OLED displays dominate cellphones and have grabbed a big piece of the high-end TV market, thanks to their crisp images, deep black levels, and wide viewing angles.
But OLED technology doesn’t do better than the alternative—LCDs—in longevity or brightness. And energy efficiency is a toss-up, depending on the images being displayed.
This year, OLED will get a big boost in all three of those metrics, when blue phosphorescent OLED (PHOLED) materials become commercially available. Universal Display Corp. (UDC), which has been chasing this technology for decades, says it has finally caught it and is about to begin mass production.
“A lot of research groups have worked on it, many people have published on it, but we believe we have it,” says Michael Hack, vice president of business development at UDC.
And while cellphone and TV manufacturers are characteristically mum about upcoming designs, they will likely rush blue PHOLEDs into their devices as soon as they can.
Fluorescent vs. Phosphorescent OLEDs
To appreciate why a blue PHOLED is such a big deal, you have to understand the difference between conventional OLEDs and the newer, phosphorescent ones. And then you have to consider how today’s OLED displays are structured.
OLEDs—organic light-emitting diodes—convert electrical energy directly into photons. They have multiple layers, including a cathode, an anode, and an emissive layer. When current passes through an OLED, electrons leave the cathode and holes leave the anode; they recombine in the central emissive layer, joining into excitons and releasing either light or heat as they decay.
Depending on how the spin states of the molecules line up, this process can generate two types of excitons: singlets and triplets. Singlets are typically outnumbered by triplets three to one, explains UDC’s Hack. That’s a problem, because in traditional—that is, fluorescent—OLEDs, only the singlets emit photons, while the triplets release their energy as heat. And that’s inefficient: Displays using fluorescent OLED materials turn only 25 percent of incoming electrical energy into light. The excess heat can eat away at a device’s life-span.
When heavy metals are added to the mix of organic compounds that make up OLEDs, Hack says, the spin states change, and even the triplet excitons emit light. The result is a phosphorescent OLED. It’s potentially 100 percent efficient and doesn’t generate damaging heat.
Phosphorescent red OLEDs have been around for about two decades; green has been available for one. But creating blue—which has the highest energy level of the three colors—has been a challenge.
“We’ve had a lot of bright chemists and physicists working on this problem for years,” Hack says. “The more energy involved, the more likely bad things can happen, so it’s been hard to get long lifetimes. But we will have it available in 2024.”
“The physics are very challenging, because those short, high-energy wavelengths of light are almost destroying the material as they are being generated,” says Jeff Yurek, vice president of marketing for Nanosys, a quantum-dot manufacturer that is now a division of Shoei Chemical. “It’s also hard to do a true 450-nanometer blue [the standard wavelength for blue], not 460 or 470. Some people had written this off as impossible to achieve.”
UDC’s PHOLEDs are to date the only ones of any color commercially available, points out analyst Robert J. O’Brien, cofounder of the market-research firm DSCC, and UDC is already supplying red and green PHOLEDs to just about every manufacturer of OLED displays. UDC isn’t saying how much of a premium blue PHOLEDs will command over traditional blue OLEDs. But Hack says that as part of a display, the cost of the OLED materials of any variety is minimal.
Blue PHOLEDs for longer cellphone battery life
What will the availability of blue PHOLEDs mean? That depends on the display that will incorporate them.
Today’s smartphone displays use OLEDs directly, without color filters. Each pixel has three OLED subpixels: red, green, and blue. At present, only the red and green subpixels use phosphorescent OLEDs, and so the blue must rely on the less-efficient fluorescent OLED. That means the blue subpixel is generally much larger than the other two, in order to produce the required brightness without reducing lifetimes.
Replacing the fluorescent blue with phosphorescent blue will mean a more balanced pixel structure and could enable higher-resolution displays in the future. In the near term, the switch will lead to an approximate 25 percent gain in efficiency; manufacturers can take advantage of this to increase battery life, reduce the size of the battery, or enable a brighter display. And in any of those cases, reducing extraneous heat generation should increase the lifetime of all the surrounding electronics.
Blue PHOLEDs enable cheaper, brighter TVs
TV manufacturers use OLEDs a little differently than phone-display makers do, and they have different approaches.
Samsung’s OLED TV displays use only blue OLEDs, creating the necessary red and green subpixels through the use of quantum dots, a material that takes in light energy at one wavelength and emits it at another. So the switch from the current fluorescent blue to phosphorescent blue will make Samsung’s displays much more efficient. It will also make them easier and cheaper to manufacture, says analyst O’Brien. To get the necessary brightness out of today’s fluorescent blue OLEDs, Samsung now uses three layers of the material. PHOLEDs will allow Samsung to cut that number to two, O’Brien says, and possibly even one. Or Samsung could choose to stick with three layers, at least in some premium models, and boost brightness significantly, says Yurek, at Nanosys.
LG, another major manufacturer of OLED TVs, builds its displays by combining layers of red, green, and blue OLEDs. The red and green layers are phosphorescent, and the blue is fluorescent, which creates white light that is then separated into subpixels using color filters. The current design requires two blue layers in the stack; PHOLED blue should allow LG to cut that to one layer.
As with cellphone displays, the switch to blue PHOLEDs in TVs should boost display longevity. Says Hack, “Lifetimes are best when devices are cool and use less current.”
Who will deploy blue PHOLEDs first?
When will displays incorporating blue phosphorescent OLEDs get into consumer hands? Possibly this year, says O’Brien, and most likely from Samsung first, for a couple of reasons.
For one, Samsung is ready, he says. The company’s researchers have been developing phosphorescent blue OLEDs for years, regularly publishing papers about their efforts. So they are likely far along in planning how they will use it in display design. These displays won’t just show up in Samsung-branded products. In addition to making displays for its own phones, Samsung supplies them to many other manufacturers. The company is the biggest display supplier for iPhones, O’Brien says.
Samsung will also likely be the first to incorporate blue PHOLEDs into TV displays, both because of its research jump and because the impacts on its all-blue OLED display will be greater than those on LG’s mixed-color approach.
Looking into the future, O’Brien sees blue PHOLEDs as making OLED displays more compelling overall. Expect them to start moving beyond phones and large-screen TVs into tablets, notebooks, and computers, where LCD technology has yet to give ground.
This article appears in the January 2024 print issue as “A Behind-the-Screens Change for OLED .”
>>> Read full article>>>
Copyright for syndicated content belongs to the linked Source : IEEE – https://spectrum.ieee.org/blue-pholed
