Deep Learning Prediction of Triplet-Triplet Annihilation Parameters in Blue Fluorescent Organic Light-Emitting Diodes.

The triplet-triplet annihilation (TTA) ratio and the rate coefficient (kTT) of TTA are key factors in estimating the contribution of triplet excitons to radiative singlet excitons in fluorescent TTA organic light-emitting diodes. In this study, deep learning models are implemented to predict key factors from transient electroluminescence (trEL) data using new numerical equations. A new TTA model is developed that considers both polaron and exciton dynamics, enabling the distinction between prompt and delayed singlet decays with a fundamental understanding of the mechanism. In addition, deep learning models for predicting the kinetic coefficients and TTA ratio are established. After comprehensive optimization inspired by photophysics, determination coefficient values of 0.992 and 0.999 are achieved in the prediction of kTT and TTA ratio, respectively, indicating a nearly perfect prediction. The contribution of each kinetic parameter of polaron and exciton dynamics to the trEL curve is discussed using various deep-learning models.

HLCT-Type Acceptor Molecule-Based Exciplex System for Highly Efficient Solution-Processable OLEDs with Suppressed Efficiency Roll-Offs.

Exciplex systems are promising candidates for thermally activated delayed fluorescence (TADF) molecules because of the small energy difference between the lowest singlet and triplet excited states (ΔEST). However, realizing high-efficiency and low-external-quantum-efficiency (EQE) roll-off in solution-processed organic light-emitting diodes (OLEDs) using an exciplex system remains a formidable challenge. In this study, two (HLCT)-type isomers with a spiro skeleton, 2-tBuspoCz-TRZ and 10-tBuspoCz-TRZ, are designed and synthesized as acceptors of exciplexes, where tert-butylspirofluorene indole is regarded as a donor and the triazine unit as an acceptor. Green exciplex emissions are observed for the 2-tBuspoCz-TRZ:TAPC and 10-tBuspoCz-TRZ:TAPC exciplexes, indicating distinct TADF characteristics with a very small ΔEST of 35 ± 5 meV. By using the TADF exciplex system based on the HLCT acceptor as an emitter, solution-processable OLEDs achieve a maximum external quantum efficiency (EQEmax) of 20.8%. Furthermore, a high EQEmax > 25% with a very low-efficiency roll-off (≈3.5% at 1000 cd m-2) is obtained for solution-processable phosphorescent devices using HLCT-based exciplexes as the host matrix of phosphors. This study paves the way for a novel strategy for designing acceptor exciplex molecules for effective TADF molecules and host matrices in solution-processable OLEDs.

Tetradentate Pt(II) Complexes with Bulky Carbazole Moieties for High-Efficiency and Narrow-Emitting Blue Organic Light-Emitting Devices.

Blue tetradentate Pt(II) complexes, Pt-tBuCz and Pt-dipCz, are synthesized by introducing carbazoles with bulky substituents for improving the rigidity and inhibiting intermolecular interactions of phosphorescent emitter. tert-Butyl and 2,6-diisopropylphenyl groups are substituted as the blocking groups at 3 position of the carbazole in Pt-tBuCz and Pt-dipCz, respectively. These new phosphorescent emitters exhibit a narrow full width at half maximum (FWHM) and a high horizontal emitting dipole orientation ratio. Pt-dipCz demonstrates a small FWHM of 24 nm, a high emitting dipole orientation ratio of 81%, and a high photoluminescence quantum yield value of 94%. As a result, the Pt-tBuCz and Pt-dipCz devices exhibited external quantum efficiencies (EQEs) of 23.7% and 25.0% with small FWHMs of 25 and 22 nm, respectively. For the Pt-dipCz device, the small FWHM and high EQE of >20% are maintained even at a doping concentration of 20 wt%. Furthermore, phosphor-sensitized organic light-emitting diodes fabricated using Pt-dipCz as a sensitizer achieved a high EQE of 31.4% with an FWHM of 18 nm. This result indicates that the 2,6-diisopropylphenyl group is a effective blocking group for Pt(II) complexes to develop highly efficient, color stable, doping concentration resistant, and efficiently sensitizing blue phosphors.

Hybridized local and charge transfer dendrimers with near-unity exciton utilization for enabling high-efficiency solution-processed hyperfluorescent OLEDs.

Achieving both high emission efficiency and exciton utilization efficiency (ηS) in hot exciton materials is still a formidable task. Herein, a proof-of-concept design for improving ηS in hot exciton materials is proposed via elaborate regulation of singlet-triplet energy difference, leading to an additional thermally activated delayed fluorescence (TADF) process. Two novel dendrimers, named D-TTT-H and D-TTT-tBu, were prepared and characterized, in which diphenylamine derivatives were used as a donor moiety and tri(triazolo)triazine (TTT) as an acceptor fragment. Compounds D-TTT-H and D-TTT-tBu showed an intense green color with an emission efficiency of approximately 80% in solution. Impressively, both dendrimers simultaneously exhibited a hot exciton process and TADF characteristic in the solid state, as was demonstrated via theoretical calculation, transient photoluminescence, magneto-electroluminescence and transient electroluminescence measurements, thus achieving almost unity ηS. A solution processable organic light-emitting diode (OLED) employing the dendrimer as a dopant represents the best performance with the highest luminance of 15090 cd m-2 and a maximum external quantum efficiency (EQEmax) of 11.96%. Moreover, using D-TTT-H as a sensitizer, an EQEmax of 30.88%, 24.08% and 14.33% were achieved for green, orange and red solution-processed OLEDs, respectively. This research paves a new avenue to construct a fluorescent molecule with high ηS for efficient and stable OLEDs.

Achieving Long-Wavelength Electroluminescence Using Two-Coordinate Gold(I) Complexes: Overcoming the Energy Gap Law.

Two-coordinate coinage metal complexes have emerged as promising emitters for highly efficient organic light-emitting devices (OLEDs). However, achieving efficient long-wavelength electroluminescence emission from these complexes remains as a daunting challenge. To address this challenge, molecular design strategies aimed at bolstering the photoluminescence quantum yield (Φ) of Au(I) complex emitters in low-energy emission regions are investigated. By varying amido ligands, a series of two-coordinate Au(I) complexes is developed that exhibit photoluminescence peak wavelengths over a broad range of 533-750 nm. These complexes, in particular, maintain Φ values up to 10% even in the near-infrared emission region, overcoming the constraints imposed by an energy gap. Quantum chemical calculations and photophysical analyses reveal the action of radiative control, which serves to overcome the energy gap law, becomes more pronounced as the overlap between hole and electron distributions (Sr (r)) in the excited state increases. It is further elucidated that Sr (r) increases with the distance between the hole-distribution centroid and the nitrogen atom in an amido ligand. Finally, multilayer OLEDs involving the Au(I) complex emitters exhibit performances beyond the borderline of the electroluminescence wavelength-external quantum efficiency space set by previous devices of coinage metal complexes.

Fused Cycloalkyl Unit-Functionalized Tetradentate Pt(II) Complexes for Efficient and Narrow-Emitting Deep Blue Organic Light-Emitting Diodes.

A blue tetradentate Pt(II) complex named Pt-tmCyCz is developed by introducing a cycloalkyl unit fused to carbazole to improve the rigidity and bulkiness of the complex. The introduction of the tetramethylcyclohexyl (tmCy) group results in a narrow full width at half maximum (FWHM), a high horizontal emitting dipole orientation, doping concentration resistant stable spectrum, and extremely small efficiency roll-off, and little concentration quenching effect. Phosphorescent organic light-emitting diodes (OLEDs) doped with Pt-tmCyCz achieve a high external quantum efficiency (EQE) of 21.5%, with a small EQE roll-off of 3.8% up to 1000 cd m-2 , a small FWHM of 24 nm, and a color coordinate of (0.132, 0.138). Moreover, Pt-tmCyCz is investigated as a sensitizer in phosphor-sensitized OLEDs using N7 ,N7 ,N13 ,N13 ,5,9,11,15-octaphenyl-5,9,11,15-tetrahydro-5,9,11,15-tetraaza-19b,20b-diboradinaphtho[3,2,1-de:1',2',3'-jk]pentacene-7,13-diamine (νDABNA) as a terminal emitter. The Pt-tmCyCz:νDABNA device achieves a high EQE of 33.9%, with a small EQE roll-off of only 8.0% up to 1 000 cd m-2 . The results demonstrate that fused tmCy group in carbazole can be an effective building block for the development of high-performance Pt(II) complexes, which can be utilized as efficient phosphors or sensitizers in OLEDs.

Highly Efficient Phosphorescent Organic Light-Emitting Diodes Using Benzo[4',5']imidazo[2',1':2,3]imidazo[4,5,1-jk]carbazole as a Rigid and Effective Electron Acceptor in Bipolar Host.

A novel bipolar host architecture was investigated to improve the external quantum efficiency (EQE) of green phosphorescent organic light-emitting diodes (PhOLEDs). The host was developed by incorporating carbazole as a hole-transport unit and fused rigid benzo[4',5']imidazo[2',1':2,3]imidazo[4, 5, 1-jk]carbazole (BzICz) as a new electron transport unit. The primary goal of the BzICz-based host design was to achieve a high triplet energy and bipolar charge transport characteristics. The green PhOLEDs fabricated using the new BzICz and carbazole-based host demonstrated a high EQE of 26.6% due to their high triplet energy and good bipolar charge transporting characteristics.

Hybridization of short-range and long-range charge transfer excited states in multiple resonance emitter.

Multiple resonance (MR) thermally activated delayed fluorescence emitters have been actively studied as pure blue dopants for organic light-emitting diodes (OLEDs) because of excellent color purity and high efficiency. However, the reported MR emitter, 2,5,13,16-tetra-tert-butylindolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[2,3-b]carbazole (tDIDCz) based on bis-fused indolocarbazole framework could not demonstrate efficient triplet-to-singlet spin crossover. In this work, we report two isomeric MR emitters designed to promote triplet exciton harvesting by reconstructing the electronic structure of tDIDCz. To manage excited states, strong electron donors were introduced at the 2,5-/1,6-position of tDIDCz. As a result, 2,5-positions managed tDIDCz shows long-range charge transfer characteristics while preserving the MR nature. Quantum chemical calculation demonstrates direct spin-orbit coupling by long-range charge transfer and spin-vibronic coupling assisted reverse intersystem crossing by short-range charge transfer simultaneously contribute to triplet-to-singlet spin crossover. Consequently, high performance blue OLED recorded a high external quantum efficiency of 30.8% at a color coordinate of (0.13, 0.13).

Color Stable Deep Blue Multi-Resonance Organic Emitters with Narrow Emission and High Efficiency.

The development of highly efficient and deep blue emitters satisfying the color specification of the commercial products has been a challenging hurdle in the organic light-emitting diodes (OLEDs). Here, deep blue OLEDs with a narrow emission spectrum with good color stability and spin-vibronic coupling assisted thermally activated delayed fluorescence are reported using a novel multi-resonance (MR) emitter built on a pure organic-based molecular platform of fused indolo[3,2,1-jk]carbazole structure. Two emitters derived from 2,5,11,14-tetrakis(1,1-dimethylethyl)indolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[3,2-b]carbazole (tBisICz) core are synthesized as the MR type thermally activated delayed fluorescence emitters realizing a very narrow emission spectrum with a full-width-at-half-maximum (FWHM) of 16 nm with suppressed broadening at high doping concentration. The tBisICz core is substituted with a diphenylamine or 9-phenylcarbazole blocking group to manage the intermolecular interaction for high efficiency and narrow emission. The deep blue OLEDs achieve high external quantum efficiency (EQE) of 24.9%, small FWHM of 19 nm, and deep blue color coordinate of (0.16, 0.04) with good color stability with increase in doping concentration. To the authors' knowledge, the EQE in this work is one of the highest values reported for the deep blue OLEDs that achieve the BT.2020 standard.

Hybridized Local and Charge-Transfer Excited-State Emitter for a Blue Organic Solid-State Laser.

The use of triplet excitons harvesting and short exciton lifetime organic emitters is important to improve the exciton utilization in organic semiconductor laser diodes. In this study, a hybridized local and charge-transfer (HLCT)-type molecule, 11-(3-(10-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)anthracen-9-yl)phenyl)-11H-benzofuro[3,2-b]carbazole (PhAnMBf), is used as an emitter for blue-emitting organic solid-state lasers (OSSLs). The short exciton lifetime and high photoluminescence quantum yield of the PhAnMBf emitter allowed the fabrication of an organic laser with an emission wavelength of 453 nm, a small full width at half-maximum of 1.2 nm, and a threshold of 105 nJ/pulse, corresponding to 44 µJ/cm2, on the distributed feedback substrate. The anthracene-based PhAnMBf material showed the potential of the HLCT emitter as an OSSL.

Development of Blue Phosphorescent Pt(II) Materials Using Dibenzofuranyl Imidazole Ligands and Their Application in Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) are energy-efficient; however, the coordinating ligand can affect their stability. Sky-blue phosphorescent Pt(II) compounds with a C^N chelate, fluorinated-dbi (dbi = [1-(2,4-diisopropyldibenzo [b,d]furan-3-yl)-2-phenyl-1H-imidazole]), and acetylactonate (acac) (1)/picolinate (pic) (2) ancillary ligands were synthesized. The molecular structures were characterized using various spectroscopic methods. The Pt(II) Compound Two exhibited a distorted square planar geometry, with several intra- and inter-molecular interactions involving Cπ⋯H/Cπ⋯Cπ stacking. Complex One emitted bright sky-blue light (λmax = 485 nm) with a moderate photoluminescent quantum efficiency (PLQY) of 0.37 and short decay time (6.1 µs) compared to those of 2. Theoretical calculations suggested that the electronic transition of 1 arose from ligand(C^N)-centered π-π* transitions combined with metal-to-ligand charge-transfer (MLCT), whereas that of 2 arose from MLCT and ligand(C^N)-to-ligand(pic) charge-transfer (LLCT), with minimal contribution from C^N chelate to the lowest unoccupied molecular orbital (LUMO). Multi-layered phosphorescent OLEDs using One as a dopant and a mixed host, mCBP/CNmCBPCN, were successfully fabricated. At a 10% doping concentration of 1, a current efficiency of 13.6 cdA-1 and external quantum efficiency of 8.4% at 100 cdm-2 were achieved. These results show that the ancillary ligand in phosphorescent Pt(II) complexes must be considered.

A Color-Tunable Alternating Current Organic Light Emitting Capacitor.

We demonstrate an alternating current (AC) driven light emitting capacitor in which the color of the emission spectra can be changed via an applied AC frequency. The device has a simple metal-oxide-semiconductor (MOS) capacitor structure with an organic emissive layer, enabling facile fabrication processing. The organic emissive layer comprises a thin, submonolayer low energy dye layer underneath a thick host matrix (∼30 nm) with higher energy emitting dyes. The emission of the lower energy dyes dominates at low frequency, while the higher energy emission of the host matrix dominates at high frequency. This simple color tunable device could be used for full-color displays and lighting in the future.

Suppression of Dexter Energy Transfer through Modulating Donor Segments of Thermally Activated Delayed Fluorescence Assistant Dopants.

Device degradation in red hyperfluorescent organic light-emitting diodes is primarily caused by exciton energy loss due to Dexter energy transfer (DET) from a thermally activated delayed fluorescence (TADF) assistant dopant to a fluorescent dopant. In this work, the donor segments in the TADF assistant dopants were delicately modulated to suppress DET for high efficiency. The derived benzothienocarbazole donors were introduced to the TADF assistant dopants instead of carbazole, and they accelerated the reverse intersystem crossing of the TADF assistant dopant and managed the DET from the TADF assistant dopant to the fluorescent dopant. As a result, the red TADF-assisted device showed a high external quantum efficiency of 14.7% and improved the device lifetime by 70% compared to a well-known TADF-assisted device.

Asymmetric Decoration of a meta-Linked Bitriazine-Based Host for Highly Efficient and Stable Blue Phosphorescent Organic Light-Emitting Diodes.

High triplet energy hosts for blue phosphorescent organic light-emitting diodes were developed by decorating a meta-linked bitriazine core with carbazole and tetraphenylsilyl functional groups. A symmetric host with two carbazole units as the two triazine units of the core and an asymmetric host with one carbazole unit and one tetraphenylsilyl unit as the two triazine units were prepared. The triplet energy of these two hosts was 2.97 eV, suitable for triplet exciton harvesting of blue phosphors. Comparing the two host designs, the asymmetric decoration of the two triazine units with carbazole and tetraphenylsilyl units was superior to the symmetric decoration of the two triazine units with two carbazoles in terms of high external quantum efficiency (EQE) and long-term device stability. A high EQE of 19.7% and a long device lifetime of 2093.6 h at 100 cd m-2 were achieved using the asymmetrical host.

Extracting Polaron Recombination from Electroluminescence in Organic Light-Emitting Diodes by Artificial Intelligence.

Direct exploring the electroluminescence (EL) of organic light-emitting diodes (OLEDs) is a challenge due to the complicated processes of polarons, excitons, and their interactions. This study demonstrated the extraction of the polaron dynamics from transient EL by predicting the recombination coefficient via artificial intelligence, overcoming multivariable kinetics problems. The performance of a machine learning (ML) model trained by various EL decay curves is significantly improved using a novel featurization method and input node optimization, achieving an R2 value of 0.947. The optimized ML model successfully predicts the recombination coefficients of actual OLEDs based on an exciplex-forming cohost, enabling the quantitative understanding of the overall polaron behavior under various electrical excitation conditions.

Solution-Processed Pure Red TADF Organic Light-Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission Color.

In spite of recent research progress in red thermally activated delayed fluorescence (TADF) emitters, highly efficient solution-processable pure red TADF emitters are rarely reported. Most of the red TADF emitters reported to date are designed using a rigid acceptor unit which renders them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To resolve this issue, a novel TADF emitter, 6,7-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-5,8-dicarbonitrile (tBuTPA-CNQx) is designed and synthesized. The highly twisted donor-acceptor architecture and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital distribution lead to a very small singlet-triplet energy gap of 0.07 eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4 µs. The peripheral t-butyl phenyl decorated quinoxaline acceptor unit and t-butyl protected triphenylamine donor unit are proven to be useful building blocks to improve solubility and minimize the intermolecular interaction. The solution-processed OLED based on tBuTPA-CNQx achieves a high external quantum efficiency (EQE) of 16.7% with a pure red emission peak at 662 nm, which is one of the highest EQE values reported till date in the solution-processed pure red TADF OLEDs. Additionally, vacuum-processable OLED based on tBuTPA-CNQx exhibits a high EQE of 22.2% and negligible efficiency roll-off.

Control of Host-Matrix Morphology Enables Efficient Deep-Blue Organic Light-Emitting Devices.

Mixing a sterically bulky, electron-transporting host material into a conventional single host-guest emissive layer is demonstrated to suppress phase separation of the host matrix while increasing the efficiency and operational lifetime of deep-blue phosphorescent organic light-emitting diodes (PHOLEDs) with chromaticity coordinates of (0.14, 0.15). The bulky host enables homogenous mixing of the molecules comprising the emissive layer while suppressing single host aggregation; a significant loss channel of nonradiative recombination. By controlling the amorphous phase of the host-matrix morphology, the mixed-host device achieves a significant reduction in nonradiative exciton decay, resulting in 120 ± 6% increase in external quantum efficiency relative to an analogous, single-host device. In contrast to single host PHOLEDs where electrons are transported by the host and holes by the dopants, both charge carriers are conducted by the mixed host, reducing the probability of exciton annihilation, thereby doubling of the deep-blue PHOLED operational lifetime. These findings demonstrate that the host matrix morphology affects almost every aspect of PHOLED performance.

Key Factor Managing the Horizontal Emitting Dipole Orientation of a Thermally Activated Delayed Fluorescence Emitter in a Mixed Host.

Horizontal emitting dipole orientation (EDO) of thermally activated delayed fluorescence (TADF) molecules in a mixed host was studied by altering the host materials and host composition of the mixed host to gain insight into the important parameter of the host governing the EDO of TADF emitters. Five different host materials were combined with 1,3-bis(carbazol-9-yl)benzene (mCP), demonstrating that the host-dopant interaction is crucial to the absolute value of the horizontal EDO of the TADF emitters, whereas the glass transition temperature (Tg) is the important parameter determining the EDO dependence upon host composition. The mixed host of mCP with a high Tg host maintained high horizontal EDO in the mCP poor host composition, while that of mCP with a low Tg host showed average horizontal EDO of two hosts. Therefore, the combination of a high Tg n-type host enabling a strong host-dopant interaction with the p-type host with the usage of the n-type-host-rich composition is effective to achieve high horizontal EDO in the mixed-host-based TADF emitting layer.

Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence.

Phosphorescent and thermally activated delayed fluorescence (TADF) blue organic light-emitting diodes (OLEDs) have been developed to overcome the low efficiency of fluorescent OLEDs. However, device instability, originating from triplet excitons and polarons, limits blue OLED applications. Here, we develop a phosphor-sensitized TADF emission system with TADF emitters to achieve high efficiency and long operational lifetime. Peripheral carbazole moieties are introduced in conventional multi-resonance-type emitters containing one boron atom. The triplet exciton density of the TADF emitter is reduced by facilitating reverse intersystem crossing, and the Förster resonant energy transfer rate from phosphor sensitizer is enhanced by high absorption coefficient of the emitters. The emitter exhibited an operational lifetime of 72.9 hours with Commission Internationale de L'Eclairage chromaticity coordinate y = 0.165, which was 6.6 times longer than those of devices using conventional TADF emitters.

Enhancing Horizontal Ratio of Transition Dipole Moment in Homoleptic Ir Complexes for High Outcoupling Efficiency of Organic Light-Emitting Diodes.

The light-emitting dipole orientation (EDO) of a phosphorescent emitter is a key to improving the external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs) without structural modification of the device. Here, four homoleptic Ir complexes as a phosphorescent emitter are systematically designed based on the molecular structure of tris(2-phenylpyridine)iridium(III) (Ir(ppy)3 ) to control the EDO. Trimethylsilane, methyl, 2-methylpropyl, and cyclopentylmethyl group substituted to pyridine ring of the ligand contribute to the improvement of the EDO from 76.5% for Ir(ppy)3 to 87.5%. A linear relationship between the EDO and the aspect ratio (geometric anisotropy factor) is founded, implying the importance of the effective area for the nonbonding force between host and dopant molecules. Also, it is investigated that the EDO enhancement mainly originates from the vertical alignment of the C3 axis of molecule in the substrate axis rather than the change in the direction of the transition dipole alignment in the molecular axis. The optical simulation reveals that the outcoupling efficiency of phosphorescent OLEDs adopting new dopants reaches 38.4%. The green OLEDs exhibiting 28.3% of EQE, 103.2 cd A-1 of current efficiency, and 98.2 lm W-1 of power efficiency are demonstrated, which is understood to have little electrical loss.