Efficient Organic Light-emitting Devices Research Articles

Mono-, di- and tri-nuclear PtII(C^N)(N-donor ligand)Cl complexes showing aggregation-induced phosphorescent emission (AIPE) behavior for efficient solution-processed organic light-emitting devices

Mono-, di- and tri-nuclear PtII(C^N)(N-donor ligand)Cl complexesPh-PAYPt1,Ph-PAYPt2andPh-PAYPt3are designed and prepared.

Efficient and stable organic light-emitting devices employing phosphorescent molecular aggregates

Stable and efficient organic light-emitting diodes (OLEDs) operating at high brightness are desirable for future high-resolution displays and lighting products. Here, we report a tetradentate Pd(ii) complex called Pd3O8-P, which has attractive optoelectronics properties. At room temperature, aggregates of Pd3O8-P exhibited a close-to-unity photoluminescent quantum yield and a short transient lifetime of 0.62 μs. A host-free Pd3O8-P yellow-orange OLED emitted light with a peak at 588 nm, a half-bandwidth of 84 nm and CIE coordinates of (0.52, 0.47), and achieved a peak external quantum efficiency (EQE) of 34.8%. The device had reduced efficiency roll-off, retaining high EQEs of 33.5% at 1,000 cd m−2 and 29.5% at 10,000 cd m−2. The estimated operational half-lifetime was 9.59 million hours at 1,000 cd m−2. The fact that the tetradentate metal complex has a triplet in the blue-emitting region may also assist the development of efficient and long-lived blue OLEDs in the future. A tetradentate Pd(ii) complex with near unity photoluminescent quantum yield is the basis of bright, long-lifetime organic LEDs.

Triptycene-imbedded thermally activated delayed fluorescence emitters with excellent film morphologies for applications in efficient nondoped and doped organic light-emitting devices

Thermally activated delayed fluorescence (TADF) materials with high luminescence efficiencies, short lifetimes, excellent thermal stability and high-quality film formation are highly desired to realize efficient organic light emitting diodes (OLEDs) with low efficiency roll-off. Herein, two asymmetric TADF molecules consisting of triptycene-fused benzophenone serving as an electron acceptor, and dimethylacridin and phenoxazine as electron donors are reported. The comparative study reveals that the introduction of triptycene scaffold strengthens the molecular rigidity and the intermolecular interactions in aggregated states, which endow the titled materials with significantly improved thermal stability, film formation quality and mechanofluorochromism behaviors. The triptycene-imbedded compound TCO-DMAC emits fast efficient delay fluorescence with photoluminescence quantum yield (PLQY) of 0.92 and short lifetime of 4.94 μs in doped evaporated film, based on which the bluish-green doped OLED achieved an external quantum efficiency (EQE) of 21.2%, a current efficiency (CE) of 57.3 cd/A, a power efficiency (PE) of 62.3 lm/W, and a maximum brightness of 15780 cd/m2. It is worth mentioning that the evaporated neat film of TCO-DMAC exhibit high PLQY of 0.69 and short delay fluorescence lifetime of 4.37 μs. The non-doped OLED achieved an external quantum efficiency (EQE) of 15.6%, a turn-on voltage of 2.8 V and a maximum brightness of 13830 cd/m2 with only minor efficiency roll-off of 3.8% at a high luminance of 1000 cd/m2.

Universal polymeric hosts adopting cardo-type backbone prepared by palladium-free catalyst with precisely controlled triplet energy levels and their application for highly efficient solution-processed phosphorescent organic light-emitting devices

A series of polymeric hosts with cardo-type backbone have been successfully developed with cheap palladium-free catalyst through controlling the special orientation of different monomers. By engineering the configuration of polymer backbones with different number and kind of the functional groups attached to the monomers, the triplet energy-levels (ETs) of these new polymeric hosts can be precisely controlled from 1.92 to 2.83 eV. Importantly, the functional groups on the monomers can afford improved and balanced charge carrier injection/transporting ability to these polymeric hosts. In addition, these polymeric hosts can show good thermal property with decomposition temperatures (Td) higher 410 °C and glass transition temperatures (Tg) higher than 170 °C. Benefitting from all these advanced properties, solution-processed phosphorescent organic light-emitting devices (PhOLEDs) based on these polymeric hosts can show impressive electroluminescence (EL) performances. The optimized blue device B2 possesses a maximum luminance efficiency (ηL) of 25.5 cd A−1, corresponding to a power efficiency (ηP) of 19.9 lm W−1 and an external quantum efficiency (ηext) of 11.3%. The maximum ηL, ηP and ηext for the optimized green device G2 are 66.9 cd A−1, 62.8 lm W−1 and 19.5%, respectively, while EL efficiencies of the optimized red device R2 can reach 11.0 cd A−1, 9.4 lm W−1 and 15.5%. All of the promising EL data achieved by these polymeric hosts show their great potential application in developing high performance polymer-based OLEDs.

A deep blue fluorescent emitter functioning as host material in highly efficient phosphorescent and hybrid white organic light-emitting devices

Abstract A deep blue fluorescent emitter of 4′, 4‴-(1H-phenanthro [9, 10-d]-imidazole-1, 2-diyl) bis (N, N-diphenyl-[1,1′-biphenyl]-4-amine) (PPI-2TPA) possessing special virtues of high triplet energy, excellent photoluminescence quantum yield and preferable bipolar transporting property was employed as host material by the first time to realize a highly efficient phosphorescent organic light-emitting device (OLED). In comparison with the reference device, the optimized device had better performance, which showed the maximum efficiencies of 40.4 cd/A and 45.3 l m/W and still kept high efficiencies of 39.7 cd/A and 37.1 l m/W at the luminance of 1000 cd/m2. Moreover, the critical current density was as high as 179.6 mA/cm2, which was about 1.8-fold that of the reference device, indicating a significant alleviation of efficiency drop at high current density in the optimized device. The emission of phosphorescent emitter originated from both energy transfer and hole-electron recombination, distinctly revealed by a systematical analysis on the device electroluminescence (EL) characteristics. Simultaneously, an efficient hybrid white OLED as well as a tandem white OLED which demonstrated quite slight color variation over a wide range of luminance were also constructed.

Managed carrier density and distribution in solution-processed emission layer to achieve highly efficient and bright blue organic light-emitting devices

In this paper, solution-processed blue organic light-emitting devices (OLEDs) with high efficiency and high luminance are realized. A blue thermally activated delayed fluorescence (TADF) material, 4CzFCN, is used as the emitting material. A host material with high hole mobility, mCP, and an electron transport material with high electron mobility and lower LUMO level, B4PyMPM, are employed to increase the electron density and balance the carrier distribution inside the emission layer. As a result, a luminance of 7625.0 cd/m 2 , a current efficiency of 49.3 cd/A, and an external quantum efficiency of up to 24% are obtained. To the best of our knowledge, these values are superior to the highest values reported for this material in the literatures. • By managing the carrier density and the carrier balance inside the EML, the solution-processed blue OLEDs performance was improved greatly. • The maximum current efficiency of the champion device is 49.3 cd/A, which is the highest efficiency reported from OLEDs with this material. • The high-efficiency device also achieved a high luminance of 7625 cd/m 2 , which is the highest luminance reported from OLEDs with this material.

Spirobiacridine-based Host Material for Highly Efficient Blue Phosphorescent Organic Light-emitting Devices

Although spirobiacridine (SBA) derivatives show a promising potential not only for thermally activated delayed fluorescent (TADF) emitters but also as host material for blue phosphorescent emitters...

Highly Efficient Microcavity Organic Light-Emitting Devices with Narrow-Band Pure UV Emission.

Ultraviolet organic light-emitting devices (UVOLEDs) may combine the tunability properties of organic materials through modifying the molecular structure and the advantages such as large-area, low-cost, and facile to realize high-performance UV sources. In the state-of-the-art UVOLEDs, the external quantum efficiencies (EQE) are more than 3%, but only a few have achieved pure UV emission and could not compromise the durability and irradiance at the same time. Portable compact UV sources with a narrow band made significant achievements in biomedical science and forensic appraisal. The microcavity effect is useful for achieving the desired narrow peak emission. In this study, asymmetric structural design with a specific distributed Bragg reflector (DBR) structure was employed to achieve narrow-band pure UV emission microcavity UVOLEDs (μC UVOLEDs). These μC UVOLEDs can realize tunable wavelength from 366 to 400 nm, with a full width at half maximum (FWHM) of 9.95-15.2 nm and a maximum irradiance of 2.79-5.63 mW/cm2. Also, the durability of the μC UVOLED has been considered, which presents a lifetime of 63.2 h under an irradiance of 0.016 mW/cm2. Moreover, the ability to identify 100 RMB with an efficient μC UVOLED has also been demonstrated. This investigation not only demonstrates the encouraging potential of narrow-band pure UVOLEDs but also provides a feasible strategy for the optimal design of μC UVOLEDs by utilizing the asymmetric structure.

Efficient and chromaticity stable green and white organic light-emitting devices with organic-inorganic hybrid materials.

Efficient inverted bottom emissive organic light emitting diodes (IBOLEDs) with tin dioxide and/or Cd-doped SnO2 nanoparticles as an electron injection layer at the indium tin oxide cathode:electron transport layer interface have been fabricated. The SnO2 NPs promote electron injection efficiently because their conduction band (−3.6 eV) lies between the work function (Wf) of ITO (4.8 eV) and the LUMO of the electron-transporting molecule (−3.32 eV), leading to enhanced efficiency at low voltage. The 2.0% SnO2 NPs (25 nm) with Ir(ddsi)2(acac) emissive material (SnO2 NPs/ITO) have an enhanced current efficiency (ηc, cd A−1) of 52.3/24.3, power efficiency (ηp, lm W−1) of 10.9/3.4, external quantum efficiency (ηex, %) of 16.4/7.5 and luminance (L, cd m−2) of 28 182/1982. A device with a 2.0% Cd-doped SnO2 layer shows higher ηc (60.6 cd A−1), ηp (15.4 lm W−1), ηex (18.3%) and L (26 858 cd m−2) than SnO2 devices or control devices. White light emission was harvested from a mixture of Cd–SnO2 NPs and homoleptic blue phosphor Ir(tsi)3; the combination of blue emission (λEL = 428 nm) from Ir(tsi)3 and defect emission from Cd–SnO2 NPs (λEL = 568 nm) gives an intense white light with CIE of (0.31, 0.30) and CCT of 6961 K. The white light emission [CIE of (0.34, 0.35) and CCT of 5188 K] from colloid hybrid electrolyte BMIMBF4–SnO2 is also discussed.

Stable and efficient phosphorescent organic light-emitting device utilizing a δ-carboline-containing host displaying thermally activated delayed fluorescence

A PHOLED with a δ-carboline-containing TADF-displaying host exhibits excellent device performance, including a high efficiency, low efficiency roll-off, and long operational lifetime.

Highly efficient flexible organic light-emitting devices based on PEDOT:PSS electrodes doped with highly conductive Pyronin B

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a promising material for utilization in flexible transparent electrodes. However, because the conductivity of PEDOT:PSS is as low as 1 S/cm or even lower, PEDOT:PSS has the inherent problem of a conductivity that is too low for it to be used in the fabrication of flexible, transparent electrodes for use in flexible organic light-emitting devices (OLEDs). Here, we report on hole injection enhancement in a highly conductive Pyronin-B-doped PEDOT:PSS electrode/hexaazatriphenylene hexacarbonitrile (HAT-CN) layer, a strong electron-accepting charge-generation layer, for use in highly efficient flexible OLEDs. While the conductivity of the pristine PEDOT:PSS is 1 S/cm, that of Pyronin-B-doped PEDOT:PSS is significantly increased to 1467 S/cm due to the dramatically enhanced screening effect for PEDOT:PSS films. The energy-band bending barrier at the interface between the Pyronin-B-doped PEDOT:PSS electrode and the HAT-CN layer is significantly decreased compared to that between the pristine PEDOT:PSS electrode and the HAT-CN layer, resulting in improved hole injection. The operating voltage and the current efficiency of the flexible OLEDs with Pyronin-B-doped PEDOT:PSS/HAT-CN electrodes (6.8 V, 26.03 cd/A) are similar to those of OLEDs with ITO/HAT-CN electrodes (7.7 V, 27.95 cd/A). The brightness of the OLEDs after 900 cycles of bending with a radius of curvature of 5 mm is 80% the initial brightness, indicative of the stability of these flexible devices under repeated bendings.

Ligand Mediated Luminescence Enhancement in Cyclometalated Rhodium(III) Complexes and Their Applications in Efficient Organic Light-Emitting Devices.

A series of luminescent cyclometalated rhodium(III) complexes have been designed and prepared. The improved luminescence property is realized by the judicious choice of a strong σ-donor cyclometalating ligand with a lower-lying intraligand (IL) state that would raise the d-d excited state and introduction of a lower-lying emissive IL excited state. These complexes exhibit high thermal stability and considerable luminescence quantum yields as high as up to 0.65 in thin film, offering themselves as promising light-emitting materials in OLEDs. Respectable external quantum efficiencies of up to 12.2% and operational half-lifetimes of over 3000 h at 100 cd m-2 have been achieved. This work demonstrates a breakthrough as the first example of an efficient rhodium(III) emitter for OLED application and opens up a new avenue for diversifying the development of OLED materials with rhodium metal being utilized as phosphors.

Highly Efficient Deep-Red Organic Light-Emitting Devices Based on Asymmetric Iridium(III) Complexes with the Thianthrene 5,5,10,10-Tetraoxide Moiety.

Highly efficient deep-red organic light-emitting devices (OLEDs) are indispensable for developing high-performance red-green-blue (RGB) displays and white OLEDs (WOLEDs). However, the shortage of deep-red emitters with high photoluminescence quantum yields (PLQYs) and balanced charge injection/transport abilities has severely restricted the performance of deep-red OLEDs. Herein, we design and synthesize four efficient emitters by combining the isoquinoline group with the thianthrene 5,5,10,10-tetraoxide group. Benefited from the introduction of the thianthrene 5,5,10,10-tetraoxide group, these Ir(III) complexes show improved electron-injection/-transport abilities. By enhancing the contribution of the triplet metal-to-ligand charge transfer (3MLCT) in emissions, the asymmetric configuration endows the related deep-red Ir(III) complexes with high PLQYs of 0.45-0.50 in solutions. More importantly, PLQYs of these Ir(III) complexes in doped host films increase up to 0.91, which is much higher than PLQYs reported for conventional deep-red Ir(III) complexes with impressive electroluminescent performance. As a result, solution-processed OLEDs based on these Ir(III) complexes exhibit deep-red emissions with Commission Internationale de L'Eclairage (CIE x, y) coordinates very close to the National Television System Committee (NTSC)-recommended standard red CIE coordinates of (0.67, 0.33). Furthermore, a deep-red OLED using the asymmetric Ir(III) complex SOIrOPh as the emitter shows outstanding performance with a peak external quantum efficiency (EQE) of 25.8%, which is the highest EQE reported for solution-processed deep-red OLEDs. This work sheds light on the great potential of utilizing the thianthrene 5,5,10,10-tetraoxide group to develop phosphorescent emitters for highly efficient OLEDs.

Multiple regulation of efficient organic light-emitting diodes using magnetic composite nanoparticles

An alternative strategy, adopting the magnetic composite nanoparticles in the hole injection layer, is proposed in this Letter to obtain efficient organic light-emitting devices (OLEDs) by simultaneously integrating the localized surface plasmon (LSP), light-scattering, and magnetic field effects (MFE). A 90% increase in the current efficiency of Alq3-based fluorescent OLEDs is obtained by using the Fe3O4@Au nanoparticles (FOA NPs), compared with the device without NPs, arising from the LSP effect induced by Au NPs and the strong light-scattering effect related to FOA NPs. Meanwhile, the current efficiency can be further increased by ∼20% under an external magnetic field (EMF) of 0.5T due to the increased singlet exciton induced by the spin injected holes via FOA NPs. Moreover, with an EMF, the current efficiency of Ir(ppy)2(acac)-based OLEDs is decreased slightly, implying that the emitting material system and mechanism have a great effect on the MFE. These results confirm the extensive use of FOA NPs in OLEDs and pave a new way to construct the FOA-NPs-based white OLEDs with both fluorescence and phosphorescence emitters, whose intensity and color can be modulated.

Highly efficient organic light-emitting devices based on multifunctional nanoparticles.

We have constructed a new approach to realize the light extraction enhancement in organic light-emitting diodes (OLEDs) by using Fe3O4@Au nanoparticles (FOA NPs). The current efficiency of 97.8cd/A at a luminance of 500 cd/m2 can be obtained for the FOA-NP-based device, while it is only 61cd/A for the control device without FOA NPs, corresponding a 60% enhancement in efficiency. The time-resolved photoluminescence and optical haze characterizations were performed to study the influence of FOA NPs on the lifetime of excitons and the light-scattering effect, respectively. These results reveal that the enhanced radical efficiency of an FOA-NP-based device is mainly ascribed to the localized surface plasmonic effects induced by Au NPs and the strong light-scattering effect related to FOA NPs, thereby achieving a double enhancement in light extraction, which paves a new way in realizing high-efficiency and controllable OLEDs.

Highly efficient and bright blue organic light-emitting devices based on solvent engineered, solution-processed thermally activated delayed fluorescent emission layer

Abstract Inefficient blue emission is one of the key bottlenecks limiting the development of solution processable displays and white light sources, which is essentially governed by the film quality of the solution-processed emission layer (EML). Here, we use an efficient blue thermally activated delayed fluorescent (TADF) emitter, bis [4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]solfone (DMAC-DPS), as EML and propose a solvent engineering strategy to achieve high efficiency and brightness. The strategy employs synergetic solvents to improve the film morphology of the solution-processed DMAC-DPS EML. The improved film quality enhances the carrier injection while reduces the surface trap charges, as revealed by the hole-/electron-only device performance and the transient electroluminescence measurements. As expected, the blue organic light-emitting devices (OLEDs) employing synergetic solvent strategy presents an increment of 22.8% in efficiency and 19.4% in luminescence, which reaches the highest efficiency of 15.76 cd/A and an efficiency of 9.63 cd/A at a luminance of 1000 cd/m2. These values are generally higher than the reported values from the blue OLEDs based on solution-processed DMAC-DPS EML.

Control of Molecular Orientation in Organic Semiconductor Films using Weak Hydrogen Bonds.

Use of the intrinsic optoelectronic functions of organic semiconductor films has not yet reached its full potential, mainly because of the primitive methodology used to control the molecular aggregation state in amorphous films during vapor deposition. Here, a universal molecular engineering methodology is presented to control molecular orientation; this methodology strategically uses noncovalent, intermolecular weak hydrogen bonds in a series of oligopyridine derivatives. A key is to use two bipyridin-3-ylphenyl moieties, which form self-complementary intermolecular weak hydrogen bonds, and which do not induce unfavorable crystallization. Another key is to incorporate a planar anisotropic molecular shape by reducing the steric hindrance of the core structure for inducing π-π interactions. These synergetic effects enhance horizontal orientation in amorphous organic semiconductor films and significantly increasing electron mobility. Through this evaluation process, an oligopyridine derivative is selected as an electron-transporter, and successfully develops highly efficient and stable deep-red organic light-emitting devices as a proof-of-concept.

Realizing an efficient warm white organic light-emitting device possessing excellent color-stability and color rendering index

Abstract We were successful in realizing an efficient warm white organic light-emitting device simultaneously exhibiting extreme color-stability and high color rendering index (CRI) just by precisely confining the recombination zone and fully utilizing the energy transfer between excitons. The warm white OLED presents maximum efficiencies of 21.4 cd/A, 25.8 lm/W and still remains at high efficiencies of 17.7 cd/A, 15.6 lm/W at the luminance of 1000 cd/m2. As the operating current density increases from 10 mA/cm2 to 60 mA/cm2, the variations in Commission Internationale de l’Eclairage (CIE) coordinates are only (0.004, 0.007) while the general CRI (Ra) maintains at 80 approximately and the special CRI R9 is higher than 20. What is more, the Duv satisfies the criteria for CRI measurements and the gamut area index (GAI) ensures the vividness of the illuminated objects, as renders the fabricated white OLED a promising candidate for high-quality solid-state lighting application. We also studied the origin of the quite slight color variations for the warm white OLED.

Improving the color-rendering index of a tandem warm white organic light-emitting device by employing a simple fabrication process.

We realized an efficient spectra-stable tandem white organic light-emitting device (WOLED) with pleasurable warm white emission and an extremely high color-rendering index (CRI) simultaneously by connecting a fluorescent blue unit and a phosphorescent dual-color unit via an easy-fabrication charge generation unit (CGU). Over a wide range of driving current density, the tandem warm white OLED exhibited general CRI (Ra) value exceeding 85 and special CRI (R9) value close to 57. In addition, Duvs are within the tolerance for CRI measurements, ensuring validity and logicality of the calculated results concerning Ra and R9. The obtained tandem OLED showed the maximum efficiencies of 38cd/A and 22.9lm/W, and still maintained high efficiencies of 35.5cd/A and 17.2lm/W at the luminance of 1000 cd/m2. Moreover, we studied the origins of both extreme color stability in dual-color OLED and extremely efficient electron injection within the CGU. To the best of our knowledge, this is the first report regarding quantitative values of both R9 and the Duv for a high-Ra tandem WOLED.

高效叠层有机电致发光器件的研制

高效叠层有机电致发光器件的研制