Oled (Organic Light Emitting Diode)
An organic light-emitting diode (OLED or Organic LED), also known as an organic EL (organic electroluminescent) 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.
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.
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. OLED’s 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.
The main component in an OLED display is the OLED emitter – an organic (carbon-based) material that emits light when electricity is applied. The basic structure of an OLED is an emissive layer sandwiched between a cathode (which injects electrons) and an anode (which removes electrons).
Why is Oled used?
Modern OLED devices use many more layers in order to make them more efficient and durable, but the basic functionality remains the same. An OLED panel itself is made from a substrate, backplane (electronics – the driver), front plane (the organic materials and electrodes as explained above) and an encapsulation layer. OLEDs are very sensitive to oxygen and moisture and so the encapsulation layer is critical.
The substrate and backplane of an OLED display are similar to those of an LCD display, but the front plane deposition is unique to OLEDs. There are several ways to deposit and pattern the organic layers. Currently most OLED displays are made using vacuum evaporation, using a Shadow Mask (FMM, Fine Metal Mask) to pattern. This is a relatively simple method but it is inefficient (a lot of material is wasted) and very difficult to scale up to large substrates. Some OLED materials are soluble, and these can be deposited using
Printing methods – mostly ink-jet printing. OLED makers hope that ink-jet printing may be a scalable, efficient and cheap way to deposit OLEDs.OLEDs are used to create digital displays in devices such as television screens, computer monitors, and 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.
Advantages of OLED
The future – flexible and transparent OLED displays
As we said, OLEDs can be used to create flexible and transparent displays. This is pretty exciting as it opens up a whole world of possibilities:
- Curved OLED displays, placed on non-flat surfaces
- Wearable OLEDs
- Foldable OLEDs and rollable OLEDs which can be used to create new mobile devices
- Transparent OLEDs embedded in windows or car windshields
- And many more we cannot even imagine today…
Flexible OLEDs are already on the market for many years (in smartphones, wearable’s and other devices) and in 2019 we have finally seen the first foldable devices – such as the Samsung Galaxy Fold and the Huawei Mate X – both are smartphones that open up to become tablet devices with foldable OLEDs. In 2019 LG also announced the world’s first rollable OLED – its 65″ OLED R TV that can roll into its base!
OLEDs aren’t perfect. First of all, it costs more to produce an OLED than it does to produce an LCD – although this should hopefully change in the future, as OLEDs has a potential to be even cheaper than LCDs because of their simple design (some believe that future OLEDs will be printed using simple ink-jet processes).
OLEDs have limited lifetime (like any display, really), that was quite a problem a few years ago. But there has been constant progress, and today this is almost a non-issue. Today OLEDs last long enough to be used in mobile devices and TVs. OLEDs can also be problematic in direct sunlight, because of their emissive nature. But companies are working to make it better, and newer AMOLEDs (such as Samsung’s super AMOLED and Super AMOLED Plus and Nokia’s CBD displays) are quite good in that respect – some even consider them superior to LCDs.
OLED difference from LCD or LED
An OLED display have the following advantages over an LCD display:
- Improved image quality – better contrast, higher brightness, fuller viewing angle, a wider color range and much faster refresh rate
- Lower power consumption
- Simpler design that enables ultra-thin, flexible, foldable and transparent display
- Better durability – OLEDs are very durable and can operate in a broader temperature range
- Life Spam is Higher in OLED over LED
Different types of Oled
There are several types of OLEDs:
- Active-matrix OLED
- Passive-matrix OLED
- Transparent OLED
- Top-emitting OLED
- Foldable OLED
- White OLED
Each type has different uses.
AMOLEDs have full layers of cathode, organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the circuitry that determines which pixels get turned on to form an image.
PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips are arranged perpendicular to the cathode strips. The intersections of the cathode and anode make up the pixels where light is emitted. External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Again, the brightness of each pixel is proportional to the amount of applied current.
Transparent OLEDs have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85 percent as transparent as their substrate. When a transparent OLED display is turned on, it allows light to pass in both directions. A transparent OLED display can be either active- or passive-matrix. This technology can be used for heads-up displays.
Foldable OLEDs have substrates made of very flexible metallic foils or plastics. Foldable OLEDs are very lightweight and durable. Their use in devices such as cell phones and PDAs can reduce breakage, a major cause for return or repair. Potentially, foldable OLED displays can be attached to fabrics to create “smart” clothing, such as outdoor survival clothing with an integrated computer chip, cell phone, GPS receiver and OLED display sewn into it.
White OLEDs emit white light that is brighter, more uniform and more energy efficient than that emitted by fluorescent lights. 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.