ORGANIC LIGHT-EMITTING DIODE

An 'organic light-emitting diode' ('OLED') is any light-emitting diode (LED) whose emissive electroluminescent layer comprises a film of organic compounds. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited. They are deposited in rows and columns onto a flat carrier by a simple "printing" process. The resulting matrix of pixels can emit light of different colors.
Such systems can be used in television screens, computer displays, portable system screens, advertising, information and indication. OLEDs can also be used in light sources for general space illumination, and large-area light-emitting elements. OLEDs typically emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point-light sources.
A great benefit of OLED displays over traditional liquid crystal displays (LCDs) is that OLEDs do not require a backlight to function. Thus they draw far less power and, when powered from a battery, can operate longer on the same charge. OLED-based display devices also can be more effectively manufactured than LCDs and plasma displays. But degradation of OLED materials has limited the use of these materials. See Drawbacks.
OLED technology was also called Organic Electro-Luminescence (OEL), before the term "OLED" became standard.

Contents

History

Related technologies

Small molecules

PLED

TOLED

SOLED

IOLED

Working principle

Advantages

Drawbacks

Technology demos

Commercial uses

See also

References

Further reading

External links

History

Bernanose and co-workers first produced electroluminescence in organic materials by applying a high-voltage alternating current (AC) field to crystalline thin films of acridine orange and quinacrine.^[1][2][3][4] In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doped anthracene.^[5]
The low electrical conductivity of such materials limited light output until more conductive organic materials became available, especially the polyacetylene, polypyrrole, and polyaniline "Blacks". In a 1963 series of papers, Weiss ''et al.'' first reported high conductivity in iodine-"doped" oxidized polypyrrole.^[6][7][8] They achieved a conductivity of 1 S/cm. Unfortunately, this discovery was "lost", as was a 1974 report^[9] of a melanin-based bistable switch with a high conductivity "ON" state. This material emitted a flash of light when it switched.
In a subsequent 1977 paper, Shirakawa ''et al.'' reported high conductivity in similarly oxidized and iodine-doped polyacetylene.^[10] Heeger, MacDiarmid & Shirakawa received the 2000 Nobel Prize in Chemistry for "The discovery and development of conductive organic polymers". The Nobel citation made no reference to the earlier discoveries.^[11]
Modern work with electroluminescence in such polymers culminated with Burroughs ''et al.'' 1990 paper in the journal ''Nature'' reporting a very-high-efficiency green-light-emitting polymer.^[12]
The OLED timeline since 1996 is well documented on oled-info.com site.^[13]

Related technologies

Small molecules

OLED technology was first developed at Eastman Kodak Company by Dr. Ching Tang using Small-molecules. The production of small-molecule displays requires vacuum deposition, which makes the production process more expensive than other processing techniques (see below). Since this is typically carried out on glass substrates, these displays are also not flexible, though this limitation is not inherent to small-molecule organic materials. The term OLED traditionally refers to this type of device, though some are using the term SM-OLED.
Molecules commonly used in OLEDs include organo-metallic chelates (for example Alq3, used in the first organic light-emitting device^[14]) and conjugated dendrimers.
Recently a hybrid light-emitting layer has been developed that uses nonconductive polymers doped with light-emitting, conductive molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longevity that they have in the SM-OLEDs.

PLED

'Polymer light-emitting diodes' (PLED) involve an electroluminescent conductive polymer that emits light when subjected to an electric current. Developed by Cambridge Display Technology, they are also known as Light-Emitting Polymers (LEP). They are used as a thin film for full-spectrum color displays and require a relatively small amount of power for the light produced. No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial inkjet printing.^[15][16] The substrate used can be flexible, such as PET.^[17] Thus, flexible PLED displays may be produced inexpensively.
Typical polymers used in PLED displays include derivatives of poly(p-phenylene vinylene) and poly(fluorene). Substitution of side chains onto the polymer backbone may determine the color of emitted light^[18] or the stability and solubility of the polymer for performance and ease of processing.^[19]

TOLED

'Transparent organic light-emitting device' (TOLED) uses a proprietary transparent contact to create displays that can be made to be top-only emitting, bottom-only emitting, or both top and bottom emitting (transparent). TOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight.

SOLED

'Stacked OLED' (SOLED) uses a novel pixel architecture that is based on stacking the red, green, and blue subpixels on top of one another instead of next to one another as is commonly done in CRTs and LCDs. This improves display resolution up to threefold and enhances full-color quality.
1439

IOLED

'Inverted OLED' (IOLED) uses a bottom cathode that can be connected to the drain end of n-channel TFT especially for the low cost a-Si TFT backplane useful in manufacturing of AMOLED display.^[20] In contrast to a conventional OLED which anode is placed on the substrate.

Working principle

An OLED is composed of an emissive layer, a conductive layer, a substrate, and anode and cathode terminals. The layers are made of special organic polymer molecules that conduct electricity. Their levels of conductivity range from those of insulators to those of conductors, and so they are called organic semiconductors.

A voltage is applied across the OLED such that the anode is positive with respect to the cathode. This causes a current of electrons to flow through the device from cathode to anode. Thus, the cathode gives electrons to the emissive layer and the anode withdraws electrons from the conductive layer; in other words, the anode gives electron holes to the conductive layer.
Soon, the emissive layer becomes negatively charged, while the conductive layer becomes rich in positively charged holes. Electrostatic forces bring the electrons and the holes towards each other and recombine. This happens closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons, (unlike in inorganic semiconductors). The recombination causes a drop in the energy levels of electrons, accompanied by an emission of radiation whose frequency is in the visible region. That is why this layer is called emissive.

The device does not work when the anode is put at a negative potential with respect to the cathode. In this condition, holes move to the anode and electrons to the cathode, so they are moving away from each other and do not recombine.
Indium tin oxide is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the polymer layer. Metals such as aluminium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the polymer layer.^[21]

Advantages

The radically different manufacturing process of OLEDs lends itself to many advantages over flat-panel displays made with LCD technology. Since OLEDs can be printed onto any suitable substrate using inkjet printer or even screen printing^[22] technologies, they can theoretically have a significantly lower cost than LCDs or plasma displays. Printing OLEDs onto flexible substrates opens the door to new applications such as roll-up displays and displays embedded in clothing.
OLEDs enable a greater range of colors, brightness, and viewing angle than LCDs, because OLED pixels directly emit light. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an "off" OLED element produces no light and consumes no power. Energy is also wasted in LCDs because they require polarizers which filter out about half of the light emitted by the backlight. Additionally, color filters in color LCDs filter out two-thirds of the light.
OLEDs also have a faster response time than standard LCD screens. Whereas a standard LCD currently has around 8 millisecond response time(though can be much lower such as 2 miliseconds), an OLED can have less than 0.01ms response time.^[23]

Drawbacks

The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs typically have lifetimes of around 5,000 hours when used for flat-panel displays, which is lower than typical lifetimes of LCD or Plasma technology – each currently rated for about 60,000 hours, depending on manufacturer and model. But recent experiments have shown that it is possible to swap the chemical component for a phosphorescent one, if the subtle differences in energy transitions are accounted for, resulting in lifetimes of up to 20,000 hours for blue PHOLEDs.^[24]
The intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.
Commercial development of the technology is also restrained by patents held by Eastman Kodak and other firms, requiring other companies to acquire a license.^[25] In the past, many display technologies have become widespread only once the patents had expired; a classic example is the aperture grille CRT.^[26]

Technology demos

At the Las Vegas CES 2007, Sony showcased 11-inch (28 cm, resolution 1,024×600) and 27-inch (68.5 cm, full HD resolution at 1920×1080) models claiming million-to-one contrast ratio and total thickness (including bezels) of 5 mm. According to news reports, Sony plans to begin releasing TVs this year.^[27]
The Optimus keyboard currently in development by the Art. Lebedev Studio is expected to use 113 48×48-pixel OLEDs (10.1×10.1 mm) for its keys.
Sony plans to begin manufacturing 1000 11-inch OLED TVs per month for market testing purposes.^[28]
On May 25 2007, Sony publicly unveiled a video of a 2.5-inch flexible OLED screen which is only 0.3 millimeters thick.^[29] The screen displayed images of a bicycle stunt and a picturesque lake while the screen was flexed.
OLEDs can be used in High-Resolution Holography (Volumetric Display). Professor Orbit showed on May 12 2007, EXPO Lisbon the potential application of these materials to reproduce three-dimensional video.
OLEDs could also be used as solid-state light sources. OLED efficacies and lifetime already exceed those of incandescent light bulbs, and OLEDs are investigated worldwide as source for general illumination; an example is the EU OLLA project.OLLA project, EU OLLA website, retrieved on July 28 2007

Commercial uses

OLED technology is used in commercial applications such as small screens for mobile phones and portable digital audio players (MP3 players), car radios, digital cameras and high-resolution microdisplays for head-mounted displays. Such portable applications favor the high light output of OLEDs for readability in sunlight, and their low power drain. Portable displays are also used intermittently, so the lower lifespan of OLEDs is less important here. Prototypes have been made of flexible and rollable displays which use OLED's unique characteristics. OLEDs have been used in most Motorola and Samsung color cell phones, as well as some Sony Ericsson phones, notably the Z610i, and some models of the Sony Walkman.Electronic News, OLEDs Replacing LCDs in Mobile Phones, April 7 2005, retrieved on July 28 2007
eMagin Corporation is the only manufacturer of active matrix OLED-on-silicon displays. These are currently being developed for the US military, the medical field and the future of entertainment where an individual can immerse themselves in a movie or a video game.

See also

★ Comparison of display technology

★ Active-Matrix OLED (AMOLED)

★ Flexible electronics

★ Light-emitting diode (LED)

★ List of light sources

★ PHOLED

★ Surface-conduction electron-emitter display (SED)

★ Field emission display (FED)

★ Nano-emissive display

★ Organic semiconductors

★ Conductive polymers

★ Molecular electronics

★ Steven Van Slyke

★ Time Multiplexed Optical Shutter (TMOS)

References

1. A. Bernanose, M. Comte, P. Vouaux, ''J. Chim. Phys.'' 1953, '50', 64
2. A. Bernanose, P. Vouaux, ''J. Chim. Phys.'' 1953, '50', 261
3. A. Bernanose, ''J. Chim. Phys.'' 1955, '52', 396
4. A. Bernanose, P. Vouaux, ''J. Chim. Phys.'' 1955, '52', 509
5. E. Gurnee, R. Fernandez, , 1965
6. R. McNeill, R. Siudak, J. H. Wardlaw, D. E. Weiss, Electronic Conduction in Polymers. I. The Chemical Structure of Polypyrrole, ''Aus. J. Chem.'' 1963, '16', 1056
7. B. A. Bolto, D. E. Weiss, Electronic Conduction in Polymers. I. The Chemical Structure of Polypyrrole, ''Aus. J. Chem.'' 1963, '16', 1056
8. B. A. Bolto, R. McNeill, D. E. Weiss, Electronic Conduction in Polymers. III. Electronic Properties of Polypyrrole, ''Aus. J. Chem.'' 1963, '16', 1090
9. J. McGinness, P. Corry, P. Proctor, Amorphous Semiconductor Switching in Melanins, ''Science'' 1974, '183', 853
10. H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x, ''J. Chem. Soc., Chem. Commun.'' 1977, 578 - 580
11. The Royal Swedish Academy of Sciences, Nobel Prize in Chemistry 2000, retrieved on July 28 2007
12. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Light-emitting diodes based on conjugated polymers, ''Nature'' 1990, '347', 539 - 541
13. OLED-Info.com, OLED history, retrieved on July 28 2007
14. C. W. Tang, S. A. VanSlyke, Organic electroluminescent diodes, ''Appl. Phys. Lett.'' 1987, '51', 913
15. T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu, J. C. Sturm, Ink-jet printing of doped polymers for organic light emitting devices, ''Appl. Phys. Lett.'' 1998, '72', 519
16. J. Bharathan, Y. Yang, Polymer electroluminescent devices processed by inkjet printing: I. Polymer light-emitting logo, ''Appl. Phys. Lett.'' 1998, '72', 2660
17. G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, A. J. Heeger, Flexible light-emitting diodes made from soluble conducting polymers, ''Nature'' 1992, '357', 477
18. A. J. Heeger, in W. R. Salaneck, I. Lundstrom, B. Ranby, ''Conjugated Polymers and Related Materials'', Oxford 1993, 27-62
19. R. Kiebooms, R. Menon, K. Lee, in H. S. Nalwa, ''Handbook of Advanced Electronic and Photonic Materials and Devices Volume 8'', Academic Press 2001, 1-86
20. Ta-Ya Chu, Szu-Yi Chen, Chao-Jung Chen, Jenn-Fang Chen and Chin H. Chen,Highly efficient and stable inverted bottom-emission organic light emitting devies, Appl. Phys. Lett. 2006, '89', 053503.
21. R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Brédas, M. Lögdlund, W. R. Salaneck, Electroluminescence in conjugated polymers, ''Nature'' 1999, '397', 121
22. D. A. Pardo, G. E. Jabbour, N. Peyghambarian, Application of Screen Printing in the Fabrication of Organic Light-Emitting Devices, ''Adv. Mater.'' 2000, '12', No. 17, 1249
23. Samsung SDI, OLED - Passive Matrix (PM), retrieved on July 28 2007
24. Cambridge Display Technology, CDT Sees Rapid Progress in Blue Polymer Lifetime, September 6, 2006, retrieved on December 19, 2006
25. OLED-Info.com, Kodak | OLED-Info, retrieved on 28 July 2007
26. Howstuffworks, What causes the faint horizontal lines on my monitor?, retrieved on July 28 2007
27. Reg Hardware, Sony develops little'n'large OLED TV panels, January 11 2007, retrieved on July 28 2007
28. CNET News, Sony to sell 11-inch OLED TV this year, April 12 2007, retrieved on July 28 2007
29. Australian IT, Sony bends video display, May 28 2007, retrieved on July 28 2007

Further reading

★ Shinar, Joseph (Ed.), ''Organic Light-Emitting Devices: A Survey''. NY: Springer-Verlag (2004). ISBN 0-387-95343-4.

★ Yersin, Hartmut (Ed.), ''Highly Efficient OLEDs with Phosphorescent Materials''. Wiley-VCH (2007). ISBN 3-527-40594-1

External links

★ OLED information and news in Chinese

★ OLED information site with news, forums, articles, images and more

★ OLED TVs: Technology Advancements Thread (AVS Forum)

★ Structure and working principle of OLEDs and electroluminescent displays

★ News and info about OLED-technology

★ OLED Design Contest

★ OLED.info - Latest technologies & innovations in the OLED industry

★ OLED applications

★ Articles about OLED Tech.

★ AMOLED, Samsung SDI (flash site)

★ OLED Products news

★ Updates on Sony OLED displays