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How to Make a Flexible Active-Matrix OLED

by iamrmishra
Flexible OLED display

A paper-thin display that can be rolled up, wrapped around objects such as phones, hammered, and still function is a pipe dream for many people. There’s also an exhibit of high-speed motion pictures with pure color and high-quality viewing from all angles while using very little power. Organic LEDs are bringing such displays closer to reality (OLEDs).

OLEDs that glow

Eastman Kodak’s Tang et al. created the first efficient small-molecule OLED devices in the 1980s. Singlet excitons emit light in these fluorescent small-molecule OLEDs. Because most excitons are triplets, the internal quantum efficiency is around 25%. Universal Display Corp. of Ewing, N.J., is developing the next generation of OLEDs, high-efficiency phosphorescent devices.

The intersystem crossing around a heavy metal atom in a glowing system converts singlet excitons to triplets. The triplet states can radiate, allowing for 100% internal quantum efficiency.

Phosphorescent OLED displays use less power than backlit active-matrix LCD displays, extending battery life for mobile devices and saving energy. Reduced drive current also reduces display operating temperature, extending display lifetime. In addition to high conversion efficiency, phosphorescent OLEDs are ideal for flexible active-matrix OLED displays. The lower drive current requirement allows for amorphous silicon and eventually organic thin-film transistors as backplane technologies, allowing for more bus-line resistance tolerance and easier manufacturing of flexible displays.

Active-matrix LCD, which works in transmissive mode, requires both substrates to be transparent. As a result, the pixel-driving circuits block light transmission, resulting in low light output efficiency. Because OLED is an emissive display technology, it is made with opaque substrates like metal foils. The cathode emits light from the top surface of the Display. As a result, OLED material can be deposited on top of pixel driving circuits, allowing for total pixel light emission and high output efficiency.

Transparent compound cathode technology combines a transparent conductive oxide and a thin metal layer to enable high-efficiency top-emission OLEDs (if both surfaces are acceptable). A compound cathode offers better efficiency, lifetime, and yield than a thin-metal-only cathode.

Substrat malle

Most glass-based active-matrix OLEDs currently use polysilicon backplanes, and commercial products are available. However, the high efficiency of the luminous material system allows the use of amorphous silicon backplanes. These have lower costs and can use the sizeable undeveloped silicon thin-film transistor manufacturing base that already serves the active-matrix LCD industry.

Improved device and circuit design address the main issue with amorphous silicon: threshold voltage shift. Organic thin-film transistors are a promising new backplane technology, especially when combined with plastic substrates. However, stability and uniformity must be addressed before the transistors can be commercialized.

Flexible substrates for active-matrix OLED displays include adjustable thin glass, thin metal foils, and plastic films. However, it is less flexible, brittle, and difficult to handle than thick glass. In addition, the thermal properties of thin plastic films limit the maximum process temperature of conventional silicon-based thin-film transistor backplanes (300 °C).

Plastics also easily absorb oxygen and water, causing OLED degradation. On the other hand, the flexible metal foil has several advantages over current flexible plastic substrates, including more excellent thermal and mechanical durability, better permeation barrier properties, and lower cost. Therefore, universal Display has chosen metal foil substrates for current work on flexible OLED displays, even though the plastic film may become a viable option in the future.

Stainless steel foil used in OLED active-matrix displays must be flattened to make it work. Coating planarized steel foils achieve it with an electrically insulating polyimide layer and surface planarizing. Its performance is comparable to amorphous silicon thin-film transistors grown on glass substrates.

Boxes that fit

The development of a flexible permeation barrier is also critical for flexible OLEDs. OLEDs degrade in the presence of oxygen and water, causing metal cathode oxidation and delamination, and electrochemical reactions within the organic layers. Glass-based OLED displays can have this problem remedied by adding a glass or metal lid with an epoxy resin bead that has been UV-cured to the Display. An OLED built-on metal foil requires a conformal encapsulation layer.

Universal Display and Vitex Systems Inc. of San Jose developed a long-life thin-film that encapsulated OLEDs on metal foil substrates. The barrier is made up of polymer and inorganic oxide layers. The polymer layers prevent defects from propagating through the multilayer structure, protect the oxide layers from mechanical damage, and provide flexibility to the overall film. The results show that thin-film encapsulated OLEDs on metal foils have a similar lifetime to OLEDs in a glass-to-glass package.

An antireflection hard-coat film is laminated over the thin-film encapsulated flexible OLED to protect further and toughen a flexible OLED display. National Starch & Chemical of Bridgewater, N.J. Wrapping a 3.25-in. laminated Display around a 1.5-in. cylinder tested its conformability. After 3600 hours, there was no delamination.

We combined an amorphous-silicon backplane from LG Philips LCD of Seoul, South Korea, with phosphorescent OLED and flexible OLED technologies. As a result, this prototype may be the world’s first high-resolution full-color active-matrix OLED in a-Si backplane technology. In addition, its thin, light, and rugged design demonstrates the commercial viability of flexible display products.

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