Fluorescent Organic Light-emitting Devices Research Articles

Highly Efficient Blue Fluorescent Organic Light-Emitting Devices Based on λ5-Phosphinine Derivatives.

Although λ5-phosphinine derivatives are known as a promising class of blue fluorescent emitters, those photoluminescent quantum yield (PLQY) values have been reached up to 92 %, however, only a few examples have been explored as an emitter for blue organic light-emitting device (OLED), and the external quantum efficiency (EQE) has been below 2.4 % so far. In this study, we newly developed two types of blue λ5-phosphinine derivatives namely CN-COCF3 and CO2Me-CHO, and investigated the photophysical properties in the solid states. The photophysical analyses in solid state films suggested that the strong electron-accepting nature of these λ5-phosphinine derivatives caused the inferior PLQY values, and the exciplex formation with the host and neighboring materials should be avoided to improve the device efficiency. By choosing suitable host and neighboring materials with deep ionization potentials, we successfully realized efficient blue fluorescent OLEDs with EQE of over 4 % and CIE (0.14, 0.18). This is among the best in λ5-phosphinine-based blue OLEDs so far.

Key Factors Governing the External Quantum Efficiency of Thermally Activated Delayed Fluorescence Organic Light-Emitting Devices: Evidence from Machine Learning.

Thermally activated delayed fluorescence (TADF) materials enable organic light-emitting devices (OLEDs) to exhibit high external quantum efficiency (EQE), as they can fully utilize singlets and triplets. Despite the high theoretical limit in EQE of TADF OLEDs, the reported values of EQE in the literature vary a lot. Hence, it is critical to quantify the effects of the factors on device EQE based on data-driven approaches. Herein, we use machine learning (ML) algorithms to map the relationship between the material/device structural factors and the EQE. We established the dataset from a variety of experimental reports. Four algorithms are employed, among which the neural network performs best in predicting the EQE. The root-mean-square errors are 1.96 and 3.39% for the training and test sets. Based on the correlation and the feature importance studies, key factors governing the device EQE are screened out. These results provide essential guidance for material screening and experimental device optimization of TADF OLEDs.

Single-layer Thermally Activated Delayed Fluorescent Organic Light-emitting Devices and Exciton Distribution Profiles

Single-layer Thermally Activated Delayed Fluorescent Organic Light-emitting Devices and Exciton Distribution Profiles

Simple-structure color-tunable fluorescent organic light-emitting devices with chromaticity difference beyond five-step McAdam ellipses

Currently, status lighting, such as signal indication and information coding, is considered as a novel potential application of organic light-emitting devices (OLEDs). Compared to conventional lighting and display applications, status lighting requires tunable colors distinguishable by the human eyes with natural appearances rather than high efficiency. Here, a color-tunable fluorescent OLED (CTFOLED) for status lighting is successfully prepared by using an ultrathin emitting layer consisting of a deep-blue host and a green guest. Through adjusting exciton concentration and forming an island growth induced exciplex interface, the CTFOLED shows a voltage-modulated emission color. Its coordinates change from (0.26, 0.61) to (0.35, 0.50) as the bias voltage increases from 4 to 10 V. The chromaticity coordinates are demonstrated to beyond five-step McAdam ellipses at the neighboring voltages (an interval of 2 V) such that the voltage-modulated color variations are easily distinguishable by the human eye. Finally, it also proposed that the CTFOLED can be used as a status light for presenting three statuses of drinking water: cold, warm and hot. This work paves a way for the application of OLEDs in status lighting.

Top-emitting thermally activated delayed fluorescence organic light-emitting devices with weak light-matter coupling

Resonance interaction between a molecular transition and a confined electromagnetic field can lead to weak or strong light-matter coupling. Considering the substantial exciton–phonon coupling in thermally activated delayed fluorescence (TADF) materials, it is thus interesting to explore whether weak light-matter coupling can be used to redistribute optical density of states and to change the rate of radiative decay. Here, we demonstrate that the emission distribution of TADF emitters can be reshaped and narrowed in a top-emitting organic light-emitting device (OLED) with a weakly coupled microcavity. The Purcell effect of weak microcavity is found to be different for TADF emitters with different molecular orientations. We demonstrate that radiative rates of the TADF emitters with vertical orientation can be substantial increased in weakly coupled organic microcavity. These observations can enhance external quantum efficiencies, reduce efficiency roll-off, and improve color-purities of TADF OLEDs, especially for emitters without highly horizontal orientation.

Highly Efficient Solution Processed Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Devices with a Mixed Hole Injection Layer

Highly Efficient Solution Processed Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Devices with a Mixed Hole Injection Layer

Efficient Green Fluorescent Organic Light-Emitting Devices with Stepwise Energy Levels

In our work, efficient devices with double-EMLs with step energy based on green fluorescent emitter C545T were reported. Due to the matched energy, TcTa and 26DCzPPy were selected as the host materials of double-EMLs. The devices with double-EMLs displayed more excellent EL efficiency and higher brightness compared with devices with single-EML, which attributed to better balance of carriers, broadening recombination zone. Finally, efficient green EL device obtained maximum CE, EQE, PE and brightness of 10.13 cd/A, 2.8%, 9.36 lm/W and 11490 cd/m2.

N-Type host materials based on nitrile and triazine substituted tricyclic aromatic compounds for high-performance blue thermally activated delayed fluorescence devices

Novel n-type host materials based on tricyclic aromatic compounds, dibenzo[b,d]furan and dibenzo[b,d]thiophene (2Trz6CNDBF and 2Trz6CNDBT), have been successfully synthesized and characterized for high-performance blue thermally activated delayed fluorescence (TADF) organic light-emitting devices (OLEDs). Dibenzo[b,d]furan and dibenzo[b,d]thiophene were utilized as central molecular building blocks to achieve excellent thermal stability and high triplet energy (ET). Nitrile and diphenyltriazine functional groups were introduced at the 6- and 2- positions of the central building blocks to achieve low-lying lowest unoccupied molecular orbital (LUMO) energy levels, high ET, and excellent electron transport properties. UV–Vis absorption, low-temperature photoluminescence, and ultraviolet photoelectron spectroscopy analysis showed that 2Trz6CNDBF and 2Trz6CNDBT possessed high ET (2.95 and 2.88 eV) and low-lying LUMO energy levels (−3.43 and −3.16 eV) that were well-matched with a blue TADF emitter, 5CzCN. Moreover, the electron-only device (EOD) result revealed that 2Trz6CNDBF and 2Trz6CNDBT had excellent electron transport properties. Blue TADF OLEDs fabricated with a p-type host material, mCBP, and n-type host, 2Trz6CNDBF or 2Trz6CNDBT, exhibited lower driving voltages (3.34 and 3.26 V, respectively) than a TADF OLED with only a p-type host material, mCBP (3.83 V). Blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT exhibited superior external quantum efficiency (ηext, 15.6 and 14.7%), current efficiency (ηce, 33.8 and 32.7 cd A−1), and power efficiency (ηpe, 25.6 and 25.7 lm W−1), respectively. The ηext and ηce of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 70% and ηpe by approximately 150% compared to those of the TADF OLED with a single p-type host, mCBP. In addition, the device lifetimes of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 1000%. The efficient electron injection by the low-lying LUMO energy level, effective exciton confinement by high ET, and high thermal stability of the film morphology provided the enhanced efficiency and lifetime for blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT.

Simultaneous realization of high-efficiency, low-drive voltage, and long lifetime TADF OLEDs by multifunctional hole-transporters

A smart high-triplet energy hole-transporter exhibits significant stability in the anion state realizing record-breaking highly efficient and long-living thermally activated delayed fluorescent (TADF) organic light-emitting devices (OLEDs).

Efficiency Enhancement in Thermally Activated Delayed Fluorescence Organic Light-Emitting Devices by Controlling the Doping Concentration in the Emissive Layer

Efficiency roll-off defined as the rapid efficiency drop by increasing the electrical current density is a major issue in the design of novel organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF). In this paper, it is shown that efficiency enhancement and suppression of efficiency roll-off in TADF OLEDs can be accomplished by broadening the exciton generation zone by precisely tuning the doping concentration of the TADF emissive material in the host matrix. This universal and simply tunable parameter can be applied to any TADF OLED to further enhance its performance characteristics. In this paper, a typical TADF OLED with realistic parameters is numerically investigated by solving the charge carrier transport and excitonic equations and taking into account all relevant processes such as triplet–triplet annihilation, triplet–polaron, and singlet–triplet quenching, and (reverse) intersystem crossing. It is shown that the internal quantum efficiency (IQE) decreases at very low and very high doping densities due to exciton distribution nonuniformity and charge balance factor reduction, respectively. Power efficiency depends on the IQE as well as the potential drop across the emissive layer and decreases at very low doping densities even for a perfectly balanced device; however, the reduction for an unbalanced device is much more substantial. For a perfectly balanced device with an emissive layer thickness of 30 nm, at a current density of 10 mAcm−2, the IQE can be increased from 18% to more than 58% by decreasing the doping concentration from ${c}= {15}$ % to 0.5%. Power efficiency reaches its peak value of 38 lm/W at 1.4% doping.

Carbazole derivatives containing one or two tetra-/triphenylethenyl units as efficient hole-transporting OLED emitters

New efficient carbazole-based emitters containing tetra-/triphenylethene units were developed for boosting efficiency of non-doped fluorescent organic light-emitting diodes. Comparable study of the properties of derivatives containing one or two tetra-/triphenylethenyl units was performed using various experimental and theoretical techniques. Depending on the substitution pattern, emitters exhibited strong blue or green emission, which was enhanced by aggregation. Compounds with two substituents showed higher glass transition temperatures (up to 120 °C) and lower ionization potentials (of ca. 5.15 eV) comparing to mono substituted derivatives. Time-of-flight hole drift mobility values of the studied compounds with two substituents reached 10−3 cm2/V at high electric fields. Non-doped fluorescent OLEDs based on carbazole derivative containing two tetraphenylethenyl units demonstrated extremely high external quantum efficiency as for simple fluorescent organic light-emitting devices, which reached 5.32%.

Degradation of PEDOT:PSS hole injection layers by electrons in organic light emitting devices

Abstract The degradation of PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) hole injection layers (HILs) under electrical bias and its effect on the stability of organic light emitting devices (OLEDs) during device operation is investigated. Phosphorescent and fluorescent OLEDs with common hole transport layers (HTLs) and PEDOT:PSS, MoO3 and PEDOT:PSS/MoO3 HILs, and hole-only devices with the same HILs and HTLs are examined. Results show that PEDOT:PSS HILs are susceptible to degradation by electrons; a mechanism that can accelerate device failure. Minimizing electron leakage to the PEDOT:PSS/HTL interface is found to play a key role in maintaining the stability of these devices. Results also show that degradation by excitons and hole accumulation at the PEDOT:PSS/HTL interface have a much less significant effect. The findings provide key insights for improved device design for OLEDs utilizing PEDOT:PSS HILs.

Ultrahighly-efficient and pure deep-blue thermally activated delayed fluorescence organic light-emitting devices based on dimethylacridine/thioxanthene-S,S-dioxide

A novel deep-blue emissive dopant material, 2,7-bis(9,9-dimethylacridine-10(9H)-yl)-9,9-diphenyl-9H-thioxanthene 10-,10-dioxide (DMA-ThX), is synthesized for use in highly-efficient thermally activated delayed fluorescence organic light-emitting devices (TADF OLEDs). The molecular design of DMA-ThX retains the overlap between the lowest singlet excited state (S1) and the lowest triplet excited state (T1) because of its twisted molecular structure. From photoluminescence emission measurements at low temperature, the difference between the S1 and the T1 energy levels of DMA-ThX is found to be 0.07 eV. Furthermore, the external quantum efficiency of the deep-blue-emitting TADF OLEDs with DMA-ThX-doped bis[2-(diphenylphosphino)phenyl] ether oxide is 18.4%, which is significant, and the Commission Internationale de l'Eclairage coordinates are (0.14, 0.14), indicative of very pure deep-blue emission.

Highly Efficient Blue Fluorescent OLEDs Based on Upper Level Triplet-Singlet Intersystem Crossing.

Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T1 ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T2 ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S1 and T2 and rather large T2 -T1 energy gap, which are expected to impede the internal conversion (IC) from T2 to T1 and facilitate the reverse intersystem crossing from the high-lying triplet state (hRISC). Through sensitizing the T2 state with ketones, the existence of the hRISC process with an ns-scale delayed lifetime is confirmed. Benefiting from this fast triplet-singlet conversion, the nondoped device based on this "hot exciton" material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll-off and CIE coordinates of (0.15, 0.13). These results reveal that the "hot exciton" path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure-blue and deep-blue fluorescent organic light-emitting devices.

Highly Efficient Fluorescent Organic Light-Emitting Devices Using a Luminescent Radical as the Sensitizer.

In traditional fluorescent organic light-emitting diodes (OLEDs), the upper limit of internal quantum efficiency (IQE) is only 25% because 75% of triplet excitons created on the fluorescent dyes are nonluminous. Here luminescent radicals are proposed as the sensitizer. Under ideal conditions, electrons and holes first recombine on the sensitizer molecule to create doublet excitons, then through energy transfer to generate singlet excitons on the fluorescent dye, and, finally, via radiative decay to emit light. The upper limit of IQE can theoretically reach 100%. As an example, the maximum external quantum efficiency (EQE) of a fluorescent OLED sensitized by a luminescent radical, TTM-1Cz, has reached 8.1%, which is much higher than the upper limit of EQE of traditional fluorescent OLEDs. Our results suggest a new route to realize highly efficient fluorescent OLEDs.

Boosting the Efficiency of Near‐Infrared Fluorescent OLEDs with an Electroluminescent Peak of Nearly 800 nm by Sensitizer‐Based Cascade Energy Transfer

AbstractA sensitization‐based cascade energy transfer channel is proposed to boost the electroluminescent performances of the solution‐processed near‐infrared organic light‐emitting devices (OLEDs) featuring an electroluminescent peak of 786 nm from a new fluorescent emitter of N4,N4,N9,N9‐tetra‐p‐tolylnaphtho[2,3‐c][1,2,5]thiadiazole‐4,9‐diamine (NZ2mDPA) with unique aggregation‐induced emission (AIE) property. The optimized device is composed of 4,4′‐N,N‐dicarbazole‐biphenyl (CBP) as the host, bis(2‐phenyl‐1,3‐benzothiozolato‐N,C2′)iridium (Ir(bt)2(acac)) as the sensitizer, and NZ2mDPA as the emitter, where the cascade energy transfer can occur via two steps realizing unexpected triplet–singlet energy transfer by the Förster mechanism. The first step features efficient triplet harvesting from CBP to Ir(bt)2(acac), and then the second step involves in resonant energy transfer from the phosphorescent sensitizer to the near‐infrared AIE emitter of NZ2mDPA, which finally endows two channels of harvesting singlet and triplet excitons. The unique scheme achieves not only more efficient Förster energy transfer but also the higher utilization efficiency of triplet excitons. As a result, the near‐infrared OLEDs can realize a factor of 2.7 enhancement of external quantum efficiency by employing the phosphor‐sensitized AIE lumogen compared with the commonly used binary host–guest system.

Functional versatile bipolar 3,3′-dimethyl-9,9′-bianthracene derivatives as an efficient host and deep-blue emitter

A new family of bipolar fluorinated 3,3′-dimethyl-9,9′-bianthracene derivatives (MBAnFs) have been synthesized, characterized and applied as deep-blue emitters and host materials in fluorescent organic light-emitting devices (OLEDs). Fine tuning the emission characteristics could be achieved by varying the position and the number of the F substituents on the two phenyl rings attached to MBAn core. Remarkably, 10,10′-bis(4-fluorophenyl)-3,3′-dimethyl-9,9′-bianthracene (MBAn-(4)-F)-based OLEDs showed an excellent performance. Both non-doped and doped multilayer OLEDs based on MBAn-(4)-F achieved the maximum external quantum efficiency (EQEmax) of 6.11% and 6.65% with Commission Internationale de L'Eclairage (CIE) coordinates of (0.155, 0.058) and (0.153, 0.055), respectively, matching the requirement of European Broadcasting Union standard of CIE (0.15, 0.06) very well. Furthermore, when doping blue-emitting p-bis(p-N,N-diphenyl-aminostyryl)-benzene (DSA-ph) into MBAn-(4)-F as a light-emitting layer (EML), the device achieved an astonishing current efficiency of 23.01 cd A−1 and a record-setting EQE of 11.52% with negligible efficiency roll-off, compared with other DSA-ph-based OLEDs. These results are attributed to the well-controlled bipolar characteristic of the MABn-(4)-F, leading to a better charge balance in the EML. In addition, the combination of the locally emissive component and twisted intramolecular charge-transfer component in one excited state can significantly enhance the singlet generation fraction in OLEDs.

Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene**Project supported by the National Natural Science Foundation of China (Grant No. 60906022), the Natural Science Foundation of Tianjin, China (Grant No. 10JCYBJC01100), the Key Science and Technology Support Program of Tianjin, China (Grant No. 14ZCZDGX00006), and the

We chose pentacene as a hole injection layer (HIL) to fabricate the high performance blue fluorescent organic lightemitting devices (OLEDs). We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120 °C. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120 °C annealed pentacene film and n-doped electron transport layer (ETL) presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m2 at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.

Suppression of Efficiency Roll-Off in Fluorescent Organic Light-Emitting Devices Through Charge Carriers and Excitons Management

In this paper, a novel design for the emission layer of the fluorescent organic light-emitting devices is presented to suppress the efficiency roll-off and maintain the high efficiency at both low and high current regimes. In the emission layer, the emitter material is blended with a manager or host material with proper charge carrier transporting and excitonic energy levels. The charge carriers and excitons undergo a carefully designed route in this manager–emitter blend so that the triplet excitons always contribute positively to the efficiency through triplety–triplet annihilation (TTA), and the competing mechanisms are severely weakened. The required characteristics of the emitter and host materials for the proper functioning of the proposed strategy are completely discussed in this paper. The proposed scheme is verified through detailed numerical simulations, and it is shown that the efficiency roll-off can be effectively suppressed. The overall efficiency depends on the TTA yield in producing singlet excitons. For the yield of 0.5, an internal quantum efficiency of 50% at the current density of 10 mAcm $^{-2}$ is computed, which increases as the triplet density increases reaching the value of 58% at the current density of 1 Acm $^{-2}$ .

High-Performance, Simplified Fluorescence and Phosphorescence Hybrid White Organic Light-Emitting Devices Allowing Complete Triplet Harvesting.

Causes of efficiency limitation in common fluorescence and phosphorescence hybrid white organic light-emitting devices (WOLEDs) are discussed, and a new device architecture is proposed to address these issues. This architecture employs a fluorescent emitting layer (EML) of blue exciplex-forming cohost, which shows broad and strong thermally activated delayed fluorescence (TADF). Hybrid WOLEDs based on this architecture not only allow complete triplet harvesting for light generation but also can achieve white light emission with high color rending indexes (CRI) using only two colors. By using 26DCzPPy:PO-T2T as the blue fluorescent EML and 26DCzPPy:Ir complexes as the phosphorescent EML, we prepared a series of two-color WOLEDs with low turn-on voltages of 2.5-3.3 V, high forward-viewing EQEs of 12.7-19.3% and high CRIs of 67-77. These results suggest this new architecture would be an effective way to achieve high performance WOLEDs with simple structures.