Exciplex‐Forming Co‐host for Organic Light‐Emitting Diodes with Ultimate Efficiency

Abstract

Phosphorescent organic light-emitting diodes (OLEDs) with ultimate efficiency in terms of the external quantum effi ciency (EQE), driving voltage, and effi ciency roll-off are reported, making use of an exciplex-forming co-host. This exciplex-forming co-host system enables effi cient singlet and triplet energy transfers from the host exciplex to the phosphorescent dopant because the singlet and triplet energies of the exciplex are almost identical. In addition, the system has low probability of direct trapping of charges at the dopant molecules and no charge-injection barrier from the charge-transport layers to the emitting layer. By combining all these factors, the OLEDs achieve a low turn-on voltage of 2.4 V, a very high EQE of 29.1% and a very high power effi ciency of 124 lm W − 1 . In addition, the OLEDs achieve an extremely low effi ciency roll-off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m − 2 .

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

“…The EQE of the Ir(mphmq) 2 tmd device was higher than that of the devices containing Ir(MDQ) 2 acac, Ir(mphq) 2 acac or fac-Ir(phq) 3 . Specifically, the Ir(mphmq) 2 tmd device showed a very high EQE of 35.6% at a luminance of 350 cd m À 2 , which is 1.5 times higher than the highest efficiency reported to date for red OLEDs and even higher than that of green OLEDs [1][2][3]19,28,29 . The efficiency roll-off was also very small, as the EQE was 35.1% at 1,000 cd m À 2 and 30.0% at 10,000 cd m À 2 .…”

“…External quantum efficiencies (EQEs) of 30% have been reported recently [1][2][3][4][5][6] ; this is considered to be the light-outcoupling efficiency limit of phosphorescent OLEDs under isotropically oriented transition dipole moments 4 . It has long been recognized that orienting the transition dipole moment of an emitter along the horizontal direction (parallel to the substrate) can enhance the outcoupling efficiency beyond that achieved under isotropic orientation, as demonstrated in polymer-based and vacuumevaporated fluorescent molecule-based OLEDs [7][8][9][10][11][12][13] .…”

“…The measured q PL 's of the dyes were 0.71 for fac-Ir(phq) 3 , 0.83 for Ir(mphq) 2 acac, 0.82 for Ir(MDQ) 2 acac and 0.96 for Ir(mphmq) 2 tmd. The co-host system was selected to take advantage of its exciplex-forming characteristics because OLEDs containing an exciplex-forming co-host have an almost perfect electron-hole balance, low driving voltage and low efficiency rolloff 1,5,22,23 . Figure 1b shows the PL spectra of vacuum-evaporated films of NPB and B3PYMPM and a co-evaporated film of NPB and B3PYMPM with a molar ratio of 1:1.…”

A look inside white OLEDs

“…We are presently investigating this issue using our tools. With all-phosphorescent OLEDs, power efficiencies beyond 100 lm/W have been demonstrated in laboratory experiments, even exceeding those of fluorescent lamps (~90 lm/W), while operating at a high luminance [9,10]. Novel concepts for efficient and stable organic light emitters are also emerging, such as organic emitters in which the energies of singlet and triplet excitons are almost identical [11].…”

Recent progress of pyrimidine derivatives for high-performance organic light-emitting devices

“…58 A green phosphorescent OLED based on bis(2-phenylpyridine)iridium(III)acetylacetonate ½IrðppyÞ 2 ðacacÞ exhibited a PE of 124 lm W −1 , EQE of 29.1%, and low turn-on voltage of 2.4 V. Similarly, the same group developed red phosphorescent OLEDs using (bis(4-methyl-2-(3,5-dimetylphenyl)quinoline))Ir(III)(tetrametylheptadionate) ½IrðmphqÞ 2 ðtmdÞ as a red emitter and B3PyMPM/N,N′-di(naphthalen-1-yl)-N,N′-diphenylbenzidine (NPD) as exciplex host material, and a resulting OLED exhibited a very high EQE of 35.6%. 12 The exciplex formation from these hosts can create an almost perfect carrier balance, realizing a low turn-on voltage of 2.1 V and negligible efficiency roll-off (EQE ¼ 30% at 10;000 cd m −2 ).…”