Multiple donor–acceptor‐type carbazole–benzonitrile derivatives that exhibit thermally activated delayed fluorescence (TADF) are the state of the art in efficiency and stability in sky‐blue organic light‐emitting diodes. However, such a motif still suffers from low reverse intersystem crossing rates (kRISC) with emission peaks <470 nm. Here, a weak acceptor of cyanophenyl is adopted to replace the stronger cyano one to construct blue emitters with multiple donors and acceptors. Both linear donor–π–donor and acceptor–π–acceptor structures are observed to facilitate delocalized excited states for enhanced mixing between charge‐transfer and locally excited states. Consequently, a high kRISC of 2.36 × 106 s−1 with an emission peak of 456 nm and a maximum external quantum efficiency of 22.8% is achieved. When utilizing this material to sensitize a blue multiple‐resonance TADF emitter, the corresponding device simultaneously realizes a maximum external quantum efficiency of 32.5%, CIEy ≈ 0.12, a full width at half maximum of 29 nm, and a T80 (time to 80% of the initial luminance) of > 60 h at an initial luminance of 1000 cd m−2.

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence-sensitized fluorescence (TSF) offer the possibility of attaining an ultimate high efficiency with low roll-off utilizing noble-metal free, easy-to-synthesize, pure organic fluorescent emitters. However, the performances of TSF-OLEDs are still unsatisfactory. Here, TSF-OLEDs with breakthrough efficiencies even at high brightnesses by suppressing the competitive deactivation processes, including direct charge recombination on conventional fluorescent dopants (CFDs) and Dexter energy transfer from the host to the CFDs, are demonstrated. On the one hand, electronically inert terminal-substituents are introduced to protect the electronically active core of the CFDs; on the other hand, delicate device structures are designed to provide multiple energy-funneling paths. As a result, unprecedentedly high maximum external quantum efficiency/power efficiency of 24%/71.4 lm W in a green TSF-OLED are demonstrated, which remain at 22.6%/52.3 lm W even at a high luminance of 5000 cd m . The work unlocks the potential of TSF-OLEDs, paving the way toward practical applications.

Maintaining high efficiency at high brightness levels is an exigent challenge for real-world applications of thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Here, versatile indolocarbazole-isomer derivatives are developed as highly emissive emitters and ideal hosts for TADF-OLEDs to alleviate efficiency roll-off. It is observed that photophysical and electronic properties of these compounds can be well modulated by varying the indolocarbazole isomers. A photoluminescence quantum yield (η ) approaching unity and a maximum external quantum efficiency (EQE ) of 25.1% are obtained for the emitter with indolo[3,2-a]carbazolyl subunit. Remarkably, record-high EQE/power efficiency of 26.2%/69.7 lm W at the brightness level of 5000 cd m with a voltage of only 3.74 V are also obtained using the same isomer as the host in a green TADF-OLED. It is evident that TADF hosts with high η values, fast reverse intersystem crossing processes, and balanced charge transport properties may open the path toward roll-off-free TADF-OLEDs.

fluorescence (TADF) as the sensitizers while CFDs as the final emitters, the limitation is successfully broken and this so-called TADF-sensitizing-fluorescence (TSF) strategy has been recognized as one of the most promising technologies for next-generation OLEDs, offering the possibility of ultimate-high device efficiency, low efficiency roll-off as well as tunable colors with all pure organic compounds. [4][5][6] Still, the performance of TSF-OLEDs needs improving so as to meet the requirement of practical applications. [7] Commonly, TSF-OLED features a typical emitting layer (EML) of a ternary system with a wide-energy-gap host, a TADF sensitizer and a CFD. Avoiding energy loss through the triplet (T 1 ) states of CFDs, formed by Dexter energy transfer (DET) from the T 1 s of the host and the TADF sensitizer, is the key for high device efficiency, which highly depends on the interplay of the components in the EMLs. In our previous work, we have revealed that CFD with electronically inert steric substituents can suppress the DET process by enlarging the distance of its electronically active core with the adjacent molecule, based on the fact that DET is a shortrange, efficient orbital-overlapping required interaction. [8] Later, such molecular design concept was also been adopted by TADF sensitizers to prevent DET. [9] But the enlarged intermolecular distance may also weaken the desired Förster energy transfer (FET). [10] To overcome this issue, we first introduced interfacial exciplex systems as the wide-energy-gap host to trigger multiple sensitizing processes, greatly enhancing FET meanwhile suppressing DET. [8,11] Consequently, the exciton loss pathways were successfully blocked and a maximum external quantum efficiency (EQE max ) as high as 24% was realized. After that, Liao and coworkers also introduced bulky exciplex system as the host to harness triplet excitons. [12] Those results validate the effectiveness of manipulating the EML components to maximize the device performances. However, the exciplex systems would increase the complexity of the device fabrication process and more importantly, the lack of efficient blue exciplex systems limits the versatility of this strategy. [7] Possessing higher up-conversion efficiencies, more flexible molecular structures and more feasible color-turnabilities than exciplex systems, single-molecular TADF materials also Comprising an emitting layer (EML) constituting a wide-energy-gap host, a thermally activated delayed fluorescence (TADF) sensitizer and a conventional fluorescent dopant, TADF-sensitizing-fluorescence organic light-emitting diodes (TSF-OLEDs) highly depend on component interplay to maximize their performance, which, however, is still under-researched. Taking the host type (TADF or non-TADF) and the recombination position (on the host or on the TADF sensitizer) into consideration, the interplay of host and TADF sensitizer is comprehensively studied and manipulated. A wide-energy-gap host with TADF and recombination of charges on it are both required to m...

Numerous efforts have been devoted to boost the efficiency of thermally activated delayed fluorescence (TADF) devices; however, strategies to suppress the device efficiency roll-off are still in urgent need. Here, a general and effective approach to suppress the efficiency roll-off of TADF devices is proposed, that is, utilizing TADF materials as the hosts for TADF emitters. Bearing small singlet-triplet splitting (ΔE) with donor and acceptor units, TADF materials as the hosts possess the potential to achieve matched frontier energy levels with the adjacent transporting layers, facilitating balanced charge injection as well as bipolar charge transport mobilities beneficial to the balanced charges transportation. Furthermore, an enhanced Förster energy transfer from the host to the dopant can be anticipated, helpful to reduce the exciton concentration. Based on the principles, a new TADF material based on indeno[2,1-b]carbazole/1,3,5-triazin derivation is synthesized and used as the universal host for the full-color TADF devices. Remarkable low efficiency roll-off was achieved with above 90% of the maximum external quantum efficiencies (EQE's) maintained even at a brightness of 2000 cd/m, along with EQE's of 23.2, 21.0, and 19.2% for orange, green, and sky-blue TADF devices, respectively. Through computational simulation, we identified the suppressed exciton annihilation rates compared with devices adopting conventional hosts. The state-of-the-art low efficiency roll-off of those TADF devices manifests the great potential of such host design strategy, paving an efficient strategy toward their practical application.