High-efficiency and low efficiency roll-off near-infrared fluorescent OLEDs through triplet fusion

Organosulfur and Organoselenium Functionalized Benzimidazo[1,2-a]quinolines: From Experimental and Theoretical Photophysics to All-Solution-Processed OLEDs

In this study, we present the synthesis of benzimidazo[1,2-a] quinoline-based heterocycles bearing organosulfur and organoselenium moieties through transition-metal-free cascade reactions involving a sequential intermolecular aromatic nucleophilic substitution (SNAr). Both sulfur and selenium derivatives presented absorption maxima located around 355 nm related to spin and symmetry allowing electronic 1π-π* transitions, and fluorescence emission at the violet-blue region (~440 nm) with relatively large Stokes shift. The fluorescence quantum yields were slightly influenced by the chalcogen, with the sulfur derivatives presenting higher values than the selenium analogs. In this sense, the quantum yields for selenium derivatives can probably be affected by the intersystem crossing or even the photoinduced electron transfer process (PET). The compounds were successfully applied in all-solution-processed organic light-emitting diodes (OLEDs), where poly(9-vinylcarbazole) was employed as a dispersive matrix generating single-layer device cells. The obtained electroluminescence spectra are a sum of benzimidazo[1,2-a]quinolines and PVK singlet and/or triplet emissive states, according to their respective energy band gaps. The best diode rendered a luminance of 25.4 cd⋅m-2 with CIE (0.17, 0.14) and current efficiency of 20.2 mcd⋅A-1, a fivefold improvement in comparison to the PVK device that was explained by a 50-fold increase of charge-carriers electrical mobility.

Molecular Engineering of Stabilized Silicon-Rosindolizine Shortwave Infrared Fluorophores

Fluorescence-based biological imaging in the shortwave infrared (SWIR, 1000-1700 nm) is an attractive replacement for modern in vivo imaging techniques currently employed in both medical and research settings. Xanthene-based fluorophores containing heterocycle donors have recently emerged as a way to access deep SWIR emitting fluorophores. A concern for xanthene-based SWIR fluorophores though is chemical stability toward ambient nucleophiles due to the high electrophilicity of the cationic fluorophore core. Herein, a series of SWIR emitting silicon-rosindolizine (SiRos) fluorophores with emission maxima >1300 nm (up to 1550 nm) are synthesized. The SiRos fluorophore photophysical properties and chemical stability toward nucleophiles are examined through systematic derivatization of the silicon-core alkyl groups, indolizine donor substitution, and the use of o-tolyl or o-xylyl groups appended to the fluorophore core. The dyes are studied via absorption spectroscopy, steady-state emission spectroscopy, solution-based cyclic voltammetry, time-dependent density functional theory (TD-DFT) computational analysis, X-ray diffraction crystallography, and relative chemical stability over time. Optimal chemical stability is observed via the incorporation of the 2-ethylhexyl silicon substituent and the o-xylyl group to protect the core of the fluorophore.

Azaacene containing iridium(III) phosphors: elaboration of the π-conjugation effect and application in highly efficient solution-processed near-infrared OLEDs

Azaacenes have attracted wide research interest due to their tremendous potential in organic electronics. However, near-infrared (NIR) light-emitting iridium(III) phosphors bearing azaacene derivatives are rarely investigated. In this contribution, two solution-processable heteroleptic iridium(III) complexes, namely DBPzIr and PPzIr, are rationally designed and synthesized, and they contain a rigid phenanthrene- or pyrene-fused diazaacene core and two peripheral groups of 4-tert-butyl-phenyl attached at the 12,13-positions in the core, respectively. The effects of the diazaacene core and appending groups on the optoelectronic properties of both complexes are systematically investigated. A dramatically red-shifted NIR emission peak at 789 nm with a photoluminescence quantum yield (PLQY) of 14% is observed in PPzIr compared with the 746 nm emission with a PLQY of 40% in DBPzIr. Taking advantage of their photophysical properties, the solution-processed device doped with DBPzIr achieves a maximum external quantum efficiency (EQEmax) of 8.00% with a radiance of 54 866 mW Sr-1 m-2 at 716 nm and the device doped with PPzIr exhibits a significantly red-shifted emission at 772 nm with an EQEmax of 3.53%. The achieved device performance is among the best values in the reported NIR-OLEDs based on iridium(III) complexes via a solution process at the same color gamut. Our study indicates that the reasonable collocation of the rigid diazaacene chelating core and flexible peripheral groups in the iridium(III) complex is of great significance in designing highly efficient NIR emitters.

Efficient and Bright Organic Radical Light-Emitting Diodes with Low Efficiency Roll-Off

Organic radicals have been of interest due to their potential to replace nonradical-based organic emitters, especially for deep-red/near-infrared (NIR) electroluminescence (EL), based on the spin-allowed doublet fluorescence. However, the performance of the radical-based EL devices is limited by low carrier mobility which causes a large efficiency roll-off at high current densities. Here, highly efficient and bright doublet EL devices are reported by combining a thermally activated delayed fluorescence (TADF) host that supports both electron and hole transport and a tris(2,4,6-trichlorophenyl)methyl-based radical emitter. Steady-state and transient photophysical studies reveal the optical signatures of doublet luminescence mechanisms arising from both host and guest photoexcitation. The host system presented here allows balanced hole and electron currents, and a high maximum external quantum efficiency (EQE) of 17.4% at 707 nm peak emission with substantially improved efficiency roll-off is reported: over 70% of the maximum EQE (12.2%) is recorded at 10 mA cm-2 , and even at 100 mA cm-2 , nearly 50% of the maximum EQE (8.4%) is maintained. This is an important step in the practical application of organic radicals to NIR light-emitting devices.

A Simple Molecular Design Strategy for Luminescent Radicals to Achieve Near-Infrared Emission

The spin-allowed doublet emission of luminescent radicals has recently attracted significant attention. However, the spectral range of most reported luminescent radical emitters and their corresponding organic light-emitting diodes (OLEDs) is confined to the red and deep red regions, with only a few extending to the near-infrared region, specifically in the context of an emission peak exceeding 800 nm. Herein, a luminescent radical, 2-(4-(bis(2,4,6-trichlorophenyl)methyl radical)-3,5-dichlorophenyl)-4-phenyl-4H-thieno[3,2-b]indole (TTM-2PTI), with NIR emission peaking at 830 nm in toluene, was obtained through attaching a 4-phenyl-4H-thieno[3,2-b]indole group to the TTM radical core. An organic light-emitting diode (OLED) utilizing TTM-2PTI as the emitter exhibits electroluminescence (EL) emission peaking at 870 nm, which is the longest EL wavelength among the doublet-emissive near-infrared (NIR) OLEDs. This work provides a simple molecular design strategy to achieve NIR emission of radicals by leveraging the lower steric hindrance and electron-donating ability of thiophene.

Progresses and Perspectives of Near-Infrared Emission Materials with "Heavy Metal-Free" Organic Compounds for Electroluminescence

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the "heavy metal-free" NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.

Achieving High Efficiency at High Luminance in Fluorescent Organic Light-Emitting Diodes through Triplet-Triplet Fusion Based on Phenanthroimidazole-Benzothiadiazole Derivatives

Achieving high efficiency at high luminance is one of the most important prerequisites towards practical application of any kind of light-emitting diode (LED). Herein, we report highly emissive organic fluorescent molecules based on phenanthroimidazole-benzothiadiazole derivatives capable of maintaining high external quantum efficiency (EQE) at high luminance enabled by triplet-triplet fusion (TTF) in doped organic LEDs. The PIBzP-, PIBzPCN-, and PIBzTPA-based devices showed EQEs of 8.27, 9.15, and 8.64 %, respectively, at luminance of higher than 1000 cd m^-2 , with little efficiency roll-off.

Non-toxic near-infrared light-emitting diodes

Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided.

Efficient Red Electroluminescence From Phenanthro[9,10-d]imidazole-Naphtho[2,3-c][1,2,5]thiadiazole Donor-Acceptor Derivatives

Red emission is one of the three primary colors and is indispensable for full color displays. Fluorescent materials that can generate efficient red electroluminescence (EL) are limited and need to be developed. In this work, we report efficient red emitters based on phenanthro[9,10-d]imidazole-naphtho[2,3-c][1,2,5]thiadiazole donor-acceptor derivatives. The molecules, abbreviated as PINzP and PINzPCN, exhibited high photoluminescence quantum yield (PLQY) up to unity in doped films. They can also reach a relatively high PLQY of ∼30% in neat films. PINzP and PINzPCN were capable of generating efficient red EL in doped devices with a maximum external quantum efficiency (EQE) of 6.96% and 5.92%, respectively.

'Light-Up' AIE-Active Materials: Self-Assembly, Molecular Recognition and Catalytic Applications

Aggregation induced emission enhancement (AIEE) is one of the most widely explored phenomena to develop 'light up' (fluorescent) materials having potential applications in the field of supramolecular chemistry, analytical chemistry and material chemistry. By applying the principles of host-guest chemistry, we have developed a variety of aggregation induced emission (AIE/AEE) active materials having specific affinity for metal ions, electron deficient/electron rich analytes. The interactions between AIE active assemblies and metal ions are further tuned to prepare nanohybrids having potential applications as catalytic/photocatalytic systems in various organic transformations under eco-friendly conditions. This account summarizes various design strategies developed in our labortary for the preparation of AIE/AEE active building blocks having sensing and catalytic applications.

A Simple and Strong Electron-Deficient 5,6-Dicyano[2,1,3]benzothiadiazole-Cored Donor-Acceptor-Donor Compound for Efficient Near Infrared Thermally Activated Delayed Fluorescence

Despite the success of thermally activated delayed fluorescent (TADF) materials in steering the next generation of organic light-emitting diodes (OLEDs), effective near infrared (NIR) TADF emitters are still very rare. Here, we present a simple and extremely high electron-deficient compound, 5,6-dicyano[2,1,3]benzothiadiazole (CNBz), as a strong electron-accepting unit to develop a sufficiently strong donor-acceptor (D-A) interaction for NIR emission. End-capping with the electron-donating triphenylamine (TPA) unit created an effective D-A-D type system, giving rise to an efficient NIR TADF emissive molecule (λ_em =750 nm) with a very small ΔE_ST of 0.06 eV. The electroluminescent device using this NIR TADF emitter exhibited an excellent performance with a high maximum radiance of 10020 mW Sr^-1  m^-2 , a maximum EQE of 6.57% and a peak wavelength of 712 nm.

Photoactivity and optical applications of organic materials containing selenium and tellurium

Sulfur-containing compounds, particularly derivatives of thiophene, are well studied for organic optoelectronic applications. Incorporating selenium or tellurium in place of sulfur imparts different physical properties due to the fundamental differences of these atoms relative to their lighter analogues. This has a profound influence on the properties of molecules and materials that incorporate chalcogens that may ultimately lead to new opportunities and applications. This mini-review will focus on the quantitative and qualitative photophysical characteristics of organic materials containing selenium and tellurium as well as their emerging applications as molecular photoactive species, including light-emitting sensors, triplet sensitizers, and beyond.

Donor-Acceptor-Donor NIR II Emissive Rhodindolizine Dye Synthesized by C-H Bond Functionalization

A NIR II emissive dye was synthesized by the C-H bond functionalization of 1-methyl-2-phenylindolizine with 3,6-dibromoxanthene. The rhodindolizine (RhIndz) spirolactone product was nonfluorescent; however, upon opening of the lactone ring by the formation of the ethyl ester derivative, the fluorophore absorbs at 920 nm and emits at 1092 nm, which are both in the NIR II region. In addition, 4-cyanophenyl- (CNRhIndz) and 4-methoxyphenyl-substituted rhodindolizine (MeORhIndz) could also be prepared by the C-H activation reaction.

Reflectance-Based Organic Pulse Meter Sensor for Wireless Monitoring of Photoplethysmogram Signal

This paper compares the structural design of two organic biosensors that minimize power consumption in wireless photoplethysmogram (PPG) waveform monitoring. Both devices were fabricated on the same substrate with a red organic light-emitting diode (OLED) and an organic photodiode (OPD). Both were designed with a circular OLED at the center of the device surrounded by OPD. One device had an OLED area of 0.06 cm2, while the other device had half the area. The gap distance between the OLED and OPD was 1.65 mm for the first device and 2 mm for the second. Both devices had an OPD area of 0.16 cm2. We compared the power consumption and signal-to-noise ratio (SNR) of both devices and evaluated the PPG signal, which was successfully collected from a fingertip. The reflectance-based organic pulse meter operated successfully and at a low power consumption of 8 µW at 18 dB SNR. The device sent the PPG waveforms, via Bluetooth low energy (BLE), to a PC host at a maximum rate of 256 kbps data throughput. In the end, the proposed reflectance-based organic pulse meter reduced power consumption and improved long-term PPG wireless monitoring.

Control of the dual emission from a thermally activated delayed fluorescence emitter containing phenothiazine units in organic light-emitting diodes

A novel TADF emitter showing dual emission is synthesized and applied to OLEDs with an external quantum efficiency of 11.5%.

Counter Anion Effect on the Photophysical Properties of Emissive Indolizine-Cyanine Dyes in Solution and Solid State

Near-infrared emissive materials with tunable Stokes shifts and solid-state emissions are needed for several active research areas and applications. To aid in addressing this need, a series of indolizine-cyanine compounds varying only the anions based on size, dipole, and hydrophilicity were prepared. The effect of the non-covalently bound anions on the absorption and emission properties of identical π-system indolizine-cyanine compounds were measured in solution and as thin films. Interestingly, the anion choice has a significant influence on the Stokes shift and molar absorptivities of the dyes in solution. In the solid-state, the anion choice was found to have an effect on the formation of aggregate states with higher energy absorptions than the parent monomer compound. The dyes were found to be emissive in the NIR region, with emissions peaking at near 900 nm for specific solvent and anion selections.

Facile access to deep red/near-infrared emissive AIEgens for efficient non-doped OLEDs

Notwithstanding the huge demand in bio-imaging and optoelectronics, the construction of highly emissive deep red/near infrared (DR/NIR) organic luminogens is still a big challenge because a narrow energy gap generally leads to low photoluminescence quantum yield. It is even more difficult to afford DR/NIR emitters in the solid state due to the aggregation caused quenching (ACQ) effect. In this work, we found that the direct attachment of a tetraphenylethylene substituted arylamine to the electron accepting 2,1,3-benzothiadiazole produces DR/NIR AIE luminogens with bright emission facilely and efficiently. And the emission wavelengths could be tuned from the red to the DR/NIR region by regulating the variety of the substituents. The long emission wavelength and high photoluminescence quantum yield of these AIEgens are ascribed to the effective intramolecular charge transfer and the suppressed intramolecular motion. Furthermore, non-doped OLEDs based on one of the AIEgens showed an EL emission at 684 nm with a large radiance of 5772 mW Sr-1 m-2 and an impressive external quantum efficiency (EQE) of 1.73%.

Efficient Near-Infrared Electroluminescence at 840 nm with "Metal-Free" Small-Molecule:Polymer Blends

Due to the so-called energy-gap law and aggregation quenching, the efficiency of organic light-emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light-emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn-on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole-based emitter and a novel indacenodithiophene-based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution-processed active layer, not leveraging triplet-assisted emission.

Distinct Mechanoresponsive Luminescence, Thermochromism, Vapochromism, and Chlorine Gas Sensing by a Solid-State Organic Emitter

In this study, we report a synthetically simple donor-acceptor (D-A)-type organic solid-state emitter 1 that displays unique fluorescence switching under mechanical stimuli. Orange and yellow emissive crystals of 1 (1O, 1Y) exhibit an unusual "back and forth" fluorescence response to mechanical force. Gentle crushing (mild pressure) of the orange or yellow emissive crystal results in hypsochromic shift to cyan emissive fragments (λ_em = 498-501 nm) with a large wavelength shift Δλ_em = -71 to -96 nm, while further grinding results in bathochromic swing to green emissive powder λ_em = 540-550 nm, Δλ_em = +40 to 58 nm. Single-crystal X-ray diffraction study reveals that molecules are packed by weak interactions, such as C-H···π, C-H···N, and C-H···F, which facilitate intermolecular charge transfer in the crystal. With the aid of structural, spectroscopic, and morphological studies, we established the interplay between intermolecular and intramolecular charge-transfer interaction that is responsible for this elusive mechanochromic luminescence. Moreover, we have also demonstrated the application of this organic material for chlorine gas sensing in solid state.

Dual phosphorescence emission of dinuclear platinum(ii) complex incorporating cyclometallating pyrenyl-dipyridine-based ligand and its application in near-infrared solution-processed polymer light-emitting diodes

Two novel mono- and binuclear cyclometalated platinum(ii) complexes of (BuPyrDPy)Pt(dpm) and (BuPyrDPy)[Pt(dpm)]₂ incorporating a pyrenyl-dipyridine-based cyclometalated ligand were synthesized and characterized, respectively. Single-crystal X-ray diffraction of the two materials revealed each complex's coordination mode; their photophysical, electrochemical as well as electroluminescent properties were also investigated. Both complexes exhibited good solubility and excellent thermal stability. (BuPyrDPy)[Pt(dpm)]₂ presented dual phosphorescence emissive character at room-temperature and showed an increased quantum efficiency compared to that of (BuPyrDPy)Pt(dpm). Density functional theory (DFT) calculations were carried out to model their photophysical process, and found a significant contribution of the second Pt center to the LUMO plot, giving the T₁ and T₂ states considerable LMCT nature, which is quite rare in metallic complexes. A device with the structure of ITO/PEDOT (40 nm)/PVK : 30 wt% OXD-7 : 16 wt% (BuPyrDPy)[Pt(dpm)]₂ (60 nm)/TPBI (30 nm)/Ba (4 nm)/Al (100 nm) showed a stable NIR emission peak at 695 nm accompanied by two shoulders at 599 nm and 762 nm, with a maximum external quantum efficiency (EQE) of 0.31% and a radiance of 26.9 mW cm^-2, which are about 2 and 1.4 times higher than those of (BuPyrDPy)Pt(dpm)-doped devices. This study provides an efficient strategy to simultaneously design novel biluminescent materials and achieve NIR emission through adjusting the emissive triplet states by introducing a second metal into an asymmetric bimetallic system.

New perylene diimide derivatives: stable red emission, adjustable property from ACQ to AIE, and good device performance with an EQE value of 4.93

Perylene diimide (PDI) derivatives, due to their special opto-electronic property, have been successfully utilized in organic field-effect transistor (OFET), solar cells, and as non-fullerene acceptor and others, while few cases in organic light-emitting diodes (OLEDs). In this work, six perylene bisimide-based red emitters, N,N'-bis(2-decyltetradecyl)-1-([1,1':3',1″-terphenyl]-5'-yl)perylene-3,4,9,10-diimide (STPH), N,N'-bis(2-decyltetradecyl)-1,7-bis([1,1':3',1″-terphenyl]-5'-yl)perylene-3,4,9,10-diimide (DTPH), N,N'-bis(2-decyltetradecyl)-1-(5'-phenyl-[1,1':3',1″-terphenyl]-4-yl)perylene-3,4,9,10-diimide (STRPH), N,N'-bis(2-decyltetradecyl)-1,7-bis(5'-phenyl-[1,1':3',1″-terphenyl]-4-yl)perylene-3,4,9,10-diimide (DTRPH), N,N'-bis(2-decyltetradecyl)-1-(4-(2,2-diphenylvinyl)phenyl)perylene-3,4,9,10-diimide (STTPE) and N,N'-bis(2-decyltetradecyl)-1,7-bis(4-(2,2-diphenylvinyl)phenyl)perylene-3,4,9,10-diimide (DTTPE), with the excellent chemical, thermal and photo-chemical stability, are synthesized through the convenient Suzuki coupling reaction, in which, the fluorescent properties can be modified easily from ACQ to AIE by just simply changing the bulky volume of the introduced aromatic substituents. After being fabricated into organic light-emitting diodes, STRPH exhibits the best performance with the maximum luminescence, power efficiency, current efficiency and external quantum efficiency of 1,948 cd m^-2, 2.04 lm W^-1, 5.85 cd A^-1, 4.93% at Commission Internationale de L'Eclairage (CIE) coordinates of (0.56, 0.34), as the result of the high efficient energy transfer and good energy match achieved in the device.

Effects of substituents on luminescent efficiency of stable triaryl methyl radicals

The factors affecting the photoluminescence quantum yields (PLQYs) of perchlorotriphenyl methyl (PTM) and tris(2,4,6-trichlorophenyl)methyl (TTM) radical derivatives were studied systematically for the first time.

Deep-Red to Near-Infrared Thermally Activated Delayed Fluorescence in Organic Solid Films and Electroluminescent Devices

The design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. A strategy was developed to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were employed as electron donor (D) and acceptor (A), respectively, to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

Pyrene-fused bisphenazinothiadiazoles with red to NIR electroluminescence

Deep red and NIR electroluminescence from pyrene-fused bisphenazinothiadiazoles.

Up to 100% Formation Ratio of Doublet Exciton in Deep-Red Organic Light-Emitting Diodes Based on Neutral π-Radical

In a neutral π-radical-based organic light-emitting diode (OLED), although the emission comes from the doublet excitons and their transition to the ground state is spin-allowed, the upper limit of internal quantum efficiency (IQE) is not clear, 50% or 100%? In this work, the deep-red OLEDs based on a neutral π-radical were fabricated. Up to 100% doublet exciton formation ratio was obtained through rational designing device structure and host-guest doping system. This indicates the IQE of neutral π-radical-based OLEDs will reach 100% if the nonradiative pathways of radicals can be suppressed. The maximum external quantum efficiency of the optimized device is as high as 4.3%, which is among the highest values of deep-red/near-infrared OLEDs with nonphosphorescent materials as emitters. Our results also indicate that using partially reduced radical mixture as emitter may be a way to solve aggregation-caused quenching in radical-based OLEDs.