Dictionary of Terms in Organic Electronics. Specific Terms from I to L.

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Light Emitting Diodes, (LEDs)

Light Emitting Diodes (LEDs) are electronic devices that produce light in solid inorganic or organic materials by means of electroluminescence under the action of electric current or field.

Organic Light Emitting Diodes (OLEDs) are LEDs that contain organic compounds in light emitting and/or carrier-transporting medium. Typical structures of mono- and multilayered OLEDs are shown on the scheme below. Sandwich-like structures include cathode made of low work-function metal (usually alloys of aluminum, magnesium, silver, or calcium); carrier-transporting and emitting layers of organic semiconductors; and transparent anode, usually made of indium-tin oxide (ITO) or other conducting inorganic oxides. Use of transparent conducting organic polymers as anode materials would be highly advantageous in terms of flexibility and prospective cost of the devices. Some modifications of polyaniline (PANI) and PEDOT-based polymers were found to be promising for this application. Conducting anode is usually covered or encapsulated with a protecting layer of glass or insulating plastic.

Intermediate organic layers usually function as both carrier-transporting and light-emitting components. A special, light-emitting organic layers may be added in multilayered OLEDs. Multiple layers are used to increase quantum efficiency of devices by means of balancing of injection of charge carriers. Since recombination of electrons and holes within an organic layer gives rise to luminescence, it is necessary that both charge carriers are simultaneously injected in equal quantities from both electrodes into organic layers. This can be achieved through optimization of both electron and hole transporting organic layers (ETL and HTL, 2- and 3-layered OLEDs on the scheme below).

Recently, a conceptually new composition of OLED devices has been proposed. So-called stacked OLEDs (SOLEDs) have been invented by Burrows, Forrest and coworkers (1997). SOLEDs are composed of two or more stacked "OLEDs" with common transparent conducting interlayer. The interlayer serves as anode for one "OLED" and the cathode for the other one. This allows for the linear increase of luminance of a device at a fixed current density. In addition, adjustment of emitting colors of each component may afford white SOLEDs prospective in use in lightning.

Variable meaning of multiplicity of layers in OLEDs appear in scientific literature. Thus, the layers that emit light may only be counted. The other layers that serve for electron/hole transport (injection) or energy transfer optimization sometimes are not considered as separate layers.

Next characteristics are used to determine an OLED efficiency:
1. Quantum efficiency at certain brightness (cd/m²), which includes:
a. Internal quantum efficiency,
b. External quantum efficiency or energy conversion efficiency
c. External-forward quantum efficiency.
4. Luminance (or brightness) (cd/m²) at certain voltage (V)
5. Maximum luminance (cd/m²) is a point when brightness may not longer increase with further increase of voltage.
6. Luminous (or current) efficiency (cd/A) at certain brightness (cd/m²).
7. Power, (or photometric) efficiency (lm/W) at certain brightness (cd/m²).
8. Chromaticity coordinates (Commission Internationale de l'Éclairage (CIE) coordinates) x, y.

OLEDs are also classified by structure of an organic conducting layer:
1). smOLEDs (small molecule OLEDs): contain relatively small organic molecules (usually oligomers) in a conductive layer.
Summary of conducting oligomers for small molecule OLEDs

2). pOLEDs or pLEDs (polymeric OLEDs): contain organic conductive polymers in a conductive layer.
Summary of conducting polymers for polymeric OLEDs

Latest developments in organic materials for OLEDs:
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Emitting layers for red OLEDs
Emitting layers for blue OLEDs
Small molecule PPV-type emitting layer
Surface-bound hole transport/injection layer based on polyamide
Surface-bound hole transport/injection layer based on modified NPB
Polymeric electron transport layer based on polyquinolines (PQs)
Solvent resistant cross-linked transport and emitting layers for pOLEDs

OLED-technology is a foundation for the construction of the most modern, efficient and prospective type of displays: electroluminescent displays.

Comprehensive scientific information on OLEDs in great detail may be found in books.
OLED business and commercialization see at oled-info.com.

Light Emitting Transistors

Light Emitting Transitors combine emission properties of LEDs with switching properties of transistors. This is a new technology with a development span within the last decade that potentially may provide new types of highly efficient integrated optoelectronic circuits and electroluminscent displays. Ability of light emitting transistors to generate two logic outputs: both electrical and light from two electrical inputs affords more flexibility in logic functioning than that of traditional transistors, which generate only one logic electrical output from two inputs.

Organic semiconductors were proved to be preferential over the inorganic ones for use in light emitting transistors due to much higher luminescence efficiency. Only few inorganic field effect (FET) and heterojunction bipolar (HBT) transistors based on silicon, gallium arsenide, or carbon nanotubes have been reported. Highly sophisticated purification techniques are necessary for successful functioning of such devices.

Several types of light-emitting organic FETs are known, among which unipolar devices possess simplest structure.
Unipolar light-emitting OFETs contain either a single-component channel with an organic semiconductor (usually a p-transporter), or a two-component channel with a pn-junction (see scheme below). Light emission from unipolar OFETs is localized and limited to either semiconductor-drain zone or pn-junction zone.

Ambipolar light-emitting OFETs are superior over the unipolar ones in terms of mobility of the light-emitting zone. The emission region in ambipolar OFETs moves across the channel depending on the gate and source-drain voltages. Multilayered (usually bilayered - scheme below) and single-component light-emitting ambipolar OFETs are known. The latter ones contain single ambipolar organic semiconductor, whose molecules include both donor and acceptor moieties.

For more comprehensive information see also scientific reviews on OFETs.



Luminescence

Luminescence is emission of light by a substance due to causes other than temperature (temperature-caused emission of light is called incandescence). The following types of luminescence are known:

A. Types of Luminescence by Mechanism:

1. Fluorescence
Fluorescence is type of luminescence that is resulted from very fast (microseconds) recombination of singlet excitons. It quickly disappears when source of excitation (light or electric current) is removed.

2. Phosphorescence
Phosphorescence is a type of luminescence that is resulted from the delayed (milliseconds to hours) recombination of triplet excitons. Emission of light from the triplets is delayed because it is forbidden to occur in the same molecule by the Pauli exclusion principle. It may occur only when the triplet exciton exchange its energy or excited electron with another molecule what is known as intersystem crossing. The process of intersystem crossing usually takes significant time to occur.

Phosphorescence usually complies with the Stokes rule, when emitted light is shifted to the lower energy region (red-shifted). However, antiStokes phosphorescence exists that is also known as delayed fluorescence. It happens when two triplets recombine to form a neutral molecule and a singlet exciton that possess higher energy. The latter recombines to emit a photon of higher energy (blue-shifted).

B. Types of Luminescence by Nature of Excitation:

1. Cathodoluminescence: emission of light by a substance under the bombardment with rays of electrons. Used in Cathode-Ray Tube Display monitors and TV sets.

2. Chemiluninescence: emission of light by a substance due to chemical reactions. This type of luminescence occurs in all biological objects. I accompanies biological decay of organic substances and some chemical reactions especially those involving oxidation-reduction mechanisms.

3. Crystalloluminescence: emission of light by a substance during a process of crystallization. No practical implication yet.

4. Electroluminescence: emission of light by a substance under the action of electric field or current.

Two general types of electroluminescence:
1.) Electroluminescence that occurs under the action of passing electric current (Lossev effect). This type of electroluminescence is commonly used in electroluminescent devices. The mechanism of this electroluminescence is different for inorganic and organic semiconductors. Inorganic semiconductors usually emit light due to a process of recombination of electrons and holes that meet together in an emitting layer or on a border between n- and p-type semiconductors (p-n-junction).
The mechanism of electroluminescence in organic semiconductors involves formation of excited molecules (excitons) followed by their recombination. This recombination may give rise to either light or heat emission (vibrational or thermal dissipation).

2.) Electroluminescence that occurs under the action of an electric field on a substance is known as "Destriau effect". This eletroluminescence occurs when a material is placed in the electric field without of directly attached electrodes.

Electroluminescence may result in either fluorescence or phosphorescence, or involve both mechanisms.

Primary parameter of electroluminescence is quantum efficiency of electroluminescence.

Detailed and comprehensive scientific information on electroluminescence of organic materials may be found in a series of books.

5. Photoluminescence: emission of light by a substance under the action of light.

Photoluminescence may occur through both fluorescence or phosphorescence, mechanisms. Photoluminescence complies to the Stokes rule (except 'delayed fluorescence' mechanism). Photoluminescence is around our everyday life: fluorescent dyes are used for production of bright textile, in traffic signs and marks, in medicine and many other fields of science and technology. It is widely used for lightening: fluorescent lamps and tubes include both electroluminescent (mercury) and photoluminescent (fluorescent coating) components.

Primary parameter of photoluminescence is quantum efficiency of photoluminescence.

6. Radioluminescence: emission of light by a substance under the bombardment with ionizing radiation (alpha or beta particles). First observed and reported by Pierre and Marie Curie. Have been used for a long time in watch and clock dials and other 'continuous' phosphorescent devices.

7. Sonoluminescence: emission of light by imploding bubbles in a liquid when excited by sound. No practical implication yet, though extremely high temperature inside of the bubbles, was proposed, to be possibly suitable for use in... no more - no less than thermonuclear fusion.

8. Triboluminescence: emission of light by a solid substance due to mechanical damage (scratching, crashing, or grounding).