Realizing 20% External Quantum Efficiency in Electroluminescence with Efficient Thermally Activated Delayed Fluorescence from an Exciplex

Effects of donor substituents on the conformational heterogeneity, photophysical, mechanochromic and electroluminescent properties of the donor-substituted fluorine-containing triphenylpyrimidines

Three derivatives of fluorinated triphenylpyrimidine with the attached carbazole, phenothiazine, or acridan donor moieties are synthesized by Buchwald-Hartwig reactions. The impact of the donor units on emissive and other properties of the compounds is reported. The compounds exhibit excellent thermal stability, competitive photophysical phenomena such as room temperature phosphorescence (RTP) appearing when compounds are molecularly dispersed in the rigid polymer matrix and thermally activated delayed fluorescence (TADF). The compounds with carbazole and phenothiazine donor moieties show the manifestation of triplet-triplet annihilation in the electroluminescence when used as emitters in organic light-emitting diodes (OLEDs). The phenothiazine-containing compound exhibit dual photoluminescence with the blue-shifted peak corresponding to the quasi-axial conformer and a red-shifted peak to the quasi-equatorial conformer. This derivative shows reversible shifts of emission spectra exceeding 100 nm due to the stable (at least 4 cycles) mechanochromic luminescence under the application of external stimuli. After grinding the emission intensity maximum is observed at 555 nm, after fuming at. ca 448 nm and after melting at 555 nm. The photoluminescence shifts and ON/OFF delayed fluorescence of the phenothiazine-based emitter occur due to the alteration between the crystalline and amorphous states. Optimization of the device structure allows to control the charge balance resulting in external quantum efficiency of up to 5.7 % observed for the OLED based on the phenothiazine-based emitter. This compound also shows the biggest response to the presence of oxygen acting as the quencher of triplet excited energy. The film of the compound doped in rigid Zeonex shows an 8.4-fold increase in emission intensity after evacuation. The optical sensor fabricated using the derivative of fluorinated triphenylpyrimidine and phenothiazine is characterized by the Stern-Volmer constant 1.37 × 10-4 ppm-1.

Progress in Research on White Organic Light-Emitting Diodes Based on Ultrathin Emitting Layers

White organic light-emitting diodes (WOLEDs) hold vast prospects in the fields of next-generation displays and solid-state lighting. Ultrathin emitting layers (UEMLs) have become a research hotspot because of their unique advantage. On the basis of simplifying the device structure and preparation process, they can achieve electroluminescent performance comparable to that of doped devices. In this review, we first discuss the working principles and advantages of WOLEDs based on UEML architecture, which can achieve low cost and more flexibility by simplifying the device structure and preparation process. Subsequently, the successful applications of doping and non-doping technologies in fluorescent, phosphorescent, and hybrid WOLEDs combined with UEMLs are discussed, and the operation mechanisms of these WOLEDs are emphasized briefly. We firmly believe that this article will bring new hope for the development of UEML-based WOLEDs in the future.

High-efficiency all-fluorescent white organic light-emitting diode based on TADF material as a sensitizer

The use of TADF materials as both sensitizers and emitters is a promising route to achieve high-efficiency all-fluorescent white organic light-emitting diodes (WOLEDs). In this study, the thermally-activated delayed-fluorescent (TADF) material DMAC-TRZ (9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine) was selected as a sensitizer for the conventional fluorescent emitter DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran), which was co-doped in a wide bandgap host of DPEPO (bis[2-(diphenylphosphino)phenyl]ether oxide) to fabricate WOLEDs. For the emitting layer of DPEPO:DMAC-TRZ:DCJTB, the DPEPO host can dilute the exciton concentration formed on the DMAC-TRZ sensitizer, which benefits the suppression of exciton quenching. The effect of the doping concentration of DCJTB on the carrier recombination and energy transfer process was investigated. With an optimized doping concentration of DCJTB as 0.8%, highly efficient WOLED was achieved with a maximum external quantum efficiency (EQE), power efficiency (PE), and current efficiency (CE) of 11.05%, 20.83 lm W-1, and 28.83 cd A-1, respectively, corresponding to the Commission Internationale de I' Eclairage (CIE) coordinates of (0.45, 0.46). These superior performances can be ascribed to the fact that the hole-trapping effect of the emitter and Dexter energy transfer (DET) from sensitizer to emitter can be suppressed simultaneously by the extremely low doping concentration.

Recombination of X-ray-Generated Radical Ion Pairs in Alkane Solution Assembles Optically Inaccessible Exciplexes from a Series of Perfluorinated para-Oligophenylenes with N,N-Dimethylaniline

We demonstrate that a series of perfluorinated para-oligophenylenes C₆F₅-(C₆F₄)_n-C₆F₅ (n = 1-3) produce exciplexes with N,N-dimethylaniline (DMA) in degassed X-irradiated n-dodecane solutions. The optical characterization of the compounds shows that their short fluorescence lifetimes (ca. 1.2 ns) and UV-Vis absorption spectra, overlapping with the spectrum of DMA with molar absorption coefficients of 2.7-4.6 × 10⁴ M^-1cm^-1, preclude the standard photochemical exciplex formation pathway via selective optical generation of the local excited state of the donor and its bulk quenching by the acceptor. However, under X-rays, the efficient assembly of such exciplexes proceeds via the recombination of radical ion pairs, which delivers the two partners close to each other and ensures a sufficient energy deposition. The exciplex emission is completely quenched by the equilibration of the solution with air, providing a lower bound of exciplex emission lifetime of ca. 200 ns. The recombination nature of the exciplexes is confirmed by the magnetic field sensitivity of the exciplex emission band inherited from the magnetic field sensitivity from the recombination of spin-correlated radical ion pairs. Exciplex formation in such systems is further supported by DFT calculations. These first exciplexes from fully fluorinated compounds show the largest known red shift of the exciplex emission from the local emission band, suggesting the potential of perfluoro compounds for optimizing optical emitters.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B)-a high triplet (T₁ =3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S₁ →S₀ CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m^-2 with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (³ LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

Enhanced Efficiency of Exciplex Emission from a 9-Phenylfluorene Derivative

The exciplex-thermally activated delayed fluorescence (exciplex-TADF) system is an excellent candidate for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) because of its more easily achieved small singlet-triplet energy splitting (ΔE_ST) and doping control. However, exciplex-TADF is still faced with the problems of low external quantum efficiency (η_ext) and unclear effect of structure modification in electron acceptors. Herein, we provide a steric hindrance increase strategy to obtain high-efficiency exciplex emissions. Through introducing a 9-phenylfluorene group into N-ethylcarbazole of the dicyano-substituted 9-phenylfluorene, an electron acceptor material with increased steric hindrance is obtained, which helps the exciplex harvest a larger driving force and higher emission efficiencies. Encouragingly, the obtained OLED displays a maximum η_ext of 25.8%, which is one of the best efficiency values among reported exciplex-OLEDs, simultaneously possessing excellent current efficiency of 83.6 cd A^-1 and power efficiency of 93.7 lm W^-1. It is expected that this work will offer a new avenue for designing electron acceptors for highly efficient exciplex emissions.

Excited-State Lifetime Modulation by Twisted and Tilted Molecular Design in Carbene-Metal-Amide Photoemitters

Carbene-metal-amides (CMAs) are an emerging class of photoemitters based on a linear donor-linker-acceptor arrangement. They exhibit high flexibility about the carbene-metal and metal-amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 10⁶ s^-1. Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin-orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device.

Advances in Blue Exciplex–Based Organic Light-Emitting Materials and Devices

Exciplexes possessing thermally activated delayed fluorescence (TADF) characteristics have received much attention in the fields of organic light-emitting materials and devices over the past decade. In general, an exciplex is a physical mixture between a donor (D) with hole transport properties and an acceptor (A) with electron transport characteristics, and the energy difference between the lowest excited singlet state and the lowest excited triplet state is usually fairly small in terms of the long-range charge-transfer process from D to A. In the processes of photoluminescence and electroluminescence, triplet excitons can be converted to singlet excitons through reverse intersystem crossing and then radiate photons to achieve TADF. As a consequence, triplet excitons can be effectively harvested, and the exciton utilization can be significantly enhanced. Up to now, a large number of exciplexes have been developed and applied to organic light-emitting devices. Notably most of them showed green or red emission, while blue exciplexes are relatively few owing to the spectrum characteristics of the large red-shift and broadened emission. In this study, the latest progress of blue exciplex–based organic light-emitting materials and devices is briefly reviewed, and future research is prospected.

Lower limits for non-radiative recombination loss in organic donor/acceptor complexes

Understanding the factors controlling radiative and non-radiative transition rates for charge transfer states in organic systems is important for applications ranging from organic photovoltaics (OPV) to lasers and LEDs. We explore the role of charge-transfer (CT) energetics, lifetimes, and photovoltaic properties in the limit of very slow non-radiative rates by using a model donor/acceptor system with photoluminescence dominated by thermally activated delayed fluorescence (TADF). This blend exhibits an extremely high photoluminescence quantum efficiency (PLQY = ∼22%) and comparatively long PL lifetime, while simultaneously yielding appreciable amounts of free charge generation (photocurrent external quantum efficiency EQE of 24%). In solar cells, this blend exhibits non-radiative voltage losses of only ∼0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, k_nr, is on the order of 10⁵ s^-1, approximately 4-5 orders of magnitude slower than typical OPV blends, thereby confirming that high radiative efficiency and low non-radiative voltage losses are achievable by reducing k_nr. Furthermore, despite the high radiative efficiency and already comparatively slow k_nr, we find that k_nr is nevertheless much faster than predicted by Marcus-Levich-Jortner two-state theory and we conclude that CT-local exciton (LE) hybridization is present. Our findings highlight that it is crucial to evaluate how radiative and non-radiative rates of the LE states individually influence the PLQY of charge-transfer states, rather than solely focusing on the PLQY of the LE. This conclusion will guide material selection in achieving low non-radiative voltage loss in organic solar cells and high luminescence efficiency in organic LEDs.

Theoretical studies on boron dimesityl-based thermally activated delayed fluorescence organic emitters: excited-state properties and mechanisms

At 300 K, S1 excitons could emit fluorescence or undergo ISC to T1, where rISC exceeds the phosphorescence emission enabling TADF.

Through-Space Charge-Transfer Emitters Developed by Fixing the Acceptor for High-Efficiency Thermally Activated Delayed Fluorescence

Through-space charge-transfer (TSCT) emitters have been extensively explored for thermally activated delayed fluorescence (TADF), but arranging various donors and acceptors into rigid cofacial conformations for various efficient TSCT TADF emitters has remained challenging. Here, we report a "fixing acceptor" design to reach various efficient TSCT TADF emitters. By chemically fixing the acceptor (benzophenone) with a rigid spiro-structure and cofacially aligning various donors with the fixed acceptor, a series of efficient TSCT TADF emitters have been developed. Single-crystal structures and theoretical calculations have verified closely packed cofacial donor/acceptor conformations and favorable TSCT in the emitters. In doped films, the emitters afford sky blue to yellow TADF emission, with high photoluminescence efficiencies up to 0.92 and reverse intersystem crossing rates up to 1.0 × 10⁶ s^-1. Organic light-emitting diodes using the emitters afford sky blue to yellow electroluminescence with high external quantum efficiencies up to 20.9%. The work opens a new avenue toward a wide variety of efficient TSCT TADF emitters.

Heptazine-Based π-Conjugated Materials for Light-Emitting

On the basis of planar and relatively rigid nitrogen-rich heterocyclic system of the heptazine core, heptazine-based π-conjugated materials have aroused widespread attention over the past decade by virtue of the fascinating electronic, optical, thermal, and mechanical properties in the fields of light-emitting, photocatalysis, sensors, environmental remediation, and so forth. However, there are still several obstacles to be solved before practical applications, such as low photoluminescence quantum efficiencies for light-emitting and weak visible absorption for photocatalysis. To further enhance various properties of heptazine-based π-conjugated materials, a series of strategies have been developed, including ingenious molecular design and modification, novel synthetic, and preparation methods. In this review, the significant progress of monomeric and polymeric heptazine-based π-conjugated materials and their applications typically in light-emitting are reviewed, which is beneficial for the acceleration of practical applications of heptazine-based materials and devices.

High-Throughput Virtual Screening of Host Materials and Rational Device Engineering for Highly Efficient Solution-Processed Organic Light-Emitting Diodes

The appropriate choice of host and electron-transporting material (ETM) plays a very crucial role in the generation and collection of radiative excitons in the desired recombination zone of organic light-emitting diodes (OLEDs). Due to the sustainable development of material organic chemistry, there is a big library of functional materials that leads to uncountable combinations of device structures, which might achieve a desirable high device performance. However, there is no appropriate methodology available for the fast virtual screening of organic materials and designing a suitable device structure. Here, we have used the electrical software package SETFOS 4.5 for high-throughput virtual screening of host materials and developed a highly efficient multistack OLED device structure. To further enhance the device performance, a co-host approach has been used, and the final device structure has also been optimized with two different ETMs. The best-optimized Ir(ppy)₃-based solution-processed green OLED device exhibited a maximum power efficiency (PE) of 83.20 lm/W and brightness of 61,362 cd/m² with a driving voltage of 2.1 V without using any light extraction outcoupling techniques, which is the best among the OLEDs in its own category. The developed device structure has also been utilized to fabricate highly efficient blue hazard-free low-color temperature OLEDs for a physiologically friendly light at night. The resultant 2083 K OLED device displayed a maximum PE of 51.4 lm/W and luminance of 44,548 cd/m² with a turn-on voltage of 2.1 V that is also 42 and 104 times safer in terms of retinal protection and ∼4 and ∼11 times safer in terms of melatonin generation when compared with those of a real candle and incandescent bulb, respectively. The observed excellent device performance may be attributed to the balanced charge carrier in the recombination zone, broader emissive layer due to a mixed-host system, less accumulation of charges at the injecting surfaces, well-aligned triplet energy and molecular orbital energy level of the host and guest, and high electron mobility and enhanced hole blocking ability of the employed ETM in the designed OLED device structure.

Long-Persistent Luminescence from an Exciplex-Based Organic Light-Emitting Diode

Organic long-persistent luminescent systems (OLPLs) exhibiting long-lasting emission after photoexcitation consist of organic electron donors and acceptors, that are widely used in organic light-emitting diodes (OLEDs). Although OLPLs and OLEDs include very similar excitonic processes, long-lasting emission has never been observed in OLEDs. This study confirms the presence of long-persistent luminescence (LPL) under electrical excitation.

Vibrational Damping Reveals Vibronic Coupling in Thermally Activated Delayed Fluorescence Materials

We investigate a series of D-A molecules consisting of spiro[acridan-9,9'-fluorene] as the donor and 2-phenylenepyrimidine as the acceptor. In two of the materials, a spiro center effectively electronically isolates the D unit from (consequently) optically innocent yet structurally influential adamantyl side groups. In a third material, adamantyl groups attached directly to the acceptor strongly influence the electronic properties. Steady-state and time-resolved photophysical studies in solution, Zeonex polymer matrix, and neat films reveal that the substituents impact the efficiency of vibronic coupling between singlet and triplet states relevant to reverse intersystem crossing (rISC) and thermally activated delayed fluorescence (TADF), without significantly changing the singlet-triplet gap in the materials. The adamantyl groups serve to raise the segmental mass and inertia, thereby damping intramolecular motions (both vibrational and rotational). This substitution pattern reveals the role of large-amplitude (primarily D-A dihedral angle rocking) motions on reverse intersystem crossing (rISC), as well as smaller contributions from low-amplitude or dampened vibrations in solid state. We demonstrate that rISC still occurs when the high-amplitude motions are suppressed in Zeonex and discuss various vibronic coupling scenarios that point to an underappreciated role of intersegmental motions that persist in rigid solids. Our results underline the complexity of vibronic couplings in the mediation of rISC and provide a synthetic tool to enable future investigations of vibronic coupling through selective mechanical dampening with no impact on electronic systems.

Exceptionally fast radiative decay of a dinuclear platinum complex through thermally activated delayed fluorescence

A novel dinuclear platinum(ii) complex featuring a ditopic, bis-tetradentate ligand has been prepared. The ligand offers each metal ion a planar O^N^C^N coordination environment, with the two metal ions bound to the nitrogen atoms of a bridging pyrimidine unit. The complex is brightly luminescent in the red region of the spectrum with a photoluminescence quantum yield of 83% in deoxygenated methylcyclohexane solution at ambient temperature, and shows a remarkably short excited state lifetime of 2.1 μs. These properties are the result of an unusually high radiative rate constant of around 4 × 105 s-1, a value which is comparable to that of the very best performing Ir(iii) complexes. This unusual behaviour is the result of efficient thermally activated reverse intersystem crossing, promoted by a small singlet-triplet energy difference of only 69 ± 3 meV. The complex was incorporated into solution-processed OLEDs achieving EQEmax = 7.4%. We believe this to be the first fully evidenced report of a Pt(ii) complex showing thermally activated delayed fluorescence (TADF) at room temperature, and indeed of a Pt(ii)-based delayed fluorescence emitter to be incorporated into an OLED.

Thermally activated delayed fluorescence exciplex emitters for high-performance organic light-emitting diodes

Owing to their natural thermally activated delayed fluorescence (TADF) characteristics, the development of exciplex emitters for organic light-emitting diodes (OLEDs) has witnessed booming progress in recent years. Formed between electron-donating and electron-accepting molecules, exciplexes with intermolecular charge transfer processes have unique advantages compared with unimolecular TADF materials, offering a new way to develop high-performance TADF emitters. In this review, a comprehensive overview of TADF exciplex emitters is presented with a focus on the relationship between the constituents of exciplexes and their electroluminescence performance. We summarize and discuss the latest and most significant developments of TADF exciplex emitters. Notably, the design principles of efficient TADF exciplex emitters are systematically categorized into three systems within this review. These progressive achievements of TADF exciplex emitters point out future challenges to trigger more research endeavors in this growing field.

Highly Efficient Thermally Activated Delayed Fluorescence via an Unconjugated Donor-Acceptor System Realizing EQE of Over 30

In this work, two novel thermally activated delayed fluorescence (TADF) emitters, 2tDMG and 3tDMG, are synthesized for high-efficiency organic light-emitting diodes (OLEDs), The two emitters have a tilted face-to-face alignment of donor (D)/acceptor (A) units presenting intramolecular noncovalent interactions. The two TADF materials are deposited either by an evaporation-process or by a solution-process, both of them leading to high OLED performance. 2tDMG used as the emitter in evaporation-processed OLEDs achieves a high external quantum efficiency (EQE) of 30.8% with a very flat efficiency roll-off of 7% at 1000 cd m^-2 . The solution-processed OLEDs also display an interesting EQE of 16.2%. 3tDMG shows improved solubility and solution processability as compared to 2tDMG, and thus a high EQE of 20.2% in solution-processed OLEDs is recorded. The corresponding evaporation-processed OLEDs also reach a reasonably high EQE of 26.3%. Encouragingly, this work provides a novel strategy to address the imperious demands for OLEDs with high EQE and low roll-off.

Modulation for efficiency and spectra of non-doped white organic light emitting diodes by combining an exciplex with an ultrathin phosphorescent emitter

Herein, structured non-doped white organic light-emitting diodes (WOLEDs) were designed by combining the emission of a blue exciplex and orange-red phosphorescent ultrathin layer. The device efficiency and spectra were modulated successfully by adjusting the thickness of the exciplex layer and ultrathin layer, respectively. Meanwhile, high efficiency with external quantum efficiency (EQE) ranging from 15% to 22%, power efficiency from 33 lm W⁻¹ to 47 lm W⁻¹ and warm white emission with correlated color temperature (CCT) from 1600 K to 2600 K were realized. The energy transfer process and emission mechanism is also discussed, and the results reveal that the efficient charge trapping and recombination contribute to the improvement of device efficiency and reduce the roll-off efficiency.

Non-doped WOLED with modulated efficiency and spectra were designed by combining blue exciplex with orange-red phosphorescent ultrathin layer.

Exciplex energy transfer through spacer: White electroluminescence with enhanced stability based on cyan intermolecular and orange intramolecular thermally activated delayed fluorescence

Capability of exciplex energy transfer through a spacer was investigated using three exciplex-forming solid mixtures which contained the well-known electron accepting 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and appropriately designed bipolar cyanocarbazolyl-based derivatives functionalized by attachment of carbazolyl, acridanyl or phenyl units. These novel cyanocarbazolyl-based derivatives were used as both the spacer and exciplex-forming donor. Efficient organic light-emitting diodes with electroluminescence in cyan-yellow region and maximum external quantum efficiency of up to 7.7% were fabricated owing to efficient thermally activated fluorescence (TADF) of the newly discovered exciplexes. An approach of exciton separation by the spacer between the studied exciplexes and selected orange TADF emitter was proposed for the fabrication of white electroluminescent devices with prolonged lifetime comparing to that of single-color exciplex-based devices. Exciplex-forming systems were tested for exciton separation between inter- and intramolecular TADF. Exciplex energy transfer through a spacer was observed on relatively long distance for one system due to the energy resonance between triplet levels of the exciplex and spacer. First time observed here exciplex energy transfer through a spacer can be useful for both improvement of device stability and obtaining of white electroluminescence.

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High-Efficiency Exciplex Light-Emitting Diodes

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light-emitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D^+• →A^-• ), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (ΔE_ST ) in 9,9'-diphenyl-9H,9'H-3,3'-bicarbazole (BCzPh):3',3'″,3'″″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1'-biphenyl]-3-carbonitrile)) (CN-T2T) (ΔE_ST = 30 meV) and BCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (ΔE_ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE_max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE_ST , and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D^+• →A^-• ) size, enhanced SOC, increased orbital polarization, and decreased ΔE_ST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.

Organic Emitters with a Rigid 9-Phenyl-9-phosphafluorene Oxide Moiety as the Acceptor and Their Thermally Activated Delayed Fluorescence Behavior

With the 9-phenyl-9-phosphafluorene oxide (PhFlOP) moiety as the acceptor (A) and various donors (D), a series of new organic emitters have been synthesized with a D-A-D configuration. Their photophysical and electrochemical behaviors and electroluminescent (EL) performances have been characterized in detail. The photophysical results have indicated that the PhFlOP-based emitters with acridyl, phenoxazyl, and phenothiazyl as donors show efficient, thermally activated delayed fluorescence (TADF) behavior, especially for the TADF emitter with the phenoxazyl donor possessing an exceptionally high rate constant of reverse intersystem crossing (k_RISC) of 6.2 × 10⁵ s^-1. It has also been found that their TADF behavior can be greatly affected by the substitution position of the donors. Different from the reported aryl phosphine oxide (APO) acceptors in TADF emitters, the PhFlOP moiety adopts a highly rigid configuration to guarantee a photoluminescent quantum yield as high as 0.80 in the 4,4'-N,N'-dicarbazolebiphenyl film, representing the top-ranking emission ability for the TADF emitters with APO-type acceptors. Benefitting from their advanced TADF performances, the doped organic light-emitting diodes/devices based on these PhFlOP-based TADF emitters can achieve exceptional EL performances with the maximum external quantum efficiency (η_ext) of 23.3%, current efficiency (η_L) of 83.7 cd A^-1, and power efficiency (η_P) of 59.1 lm W^-1. These encouraging EL data show the great potential of the PhFlOP moiety in developing highly efficient TADF emitters.

Tricomponent Exciplex Emitter Realizing over 20% External Quantum Efficiency in Organic Light-Emitting Diode with Multiple Reverse Intersystem Crossing Channels

With the naturally separated frontier molecular orbitals, exciplexes are capable of thermally activated delayed fluorescence emitters for organic light-emitting diodes (OLEDs). And, the current key issue for exciplex emitters is improving their exciton utilization. In this work, a strategy of building exciplex emitters with three components is proposed to realize multiple reverse intersystem crossing (RISC) channels, improving their exciton utilization by enhancing upconversion of nonradiative triplet excitons. Accordingly, a tricomponent exciplex DBT-SADF:PO-T2T:CDBP is constructed with three RISC channels respectively on DBT-SADF, DBT-SADF:PO-T2T, and CDBP:PO-T2T. Furthermore, its photoluminescence quantum yield and rate constant of the RISC process are successfully improved. In the OLED, DBT-SADF:PO-T2T:CDBP exhibits a remarkably high maximum external quantum efficiency (EQE) of 20.5%, which is the first report with an EQE over 20% for the OLEDs based on exciplex emitters to the best of our knowledge. This work not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of the tricomponent exciplex strategy for further development of exciplex emitters.

Exciplex-Based Electroluminescence: Over 21% External Quantum Efficiency and Approaching 100 lm/W Power Efficiency

Benzimidazole-triazine-based electron acceptor PIM-TRZ with high triplet exited-state energy and strong electron-transport ability was newly developed. A series of highly efficient exciplex emitters have been fabricated. The TAPC:PIM-TRZ (TAPC: di-[4-( N, N-ditoly amino)-phenyl]cyclohexane) film shows a high photoluminescence (PL) quantum yields (PLQY, Φ_f) of 93.4%, and the device based on TAPC:PIM-TRZ exhibits a low turn-on voltage of 2.3 V, high maximum efficiency of 71.2 cd A^-1 (current efficiency, CE), 97.3 lm W^-1 (power efficiency, PE), and 21.7% (external quantum efficiency, EQE), as well as a high EQE of 16.2% at a luminance of 5000 cd m^-2. The device displays the highest efficiency among reported organic light-emitting devices with an exciplex film as the emitting layer. Furthermore, a green device is also fabricated with a TAPC:PIM-TRZ cohost using C545T (C545T: (10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1 H,5 H,11 H-benzopyrano[6,7-8- I, j]quinolizin-11-one)) as the dopant, and the highest CE, PE, and EQE are 68.3 cd A^-1, 86.6 lm W^-1, and 20.2%, respectively.