Blue Phosphorescent Dopant Research Articles

Modified t-butyl in tetradentate platinum (II) complexes enables exceptional lifetime for blue-phosphorescent organic light-emitting diodes

In blue phosphorescent dopants, the tetradentate platinum(II) complex is a promising material showing high efficiency and stability in devices. However, metal-metal-to-ligand charge transfer (MMLCT) formation leads to low photo-luminescence quantum yields (PLQYs), wide spectra, and intermolecular interaction. To suppress MMLCT, PtON-tb-TTB and PtON-tb-DTB are designed using theoretical simulation by modifying t-butyl in PtON-TBBI. Both materials effectively suppress MMLCT and exhibit high PLQYs of 99% and 78% in 5 wt% doped film, respectively. The PtON-tb-TTB and PtON-tb-DTB devices have maximum external quantum efficiencies of 26.3% and 20.9%, respectively. Additionally, the PtON-tb-DTB device has an extended lifetime of 169.3 h with an initial luminescence of 1200 nit, which is 8.5 times greater than the PtON-TBBI device. Extended lifetime because of suppressed MMLCT and smaller displacement between the lowest triplet and triplet metal-centered states compared to other dopants. The study provides an effective approach to designing platinum(II) complexes for long device lifetimes.

Symmetric “Double Spiro” Wide Energy Gap Hosts for Blue Phosphorescent OLED Devices

AbstractWide energy gap materials dispiro[fluorene‐9,9′‐anthracene‐10′,9″‐fluorene] (SAS) and dispiro[xanthene‐9,9′‐anthracene‐10′,9″‐xanthene] (XAX) containing double spiro–carbons, are introduced as hosts for blue phosphorescent organic light‐emitting diodes (PHOLEDs). Both SAS and XAX are free of heteroatomic exocyclic bonds, which are implicated in limiting the stability of blue PHOLEDs. The materials are synthesized in gram‐scale quantities through short and efficient paths. They have large energy gaps (≥5.0 eV) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and correspondingly have high triplet energies in solid state (ET ≈ 3.0 eV). Analysis of devices using SAS and XAX as host materials with the blue phosphorescent dopant fac‐tris(N,N‐di‐p‐tolyl‐pyrizinoimidazol‐2‐yl)iridium(III) (Ir(tpz)3), shows that charges are transported and trapped by the dopant, which subsequently forms excitons directly on the phosphor. As a result, luminescence quenching pathways are suppressed which leads to blue phosphorescent devices with high (≈18%) external quantum efficiency. Thus, SAS and XAX serve as promising host materials, with high triplet energies suitable for blue PHOLEDs.

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.

Highly efficient blue, orange and red PhOLEDs with low roll-off of efficiency using a carbazole dendritic thermally activated delayed fluorescence (TADF) material as host

Numerous efforts have been devoted to improve the device efficiency of phosphorescent organic light-emitting diodes (PhOLEDs), however, strategies to suppress the device efficiency roll-off are still in urgent need. Here, a general and effective approach to boost the efficiency and suppress the efficiency roll-off of PhOLEDs is proposed, that is, utilizing TADF material with high triplet energy (ET) as the host for blue, orange and red phosphorescent dopants is proposed. In this work, we investigate the performance of these PhOLEDs based a new TADF host phenylbis(4-(3,3″,6,6″-tetra-tert-butyl-9′H-[9,3′:6′,9″-tercarbazol]-9′-yl)phenyl)phosphineoxide (TPPOCz). The blue device based on the TPPOCz host exhibit efficiencies of 43.6 cd A−1 and 20.4%, and small efficiency roll-off of 2.1% and 2.9% for CE and EQE from the peak efficiencies to these at 1000 cd m−2. We compared the performances with that reported in literature, and confirmed these devices in this work are among the best of PhOLEDs. These results can be ascribed to the rapid reverse intersystem crossing (RISC) process and high triplet energy (ET). These excellent performances indicating the potential of TADF host for the realization of highly efficient and stable devices in future.

Blue Phosphorescent Organic Light‐emitting Diode Using a Novel Starburst‐type Donor and Acceptor Material as a Co‐host

Starburst‐type hole‐transporting and electron‐transporting materials, tris(3‐(9H‐carbazol‐9‐yl)phenyl)amine ( TmCTA ), 2,4,6‐tri([1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (1), 2,4,6‐tris(3′‐methyl‐[1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (2), and 2,4,6‐tris(3′,5′‐dimethyl‐[1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (3) having high triplet energies and excellent thermal stabilities were synthesized. Deposited films of TmCTA , with 1, 2, or 3 as hosts, were doped with the blue phosphorescent dopant bis[2‐(4,6‐difluorophenyl)pyridinato‐C2 ,N](picolinato)iridium(III) (FIrpic) and their device characteristics were evaluated. In co‐deposited films, the molecular weight ratio of TmCTA to 1, 2, or 3 was 1: 1, and these co‐hosts formed exciplexes. By comparison with those of single‐host systems, the PL spectra of the co‐deposited films were red‐shifted, suggesting that exciplexes are formed by the two constituent molecules. When doped with 10% FIrpic, TmCTA :1 device showed an external quantum efficiency (EQE) of 14.56% (maximum 16.46%) at 1000 cd/m2; similar results were obtained for the remaining films. The EQE roll‐offs in the co‐host systems are much smaller than those of single‐host device using TmCTA .

ChemInform Abstract: Pd‐Catalyzed Intramolecular Heck Reaction, C(sp2)—H Activation, 1,4‐Pd Migration, and Aminopalladation: Chemoselective Synthesis of Dihydroindeno[1,2,3‐kl]acridines and 3‐Arylindoles.

Palladium-catalyzed intramolecular Heck reaction and aminopalladation of N-(2-(1-phenylvinyl)phenyl)aniline for the efficient synthesis of dihydroindeno[1,2,3-kl]acridines and 3-arylindoles via tuning of the phosphine ligands and solvents under two optimized conditions are reported. The reaction follows a 1,4-Pd migration, aminopalladation, C(sp2)–H activation, as well as five- and six-membered-ring fusion to form different products. The dihydroindeno[1,2,3-kl]acridine derivatives showed higher triplet energy (ET) levels than common blue phosphorescent dopant and may serve as good host candidates for blue triplet emitters.

Pd-Catalyzed Intramolecular Heck Reaction, C(sp2)–H Activation, 1,4-Pd Migration, and Aminopalladation: Chemoselective Synthesis of Dihydroindeno[1,2,3-kl]acridines and 3-Arylindoles

Palladium-catalyzed intramolecular Heck reaction and aminopalladation of N-(2-(1-phenylvinyl)phenyl)aniline for the efficient synthesis of dihydroindeno[1,2,3-kl]acridines and 3-arylindoles via tuning of the phosphine ligands and solvents under two optimized conditions are reported. The reaction follows a 1,4-Pd migration, aminopalladation, C(sp(2))-H activation, as well as five- and six-membered-ring fusion to form different products. The dihydroindeno[1,2,3-kl]acridine derivatives showed higher triplet energy (ET) levels than common blue phosphorescent dopant and may serve as good host candidates for blue triplet emitters.

Synthesis and Characterization of New Dibenzothiophene-based Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes

A series of bipolar host materials containing dibenzo[b,d]thiophene (DBT) or dibenzo[b,d]thiophene 5,5-dioxide core were successfully synthesized, and their physical, photophysical, and electrochemical properties were investigated. The three host materials showed well-localized electron distribution at the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states, as evidenced by theoretical calculations. Triplet energies of the new host materials are higher than 2.6 eV, and the blend film with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as a blue phosphorescent dopant showed highly efficient energy transfer between the host material and dopant.

P‐135: Recombination Zone Monitoring of the Blue Phosphorescent Organic Light‐Emitting Diodes during Lifetime Test

We studied the recombination zone shift during lifetime measurement in phosphorescent blue organic light‐emitting devices with a sensing layer. 3,3‐Di(9H‐carbazol‐9‐yl)biphenyl was used as a host material for a blue phosphorescent dopant and a red sensing dopant. The red sensing layer was inserted at a thickness of 2 nm at three different positions of the emitting layer to follow the recombination zone change before and after life‐time measurement by monitoring the electroluminescence spectra. We found that the recombination zone is shifted to electron transport layer side according to the luminance decrease of the device via direct monitoring of the recombination zone shift. .

An efficient dual-emissive-layer white organic light emitting-diode: Insight into device working mechanism and origin of color-shift

By using a single host for both blue and orange phosphorescent dopants, a simple and efficient white organic light emitting-diode is reported. The dual-emissive-layer white device achieves a peak external quantum efficiency of 16.9±0.9% and power efficiency of 44.1±2.3lm/W without out-coupling enhancement. Analysis of the device working mechanism determines that the blue dopant molecules can form a bridge to facilitate electron transport into the adjacent orange emitting-layer. The orange emission originates from both the direct electron trapping by the orange dopant and incomplete blue–orange energy transfer mechanisms. The origin of the voltage-dependent color shift of the device is quantitatively determined according to the working mechanism. Possible solution to reducing the color-shift is also provided based on the calculation and analytical results.

Wide bandgap materials design toward tunable bandgap and increased carries injection property: silane interrupting π-conjugation together with peripheralcarbazolyl substituents

Universal host for blue, green and red phosphorescent dopants is achieved by rational molecule design with silane interrupting π-conjugation and non-conjugated peripheralcarbazolyl substituents.

An Exciplex Forming Host for Highly Efficient Blue Organic Light Emitting Diodes with Low Driving Voltage

The exciplex forming co‐host with phosphorescent dopant system has potential to realize highly efficient phosphorescent organic light emitting didoes (PhOLEDs). However, the exciplex forming co‐host for blue phosphorescent OLEDs has been rarely introduced because of higher triplet level of the blue dopant than green and red dopants. In this work, a novel exciplex forming co‐host with high triplet energy level is developed by mixing a phosphine oxide based electron transporting material, PO‐T2T, and a hole transporting material, N,N′‐dicarbazolyl‐3,5‐benzene (mCP). Photo‐physical analysis shows that the exciplexes are formed efficiently in the host and the energy transfer from the exciplex to blue phosphorescent dopant (iridium(III)bis[(4,6‐difluorophenyl)‐pyridinato‐N,C2′]picolinate; FIrpic) is also efficient, enabling the triplet harvest without energy loss. As a result, an unprecedented high performance blue PhOLED with the exciplex forming co‐host is demonstrated, showing a maximum external quantum efficiency (EQE) of 30.3%, a maximum power efficiency of 66 lm W−1, and low driving voltage of 2.75 at 100 cd m−2, 3.29 V at 1000 cd m−2, and 4.65 V at 10 000 cd m−2, respectively. The importance of the exciton confinement in the exciplex forming co‐host is further investigated which is directly related to the performance of PhOLEDs.

Highly efficient, color-stable white top-emitting organic light-emitting diodes

Top-emitting white organic light-emitting diodes (WOLEDs) have potential applications in lighting and full color dis- plays due to the potential realization of a large aperture ratio and a high resolution when recombining top-emitting WOLEDs with active-matrix driving circuits. In the present paper, the authors fabricated top-emitting WOLEDs based on the red/blue dual-phosphorescent emitting layers and improved luminous efficiency and color stability in WOLEDs through inserting an tris(pheny-pyrazole)iridium (Ir(ppz)3) thin film as an electron-blocking layer between the red layer and the blue one. The insertion of an Ir(ppz)3 thin film can easily generate white emission through the enhancement of doping concentration of the red dopant, thus reducing the serious requirements of manufacture technology and improving the reproducibility of process technology. In addition, the authors analyzed the mechanism of color stability, optimized the concentrations of the red and blue phosphorescent dopants, and realized a luminous efficiency of as high as 7.9 cd · A(-1) in the authors' top-emitting WOLED. The white emission lies in the warm-white region with a very small chromaticity change of (0.006, 0.01) under a wide brightness range of 87-2,403 cd · m(-2).

Effect of interfacial mixed layer in blue phosphorescent organic light-emitting diodes

Blue phosphorescence organic light-emitting diodes (PHOLEDs) with interfacial mixed layer (IML) were fabricated to observe their electrical characteristics. The IML was composed of mCP as host material in the emissive layer (EML) and 3TPYMB as material for the electron transport layer (ETL) with the blue phosphorescent dopant FIrpic. PHOLEDs with IML were improved by 41% of current density and 35% of luminous efficiency compared to PHOLEDs without IML. The electron and energy transition mechanisms between EML and ETL in the presence of IML were confirmed through I–V–L characteristics and blue color coordinates.

Investigation of blue phosphorescent organic light-emitting diode host and dopant stability

By incorporating prospective materials for blue PHOLED emissive layers into model OLEDs, we have investigated how hole transport in a prototypical blue phosphorescent emitter, FIrpic (bis(4,6-difluorophenyl-pyridinato-N,C2) picolinate Iridium) doped mCBP (4′-bis(3-methylcarbazol-9-yl)-2,2′-biphenyl), can impact PHOLED device operational stability. We found the host mCBP to be stable in supporting hole transport, but unstable with respect to electron–hole recombination. As a dopant, FIrpic was found to be unstable with respect to both hole transport and charge recombination processes. Our results indicate that FIrpic doped mCBP is unsuitable for use as an emissive layer in OLED devices and we provide a general strategy to screen materials and better understand their stability.

Carbazolyldibenzofuran-type high-triplet-energy bipolar host material for blue phosphorescent organic light-emitting diodes

A high-triplet-energy material, 9′-(4,4′-(phenylphosphoryl)bis(dibenzo[b,d]furan-6,2-diyl))bis(9H-carbazole) (44DFCzPO), was synthesized as a bipolar host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). 44DFCzPO was synthesized by the selective lithiation of the 4-position of a carbazole-modified dibenzofuran, followed by phosphorylation. 44DFCzPO showed a high triplet energy of 2.91eV for energy transfer to a blue phosphorescent dopant, and bipolar charge transport properties for balanced hole and electron density in the emitting layer. A high quantum efficiency of 16.7% at a low doping concentration of 3% was obtained using 44DFCzPO as the host in blue PHOLEDs.

Electroluminescence enhancement in blue phosphorescent organic light-emitting diodes based on different hosts

Blue phosphorescent organic light-emitting diodes (OLEDs) are fabricated by utilizing the hole transport-type host material of 1,3-bis(carbazol-9-yl)benzene (MCP) combined with the electron transport-type host material of 1,3-bis (triphenylsilyl) benzene (UGH3) with the ratios of 1:0, 8:2 and 6:4, and doping with blue phosphorescent dopant of bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic). The device with an optimum concentration proportion of MCP:UGH3 of 8:2 exhibits the maximum current efficiency of 19.18 cd/A at luminance of 35.71 cd/m2 with maintaining Commission Internationale de L’Eclairage (CIE) coordinates of (0.1481, 0.2695), which is enhanced by 35.7% compared with that of 1:0 with (0.1498, 0.2738). The improvements are attributed to the effective carrier injection and transport in emitting layer (EML) because of mixed host materials. In addition, electron and exciton are confined in the EML, and 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) and Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) have the high lowest unoccupied molecular orbital (LUMO) energy level and triplet exiton energy.

Efficient inverted bottom-emission blue phosphorescent organic light-emitting diodes with a ytterbium-doped electron injection layer

An efficient electron injection layer (EIL) for inverted bottom-emission organic light-emitting diodes (IBE-OLEDs) is developed by doping ytterbium (Yb) into an organic electron transport material of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB). The Yb-doped EIL in the IBE-OLEDs having iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2′) picolinate (FIrpic) as a blue phosphorescent dopant shows a lower turn-on voltage (∼4.5 V) and about a 6.6 times higher efficiency (∼8.6% and 15.3 cd/A at 0.15 mA/cm2) compared with the device without the Yb-doped EIL. Furthermore, the electroluminescence spectrum of the device with the Yb-doped EIL is about the same as that of the device without the Yb-doped EIL. This result indicates that the Yb-doped EIL improves the electron injection from the ITO cathode to the organic electron transport layer. Therefore, the Yb-doped EIL can be utilized as an effective electron injection layer in the OLEDs.

Fluorinated Carbazole Derivatives as Wide-Energy-Gap Host Material for Blue Phosphorescent Organic Light-Emitting Diodes

2,7-Difluo-carbazole and 2,4,5,7-tetrafluoro-carbazole were synthesized as new building blocks of wide-energy-gap host material for phosphorescent organic light-emitting diodes (PHOLEDs). These fluorinated positions in the carbazole ring were determined on the basis of density functional theory calculation results. Spectroscopic analyses supported the hypothesis that poly(N-vinyl-2,7-difluoro-carbazole) (2,7-F-PVK) with the fluorinated pendant group possessed a wide energy gap, leading to the exciton energy confinement on the blue phosphorescent dopant as well as nonsubstituted poly(N-vinyl-carbazole) (PVK). 2,7-F-PVK was used in solution-processed blue PHOLED to achieve 27 cd/A at 760 cd/m2, which is 1.8 times higher than that of nonsubstituted PVK. We assumed that the replacement of nonsubstituted PVK with 2,7-F-PVK improved the charge balance in the emission layer, while keeping the exciton confinement effect. The fluorination of the carbazole ring is a useful molecular design strategy for wide-energy-gap host material.

Highly efficient white phosphorescent organic light emitting diodes using a mixed host structure in deep blue emitting layer

Highly efficient phosphorescent white organic light-emitting diodes (PHWOLEDs) were developed using a deep blue phosphorescent emitter doped into a mixed host of high triplet energy host materials. The deep blue emitting layer was combined with a red:green emitting layer to fabricate PHWOLEDs. A high quantum efficiency of 19.5% with a color coordinate of (0.29,0.38) and 19.8% with a color coordinate of (0.39,0.46) were achieved in the PHWOLEDs using the mixed host emitting layer doped with a deep blue phosphorescent dopant. In addition, a low optimum doping concentration below 5% in red, green and blue dopants was realized in the PHWOLEDs.