Esipt Materials With High-efficiency Exciton Utilization And Their Applications In OLED

Low cost,high-efficiency and highly stable white light emitting diodes（WOLEDs）are always one of the hot issues in the field of organic optoelectronics.Traditional WOLED realizes white light emission based on incomplete energy transfer between different color luminous components,but faces the problem that the energy transfer degree between different emitters is difficult to finely control,resulting in unstable color coordinates,low reproducibility and low production yield.In addition,according to spin quantum theory,only 25%singlet excitons can be utilized for most fluorescent molecules,resulting in the maximum external quantum efficiency（EQE）of the fluorescent OLED devices gnenerally not exceeding 5%.In comparation,the phosphorescent and thermally activated delayed fluorescence（TADF）materials were able to achieve 100%extion efficiency due to full use of triplet exitons.However,the phosphorescent materials are rare and expensive,whereas the TADF materials exhibit low luminous efficiency due to space charge separation and significant efficiency roll-off due to triplet-involved annihilation caused by the teiplet cumulaton during its long-lifetime.This also limits the further development of WOLEDs.In recent years,"hot-exciton"materials which can rapidly undergo fast reverse intersystem crossing through high energy levels have attracted numorous attentions because of their advantages of both high exciton utilization rate and prohibited efficiency roll-off.Herein two excited-state intramolecular proton transfer（ESIPT）yellow fluorescent material HDAPD-1 and bluish-green fluorescent material HDAPD-2 with high efficiency of exciton utilization（70%to 88%）of Hybridized Local Excited and Charge-Transfer（HLCT）states by use of fast high-level reverse intersystem crossing have been designed and synthesized.Notably,both compounds exhibit anti-kasha characteristics of simultaneous emissions in solutions from high energy level singlet state（S₂or S₃）as well as lowest singlet state（S₁）.OLED Device based on HDAPD-1 shows yellow light emission with a Commission Internationale d’Eclairage（CIE）of（0.4513,0.4521）,and maximum EQE of 4.12%,which is among the best performance in yellow ESIPT fluorescent OLEDs.HDAPD-2 remains such dual emissions in films,with its blue-white OLED exhibiting a CIE coordinate of（0.2317,0.3418）and maximum EQE up to 5.60%.This is among the highest performance for single molecular white emitting fluorescent materials.By fabricating nonenergy-transfer white emitting films consisting of different ratios of yellow emitting HDAPD-1 with sky-blue TADF emitter of DMAC-DPS,high-performance and color-stable non-energy-transferred single-EML cold white and pure-white WOLEDs have been realized,with CIE coordinates of（0.2589,0.3567）and（0.3209,0.3225）,and maximum EQEs of 13.3%and9.66%,respectively.This work pave ways for realizing simple structured low-cost,high efficiency,highly reproducible and color-stable WOLEDs.