Module 32: Flexible and Conformable Displays
32.1. Introduction
There is currently a significant interest in replacing the glass substrates on displays
with plastic substrates for certain display applications. The ability to create a
lightweight, flexible display is very attractive for applications that require inexpensive
and robust displays.
There have been many flexible displays concepts envisioned by the display
community. The figure below demonstrates one of most attractive flexible display
concepts as an example of a futuristic product and also puts it into historical
perspective.
The CRT is now more than 100 years old and is still the dominant desktop monitor for
computing. There are now a significant number of LCD desktop monitors in the
market because <20" diagonal screens are affordable. The active matrix LCD
completely dominates the laptop market. the concept product illustrated in the above
figure is a plastic that rolls out of a handle for use and rolls back into the handle for
easy and convenient carrying. This rollable display has also been conceived as
rolling out of a pen for PDA applications.
32.2. Transmission vs Reflection
Then do not always go hand-in-hand, but when most people think about flexible
displays, they also think about really low power and inexpensive display mediums.
The figure below compares a transmissive LCD with a reflective portable LCD. The
basic difference is that the transmissive display models a backlight and the reflective
display modulates reflected ambient light. In most cases, reflective displays are
largely manufactured for the portable market where power and weight are minimized.
There have been plenty of demonstrations of OLEDs in flexible display configurations
as well.
The positive attributes of reflective displays are shown in the figure below. The bar
chart also demonstrates the approximate power savings - a factor of two when the
backlight is removed and enormous power savings of the material has bistable
memory (e.g. Gyricon, e-ink, and cholesteric liquid crystals).
32.3. Converging Technology
One interesting aspect of display technology in recent years is that there has been
significant research and development investment in reflective display technology and
plastic substrate technology.
The figure below shows a schematic on the convergence of these two technologies,
which can truly enable the rollable display concept.
There are a number of issues with reflective display technology and flexible
substrates that still need to be improved in order to realize such a far reaching
applications the unraveling pen display. One of the most attractive features of a
plastic displays in the possibility of using a roll-to-roll manufacturing process. If the
display materials are compatible with such a process, the roll-to-roll manufacturing
process may be feasible.
With this high-throughput manufacturing process, it may be possible to create very
inexpensive displays.
32.4. The E-Paper Chase
The unraveling display application is certainly a wildly different application that can be
enabled by a flexible substate technology, but electronic paper is equally compelling.
Creating a paper surrogate is no easy task - paper is the ultimate display medium.
Paper is foldable, rollable, and unbreakable, with a 180° viewing angle and a nice
Lambertian reflection.
There are a few contenders for e-paper - namely those display materials that are
reflective and bistable (e.g. Gyricon, e-ink, and cholesteric LCDs) The cholesteric
LCD is very mature and has the potential for full color. It has very attractive features
such as high contrast (50:1), 40% reflectance at the Bragg wavelength, th, relative
low switching voltage compared to other bistable reflective displays. One of the most
attractive features about it is tat it has a well defined threshold and therefore passive
matrix driving schemes can be used. A downfall of this technology is that it is difficult
to achieve a white Lambertian appearance with one layer.
The Gyricon technology consisting of a dispersion of bichromal balls in a elastomer
matrix is very attractive because the materials to make such a device are
inexpensive. The attractive features of the Gyricon displays are its Lambertian
reflection and bistability. the disadvantages of Gyricon are that the drive voltages are
relatively high for moderate reflectances. In addition, it is a thresholdless technology
and would therefore need an active matrix for a medium to high resolution
applications.
E-ink is another potential for paper surrogate applications that is based on
electrophoretic materials technology. E-inks encapsulate the electrophoretic particles
to greatly improve robustness and stability of electrophoretic displays. They have a
Lambertian reflectance that approaches 40%. The disadvantage is that the material
is thresholdless and would therefore need an active matrix for medium for high
resolution applications.
An intriguing technology that is currently being developed at Philips, involves a
'paintable' electro-optic medium. A dispersion of LC and polymer is coated on a
plastic substrate and irradiated with U light as shown below. The irrradiation is slow
such that the liquid crystal will diffuse to the bottom surface and the polymer will form
a top substrate as shown here.
It is an exciting technology since it is only one substate and therefore very thin. In
order to support the top polymer layer, they can create pixel walls between the layer
as shown below. This technology has great potential.
Organic light light emitting diode (OLED) technology has also been shown to be
adaptable to plastic as shown below. OLED are attractive since they are an emissive
technology and a bright display is possible. Here is an example of what they look like:
There is however one downfall and that is that OLEDs seem to be quite sensitive to
oxygen and moisture that diffuse through the plastic. Additional barrier layers are
required to minimize the penetration of these impurities into the OLED material.
Another technology is based on electroluminescent materials applied to a screen-like
substrate, Developed by Visson, electrical pulses are applied to the rows and
columns of the screen. As shown below, light emission is generated at the
intersection.
Another technology, based on interference, is being developed by Iridigm. This
deformable membrane technology reflects bright iridescent colors or can be black &
white.
Full color is possible with Iridigm technology and the switching behavior and the
deformable membrane has a well defined threshold so multiplexing is possible.
32.5. Flexible Display Technology
Creating a flexible conducting substrate is key to all technologies described above.
Currently, there are two solutions, Iridium-tin-oxide (ITO) on plastic and conducting
polymer on plastic as shown below:
ITO on plastic (polyester or PET) currently has strong performance properties
(transmission > 90% and sheet resistance < 50
) comparable to ITO on glass.
However, the ITO cracks where subjected to strain (~2%). Conducting polymer on
PET, on the other hand, can sustain very high degrees of strain (>10%) but current
materials exhibit transmissions (<85%) and sheet resistances 100-1000
.
When a flexible ITO substrate substate is flexed cracking can occur in tension and
buckling or cracking can occur in compression as shown below.
Images from a scanning electron microsphere on SEM are shown below to
demonstrate the cracking. A close-up of an ITO crack is also shown in the following
figure.
In order to quantify the degree of cracking and how it influences the resistance, a
miniature Inston (stress-strain measurement) is used. The figure above shows a
tremendous increasing resistance at 2& strain. It is interesting to note that after 2%
strain, if the substrate is allowed to relax, the resistance is never restored to its
original zero strain values as shown above. This suggest that once cracking occurs
and the resistances get large, the substate does not 'heal' and therefore is unusable.
Therefore, flexible applications must not except this value or catastrophic failure will
occur.
The figure below is another potential way to characterize ITO on PET. It is known as
bulge testing, which introduces a biaxial strain on the material. An SEM image shows
the complex cracking pattern as the material is subjected to a bulge.
The figure below compares the ITO and PET to conducting polymer on PET directly.
One can see that the conducting polymer is much less susceptible to strain, but its
resistance is much less attractive.
32.6. Active Matrix on Plastic
There are two schools of thought when it comes to enabling active matrix technology
on plastic substrates as shown below. One option is to raise the processing
temperatures of the substrate so that it is compatible with inorganic TFT technology.
The other is to develop new TFT technology that can be processed at low
temperatures. There are both inorganic and organic TFT approaches at low
temperatures, but the performance parameters are inferior to the high temperature
inorganic TFTs.
32.7. Conformable Displays
Conformable displays (displays that can be deformed around curved surfaces) may
also open up new product categories for displays. A conformable display can be as
simple as securing a flexible display to a curved surface. The display will have to
withstand the internal stresses over time introduced by the conformed deformation
conformed display may find applications in wrist band applications as shown below,
or they may be integrated into clothing for example.
Another interesting off-spring of conformal displays is to permanently capture the
deformation in the substrates as shown below.
The glass transition temperatures Tg, of PET for example is around 90°C. If the
substate is conformed (spiral deformation above) and heated to temperatures above
Tg, and cooled down under deformation, the spiral will remain indefinitely.
What is attractive about this process is shown in the graph above. The PET substrate
is strained to 8% and then heated above Tg. The graph clearly shows the stress
relaxing out of the substrate over time.
This concept was tested on a Polymer Dispersed Liquid Crystal display shown below.
When the voltage is on, the liquid crystal droplets align creating and indexing
matching condition. In this state the display is transparent. In the off-voltage state, the
droplets are randomly oriented thereby creating light scattering state (milky white)
Since the liquid crystal is encapsulated in a polymer so it is extremely robust and can
survive the conforming and heating process as shown below. The display is first
manufactured in its planar state (step 1), subjected to strain (step 2), heated above
Tg (step 3), and cooled to low temperature.
The figure below shows some small 1" x 5" laboratory samples subject to the process
under different deformations.
What is even more impressive is that the display works after this process as
demonstrated below. In fact, the threshold voltage or drive voltage decreases, the
hysteresis is reduced and the contrast decreases. Most of these observations can be
described by a shrinking cell gap (10-20%) during the process.
Permanently conformed displays are possible with other display materials that can
sustain the process.
32.8. Summary
Flexible and conformable display is attractive
and may open up many new product categories
that currently do not exist, such as rollable and
foldable displays, and electronic paper. There
are still many challenges ahead but the industry
is determined to create a viable flexible display
technology.