Organic Light-Emitting Diodes (OLEDs) have become a leading display technology. The luminescent material is a core component of OLEDs. Thermally activated delayed fluorescence (TADF) materials have emerged as promising emitters for achieving high-efficiency OLEDs. Blue TADF-OLEDs confront a more pronounced efficiency roll-off and material degradation issue compared to their green and red counterparts, owing to the formation of high-energy excitons through bimolecular reactions of long-lived excited states. 

To pave the way for future TADF-OLED applications, it is crucial to develop advanced blue TADF molecules with extremely short exciton lifetimes, ideally in the nanosecond scale. Simultaneously achieving a high photoluminescence quantum yield (PLQY), ultrashort exciton lifetime, and suppressed concentration quenching in TADF materials is desirable yet challenging. 

In a study published in Adv. Mater., the research group led by Prof. LU Canzhong from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, reported a new strategy for designing high-performance blue TADF materials. They developed a blue TADF material with high emission efficiency, nanosecond emission lifetime, and effectively suppressed concentration quenching, enabling the realization high-performance nondoped and doped blue OLEDs. 

The researchers designed and synthesized a acceptor-donor-acceptor (A-D-A) type TADF molecule, namely 2BO-sQA, bearing a dispirofluorene-quinolinoacridine (sQA) donor and two cofacial oxygen-bridged triarylboron (BO) acceptors which are nearly orthogonal to the sQA donor. On one hand, this unique molecular structure facilitates achieving an extremely narrow singlet-triplet energy gap (ΔE_ST = 0.01 eV), thereby accelerating reverse intersystem crossing (k_RISC ≈ 2×10⁶) and achieving ultra-short emission lifetimes (476 ~ 817 ns). Such a short delayed fluorescence lifetime is uncommon in high-efficiency TADF materials and is expected to be beneficial in mitigating exciton annihilation in the emitting layer and suppressing efficiency roll-off at high luminance levels. On the other hand, the molecular rigidity and intramolecular interactions effectively inhibit non-radiative transitions, ensuring high emission efficiency. Moreover, rigid three dimensional A-D-A molecular architecture can effectively suppress aggregation caused emission quenching at high doping concentrations. The films containing 2BO-sQA, with concentrations ranging from 10 wt% to 100 wt%, exhibit high PLQYs of 99% to 86%. 

The OLEDs utilizing 2BO-sQA as the terminal luminescent emitter demonstrated excellent electroluminescent characteristics, with no significant dependence on concentration, maintaining maximum external quantum efficiencies (EQE_max) at over 30% for doping concentrations ranging from 10 to 70 wt%. Remarkably, the nondoped blue OLED achieved a record-high maximum EQE of 26.6% with a small efficiency roll-off of 14.0% at a luminance of 1000 cd m^-2. 

Hyperfluorescence OLEDs employing 2BO-sQA as the sensitizer and v-DABNA as the terminal dopant realized narrowband deep-blue emission with high EQEs of up to 32.3%. Moreover, the EQE roll-off at 1000 cd m^-2 decreased significantly from 40.1% for the control device to 14.5% for the hyperfluorescence OLED with 40 wt% 2BO-sQA, likely attributed to the short exciton lifetime of the sensitizer. These results demonstrate the potential of 2BO-sQA as a sensitizer for achieving high-performance deep blue hyperfluorescence OLEDs. 

This study sheds new light on the development of ideal blue TADF materials that  simultaneously exhibit high emission efficiency, ultrashort exciton lifetime, and suppressed concentration quenching, which are highly desirable for achieving high-performance blue OLEDs. 

Performance Overview (Image by Prof. LU’s group) 

Contact: 

Prof. LU Canzhong 

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

Email: czlu@fjirsm.ac.cn;