Flexible Organic Light-Emitting Diode Abstract :- A flexible organic light emitting diode (FOLED) is a type of organic light-emitting diode (OLED) 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. Currently the focus of research in industrial and academic groups.

2013

Group Members

Lokendra Ahuja Y11UC125

Mridul Gupta Y11UC140

Achal Khunteta Y11UC013

Chanish Agrawal Y11UC076

Introduction

An exciting technology has been available in many small devices such as cell phones and digital camera displays for the last 13 years. Soon it may available for use in larger standard office and home entertainment displays. The technology is organic light emitting diode (OLED). It is possible that in the next 2-3 years you may see an 80” OLED in your living room or board room that only requires 10 or less volts of power to operate. OLED display devices use organic carbon-based films, sandwiched together between two charged electrodes. One is a metallic cathode and the other a transparent anode, which is usually glass. Online encyclopedia, Wikipedia, defines an organic compound as “any member of a large class of chemical compounds whose molecules contain carbon, with the exception of carbides, carbonates, carbon oxides and gases containing carbon.” The basic components and Structure of an OLED are(Fig1): • Substrate. This is support for the OLED. • Anode. The anode removes electrons when a current flows through the device. • Organic layers. These layers are made of organic molecules or polymers. - Conducting layer. This layer is made of organic plastic molecules that send electrons out from the anode. - Emissive layer. This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made. • Cathode (may or may not be transparent depending on the type of OLED). The cathode injects electrons when a current flows through the device.

Fig1 : OLED Structure

Applying the organic layers to the substrate can be accomplished in three ways: 1. Vacuum Deposition or Vacuum Thermal Evaporation (VTE) :- In a vacuum chamber, the organic molecules are evaporated through a slow heat process and then allowed to condense as thin films onto a cooled substrate. This is a very inefficient and expensive process. 2. Organic Vapor Phase Deposition (OVPD) :- This process employs an inert carrier gas (such as nitrogen) to precisely transfer films of organic material onto a cooled substrate in a hot-walled, low-pressure chamber. The precise transfer and ability to better control film thickness translates to lower material cost and higher production throughput. 3. Inkjet Printing :- OLEDs are sprayed onto the substrate the same way our desktop inkjet printer sprays ink onto paper. This greatly reduces the cost of manufacturing OLEDs and allows for printing on very large films. This allows for a much lower cost and larger home displays and PIPD products. One of the major benefits of OLEDs is their low power consumption when compared to traditional LEDs or LCDs. OLEDs also do not require backlighting to function, which in addition to using less power, also lowers manufacturing costs. Even with all the layers that make up an OLED, this is an emissive technology – meaning it generates its own light. An OLED display is very thin and compact, typically has a viewing angle of 160 degrees and will operate on as little as 2 volts. Imagine today’s typical 60” flat-screen display, but instead of an 8-in. thick, 250-lb. plasma display or a 65-lb. LCD, your 60” OLED display is only 1/2” thick and weighs roughly 30 lbs.!

How do OLEDs work?

As previously mentioned, OLEDs are an emissive technology, which means they emits light instead of diffusing or reflecting a secondary source, as LCDs and LEDs currently do. Below is a graphic explanation of how the technology works.(Fig 2)

Fig3 : OLED Working

Types of OLEDs There currently are six types of OLED screens, each designed for a different type of use. The types are: 1. Passive Matrix OLEDs (PMOLEDs) (Fig 3) have strips of cathode, organic layers and strips of anode. The anode strips are arranged perpendicular to the cathode strips. The intersections of the cathode and anode make up the pixels where light is emitted. External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Again, the brightness of each pixel is proportional to the amount of applied current.

Fig 4 : Passive Matrix OLED

PMOLEDs are easy to make, but they consume more power than other types of OLED, mainly due to the power needed for the external circuitry. PMOLEDs are most efficient for text and icons and are best suited for small screens (2- to 3-inch diagonal) such as those you find in cell phones, PDAs and MP3 players. Even with the external circuitry, PMOLEDs consume less battery power than the LCDs that are currently used in these devices.

2. Active-matrix OLEDs (AMOLEDs) (Fig 4) have full layers of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image.

AMOLEDs consume less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. AMOLEDs also have faster refresh rates suitable for video. The best uses for AMOLEDs are computer monitors, large-screen TVs and electronic signs or billboards.

Fig 5: Active Matrix OLED (AMOLED)

3. Transparent OLEDs (Fig 5) have only transparent components(substrate, cathode and anode) and, when turned off, are up to 85% as transparent as their substrate. When a transparent OLED display is turned on, it allows light to pass in both directions. A transparent OLED display can be either active- or passive-matrix. This technology can be used for heads-up displays.

Fig 6: Transparent OLED’s

4. Top-emitting OLEDs (Fig 6) have a substrate that is either opaque or reflective. They are best suited to active-matrix design. Manufacturers may use top-emitting OLED displays in smart cards.

Fig 7: Top- Emitting OLED’s

5. Foldable/Flexible OLEDs (Fig 7) have substrates made of very flexible metallicfoils or plastics. Foldable OLEDs are very lightweight and durable. Their use in devices such as cell phones and PDAs can reduce breakage, a major cause for return or repair. Potentially, foldable OLED displays can be sewn into fabrics for “smart” clothing, such as outdoor survival clothing with an integrated computer chip, cell phone, GPS receiver and OLED display sewn into it.

Fig 8 : Foldable OLED’s

6. White OLEDs (Fig 8) emit white light that is brighter, more uniform and more energy efficient than that emitte by fluorescent lights. White OLEDs also have the true-color qualities of incandescent lighting. Because OLEDs can be made in large sheets, they can replace fluorescent lights that are currently used in homes and buildings. Their use could potentially reduce energy costs for lighting.

Fig 9 : White OLED’s

Flexible Display Prototypes The primary reason for the interest in flexible displays also arises from the early success of prototypes manufactured by a number of organizations. A number of display system papers are also presented, including a high-resolution LCD based on amorphous silicon, a full-color LCD based on low-temperature polysilicon, and an OLED display that was manufactured using a TFT transfer process ,Clearly the ability to prototype high-quality flexible display prototypes has had a tremendous impact on the continued interest and investment in the flexible display arena. The prototypes show the industry what is possible. Flexible OLED based displays Concepts

1. Nokia Morph and Kinetic concepts :- Nokia first conceptualized the application of flexible OLED displays in mobile phone with the Nokia Morph concept mobile phone. Released to the press in February 2008, the Morph concept was project Nokia had co-developed with the University of Cambridge. Nokia renewed their interest in flexible mobile devices again in 2011 with the Nokia Kinetic concept.

2. Sony :- Sony Electronics expressed interest for research and development towards a flexible display video display since 2005. They mainly worked upon TFT driven OLED displays.

3. Samsung :- Samsung prototypes incorporates the flexible AMOLED display technology. They have named their technology as Youm.

4. ASU :- Arizona State University in partnership with Army research lab announced that it has successfully manufactured the world's largest flexible OLED display using TFT technology.

Fig 10 : Developed Prototypes

OLED Advantages

• Pocket Friendly: In this technology, there is no use of glass which makes it practically unbreakable. The plastic, organic layers of an OLED are0T 0T5Tthinner, lighter and more flexible0T5T 0Tthan the crystalline layers in an LED or LCD. Smartphones with this technology will be more0T 0T1Tpocket friendly0T1T 0Tas they will be slimmer, lighter and will be less prone to scratches. In terms of design, a flexible display would allow for much greater variety in the shapes and form factors of our smartphones. The display can be edge-to-edge, with no need for a bezel or border. It could even bend around a rigid device to display specific data on the edge, or provide touch sensitive controls, so that they don’t have to be accommodated on the main display. It would also open up new possibilities in the accessory market.

• Energy Efficient:1T OLED screens are more energy efficient1T. The single greatest reason that

people are interested in OLED screens on mobile phones is that these screens are far more energy-efficient than LCD screens. These screens do not require any sort of backlight to be lit which significantly reduces the consumption of power when using the phone. This means that the battery of the phone can last longer without requiring recharging. As phones become increasingly complex, handset designers need to do all that they can to reduce the drain on the battery.

• Better Viewing Experience:1T OLED screens are more vibrant1T. People who want a phone

experience that is pleasing to the eye tend to enjoy OLED screens because the colors on these screens are more vibrant than those on traditional LCD screens. The way that the OLED screen pixels directly emit light allows for a better viewing experience in terms of the contrast, brightness, and coloring on the screens. Additionally, the technology means that the colors are less affected when the viewing angle changes than what we see on LCD screens so you can hold your phone in different ways and still get the same viewing xperience.

• Less Response Time:1T OLED screens may be faster1T. The response time for this technology is slightly quicker than that of LCD screens which could make the user experience better on these phones.

• Potential of Lower Cost: The process of making OLED screens seems to have the potential

to be less costly than that for LCD screens. At the current time, manufacturing costs are high because it’s a new technology but it is believed that future development of this technology could lead to a reduced cost for these screens. There is some possibility that handset makers will pass on that savings to consumers with lower-priced phones although this is by no means guaranteed in the market.

OLED Disadvantages • Shorter Life Span:1T OLED screens have a shorter life span1T. The OLED screen is not designed

to last as long as the LCD screen will. This means that people need to replace these phones more quickly than they must replace phones today. This could be considered a problem.

• Water Damage:1T OLED screens are more prone to water damage1T. Even the slightest contact

can ruin the display permanently. This is especially true in the case of mobile phone screens and other portable device screens. You shouldn’t have your phone near water anyway but you’ll find that the LCD screen can withstand some water damage better than the OLED screen can.

• Outdoor Performance:1T LED screens may be harder to see in direct sunlight1T. If you’re going

to be using your phone outdoors or in sunny rooms then you may find that the improved user experience is detracted from since you can’t see these screens nearly as clearly as LCD screens when you are viewing them in direct sunlight.

• Power Consumption: While an OLED will consume around 40% of the power of an LCD

displaying an image which is primarily black, for the majority of images it will consume 60–80% of the power of an LCD: however it can use over three times as much power to display an image with a white background such as a document or website.0T 0TThis can lead to reduced real-world battery life in mobile devices when white backgrounds are used.

• Current Costs: OLED manufacture currently requires process steps that make it extremely

expensive. Specifically, it requires the use of Low-Temperature Polysilicon backplanes. LTPS backplanes in turn require laser annealing from an amorphous silicon start, so this part of the manufacturing process for AMOLEDs starts with the process costs of standard LCD, and then adds an expensive, time-consuming process that cannot currently be used on large-area glass substrates.

• Hardware Compatibility issues: With smartphones, Companies may be able to create the

flexible display, but what about the internal hardware and the battery? Early designs look like they have a flexible display attached to a solid unit of some kind that contains the rest of the hardware. The first wave of releases will certainly not be fully flexible devices and it may be some time before we move beyond that model.

Possible application areas

It is generally accepted that one of the drawbacks of wearable computers is their limited I/O capabilities. Wearable devices used by large audiences such as PDAs, cell phones, and wristwatches can be revolutionized by having large flexible OLED displays that roll up and fit inside. Wearable devices used by different professions can benefit from flexible displays. Military personnel could use wrist or thigh mounted computers with foldable displays and helmets with transparent displays. Flexible displays can also be mounted on their clothing. Our interviews with medical personnel indicate that they want to wear compact equipment, but would like the added information that a flexible display could provide. So doctor’s equipment such as stethoscopes, otoscopes, ophthalmoscopes, blood pressure cuffs, etc. would be good candidates for flexible displays. Sports players could receive strategy and play information on displays mounted on clothing. Clearly it is possible to categorize the application of flexible OLED technology for several other professions. Over time, flexible displays could be incorporated into designer clothes to make fashion statements and dynamically change the look of clothes and accessories. One common wearable form factor, a pen, is often advertised as the ultimate device that will adopt flexible displays. At present the bend radius for most prototypes is around one inch. The display bend radius would have to reduce to few mm to fit into a pen form factor. The bend radius would have to reduce even further before the form factor shown in Popular Science [4] can be realized since the display rolls up and then folds in half. With flexible displays, the form factor of wearable devices will be constrained more by the size of the input mechanisms than by display size. By increasing the adoption of voice input for interaction, these devices can be made even smaller. As we mention in Section 4, flexible displays could be used as input devices if they are augmented with mechanisms to detect the amount of flexion. Curved displays are also ideal for immersive applications. Personal movie players may use domes or half cylinders to show movies that have IMAX-type immersive experiences. Display software would have to know the curvature of the display and the location of the viewer to draw the right image. Fortunately, work related to texture mapping in 3D graphics can be used in this regard. Single Unfurled :

We first look at novel ways of using a single flexible display that can be rolled up completely and stored inside the device. An interesting trade-off is to allow variable unrolling of the

display. Such a capability allows the application designer to provide the right level of detail based on the current display size. These picture show how a flexible display may be added to a pager like the BlackberryTM.

In the default view, when only a small part of the display is unrolled, a calendar application may just show the current appointment. If the user wishes to get more detail he can partially unfurl the display and get a summary of each appointment for the day. Unfurling the display fully may get even more detail and show things such as the location of the appointment, necessary phone numbers, and other contextfor the appointment. Applications designed to work with such rollable displays need to be adapt themselves to the extent to which the display is unfurled. Two sided unfurled : The next step in this direction is to make the flexible display two-sided. Since flexible displays can be less than 1 mm thick, making a display two sided does not add much weight or volume to the device but adds convenience. When the two sides face different users, the information may be presented differently. For instance, one side could show a presentation to an audience while the other side shows notes and the presentation. When used by a single user, applications may judiciously place information on one side or the other to control how readily the user can view the content. Information that is non-critical and distracting can be sent to the back side and an optional notification can be sent on the visible side informing the viewer that information is available on the other side. Consider an example illustrating the addition of a two sided rollable OLED display to a cell phone. As shown in Figure 7 and Figure 8, one side shows a crossword puzzle and the other side could have the solution. An email application could send the sender’s name on the visible side and the subject and content could be rendered on the other side.

Multiple unfurled : A prototype flexible display built by Universal Displays has a thickness of 0.7mm and a bend radius of 1 inch. Based on the above numbers, having more than one rolled display does not significantly increase dimensions of the device. A 10 inch display would need 10/(2π x 1) or approximately 1.5 revolutions and add only 1-2mm to the cylinder on to which the is rolled. Therefore it is reasonable to think about using multiple rollable displays per device that can be viewed simultaneously. Multi-head displays have shown significant benefit in traditional computing. For example cutting and pasting from one window to another is much simpler when the two windows (too large to fit in one display) are on two different displays attached to the same computer. Having multiple displays rolled up introduces a few more mechanical challenges that however can be surmounted. Paradigms of multiple desktops are easily realized when multiple display surfaces are available. For example as shown in Figure 9, one screen can be used as a work area for editing documents, and another screen can be used for receiving notifications, instant messages and so on. As shown in Figure 10, the two displays could be rolled on a single spool inside the device. Alternatively, the two displays could be positioned above the keyboard as shown in Figure 11. The power really increases when the multiple displays are themselves two sided. Applications such as electronic books shown in Figure 12 become possible. In addition we see potential for reducing context swapping and simplifying the UI navigation since multiple displays contain information and switching from one to another is possible with a simple turning of the page.

Additional design issues

Designing a device with a flexible display requires several mechanical and thermal developments as well. As flexibility of the display increase, (i.e., bend radius reduces), it becomes important to support the display when it is unfurled. One way could be using telescoping support bars/trusses or special materials that can be polarized to increase rigidity. In order to save power, CPUs in wearable computers prefer to go into low power states at every available opportunity. For usability, the display should have an option where it maintains its contents even when the CPU has gone to sleep. If this feature is supported, the CPU may display a page of information and immediately enter a lower power state till the next event. Since the flexible display may be used in various states of unrolling, it should be possible to detect the extent of unroll and also save energy by turning off the circuitry associated with the non-visible portions of the display. The degree to which the display is unrolled should be passed on to applications through an interface. The preferred mode of usage is to roll up the display immediately after use. So any heat generated while the display is in use should be rapidly dissipated to avoid damaging the display or other circuits inside the wearable The telescoping bars could also possibly conduct away excess heat. The spindle for the display spool can also conduct away excess heat. Applications may also adaptively reduce heat generation, perhaps by reducing the brightness when the user is expected roll up the display soon. Flexible displays should be able to tolerate a large number of roll and unroll cycles in order to be used in this

manner. Displays should preferably have sensing mechanisms that measure any degradation as a result of roll/unroll cycles or usage and applications should be able to adapt to display degradation, if any. Sensors to detect the curvature of the display at different locations could be used by applications to tailor content that is presented. One possible mechanism is to use strips of piezoelectric materials. For example, more private views may be provided when the curvature of the display is higher. With reliable indications of curvature, it may be possible for a user to flex and unflex the display as an input mechanism. We feel this mechanism is superior to a touch screen since the user gets a tactile feedback when he flexes the display. For example a gesture similar to turning a page in a book, as shown in Figure 13, may be used to scroll the information presented on the display. As shown in folding the top right portion of the display could scroll up or perform some other action.

Flexible displays in the environment

Wearable devices can augment their capabilities if they can easily leverage other computing and display devices in the environment. We also described how enhanced user experience is possible when displays are elevated to first class network objects that can advertise their services, and such services are leveraged by handhelds or other portable computers that users carry. Environmental devices and personal wearables establish symbiotic relationships to enable the user to interact with displays at greater levels of ease and comfort. With flexible OLED technology, it becomes possible for many more surfaces in the environment to sport displays. Flexible OLED properties such as impact resistance and temperature tolerance enable us to deploy displays in environments where traditional display devices may be considered to be too fragile. The ability to unfurl the display, use it and store it away compactly is a key advantage that flexible displays offer. A usage situation that is commonly known is the unfurling of a screen to project the image from an LCD projector. The ability to roll away the screen when not required allows us to place something else on the wall behind the screen (such as a chalk board, for instance). With flexible OLED displays, several environmental surfaces can support multiple uses. For instance there may be paintings, photographs or even a window on a

portion of wall that is temporarily obscured when the user needs to use a display. Roll up OLED displays may be used on a refrigerator door to show recipes, shopping lists, etc., when needed. One may consider adding mechanisms that enable network connected displays to unfurl by themselves to notify users of events that have occurred. The movement of an unfurling display may be more capable of attracting user attention compared to a fixed display and is less annoying than an audio alert.

Conclusions

We summarized the recent advances in flexible OLED display technology and have explored the ways in which one might add flexible OLED displays to wearable computers. We believe that flexible OLEDs open up several interesting opportunities to make wearable computers lighter, more compact, more energy efficient, as well as more usable. We discussed several configurations of displays and how these might be used. We hope the wearable computing community will play a key role in influencing flexible OLED technology and accelerate the realization of the immense potential of such displays.

References

1. http://en.wikipedia.org/wiki/Flexible_organic_light-emitting_diode 2. http://en.wikipedia.org/wiki/oled 3. Savage, N. (1999) Flexible displays – electronic paper coming to market. Laser Focus World. 35, 42–46 4. W. E. Howard Organic Displays Coming to Market. Scientific American, Jan 2004. 5. J. N. Bardsley, International OLED Technology Roadmap: 2001-2010, U. S. Display Consortium