Flexible AMOLEDs for Low-Power, Rugged Applications

Flexible AMOLEDs equipped with phosphorescent OLEDs are well-positioned for low-power, rugged, full-color video applications. Replacing glass with flexible substrates and thin-film encapsulation makes displays thinner, lighter, and non-breakable – all attractive features for portable applications. With enhanced flexibility and low power consumption, a range of revolutionary opportunities are being created.

by Ruiqing Ma, Mike Hack, and Julie J. Brown

FLEXIBLE DISPLAYS refer to those that can be fabricated and perform well on non-glass substrates such as plastic films or metal foils. There are two main categories of flexible-display technologies: reflective bistable types, such as cholesteric liquid-crystal displays and electrophoretic displays, and full-color video-rate organic light-emitting-diode (OLED) displays. While bistable reflective displays are ideally suited for e-reader applications and have the potential to replace printed-paper media, OLED displays are a much better choice for high-information-content full-color video applications because of their high contrast ratio, inherent ultra-fast switching, and vivid visual full-color appearance. Additionally, because of their simple, organic thin-film design, OLEDs can be easily built on flexible substrates. Finally, the incorporation of phosphorescent OLED (PHOLED) technology enables OLED displays to have a lower power consumption than conventional active-matrix liquid-crystal displays (AMLCDs) in video-mode operation.

A roadmap for flexible-display development in both bistable reflective and OLED displays is shown in Fig. 1.

The authors envision a five-phase roadmap for the PHOLED display: (1) low power – the displays are actually built on rigid glass and offer low power and thin form factor; (2) rugged – glass is eliminated so the displays are thin and non-breakable and may have limited flexibility, (3) bendable – displays are bendable and conformable, (4) rollable – the displays in this phase are extremely flexible in one dimension so they can be rolled around a cylinder with a small diameter, and (5) free form – this is the ultimate phase of flexibility, in which displays can be made into any form, just like paper.

Commercial products based on flexible-display technologies are currently in the low-power phase. There are already several e-readers and small-to-medium-sized AMOLED displays and televisions on the market. The chosen display technologies all offer low power consumption. Bistable reflective displays save power for e-reader applications because no power is needed to maintain a static image and the display does not emit light. For full-color video applications, AMOLEDs with PHOLED materials consume much less power than AMLCDs.

Rugged Applications

The second phase in the evolution of flexible displays has significant importance because it represents a new era in terms of flexibility. With the glass substrate replaced by rugged substrate materials, displays will be thinner, lighter, non-breakable, and safer – all extremely attractive features for portable applications. (Note that in Phase 2, the display itself may still have a flat-use form factor.) Many e-reader users are already nervous about their glass devices sitting next to hard objects or being dropped on the floor. A rugged display will resolve this problem.

Rugged AMOLED displays are increasingly being considered for a range of military and commercial applications. The first example involves a portable communications system incorporated into a wearable wrist display unit, as shown in Fig. 2(a). This system enables the user to view a variety of information, including real-time video, with hands-free operation. To demonstrate this technology, Universal Display Corp. (UDC), with its partners LG Display (LGD) and L3 Communications Display Systems (L3), fabricated a low-power-consumption wearabledisplay wrist unit,¹ which is thin, lightweight, and non-breakable, as shown in Fig. 2(b). In its present version, the flexible PHOLED wrist unit receives video signals from a separate interface electronics unit also worn by the user.

Fig. 1: Device evolution proceeds from left to right in this chart of flexible-display developments for bistable reflective displays (top) and full-color AMOLED video displays (bottom).

 (a)     (b)

Fig. 2: (a) At left, an artistic rendering shows a complete portable communications system with a wearable display and (b) at right, a user models a wearable prototype.

Another potential product is a non-glass ejection-safe digital display designed to replace printed-paper maps and checklists on pilots' knees in tactical cockpits. This will give the pilot access to a large amount of information in digital form without the need to carry a large number of paper documents. When the displays are flexible, they can be rolled up to save space. Moreover, removing the glass from a display system can allow a pilot to safely eject from an aircraft with the display still available for use outside of the aircraft. One can also envision applications of such a non-breakable display in the consumer marketplace.

Yet another example is a full-color flexible hands-free personal-digital-assistant (PDA) display to increase the operational capability of its user for both daytime and covert nighttime functionality. The PHOLED display could be made to have both visible and near-infrared (NIR) emitting pixels. The visible pixels would be operated during the day and the NIR pixels at night with the visible pixels turned off and viewed through the use of night-vision equipment. The rugged high-information-content display would not give away the user's position during nighttime situations. This dual-mode operation will be a unique attribute of the display. A prototype display with both visible and IR capability² is shown in Fig. 3.

A hands-free PDA would have numerous consumer uses as well, some of which may not even be contemplated until such a display is readily available.

Low-Power PHOLED

Through the use of phosphorescent OLED (PHOLED) technology for the pixel design, AMOLED displays can offer lower power consumption than AMLCDs. In conventional fluorescent OLEDs, the internal quantum efficiency is limited to approximately 25% because only the singlet excitons recombine to emit light. In a PHOLED system, how-ever, heavy-metal atom centers enable efficient spin-orbit coupling.³ The spin-orbit coupling allows both singlet and triplet excitons to be harvested as phosphorescent radiation in the guest-host systems, leading to internal quantum efficiencies (IQEs) of up to 100%.⁴

The IQE is the percentage of injected electrons that form photons and is directly related to the power consumption of the OLED device. Another key attribute of OLEDs is lifetime, which limited their early adoption in commercial products but has improved significantly in recent years. One typical measurement of OLED lifetime is LT50, defined as the time it takes an OLED, under constant current, to reach 50% of its initial luminance level (typically at 1000 nits).

The very high IQE makes PHOLED technology ideal for both flat-panel-display and solid-state-lighting applications, and close to 100% internal emission efficiency has now been reported for all primary colors. Additionally, phosphorescent device lifetimes have rapidly increased to the point where they are competitive with the best in the industry. Two recent examples of red-PHOLED performance includes bottom-emission devices with 1931 CIE coordinates of (0.67,0.33) and (0.65,0.35) and a luminous efficiency and (LT50) lifetime at 1000 nits of 22 and 24 cd/A and 200,000 and 300,000 hours, respectively. Very recent progress in bottom-emission green PHOLEDs has achieved a luminous efficiency of 63 cd/A and an LT50 lifetime at 1000 nits of 500,000 hours. Advancements are continuously being made in blue-PHOLED device performance towards commercial entrylevels. Most recent progress has been reported with an all-phosphorescent system for warm-white lighting applications with a long-lived phosphorescent light-blue emitter-host material set. At 1000 cd/m², the 2 x 2-mm² phosphorescent white-OLED (WOLED) pixel has a power efficacy of 79 lm/W, CRI of 80, CCT = 2910K, CIE = (0.456, 0.430), and lifetime LT70 = 30,000 hours.⁵

Fig. 3: A prototype PDA is shown in visible mode (left) and IR mode (right). The IR mode is imaged through an IR monocular which replaces IR emission with green emission.

Low power consumption is a key display requirement for mobile applications. The use of PHOLED technology will enable displays to have a lower power consumption than that of backlit AMLCDs, significantly extending battery life for mobile devices and providing significant power savings compared to that of fluorescent OLED technology. Figure 4 shows simulations of the power consumption of a 3.5-in. display showing video content operat-ing at 360 cd/m² for various technology options.

Also shown for comparison is the power consumption of an equivalent AMLCD operating at 450 cd/m², having perceived visual performance similar to that of an AMOLED operating at 360 cd/m² because of the higher contrast ratios of OLED displays. While the AMLCD backlight consumes 550 mW, the plot shows power savings that can be achieved by incorporating PHOLED technology. First, using just a red PHOLED subpixel, power consumption can be reduced to 428 mW; the addition of green-PHOLED subpixels leads to a further power reduction to 269 mW, and the use of an all-PHOLED system will reduce the power consumption to less than 175 mW.

The high conversion efficiency of PHOLEDs has additional benefits to AMOLED technology, and particularly to flexible AMOLED displays. The lower-drive-current requirement of PHOLEDs will make it easier to use lower mobility backplanes such as a-Si (and perhaps eventually organic TFTs). These technologies will be very important as they enable backplanes to be fabricated at lower temperatures than that of conventional low-temperature polysilicon (LTPS), facilitating the launch of AMOLEDs on plastic substrates. In addition, the lower-drive-current requirement of PHOLEDs also reduces the display operating temperature, which will extend the display operational lifetime. The higher efficiencies lead to at least a three-times reduction in display temperature rise compared to that of a fluorescent OLED display.⁶ This is an important consideration for mobile devices. Lower pixel currents will also allow for higher bus-line resistances, enabling thinner metallization, which will also simplify the manufacture of displays on flexible substrates.

Rugged AMOLED Fabrication

Two critical technologies that enable flexible AMOLEDs are flexible active-matrix backplanes and thin-film encapsulation.

Flexible Backplanes: The two material groups being considered as substrates for rugged AMOLED displays are metal foils and plastic films. Thin plastic films have excellent flexibility; however, their moisture-barrier property is poor and their thermal properties limit the maximum process temperature of the backplanes. Conventional Si-based TFT-backplane processes for AMOLED displays require temperatures in excess of 250°C, which cannot be supported by current available plastic-substrate systems. Flexible metal foils offer a number of desirable advantages, including reasonable flexibility, enhanced thermal and mechanical durability, and excellent permeation barrier property, but the surface tends to be rough and the materials are opaque.

Currently, most glass-based AMOLEDs are fabricated using poly-Si backplanes, and several commercial products are already in the marketplace. Poly-Si has high mobility and the advantage of enabling the integration of driver circuits around a display periphery. This is advantageous for small arrays because it significantly reduces the cost of the driver electronics. However, with the use of poly-Si TFT backplanes, the variation in TFT threshold voltages can cause significant variations in image intensity across a display. Improved display uniformity can be obtained by employing correction schemes, either external or internal to each pixel. While poly-Si is proving itself to be a viable AMOLED backplane technology, the high efficiency of the phosphorescent material system enables the use of a-Si backplanes. (For more information on poly-Si and OLEDs, see "Emerging Technologies for the Commercialization of AMOLED TVs" in the September 2009 issue of Information Display.) The advantages of a-Si backplanes for AMOLED production are excellent uniformity, lower cost, and the ability to leverage off the large a-Si TFT manufacturing base that already serves the AMLCD industry. In addition, a-Si TFTs perform better under strained conditions due to the amorphous nature that makes them better suited for flexible applications. The main issue with a-Si is the shift of threshold voltage over time. Some of the on-going efforts to address this challenge include (1) improvement in the transistor design and fabrication process – one example is to use an inverted OLED structure, (2) pulsed driving or negative bias – these have been proven to be effective in reducing the threshold-voltage shift,⁷ and (3) compensation circuit – this requires a complex circuit at the pixel level (more than four transistors).⁸

Fig. 4: A simulated power-consumption roadmap for a 3.5-in. AMOLED display uses different combinations of fluorescent and phosphorescent OLED technologies. Assumptions are 360 cd/m² and a 40% video rate. Also shown is the equivalent power consumption for backlit AMLCD at 450 cd/m².

Some other backplane technologies being investigated, which may also prove viable for flexible displays, include oxide TFTs, microcrystalline-silicon TFTs, and organic TFTs. Researchers around the world have been actively working in this field, and progress has been made over the years. These include an AMOLED driven by LTPS on metal foil developed by Samsung,⁹ an AMOLED driven by OTFT on plastic developed by Sony,¹⁰ an oxide-driven AMOLED on metal foil developed by LGE, and an a-Si driven all-phosphorescent AMOLED on metal foil developed by UDC–LGD–L3.

Thin-Film Encapsulation: The other critical component for flexible OLEDs is the development of a flexible permeation barrier. OLEDs degrade as a result of exposure to atmospheric oxygen and water, causing oxidation and de-lamination of the metal cathode, as well as detrimental electro-chemical reactions within the organic layers. Because most OLED work to date has been focused on the development and manufacture of glass-based displays, the degradation problem has been addressed by sealing the display in an inert atmosphere using a glass or metal lid attached by a bead of UV-cured epoxy resin. For flexible displays, the rigid glass covers cannot be used, so a flexible encapsulation solution is needed. For OLEDs built on plastic, both sides of the OLEDs need barrier protection. Because metal is an ideal oxygen/ water permeation barrier, OLEDs fabricated on metal foil only require encapsulation after display fabrication.

UDC has demonstrated encapsulated flexible PHOLED displays on metal substrates using a thin-film-encapsulation system developed by Vitex Corp. Vitex is pursuing a multi-layered barrier consisting of alternating layers of polymer and inorganic oxide layers that can be deposited as a monolithic encapsulation over an OLED cathode. By incorporating these multiple layers, the polymer films "decouple" any defects between the oxide layers, thereby preventing propagation of defects through the multi-layered structure and allowing the organic films to protect the barrier layers from mechanical damage.

More recently, a novel permeation barrier has been reported that uses a single-chamber plasma-enhanced chemical-vapor-deposition (PECVD) process capable of depositing a low-stress transparent single-layer barrier film for flexible OLED encapsulation.¹¹The barrier is a single-phase hybrid material deposited by PECVD from hexamethyl disiloxane and oxygen that combines the hermeticity of SiO₂ with sufficient toughness similar to that of a silicone polymer, such that it appears not to form microcrack permeation paths. In accelerated storage tests at 65°C and 85% relative humidity, the half-life of OLEDs coated with this barrier has exceeded 1 year. Figure 5 shows the results of pixel photos and active area versus ageing.

Preliminary results of this work show great promise from both performance and manufacturing-process simplicity.

UDC Flexible AMOLED Prototypes

UDC, working with LGD and L3, demonstrated a 4-in. QVGA 100-ppi top-emission AMOLED display on a-Si TFT backplanes built on metal foils at Display Week 2007, as shown in Fig. 6(a). One year later, the company was able to obtain much better display performance, and the 4-in. QVGA full-color video prototype consumed less than 1 W at 100 cd/m² full white, as shown in Fig. 6(b). At SID '08, UDC also demonstrated an ultra-thin AMOLED having a total thickness of ~30 μm.¹² Flexibility results on this razor-thin display showed that it operated when conformed to a tight diameter of only 5 mm, as shown in Fig. 6(c). This work demonstrated the possibility of achieving a rollable AMOLED display at some point in the future.

At CES '09, the team demonstrated the world's first wrist-worn communications device built upon a flexible low-power-consumption full-color AMOLED. The device offers the wearer the ability to see high-information-content video-rate information in a thin-and-rugged-form-factor 4-in. QVGA display, conformed around a human wrist, as shown in Figs. 2(b) and 6(d).

At SID '09, the company published its result of 4-in. QVGA flexible AMOLED displays on temporary bonded polyethylene naphthalate substrates with 180°C a-Si:H TFTs, as shown in Fig. 6(e).¹³ This work was in collaboration with the Flexible Display Center at Arizona State University, and it demonstrated the viability of plastic substrates for flexible AMOLEDs. At FPD 2009, UDC demonstrated a 4.3-in. HVGA full-color display in collaboration with LG Display, as shown in Fig. 6(f). The display was built on 76-μm-thick metal foil with a total thickness of 0.3 mm and can be bent in both inward and outward directions. It offers 256 gray-scale levels per color (8 bit) and can portray a variety of images, including full-motion video. The specifications of the finished flexible display are shown in Table 1.

Fig. 5: Shown are the results of 65°C/85% RH ageing of 2-mm² test pixels encapsulated using single-layered thin-film encapsulation. TOLED and BOLED stand for top- and bottom emission OLEDs, respectively. TOLED2, BOLED4, and BOLED 5 represent different samples.

Summary and Outlook

Today, AMOLEDs on glass substrates are gaining traction in the marketplace. And significant progress is being made to achieve low-power AMOLED displays by using PHOLED technology. Building upon this platform, we believe that AMOLEDs in non-breakable substrates should soon be one of thenext growth areas. Although there is no current commercial product, the case for flexible dis-plays to be used for rugged applications is very convincing, as demonstrated by the examples and prototypes described in this article. Without glass, the display will be thinner, lighter, and non-breakable, all very attractive features for portable devices for both military and consumer applications. To make this happen, the two key challenges are the demonstration of a reliable, high-performance backplane technology on a flexible substrate and the development of a high-yield manufacturable thin-film encapsulation process.

Looking forward, the authors expect in the near future that flexible AMOLEDs will be first adopted in specialized applications in which rugged displays are needed. At the same time, the flexible AMOLED will keep improving in its optical performance, lifetime, and flexibility, and move into the rollable and ultimately free-form phases. With enhanced flexibility coupled with low power consumption, a range of revolutionary displays can be created: wrist-based PDAs, camcorders with a flexible screen (Fig. 7), cell phones with roll-out screens, and TVs that can be carried around as scrolls.

Fig. 6: (a)-(f) Some recent prototypes demonstrated by UDC and its partners.

Fig. 7: A camcorder concept with a flexible AMOLED screen (designed by Emory Krall from Universal Display Corp.).

Acknowledgments

The authors gratefully acknowledge the financial support from the Communications-Electronics Research, Development, and Engineering Center (Raymond Schulze), the Air Force Research Laboratory (AFRL) (Darrel Hopper, Gurdial Saini), and the Army Research Laboratory (David Morton and Eric Forsythe). The authors thank their collaborators at LG Displays, L3 Communications Display Systems, Flexible Display Center at Arizona State University, and Kyung Hee University (Prof. Jin Jang).

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