Intro Til Ref

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Stretchable OLED display device

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Chapter 1

Introduction

A stretchable organic light emitting diode (SOLED) incorporating a

stretchable substrate on which the electroluminescent organic semiconductor is deposited.

This enables the device to be stretched while still operating.

Stretchable OLEDs also create great potential to improve many of the electronics

we use today. Video displays are now rigid, but might soon be able to “crumple like a

handkerchief and be pulled out of your pocket when you need it,” says Lawrence

Gasman, Co-founder and Principal Analyst at Nanomarkets — a market research firm

focusing on energy and electronics enabled by advanced materials.

Developed by a team at UCLA led by Qibing Pei, a professor of Materials

Science and Engineering, the first fully stretchable OLED was achieved by layering a

polymer electrode into a light-emitting plastic that remains conductive even while being

pulled and elongated like a piece of chewing gum.

http://www.ecomagination.com/nanomarkets.net

http://www.ecomagination.com/nanomarkets.net




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Chapter 2

What is OLED?

OLED (Organic Light Emitting Diodes) is a flat light emitting technology, made

by placing a series of organic thin films between two conductors. When electrical current

is applied, a bright light is emitted. OLEDs can be used to make displays and lighting.

Because OLEDs emit light they do not require a backlight and so are thinner and more

efficient than LCD displays (which do require a white backlight).

Already a number of companies are helping to make OLEDs commercially

viable in a variety of products, such as Samsung mobile phones and Sony high definition

televisions.

Energy-efficient and long lasting, OLEDs are the go-to light source for a new

environmentally aware, post-incandescent era. Already capable of being produced as thin

and floppy as a sheet of paper, now scientists are taking existing OLED technology a step

further, from bendable to the first fully stretchable OLED.

2.1 Advantages of OLED:

Lower power consumption.

OLEDs have a potential to be even cheaper than LCDs because of their simple

design.

Faster refresh rate and better contrast.

Greater brightness - The screens are brighter, and have a fuller viewing angle.

Exciting displays - new types of displays, that we do not have today, like ultra-thin,

flexible or transparent displays.

Better durability - OLEDs are very durable and can operate in a broader temperature

range.

Lighter weight - the screen can be made very thin, and can even be 'printed' on

flexible surfaces.




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2.2 Disadvantages of OLED:

OLEDs aren't perfect.

Today it costs more to produce an OLED than it does to produce an LCD

OLEDs have limited lifetime but this is almost a non-issue.

OLEDs can also be problematic in direct sunlight, because of their emissive nature.

Figure 2.1 Structure of OLED




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Chapter 3

Flexible OLEDs

It turns out that because OLEDs are thin and simple - they can be used to create

flexible and even transparent displays. This is pretty exciting as it opens up a whole world

of possibilities:

Curved OLED displays, placed on non-flat surfaces

Wearable OLEDs

Transparent OLEDs embedded in windows

OLEDs in car windshields

Stretchable OLEDs in displays

New designs for lamps

Several companies are working towards such displays. In fact TDK is already

producing simple transparent OLEDs, and Lenovo's S-800 phone is the first product to

use them.

A flexible organic light emitting diode (FOLED) incorporating a flexible

plastic substrate on which the electroluminescent organic semiconductor is deposited.

This enables the device to be bent or rolled while still operating.

Figure 3.1 Flexible OLED display




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Chapter 4

Stretchable Displays

Researchers at the University of Tokyo have moved a step closer to displays and

simple computers that you can wear on your sleeve or wrap around your couch. And they

have opened up the possibility of printing such devices, which would make them cheap.

Takao Someya, an electrical-engineering professor, and his colleagues make a

stretchable display by connecting organic light-emitting diodes (OLEDs) and organic

transistors with a new rubbery conductor.

Figure 4.1 New printable elastic conductors made of carbon nanotubes are used to

connect OLEDs in a stretchable display that can be spread over a curved surface.




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The researchers can spread the display over a curved surface without affecting

performance. The display can also be folded in half or crumpled up without incurring any

damage.

In a previous Science paper, the researchers used their elastic conductor a mix of

carbon nanotubes and rubber to make a stretchy electronic circuit. The new version of the

conductor, described online in Nature Materials, is significantly more conductive and can

stretch to more than twice its original size. What's more, it can be printed. Combined with

printable transistors and OLEDs, this could pave the way for rolling out large, cheap,

wearable displays and electronics.

Bendy, flexible electronics that can be rolled up like paper are already available.

But rubber-like stretchable electronics offer the additional advantage that they can cover

complex three-dimensional objects. "With a sheet of paper, you can wrap a cylinder or a

cone, but that's pretty much it," says John Rogers, a professor of materials science and

engineering at the University of Illinois at Urbana-Champaign. "You can't wrap a body

part, a sphere."

To make such materials, researchers have tried several approaches. Rogers

uses ultrathin silicon sheets to make complex circuits on stretchy surfaces he recently

demonstrated aspherical camera sensor using the circuits. Others have made elastic

conductors using graphene sheets or by combining gold and rubbery polymers.

One challenge in creating stretchable electronics is to develop an electrode that

maintains its conductivity when deformed. To achieve this property, some researchers

have turned to carbon nanotubes because they are stretchable, conductive, and appear

transparent in thin layers, letting light shine through. However, for carbon nanotubes to

hold their shape, they must be attached to some surface. Coating carbon nanotubes onto aplastic backing has not worked well, because the nanotubes slide off or past each other

instead of stretching with the plastic. While some researchers have gotten around this

problem, they still were not able to make a completely stretchable OLED.




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Figure 4.2 Structure for stretchable displays

4.1 Displays using printing technology:

The new carbon nanotube conductor offers the advantage of being printable.

"The main advance is that they're able to print elastic conductors that are highly

conductive and highly stretchable," says Stephanie Lacour, who studies stretchable

electronic skin at the University of Cambridge, in England. "Printing is cheap, and it

allows you to cover large area substrate." engineers at the University of California, Los

Angeles, (UCLA) have taken a step toward these handy electronics by creating the first

fully stretchable organic light-emitting diode (OLED). Previously, researchers had only

been able to create devices that are bendable but can't stretch, or stretchable pieces that

connect smaller, rigid LEDs (Figure 4.3 and 4.4).




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Figure 4.3 LED matrix being stretched

Figure 4.4 LED matrix




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To make their device entirely pliable, the UCLA researchers devised a novel

way of creating a carbon nanotube and polymer electrode and layering it onto a

stretchable, light-emitting plastic. To make the blended electrode, the team coated carbon

nanotubes onto a glass backing and added a liquid polymer that becomes solid yet

stretchable when exposed to ultraviolet light. The polymer diffuses throughout the carbon

nanotube network and dries to a flexible plastic that completely surrounds the network

rather than just resting alongside it. Peeling the polymer-and-carbon-nanotube mix off of

the glass yields a smooth, stretchable, transparent electrode.

Figure 4.5 Organic ink that is used in printing technology

"The infusion of the polymer into the carbon nanotube coatings preserved the

original network and its high conductance," says Qibing Pei, professor of materials

science and engineering and principal investigator of the project.




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Figure 4.6 OLED display being stretched at UCLA

Figure 4.7 The stretchable OLED shown at longitudinal strain of 0 percent,20 percent, and 45 percent.

To create the stretchable display, the team sandwiched two layers of the carbon

nanotube electrode around a plastic that emits light when a current runs through it. The

team used an office laminating device to press the final, layered device together tightly,

pushing out any air bubbles and ensuring that the circuit would be complete when

electricity was applied. The resulting device can be stretched by as much as 45 percent

while emitting a coloured light.




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"The fact that the fabricated OLED can work under stretched conditions is quite

impressive," says Jay Guo, a professor of electrical engineering at the University of

Michigan who works on manufacturing plastic electronics.

The proof-of-concept device is a two-centimeter square with a one-centimeter

square area that emits a sky-blue light. This week, the group published an additional

paper showing that swapping in more-conductive silver nanowires for carbon nanotubes

in a similar process made a more efficient light-emitting diode.

This work is interesting and significantly different from past work, according

to John Rogers, a professor of materials science at the University of Illinois who develops

stretchable, deformable electronics.

Another benefit of the electrode is that it is less likely to short out. "Typically,

carbon nanotube film is rough, so that can cause shorting in electronic devices,"

says Zhenan Bao, a Stanford professor of chemical engineering who works on stretchable

solar cells. "Using this method, they ended up with a relatively flat surface that can be

used for an electrode."

She adds that the stretchable electronics demonstrated thus far lose conductivity

after being stretched too far or too many times, so more research is needed in this area.

"We are still some ways off from having high-performance, really robust,

intrinsically stretchable devices," says Bao, but "with this work and those from others, we

are getting closer and closer to realizing this kind of sophisticated and multifunctional

electronic skin."




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The mechanics define the wavelengths and the ribbons are intimately bonded to

the PDMS along their entire lengths. To achieve high stretchability, the PDMS can be

selectively activated by UV/ozone treatment so that only certain regions of the ribbons

bond strongly to PDMS. Upon releasing the prestrain in PDMS, the weakly bonded areas

of ribbons delaminate from the PDMS and form bridge like structures that are capable of

accommodating strains of up 100 % (Figure 5.2).

Figure 5.1 Schematic illustration of procedures for fabricating wavy and buckled

semiconductor nano ribbons on elastomeric PDMS substrates

Figure 5.2 Scanning electron micrographs of wavy Si and buckled GaAs ribbons




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In practical embodiments, such structures are encapsulated on top with

additional PDMS to eliminate the air gaps and to provide fully reversible stretching

behaviours.

These "wavy" structures of inorganic semiconductors on PDMS can be

reversibly stretched or compressed changes in amplitude and wavelengths accommodate

the externally applied strain. Figure 5.3 shows the response of Si ribbons to strain. When

the initially wavy Si ribbons (middle) are compressed, the amplitudes increase and

wavelengths decrease (top). The opposite is true for stretching (bottom). For functional,

stretchable electronic devices on PDMS, all the device processing steps, especially those

such as doping and contact metallization that can require high temperatures, are

performed on the source wafer. Subsequently, ribbons with integrated device layers areconfigured into wavy geometries using the processes mentioned above. Upon applying

compressive or tensile strains of10%, these devices show good electrical performance

without significant changes.

Figure 5.3 Images of wavy Si ribbons formed on a PDMS substrate.




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Figure 5.4 Optical images of 2D wavy structures in silicon nanomembranes at

various stages of biaxial compression, ranging from 0% to 3.8%.




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Figure 5.5 Printed elastic conductor




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Chapter 6

Why OLEDs for stretchable displays?

When evaluating a display, response time is the amount of time it takes for a

pixel to transition from one value to another and back. Low response times are essential

for better displays. Current LCD tech is simply unable to provide fast enough response

times to rival the smooth, clear and fluid movement provided by CRT’s of the past. This

problem is less prevalent with LCD monitors, but even the fastest of large LCD

televisions currently available are easily overmatched moreover these LCDs, CRTs have

a lot of complications when stretched.

The addition of input lag resulting from post processing techniques (such as

dynamic contrast and “true motion” features) used to improve image quality often

introduces further complications. What this means is fast moving images such as those

found in many video games are morphed into a blurry, eye straining mess. In many cases

hardcore gamers will find fast paced games like first person shooters practically

unplayable on LCD because they put the gamer at a competitive disadvantage against

those using faster, more responsive displays.

This is where OLEDs will shine in stretchable displays. Unlike LCD tech, which

suffer from response times ranging from 2 to 16 ms (milliseconds) and higher, OLED can

provide silky smooth motion video with response times below 1 microsecond (around .01

milliseconds). Input lag should also be eliminated, as OLED displays will require little to

no extra processing to improve contrast ratios and image quality the way LCD panels do.

Casual gamers may not notice or even care about this increased performance, but many

players will jump at the chance to reduce any lag in their gaming experience introducedby electronics.




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Chapter 7

Stretchy Advantages

The main advantage is these new carbon nanotube conductors are printable and

stretchable. Also, if the material can be used for both OLED and pressure sensors, they can most

likely be combined to create stretchable touch screens that we can wear. This could be very

advantageous to the everyday gadgets we carry around.

Advertisers would love stretchable displays. They could put images on more interesting

objects, rather than being limited to flat surfaces for video and image displays. Being able to

advertise on a relevant object to the product they're marketing would be awesome.

The stretchable wiring could make many other applications possible. Researchers could

use it to make sensitive artificial skin for robots or prosthetic limbs. Instead of OLEDS, they

would use pressure sensors on the printed conductor. Also, the electrodes could be used in

implantable medical devices to study or repair body organs. This would perhaps be the best use of

the stretchy wiring and displays; medical technology can always be improved. If this can be

developed into something that saves life’s or improves the quality of people’s lives, I think it is

imperative that it is developed further.




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Conclusion

The stretchable display technology which is being extensively tested by

operators is a step forward in the way to the future portable displays. However, several

challenges are yet to be solved. Several solutions are already on the stage, but

experimental analysis is needed to reveal if they are realistic and efficient. It would be

interesting to perform comparisons also through theoretical performance analysis to

compare between the currently used displays and these future displays.

Stretchable display technology is still in the early stages of development but it is

likely to be something that we are going to hear a lot about in the near future. As the

various trails taking place around the world begin to produce results, different groups and

organizations will make announcements about how they plan to move forward with the

technology. While most consumers will not be looking at using this technology

immediately, it is something that is highly likely to be part of their technological

vocabulary in the years to come.




References

1. Future concepts, “Stretchable OLED display” - http://www.itechfuture.com

2. Matthew Humphries, "Stretchable, printable, cheap OLED display created"

- http://www.geek.com

3. Prachi Patel, "Stretchable displays" Technology Review -

http://www.technologyreview.in

4. Dan Nosowitz, “Stretchable Electronics” - http://www.popsci.com

5. UCLA research, “Stretchable display” - http://www.ucla.edu

6. The University of Tokyo, “Printed Organic Transistors for Stretchable

Electronics” - http://www.ectc.net

7. Dr. Blessing,” International Workshop on Flexible & Stretchable Electronics”

-http://www.stella-project.de

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