Organic molecules for efficient bendable displays and lighting
Bendable, lightweight, low-cost, and highly efficient displays could transform our daily lives, enhancing our ability to interact with information. Organic LEDs (OLEDs) can enable such displays and lighting because they use extremely thin (∼100nm) layers of organic semiconducting molecules to emit light. In an operating OLED, an energetic state called an exciton forms when a high-energy electron arrives at a molecule missing a lower-energy electron. The exciton releases its energy as light when its electron moves to the available low-energy state.
Two different types of excitons are created, 25% of them singlets and 75% triplets. The first OLEDs emitted light only from singlets through fluorescence—see Figure 1(a)—limiting the internal quantum efficiency (the fraction of charge in the current converted to photons) to 25%. Second-generation materials achieved 100% efficiency by converting singlets into triplets and emitting from these through phosphorescence: see Figure 1(b). However, this approach is often less stable and requires expensive rare earth metals. To overcome these limitations and achieve high efficiency, we have developed new materials and devices that use thermally activated delayed fluorescence (TADF).
Although the energy difference (ΔEST) between singlets and triplets in conventional fluorescent materials is large, we have successfully designed TADF materials to have small ΔEST so that the low-energy triplets can ‘up’-convert into singlets by absorbing small amounts of heat from the surroundings: see Figure 2(a).1 Thus, 100% of excitons can contribute to fluorescence, with prompt emission from the initial singlets and delayed emission from triplets that are up-converted. However, singlets can also down-convert into triplets before fluorescence, so excitons often go through several conversion cycles before emission. This has prompted concerns that the resulting long lifetime of the energetic excitons could accelerate device degradation.
To prevent this, we propose a new system, TADF-assisted fluorescence (TAF), combining TADF materials to generate singlets and conventional fluorescent materials to emit light.2 As shown in Figure 2(b), the TADF molecules harvest excitons, convert them into singlets, and then transfer them to the fluorescent molecules by energy transfer, which is very fast for optimized combinations. Since molecular design for small ΔEST generally leads to slower fluorescence compared to conventional fluorescent materials, we can decrease exciton lifetime by quickly shuttling the up-converted singlets to the fluorescent molecules for fast emission before down-conversion.
We studied the TAF system using (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN-Me) (which has a high up-conversion rate) as the TADF material, and tetra(t-butyl)rubrene (TBRb) as the fluorescent emitter.3 Incorporating TBRb in a 4CzIPN-Me OLED changed the emission from 4CzIPN-Me to TBRb while approaching an efficiency of 100%. The device lifetime when operated at a constant current improved for TAF compared to TADF: see Figure 2(c). We calculated that the density of triplets decreases significantly by adding TBRb, which also reduces the drop in efficiency at higher brightness.
Another advantage of the TAF system is that it is possible to transfer excitons to many different existing fluorescent emitters. Exploiting this, we fabricated a white OLED using the blue TADF emitter bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMACDPS), which transfers some of its energy to red tetraphenyldibenzoperiflanthene (DBP) and green 9,10-bis[N,N-di-(p-tolyl)-amino]anthracene (TTPA) fluorescent emitters in a separate layer: see Figure 3(a).4 Furthermore, we can tune the energy transfer and emission spectrum based on the emitter concentration: see Figure 3(b).
New TADF materials enable highly efficient OLEDs by converting triplets to singlets, and without using rare earth metals. Combining TADF and existing fluorescent materials in TAF, we can produce a wide range of colors and improve stability. Moving forward, we will develop new molecules and architectures to further improve device lifetime, achieve pure red and blue emission, and reduce degradation caused by oxygen and water.
This research was supported by the Japan Science and Technology Agency under the Exploratory Research for Advanced Technology research funding program through the Adachi Molecular Exciton Engineering Project.
Chihaya Adachi's career-long study of organic LEDs (OLEDs) includes early fluorescent devices during his doctorate at Kyushu University, and some of the first highly efficient phosphorescent OLEDs while at Princeton University. He has been a professor at Kyushu since 2006 and director of OPERA since 2010.