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OLED displays dominate the mobile phone market and have captured a large chunk of the high-end TV market thanks to their crisp images, deep blacks and wide viewing angles. But OLED technology is no better than the alternative – liquid crystal – in terms of durability and brightness. And energy efficiency depends on the displayed images.
OLED will get a major boost on all three fronts this year when blue phosphorescent OLED (PHOLED) materials become commercially available. Universal Display Corp. (UDC), which has been developing this technology for decades, says it has finally caught it and is about to begin mass production, reports Spectrum IEEE.
Fluorescent vs phosphorescent OLED
To understand why blue OLED displays are a big deal, you need to understand the difference between regular OLED displays and the newer, phosphorescent ones.
OLED – Organic light emitting diodes convert electrical energy directly into photons. They have several layers, including a cathode, anode and an emitting layer. When current passes through the OLED, electrons leave the cathode and holes leave the anode; they recombine in the central emission layer, combining into excitons and releasing light or heat during decay.
Depending on how the spin states of the molecules are aligned, this process can generate two types of excitons: singlets and triplets. Singlets typically outnumber triplets by a ratio of three to one. This is a problem because in traditional fluorescent LEDs, only the singlets emit photons, while the triplets release their energy as heat. And this is inefficient: displays using fluorescent OLED materials convert only 25% of the input electrical energy into light. Excess heat can eat up the life of the device.
When heavy alloys are added to the consistency of organic compounds that make up OLEDs, the spin state changes and triplet excitons emit light. The result is a phosphorescent OLED. It is potentially 100% efficient and does not generate harmful heat.
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Phosphorescent red OLEDs have been around for about two decades, and green OLEDs have been around for decades. But creating blue—which has the highest energy level of the three colors—was a difficult task.
Blue OLED displays will increase battery life of phones
What does it mean to have blue LEDs? It depends on the screen in which they will be used. Modern smartphone displays use OLED displays directly without color filters. Each pixel has three OLED subpixels: red, green and blue. For now, only the red and green subpixels use phosphorescent OLEDs, so blue must rely on a less efficient fluorescent OLED. This means that the blue subpixel is typically much larger than the other two in order to provide the necessary brightness without reducing the lifespan.
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Replacing fluorescent blue with phosphorescent blue means a more balanced pixel structure and could enable higher resolution displays in the future. In the long term, this transition will lead to approximately 25% increase in efficiency; Manufacturers can use this to improve battery life, reduce battery size, or make the display brighter. And in any of these cases, reducing extraneous heat should increase the life of all surrounding electronics.
PHOLED displays could create cheaper, brighter TVs
TV manufacturers use OLED slightly differently than phone display manufacturers, they have different approaches. Samsung OLED TV displays use only blue LEDs, creating the necessary red and green subpixels using quantum dots, a material that absorbs light energy at one wavelength and emits it at another. So, moving from the current fluorescent blue to phosphorescent blue will make Samsung displays much more efficient. This will also simplify and reduce the cost of their production. To get the necessary brightness from today's fluorescent blue OLED displays, Samsung now uses three layers of material. PHOLED will allow Samsung to reduce this number to two, and perhaps even to one.
Who will be the first to introduce blue PHOLED displays?
It may be 24 years since such a display will be ready, and most likely Samsung will be the first to do it. The company's researchers have been developing phosphorescent blue OLED displays for years, regularly publishing articles about their developments.