Organic LEDs
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OLEDs are solid-state semiconductor devices composed of thin films of organic molecules that create light with the application of electricity. They are 100 to 500 nm thick or about 200 times smaller than human hair.
Advantages: OLEDs can provide brighter, crisper displays on electronic devices and use less power than conventional LEDs and LCDs (liquid Crystal displays).
The color of light depends on the type of organic molecule in the emissive layer. Manufacturers place several types of organic films on the same OLED to make colored display.
The intensity or brightness of the light depends on the amount of electrical current applied: the more current, the brighter the light.
Unlike LEDs, which are small-point light source, OLEDs are made in sheets that are diffuse-area. OLED technology is developing rapidly and there are handful of products offering with efficacy, lifetime, or color quality specs that are comparable to LEDs.
Types of OLEDs: They are several types of OLEDs
a) Passive-Matrix OLEDs (PMOLED): It consist of strips of cathode, organic layers, and strips of anodes. The anode strips are arranged perpendicular to the cathode strips. The intersection of the cathode and anode make up the pixels where light is emitted. External circuit applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. Brightness of each pixel is proportional to the amount of applied current.
Advantages: Easy to make
Limitations: Consumes more powers than other types of OLED, mainly due to power needed for external circuit.
Application: Suitable for text and icons and thus are best suited for screens (2 to 3 inch) such as those used in cell phones, PDAs, and MP3 players.
Note: Even with external circuitry, passive matrix OLEDs consume less battery power than the LCDs that currently power these devices.
b) AMOLED (Active-matrix OLED): They have full layers of cathode, organic molecules and anode, but the anode layers overlay a thin film transistor (TFT) array that forms a matrix. The TFT array itself is a circuitry that determines which pixels get turned on to form an image.
Advantages:
- Consumes less power than PMOLEDs because TFT array requires less power than external circuitry, so they are efficient for large displays.
- They also have faster refresh rates suitable for videos.
Applications: Computer Monitor, large-screen TVs and electronic signs or billboards.
c) Transparent OLEDs: They have only transparent components (substrate, cathode, anode) and, when turned off, are upto 85% as transparent as their substrate.
When a transparent OLED display is turned on, it allows light to pass in both directions.
It can be PMOLED or AMOLED. This technology may be used for heads-up displays.
d) Foldable OLEDs: They have substrate made of very flexible metallic foils or plastics. They are lightweight and durable. Their use in devices such as cell phone and PDAs can reduce breakage, a major cause of phone repairs. They can also be used for making smart clothing.
Advantages of OLEDs:
- OLEDs can be configured as large-area, more diffuse light sources whose soft light can be viewed directly. This eliminates the need of shades, diffusers, lenses, or parabolic shells.
- This diffused light allows them to be used very close to the task surface without creating glare for the user.
- OLEDs can be made very thin, increasing their eye appeal and allowing for easy attachment to the surface of walls and ceilings.
- The Plastic, organic layers of an OLED are thinner, lighter, and more flexible than the crystalline layers in LED or LCD.
- OLEDS are brighter than LEDs.
- Because the organic layers of an OLED are much thinner than the corresponding inorganic crystal layer of an LED, the conductive and emissive layer of an OLED can by multilayered. Further, it doesn’t require glass for support (which is needed by LED)
- OLEDs are much more energy efficient.
- OLEDs don’t require backlighting like LCDs. LCDs work selectively blocking areas of backlighting to make the images that you see, while OLEDs generate light themselves. Because OLEDs don’t require backlighting, they consume much less power than LCDs (most of the LCD power goes to the backlighting). This is specially important for battery-operated devices such as cell phones.
- OLEDs are easier to produce and can be larger in size. Because OLEDs are essentially plastic, they can be made into large, thin sheets.
- OLEDs have larger field view, about 170 degrees. Because LCDs work by blocking light, they have an inherent viewing obstacle from certain angles.
- OLEDs can be made up of almost any shape and can be deposited on flexible substrates.
Limitations of OLEDS:
- Lifetime: Blue organics currently have much shorter lifetime.
- Expensive manufacturing
- Water can easily damage OLEDS.