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OLEDs are energy conversion devices (electricity-to-light) based on electroluminescence. Electroluminescence is the emission of light from materials in an electric ¬eld. In 1960, hole injection into an organic crystal was first observed by Martin Pope and his group in anthracene. Three years later, they also observed electroluminescence (EL) from single crystal anthracene and an impurity-doped one under direct current. Despite the high quantum ef¬ciency obtained with such organic crystals, no applications emerged due to the requirement of high working voltage(above 400 V) for visible emission. Subsequently, Helfrich and Schneider achieved double injection recombination EL in anthracene single crystal using hole and electron injecting electrodes with voltages reduced to ~60 V for observable emission.
In 1987, C. Tang and Van Slyke from Eastman Kodak devised a novel heterostructure- double layered device containing active "small molecules". The two thin-film organic layers independently were responsible for hole and electron transport. The device provided a low operating voltage (<10V), good brightness (>1000cd/m2) and respectable luminous efficiency (1.5 lm/W), research gained the momentum. Additionally, the device showed rectifier behavior, giving rise to the term OLED (organic light emitting diode). This discovery stimulated explosive development of this field.
OLED -WHAT IS IT?
OLED is an emissive technology; they emit light instead of diffusing or reflecting a secondary source.
OLED is an acronym for Organic Light Emitting Diode. OLED is a self light-emitting technology which consists of a number of semiconducting organic layers sandwiched between two electrodes at least one of them being transparent. Transparent electrode is composed of electric conductive transparent Indium Tin Oxide (ITO) coated glass substrate.
A simplified device structure is shown in following figure. The device on the left has one transparent electrode and emits light on one side only. The device on the right uses both the electrodes as transparent ones and it emits light in both top and bottom direction.
OLEDs are extremely thin, practically 2- dimensional multi-layer devices. The thickness of all the active layers combined is only of the order of one hundred nanometers. This is extremely useful in applications where space is a premium, such as in aircrafts. Also, these devices can work in subzero temperatures and hence can be significance for military applications as well.
OLED devices have no restriction on the size and shape. Every conceivable shape, including flexible ones, can be provided. The devices can be in form of fibers, and woven to fabrics. They can be in form of bent or rolled films or constitute the surface of spheres. For lighting applications, thin flat sheets possibly using thin glass substrates can be used.
MATERIALS FOR OLED
OLEDs consist of multiple layers - Anode, Hole Injection Layer(HIL), Hole Transport Layer(HTL), Emission Layer(EML), Electron Transport Layer(ETL), Electron Injection Layer(EIL) and the cathode. The multi-layer structure is shown in the following figure
An extremely thin layer of indium - tin oxide (extremely thin, because it has to be optically transparent), is used as anode. Low work-function metals such as Mg, Li, and their alloys with Ag, and in some cases Al, are now used as cathodes.
Several types of organic materials are used in the functional layers. Since all emission materials are not good for electron or hole transport, different materials for different functions are used. Also, stability of single material emission layer is not good, dye dopants are used for stabilized OLED emission and color tuning.
Electron Transport Materials
Most prominent OLED material is Alq3. Not only is it a very good emissive material, it is also a good electron transport material. Another electron transport material used is ADN.
Hole Transport Materials
There are several established hole transport materials, but the mature ones are NPD and TPD; both are used in OLEDs.
Main purpose of the dyes is - color tuning and color stabilization.
Rubrene is used to dope Alq3 for yellow emission. DCM II and DCJTB are used to dope Alq3 for red emission. C545 is used for green color stabilization and Perlene is used for blue color stabilization.
Based on the two classes of electroluminescence materials used in organic light emitting devices, two types of OLEDs are available. Electroluminescence is similar in both types; the difference is in the deposition of the organic films
Polymer based LEDs(PLED)
Polymers are bigger molecules and hence cannot be thermally deposited. Polymer OLEDs are made by depositing the polymer materials on substrates through inkjet printing process or other solution processing methods(also referred to as 'wet process') under ambient conditions. They are used for fabrication of large size screens.
Polymeric OLED devices usually have fewer layers. The electro-active polymers may serve multiple functions: both electron and hole transport and light emission, even though dopant emitters can be used to tune the color. The electron transporting polymer and hole transporting polymer may be in one or two separate layers. Polymer device structure is shown below. It is bi-layer structure made from solution.
Conducting polymers are
Emissive polymers are
Small molecule materials (SM-OLED)
Small molecular OLEDs are made by vacuum evaporating (also referred to as 'dry process' small molecules to the substrate. Since small molecules do not exhibit any orientating property and therefore form amorphous films.
Small molecular device structure is shown below. It is a multilayer structure made all in vacuum.
Hole Transport small molecules are
Arylamines, starburst amines
Emissive small molecules are
Transfer material, Emission Layer material and choice of electrode is the key factors that determine the quality of OLED components.
OLED -HOW DO THEY WORK?
As explained in previous section, OLED consists of multiple layers; each layer is responsible for a certain function.
When forward bias is applied on the electrodes, electric fields of the order of 105 - 107 V/cm are generated in the active layers though applied voltages are low, from 2.5 to ~ 20 V. These high electric fields cause injection of charges across the electrode / active layers interfaces. Holes are injected from the transparent anode and electrons are injected from the cathode. Sometimes, there is difficulty in injecting carriers into the organic layer from the inorganic contacts. So, to facilitate charge injection, Hole Injection Layer and Electron Injection Layer are used in the structure.
Injected holes and electrons from the anode and cathode move inside the material (typically by hopping) and then recombine in the emission layers to form excitons, after which electroluminescence occurs. Radiative relaxation of the excitons generates photons, part of which exit from the transparent side of OLEDs.
The materials that are used to bring the charges to the recombination sites
are usually, but not always, poor photon emitters (most of the excitation energy is released as heat). Therefore, suitable dopants are added, which first transfer the energy from the original excitons, and release the energy more efficiently as photons
The energy level diagram is shown below
Since charge carrier transport relies on hopping process, the conductivity of organic semiconductors is several orders of magnitude lower than that of inorganic counterparts. Also concept of energetic bands is not applicable to organic electronics. Instead of valence and conduction bands, highest occupied and lowest unoccupied molecular orbital levels (HOMO and LUMO) are used.
The color of the photon is a function of the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the electroluminescent molecule. The wavelength of the light emission can thus be controlled by the extent of the conjugation in the molecule or the polymer.
The emission color is a material property. Thus, by stacking several different emitting layers in a single device the total emission can be tuned to virtually every colour including white at any color temperature.
OLED DISPLAY -TYPES
Displays are often classified as Active Matrix and Passive Matrix and so does the OLED
Passive Matrix OLEDs(PMOLED)
Passive Matrix OLEDs (PMOLEDs) consists of an array of strips of cathode and an array of strips of anode. Sandwitched between the two is the organic layer. The anode strips are arranged perpendicular to the cathode strips thereby forming a row and column matrix. Pixels are formed at the intersections of the cathode and anode; the pixels are the points where photons are emitted
To illuminate a particular pixel, external circuitry applies current to the row line of anode and column line of the cathode. Thus, the desired pixels can be turned on and off. The brightness of the pixel is governed by the amount of current through the pixel
Passive matrix are low cost and easy to make but they consume more power than other types of OLEDs due to presence of external circuitry. However, power consumption of PMOLEDs is smaller than that of LCDs.
PMOLEDs can be manufactured economically for small sizes; standard sizes for colour PMOLED are 0.95" and 1.5" and are best suited for small displays of cellphones, PDAs, MP3 players,etc.
Active Matrix OLEDs(AMOLED)
Huge amount of current is required to achieve adequate brightness in passive matrix OLEDs. This necessitates use of large drive voltages leading to increased power dissipation, more flickering and shortened lifetimes.
AMOLEDs uses active matrix addressing, where each pixel is defined by its own electrode and driven by circuitry comprising of thin film transistor and capacitors. The anode is then placed on top of this active-matrix circuitry and the counter electrode, which is not patterned, acts as a ground electrode. In such a device the capacitor is aimed at retaining the information during a frame period.
TFT array determines which pixel to turn on or off. By controlling the amount of current through the TFT, brightness is controlled.
AMOLEDs consume lesser power and hence are suited for large displays like TV screens, computer monitors, etc.
OLEDs Displays are also classified as follows
Transparent OLEDs use transparent anode and cathode as well as transparent substrate. They can be either passive matrix or active matrix. When turned ON, transparent OLED allows passage of light in both directions.
Top-emitting OLEDs uses a substrate that is either opaque or reflective. They use active matrix OLEDs
Foldable OLEDs have substrates made of very flexible metallic foils or plastics. Foldable OLEDs are very lightweight and durable.
They can be used in devices such as cell phones and PDAs to reduce breakage. They can also be sewn into fabrics to form smart clothing.
Compared to fluorescent lights, White OLEDs emit white light that is brighter, more uniform and more energy efficient. White OLEDs also have the true-color qualities of incandescent lighting.
Because OLEDs can be made in large sheets, they can replace fluorescent lights that are currently used in homes and buildings. Their use could potentially reduce energy costs for lighting.
OLEDs offer many advantages over both LCDs and LEDs, including:
The organic layers of an OLED are thinner and lighter than the crystalline layers in an LED or LCD. At present, the thickness of OLEDs is less than 2 mm whereas LCD thickness is 4-6 mm. Thickness of OLEDs is likely to go down further.
As OLEDs use plastic substrate instead of glass (used for LCD), OLEDs can be flexible/foldable.
Viewing angles of LCDs have increased significantly to about 170 degrees but at a poor contrast ratio. Because LCDs work by blocking light, they have an inherent viewing obstacle from certain angles. OLEDs produce their own light, so they have a much wider viewing range. OLEDs are in good approximation Lambertian surface emitter, which means that when viewed from any angle, they have the same apparent radiance.
Fast Response Time
OLEDs have very response time of the order of tens of microseconds compared to milliseconds for LCDs. This is important for high speed video.
Because OLEDs do not require backlighting, they consume much less power than LCDs (most of the LCD power goes to the backlighting). This is especially important for battery-operated devices such as cell phones.
â€¢ Emissive technology
OLEDs do not require backlighting like LCDs. LCDs works by selectively blocking areas of the backlight to make the images that you see, while OLEDs light themselves.
â€¢ Ease of fabrication
OLEDs are easier to produce and can be made to larger sizes. Because OLEDs are essentially plastics, they can be made into large, thin sheets. It is much more difficult to grow and lay down so many liquid crystals.
OLED emission is harmless in terms of eye safety.
OLED seem to be the perfect technology for all types of displays; however, they do have some issues, including:
â€¢ Lifetime: While red and green OLED films have long lifetimes (10,000 to 40,000 hours), blue organics currently have much shorter lifetimes (only about 1000 hours).
â€¢ Manufacturing: Processes are expensive right now.
â€¢ Water: Water can easily damage OLEDs. The organic layers have to be protected against air as they are sensitive to moisture and oxygen and decompose when exposed. Hence they need proper encapsulation
Small molecule Passive matrix display products
Small molecule Active matrix display products
Polymer based Active matrix display products
Polymer based Passive matrix display products
R&D activities in OLEDs is progressing at a fast rate and soon, OLEDs may be visible in heads-up displays(HUD), automotive dashboards, home and office lighting, for flexible displays. Because of the numerous advantages enumerated in previous sections, this will be the technology of choice for displays.
In addition, OLEDs offer several unique features which set them apart from current conventional light sources. They can be very thin, very light weight and they are a non-glaring area light source. They offer high color quality and turn on instantly when a current is applied. In contrast to conventional light sources, OLED lighting modules provide extensive light of high colour quality that is very pleasant to the human eye. They do not contain UV radiation which means they do not bear any risk for eye safety. They have potential to be as efficient and long living or better than fluorescent lamps while 100% mercury-free. All this implies is that OLEDs will be among the most efficient light sources in future.
OLEDs have all the attributes to effectively compete with incandescent and fluorescent lighting. OLEDs will also create new lighting possibilities by enabling large area illumination sources, panels, ceilings, walls, partitions, fabrics etc. OLEDs operate at very low voltages, of the order of 3 - 5 V and therefore, the introduction of OLEDs as sources of light will effect a major paradigm shift in the lighting industry
However, there are still many technical obstacles that have to be overcome before OLEDs become a viable alternative to fluorescent and incandescent lighting.