Efficient Phosphorescent OLEDs Research Articles

Narrow-emitting pyridoimidazole functionalized N-heterocyclic carbene based tetradentate Pt(II) complexes for blue phosphorescent organic light-emitting diodes

Two novel classes of N-heterocyclic carbene (NHC) based tetradentate Pt(II) complexes possessing bulky benzyl groups and pyridoimidazole ligands were designed and synthesized to produce efficient blue phosphorescent organic light-emitting diodes (PHOLEDs). Two blue Pt(II) complexes are reported, namely, platinum (II) [2-(3-(1-(4-(tert-butyl)benzyl)-1H-imidazo [4,5-b]pyridin-3(2H)-yl)phenoxy)-9-(pyridin-2-yl)-9H-carbazole] (Pt(t-BuBnOCzPy)) and platinum (II) [3-(3-(1-benzyl-1H-imidazo [4,5-b]pyridin-3(2H)-yl)phenoxy)-9-(4-(tert-butyl)pyridin-2-yl)-9H-carbazole] (Pt(BnOCz4t-BuPy)), respectively. Notably, pyridoimidazole and bulky benzyl groups efficiently reduced steric hindrance, vibrational peaks, and intermolecular interactions. The photophysical and electrochemical properties of the two Pt(II) complexes were analytically examined experimentally and theoretically. The complexes exhibited blue electroluminescence (EL) emission at 466 nm with a small full-width half maximum (FWHM) of 26 nm and completely suppressed vibrational emission peaks. Blue PHOLEDs, at 5 wt% Pt(t-BuBnOCzPy) and Pt(BnOCz4t-BuPy) doping ratios exhibited maximum external quantum efficiencies (EQEmax) of 19 and 18.6 %, maximum power efficiencies (PEmax) of 25.5 and 24.9 cd m−2, and maximum current efficiencies (CEmax) of 24.3 and 24.3 cd m−2 with Commission International de I'Eclairage (CIEx,y) color coordinates of (0.13, 0.17) and (0.13, 0.17), respectively. These superior results suggest that the designed benzylated NHC pyridoimidazole tetradentate Pt(II) complexes provide a novel means of reducing steric hindrance and intermolecular interactions.

Novel bipolar hosts for highly efficient green phosphorescent organic light-emitting diodes with slow efficiency roll-off

Organic light-emitting diodes (OLED) based on phosphorescent emitter promise 100 % internal quantum efficiency (IQE) theoretically, showing extensive applications in various fields. The guest phosphors are usually doped into appropriate host matrices to minimize unnecessary excitons quenching. Herein, three novel bipolar host materials (H1–H3) were developed. Pyridine or pyrimidine was selected as the electron-transporting n-type unit and indolocarbazole as hole-transporting p-type unit. Dibenzofuran was grafted onto the central pyridine or pyrimidine to enhance the electron-transporting ability based on its electron-deficient feature. Furthermore, dibenzofuran was also useful to enhance the thermal and amorphous stabilities of the generated host materials. These host materials are characterized by high glass transition temperatures over 150 °C and suitable energy levels. Thanks to strengthened microcavity effect, the top-emitting green phosphorescent OLEDs containing these hosts in emitting layer realized quite narrow spectra with full width at half maxima (FWHM) of ≤27 nm. Low turn on voltage of 2.01 V, high external quantum efficiency (EQE) up to 41.1 % and pretty stable emission color with ΔCIEy≤0.002 were achieved for these green devices. Moreover, these devices are characterized by extraordinary efficiency stability. A high EQE of 34.9 % was still remained at extremely high brightness of 10000 cd/m2, corresponding to an efficiency roll-off of less than 15 %. These host materials may find practical applications in highly efficient and stable OLED displays.

Highly efficient phosphorescent organic light-emitting diodes with low turn-on voltages using N-phenylcarbazole/pyrimidine-based bipolar host materials

Herein, three novel bipolar host materials with D-π-A structure, termed 2CzPm, 3CzPm, and 9CzPm, were successfully synthesized by incorporating a pyrimidine moiety as an accepter and N-phenylcarbazole group as a donor through different connection sites. The three hosts exhibited good thermal stability with a glass transition temperature (Tg) of 104, 103, and 102 °C, respectively. They also possessed high enough triplet energy level (ET = 2.58–2.75 eV) to be efficient hosts for red and green phosphorescent organic light-emitting diodes (PhOLEDs) (ETdopants <2.5 eV). Moreover, considerable hole and electron current density in the conventional voltage range were observed, suggesting their good bipolar transport properties. When utilized in green and red PhOLEDs, the devices hosted by 9CzPm exhibited the best electroluminescence (EL) performance with low turn-on voltages (Von) of 2.6 and 2.5 V, and maximum external quantum efficiency (EQEmax) of 21.79% and 25.03%, respectively, indicating that pyrimidine moiety is a promising building block for bipolar host materials.

New ionic iridium(III) complexes based on methoxyl-modified 1-phenylisoquinoline for solution-processed red organic light-emitting diodes

Three transition-metal phosphorescent iridium(Ⅲ) complexes (Ir1, Ir2, and Ir3) are designed and synthesized with 1-phenylisoquinolines modified by methoxy as the main ligands and o-phenanthroline as the auxiliary ligands. The introduction of different amounts of methoxy can change the luminescence color of the complexes, and Ir1, Ir2, and Ir3 show orange, dark red, and red phosphorescence under the excitation of 365 nm UV lamp, peaking at 588, 648, and 622 nm, respectively. Ionic complexes are conducive to the efficient solution-processed phosphorescent red organic light-emitting diodes (OLEDs). Among the three devices, the Ir2-based device achieved a relatively high external quantum efficiency, current efficiency, and power efficiency of 1.12 %, 0.75 cd/A, and 0.37 lm/W, respectively. These results indicated that polymethoxy-based ionic Ir(III) complexes have great potential for the fabrication of red OLEDs with wet method.

Highly Efficient Phosphorescent Organic Light-Emitting Diodes Using Benzo[4',5']imidazo[2',1':2,3]imidazo[4,5,1-jk]carbazole as a Rigid and Effective Electron Acceptor in Bipolar Host.

A novel bipolar host architecture was investigated to improve the external quantum efficiency (EQE) of green phosphorescent organic light-emitting diodes (PhOLEDs). The host was developed by incorporating carbazole as a hole-transport unit and fused rigid benzo[4',5']imidazo[2',1':2,3]imidazo[4, 5, 1-jk]carbazole (BzICz) as a new electron transport unit. The primary goal of the BzICz-based host design was to achieve a high triplet energy and bipolar charge transport characteristics. The green PhOLEDs fabricated using the new BzICz and carbazole-based host demonstrated a high EQE of 26.6% due to their high triplet energy and good bipolar charge transporting characteristics.

Low-trigger voltage and efficient phosphorescent organic light-emitting diodes with bipolar TADF host

Thermally activated delayed fluorescence (TADF) materials have continued to attract interest due to their rapid improvements in efficiency, non-toxic and potential to be manufactured at energy-saving organic light-emitting diode for full-color display and solid-state lighting. Here, The bipolar and electroluminescence (EL) characteristics of TADF host, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), were investigated. By incorporating the phosphorescent tris[2-(p-tolyl)pyridine]iridium(III) [Ir(mppy)3] dopant into the matrix, DPDDC showed more excellent EL performance compared with the typical 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. The turn-on voltage for phosphorescent organic light-emitting diodes (PhOLEDs) was decreased to 2.2 V from 2.5 V by exploiting the TADF host instead of CBP. Crucially, we found that the ratio of Ir(mppy)3 in TADF host plays a significant role in EL process. Remarkable low efficiency roll-off was achieved with above 90% of the current efficiency maintained even at a brightness of 3000 cd m−2, along with high power efficiency of 84.8 lm·W−1 and the external quantum efficiency of 20.6% for DPDDC: 5 wt% Ir(mppy)3-based device.

A symmetrical carbazole/benzonitrile-based high triplet energy bipolar host designed by a para-linkage strategy for efficient blue and green phosphorescent organic light-emitting diodes

Based on a para-linkage concept, a symmetrical carbazole-benzonitrile derivative of 4-(9H-carbazol-9-yl)-3,5-diphenyl-2,6-difluoronitrile (CzPhFBN) was designed and prepared as a host for phosphorescent organic light-emitting diodes (PhOLEDs). In the CzPhFBN molecular, a cyano (CN) unit and a carbazole (Cz) unit were attached to the central phenyl ring at the para positions. Simultaneously, the Cz group was surrounded by two phenyl units to increase structure distortion for high triplet energy and CN unit was surrounded by two F units to improve electron transporting behavior. The photophysical properties and carrier transport behavior of CzPhFBN were investigated. The results demonstrated that CzPhFBN offered a high triplet energy of exceeding 3.0 eV and bipolar carrier transporting property. By incorporating CzPhFBN as the host, the blue FIrpic- and green Ir(ppy)3-based PhOLEDs with a common device configuration delivered the maximum efficiencies of 11.1%/19.7 cd A−1/12.1 lm W−1 and/13.1%/43.6 cd A−1/28.6 lm W−1, respectively. These results indicate that this study provides a potential design strategy of high triplet energy bipolar host materials with a simple and elegant structure.

Asymmetric Decoration of a meta-Linked Bitriazine-Based Host for Highly Efficient and Stable Blue Phosphorescent Organic Light-Emitting Diodes

High triplet energy hosts for blue phosphorescent organic light-emitting diodes were developed by decorating a meta-linked bitriazine core with carbazole and tetraphenylsilyl functional groups. A symmetric host with two carbazole units as the two triazine units of the core and an asymmetric host with one carbazole unit and one tetraphenylsilyl unit as the two triazine units were prepared. The triplet energy of these two hosts was 2.97 eV, suitable for triplet exciton harvesting of blue phosphors. Comparing the two host designs, the asymmetric decoration of the two triazine units with carbazole and tetraphenylsilyl units was superior to the symmetric decoration of the two triazine units with two carbazoles in terms of high external quantum efficiency (EQE) and long-term device stability. A high EQE of 19.7% and a long device lifetime of 2093.6 h at 100 cd m-2 were achieved using the asymmetrical host.

A new multifunctional fluorescent molecule for highly efficient non-doped deep-blue electro-fluorescence with high color-purity and efficient phosphorescent OLEDs

A multifunctional molecule DTPCZTZ was synthesized, which can not only realize high efficiency nondoped deep blue device, but also achieve high efficiency phosphorescent devices.

Highly efficient single emitter white phosphorescent organic light-emitting diodes based on Pt(ii) emitters

We fabricate highly efficient white platinum(ii) emitters with high CRI for phosphorescent OLEDs using new tetradentate ligands prepared with difluoropyrido-pyridyl and meta-xylyl amine linkers.

Highly Efficient Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Based on Co-Dopant and Co-Host System.

The low-lying HOMO level of the blue emitter and the interfacial miscibility of organic materials result in inferior hole injection, and long exciton lifetime leads to triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), so the efficiencies of blue phosphorescent organic light-emitting diodes (PhOLEDs) are still unsatisfactory. Herein, we design co-host and co-dopant structures to improve the efficiency of blue PhOLEDs by means of solution processing. TcTa acts as hole transport ladder due to its high-lying HOMO level, and bipolar mCPPO1 helps to balance carriers’ distribution and weaken TPA. Besides the efficient FIr6, which acts as the dominant blue dopant, FCNIrPic was introduced as the second dopant, whose higher HOMO level accelerates hole injection and high triplet energy facilitates energy transfer. An interesting phenomenon caused by microcavity effect between anode and cathode was observed. With increasing thickness of ETL, peak position of electroluminescence (EL) spectrum red shifts gradually. Once the thickness of ETL exceeded 140 nm, emission peak blue-shifts went back to its original position. Finally, the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of blue phosphorescent organic light-emitting diode (PhOLED) went up to 20.47 cd/A, 11.96 lm/W, and 11.62%, respectively.

A simple bipolar host material based on triphenylamine and pyridine featuring σ-linkage for efficient solution-processed phosphorescent organic light-emitting diodes

Solution-processable host materials play an important role in achieving highly efficient and cost-effective phosphorescent organic light-emitting diodes (PhOLEDs). This paper designed and prepared a simple solution-processable host material of TPA-PyF2 based on triphenylamine and pyridine to fabricate efficient solution-processed PhOLEDs. TPA-PyF2 featured an σ-linkage between the electron donor and acceptor units, which not only served to limit the conjugation length of the molecule and maintain a high triplet energy level, but also endowed the host molecule with a three-dimensional structure to enable high film morphological stability for solution processing. As a result, TPA-PyF2 exhibited a high triplet energy level (2.82 eV), high thermal stability, bipolar charge transfer, and excellent film morphology. Blue solution-processed OLED with a simple double-layer structure based on TPA-PyF2 host achieved a maximum current efficiency, power efficiency, and luminance of 17.2 cd/A, 7.1 lm/W, and 11519 cd/m2, respectively, with a small efficiency roll-off of only 9.1% at the practical luminance of 1000 cd/m2. This work reveals that the σ-linkage strategy is promising in designing efficient solution-processable host materials for high-efficiency PhOLEDs.

Solution-processed sky-blue phosphorescent organic light-emitting diodes based on 2-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene-8-yl)-4-(trimethylsilyl)pyridine chelated iridium complex

ABSTRACT A new 2-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene-8-yl)-4-(trimethylsilyl)pyridine-based iridium complex was synthesized for efficient solution-processed sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). The effect of dioxaboranaphthoanthracene instead of phenyl with the electron-accepting group, as well as the bulky pyridine with the trimethylsilyl group, on the ligand was investigated. The new dopant was found to have an extremely high photoluminescence quantum yield of 94% when doped in an emissive layer. As a result, the solution-processed blue PhOLED consisting of a simple structure without any interlayer exhibited remarkable light-emitting performance with an external quantum efficiency of 8.93% and a current efficiency of 23.56 cd/A.

Mechanism of Ir(ppy)3 Guest Exciton Formation with the Exciplex-Forming TCTA:TPBI Cohost within a Phosphorescent Organic Light-Emitting Diode Environment

Cohosts based on hole transporting and electron transporting materials often act as exciplexes in the form of intermolecular charge transfer complexes. Indeed, exciplex-forming cohosts have been widely developed as the host materials for efficient phosphorescent organic light-emitting diodes (OLEDs). In host–guest systems of OLEDs, the guest can be excited by two competing mechanisms, namely, excitation energy transfer (EET) and charge transfer (CT). Experimentally, it has been reported that the EET mechanism is dominant and the excitons are primarily formed in the host first and then transferred to the guest in phosphorescent OLEDs based on exciplex-forming cohosts. With this, exciplex-forming cohosts are widely employed for avoiding the formation of trapped charge carriers in the phosphorescent guest. However, theoretical studies are still lacking toward elucidating the relative importance between EET and CT processes in exciting the guest molecules in such systems. Here, we obtain the kinetics of guest excitation processes in a few trimer model systems consisting of an exciplex-forming cohost pair and a phosphorescent guest. We adopt the Förster resonance energy transfer (FRET) rate constants for the electronic transitions between excited states toward solving kinetic master equations. The input parameters for calculating the FRET rate constants are obtained from density functional theory (DFT) and time-dependent DFT. The results show that while the EET mechanism is important, the CT mechanism may still play a significant role in guest excitations. In fact, the relative importance of CT over EET depends strongly on the location of the guest molecule relative to the cohost pair. This is understandable as both the coupling for EET and the interaction energy for CT are strongly influenced by the geometric constraints. Understanding the energy transfer pathways from the exciplex state of cohost to the emissive state of guest may provide insights for improving exciplex-forming materials adopted in OLEDs.

Electroplex hosts for highly efficient phosphorescent organic light-emitting diodes with extremely small efficiency roll-offs

• Four bipolar molecules were designed and synthesized for accessing exciplex and electroplex host systems. • Highly efficient green PhOLED with EQE max of 29.9% was achieved. • Extremely small efficiency roll-offs (1.3% at 1,000 cd m −2 ; 5.4% at 5,000 cd m −2 ) were obtained. Host materials are of great significance to achieve high-performance host–guest organic light-emitting diodes (OLEDs) through restraining the exciton annihilation in the emissive layer. Herein, four bipolar molecules bearing benzothienocarbazole ( BTCz )/benzothienocarboline ( BTCb ) and benzimidazole ( Bi ) moieties were designed and synthesized. Using them as hosts, green phosphorescent OLEDs (PhOLEDs) with maximum current efficiency (CE max ) of 90.8 cd A −1 , power efficiency (PE max ) of 76.5 lm W −1 , and external quantum efficiency (EQE max ) of 27.9% were obtained. More importantly, it was found that these four molecules could be used as electron-donating materials to interact with electron-accepting triazine derivatives to form electroplexes. Employing these electroplexes as hosts, highly efficient green PhOLEDs with CE max of 104.4 cd A −1 , PE max of 85.1 lm W −1 , EQE max of 29.9%, and extremely small efficiency roll-offs (EQE = 29.5%, 1.3% roll-off at 1,000 cd m −2 ; EQE = 28.3%, 5.4% roll-off at 5,000 cd m −2 ) were achieved. The excellent device performances fully exhibit the promising potential of electroplexes as hosts for high-performing OLEDs.

Deep-Red and Near-Infrared Iridium Complexes with Fine-Tuned Emission Colors by Adjusting Trifluoromethyl Substitution on Cyclometalated Ligands Combined with Matched Ancillary Ligands for Highly Efficient Phosphorescent Organic Light-Emitting Diodes.

Six novel Ir(C^N)2(L^X)-type heteroleptic iridium complexes with deep-red and near-infrared region (NIR)-emitting coverage were constructed through the cross matching of various cyclometalating (C^N) and ancillary (LX) ligands. Here, three novel C^N ligands were designed by introducing the electron-withdrawing group CF3 on the ortho (o-), meta (m-), and para (p-) positions of the phenyl ring in the 1-phenylisoquinoline (piq) group, which were combined with two electron-rich LX ligands (dipba and dipg), respectively, leading to subsequent iridium complexes with gradually changing emission colors from deep red (≈660 nm) to NIR (≈700 nm). Moreover, a series of phosphorescent organic light-emitting diodes (PhOLEDs) were fabricated by employing these phosphors as dopant emitters with two doping concentrations, 5% and 10%, respectively. They exhibited efficient electroluminescence (EL) with significantly high EQE values: >15.0% for deep red light0 (λmax = 664 nm) and >4.0% for NIR cases (λmax = 704 nm) at a high luminance level of 100 cd m−2. This work not only provides a promising approach for finely tuning the emission color of red phosphors via the easily accessible molecular design strategy, but also enables the establishment of an effective method for enriching phosphorescent-emitting molecules for practical applications, especially in the deep-red and near-infrared region (NIR).

Highly efficient blue and white phosphorescent organic light-emitting diodes with low-efficiency roll-off utilizing thermally activated delayed fluorescent-based co-host architecture

Highly efficient blue phosphorescent organic light-emitting diodes (PhOLEDs) with reduced efficiency roll-off are still challenging. A blue phosphor of (cyanato-κO)[3,5-difluoro-2-(2-pyridinyl-κN)phenyl-κC][[1,1'-(1,3-phenylene-κC2)bis[3-butyl-2,3-dihydro-1H-imidazolato-κC2]](5-)]-iridium complex is utilized as the blue emitter to fabricate highly efficient blue PhOLEDs. Especially, it is first utilized for white PhOLEDs with superior device performance. The realized blue PhOLED employing a novel TADF co-host system achieves maximum power efficiency (PE) of 43.3 lm/W and the PE still remains 26.3 lm/W at 1000 cd/m2, which are 1.9 and 2.5 times higher than that of blue PhOLED using conventional phosphor single-host structure. Furthermore, the current efficiency (CE) roll-off of the blue PhOLED with TADF co-host architecture is only 2.2% at 1000 cd/m2, which can be assigned to the balanced charge carriers injection and transport. The final white device obtained the maximum CE of 46.1 cd/A, PE of 48.1 lm/W, and a low roll-off of 1.9% at 1000 cd/m2. We also systemically investigate the energy transfer mechanism and exciton distribution of the devices. It was found that our proposed device structure could effectively promote the Förster energy transfer mechanism and the even distribution of excitons. We firmly believe that our work may bring light to the future development of highly efficient PhOLEDs with simultaneous low roll-off.

Computational Protocol to Calculate the Phosphorescence Energy of Pt(II) Complexes: Is the Lowest Triplet Excited State Always Involved in Emission? A Comprehensive Benchmark Study.

The reliable calculation of phosphorescence energies of phosphor materials is at the core of designing efficient phosphorescent organic light-emitting diodes (PhOLEDs). Therefore, it is of paramount importance to have a robust computational protocol to perform those calculations in a black-box manner. In this work, we use Domain-Based Local Pair Natural Orbital Coupled Cluster theory with single, double, and perturbative triple excitation (DLPNO-CCSD(T)) calculations to attain the phosphorescence energies of a large pool of Pt(II) complexes. Several approaches to incorporate relativistic effects in our calculations were tested. In addition, we have used the DLPNO-CCSD(T) values (i.e., our best theoretical values) to assess the performance of different flavors of density functional theory including pure, hybrid, meta-hybrid, and range-separated functionals. Among the tested functionals, the M06HF functional provides the best values compared with the DLPNO-CCSD(T) ones, with a mean absolute deviation (MAD) value of 0.14 eV. In its turn, and thanks to the increased accuracy achieved in the calculation of phosphorescence energies, we also demonstrate that not all of the investigated complexes emit from their lowest-lying triplet state (T1). The outlier complexes include different complex photophysical scenarios and both Kasha and anti-Kasha types of complexes. Finally, we provide a general computational protocol to pre-screen whether T1 is actually the emissive state and to accurately calculate the phosphorescence energies of Pt(II) complexes.

Highly efficient blue phosphorescent organic light-emitting diodes with low operation voltage

In this work, we report an efficient blue organic electroluminescent device based on phosphorescent emitter iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which displayed excellent thermal stability. To balance carriers’ distribution and reduce operation voltage, double light-emitting layers device structure was designed by selecting 4,4′,4′-tris(N-carbazolyl)triphenylamine (TcTa) and 2,6-bis(3-(9Hcarbazol-9-yl)phenyl)phyridine (26DCzPPy) as host materials because of their stepwise energy levels. By optimizing the doping concentration of FIr6 to be 26 wt%, well balanced carriers’ distribution on emitter molecules and broadening recombination zone were realized. Finally, high performance blue device with slow efficiency roll-off and high current efficiency up to 56.83 cd/A (obtained at 700 cd/m2) was realized. At the certain brightness of 1000 (3.9 V) and 5000 cd/m2 (4.8 V), the same device retained efficiencies of 56.33 and 51.45 cd/A, respectively.

Novel difluorenyl substituted 1,3,5-triazine and carbazole based bipolar host materials with high thermal stability for efficient green phosphorescent organic light-emitting diodes (PhOLEDs)

In this work, two novel compounds 9-(4'-(4,6-bis(9,9-dimethyl-9H-fluoren-2-yl)-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-4-yl)-9H-carbazole (TPBDF-Cz1) and 3-(4-(4,6-bis(9,9-dimethyl-9H-fluoren-2-yl)-1,3,5-triazin-2-yl)phenyl)-9-phenyl-9H-carbazole (TPBDF-Cz2) were synthesized and fully characterized by taking advantage of carbazole, 1,3,5-triazine and fluorene groups. Both materials exhibit extremely excellent thermal properties, appropriate HOMO and LUMO energy levels and high triplet energy. With the beneficial physical behaviors through measuring their optical and electrochemical properties, green PhOLEDs with the two materials acting as host were designed and fabricated. Both devices reveal a low turn-on voltage of 3.0 V and good electroluminescence performance owing to the balanced hole and electron transport mobilities. Besides, it was found that the TPBDF-Cz1-based device is slightly more efficient than the TPBDF-Cz2-based one because of higher carrier transport ability of TPBDF-Cz1. The relevant results indicate TPBDF-Cz1 opens up a new perspective for designing hosts for high performance PhOLEDs.