High Maximum External Quantum Efficiency Research Articles

Deepening Insights into Aggregation Effect of Intermolecular Charge‐Transfer Aggregates for Highly Efficient Near‐Infrared Non‐Doped Organic Light‐Emitting Diodes over 780 nm

AbstractThough urgently needed, high‐efficiency near‐infrared (NIR) organic light‐emitting diode (OLED) is still rare due to the energy‐gap law. Formation of intermolecular charge‐transfer aggregates (CTA) with nonadiabatic coupling suppression can decelerate non‐radiative decay rates for high‐efficiency NIR‐OLEDs. However, the aggregation effect of CTA is still not fully understood, which limits the rational design of CTA. Herein, two CTA molecules with a same π‐framework but different terminal substituents are developed to unveil the aggregation effect. In highly ordered crystalline states, the terminal substituents substantially affect the molecular packing motifs and intermolecular charge‐transfer states, thus leading to distinct photophysical properties. In comparison, in amorphous states, these two CTA demonstrate similar photophysical behaviors and properties due to their similar molecular packing and intermolecular interactions as evidenced by molecular dynamics simulations. Importantly, the formations of amorphous CTA trigger multifunction improvements such as aggregation‐induced NIR emission, aggregation‐induced thermally activated delayed fluorescence, self‐doping and self‐host features. The non‐doped OLEDs demonstrate NIR emissions centered at 788 and 803 nm, and high maximum external quantum efficiencies of 2.6% and 1.5% with small efficiency roll‐off, respectively. This study provides deeper insight into the aggregation effect of CTA and lays a foundation for the development of high‐efficiency NIR non‐doped OLEDs.

Tetradentate C∧C∧N∧N Ligand-Containing Gold(III) Complexes with Orange to Deep-Red Thermally Activated Delayed Fluorescence (TADF) and Their Application in Organic Light-Emitting Devices

A new class of thermally activated delayed fluorescence (TADF) tetradentate C∧C∧N∧N ligand-containing gold(III) complexes containing acridinyl moieties has been designed and synthesized. These complexes exhibit orange-red to deep-red emission with photoluminescence quantum yields (PLQYs) of up to 0.76 in solid-state thin films. Short excited-state lifetimes of ≤2.0 μs and large radiative decay rate constants (kr) in the order of 105 s-1 have also been found in the complexes. High-performance solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) based on these complexes have been fabricated, demonstrating high maximum external quantum efficiencies (EQEs) of 12.2 and 12.7%, respectively, which are among the best values ever reported for red-emitting gold(III)-based OLEDs. In addition, satisfactory operational half-lifetime (LT50) values of up to 34,058 h have been attained in these red-emitting devices. It is found that the operational stability is strongly dependent on the choice of functional groups on the acridinyl moieties, of which the incorporation of -O- and -S- linkers can effectively prolong the LT50 value by an order of magnitude. The TADF properties of the complexes are substantiated by the hypsochromic shift in emission energies and the remarkable enhancement in the emission intensity upon increasing temperature. The TADF properties have also been supported by temperature-dependent ultrafast transient absorption studies, with the direct observation of reverse intersystem crossing (RISC) and the determination of the activation parameters for the very first time, together with their excited-state dynamics.

Orbital Symmetry Engineering in Fused Polycyclic Heteroaromatics toward Extremely Narrowband Green Emissions with an FWHM of 13nm.

Multiresonance (MR) molecules generally face spectral broadening issues with redshifted emissions. Thus, green emitters with full widths at half maximum (FWHMs) of <20nm are rarely reported, despite being highly desired. Herein, by properly fusing indolo(3,2,1-jk)carbazole (ICZ) and naphthalene moieties, green MR emitters are reported, which have FWHMs of merely 13nm (0.064eV) and 14nm (0.069eV) in dichloromethane, accompanied by high photoluminescence quantum yields of >95%, which represent not only the smallest FWHMs among all green MR emitters but also the first green emitters based on ICZ MR derivatives. Theoretical studies reveal that the orbital interactions between the antisymmetric sites of the segments play an important role in extending the conjugation length in the fusion architectures while simultaneously maintaining a small FWHM. The corresponding organic light-emitting diodes exhibit green emission peaks at 508-509nm and the first green electroluminescence FWHM of <20nm ever reported. Benefiting from the preferential horizontal dipole orientation, a high maximum external quantum efficiency of up to 30.9% is obtained, which remains at 28.9% and 23.2% under luminances of 1000 and 10000 cdm-2 , respectively, outperforming most reported green devices based on narrowband emitters.

Triazolotriazine-based mixed host for pure-red phosphorescent organic light-emitting diodes exhibiting ultra-low efficiency roll-off

The development of high-performance pure-red phosphorescent organic light-emitting diodes (Ph-OLEDs) especially with low efficiency roll-off at high brightness is still an appealing yet challenging task. Despite success of well-documented 1,3,5-triazine (TRZ)-based thermally activated delayed fluorescence (TADF) hosts in green Ph-OLEDs, the limited electron-withdrawing ability of the TRZ acceptor makes the TADF hosts unsuitable enough for usage in the assembly of high-performance red Ph-OLEDs. Herein, pure-red Ph-OLEDs with ultra-low efficiency roll-off at luminance above 10000 cd m−2 are achieved by developing two triazolotriazine-based TADF hosts (23aIC-TAZTRZ and 32aIC-TAZTRZ) with balanced bipolar transporting property, good morphological stability and high photoluminescence quantum yield in neat film. Particularly, the phosphor-doped 23aIC-TAZTRZ and 32aIC-TAZTRZ emitting layers display high horizontal orientation factors (Θ//) of 84% and 85%, respectively. Ph-OLEDs utilizing 50 wt% 23aIC-TAZTRZ and 50 wt% 32aIC-TAZTRZ as the mixed host exhibit pure-red emission with Commission Internationale de l’Eclairage (CIE) coordinate of [0.66, 0.34], high maximum external quantum efficiency (EQEmax) of 27.4% and ultra-low efficiency roll-off at 10000 cd m−2 (EQE10000 = 23.9%). This work not only discloses the potential of triazolotriazine acceptor for the construction of efficient TADF hosts, but also provides alternative candidates to TRZ-based hosts for the assembly of high-performance red Ph-OLEDs.

Efficient pure blue hyperfluorescence devices utilizing quadrupolar donor-acceptor-donor type of thermally activated delayed fluorescence sensitizers

The hyperfluorescence (HF) system has drawn great attention in display technology. However, the energy loss mechanism by low reverse intersystem crossing rate (kRISC) and the Dexter energy transfer (DET) channel is still challenging. Here, we demonstrate that this can be mitigated by the quadrupolar donor-acceptor-donor (D-A-D) type of thermally activated delayed fluorescence (TADF) sensitizer materials, DBA-DmICz and DBA-DTMCz. Further, the HF device with DBA-DTMCz and ν-DABNA exhibited 43.9% of high maximum external quantum efficiency (EQEmax) with the Commission Internationale de l'Éclairage coordinates of (0.12, 0.16). The efficiency values recorded for the device are among the highest reported for HF devices. Such high efficiency is assisted by hindered DET process through i) high kRISC, and ii) shielded lowest unoccupied molecular orbital with the presence of two donors in D-A-D type of skeleton. Our current study provides an effective way of designing TADF sensitizer for future HF technology.

Intramolecular Cyclization: A Convenient Strategy to Realize Efficient BT.2020 Blue Multi-Resonance Emitter for Organic Light-Emitting Diodes.

Rationally tuning the emission position and narrowing the full width at half-maximum (FWHM) of an emitter is of great importance for many applications. By synergistically improving rigidity, strengthening the resonant strength, inhibiting molecular bending and rocking, and destabilizing the HOMO energy level, a deep-blue emitter (CZ2CO) with a peak wavelength of 440 nm and an ultranarrow spectral FWHM of 16 nm (0.10 eV) was developed via intramolecular cyclization in a carbonyl/N resonant core (QAO). The dominant υ0-0 transition character of CZ2CO gives a Commission Internationale de I'Éclairage coordinates (CIE) of (0.144, 0.042), nicely complying with the BT.2020 standard. Moreover, a hyper-fluorescent device based on CZ2CO shows a high maximum external quantum efficiency (EQEmax ) of 25.6 % and maintains an EQE of 22.4 % at a practical brightness of 1000 cd m-2 .

Intramolecular Cyclization: A Convenient Strategy to Realize Efficient BT.2020 Blue Multi‐Resonance Emitter for Organic Light‐Emitting Diodes

AbstractRationally tuning the emission position and narrowing the full width at half‐maximum (FWHM) of an emitter is of great importance for many applications. By synergistically improving rigidity, strengthening the resonant strength, inhibiting molecular bending and rocking, and destabilizing the HOMO energy level, a deep‐blue emitter (CZ2CO) with a peak wavelength of 440 nm and an ultranarrow spectral FWHM of 16 nm (0.10 eV) was developed via intramolecular cyclization in a carbonyl/N resonant core (QAO). The dominant υ0‐0transition character of CZ2CO gives a Commission Internationale de I’Éclairage coordinates (CIE) of (0.144, 0.042), nicely complying with the BT.2020 standard. Moreover, a hyper‐fluorescent device based on CZ2CO shows a high maximum external quantum efficiency (EQEmax) of 25.6 % and maintains an EQE of 22.4 % at a practical brightness of 1000 cd m−2.

Dihydrophenazine-derived thermally activated delayed fluorescence emitters for highly efficient orange and red organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) emitters in the long wavelength region usually suffer from severe nonradiative decay according to the energy-gap law, and show relatively large energy split (ΔEST) between the lowest singlet and triplet excited states which compromise efficient up-conversion. Here, we report three orange and red TADF emitters comprising 5, 10-disubstituted phenazine electron-donor and quinoxaline-derived electron-acceptors, which show excellent TADF properties and yield efficient organic light-emitting diodes (OLEDs). These symmetrical acceptor-donor-acceptor (A-D-A) type and rodlike molecules concurrently exhibit high photoluminescence quantum yields up to 85% and fast reverse intersystem crossing. Experimental and theoretical studies reveal that these excellent photophysical properties are attributed the synergistic effect of structural rigidity, steric hindrance and strength of the donor and acceptor units. Doped OLEDs based on these emitters achieved high maximum external electroluminescence quantum efficiencies up to 19.9%, and maximum luminance surpassing 31240 cd/m2.

Organic Light‐Emitting Diodes Based on Silandiol‐Bay‐Bridged Perylene Bisimides

AbstractPerylene bisimides (PBIs) are among the best fluorophores but have to be enwrapped for optoelectronic applications by large and heavy substituents to prevent their ππ‐stacking, which is known to accelerate non‐radiative decay processes in the solid state. Here, light‐weight di‐tert‐butylsilyl groups are introduced to bridge 1,12‐dihydroxy and 1,6,7,12‐tetrahydroxy PBIs to afford sublimable dyes for vacuum‐processed optoelectronic devices. For both new compounds, this substitution provides a twisted and shielded perylene π‐core whose, via OSiObridges, rigid structure affords well‐resolved absorption and emission spectra with strong fluorescence in solution, as well as in the solid state. The usefulness of these dyes for vacuum‐processed optoelectronic devices is demonstrated in organic light‐emitting diodes (OLEDs) that show monomer‐like emission spectra and high maximum external quantum efficiency (EQEmax) values of up to 3.1% for the doubly silicon‐bridged PBI.

Pyrene‐Based Emitters with Ultrafast Upper‐Level Triplet–Singlet Intersystem Crossing for High‐Efficiency, Low Roll‐Off Blue Organic Light‐Emitting Diode

AbstractThe reverse intersystem crossing (RISC) process from triplet to singlet manifold can efficiently improve the efficiencies of organic light‐emitting diodes (OLEDs). The relevant RISC rate (kRISC) is one of the key factors that affect the efficiency roll‐off and stability of devices. Here, a new blue “hot exciton” material, CPPCN, with pyrene as the core, is designed and synthesized. Photophysical studies indicate there are multiple high‐lying triplet states near the lowest singlet excited state (S1) of CPPCN, which facilitate the RISC from the high‐lying triplet state (hRISC) to the S1 state. The corresponding hRISC rate constant is determined to be as high as 2.86 × 108 s−1, a value some two to three orders of magnitude higher than typical thermally activated delayed fluorescence materials emitters. Benefiting from this ultrafast hRISC process, non‐doped blue OLED based on CPPCN reaches a high maximum external quantum efficiency (EQE) of 11.64% with suppressed efficiency roll‐off (begin roll‐off at a high brightness of 3732 cd m−2 and retains EQE &gt;8.5% at 10 000 cd m−2). The T95 lifetime of the CPPCN‐based device with a structure of commercial applications is evaluated to be ≈368 h at 1000 cd m−2.

Correlating Exciton Dynamics of Thermally Activated Delayed-Fluorescence Emitters to Efficiency Roll-Off in OLEDs

Physical parameters, such as decay rates and photoluminescence (PL) quantum efficiencies, associated with thermally activated delayed fluorescence (TADF) emitters play a critical role in determining the performance of organic light-emitting diodes (OLEDs). Herein, we investigate the impact of TADF decay rates and PL quantum efficiencies on external quantum efficiency (EQE) roll-off. Our analysis reveals that a high EQE with suppressed efficiency roll-off in TADF OLEDs can be achieved by shortening the delayed lifetime with an increasing delayed contribution or by shortening the prompt lifetime with increasing prompt contribution simultaneously. We further show that TADF compounds with long delayed and/or prompt lifetimes can give rise to multiple spin cycling and subsequently result in significant exciton quenching and efficiency roll-off in OLEDs. These results provide universal selection criteria for TADF emitters in OLEDs to concurrently achieve a high maximum EQE and low-efficiency roll-off.

One-Shot Synthesis of B/N-Doped Calix[4]arene Exhibiting Narrowband Multiple Resonance Fluorescence.

A novel macrocycle of B/N-doped calix[4]arene (C-BN) was synthesized by a one-shot double boronation. Owing to the structural tension and electron-donating properties of the nitrogen atoms in the macrocycle, reaction selectively proceeds between the adjacent benzene rings outside the macrocycle. C-BN shows a highly centrosymmetric structure with two multiple resonance (MR) fragments bridged by tertiary amine groups at the 1,3 positions of the benzene ring. Benefiting from the large intermolecular distance (>4.6 Å) between adjacent MR-emitting cores, C-BN also exhibits excellent narrowband emitting features against aggregation-induced quenching and spectrum broadening. Optimized organic light-emitting diode devices based on C-BN exhibit high maximum external quantum efficiencies of 24.7-26.6 % and small full width at half maximums of 25-28 nm over a wide doping range of 1-12 wt %.

One‐Shot Synthesis of B/N‐Doped Calix[4]arene Exhibiting Narrowband Multiple Resonance Fluorescence

AbstractA novel macrocycle of B/N‐doped calix[4]arene (C‐BN) was synthesized by a one‐shot double boronation. Owing to the structural tension and electron‐donating properties of the nitrogen atoms in the macrocycle, reaction selectively proceeds between the adjacent benzene rings outside the macrocycle. C‐BN shows a highly centrosymmetric structure with two multiple resonance (MR) fragments bridged by tertiary amine groups at the 1,3 positions of the benzene ring. Benefiting from the large intermolecular distance (&gt;4.6 Å) between adjacent MR‐emitting cores, C‐BN also exhibits excellent narrowband emitting features against aggregation‐induced quenching and spectrum broadening. Optimized organic light‐emitting diode devices based on C‐BN exhibit high maximum external quantum efficiencies of 24.7–26.6 % and small full width at half maximums of 25–28 nm over a wide doping range of 1–12 wt %.

The Role of Balancing Carrier Transport in Realizing an Efficient Orange-Red Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diode.

Simultaneously realizing improved carrier mobility and good photoluminescence (PL) efficiency in red thermally activated delayed-fluorescence (TADF) emitters remains challenging but important. Herein, two isomeric orange-red TADF emitters, oPDM and pPDM, with the same basic donor-acceptor backbone but a pyrimidine (Pm) attachment at different positions are designed and synthesized. The two emitters show similarly good PL properties, including narrow singlet-triplet energy offsets (0.11 and 0.15 eV) and high photoluminescence quantum yields (ca. 100 and 88%) in doped films. An orange-red organic light-emitting diode (OLED) employing oPDM as an emitter achieves an almost twice as high maximum external quantum efficiency (28.2%) compared with that of a pPDM-based OLED. More balanced carrier-transporting properties are responsible for their contrasting device performances, and the position effect of the Pm substituent leads to significantly distinct molecular packing behaviors in the aggregate states and different carrier mobilities.

Stable Tetradentate Gold(III)-TADF Emitters with Close to Unity Quantum Yield and Radiative Decay Rate Constant of up to 2× 106 s-1 : High-Efficiency Green OLEDs with Operational Lifetime (LT90 ) Longer than 1800 h at 1000cd m-2.

High maximum external quantum efficiency (EQEmax ), small efficiency roll-offs, and long operational lifetime at practical luminances are three crucial parameters for commercialization of organic light-emitting diodes (OLEDs). To simultaneously achieve these goals, it is desirable to have the radiative decay rate constant (kr ) as large as possible, which, for a thermally activated delayed fluorescent (TADF) emitter, requires both a large S1 →S0 radiative decay rate constant (kr S ) and a small singlet-triplet energy gap (ΔEST ). Here, the design of a class of tetradentate gold(III) TADF complexes for narrowing the ΔEST while keeping the kr S large is reported. The as-synthesized complexes display green emission with close to unity emission quantum yields, and kr approaching 2× 106 s-1 in thin films. The vacuum-deposited green OLEDs based on 1 and 4 demonstrate maximum EQEs of up to 24 and 27% with efficiency roll-offs of 5.5 and 2.2% at 1000cd m-2 , respectively; the EQEs maintain high at 10000cd m-2 (19% (1) and 24% (4)). A long LT90 device lifetime of 1820h at 1000cd m-2 for complex 1 is achieved, which is one of the longest device lifetimes of TADF-OLEDs reported in the literature.

A new amine‒carbonyl fused emitter with hybridized excited state for green OLED with high luminance and low efficiency roll-off

In this work, a novel green emitter, namely 16,16-dimethylbenzo[a]quinolino[3,2,1-de]acridin-10(16H)-one (GACQ), with amine‒carbonyl fused structure has been designed and synthesized. The single-crystal structure analysis confirmed that the fused structure shows non-planar conformation with dihedral angle of 48.97° from the front-view direction. The photophysical and theoretical results confirmed that GACQ exhibited hybridized local and charge-transfer (HLCT) feature with strong green emissions of 513 and 535 nm in solution and film, respectively. The non-doped device based on GACQ achieved high luminance (15810 cd m−2) and good efficiency stability with negligible external quantum efficiency (EQE) roll-off. Furthermore, the doped device exhibited high maximum EQE of 4.8% with CIE coordinate of (0.38, 0.58).

Novel Polycyclic Fused Amide Derivatives: Properties and Applications for Sky-blue Electroluminescent Devices.

Aromatic imide derivatives play a critical role in boosting the electroluminescent (EL) performance of organic light-emitting diodes (OLEDs). However, the majority of aromatic imide-based materials are limited to long wavelength emission OLEDs rather than blue emissions due to their strong electron-withdrawing characteristics. Herein, two novel polycyclic fused amide units were reported as electron acceptor to be combined with either a tetramethylcarbazole or acridine donor via a phenyl linker to generate four conventional fluorescence blue emitters of BBI-4MeCz, BBI-DMAC, BSQ-4MeCz and BSQ-DMAC for the first time. BSQ-4MeCz and BSQ-DMAC based on a BSQ unit exhibited higher thermal stability and photoluminescence quantum yields than BBI-4MeCz and BBI-DMAC based on a BBI unit due to their more planar acceptor structure. The intermolecular interactions that exist in the BSQ series materials effectively inhibit the molecular rotation and configuration relaxation, and thus allow for blue-shifted emissions. Blue OLED devices were constructed with the developed materials as emitters, and the effects of both the structure of the polycyclic fused amide acceptor and the electron donor on the EL performance were clarified. Consequently, a sky-blue OLED device based on BSQ-DMAC was created, with a high maximum external quantum efficiency of 4.94% and a maximum luminance of 7761 cd m−2.

Efficient deep red and NIR OLEDs based on Ir(III) complexes fabricated by evaporation and solution processing

Solution processing of organic light-emitting devices (OLEDs) becomes an attractive approach due to its low fabrication cost and accessibility to a broad range of materials. Near-infrared (NIR) OLEDs emerge as a practicable technological medium that can be widely used in phototherapy, optical imaging, remote communication, secured display and so on. We report here a couple of high-performance deep red OLEDs fabricated by evaporation processing and a series of NIR OLEDs fabricated by solution processing, in which the devices attain high maximum external quantum efficiencies (EQEs) of 12.68 %, 12.49 % and 3.80 % with the electroluminescence peaks at 673, 692 and 706 nm, respectively. The effects of functional substituents and p–π conjugation on the optoelectronic properties of these new Ir(III) phosphors have also been investigated. It can provide a new insight into developing highly efficient evaporation-processed and solution-processed OLEDs based on Ir(III) complexes with deep red and NIR emissions.

A Chiral Dual-Core Organoboron Structure Realizes Dual-Channel Enhanced Ultrapure Blue Emission and Highly Efficient Circularly Polarized Electroluminescence.

The realization of luminescent materials with narrowband and circularly polarized luminescence (CPL) is of great significance for the development of future optical and photonic devices. Herein, through a steric-hindrance-assisted dual-core strategy, two pairs of chiral dual-core multiple resonance thermally activated delayed fluorescence (MR-TADF) materials (R/S-DOBN and R/S-DOBNT) are directly constructed by the bonding of two organoboron MR-TADF monocores (SOBN and SOBNT) with carbazole/3,6-di-tert-butyl-9H-carbazole and phenol derivative as donors, realizing obvious CPL and narrowband emissions. Furthermore, the dual-core effect in the prepared R/S-DOBN and R/S-DOBNT increases the transition oscillator strength two times more than that of a monocore structure, while maintaining the ultrapure blue emissions peaking at 453 and 459nm with a narrower full-width at half-maximum of 21nm through reorganization energy reduction. The circularly polarized organic light-emitting diodes based on the enantiomers exhibit ultrapure blue emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.14, 0.10) and (0.13, 0.12), high maximum external quantum efficiencies of 23.9% and 25.6%, and obvious circularly polarized electroluminescence with dissymmetry factors (|gEL |) ≈ 10-3 .

Development of Pure Green Thermally Activated Delayed Fluorescence Material by Cyano Substitution.

Multiple resonance (MR)-effect-induced thermally activated delayed fluorescence (TADF) materials have garnered significant attention because they can achieve both high color purity and high external quantum efficiency (EQE). However, the reported green-emitting MR-TADF materials exhibit broader emission compared to those of blue-emitting ones and suffer from severe efficiency roll-off due to insufficient rate constants of reverse intersystem crossing process (kRISC ). Herein, a pure green MR-TADF material (ν-DABNA-CN-Me) with high kRISC of 105 s-1 is reported. The key to success is introduction of cyano groups into a blue-emitting MR-TADF material (ν-DABNA), which causes remarkable bathochromic shift without a loss of color purity. The organic light-emitting diode employing it as an emitter exhibits green emission at 504nm with a small full-width at half-maximum of 23nm, corresponding to Commission Internationale d'Éclairage coordinates of (0.13, 0.65). The device achieves a high maximum EQE of 31.9% and successfully suppresses the efficiency roll-off at a high luminance.