High-efficiency Organic Light-emitting Diodes Research Articles

Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence.

Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes. We have proposed a model to describe reverse intersystem crossing (rISC) in donor–acceptor charge transfer molecules, where spin–orbit coupling between singlet and triplet states is mediated by one of the local triplet states of the donor (or acceptor). This second order, vibronically coupled mechanism describes the basic photophysics of TADF. Through a series of measurements, whereby the energy ordering of the charge transfer (CT) excited states and the local triplet are tuned in and out of resonance, we show that TADF reaches a maximum at the resonance point, substantiating our model of rISC. Moreover, using photoinduced absorption, we show how the populations of both singlet and triplet CT states and the local triplet state change in and out of resonance. Our vibronic coupling rISC model is used to predict this behaviour and describes how rISC and TADF are affected by external perturbation.

ChemInform Abstract: Recent Progress in Functionalized Electrophosphorescent Iridium(III) Complexes

AbstractReview: 67 refs.

Aggregation-induced delayed fluorescence from phenothiazine-containing donor–acceptor molecules for high-efficiency non-doped organic light-emitting diodes

Highly luminescent donor–acceptor molecules based on a phenothiazine donor unit coupled with a xanthone or benzophenone acceptor unit were developed for use in organic light-emitting diodes (OLEDs). While both molecules are almost non-luminescent in pure tetrahydrofuran (THF) solution, a strong yellow delayed fluorescence was observed upon their aggregation in THF/water mixtures or in neat films. This result demonstrates the unique aggregation-induced delayed fluorescence (AIDF) characteristics of these molecules. OLEDs using these AIDF materials as a non-doped emission layer achieved high external electroluminescence quantum efficiencies of up to 11%, which exceeds the theoretical maximum for conventional fluorescent OLEDs. Highly luminescent donor–acceptor molecules based on a phenothiazine donor unit coupled with a xanthone or benzophenone acceptor unit were developed. While both molecules are almost non-luminescent in pure tetrahydrofuran (THF) solution, a strong yellow delayed fluorescence was observed upon their aggregation in THF/water mixtures and in neat films. This result demonstrates the unique aggregation-induced delayed fluorescence (AIDF) characteristics of these molecules. Organic light-emitting diodes (OLEDs) using these AIDF materials as a non-doped emission layer achieved high external electroluminescence quantum efficiencies of up to 11%, which far exceeds the theoretical maximum for conventional fluorescent OLEDs.

Synthesis and characterization of red phosphorescent iridium(Ⅲ) complexes based on electron-acceptor modulation of main ligand for high efficiency organic light-emitting diodes

ABSTRACTA new series of red phosphorescent iridium(Ⅲ) complexes, (PT-TFP)2Ir(tmd), (PT-P)2Ir(tmd), and (PT-MP)2Ir(tmd) were synthesized for highly efficient red emitter of the organic light-emitting diodes (OLEDs). The main ligands consisted of electron-donor (phenanthren) and electron-acceptor (trifluoromethyl pyridine, pyridine, and methyl pyridine) were synthesized by Suzuki coupling reaction. The iridium(Ⅲ) complexes based on main ligands and 2,2,6,6-tetramethyl-3,5-heptanedione(tmd) ancillary ligand were synthesized by Nonoyama reaction. Their luminescence properties were investigated by UV-visible spectroscopy and photoluminescence (PL) spectroscopy. The OLEDs devices were manufactured by vacuum deposition and These OLEDs devices were characterized by investigation of current density-voltage-luminance (J-V-L), current efficiency, power efficiency, external quantum efficiency (EQE), EL spectrum. The Ir(Ⅲ) complexes will be used as a solution process dopant for the hybrid OLED devices.

High Efficiency and Stable Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Emitter**Supported by the National Natural Science Foundation of China under Grant Nos 61420106002, 51373189, 61178061, and 61227008, the Hundred Talents Program of the Chinese Academy of Sciences, the National Basic Research Program of China under Grant No 2014CB932600, and the Start-Up Fund of the Technical Institute of

High efficiency, stable organic light-emitting diodes (OLEDs) based on 2-pheyl-4′-carbazole-9-H-Thioxanthen-9-one-10,10-dioxide (TXO-PhCz) with different doping concentration are constructed. The stability of the encapsulated devices are investigated in detail. The devices with the 10 wt% doped TXO-PhCz emitter layer (EML) show the best performance with a current efficiency of 52.1 cd/A, a power efficiency of 32.7 lm/W, and an external quantum efficiency (EQE) of 17.7%. The devices based on the 10 wt%-doped TXO-PhCz EML show the best operational stability with a half-life time (LT50) of 80 h, which is 8 h longer than that of the reference devices based on fac-tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3). These indicate excellent stability of TXO-PhCz for redox and oxidation processes under electrical excitation and TXO-PhCz can be potentially used as the emitters for OLEDs with high efficiency and excellent stability. The high-performance device based on TXO-PhCz with high stability can be further improved by the optimization of the encapsulation technology and the development of a new host for TXO-PhCz.

High-efficiency diphenylsulfon derivative-based organic light-emitting diode exhibiting thermally-activated delayed fluorescence

Novel thermally activated delayed fluorescence (TADF) material with diphenyl sulfone (DPS) as an electron acceptor and 3,6-dimethoxycarbazole (DMOC) and 1,3,6,8-Tetramethyl-9H-carbazole (TMC) as electron donors were investigated theoretically for a blue organic light emitting diode (OLED) emitter. We calculate the energies of the first singlet (S1) and first triplet (T1)-excited states of TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state using a dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. The calculated {\Delta}EST values of TMC-DPS (0.094 eV) was smaller than DMOC-DPS (0.386 eV) because of the large dihedral angles between the donor and accepter moieties. We show that TMC-DPS would have a suitable blue OLED emitter, because it has a large dihedral angle that creates a small spatial overlap between the HOMO and the LUMO and, consequently, the small {\Delta}EST and the emission wavelength of 2.82 eV (439.9 nm).

High efficiency organic light-emitting diodes with in situ synthesized Cu(I) complex emitter: How to do chemical reaction in a vacuum chamber?

Cu(I) complexes are considered as idea emitters in organic light-emitting diodes (OLEDs) due to their low cost and theoretical high internal quantum efficiency, which are two important issues have to be concerned for OLEDs commercialization. However, most Cu(I) complexes are unstable toward sublimation and hence not amenable to the vacuum deposition method typically used to fabricate OLEDs. To solve this problem, a codeposition route that involves codeposition of CuI and pyridine derivative has been proposed to synthesis Cu(I) complex emitter in situ. Since chemical reactions were conducted in a vacuum chamber, we systematically studied the effect of reactant chemical structure, reaction ratio, and deposition rate on the in situ synthesized Cu(I) complex and its application as an emitter in OLEDs. With an optimal chemical reaction condition, the device showed a high external quantum efficiency (EQE) up to 14.2% at a brightness of 100 cd/m2, corresponding to a current and power efficiency of 45.2 cd/A and 33.3 lm/W, respectively. The performance is comparable to those efficient OLEDs with iridium complex emitter, while using a CuI dopant that having only one ten-thousandth of price to bis(2-phenylpyridine) (acetylacetonate)iridium.

Recent progress in functionalized electrophosphorescent iridium(III) complexes

Abstract Iridium(III) complexes are one of the most important electrophosphorescent dyes with tunable emissions in the range of visible and near infrared lights, high photoluminescence yields and short lifetimes for high-efficiency organic light-emitting diodes (OLED) with 100% exciton harvesting. This review summarizes the recent development of electroluminescent Ir 3+ complexes functionalized with host-featured carrier-transporting groups, with emphasis on correlations between functionalization, optoelectronic properties and device performance. According to the introducing approaches, the complexes were sorted with conjugated and aliphatic linkages, as well as the types of functional groups. The modification effect on physical properties and the state-of-the-art device performances were discussed.

High efficiency and simplified white organic light-emitting diode based on a single-host emission layer

Efficient white organic light-emitting diodes (WOLEDs) with high stability are highly desired nowadays to produce devices that meet the commercial illumination requirement. WOLEDs based on single-host emission layer, is a preferred route to simplify device structures and thus lower the production cost. In this work, we report on single-host WOLEDs based on bipolar material, 2,6-bis[(3-carbazol-9-yl)phenyl)]pyridine (26DCzPPy), as the host, and Iridium (III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (FIrpic), [3,2-c]pyridinato-N,C2′) acetylacetonate (PO-01), as blue and yellow dopants, respectively. The devices exhibit not only higher efficiency, but also superior stability compared to the conventional devices and doping-free devices. Detailed analysis is conducted to clarify the underlying reasons. This work enriches the mechanism understanding of single-host devices and paves the way for development of high performance WOLEDs.

Solution‐Processed Double‐Layer Electron‐Transport Layer for Conventional Blue Phosphorescent Organic Light‐Emitting Diodes

Zinc oxide (ZnO) is a potential material for the electron‐transport layer (ETL) in organic light‐emitting diodes (OLEDs). However, ZnO may induce exciton dissociation at the emission layer (EML)/ZnO interface, which lowers the device efficiency. Here, polyethyleneimine (PEI)‐doped lithium acetylacetonate (Li(acac)) is used as a hybrid interfacial layer (IL) between a ZnO nanoparticle (NP) ETL and a phosphorescent EML, with a thermally evaporated aluminum film as the cathode contact on the top. To avoid the IL being dissolved, ZnO NPs were re‐dispersed in octane (an orthogonal solvent for most ILs) and doped with trans‐polyisoprene (PI) to prevent aggregation on the film. The hybrid IL markedly improves electron injection and transport and suppresses triplet exciton quenching at the EML/ZnO interface, resulting in a lower driving voltage and doubled luminous efficiency compared with those of the control device without IL. ZnO NPs can serve as an ETL in conventional OLEDs cooperating with PEI‐doped Li(acac) as an IL and this research provides an easy way to construct a stable double‐layer ETL configuration in high‐efficiency OLEDs.

P-155: Synthesis and Characterization of Red Phosphorescent Iridium(III) Complexes based on Electron-Acceptor Modulation of Main Ligand for High Efficiency Organic Light-Emitting Diodes

A new series of red phosphorescent iridium(lll) complexes, (PT-MQ)2Ir(tmd), (PT-P)2Ir(tmd), and (PT-TFP)2Ir(tmd) were synthesized for highly efficient red emitter of the organic light-emitting diodes (OLEDs). The devices give very deep red EL spectra with much narrower full width at half maximum (FWHM). Maximum current efficiency of devices is 8.56 Cd/A, 5.52 Cd/A, and 2.62 Cd/A respectively.

24-4: Investigation of Effect of Triplet-Triplet Annihilation and Molecular Orientation on External Quantum Efficiency of Ultrahigh-Efficiency Blue Fluorescent Device

The effects of triplet-triplet annihilation (TTA) and molecular orientation were quantitatively investigated for a high-efficiency blue fluorescent organic light-emitting diode. Both the efficient TTA generation by improving carrier balance and the horizontal orientation of the transition dipole moment of the blue dopant realized ultrahigh external quantum efficiency (approximately 12%).

Highly Efficient White Organic Light-Emitting Diodes with Ultrathin Emissive Layers and a Spacer-Free Structure.

Ultrathin emissive layers (UEMLs) of phosphorescent materials with a layer thickness of less than 0.3 nm were introduced for high-efficiency organic light-emitting diodes (OLEDs). All the UEMLs for white OLEDs can be prepared without the use of interlayers or spacers. Compared with devices fabricated with interlayers inserted in-between the UEMLs, our spacer-free structure not only significantly improves device efficiency, but also simplifies the fabrication process, thus it has a great potential in lowering the cost of OLED panels. In addition, its spacer-free structure decreases the number of interfaces which often introduce unnecessary energy barriers in these devices. In the present work, UEMLs of red, green and blue-emitting phosphorescent materials and yellow and blue phosphorescent emitters are utilized for the demonstration of spacer-free white OLEDs. Upon optimization of the device structure, we demonstrated spacer-free and simple-structured white-emitting OLEDs with a good device performance. The current and power efficiencies of our white-emitting devices are as high as 56.0 cd/A and 55.5 lm/W, respectively. These efficiencies are the highest ever reported for OLEDs fabricated with the UEML approach.

Derivative of oxygafluorene and di-tert-butyl carbazole as the host with very high hole mobility for high-efficiency blue phosphorescent organic light-emitting diodes

Derivative of oxygafluorene and di-tert-butyl carbazole was synthesized as the host material for high-efficiency blue phosphorescent organic light-emitting diodes. It exhibited moderate thermal stability with 5% weight loss temperature of 337 °C and glass-forming ability with the glass-transition temperature of 65 °C. The solid-state ionization potential of di-tert-butyl carbazolyl-substituted oxygafluorene was found to be 5.54 eV as established by electron photoemission spectrometry. It showed low-dispersity hole transport with time-of-flight hole mobilities of 10−2 cm2 V−1 s−1 observed at high electric fields. Blue devices based on the synthesized compound showed high external quantum efficiency up to 16.5% and low efficiency roll-off.

Extremely high-efficiency and ultrasimplified hybrid white organic light-emitting diodes exploiting double multifunctional blue emitting layers.

Numerous hybrid white organic light-emitting diodes (WOLEDs) have recently been developed. However, their efficiency is not comparable to that of their best all-phosphorescent WOLED counterparts, and the structures are usually complicated, restricting their further development. Herein, a novel concept is used to achieve a hybrid WOLED, whose crucial feature is the exploitation of double multifunctional blue emitting layers. The three-organic-layer WOLED exhibits a total efficiency of 89.3 and 65.1 lm W–1 at 100 and 1000 cd m–2, respectively, making it the most efficient hybrid WOLED reported in the literature so far. Significantly, the efficiencies of hybrid WOLEDs have, for the first time, been demonstrated to be comparable to those of the best all-phosphorescent WOLEDs. In addition, the device exhibits the lowest voltages among hybrid WOLEDs (i.e., 2.4, 2.7 and 3.1 V for 1, 100 and 1000 cd m–2, respectively). Such remarkable performance achieved from such an ultrasimplified structure opens a new path toward low-cost commercialization.

High-efficiency and superior color-stability white phosphorescent organic light-emitting diodes based on double mixed-host emission layers

We demonstrate high-efficiency and superior color-stability white phosphorescent organic light-emitting diodes based on double blue mixed-host emission layers (EMLs) with different mixed ratios. The key feature of the concept is to introduce double blue mixed-host EMLs with an orange ultrathin layer sandwiched between them. The improved white device without spacer or interlayer achieves superior color-stability and reduced efficiency roll-off, which are consistent with the good ambipolar conductivity of the mixed-host layer. Moreover, peak efficiency of 40.8 lm/W and low turn-on voltage of 2.71 V are realized. The double mixed-host EMLs concept proves to be quite useful in achieving excellent device performance.

Design of C^NN type iridium(iii) complexes towards short-wavelength emission for high efficiency organic light-emitting diodes

The first blue-emitting C^NN based Ir-complex was synthesized and applied to PhOLEDs, with efficiencies of 20.1% and 41.6 cd A−1.

Solution-processed nickel oxide nanoparticles with NiOOH for hole injection layers of high-efficiency organic light-emitting diodes.

Nickel oxide (NiOx) nanoparticles (NPs) were synthesized by a solution-based method and NP films were used as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). To evaluate the hole injection functionality of the NiOx NP HIL, we compared the performance of OLEDs with three types of HILs: spin-coated PEDOT:PSS, thermally evaporated HAT-CN, and spin-coated NiOx NP films. The considerably high component ratio of NiOOH on the air-annealed NiOx NP film surface results in an enhanced hole injection functionality even without UV-ozone treatment. Consequently, the OLEDs using the NiOx NP HILs show significantly higher performances than those of the OLEDs using PEDOT:PSS along with a more than doubled lifetime. Moreover, the OLEDs using the NiOx NP layers show higher external quantum efficiency (EQE), and current and power efficiency values than those of the OLEDs using HAT-CN at a high luminance level. Most notably, the device shows considerably higher current and power efficiency values than those of the recently reported state-of-the-art OLEDs using other types of metal-oxide or metal-based HILs.

High-Efficiency Dicyanobenzene-Based Organic Light-Emitting Diodes Exhibiting Thermally Activated Delayed Fluorescence

We have designed novel thermally activated delayed fluorescence (TADF) materials, 2DMACPN and 2DMACTPN, with 9,9-dimethyl-9,10-dihydroacridine (DMAC) as an electron donor and dicyanobenzene as an electron acceptor. We obtain the zero-zero transition energies of the first excited singlet (S1) and first triplet excited (T1) states of TADF materials by performing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations on the ground state using a dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. The calculated ΔEST values of 2DMACPN (0.019 eV) and 2DMACTPN (0.023 eV) were smaller than those of 2CzPN (0.363eV) and 2CzTPN (0.178eV) because of the large dihedral angles between the plane of DMAC and the connected phenyl rings of PN and TPN. We show that 2DMACPN would have the highest TADF efficiency among the four compounds because it has the largest dihedral angle, which creates a small spatial overlap between the HOMO and the LUMO, and consequently the smallest ΔEST.

High-Efficiency Bottom-Emitting Organic Light-Emitting Diodes with Double Aluminum as Electrodes**Supported by the Nanjing University of Telecommunications and Posts under Grant Nos NY212010 and NY212034, the National Natural Science Foundation of China under Grant Nos 91233117 and 51333007, the Natural Science Fund in Jiangsu Province under Grant No BK2012834, the National Basic Research Program of China under Grant No 2015CB932200, and

Bottom-emitting organic light-emitting diodes (BOLEDs), using Al/MoO3 as the semitransparent anode and LiF/Al as the reflective cathode and Alq3 as the emitter, are fabricated. At the same time, the performance improvement of the BOLEDs having a capping layer inserted between the semitransparent anode and the glass substrate is studied. The optimized microcavity BOLED shows a current efficiency (5.49 cd/A) enhancement of 10% compared with a conventional BOLED based on ITO (5.0 cd/A). Slight color variation is observed in 120° forward viewing angle with 50 nm BCP as the capping layer. Strong dependence of efficiency on Al anode thickness and the thickness and refractor index of the capping layer is explained. The results indicate that the BOLEDs with the double-aluminum electrode have potential practical applications.