ELECTRONIC PAPER”:  Organic Light Emitting Diodes

It’s All In The Way We See Things:

If ever a technology has begged to be disrupted, it is Liquid Crystal Displays.  Invented in 1963 and envisioned as a slimmed-down replacement for bulky cathode ray tubes or as screens for wall mounted televisions – a use never realized due to problems scaling up to large surfaces – liquid crystal displays have instead become the standard for everything from watches to laptop computers.  Despite this, however, remains high production and commercial expenses that have never come down enough to successfully mass market these displays, leaving the technology vulnerable to new innovations.

With the imaging appliance revolution underway, the need for more advanced handheld devices that will combine the attributes of a computer, PDA, and cell phone is increasing and the flat-panel mobile display industry is searching for a display technology that will revolutionize the industry.  The need for new lightweight, low-power, wide viewing angled, handheld portable communication devices have pushed the display industry to revisit the current flat-panel digital display technology used for mobile applications.  Struggling to meet the needs of demanding applications such as e-books, smart networked household appliances, identity management cards, and display-centric handheld mobile imaging devices, the flat panel industry is now looking at new displays known as Organic Light Emitting Diodes (OLED).

What Is Organic Light Emitting Diodes (OLED)?

Organic Light Emitting Diode technology, pioneered and patented by Kodak/Sanyo, enables full color, full-motion flat panel displays with a level of brightness and sharpness not possible with other technologies. 

Unlike traditional LCD’s, OLED’s are self-luminous and do not require backlighting, diffusers, polarizers, or any of the other baggage that goes with liquid crystal displays.  Essentially, the OLED consists of two charged electrodes sandwiched on top of some organic light emitting material.  This eliminates the need for bulky and environmentally undesirable mercury lamps and yields a thinner, more versatile and more compact display.  Their low power consumption provides for maximum efficiency and helps minimize heat and electric interference in electronic devices.  Armed with this combination of features, OLED displays communicate more information in a more engaging way while adding less weight and taking up less space.

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

Passive Displays: 

            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 Displays:

            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.

How It Works:

            The basic OLED cell structure consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode.  The organic layers comprise a hole-injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer.  When an appropriate voltage (typically between 2 and 10 volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electro luminescence).  The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device.

Advantages:

- Robust Design - OLED’s are tough enough to use in portable devices such as cellular phones, digital video cameras, DVD players, car audio equipment and PDA’s.

- Viewing Angles – Can be viewed up to 160 degrees, OLED screens 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’s are paper-thin.  Due to the exclusion of certain hardware goods that normal LCD’s require, OLED’s are as thin as a dime.

- 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’s.  Can be produced out of plastic and is bendable.  Also, OLED’s do not need lamps, polarizers, or diffusers.

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

  OLED                        LCD

Disadvantages:

- 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.  After a month of use, the screen becomes nonuniform.  Reds, and blues die first, leaving a very green display.  100,000 hours for red, 30,000for green and 1,000 for blue.  Good enough for cell phones, but not laptop or desktop displays.

- Overcoming LCD’s – LCD’s 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 LCD’s.  They will also continue to improve displays and search for new ways to reduce production costs.

Future Outlook:

            The OLED technology faces a bright future in the display market, as the ever-changing market environment appears to be a global race to achieve new success.  Eventually, the technology could be used to make screens large enough for laptop and desktop computers.  Because production is more akin to chemical processing than semiconductor manufacturing, OLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, and even head wearable displays.

            The OLED market appears to be expanding at a rapid pace.  Sales of passive OLED displays rose from $2 million to $18 million this year.  Projected sales by 2005 are expected to reach $717 million, with active matrix sales accounting for half of that.

Summary:

            The Organic Light Emitting Diode forms of display still have many obstacles to overcome before it’s popularity and even more importantly, its reliability are up to par with standards expected by consumers.  Although the technology presents itself as a major player in the field of displays, overcoming these obstacles will prove to be a difficult task.  However, the OLED’s advantages over LCD’s and future outlook have many in the industry goggle-eyed at the realm of possibilities.  For all we know and can hope for…OLED’s could change the ways in which we see things.