High efficiency, low efficiency roll-off and long lifetime fluorescent white organic light-emitting diodes based on strategic management of triplet excitons via triplet-triplet annihilation up-conversion and phosphor sensitization

The simultaneous realization of high efficiency, low efficiency roll-off, long lifetime and stable electroluminescence (EL) spectra in fluorescent white organic light-emitting diodes (WOLEDs) is still a huge challenge. Here, we have used a triplet-triplet annihilation up-conversion (TTA-UC) material as the blue emission layer and a phosphor sensitized fluorescent emitter as the red emission layer and introduced a bipolar interlayer between them to fabricate high efficiency, low efficiency roll-off and long lifetime fluorescent WOLEDs. It is clearly seen that the bipolar interlayer not only efficiently distributed the singlet and triplet excitons, but also significantly stabilized the EL spectra. Thus, the resulting two-color fluorescent WOLEDs exhibited the maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 26.9 cd A(-1), 22.3 lm W(-1)and 12.8%, and retained 25.9 cd A(-1), 16.9 lm W(-1)and 12.1% at a luminance of 1000 cd m(-2)and 23.1 cd A(-1), 10.6 lm W(-1)and 10.7% at a luminance of 10 000 cd m(-2), respectively, and the operational lifetime LT50(50% decay, initial luminance of 1000 cd m(-2)) was also up to 984 h. It can be seen that these devices also showed stable EL spectra with a Commission Internationale de I'Eclairage (CIE) of (0.51, 0.41) from 1000 cd m(-2)to 5000 cd m(-2)luminance. Furthermore, upon further introducing a green fluorescent emitter the fabricated three-color fluorescent WOLEDs also showed high efficiencies of 15.5 cd A(-1), 13.0 lm W(-1)and 8.1% and stable EL spectra with a CIE of (0.43, 0.36) from 1000 cd m(-2)to 5000 cd m(-2)luminance. More importantly, their CE and EQE exhibited an increasing trend with luminance, a negligible efficiency roll-off. This design strategy provides a potential route towards high performance fluorescent WOLEDs.