TADF Emitters Research Articles

Acceleration of reverse intersystem crossing in multi-resonance TADF emitter

Acceleration of reverse intersystem crossing in multi-resonance TADF emitter

Enhancing emission performance of red TADF emitters via the introduction of electronically inert pendant

In addition to emission efficiency and colors, emission characteristics such as preferentially horizontal emitting dipole orientation and resistance against concentration quenching are also highly desired for high-performance thermally activated delayed fluorescence (TADF) emitters. In this work, we report a simple and yet effective strategy for enhancing such emission performance of the pure red-to-near infrared (NIR) TADF emitter by introducing bulky aryl derivatives (m-xylene) to the efficient emission core. Incorporating m-xylene groups as the electronically inert pendants to the acceptor unit of the TADF emitter TPA–CN–N4-CH3 successfully retains favorable emission properties (e.g., pure red color chromaticity, nearly unitary photoluminescence quantum yield etc.) of the parent compound TPA–CN–N4 and yet also renders significantly higher horizontal emitting dipole ratio Θ// of ∼85 % with the more extended molecular configuration and reduced concentration quenching at very high concentrations in thin films. As a result, OLEDs adopting TPA–CN–N4-CH3 can demonstrate rather high EQEs of 29.8–32.1 % for pure red emission (625–650 nm) at low doping concentrations and decent EQEs of 18.9–27.5 % in the deep red to NIR range (650–700 nm) at higher doping concentrations. The results of this work shall provide a useful reference for development of efficient TADF emitters.

Regulating the conformation of phenoselenazine enable efficient TADF emitter with rapid reverse intersystem crossing rate

Triplet exciton utilization plays a crucial role in determining the performance of organic light-emitting diodes (OLEDs). Pure organic thermally activated delayed fluorescence (TADF) emitters based on donor–acceptor architecture are promising emitters for their low cost and high efficiency. However, many TADF emitters exhibit slow reverse intersystem crossing process and therefore suffer from large efficiency roll-off in the electroluminescence device, which hinders their practical application for the advanced OLEDs operating at high brightness. Herein, through regulating the conformation of the heavy-atom containing phenoselenazine moiety in a donor–acceptor molecular framework with a methyl group, we have constructed a TADF emitter with both small singlet–triplet splitting energy and pronounced heavy-effect. The designed emitter MCPPSeZ shows decent photoluminescence quantum yield of 67 % together with rapid reverse intersystem crossing rate of 4.2 × 106 s−1. Consequently, the corresponding yellow light-emitting electroluminescence device realize good external quantum efficiency (EQE) of 21.4 % and significantly efficiency roll-off, with EQEs remain 19.5 % and 13 % at the brightness of 1000 and 10 000 cd m−2, respectively. Moreover, using MCPPSeZ as the sensitizer for a red multi-resonance emitter, the narrowband emissive hyperfluorescent OLEDs are fabricated, which exhibit an EQE over 20 % accompanied by suppressed efficiency roll-off.

Enhancing blue TADF narrow-band emission via tandem OLEDs with optical modeling simulation

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials, based on polycyclic aromatic frameworks, have shown promise in achieving narrow-band emission and high luminous efficiency. In this study, we investigate the performance of a blue MR-TADF material, 2′2'2''-(1,3,5-triazine-2,4,6-triyl) tris(9-(2-(naphthalen-2-yl)phenyl)-9H-carbazole) (TBN-TPA), in tandem OLED devices. Single emitter unit and two emitter unit tandem OLED devices with blue MR-TADF material were explored with optical modeling simulation. More importantly, the tandem architecture assists in realizing narrow-band emission and high color purity via optical interference and microcavity effects, which are essential for meeting BT 2020 standards. TBN-TPA achieves narrow full width at half maximum down to 24 nm and CIE coordinates approaching the blue region in tandem devices. Our work highlights the significant advantages of combining tandem architectures and emerging MR-TADF emitters for developing high-performance OLEDs with both high efficiency and wide color gamut. This is the first demonstration of using tandem OLED architecture to improve both efficiency and color purity of a blue multi-resonance TADF emitter. Further research on optimizing tandem structures and designing advanced MR-TADF materials will promote the applications of OLED displays.

Ortho-Carborane Decorated Multi-Resonance TADF Emitters: Preserving Local Excited State and High Efficiency in OLEDs.

A novel class of o-carboranyl luminophores, 2CB-BuDABNA (1) and 3CB-BuDABNA (2) is reported, in which o-carborane moieties are incorporated at the periphery of the B,N-doped multi-resonance thermally activated delayed fluorescence (MR-TADF) core. Both compounds maintain the inherent local emission characteristics of their MR-emitting core, exhibiting intense MR-TADF with high photoluminescence quantum yields in toluene and rigid states. In contrast, the presence of the dark lowest-energy charge transfer state, induced by cage rotation in THF, is suggested to be responsible for emission quenching in a polar solvent. Despite the different arrangement of the cage on the DABNA core, both 1 and 2 show red-shifted emissions compared to the parent compound BuDABNA (3). By utilizing 1 as the emitter, high-efficiency blue organic light-emitting diodes (OLEDs) are achieved with a remarkable maximum external quantum efficiency of 25%, representing the highest reported efficiency for OLEDs employing an o-carboranyl luminophore as the emitter.

Secondary donor-acceptor group enable efficient pure green organic light-emitting devices based on multi-resonance TADF emitters

Multi-resonance thermally activated delayed fluorescence (MR-TADF) presents great potential for applications in high-quality organic light emitting diodes (OLEDs) displays due to their superb properties of high color purity and high quantum efficiencies. Nonetheless, most reported MR-TADF emitters show unsatisfactory devices performances with severe efficiency roll-off resulted from their inherent issues of slow reverse intersystem crossing rate (kRISC) and aggregation-caused quenching (ACQ). Herein, we propose a novel strategy by employing quinazoline derivatives as secondary acceptor (PQ) and secondary donor–acceptor (PQCz) to provide intermediate locally excited triplet (3LE) states and then enhance spin-orbital coupling (SOC). In addition, the secondary donor–acceptor group PQCz in tCzBN-PQCz can suppress intermolecular interaction and provide dual long-range charge transfer (LRCT) channels to further enhance kRISC. Therefore, tCzBN-PQ and tCzBN-PQCz exhibit high kRISC of 5.6 × 105 s−1 and 7.3 × 105 s−1 with large photoluminescence quantum yield (PLQY) of 89 % and 95 % respectively. Consequently, OLED based on tCzBN-PQ and tCzBN-PQCz show excellent external quantum efficiency (EQE) of 30.2 % and 35.1 % with small efficiency roll-off and small full width at half maxima (FWHM) of 37 nm and 34 nm respectively. Especially, tCzBN-PQCz exhibits better performances and less-sensitive to the doping concentration which proves the effectiveness of this secondary donor–acceptor group.

The role of a small molecular dipole moment for efficient non-doped deep blue thermally activated delayed fluorescence emitters

Non-doped and solution-processable deep blue emitters can greatly reduce the manufacturing cost of organic light-emitting diodes (OLEDs). However, influences of parameters concerning intermolecular interactions in solid state have not yet been explored. Herein, we proposed molecular dipole moment as a pre-experimental parameter to evaluate an emitter’s performance in the solid state. By extracting efficiency data of OLEDs based on blue-emitting molecules from literature and calculating the dipole moments of these molecules, we found a clear inverse relationship. Three deep blue TADF emitters with similar molecular emitting properties were designed and synthesized. The PL efficiency of their solid films and device performance was found to decrease as the molecular dipole increased. Specifically, TDBA-SPQ with the smallest dipole moments of 1.86 D delivered much better performance in both vacuum-processed and solution-processed non-doped OLEDs with record-high EQEmax of 22.7% and 15.2% with CIE-y coordinates of 0.136 and 0.123, respectively.

Improving the quantum yield of luminescence for three-coordinated gold(i) TADF emitters by exploiting inversion symmetry and using perhaloaryl ligands

New perhalophenyl gold(i)–tetraphosphino complexes are designed as highly efficient luminescent molecules. The electronegativity of the aryl groups and the symmetry of the molecules enable the formation of species with improved quantum yields at RT.

Effect of tert-butyl substitution on controlling the orientation of TADF emitters in guest–host systems

We compare how changes in the structure of an acceptor–donor–acceptor TADF emitter decorated with tert-butyl groups affect alignment. The “champion” material is ICzTRZ with tBu-substitution at the distal phenyl rings of the triazine acceptors.

High-performance deep-blue electroluminescence from multi-resonance TADF emitters with a spirofluorene-fused double boron framework.

The development of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials in the deep-blue region is highly desirable. A usual approach involves constructing an extended MR-TADF framework; however, it may also intensify aggregate-caused quenching issues and thereby reduce device efficiency. In this study, we develop a molecular design strategy that fuses the MR-TADF skeleton with 9,9'-spirobifluorene (SF) units to create advanced deep-blue emitters. The SF moiety facilitates high-yield one-shot bora-Friedel-Crafts reaction towards an extended skeleton and mitigates interchromophore interactions as a steric group. Our findings reveal that orbital interactions at the fusion site significantly influence the electronic structure, and optimizing the fusion mode allows for the development of emitters with extended conjugation length while maintaining non-bonding character. The proof-of-concept emitter exhibits narrowband emission in the deep-blue region, a near-unity photoluminescence quantum yield, and a fast k RISC of 2.4 × 105 s-1. These exceptional properties enable the corresponding sensitizer-free OLED to achieve a maximum external quantum efficiency (EQEmax) of 39.0% and Commission Internationale de l'Eclairage (CIE) coordinates of (0.13, 0.09). Furthermore, the hyperfluorescence device realizes an EQEmax of 40.4% with very low efficiency roll-off.

Achieving efficient and stable blue thermally activated delayed fluorescence organic light-emitting diodes based on four-coordinate fluoroboron emitters by simple substitution molecular engineering.

Achieving both high efficiency and high stability in blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is challenging for practical displays and lighting. Here, we have successfully developed a series of sky-blue to pure-blue emitting donor-acceptor (D-A) type TADF materials featuring a four-coordinated boron with 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligands, i.e.1-8. Synergistic engineering of substituents on the phenyl bridge as well as the electronic properties and the attached positions of heteroatom N-donors not only enables fine-tuning of the emission colors, but also modulates the nature and energies of their triplet excited states that are important for the reverse intersystem crossing (RISC). Particularly for the compound with two methyl substituents on the phenyl bridge (compound 8), RISC is significantly facilitated through the vibronic coupling of the energetically close-lying triplet charge transfer (3CT) and the triplet local excited (3LE) states, when compared to analogue 7. Efficient sky-blue to pure-blue OLEDs with electroluminescence peaks (λ EL) at 460-492 nm have been obtained, in which ca. five-fold higher external quantum efficiencies (EQEs) of 18.9% have been demonstrated by 8 than that by 7. Moreover, ca. thirty times longer device operational half-lifetimes (LT50) of 9113 hours for 8 than that for 7 as well as satisfactory LT50 reaching 26 643 hours for 6 at an initial luminance of 100 cd m-2 have also been demonstrated. To the best of our knowledge, these results represent one of the best high-performance blue OLEDs based on tetracoordinated boron TADF emitters. Moreover, the design strategy presented here has provided an attractive strategy for enhancing the device performance of blue TADF-OLEDs.

Pyrazino[2,3-f][1,10]phenanthroline-based color-tunable thermally activated delayed fluorescence emitters with AIE characteristics for high-efficiency organic light-emitting diodes

Colour tunable TADF emitters with strong AIE characteristics exhibited EQEs above 20%.

Narrow emission band ultraviolet/deep-blue thermally activated delayed fluorescence emitters modified with carbazole/carboline as a donor

Four high-efficiency ultraviolet/deep-blue narrow TADF emitters based on carbazole or carboline donor derivatives and their applications in ultraviolet/deep-blue OLEDs are demonstrated.

“Flagella effect” of thermally activated delayed fluorescence emitter dominating the efficiency of non-doped solution-processed OLEDs

A series of TADF emitters was developed by adjusting the length and mold of flexible chains, resulting in a huge difference in the PLQY, carrier mobility balance, and morphology in the pristine film and EQEmax of non-doped solution-processed OLEDs.

MR-TADF liquid crystals: towards self assembling host-guest mixtures showing narrowband emission from the mesophase.

Creating (room temperature) liquid crystalline TADF materials that retain the photophysical properties of the monomolecular TADF emitters remains a formidable challenge. The strong intramolecular interactions required for formation of a liquid crystal usually adversely affect the photophysical properties and balancing them is not yet possible. In this work, we present a novel host-guest approach enabling unperturbed, narrowband emission from an MR-TADF emissive core from strongly aggregated columnar hexagonal (Colh) liquid crystals. By modifying the DOBNA scaffold with mesogenic groups bearing alkoxy chains of different lengths, we created a library of Colh liquid crystals featuring phase ranges >100 K and room temperature mesomorphism. Expectedly, these exhibit broad excimer emission from their neat films, so we exploited their high singlet (S1 ∼2.9 eV) and triplet (T1 ∼2.5 eV) energies by doping them with the MR-TADF guest BCzBN. Upon excitation of the host, efficient Förster Resonance Energy Transfer (FRET) resulted in almost exclusive emission from BCzBN. The ability of the liquid crystallinity of the host to not be adversely affected by the presence of BCzBN is demonstrated as is the localization of the guest molecules within the aliphatic chain network of the host, resulting in extremely narrowband emission (FWHM = 14-15 nm). With this work we demonstrate a strategy for the self-assembly of materials with previously mutually incompatible properties in emissive liquid crystalline systems: strong aggregation in Colh mesophases, and narrowband emission from a MR-TADF core.

A fluorene-bridged double carbonyl/amine multiresonant thermally activated delayed fluorescence emitter for efficient green OLEDs.

Herein, we report a fluorene-bridged double carbonyl/amine-based MR TADF emitter DDiKTa-F, formed by locking the conformation of the previously reported compound DDiKTa. Using this strategy, DDiKTa-F exhibited narrower, brighter, and red-shifted emission. The OLEDs with DDiKTa-F emitted at 493 nm and showed an EQEmax of 15.3% with an efficiency roll-off of 35% at 100 cd m-2.

Solution-Processed Wide-Color-Tunable OLEDs From Green to Red With Multimixed Hosts Using an Aggregation-Induced TADF Emitter

Solution-Processed Wide-Color-Tunable OLEDs From Green to Red With Multimixed Hosts Using an Aggregation-Induced TADF Emitter

A red thermally activated delayed fluorescence emitter with mitigated efficiency roll-off via a π-stacked multiple donor–acceptor structure

The TADF emitter TCPP-DTPA with a multilayered π-stacked structure accelerates the RISC process and achieves extremely low-efficiency roll-off in red OLEDs.

Donor-modified asymmetric N/B/O multi-resonance TADF emitters for high-performance deep-blue OLEDs with the BT.2020 color gamut

Spiro-acridine modified N/B/O type MR-TADF emitters realized deep blue fluorescence with an EQEmax of 20.5% and CIE (0.147, 0.048), which is state-of-the-art efficiency for N/B/O type deep-blue MR emitters with CIEy approaching the BT.2020 standard.

A thermally activated delayed fluorescent platinum(ii) complex for red organic light emitting diodes with high efficiencies and small roll-off

A design of Pt(ii)–TADF emitters based on metal-perturbed intraligand charge-transfer excited states towards high-performance red organic light-emitting diodes is presented.