Understanding and Manipulating the Interplay of Wide‐Energy‐Gap Host and TADF Sensitizer in High‐Performance Fluorescence OLEDs

Abstract

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...

Strategically Modulating Carriers and Excitons for Efficient and Stable Ultrapure‐Green Fluorescent OLEDs with a Sterically Hindered BODIPY Dopant

“…In our previous work, we have demonstrated the modulation of charges recombination by optimizing the energy level of constituents in EML with different host materials. [ 24 ] Here, those results demonstrate the importance of the adjacent layers in manipulating charges recombination.…”

“…In previous works, multiple sensitizing processes were realized by varying the constituents in the EML. [ 24 ] Here, it is proved that modulating the energy level of the adjacent layers can also manipulate charges recombination zone for multiple sensitizing processes. Besides, a remarkable high PE max of 85.7 lm W −1 was also realized for D2, which remained 78.3 and 58.5 lm W −1 at 1000 and 5000 cd m −2 .…”

All‐Fluorescence White Organic Light‐Emitting Diodes Exceeding 20% EQEs by Rational Manipulation of Singlet and Triplet Excitons

“…The yellow sensitized efficiency is also among the best OLEDs based on TBRb as the fluorescence dye. [ 26,27,40 ]…”

“…[ 26 ] This technique has made promising progress in monochromatic emitting dyes with relatively low‐lying states, such as green, yellow, and red fluorophores. [ 27–29 ] Thus, it seems reasonable and accessible to achieve all‐fluorescence WOLEDs by incorporating sensitized and complementary fluorescence dyes for high efficiency. Nevertheless, very few efficient all‐fluorescence WOLEDs are successfully designed on the basis of this strategy, [ 30,31 ] which may be attributed to the following reasons: i) the energy level of deep blue fluorescence emitter is difficult to be matched with the appropriate TADF sensitizer; ii) meanwhile, the relatively low‐lying triplet energy level of blue fluorescent emitters () could induce triplet exciton quenching or detrimental Dexter energy transfer with non‐radiative process, thus reduce the exciton utilization ratio for radiative transition of other color‐complementary emitters; iii) furthermore, the trap effect and accumulation of triplet excitons would accelerate triplet–triplet annihilation (TTA), triplet–polaron annihilation (TPA) degradation; [ 32,33 ] iv) unlike a monochromatic OLED, it is also important but difficult to maintain rational and stable exciton allocation under different current injections, thereby induces a steady CIE coordinates for WOLED systems composed of multiple emissive units.…”

Recent Progress in Emerging Near-Infrared Emitting Materials for Light-Emitting Diode Applications

“…In view of the abundant conventional fluorescent dopant (CFD) mentioned earlier, it is feasible to use TADF materials as hosts or sensitizers and CFDs as dopants to fabricate emitting layers in OLEDs. [96][97][98][99][100] In this way, holes and electrons recombine in the host or sensitizer to form singlet and triplet excitons at a 1:3 ratio and then triplet excitons absorb the heat in the environment to form singlet excitons via a reversed intersystem crossing (RISC) due to the relatively small ΔE ST . Radiative excitons are transferred to a fluorescent or phosphorescent dopant and then radiative decay for emitting photons through Förster resonance energy transfer (FRET, a nonradiative mechanism of energy transfer from a fluorophore in the excited state to a fluorophore in the ground state via dipole-dipole coupling) in PL or a charge carrier trapping mechanism in EL.…”