For the whole life of the technology, blue has been a problem for OLEDs. Over 20 years ago, I caused consternation at a CDT press event when the firm was hyping up ‘rollable TVs’ (coming anytime soon, now, of course) when I asked “What about blue?” as there was no blue material in the firm’s portfolio. But scientists have been busy….
Some of the busiest have been the researchers in Korea, where display makers identified OLED as the key technology to distance the Korean LCD makers from the Chinese, when it became apparent that the Chinese were eventually going to dominate LCD. The two Korean companies, Samsung & LG, worked in isolation and Samsung developed its flexible OLED business (where it has 90%+ market share) and LG used the Kodak patents and its process skills to develop WOLEDs for TV (with 100% market share). Each has struggled in the other’s market.
However, blue has remained the challenge, with the need for a good efficient blue limiting the potential output brightness in smartphones which really still need better performance in strong sunlight and allowing LCD a major advantage in brightness and lifetime in the TV market. Further, the lack of a good deep blue has been a problem for WOLED costs as LG has to use two different blues – one deep blue where you really, really need it, with a light blue emitter with better lifetime and efficiency used most of the time. (Of course, blue is also a problem for QD OLED as Peter Palomaki pointed out in his recent article Singing the EL-QLED Blues). Blue, as with LED, is often a display challenge.
Anyway, among the busy scientists were a group at Pusan University in Korea and they have just announced the development of a new phosphorescent blue material in “External Quantum Efficiency Exceeding 24% with CIEy Value of 0.08 using a Novel Carbene?Based Iridium Complex in Deep?Blue Phosphorescent Organic Light?Emitting Diodes” in the journal, Advanced Materials. That’s important because OLED materials are either fluorescent or phosphorescent. Fluorescent materials were developed first and are easier to make than phosphorescent, but the phosphorescent materials are four times more efficient.
Up to now, the commercialisation of OLEDs has been based on phosphorescent red and green materials, but fluorescent blue and that limits the peak brightness if you want a full colour display with balanced colour. It also means that device designers typically have to drive the blue channel as hard as possible to get the brightness, and that has a significant effect on lifetimes. Companies such as Kyulux in Japan and Cynora in Germany have been beavering away to create blues with the efficiency of phosphorescence, but the promised milestones keep receding.
Trilemmas
As I have written before, I love finding new ‘Trilemmas’ (On the Horns of a Trilemma!), that is to say, a situation where there are three factors and, at best, you can pick two from three, but you can never have all three. One that I didn’t mention in that article is from (real) estate agents. We would all like a property that is in a neighbourhood that is safe, lively and cheap. Agents would say…
- If it’s cheap and lively it won’t be safe
- If its cheap and safe, it won’t be lively
- If its safe and lively, it won’t be cheap.
You get the idea….
Well, the OLED developers have the same problem based on efficiency, lifetime and brightness, so you very rarely see all three quoted together by developers. They will trumpet their lifetime, without mentioning brightness or their lifetime without the efficiency. The difficult bit is getting all three at the same time (and with blue OLED, you also need the correct blue wavelength, to add complication).
The Pusan group have achieved high (24.8%) efficiency. The blue may be a bit shorter than desired by vendors (Cynora has said in the past that around 460nm is ideal) at between 444 and 448nm in the devices made, but with a CIEy value at around the desired 0.15. Peak brightness of the devices was 6453 cd/m² in one sample and 5,247 cd/m² in another. So, brightness and efficiency are there…
As the paper was heavy with chemistry that is beyond my pay grade, I reached out to Peter Palomaki, (not an OLED expert, but a chemist and QD expert) and he pointed out that the peak efficiency was not measured at the brightness of 1,000 cd/m². He also identified that the spectrum has a relatively broad peak width at around 60-70nm (where QDs do particularly well).
Getting back to our trilemma, efficiency and brightness are OK, but what about lifetimes? Lifetimes are still very short and the group said that one of the devices had a 400 minute LT50 time at 500 cd/m² – and that was ” seven times longer than devices with a low dopant ratio”. So, the new materials are bright and efficient, but have a short life. Hmmmm…. the trilemma strikes again! However, two out of three is not bad, although Cynora will tell you that there is a huge difference between having two factors solved, but not the third! (BR)
