An organic light-emitting diode (OLED or organic LED), also known as organic electroluminescent (organic EL) diode, is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as smartphones, handheld game consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications.

OLED

OLED

There are two main families of OLED: those based on small molecules and those employing

polymers. Adding mobile ions to an OLED creates a light-emitting electrochemical cell (LEC) which has a slightly different mode of operation. An OLED display can be driven with a passive-matrix (PMOLED) or active-matrix (AMOLED) control scheme. In the PMOLED scheme, each row (and line) in the display is controlled sequentially, one by one, whereas AMOLED control uses a thin-film transistor backplane to directly access and switch each individual pixel on or off, allowing for higher resolution and larger display sizes.

Although its name looks similar, the OLED is fundamentally different from its cousin the LED used for general lighting and for which the Nobel Prize was awarded in 2014. The LED is based on a p-n diode structure. In a LED, doping is used to create p- and n- regions by changing the conductivity of the host semiconductor. The OLED is not a p-n structure. Doping of OLEDs is used to increase radiative efficiency by direct modification of the quantum-mechanical optical recombination rate. Doping is additionally used to determine the wavelength of photon emission. OLED doping is discussed further on in this article.

 

An OLED display works without a backlight because it emits visible light. Thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions (such as a dark room), an OLED screen can achieve a higher contrast ratio than an LCD, regardless of whether the LCD uses cold cathode fluorescent lamps or an LED backlight. OLED displays are made in the same way as LCDs, but after TFT (for active matrix displays), addressable grid (for passive matrix displays) or ITO segment (for segment displays) formation, the display is coated with hole injection, transport and blocking layers, as well with electroluminescent material after the 2 first layers, after which ITO or metal may be applied again as a cathode and later the entire stack of materials is encapsulated. The TFT layer, addressable grid or ITO segments serve as or are connected to the anode, which may be made of ITO or metal. OLEDs can be made flexible and transparent, with transparent displays being used in smartphones with optical fingerprint scanners and flexible displays being used in foldable smartphones.

Practical OLEDs

Chemists Ching Wan Tang and Steven Van Slyke at Eastman Kodak built the first practical OLED device in 1987. This device used a two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer; this resulted in a reduction in operating voltage and improvements in efficiency.

Research into polymer electroluminescence culminated in 1990, with J. H. Burroughes et al. at the Cavendish Laboratory at Cambridge University, UK, reporting a high-efficiency green light-emitting polymer-based device using 100 nm thick films of poly(p-phenylene vinylene). Moving from molecular to macromolecular materials solved the problems previously encountered with the long-term stability of the organic films and enabled high-quality films to be easily made. Subsequent research developed multilayer polymers and the new field of plastic electronics and OLED research and device production grew rapidly. White OLEDs, pioneered by J. Kido et al. at Yamagata University, Japan in 1995, achieved the commercialization of OLED-backlit displays and lighting.

In 1999, Kodak and Sanyo had entered into a partnership to jointly research, develop, and produce OLED displays. They announced the world’s first 2.4-inch active-matrix, full-color OLED display in September the same year. In September 2002, they presented a prototype of 15-inch HDTV format display based on white OLEDs with color filters at the CEATEC Japan.

Manufacturing of small molecule OLEDs was started in 1997 by Pioneer Corporation, followed by TDK in 2001 and Samsung-NEC Mobile Display (SNMD), which later became one of the world’s largest OLED display manufacturers – Samsung Display, in 2002.

The Sony XEL-1, released in 2007, was the first OLED television. Universal Display Corporation, one of the OLED materials companies, holds a number of patents concerning the commercialization of OLEDs that are used by major OLED manufacturers around the world.

On 5 December 2017, JOLED, the successor of Sony and Panasonic’s printable OLED business units, began the world’s first commercial shipment of inkjet-printed OLED panels.