Next StepFlexible Optical and Electronic Device Manufacturing Facility

• Wright Capital Grant, $1.6M (State of Ohio)

• $1.6M match from industry and KSU

• Add to existing Resource Facility

• Provide centralized facility for development and

prototyping of flexible devices.


Goal

• Develop materials required for flexible displays– optimized liquid crystals

– organic semiconductors

– conducting polymers

– optimized substrates

• Develop fabrications techniques– printing electrodes

– applying thin films

– lamination

– cutting


PlanEstablish a facility for the research and development

of flexible electronic devices

• Printing• Coating• Lamination• Cutting• Electronics• Materials Synthesis


Planned Research

• Evaluation of available substrates

• Development of printing techniques for electrodes

• Fabrication of printed flexible VGA display using only commercial materials.


KSU/UA HANA

Start-Ups bring Innovation and JobsBuilt on an Effective Academic Industrial Collaboration

LXD, Inc.

Poly Displays

Akron Polymer Systems


Utilize the unique skills in the region in polymers/liquid

crystals and printing to spawn a new

industry in flexible displays and related electronic devices.

liquid crystal institute john west kent state university april 22, 2004

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