Advancements in OLEDs continue to pave way for future of display technology
When it comes to display and lighting technology, organic light emitting diodes, or OLEDs as they’re better known, are undergoing rapid technological advancement and show great promise for replacing both LCD and LED lighting in the future.
15-inch OLED TV from LG (Via: gizmag.com)
They offer much better brightness, require less power, and are far cheaper to produce. If you haven’t heard of OLEDs before, read on for some of the fundamental basics to this emerging technology.
What are OLEDs?
First invented by Eastman Kodak in the early 1980s, an OLED is a solid-state semiconductor device with several parts to it, including a substrate, cathode, anode, various organic layers, conducting layer, and emissive layer. Depending on the manufacturer and type of OLED you’re looking for, the materials used to create these parts can vary.
Generally speaking, the polymers or organic materials that make up the various organic layers of the OLED are electrically conductive as a result of the delocalization of pi electrons. This is caused by conjugation over all or part of the individual molecules. Actual conductivity levels can range anywhere from being an insulator to a conductor.
How do they work?
The process of OLEDs emitting light is called electrophosphorescence. Thin films of organic material are positioned between two charged electrodes, one a metallic cathode and the other a transparent anode. These films consist of a hole-injection layer, hole-transport layer, an emissive layer, and an electron-transport layer. For reference, the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of these organic semiconductors are similar to the valence and conduction bands of inorganic semiconductors. The entire device sits atop a substrate, which can be made of glass, plastic, or other material.
General structure of the OLED (Via: dailymail.co.uk)
When a voltage is applied to the OLED, the current flows from the cathode to the anode: Electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This process, that is, the movement of the electrons, creates electron holes in the conductive layer. At the boundary between the conductive and emissive layers, these electron holes and electrons are brought back together. They recombine to form an exciton: a bound state of the electron and electron hole. The decay of this excited state causes a relaxation of the energy levels of the electrons which, in turn, results in an emission of photons.
Some additional details . . .
Since OLEDs are made up of films of organic material, they are very thin, typically falling between 100 and 500 nanometers thick. For comparison, that’s approximately 200 times smaller than a strand of your hair.
Color depends on the type of organic molecule used in the emissive layer. Manufacturers generally place multiple types of organic films on the same OLED which can effect this. Also, the brightness of the OLED depends on the amount of electrical current applied: the more current, the brighter the light.
OLED matrix options:
OLED passive matrix and OLED active matrix. (Via: vankeizer.com)
Passive-matrix OLED (PMOLED): have strips of cathode, then organic layers, then strips of anode. The anode strips are arranged perpendicular to the cathode strips and the intersections of the cathode and anode make up the pixels where the photons are emitted. PMOLEDs are easy to make, but they require external circuitry. As a result, they consume more power than an active-matrix OLED (below.) They are an efficient option when it comes to text and icon display, and are best suited for smaller screens.
Active-matrix OLED (AMOLED): have full layers of the cathode, organic molecules, and anode, with the anode being overlaid with a thin film transistor (TFT) to form the matrix. The TFT’s array determines which pixels get turned on to form an image. AMOLEDs are more energy efficient than PMOLEDs because the TFT array requires less power than external circuitry. This makes them a better fit for larger displays like computer monitors, TVs, and billboard displays.
Types of OLEDs:
Bottom or top emission OLED : Bottom emission uses a transparent or semi-transparent bottom electrode to get light through a transparent substrate. Top emission uses a transparent or semi-transparent top electrode to emit light directly.
Foldable OLED : Substrate is made of flexible metallic foils or plastics. They are lightweight and considered very durable.
Transparent OLED : Use transparent or semi-transparent contacts on both sides to create displays that can be top and bottom emitting. Offers great contrast.
Graded Heterojunction OLED : Allows for the charge injection to be stabilized while also reducing the distance that an electron needs to travel to an electron hole. This increases the OLED’s quantum efficiency, which results in lower power consumption and better color display.
Stacked OLED : Unique pixel architecture that stacks red, green, and blue subpixels on top of one another (instead of next to each another). This provides better color scope and depth, and also reduces pixel gap.
Manufacturing:
As opposed to LCD displays, which are made up of separate layers, OLEDs are monolithic. Each layer is deposited on to the other, to create a single, solid unit.
There are three ways to manufacture OLEDs:
1. Inkjet printing : Organic materials get sprayed onto the substrate. Fast and efficient, this process allows for OLEDs to be printed on large films, which greatly reduces production costs.
2. Organic vapor phase deposition : In a low-pressured, hot-walled reactor chamber, a carrier gas transports evaporated organic molecules onto a cooled substrate, where they’re condensed into thin films. The use of the carrier gas is very efficient, and helps save costs.
3. Vacuum deposition : In a vacuum chamber, organic molecules are evaporated and then condensed as thin films onto the cooled substrate. Without the carrier gas used in organic vapor phase deposition, this process can actually be a bit inefficient and somewhat expensive.
LCD backlight vs. no backlight on the OLED:
When it comes to visual display, LCDs require backlighting. The way they work is they selectively block out areas of the backlight to make images more pronounced. The problem with this is that since LCDs are purposely blocking out light, there’s an inherent viewing obstacle from certain angles.
OLED displays work without a backlight. They are emissive devices; that is, they emit light rather than transmit or reflect it. Even in low ambient light conditions, an OLED screen offers much higher contrast than an LCD.
LCD display vs OLED display (Via: oled-display.net)
Additional advantages:
• Substrate is flexible, which offers the possibility of new display options
• Plastic can be used as opposed to glass, which is good because glass absorbs some light
• Since manufacturers are working mostly with plastics, OLEDs tend to be easier to produce
• Wider viewing range (approximately 170°)
Some disadvantages:
• Water can damage the device and limit flexibility
• Displays can be damaged by prolonged exposure to UV light
• Pixel brightness fades over time and the varied lifespan of the dyes can cause a discrepancy between red, green, and blue intensity; this could, in turn, lead to screen burn-in.
• Blue OLEDs are inefficient: red (625 nm) and green (530 nm) diodes have shown external quantum efficiency values of 20% and 19%; blue diodes (430 nm), on the other hand, are much lower, with a reported maximum external quantum value between 4% and 6%
Outlook:
The OLED’s incredible versatility, coupled with its cost and energy efficiency, make it an exciting product to follow. Developers have already brought about several new design and display lighting technologies using OLEDs, and promise much more to come. ■
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