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What is OLED? How does it work?

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Introduction: What is OLED? How does it work?

OLED technology was invented and patented by Eastman Kodak/Sanyo in the early 1980s. In short, OLED stands for organic light-emitting diode – a display device that sandwiches carbon-based films between two charged electrodes, one a metallic cathode and one a transparent anode, usually being glass. The organic films consist of a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When voltage is applied to the OLED cell, the injected positive and negative charges recombine in the emissive layer and create electro luminescent light. Unlike LCDs, which require backlighting, OLED displays are emissive devices - they emit light rather than modulate transmitted or reflected light.

Types of OLED

There are two forms of OLED displays: Passive-matrix and Active-matrix.

Passive-matrix

The passive-matrix OLED display has a simple structure and is well suited for low-cost and low-information content applications such as alphanumeric displays. It is formed by providing an array of OLED pixels connected by intersecting anode and cathode conductors.

Organic materials and cathode metal are deposited into a “rib” structure (base and pillar), in which the rib structure automatically produces an OLED display panel with the desired electrical isolation for the cathode lines. A major advantage of this method is that all patterning steps are conventional, so the entire panel fabrication process can easily be adapted to large-area, high-throughput manufacturing.

To get a passive-matrix OLED to work, electrical current is passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers attached to each row and column. An external controller circuit provides the necessary input power, video data signal and multiplex switches. Data signal is generally supplied to the column lines and synchronized to the scanning of the row lines. When a particular row is selected, the column and row data lines determine which pixels are lit. A video output is thus displayed on the panel by scanning through all the rows successively in a frame time, which is typically 1/60 of a second.

Active-matrix

In contrast to the passive-matrix OLED display, active-matrix OLED has an integrated electronic back plane as its substrate and lends itself to high-resolution, high-information content applications including videos and graphics. This form of display is made possible by the development of polysilicon technology, which, because of its high carrier mobility, provides thin-film-transistors (TFT) with high current carrying capability and high switching speed.

In an active-matrix OLED display, each individual pixel can be addressed independently via the associated TFT’s and capacitors in the electronic back plane. That is, each pixel element can be selected to stay “on” during the entire frame time, or duration of the video. Since OLED is an emissive device, the display aperture factor is not critical, unlike LCD displays where light must pass through aperture.

Therefore, there are no intrinsic limitations to the pixel count, resolution, or size of an active-matrix OLED display, leaving the possibilities for commercial use open to our imaginations. Also, because of the TFT’s in the active-matrix design, a defective pixel produces only a dark effect, which is considered to be much less objectionable than a bright point defect, like found in LCD’s.

Advantages of OLED

--> Robust Design – OLED are sturdy enough to be employ in portable devices such as cellular phones, digital video cameras, digital audio devices, DVD players, car audio equipment and PDAs.

--> Viewing Angles – OLED can be viewed up to 160 degrees with screens that can provide a clear and distinct image, even in bright light.

--> High Resolution – High information applications including videos and graphics, active-matrix OLED provides the solution. Each pixel can be turned on or off independently to create multiple colors in a fluid and smooth edged display.

--> “Electronic Paper” – OLED are paper-thin because of the exclusion of certain hardware components, which a normal LCD would employ.

--> Production Advantages – Up to 20% to 50% cheaper than LCD processes. Plastics will make the OLED tougher and more rugged. The future quite possibly could consist of these OLED’s being produced like newspapers, rather than computer “chips”.

--> Video Capabilities – They hold the ability to handle streamlined video, which could revolutionize the PDA and cellular phone market.

--> Hardware Content – Lighter and faster than LCD. Can be produced out of plastic and is bendable. Also, OLED do not need lamps, polarizer or diffusers.

--> Power Usage – Takes less power to run (2 to 10 volts).

Disadvantages of OLED

--> Engineering Hurdles – OLED’s are still in the development phases of production. Although they have been introduced commercially for alphanumeric devices like cellular phones and car audio equipment, production still faces many obstacles before production.

--> Color – The reliability of the OLED is still not up to par. It has been reported that the screen may become non-uniform after a month of use. Generally, if the component itself is faulty, reds and blues die first, leaving a very green display – 100,000 hours for red, 30,000 for green and 1,000 for blue. Accordingly, OLED is sufficient for cellular phone but inadequate for laptop or desktop display incorporation due to the stated issue.

--> Overcoming LCDs – LCDs have predominately been the preferred form of display for the last few decades. Tapping into the multi-billion dollar industry will require a great product and continually innovative research and development. Furthermore, LCD manufacturers will not likely fold up and roll over to LCDs. They will also continue to improve displays and search for new ways to reduce production costs.

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*information from the preceding sections are taken from ELECTRONIC PAPER: Organic Light Emitting Diodes

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Sony's OLED display

*Press release file

SONY BEGINS MASS PRODUCTION OF FULL-COLOR ORGANIC

LIGHT EMITTING DIODE (OLED) DISPLAYS

New Thin Screens for Mobile Devices Realize CRT-Quality Picture Clarity and Color Gamut

TOKYO, Japan, Sept. 14, 2004 - This month, Sony Corporation will commence mass production of a full-color Organic Light Emitting Diode (hereafter OLED) display, which will be first implemented in the new Sony CLIE 'PEG-VZ90' personal entertainment handheld (also announced today for the Japanese market).

clie01.jpg

Photo: OLED display introduced in CLIE 'PEG-VZ90' handheld

OLED display is a self-luminous display that does not require a back-light, offering high contrast ratio, a quick response time and wide viewing angle-all in a package slimmer than current LCD modules. To enhance this display, Sony has employed its unique Super Top Emission technology for outstanding brightness and greater color gamut. These translate into image quality and clarity that could previously viewed only on CRT (cathode ray tube) displays.

The new 3.8-inch (or 9.7 cm) screen measures 2.14 mm thin, and makes it possible for users to enjoy a variety of high-quality content such as TV programs and digital still images on mobiles products such as the new CLIE VZ90 handheld without compromising on the viewing experience.

Key Features

1.Super Top Emission

As the name implies, Super Top Emission leads light emission from the upper side of the organic material, realizing a brightness level of 150cd per square meter. Also, by optimizing the thickness of the organic layer for each of the color components, RGB, Sony realized multi-interference of reflecting light. Additionally, with the cavity multi-reflection interference structure which enables emission of colors with high purity, bright and brilliant color gamut comparable to that of CRT is realized

clie02-e.gif

Note: left: Bottom Emission Structure; right: Super Top Emission Structure

2.Excellence in response time, viewing angle and contrast ratio

As OLED works with self-luminous organic materials, it has outstanding response time, without producing any afterimage even when displaying moving images (movies). Also with wide viewing angle and contrast ratio as high as 1000:1, high quality images can be realized on mobile products which are used in various occasions.

Main specifications for OLED display panel introduced in <<PEG-VZ90>>

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