Last week in the Display Supply Chain Monitor I reported that LGD was considering the use of a TADF (Thermally Activated Delayed Fluorescence) blue emitting material in their OLED devices (DCSM 07.10.2017, “LG Aiming to Adopt Blue TADF Emitter in 2018”). To follow up on this topic with more depth, I had a discussion with Dr. Andreas Haldi, CMO of Cynora, the likely supplier of such a material.
A short description of TADF may be helpful. TADF is one of several different ways to enable OLED material to emit light, as shown in Figure 1. In Cynora’s description, TADF is described as a third-generation material for OLED emission, following Fluorescence and Phosphorescence.
Figure 1: Thermally Activated Delayed Fluorescence – TADF (Cynora)
In all the mechanisms of OLED emission, the task is to take the electrical energy associated with an electron-hole pair (at the top of the chart) into light energy from the organic molecule. Both phosphorescence and TADF make use of Intersystem Crossing (ISC) to capture all of the energy and direct it to emit a photon. TADF also utilizes Reverse Intersystem Crossing (RISC) but redirects the energy into the same emission mechanism as fluorescence. While both phosphorescence and TADF can theoretically reach 100% internal quantum efficiency (IQE), as a practical matter for displays IQE is not a useful metric, and companies in the industry talk about external quantum efficiency, or EQE.
Cynora has been steadily improving its performance for blue TADF, and last week announced a new milestone reached in June. Back in April, Cynora announced that it expects “to introduce a market-ready material by December, with an EQE above 15% and with more than 100h lifetime (LT97) at a typical display brightness and a CIEy color coordinate of <0.1.” At the SID Business Conference in May, they stated that their blue emitter has achieved 15% external quantum efficiency (EQE) at “1000 cd/m² with an emission peak at < 470 nm and a LT97 of > 90 hours (at 700 cd/m²) on a device level.”
I distinctly recall that Mike Hack of Universal Display Corporation commented that “you should not underestimate how difficult it can be to get 10 nanometers”, referring to the difference between Cynora’s claimed performance of 470nm and the target of 460nm. However, in a paper presented last week at the ERPOS conference in the UK, Cynora announced that they had made it halfway there, achieving a 15% EQE with a emission peak at 464nm and a full-width half-maximum (FWHM) of 58nm, although the lifetime performance degraded to 15 hours and will require optimization, according to Haldi.
Figure 2: Cynora Hits New Milestone in TADF Blue Performance
In my discussion with Dr. Haldi, I asked for an explanation of the target performance for lifetime. On its surface, the specification of more than 100 hours to 97% brightness seems woefully inadequate. With an exponential decay, this implies that at 1000 hours emission is more than 25% lower than the start, and at 5000 hours you’ve lost almost 80% of the initial emission. Smartphones will often be used for 6 hours per day, which is 2000 hours per year. LCDs are routinely capable of 50,000 hours to get to 50% brightness; while that may be overkill, in a consumer device the minimum expectation is 10,000 hours of good performance.
Haldi related several factors that explained this seeming discrepancy. The 100h/LT97 spec is specifically constructed as an accelerated lifetime spec to overcome the practical limitation of testing device life – a true test of a 50,000 hour lifetime spec would take almost six years to conduct. First, in a real device, the average brightness of blue will be substantially lower than 700 nits. Second, although the brightness may have a decay, if the decay is predictable it can be compensated (and this compensation is built into devices today). Finally, it is expected that materials used for mass production will have a longer life than those produced in an R&D setting, because the R&D tools and equipment are subject to many changes that can contaminate the material, while mass production systems are maintained in a cleaner state.
A similar question on the specification involves the color point, where the initial claim of Cynora (to achieve a CIEy color coordinate of <0.1 would seem to be inadequate to reach DCI-P3, let alone BT 2020 performance. The specified blue color point for DCI-P3 is (0.150, 0.060) and for BT 2020 it is (0.131, 0.046), as shown in Figure 3, which is close to a monochromatic blue at ~465nm.
Figure 3: BT 2020 Specification on CIE 1931 Color Coordinates
It should be apparent from Figure 3 that small changes in the peak emission affect the CIEy point, and according to Haldi each few nm of change achieves a big jump in CIEy. With reference to the milestones above in Figure 2, the April material had a CIEy of 0.29, with the June material at 0.20. Still these numbers seem nowhere close to the BT2020 spec, but again, according to Haldi, the differences between the R&D environment and performance in the device favors mass production. First, the R&D performance quoted refers to bottom emission, and a CIEy at 0.100 for bottom emission will translate to a CIEy of 0.050 at top emission. Second, for TV, the RGB filter used on LGD’s WOLED structure helps narrow the color point to achieve 0.05.
So what about Hack’s comment on the difficulty of achieving 460nm? According to Haldi, the differences between the phosphorescent mechanism pursued by UDC and the TADF mechanism favored by Cynora favor the TADF approach. In phosphorescence, the higher energy involved in blue emission acts to destabilize the host material, so improvements in color point go hand-in-hand with reduced lifetime. TADF has many more unexplored avenues for research compared to phosphorescence, and the fluorescence mechanism does not harm the surrounding material (as much).
Assuming that they actually achieve their goal of commercialization, Cynora’s blue TADF would represent a major efficiency improvement, and would disrupt the fluorescent emitter material business of Idemitsu Kosan. With respect to efficiency, the TADF performance of 15% EQE represents roughly a factor 2x compared to the incumbent fluorescent emitters, which have an EQE in the 7-9% range today. This is a moving target, though, because fluorescent emitters (presumably from Idemitsu Kosan) have been steadily improving over the last few years. Similarly, Haldi says that TADF will be able to squeeze out more EQE with improvements over time.
Longer term, Cynora plans to introduce green and red TADF emitters, which would represent a direct threat to UDC’s phosphorescent emitter business. Haldi said that up to now, Cynora has spent little time or effort on green or red, concentrating on the biggest opportunity space for OLED in blue, but once they have a business in blue, the other colors will be easier. Haldi mentioned that in Cynora’s research toward achieving a blue emitter, they have already found some materials that are closer to green, and these materials have more efficiency and lifetime. Reaching BT2020 may be a big challenge, though, because it requires a very narrow emission peak (small FWHM) and this is not the strong point for TADF (nor for phosphorescence). With red, they have a different challenge, because with a wide emission spectrum some of the emitted light is infra-red, and therefore the efficiency gets reduced.
When they introduce TADF green and red, Cynora will target to have a better performance product than UDC, Haldi said. Phosphorescent green has a lifetime issue, and phosphorescent red had an efficiency issue, so there are opportunities to have a higher performance product in each case. Further, panel makers are looking to avoid the licensing payments to UDC, and TADF is expected to be introduced without licensing fees.
With respect to manufacturing, Cynora plans to adopt the same outsourcing approach taken by UDC. UDC has contracted with PPG for manufacture of phosphorescent OLED emitter materials, and Cynora has been working with several companies, and will select a partner when needed. The German chemical industry is well suited to this type of work, Haldi said, and the capacity to make the materials will not be any problem. Cynora is looking at 2019 as the real milestone for production, with the expectation that if they have a product available in December 2017, both panel makers and then device makers will require at least 6 months to optimize their products to use TADF.
Finally, Haldi noted that Cynora is looking forward to their TADF Symposium on September 7 in Frankfurt. DSCC’s Yoshio Tamura will be presenting our view of OLED Supply/Demand, and a number of speakers from industry and academia will present the latest findings in the field. Based on the progress of Cynora in recent months and viewing their December milestone, we may see them announce another breakthrough there.
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