See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/231184157
Improvement of Dichroic Polymer Dispersed Liquid Crystal (PDLC)
Performance for Flexible Display Using Lift-off Technique
Conference Paper · January 2005
CITATION
READS
1
207
5 authors, including:
Akio Yasuda
Sony Corporation
177 PUBLICATIONS 6,462 CITATIONS
SEE PROFILE
All content following this page was uploaded by Akira Masutani on 29 May 2014.
The user has requested enhancement of the downloaded file.
Improvement of Dichroic Polymer Dispersed Liquid Crystal
(PDLC) Performance for Flexible Display
Using Lift-off Technique
A. Masutani, T. Roberts, B. Schüller, A. Sakaigawa*, and A. Yasuda
Sony Deutschland GmbH, Hedelfinger Str. 61, 70327 Stuttgart, Germany
* Sony Corporation, Atsugi Tec. 4-14-1 Asahi-cho, Atsugi-shi, Kanagawa, 243-0014, Japan
ABSTRACT
The performance of dichroic sponge polymer
dispersed liquid crystals (SPDLC) display can be
improved further by incorporating a lift-off method
using a substrate covered with a fluorinated-organosilane. The resulting cells achieve
improved contrast, improved response time,
smaller switching voltages and better uniformity,
compared with the previously reported dichroic
SPDLCs.
The
display
exhibits
near
magazine-standard reflectivity and contrast ratio.
Furthermore,
the
lift-off
method
enables
rubbing-free, low temperature, roll-to-roll processing techniques. Such displays are suitable for flexible solvent-sensitive, organic thin-film transistors
(TFTs).
INTRODUCTION
A polymer dispersed liquid crystal (PDLC) cell
consists of a thin film sandwiched between
transparent electrodes [1, 2]. Such films are composed of a polymer matrix within which small droplets of a liquid crystal (LC) are dispersed. PDLC
films usually possess a transparent on-state and a
scattering off-state. By doping PDLC with dichroic
dyes, the films then exhibit an absorbing off-state
(a)
V
and a transparent on-state [1]. Such films, known
as Dichroic PDLCs (D-PDLCs), have the potential
to outperform conventional reflective-type twisted
nematic (TN) LC displays in some applications
because they have no polarisers, leading to increased reflectivity and viewing angle [3]. Schematic diagrams of a D-PDLC in the off-state (a)
and on-state (b) are shown in Fig. 1. The white
rods represent the LC molecules, while the black
rods represent the dichroic dyes. In the off-state,
the film appears both scattering and coloured due
to light absorption by the dichroic dye. In the
on-state, the directors of the droplets align with the
field, which makes the film transparent and colourless.
One of the most common ways to prepare
PDLCs is via the photo-induced phase separation
(PIPS) method [3, 4]. In this method, irradiating UV
light to a mixture of LC, dye and pre-polymer induces a phase separation. The result is the formation of LC and dye droplets in a polymer matrix.
However, conventional D-PDLCs suffer from low
contrast ratio and low on-state reflectivity, because
of (1) degradation of dye/LC during fabrication, (2)
dye interfering with the curing process, (3) trapping
of dye/LC in the polymer matrix – thus making
the dye/LC unresponsive to the electric field [5-10].
In our previous work [11, 12], we developed a
simple processing method, which we dubbed the
“Split” method, to overcome the aforementioned
problems (Fig. 2a). The technique is as follows:
firstly a pure PDLC cell with LC without dye is
made – this defines the morphology of the film in
the device; secondly, the substrates are split apart;
(a)
(b)
Glass
Glass
Glass
Glass
V
(b)
Glass
Glass
Glass
Glass
Glass
Glass
Cover
substrate
Glass/PET
Lift-off
250?msubstrate
PET film
Base
substrate
Glass/PET
Fig. 1 Dichroic PDLC in the off-state (a)
and on-state (b)
Glass
Glass
Base
substrate
Glass/PET
Base
substrate
Glass/PET
Fig. 2 Schematic of (a) Split-SPDLC and
(b) LO-SPDLC fabrications
Fabrication of D-PDLC by Lift-off method
For the fabrication of D-PDLC with the lift-off
(LO) method, we followed mostly the previously
reported recipe [11, 12]. Firstly, 78.9 wt% TL213
LC and 21.1 wt% PN393 pre-polymer were mixed
together with small amount of 8µm spacers. TL213
is a nematic LC mixture from Merck, with an
extraordinary refractive index (ne) of 1.77, an ordinary refractive index (no) of 1.53, ∆n (=ne-no) of
0.24 (589nm at 20ºC), and a dielectric anisotropy
(∆ε) of 5.7. PN393 is a UV curable polymer from
FFL Funktionsfluid GmbH, with a refractive index
of 1.47.
The solution was sandwiched between a base
substrate and a LO substrate. The base substrate
was an ITO coated glass substrate, which was
weakly pre-treated with an ozone-plasma (100W
for 10 min) to make the surface hydrophilic. The
LO substrate was a glass substrate which has
been functionalised with a fluorinated silane layer
to render the surface hydrophobic. The silanisation
was undertaken by immersing the LO substrate in
20µl
SIH5840.0
(Heptadecafluoro1,1,2,2,
-tetrahydrodecyl dimethylchlorosilane) in a vacuum
desiccator at room temperature (c.a. 22°C) for 30
min. The LO substrate was allowed to dry in ambiOn-state
Off-state
Transmittance [%]
100
80
60
40
20
0
10µm
(a)
(b)
Fig. 4 Optical microscope photos of
(a) LO-SPDLC and (b) Split-SPDLC
ent conditions in a fume hood before use.
The phase separation of the TL213-PN393 solution was initiated by irradiating the cell with
2
10 mW/cm 365nm UV for 2 minutes at 23°C.
Then the LO substrate was separated slowly from
the PDLC film. Due to the hydrophobic surface
treatments, the film only adheres to the hydrophilic
base substrate. Subsequently the LC in the PDLC
film was fully dissolved from of the polymer matrix
by washing with methanol. The solvent was then
removed by placing the cell under vacuum in an
oven at 80ºC for 3 hours. The end result was an
open porosity sponge consisting of a polymer matrix with air cavities (voids) on the base substrate.
Then Black-4 dye (B4) doped TL203 LC was sandwiched between the base substrate and either (1)
ITO coated glass cover substrate, or (2) a diffuse
layer coated TFT. The different substrates were
used for transmittance measurements or for
reflectivity measurements respectively. The B4 dye
(from Mitsubishi Chemical) consists of a mixture of
six different azo and anthraquinone dyes. TL203 is
a nematic LC mixture from Merck with nematic to
isotropic temperature (TNI) of 77°C, with an ne of
1.73, an no of 1.53, ∆n of 0.20 (589 nm at 20°C),
and a ∆ε of 11. Finally, the cell was heated to 90°C
in vacuum oven for 10 min to reduce flow alignment defects in the LC. We referred to the final
device as a “dichroic LO SPDLC.”
Comparison of different fabrication methods
for D-PDLCs
PDLC test cells made with different fabrication
methods were compared. The cells were denoted
as “PDLC” for the cell made with the conventional
Applied Voltage [V]
thirdly, the first LC is replaced with a dye-doped
LC; finally, the display is reassembled. We called
such displays dichroic Sponge PDLC (D-SPDLC),
and we demonstrated displays with favourable
viewing properties, such as a reflectivity of 98%
and a contrast ratio (CR) of 8 [11]. Despite this
high performance, better ways of fabricating the
display were desirable because the Split method
was: (1) not homogeneous enough, (2) not suitable for a mass production, (3) not compatible with
the additional diffuse layer developed for
wide-view TFT [12], and (4) not suitable for a solvent-sensitive substrate, such as organic TFTs.
This report presents an alternative approach to
solve the above shortcomings, by lifting-off the
PDLC film uniformly using anti-sticking lift-off substrate and then transferring the base substrate to a
desired cover substrate, such as a TFT backplane
(Fig. 2 b).
9
8
7
6
5
4
3
2
1
0
V10
V90
PDLC
PDLC
Split
SPDLC
Split DSPDLC
LO
SPDLC
LO DSPDLC
Fig. 3 On- and off-state transmittance
Split
SPDLC
Split DSPDLC
LO
SPDLC
LO DSPDLC
Fig. 5 Applied voltage required for 10% (V10)
and 90% (V90) of the On-state transmittance
Rise Time at V90
600
500
400
300
200
LO-SPDLC refilled with
4% B4 doped TL203
Conventional TN TFT
100
White-state
Reflectivity [%]
Responce Time [ms]
Decay Time at V90
80
60
40
20
0
100
0
0
PDLC
Split SPDLC
Split DSPDLC
LO SPDLC
LO DSPDLC
Fig. 6 Rise time when V90 is applied, and
decay time when V90 is removed
PIPS method, “Split SPDLC” for the cell made with
the Split method, and “LO SPDLC” for the cell
made with the LO method. For comparison, Split
and LO SPDLCs doped with 3wt% B4 dye were
also made, these are denoted as “Split D-SPDLC”
and “LO D-SPDLC” respectively.
Fig. 3 shows how the On- and Off-state
transmittance (Ton & Toff) differs between the cells.
Ton stays almost constant between 82 and 83% for
conventional PDLCs, Split and LO SPDLCs with
no B4 dopant. Because of the absorption by the
dye, both D-SPDLCs have approximately 5%
lower Ton than the non-doped materials. Toff of the
Split SPDLCs is approximately 8% higher than the
conventional PDLC, which means that the Split
SPDLC is not as scattering as the conventional
PDLC. This is partly due to inhomogeneities within
the SPDLC film. In contrast, the LO method produced more homogeneous films; which thus led to
more efficiently scattering films, as can be seen by
the reduction in Toff. Furthermore directly, the
microscope photographs (Fig. 4) show that the LO
SPDLC has more uniform droplet morphology than
compared to the Split SPDLC.
Fig. 5 shows the applied voltages required for
10% (V10) and 90% (V90) of the transmittance
when Ton is taken as 100%. The V90 of both
SPDLCs is noticeably increased compared to the
conventional PDLC. This is probably due to the
increased polymer-LC interaction caused by the
use of different LCs and the removal/refilling process. Interestingly, LO SPDLC has c.a. 1V reduction in V90 compared to the Split SPDLC. The
same trend could also be seen with the doped
SPDLCs. The cause for the reduction in the
switching voltage is expected, and is likely to be
because of the improved droplet uniformity and
slightly thinner cell gap because the LO film is
more homogeneous. As we have previously reported [11], the reduction in V90 by 1~1.3V when
B4 dyes are added to SPDLCs can be observed.
The cause of this is yet to be fully understood.
The variations in rise and decay times (ton & toff)
when V90 is applied are shown in Fig. 6. Rise time
20
40
60
Angle of Incident Light [degrees]
80
Fig. 7 Reflectivity of D-SPDLCs compared
with TN TFT at their white-states
is the time taken from when the voltage is applied,
to when the cell transmittance reaches 90% of |Ton
- Toff|. Decay time is the time taken from when the
applied voltage is turned off, to when the cell transmittance reaches 10% of |Ton - Toff|. Again, the
same trends as previously reported [11] were observed; specifically that compared with PDLCs,
both SPDLCs successfully showed reductions in
both ton and toff. The response times for SPDLCs
prepared by the Split and LO methods are
comparable. However, when the SPDLCs are
doped with 3wt% B4, the LO D-SPDLC ton did not
show as much increase as the Split D-SPDLC.
The reason for this improvement with the LO
method has still to be fully explained, we expect it
to be because of the improved droplet uniformity
and film thickness.
Characterisation of Reflective D-PDLC Display
Prepared by Lift-off method
As well as transmissive display cells, dichroic
LO SPDLCs were also fabricated on a reflective
TFT substrate. An additional diffuse layer coated
on the substrate suppressed the metallic glare and
improved the viewing angle dependency of the
TFT’s existing diffuse reflector. Fabricating
D-SPDLC on such diffuse layer was not directly
possible with the Split method because the diffuse
layer on the substrate was often removed or damaged during the splitting process.
The reflectivity of the LO D-SPDLC cell was
measured and compared with a commercially
available reflective-type TN TFT display. The
detector was set at 0º (surface normal) while the
incident parallel white light was moved from 15º to
70º. The normalization value of 100% reflectance
was taken using a Diffusing White Standard (Labsphere SRS 99-020). When the incident light was
at 30º, the Dichroic LO SPDLC achieved a
reflectivity of 66% in its on-state and 4.8% in the
off-state (Fig. 7). At this angle the contrast ratio,
defined as (Reflectivity at white-state)/(Reflectivity
at black-state), for the Dichroic LO SPDLC panel
was 13.8. This value is close to that of magazine
(Fig. 8).
Contrast Ratio
30
25
20
15
10
5
0
Poster
TN-LCD
D-SPDLC
Magazine
Newspaper
0
10 20 30 40 50 60 70 80 90 100
Reflectivity [%]
Fig. 8 Position map of papers and D-SPDLC
Conclusion
By creating a new type of D-PDLC film and the
development of a novel lift-off method, we have
obtained a high reflectivity, high contrast, polariser-free display. Compared with existing This new
method enables: (1) homogeneous display,
(2) processes which are compatible with existing
conventional TFT fabrication process, (3) improved
compatibilities with novel diffusing layers [12] on
TFTs, and (4) the use of solvent-sensitive TFTs,
such as the materials used in flexible/organic TFTs.
We have also found that the Dichroic LO SPDLC
has improved shorter rise times and smaller
switching voltages than compared to the Dichroic
Split SPDLC. We have demonstrated the application of the Dichroic LO SPDLC to a reflective TFT
substrate. The contrast ratio achieved was 14 and
the reflectivity was 66%. Furthermore, this method
allows a study of dye absorption enhancement by
scattering because the PDLC’s morphology and
electro-optical properties can be precisely tuned
during the first step of the fabrication process.
Such methods and materials are expected to thus
have the potential to achieve a bright, high contrast, fast-switching and flexible paper-like display.
Acknowledgements
We would like to express our sincere gratitude
to Professor David Bloor, Dr. Nigel Clark and
Dr. Lars-Olof Pålsson from the University of Durham (UK) for fruitful discussions throughout the
development of these materials and processes.
[1]
[2]
[3]
[4]
REFERENCES
J.L. Fergason, 1983, 4616903, US Patent.
J.L. West, "Phase-separation of liquid-crystals
in thermoplastics". Abstracts of Papers of the
American Chemical Society, 1987. 194: p.82.
P.S. Drzaic, "Polymer dispersed nematic liquid-crystal for large area displays and light
valves", J. App. Phys., 1986. 60 (6):
p.2142-2148.
J.W. Doane, N.A. Vaz, B.G. Wu, and S.
Zumer, "Field controlled light-scattering from
nematic microdroplets", App. Phys. Lett., 1986.
View publication stats
48 (4): p.269-271.
G.M. Zharkova, S.A. Streltsov, and V.M.
Khachaturyan, "Guest-host effect in polymer-dispersed nematic liquid-crystals", J.
Struct. Chem., 1993. 34 (6): p.930-933.
[6] P.S. Drzaic, Liquid crystal dispersions. Series
on liquid crystals, ed. H.L. Ong. Vol. 1. 1995,
Singapore: World Scientific Publishing.
[7] J.J. Wu, C.M. Wang, and S.H. Chen, "Effects
of dichroic dye in UV-cured polymer dispersed
liquid crystal films", Jpn. J. App. Phys. Pt. 1 Reg. Pap., 1996. 35 (5A): p.2681-2685.
[8] P.S. Drzaic, "Recent progress in dichroic polymer-dispersed liquid crystal materials", Pure
App. Chem., 1996. 68 (7): p.1435-1440.
[9] P.S. Drzaic, R.C. Wiley, and J. McCoy, Proc.
SPIE, 1989. 1080: p.41.
[10] L. Bouteiller and P. LeBarny, "Polymer-dispersed liquid crystals: Preparation, operation and application", Liq. Cryst., 1996. 21
(2): p.157-174.
[11] A. Masutani, A. Roberts, B. Schüller, A.
Sakaigawa and A. Yasuda, "Improved
Performance of a Novel Polariser-Free Dye
Doped Polymer Dispersed Liquid Crystal for
Reflective Display", Journal of the SID, Vol.
12/3
[12] A. Masutani, M. Dürr, A. Roberts, B. Schüller,
A. Sakaigawa and A. Yasuda, "A Method of
Controlling the Diffusing Reflector Properties
for Reflective Displays", AD/IMID '04 Conference Proceedings (2004.8, Daegu, Korea)
[5]