Sky-blue Phosphorescent Organic Light-emitting Diodes Research Articles

Sky-blue tri-cyclometalated heteroleptic iridium (III) phosphors: Influence of ancillary ligand

Two sky-blue tri-cyclometalated heteroleptic iridium complexes with the Ir(C^N)2(C1^N1) framework, namely M1 and M2, were developed with (3,5-bis(trifluoromethyl)phenyl)pyridine (cf3ppy) as the main cyclometalating ligand and 3-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine (pt) or 2′,6′-difluoro-2,3′-bipyridine (dfbpy) as the ancillary cyclometalating ligand. The effect of the C1^N1 ancillary ligand on the photophysical, thermal, electrochemical properties as well as the device performances were studied comparatively. It was revealed that the ancillary ligand pt favored to suppress the longer-wavelength vibronic band intensity and thus enhanced the blue color purity of the tris-cyclometalated iridium complex M1. Sky-blue phosphorescent organic light-emitting diodes (OLEDs) were fabricated with these iridium phosphors as doped emitter. The M1-based OLED exhibited better performance with improved color purity and reduced intensity at long-wavelength vibronic band, and a maximum current efficiency and external quantum efficiency of 28.2 cd/A and 12.5 %. This study provides a valid molecular design strategy to control the intensity of the shoulder vibronic sub-band for optimized color purity of cyclometalated iridium phosphors.

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.

New trifluoromethyl modified iridium(III) complex for high-efficiency sky-blue phosphorescent organic light-emitting diode

A new and efficient trifluoromethyl modified sky-blue iridium(III) coordination compound (Ir-CF3) is well designed and prepared by using bis(trifluoromethyl) modified 4-methyl-2-phenylpyridine as the cyclometalating ligand and 2-(1H-imidazol-2-yl)pyridine as the auxiliary ligand. Ir-CF3 shows intensive sky-blue phosphorescence (465 nm with 494 nm as shoulder), high solution photoluminescence efficiency (0.22). The incorporation of trifluoromethyl moiety into the complex not only enhances the robust chemical stability but also provides the steric hindrance beneficial to organic light-emitting diodes. The sky-blue OLEDs based on this complex show narrow emission band with full width at half maxima of 54 nm. The peak power efficiency, current efficiency and external quantum efficiency of electroluminescence are as high as 36.05 lm/W, 51.45 cd/A and 21.74%, respectively.

Low-driving-voltage sky-blue phosphorescent organic light-emitting diodes with bicarbazole-bipyridine bipolar host materials

Two bipolar host containing bipyridine and bicarbazole exhibited rapid and balanced charge transportation, which resulting the extremely low turn-on voltages of 2.3 and 2.4 V, respectively, and the high efficiency of 23.7% and 52.6 lm W−1.

Bistriazoles with a Biphenyl Core Derivative as an Electron-Favorable Bipolar Host of Efficient Blue Phosphorescent Organic Light-Emitting Diodes.

High-quality host materials are indispensable for the construction in the emitting layer of efficient organic light-emitting diodes (OLEDs), especially in a guest and host system. The good carrier transport and energy transfer between the host and emitters are out of necessity. In this work, a wide bandgap and bipolar organic compound, 2,2′-bis(4,5-diphenyl-(1,2,4)-triazol-3-yl)biphenyl (BTBP), conjugating two electron-transporting triazole moieties on a hole-transporting biphenyl core, was synthesized and characterized. The wide bandgap of 4.0 eV makes the promise in efficient energy transfer between the host and various color emitters to apply as the universal host, especially for blue emitters. The close electron and hole mobilities perform the same order of 10–5 cm2·V–1·s–1, identified as bipolar behavior and benefited for carrier balance at low bias. Although carrier transportation belongs to bipolar behavior at a low electrical field, the electron mobility is much faster than the hole one at a high electrical field and belongs to electron-transporting behavior. Employing the BTBP as the host matrix mixed with a phosphor dopant, iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2]picolinate, a high-efficiency sky-blue phosphorescent organic light-emitting diode (OLED) was achieved with a maximum current efficiency of 65.9 cd/A, maximum power efficiency of 62.8 lm/W, and maximum external quantum efficiency of 30.2%.

High-performance sky-blue phosphorescent organic light-emitting diodes employing wide-bandgap bipolar host materials with thermally activated delayed fluorescence characteristics

Abstract Up to date, several kinds of thermally activated delayed fluorescence (TADF) host molecules have been rapidly developed to fulfill the increasing demand of high-efficiency phosphorescent organic light-emitting diodes (PHOLEDs). Herein, three wide-bandgap TADF materials BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz are newly designed and synthesized as host materials for sky-blue PHOLEDs. By modulation of carbazole-units in the molecular skeleton, we find that the optimized molecules can realize both high triplet energies and balanced carrier transport properties with obvious TADF properties. The sky-blue PHOLEDs with ultralow phosphor doping ratio employing BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz as host materials demonstrate really high performance with alleviated roll-offs and achieve impressive current efficiencies (CEs) of 46.5 cd A−1, 50.0 cd A−1, and 34.5 cd A−1, power efficiencies (PEs) of 40.6 lm W−1, 46.8 lm W−1, and 34.8 lm W−1, and external quantum efficiencies (EQEs) of 22.2%, 24%, and 15.8%, respectively. More encouragingly, even at an applied luminance of 1000 cd m−2, the EQEs could maintain high levels of 21.0%, 21.5% and 12.8%, respectively, which represents the potential utility value of the new family of these molecules in blue PHOLEDs. Our molecular design strategy provides a practical way to construct promising wide-bandgap TADF host materials with an excellent combination of TADF property, high triplet energy, and balanced carrier-transporting ability for high-performance devices.

Roles of Ancillary Chelates and Overall Charges of Bis-tridentate Ir(III) Phosphors for OLED Applications.

A series of charge-neutral bis-tridentate Ir(III) complexes (1, 3, and 4) were prepared via employing three distinctive tridentate prochelates, that is, (pzptBphFO)H2, [(phpyim)H2·(PF6)], and [(pimb)H3·(PF6)2], which possess one dianionic pzptBphFO, together with a second monoanionic tridentate chelate, namely, (pzptBphFO)H, phpyim, and pimb, respectively. Moreover, a homoleptic, charge-neutral complex 2 was obtained by methylation of chelating (pzptBphFO)H of 1 in basic media, while closely related cationic complexes 5-7 were obtained by further methylation of the remaining pyrazolate unit of previously mentioned neutral complexes 2-4, followed by anion metatheses. All of these Ir(III) metal complexes showed a broadened emission profile with an onset at ∼450 nm, a result of an enlarged ligand-centered ππ* transition gap, but with distinct efficiencies ranging from 0.8% to nearly unity. Comprehensive spectroscopic and computational approaches were executed, providing a correlation for the emission efficiencies versus energy gaps and between the metal-to-ligand charge transfer/ππ* emitting excited state and upper-lying metal-centered dd quenching state. Furthermore, Ir(III) complexes 3 and 4 were selected as dopant emitters in the fabrication of sky-blue phosphorescent organic light-emitting diodes, affording maximum external quantum efficiencies of 16.7 and 14.6% with CIEx,y coordinates of (0.214, 0.454) and (0.191, 0.404) at a current density of 102 cd/m2, respectively. Hence, this research highlights an inherent character of bis-tridentate Ir(III) complexes in achieving high phosphorescence quantum yield at the molecular level.

Sky-blue phosphorescent organic light-emitting diode with superior performance based on novel chlorine functionalized iridium(III) complex

A novel and highly efficient chlorine functionalized iridium(III) complex is designed and synthesized. The complex shows intensive sky-blue phosphorescence (with a peak of 492 nm and a shoulder at 524 nm), high photoluminescence efficiency (0.78) and moderate full width at half maximum (62 nm). The aromatic chlorine introduced into the complex provides the robust chemical stability and effective sky-blue phosphorescence for organic light-emitting diodes (OLEDs). The maximum power efficiency, current efficiency and external quantum efficiency for the complex based OLED are up to 48.46 lm/W, 55.04 cd/A and 18.47%, respectively.

Sky-blue phosphorescent organic light-emitting diodes with dibenzo-24-crown-8 substituted iridium(III) complexes as the dopants

We report several sky-blue phosphorescent organic light-emitting diodes (PhOLEDs) fabricated by spin-coating technique, using dibenzo-24-crown-8 unit-based iridium complexes (B1, B2 and B3) as the dopants and poly(N-vinylcarbazole) (PVK) as the polymer host. Among these, the single emissive layer PhOLEDs from B3 exhibited the best electroluminescent performances with a maximum luminous efficiency (LEmax) of 10.7 cd A−1 at a current density of 4.6 mA cm−2 and a maximum brightness (Lmax) of 4604 cd m−2. Compared to B1 and B3, the devices with B2 as dopant displayed much lower device's performances (LEmax = 4.2 cd A−1 and Lmax = 1433 cd m−2) as the presence of p-terphenyl unit in the auxiliary ligand of B2, the triplet energy level of which is slightly less than that of iridium (III) bis[(4,6-difluorophenyl)pyridinato-N,C2′] picolinate (FIrpic). This present work demonstrates that the triplet energy level of the cyclometalated and/or auxiliary ligands of the iridium complexes should be considered to design novel electrophosphorescent materials for highly efficient PhOLEDs.

Exploiting Photo- and Electroluminescence Properties of FIrpic Organic Crystals.

In this work, we investigate the optical and structural properties of the well-known triplet emitter bis(4',6'-difluorophenylpyridinato)-iridium(III) picolinate (FIrpic), showing that its ability to pack in two different ordered crystal structures promotes attractive photophysical properties that are useful for solid-state lighting applications. This approach allows the detrimental effects of the nonradiative pathways on the luminescence performance in highly concentrated organic active materials to be weakened. The remarkable electro-optical behavior of sky-blue phosphorescent organic light-emitting diodes incorporating crystal domains of FIrpic, dispersed into an appropriate matrix as an active layer, has also been reported as well as the X-ray diffraction, nuclear magnetic resonance, electro-ionization mass spectrometry, and scanning electron microscopy analyses of the crystalline samples. We consider this result as a crucial starting point for further research aimed at the use of a crystal triplet emitter in optoelectronic devices to overcome the long-standing issue of luminescence self-quenching.

Modulation of the Physicochemical Properties of Donor-Spiro-Acceptor Derivatives through Donor Unit Planarisation: Phenylacridine versus Indoloacridine-New Hosts for Green and Blue Phosphorescent Organic Light-Emitting Diodes (PhOLEDs).

This work reports a detailed structure-property relationship study of a series of efficient host materials based on the donor-spiro-acceptor (D-spiro-A) design for green and sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). The electronic and physical effects of the indoloacridine (IA) fragment connected through a spiro bridge to different acceptor units, namely, fluorene, dioxothioxanthene or diazafluorene moiety, have been investigated in depth. The resulting host materials have been easily synthesised through short, efficient, low-cost, and highly adaptable synthetic routes by using common intermediates. The dyes possess a very high triplet energy (ET ) and tuneable HOMO/LUMO levels, depending on the strength of the donor/acceptor combination. The peculiar electrochemical and optical properties of the IA moiety have been investigated though a fine comparison with their phenylacridine counterparts to study the influence of planarisation. Finally, these molecules have been incorporated as hosts in green and sky-blue PhOLEDs. For the derivative SIA-TXO2 as a host, external quantum efficiencies as high as 23 and 14 % have been obtained for green and sky-blue PhOLEDs, respectively.

Donor/Acceptor Dihydroindeno[1,2-a]fluorene and Dihydroindeno[2,1-b]fluorene: Towards New Families of Organic Semiconductors.

New families of donor/acceptor semiconductors based on dihydroindeno[1,2-a]fluorene and dihydroindeno[2,1-b]fluorene are reported. Due to the spiro bridges, this new generation of dihydroindenofluorenes allows a spatial separation of HOMO and LUMO, which retains the high ET value of the dihydroindenofluorene backbone and excellent physical properties. This control of the electronic and physical properties has allowed a second generation of dihydroindeno[1,2-a]fluorene to be obtained with strongly enhanced performance in green and sky-blue phosphorescent organic light-emitting diodes (PhOLEDs) relative to the first generation of materials. To date, this is the highest performance ever reported for a blue PhOLED by using a dihydroindenofluorene derivative. Through this structure-property relationship study, a remarkable difference of performance between syn and anti isomers has also been highlighted. This surprising behaviour has been attributed to the different symmetry of the two molecules, and highlights the importance of the geometry profiles in the design of host materials for PhOLEDs.

A host material consisting of phosphinic amide for efficient sky-blue phosphorescent organic light-emitting diodes

A new bipolar host material (PBCz-PO) consisting a phosphinic amide is developed to show a resonance effect. It possesses quite high triplet energy (2.76eV), and this high triplet energy make it suitable to be blue host in phosphorescent organic light-emitting diodes (PHOLEDs). The device with PBCz-PO as a host and FIrpic as a dopant achieved a maximum current efficiency (ηc) of 31.5cd/A, a maximum power efficiency (ηp) of 31.0lm/W and a maximum external quantum efficiency (EQE) of 13.4% as well as low driving voltage.

4-Pyridyl-9,9′-spirobifluorenes as Host Materials for Green and Sky-Blue Phosphorescent OLEDs

We report herein new pyridine-substituted spirobifluorene (SBF) dyes, i.e., 4-(9,9′-spirobi[fluoren]-4-yl)pyridine (4-4Py-SBF), 3-(9,9′-spirobi[fluoren]-4-yl)pyridine (4-3Py-SBF), and 2-(9,9′-spirobi[fluoren]-4-yl)pyridine (4-2Py-SBF), built on the association of the 4-substituted spirobifluorenyl core and various regioisomers of pyridine. These organic semiconductors possess high triplet energy levels (ET around 2.7 eV) in accordance with their use as hosts for green and sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). These dyes have been synthesized from the 4-bromo-spirobifluorene (4-Br-SBF) platform, obtained from a new and efficient synthetic approach using the promising building block 4-bromofluorenone as a key intermediate. Synthesis, structural, thermal, electrochemical, and photophysical properties of the three dyes have been investigated in detail and compared to other model compounds, namely, 4-phenyl-SBF (4-Ph-SBF), 2-phenyl-SBF (2-Ph-SBF), and 2,4-pyridyl-SBF (2-4Py-SBF), in order to precisely study the influence of (i) the pyridine unit, (ii) the position of the nitrogen atom within the pyridyl core (in position 4, 3, or 2), and (iii) the substitution at the C4 position of SBF. This rational structure–properties relationship study sheds light on the effect of the substitution in position 4 of the SBF core and may pave the way to the development of such materials in electronics. Finally, the high performance of green PhOLEDs, ca. 63 cd/A, and of sky-blue PhOLED, ca.16 cd/A, clearly evidence the potential of these new SBF derivatives as hosts for phosphorescent dopants.

Ortho-, meta-, and para-dihydroindenofluorene derivatives as host materials for phosphorescent OLEDs.

This work reports the first structure-properties relationship study of ortho [2,1-c]-, meta [1,2-a]-, and para [1,2-b]dihydroindenofluorenes, highlighting the influence of bridge rigidification on the electronic properties. This study has made it possible to devise an extended π-conjugated molecule with both a high triplet state energy level and excellent thermal and morphological stability. As a proof of concept, dihydroindenofluorenes were used as the host in sky-blue phosphorescent organic light-emitting diodes (PhOLEDs) with high performance.

Novel benzimidazole derivatives as electron-transporting type host to achieve highly efficient sky-blue phosphorescent organic light-emitting diode (PHOLED) device.

The development of benzimidazole substituted biphenyls as electron-transporting hosts for bis[2-(4,6-difluorophenyl)pyridinato-C(2),N](picolinato)iridium(III) is reported. Under the optimized conditions, the organic light-emitting diode (OLED) achieves the maximum current efficiency of 57.2 cd/A, power efficiency of 50.4 lm/W, and external quantum efficiency 25.7%.