Phosphorescent Organic Light Research Articles

Effect of β-Ketoiminato Ancillary Ligand Modification on Emissive Properties of New Iridium Complexes.

A series of new bis(benzo[h]quinolinato) Ir(III) complexes with modified β-ketoiminato ancillary ligands were synthesized, and their electrochemical, photophysical properties were determined with the support of theoretical calculations. Moreover, all the synthesized heteroleptic Ir(III) complexes were examined as dopants of the host-guest type emissive layers in solution-processed phosphorescent organic light emitting diodes (PhOLEDs) of a simple structure. As expected on the basis of voltammetry measurements as well as DFT calculations, all the compounds appeared to be green emitters. Their examination showed that alteration of β-ketoiminato ligand structure causes frontier orbitals' energy levels to be slightly changed, while significantly affecting photoluminescence and electroluminescence efficiencies of iridium phosphors containing these ligands. It was also found that modification of ancillary ligands might enhance charge trapping on the dopant, thus increasing its efficiency, especially in electroluminescence. From among the iridium complexes studied, the compound bearing 1-naphthyl group bonded to the nitrogen atom of the ancillary ligand proved to be the most efficient emitter. The PhOLED fabricated on the basis of this dopant has reached a luminance level of 16000 cd/m2, current efficiency close to 12 cd/A, and an external quantum efficiency around 3.2%.

Inverted organic light-emitting devices using a charge-generation unit as an electron injector

Abstract Efficient green fluorescent and phosphorescent inverted organic light emitting devices (IOLEDs) with a non-doped charge generation unit (CGU) of NPB/MoO3/Al/LiF as an electron injector have been developed. It is found that the electrons are generated effectively at the NPB/MoO3 interface and injected from MoO3 to the electron transporting layer via assistant electron injection layers of Al/LiF. More importantly, the performance of the devices shows insensitivity to the work function of the used cathode. Under the optimal structure of NPB(10 nm)/MoO3(5 nm)/Al(3 nm)/LiF(1 nm), the green fluorescent IOLEDs present a maximum current efficiency (CEmax) of 3.5 cd/A. Furthermore, the green phosphorescent IOLEDs based on this CGU show a turn-on voltage of 3.8 V, a CEmax of 54.6 cd/A and a maximum external quantum efficiency of 15.8%. The results suggest that the introduction of non-doped CGU in IOLEDs provides a promising strategy to integrate IOLEDs with the n-channel thin film transistors for applications in active-matrix OLEDs.

Triplet energy boosting ternary exciplex host for improved efficiency in deep blue phosphorescent organic light emitting diode

Abstract A high triplet energy exciplex host derived from a ternary mixture of hosts was developed for efficient energy transfer to a deep blue phosphor by suppressing reverse energy transfer. In the thermally activated delayed fluorescent type exciplex host of 4,4′-bis(carbazol-9-yl)-2,2′-dimethylbiphenyl (CDBP) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), the (oxybis(2,1-phenylene))bis(diphenylphosphine oxide) (DPEPO) was added as a third component to increase the triplet energy of the exciplex host. The addition of DPEPO in the CDBP:PO-T2T exciplex host increased the triplet energy of the exciplex from 2.88 eV to 2.97 eV, and the photoluminescence quantum yield of the deep blue phosphor doped exciplex from 0.77 to 0.86. The triplet energy boosting function of the DPEPO improved the maximum external quantum efficiency of the deep blue phosphorescent organic light-emitting diode from 12.8% to 16.6% at a DPEPO content of 70%.

Optimization of Transparent Organic Light-Emitting Diodes by Simulation-Based Design of Organic Capping Layers.

In this work, we use the transfer matrix method to optimize TPBi capping layers deposited on organic light emitting diodes with respect to light extraction and transmittance. The green transparent organic light emitting diodes comprise three organic semiconductors (CBP, Ir(ppy)₃ and TPBi) forming an efficient simplified phosphorescent organic light emitting diode stack. A transparent cathode of 2 nm Cs₂CO₃, 2 nm Al and 16 nm Au is deposited by thermal evaporation. The diode stack as well as the capping layer are deposited by organic vapor phase deposition. The refractive indices and extinction coefficients of all materials in the transparent organic light emitting diodes (glass, indium tin oxide, organic semiconductors and cathode) are determined using spectroscopic ellipsometry combined with optical transmittance and reflectance measurements. With these spectrally resolved data, we calculate the transmittance of transparent organic light emitting diodes with TPBi capping layers of different thicknesses. The results were validated with high accuracy in the visible spectral range and beyond (360 nm-1000 nm) by a series of experiments. By choosing a TPBi capping layer of optimized thickness (here 50 nm), we fabricated transparent organic light emitting diodes with an optical transmittance which was strongly enhanced from 47% (reference without capping layer) to 65%, measured at 555 nm.

Design of Phosphorescent Organic Light Emitting Diodes (OLED) Host Materials Based on Carbazole: Quantum Chemistry Study on Carrier Transport Properties of Oxazole Derivatives

Design of Phosphorescent Organic Light Emitting Diodes (OLED) Host Materials Based on Carbazole: Quantum Chemistry Study on Carrier Transport Properties of Oxazole Derivatives

Bicarbazole/nitrogen heterocycle based bipolar host materials for efficient green phosphorescent organic light-emitting diodes

In this paper, three novel bipolar host materials, 9,9′-Diphenyl-6-(4-pyridinyl)-3,3′-bicarbazole(BCz4Py), 9,9′-Diphenyl-6-(3-pyridinyl)-3,3′-bicarbazole(BCz3Py) and 9,9′-Diphenyl-6-(5-pyrimidinyl)-3,3′-bicarbazole(BCz5Pm), were synthesized by incorporation of bicarbazole as donor with pyridine/pyridimide based N-hexahydroxy nitric heterocycle as acceptor units. The photophysical, thermal and electrochemical properties of these three materials were fully investigated. These compounds all exhibited superior thermal stability with high glass-transition temperature (Tg > 130 °C) and decomposition temperature (Td > 400 °C). Appropriate HOMO (highest occupied molecular orbitial) -LUMO (lowest unoccupied molecular orbital) levels and triplet energy (2.74–2.75 eV) of these compounds make them suitable hosts for green iridium pohosphor Ir(ppy)3 in phosphorescent organic light emitting devices (PhOLEDs). Among them, the BCz3Py hosted device exhibited outstanding performance with maximum external quantum efficiency (EQE) of 14.9%, maximum current efficiency (CE) of 49.9 cd/A, and maximum power efficiency (PE) of 29.1 lm/W, suggesting that BCz3Py is a promising bipolar material for high-efficiency green PhOLEDs.

Three new carbazole derivatives with high thermal stability as host for efficient green phosphorescent organic-light emitting diodes

Three host materials, DCzPh, DCzPy and DCzPm, were developed for phosphorescent organic light emitting devices (PhOLEDs). These three compounds exhibit high triplet energy level (ET > 2.7 eV), suitable glass transition temperatures (Tg > 130 °C), appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, and balanced charge-transporting properties. Green PhOLEDs were fabricated by utilizing these three materials as hosts and the efficiencies were satisfactory. Device G3 based on DCzPm exhibited high efficiency with the maximum external quantum efficiency (EQE) of 17.2%, and even at the brightness of 1000 cd m−2, the EQE reached 17.1%, demonstrating its low efficiency roll-off. The above results indicate that these host materials possess great commercial potential in OLED applications.

12‐1: Analysis of Key Factors Affecting the Lifetime of Blue Phosphorescent OLED Using CN Modified Blue Host Materials

Three host materials were synthesized for blue phosphorescent organic light emitting diodes (OLEDs) by combining carbazolylcarbazole and benzonitrile. Each material has different CN positions relative to the carbazolylcarbazole donor as ortho,‐meta, and para. These materials were used for finding fundamental factors of lifetime degradation of blue OLEDs. By comparing the characteristics of the three hosts with the CN unit at ortho, meta and para positions, we studied the effects of the CN position on the lifetime of the blue phosphorescent OLEDs. We calculated bond dissociation energy of the three hosts and energy transfer to find key factors about degradation. We calculated Forster energy transfer rate from host to dopant by measuring parameters for the calculation. The correlation of the device lifetime with the parameters revealed that the bond dissociation energy and Forster energy transfer rate are critical to the device lifetime of the blue phosphorescent OLEDs.

Improved carrier injection and balance in solution-processed blue phosphorescent organic light emitting diodes based on mixed host system and their transient electroluminescence

Solution processed blue phosphorescent organic devices based on mixed host system were fabricated in this paper. A set of blue phosphorescent organic light emitting diodes (PhOLEDs) utilizing two small molecular materials of 3,3-Di(9H-carbazol-9-yl) biphenyl (mCBP) and 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) as mixed host doped with phosphor bis[(4,6-difluorophenyl)-pyridina-to-N,C2](picolinate) iridium(III) (FIrpic) are fabricated. The results show that the performance of mixed host system devices is improved compared with two single host devices. The max luminance of 23,340 cd/m2 is achieved in the mCBP:TCTA (2:1) mixed host device while the 14,400 cd/m2 in mCBP single host devices and the 12,968 cd/m2 in TCTA, and the current efficiency is improved from 8.17 cd/A(mCBP), 9.61 cd/A (TCTA) to 13.62 cd/A at the luminance of 1000 cd/m2. The transient electroluminescence measurement was utilized to detect the carrier injection between two kinds of PhOLEDs and penetrate how the trap charges works in the mixed host devices. This presents that the mixed host system contributes to the hole injection and the equilibrium of the carrier transport, reduces trapped charges, thus brings about high efficiency devices.

Triphenylamine-functionalized iridium(III) complexes for near-infrared phosphorescent organic light emitting diodes

With the aim of enhancing the efficiency of near infrared (NIR) organic light-emitting devices (OLEDs), two new triphenylamine-functionalized NIR iridium(III) complexes by covalently attaching the functional moieties (tBu and F) complexes (tBuTPA-BTz-Iq)2Irpic and (FTPA-BTz-Iq)2Irpic were designed and synthesized (TPA = triphenylamine; BTz = benzotriazole; Iq = isoquinoline). It was found that compared to their parent iridium complex (TPA-BTz-Iq)2Irpic, the addition of either butyl or fluorine groups into the ligand play an important role in improving the optophysical and electroluminescent characteristics. In solution-processed phosphorescent OLEDs, the (tBuTPA-BTz-Iq)2Irpic-based devices achieved a maximum external quantum efficiency (EQEmax) of 0.66% at 716 nm, while the (FTPA-BTz-Iq)2Irpic-based devices achieved an EQEmax of 0.48% at 710 nm. The EQEmax level is 2.28 or 1.65 times higher than that in the (TPA-BTz-Iq)2Irpic-based devices (0.29% at 712 nm), respectively.

Rapid Multiscale Computational Screening for OLED Host Materials.

The design of new host materials for phosphorescent organic light emitting diodes (OLEDs) is challenging because several physical property requirements must be met simultaneously. A triplet energy ( ET) higher than that of the chosen emitting dopant, appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels, good charge carrier transport, and high stability are all required. Here, computational methods were used to screen structures to find the most promising candidates for OLED hosts. The screening was carried out in three Tiers. The Tier 1 selection, based on density functional theory calculations, identified a set of eight molecular structures with ET > 2.9 eV, suitable for hosting blue phosphorescent dopants such as iridium(III)bis((4,6-di-fluorophenyl)-pyridinato-N,C2')picolinate. Phenanthro[9,10- d]imidazole was chosen as the starting point for the Tier 2 selection. Thirty-seven unique molecular structures were enumerated by isoelectronic nitrogen transmutation of up to two CH fragments of the phenanthrene. Three molecules, that is, imidazo[4,5- f]-phenanthrolines with nitrogens at the 1,10-, 3,8-, and 4,7-positions, were selected for Tier 3, which involved the use of molecular dynamics simulations and electron coupling calculations to predict differences in charge transport between the three materials. The three were explored experimentally through synthesis and device fabrication. The singlet, triplet, and frontier orbital energies computed using single-molecule density functional theory calculations ( Tiers 1 and 2) were consistent with the experimental values in a fluid solution, and the multiscale modeling scheme ( Tier 3) correctly predicted the poor device performance of one material. We conclude that screening host materials using only single-molecule quantum mechanical data was not sufficient to predict whether a given material would make a good OLED host with certainty; however, they can be used to screen out materials that are destined to fail due to low singlet/triplet energies or a poor match of the frontier orbital energies to the dopant or transport materials.

Molecular design and synthetic approach to C2,C3,C4-modified carbazoles: high triplet energy bipolar host materials for efficient blue phosphorescent organic light emitting diodes.

Herein, we report the first examples of the design of and synthetic approach to C3/C4 and C2/C3/C4-modified carbazole based high triplet energy bipolar host materials for blue phosphorescent organic light emitting diodes and demonstrated excellent device performance with the maximum external quantum efficiency as high as 25.3% and current efficiency of 48.1 cd A-1.

New blue phosphorescence from trifluorosulfonyl-substituted iridium complexes

For color-pure phosphorescence organic light emitting diodes (PHOLEDs), we synthesized the trifluoromethyl sulfonyl substituted ligand and two new blue Iridium complexes with perfluoro-sulfonyl group (SOCF3pic and SOCF3mpic). The both iridium complexes showed similar photophysical and thermal properties while the HOMO energy leves of two Iridium complexes slightly differnet to be −5.84 eV for SOCF3pic and −5.74 eV for SOCF3mpic, respectively. The device with the SOCF3mpic dopant exhibited higher device efficiencies of 7.17 cd/A and 3.7% compared to that with the SOCF3pic dopant (4.03 cd/A and 2.78%). Two-dimensional GIXD images of scattered X-ray intensity from surface to full depth for SOCF3pic-doped and SOCF3mpic-doped TCTA:4PTPS films were characterized for preferential orientation along the out-of-plane direction.

Decoration of Dibenzofuran Using Cyanocarbazole via 6-Position as a Molecular Design Approach for High-Triplet-Energy Bipolar Host Materials.

In this study, two new dibenzofuran derivatives featuring one or two cyanocarbazole units, 6-(dibenzo[b,d]furan-4-yl)-9-phenyl-9H-carbazole-3-carbonitrile (mBFCzCN) and 6,6'-(dibenzo[b,d]furan-4,6-diyl)bis(9-phenyl-9H-carbazole-3-carbonitrile) (dBFCzCN), were developed as host materials for phosphorescent organic light emitting diodes (PhOLEDs). A new molecular design connecting the cyanocarbazole to the dibenzofuran using the cyanocarbazole 6-position instead of its 9-position was created, and the effects of number of cyanocarbazole units in the dibenzofuran building block on the photophysical and electroluminescence properties were investigated in detail. The mBFCzCN compound revealed high triplet energy (2.78 eV) than that of dBFCzCN (2.68 eV) and good bipolar charge transporting properties. The potential of these materials as hosts for blue and green PhOLEDs was investigated using bis(4,6-(difluorophenyl)pyridinato-N,C2' )picolinate iridium(III) (FIrpic) and tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3 ) dopants, respectively. The results indicated that the mBFCzCN with one cyanocarbazole unit showed better device performance than the dBFCzCN with two cyanocarbazole units in the blue and green devices. High external quantum efficiencies of 19.0 and 21.2 % were demonstrated in the blue and green PhOLEDs with the mBFCzCN host due to its high triplet energy and good bipolar charge transporting characteristics.

Triazine-dibenzocarbazole based bipolar host materials for highly luminescent green and yellow phosphorescent organic light emitting diodes

Two new bipolar host materials, 7-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7H-dibenzo[a,g]carbazole (TRZ-DBC1) and 7-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7H-dibenzo[a,i]carbazole (TRZ-DBC2) were designed and synthesized between dibenzocarbazole donor and triazine acceptor. The both materials revealed excellent thermal stabilities and good bipolar characteristics. We have fabricated yellow and green phosphorescent OLED devices to study the host performances. The TRZ-DBC1 based yellow device showed excellent current efficiency of 71.4 cd/A, while CBP based reference device showed lower efficiency of 58.0 cd/A. The external quantum efficiency of TRZ-DBC1 (25.4%) and TRZ-DBC2 (22.4%) were outstanding when compare to reference device (19.1%). The TRZ-DBC2 used as host material for green phosphorescent OLED due to its higher triplet energy of 2.71 eV. The green phosphorescent OLED exhibited excellent current efficiency of 75.9 cd/A, and excellent external quantum efficiency of 24.7%. These two bipolar host materials would be promising candidates for green and yellow phosphorescent OLEDs for display application.

Exciton‐Induced Degradation of Hole Transport Layers and Its Effect on the Efficiency and Stability of Phosphorescent Organic Light‐Emitting Devices

AbstractThe effect of exciton‐induced degradation of hole transport layers (HTLs) and its influence on efficiency and stability of phosphorescent organic light emitting devices (PhOLEDs) are investigated. In order to be able to isolate and study the effect of excitons on HTLs, UV illumination as a means to expose them to exciton stress is used. Results reveal that exciton stress of only the HTLs can lead to a significant deterioration in the electroluminescence external quantum efficiency and stability of PhOLEDs, revealing the detrimental role of exciton‐induced degradation of HTLs in limiting the device performance. The creation of quenchers in HTLs and the diffusion of excitons from the HTL to the EML appear to play roles in this degradation mechanism. Observations reveal that exciton‐induced degradation of HTLs more strongly impacts PhOLEDs than their fluorescent counterparts, revealing the more critical role that HTLs play in influencing their stability and pointing to the role of triplet excitons in this phenomenon. Observations also suggest that increasing the exciton stability of HTLs or reducing exciton lifetime in them can help increase device stability. The findings uncover a new degradation mode in PhOLEDs and provide key insights for device design for realizing better performance and stability.

Synthesis and characterization of heptaflurosulfonyl-substituted iridium complexes for blue phosphorescent organic light emitting diodes

We synthesized the heptaflurosulfonyl-substituted iridium complexes with SOCF7pic, SOCF7mpic, and SOCF7taz as a dopants for blue phosphorescent organic light emitting diodes (PHOLEDs). The SOCF7pic, SOCF7mpic, and SOCF7taz showed high thermal stability with thermal decomposition temperature of 395, 396, and 411 °C by the TGA measurement. Using newly synthesized dopants, blue emissive PHOLEDs were fabricated by using co-host materials during vacuum process. The devices with SOCF7pic or SOCF7mpic dopant exhibited similar current efficiencies (1.55 cd/A vs. 1.25 cd/A) and power efficiencies (1.24 lm/W vs. 1.0 lm/W). It showed relatively higher than the SOCF7taz doped device efficiencies with 1.0 cd/A and 0.6 lm/W. The high efficiencies of the SOCF7pic and SOCF7mpic doped devices compared with the SOCF7taz doped device are caused by the improved electrons and holes injection into the emitting layer with well-aligned HOMO-LUMO levels or relatively uniform surface morphology.

Efficient blue phosphorescent organic light emitting diodes based on exciplex and ultrathin Firpic sandwiched layer

Abstract The high efficient blue phosphorescent organic light emitting diodes (PHOLEDs) employing ultrathin Firpic layer sandwiched between electron donor mCP and electron acceptor TmPyPB layers were fabricated, and the dependence of the electroluminescence (EL) performance on the ultrathin Firpic layer thickness were investigated. It was found that the Firpic layer thickness imposed vital influence on the OLEDs EL efficiency and the one with the 0.3 nm Firpic demonstrated the optimal power efficiency (PE) of 50 lm/w and the external quantum efficiency (EQE) close to 18%. The sandwiched ultrathin Firpic layer would not impede the formation of the exciplex between mCP and TmPyPB, and the balance between the exciplex formation and exciplex decaying showed strong dependence on Firpic layer thickness and imposed vital influence on the energy transfer, fluorescence and phosphorescence inside the emissive layer. There were three radiative decaying processes involved, i.e. the prompt and delayed fluorescence (PF and DF) of the exciplex as well as the phosphorescence (PH) of the Firpic, and the final EL relied on the competition of those processes. The optimal EL performance might be attributed to the combining benefits of the balanced PF, DF and PH decaying, alleviated Dexter-transfer-based triplet-triplet annihilation (TTA) and the triplets diffusion suppressed architecture.

The role of electron-transporting Benzo[f]quinoline unit as an electron acceptor of new bipolar hosts for green PHOLEDs

We prepared three new compounds [3,6-di(9H-carbazol-9-yl)phenanthrene (3,6-DCP), 2,9-di(9H-carbazol-9-yl)benzo[f]quinoline (2,9-DCBQ), and 3,9-di(9H-carbazol-9-yl)benzo[f]-quinoline (3,9-DCBQ)] containing phenanthrene or benzo[f]quinoline as an electron-withdrawing moiety and a carbazole as electron-donating moiety, respectively, as bipolar hosts for green phosphorescent organic light emitting diodes (PHOLEDs). We intentionally substituted nitrogen atom to the C-3 position of phenanthrene moiety to prepare benzo[f]quinolinegroup. And, we found that it allowed better electron transporting behavior than the phenanthrene moiety. Meanwhile, the benzo[f]quinoline/phenanthrene core moieties significantly improved the thermal stability of those host materials, which exhibited glass transition and decomposition temperatures of 132–139 and 395–427 °C, respectively. The green PHOLEDs which were fabricated with those host materials showed the lowest operating voltage of 4.7 V at 1000 cd/m2 when we used 3,9-DCBQ. Very interestingly, it has an asymmetric structure with completely separated HOMO and LUMO in space. In contrast, 3,6-DCP having phenanthrene and carbazole moieties showed much higher operating voltage of 6.1 V which imply that replacing nitrogen at the C-3 position of phenanthrene improves carrier transport, that is, electron transporting behavior. As a result, the 3,9-DCBQ-based PHOLED showed the best overall performance, exhibiting current and power efficiencies of 48.5 cd/A and 20.6 lm/W, respectively.

Effect of Triplet Confinement on Triplet–Triplet Annihilation in Organic Phosphorescent Host–Guest Systems

AbstractThe efficiency of phosphorescent organic light emitting diodes (OLEDs) shows a decrease with increasing luminance (“roll‐off”). One of the contributions to the roll‐off is triplet–triplet annihilation (TTA). TTA is the process of energy transfer from one triplet exciton to another, after which the excited exciton decays nonradiatively to the lowest triplet state. In this study, the TTA‐rate is measured for a large number of emissive materials consisting of a small concentration of phosphorescent “guest” molecules, with emission colors across the entire visible range, embedded in various host materials. It is found that the TTA‐rate does not only depend on the direct interaction rate between the excitons on the guest molecules, but also on the difference in triplet energy ΔE T of the host and guest molecules: when ΔE T is smaller than about 0.20 eV, diffusion of excitons via the host molecules leads to a significant enhancement of the TTA‐rate. By varying the guest concentration and using kinetic Monte Carlo simulations, the roles of the direct interaction, guest‐mediated diffusion, and host‐mediated diffusion are disentangled.