Blue Organic Light-emitting Devices Research Articles

Highly Efficient Blue Fluorescent Organic Light-Emitting Devices Based on λ5-Phosphinine Derivatives.

Although λ5-phosphinine derivatives are known as a promising class of blue fluorescent emitters, those photoluminescent quantum yield (PLQY) values have been reached up to 92 %, however, only a few examples have been explored as an emitter for blue organic light-emitting device (OLED), and the external quantum efficiency (EQE) has been below 2.4 % so far. In this study, we newly developed two types of blue λ5-phosphinine derivatives namely CN-COCF3 and CO2Me-CHO, and investigated the photophysical properties in the solid states. The photophysical analyses in solid state films suggested that the strong electron-accepting nature of these λ5-phosphinine derivatives caused the inferior PLQY values, and the exciplex formation with the host and neighboring materials should be avoided to improve the device efficiency. By choosing suitable host and neighboring materials with deep ionization potentials, we successfully realized efficient blue fluorescent OLEDs with EQE of over 4 % and CIE (0.14, 0.18). This is among the best in λ5-phosphinine-based blue OLEDs so far.

Tetradentate Pt(II) Complexes with Bulky Carbazole Moieties for High-Efficiency and Narrow-Emitting Blue Organic Light-Emitting Devices.

Blue tetradentate Pt(II) complexes, Pt-tBuCz and Pt-dipCz, are synthesized by introducing carbazoles with bulky substituents for improving the rigidity and inhibiting intermolecular interactions of phosphorescent emitter. tert-Butyl and 2,6-diisopropylphenyl groups are substituted as the blocking groups at 3 position of the carbazole in Pt-tBuCz and Pt-dipCz, respectively. These new phosphorescent emitters exhibit a narrow full width at half maximum (FWHM) and a high horizontal emitting dipole orientation ratio. Pt-dipCz demonstrates a small FWHM of 24nm, a high emitting dipole orientation ratio of 81%, and a high photoluminescence quantum yield value of 94%. As a result, the Pt-tBuCz and Pt-dipCz devices exhibited external quantum efficiencies (EQEs) of 23.7% and 25.0% with small FWHMs of 25 and 22nm, respectively. For the Pt-dipCz device, the small FWHM and high EQE of >20% are maintained even at a doping concentration of 20 wt%. Furthermore, phosphor-sensitized organic light-emitting diodes fabricated using Pt-dipCz as a sensitizer achieved a high EQE of 31.4% with an FWHM of 18nm. This result indicates that the 2,6-diisopropylphenyl group is a effective blocking group for Pt(II) complexes to develop highly efficient, color stable, doping concentration resistant, and efficiently sensitizing blue phosphors.

Performance enhancement of blue organic light-emitting devices by inserting BPhen: Alq3 as an n-doping electron transport layer

An n-doping electron transport layer (ETL) that consists of 4-7-diphenyl-1, 10-phenanlhroline (BPhen) and tris(8-hydroxyquinoline) aluminum (Alq3) was applied to enhance the performance of blue organic light-emitting diodes (OLEDs). The resulting n-doping OLED afforded a maximum luminance efficiency of 8.24 cd/A, which was higher than the devices using either BPhen or Alq3 as ETL by 56.9 % and 91.2 %, respectively. In addition, this device was found to possess a higher maximum power efficiency of 5.17 Lm/W compared to the blue OLEDs without such n-doping ETL concentrations. By performing the hole- and electron- ‘only’ devices and the density functional theory (DFT) calculations, we show that the enhancement of the device performance can be attributed to increased electron injection/transport at ETL interfaces and hence improved carrier balance in emission layer (EML).

Recent Advances in Triplet–Triplet Annihilation-Based Materials and Their Applications in Electroluminescence

Efficient and stable blue organic light-emitting devices (OLEDs) represent one of the most important components in future solid-state lighting and full color displays. Although red and green OLEDs have been successfully commercialized, it remains challenging to realize blue OLEDs showing high efficiency and high stability simultaneously, restricting their commercialization. Triplet–triplet annihilation (TTA)-based materials have been emerging as a new category of electroluminescent or host materials with promising potential for highly efficient and stable blue devices due to their appealing photophysical properties (e.g., ability to generate emissive singlet excited states through TTA process, high photoluminescence quantum yield, ease of molecular design, etc.). Recently, research on TTA-based materials toward high-performance blue OLEDs has received an increasing attention. In this review, recent research advances on the molecular design strategies, photophysical properties of TTA-based materials (both blue emitters and host materials), mechanism of TTA-based electroluminescence, as well as their OLED applications are presented comprehensively. Moreover, potential perspectives with future research opportunities in this field are also described. We believe that the emerging TTA-based materials will bring more opportunities for developing a large family of novel photofunctional materials showing appealing and tunable photophysical properties, thus providing new opportunities for high-efficiency and stable blue electroluminescence in the future.

Controlling the electronic structures of triphenylene based sky blue TADF emitters by chemical modifications for high efficiency with shorter emission lifetimes

Chemically modified triphenylene derivatives are one of the attractive candidates as an acceptor unit for realizing efficient blue thermally activated delayed fluorescence (TADF) emitters due to the rigid structure and the high triplet energy of 2.9 eV however, only a limited number of examples with low external quantum efficiency (EQE) below 10% are reported to date. Here, we developed three types of blue TADF emitters based on modified triphenylene acceptor units. These emitters showed sky blue emission with the emission peak wavelength around 470 nm, high photoluminescent quantum yield (PLQY) of ∼88%, and a short delayed lifetime (τd) of ∼7.7 μs. These emitters realized efficient sky blue organic light-emitting device with high EQE of over 20%.

Room-temperature corrugated indium zinc oxide anode to achieve high-efficiency blue phosphorescent organic light-emitting diodes

A corrugated indium zinc oxide thin film was fabricated on a glass substrate at room temperature for use in a blue phosphorescent organic light-emitting device (OLED) to facilitate device efficiency in term of the external quantum efficiency (EQE) of the device, which increased from 28.77% to 34.33% after the fabrication of the aforementioned layer. Furthermore, a remarkably high EQE of 52.51% was achieved through external light extraction by attaching a hemispheric macrolens on the substrate of the device. Consequently, the efficiency of the developed blue phosphorescent OLED became 1.83 times higher than that of a planar counterpart. Moreover, this finding was verified through theoretical analyses, that indicated the underlying light extraction physics was attributed to the efficient extraction of trapped light from the surface plasmon polariton, waveguide, and substrate modes. (submitted to Organic Electronics).

Research on Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub> doped polyfluorene organic light-emitting devices

Since the breakthrough by Tang et al. in 1987, organic light-emitting devices (OLEDs) have attracted extensive attention in the industries and academic research communities. OLEDs have many promising characteristics, such as self-illumination, lower power consumption, easy fabrication and so on. It has a broad development prospect in high resolution display and other fields. For RGB color OLED display technology, blue light organic material is very important. Polyfluorene (PFO) is a kind of rigid planar biphenyl structure compound in all kinds of OLEDs blue light materials. However, PFO has a very big disadvantage: the long wave shift of the light-emitting peak of the electroluminescent device will produce the green light-emitting band that should not have appeared. This seriously affects the saturation color purity of PFO devices, and also seriously restricts the industrialization process. In this paper, the molecular magnetic material [Fe(NH<sub>2</sub>trz)<sub>3</sub>· (BF<sub>4</sub>)<sub>2</sub>] is used to solve this problem. ITO/PEDOT:PSS (30 nm)/PFO:Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub> (65 nm)/CsCl (0.6 nm)/Al (120 nm) devices were fabricated on ITO glass substrate. It is the first time to report the strong pure blue emission of PFO by using the special electronic spin state modulation of Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub>. The influence of Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub> on the photoelectric properties of PFO was studied in detail by analyzing the PL and EL characteristics of PFO and PFO:Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub>. Under the bias voltage of 4 V to 9 V, the device without doping Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub> emits very strong green light. The central peak wavelength is 553 nm, and the color coordinates are (0.33, 0.45). Moreover, with the constant change of voltage, the green light-emitting band is always much larger than the blue light-emitting band. However, the obvious difference is that Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub> doped device emits strong blue light, the peak wavelength is 438 nm, and the color coordinates (0.23, 0.22), which is completely consistent with the peak wavelength of the PL spectrum of the PFO film; the green light-emitting band of the PFO is successfully suppressed; with the change of the electric voltage, the proportion of the blue light part of the device spectrum in the whole EL spectrum is almost unchanged. The photoconductivity effect of undoped Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub> device is further studied by means of the integrated opto-electro-magnetic measurement technology. Under different bias voltage, it is found that there is almost no excimer in PFO:Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub>. This study solves the problem of green light of polyfluorene, which has puzzled the industry for many years, and provides a reliable way for the industrialization of polyfluorene used in blue OLED. The mechanism of Fe(NH<sub>2</sub>trz)<sub>3</sub>·(BF<sub>4</sub>)<sub>2</sub> blocking the abnormal green emission of PFO was discussed by using the theory of luminescence dynamics.

The [3+2] Annulation of CF3-Ketimines by Re Catalysis: Access to CF3-Containing Amino Heterocycles and Polyamides.

SummaryTransition metal catalyzed [3 + 2] annulation of imines with double bonds via directed C-H activation offers a direct access to amino cyclic motifs. However, owing to weak coordination and steric hindrance, trifluoromethylated ketimines have been an unaddressed challenge for TM-catalyzed annulations. Here, a rhenium-catalyzed [3 + 2] annulation of trifluoromethylated ketimines with isocyanates via C(sp2)-H activation has been disclosed. This approach provides an efficient platform for rapid access to a privileged library of CF3-containing iminoisoindolinones and polyamides by utilizing challenging CF3-ketimines as the annulation component. The capability of gram scale synthesis, the post-functionalization of the cyclization adduct, the derivation of complex natural molecules and the facile synthesis of polyamides highlight a diversity of synthetic potential of the current methodology.

Tuning molecular emission of organic emitters from fluorescence to phosphorescence through push-pull electronic effects

Organic emitters with persistent phosphorescence have shown potential application in optoelectronic devices. However, rational design and phosphorescence tuning are still challenging. Here, a series of metal-free luminophores without heavy atoms and carbonyl groups from commercial/lab-synthesized carbazole and benzene were synthesized to realize tunable molecular emission from fluorescence to phosphorescence by simply substituent variation. All the molecules emit blue fluorescence in both solution and solid state. Upon removal of excitation source, the fluorinated luminophores show obvious phosphorescence. The lab-synthesized carbazole based molecules exhibit a huge lifetime difference to the commercially purchased ones due to the existence of isomer in the latter samples. The small energy gap between singlet and triplet state and low reorganization energy help enhance intersystem crossing to contribute to a more competitive radiative process from triplet to ground state. Blue and white organic light-emitting devices are fabricated by using fluorinated luminophore as emitting layer.

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 and wide-color-gamut organic light-emitting devices using external microcavity

An external microcavity structure was introduced to the stacked cathode in organic light-emitting devices (OLEDs), and its influence on the luminescent characteristics was investigated. From the detailed optical analysis that was conducted, it was found that the surface plasmon loss in a metal cathode can be reduced to about one-fifth by using a semi-transparent thin-film cathode, and can be extracted outside through the microcavity effect after being converted to the thin-film waveguide mode. No less than 50% of the optical power in dipole emission was successfully utilized as the external and substrate modes. The luminous efficiency increased about 1.4-fold, and the color purity of the luminance also improved. As a trial preparation of red, green, and blue OLEDs with the external microcavity structure, the color gamut improved greatly and approached the BT.2020 national standard. The effect of the external microcavity will be discussed from the viewpoint of multi-scale optical analysis.

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.

Periphery Design of Macrocyclic Materials for Organic Light-Emitting Devices with a Blue Phosphorescent Emitter.

Cyclo- meta-phenylenes were modified with trifluoromethyl groups at their periphery to create host materials suitable for use in blue phosphorescent organic light-emitting devices. The periphery design resulted in molecules with high triplet-state energies, which were required to support the blue emission from Ir phosphors. As a result, an external quantum efficiency of 9.9% was achieved. The most successful host, a pentameric congener, preferred CF-π/CH-π interactions in its crystalline packings, which could be beneficial for the host performance.

High-Efficiency Blue Phosphorescent Organic Light-Emitting Devices with Low Efficiency Roll-Off at Ultrahigh Luminance by the Reduction of Triplet-Polaron Quenching.

High-performance phosphorescent organic light-emitting devices (PhOLEDs) at high luminance are still a remaining problem that needs to be solved, especially blue PhOLEDs. Here, 5-(5-9 H-carbazol-9-yl)pyridin-2-yl)-8-(9 H-carbazol-9-yl)-5 H-pyrido[3,2- b]indole (p2PCB2CZ) with excellent characteristics as a host is designed to realize a novel host-guest system without hole trapping effect in blue PhOLEDs. The device in which p2PCB2CZ and bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic) is used as host and guest, respectively, is proposed to improve the performances of blue PhOLEDs at high luminance, especially at ultrahigh luminance (>30000 cd/m2). The maximum external quantum efficiency (EQE) of this type of blue PhOLEDs is 19.2%, while the maximum EQE of the reference blue PhOLEDs is 18.7%. Nevertheless, the p2PCB2CZ-based devices exhibit significant advantages at high luminance, because its EQE still attains to 10.8% even when the luminance increases to 30000 cd/m2, which is 1.67 times that of the reference device. From measurements based on steady-state and time-resolved spectroscopies, the reduction of triplet-polaron quenching in p2PCB2CZ-based devices is proved to be the main reason for improving the performances of blue PhOLEDs at high luminance.

Carrier transport manipulation for efficiency enhancement in blue phosphorescent organic light-emitting devices with a 4,4′-bis(N-carbazolyl)-2,2′-biphenyl host

The EQE of CBP-based blue PhOLEDs is effectively enhanced by achieving a more balanced distribution of carriers in the EML.

Highly efficient blue organic light-emitting devices based on an exciplex host with thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) resulting from the harvesting of triplet excitons has currently emerged as an excellent candidate for enhancing the efficiency of organic light-emitting devices (OLEDs). Highly efficient blue OLEDs based on an exciplex host with carbazole/thioxanthene-S, S-dioxide (EBCz-ThX) and 2-phenyl-bis-4, 6-(3, 5-di-4-pyridylphenyl) pyrimidine (B4PYPPM) acting as a blue TADF emitter were fabricated. The maximum values of the current and the power efficiency for the blue OLEDs with an EBCz-ThX:B4PYPPM exciplex host were 22.46 cd/A and 28.23 lm/W, respectively. The power efficiency of blue OLEDs with an exciplex host was much higher than that of conventional blue OLEDs. The efficiency enhancement of the blue OLEDs based on an exciplex system with a TADF emitter was attributed to the efficient up-conversion of the triplet excitons in the EBCz-thx:B4PYPPM and to the efficient energy transfer from the exciplex host to the blue TADF emitter.

Low color temperature, high color rendering index candlelight style white organic light-emitting devices with a fac-tris (mesityl-2-phenyl-1H-imidazole) iridium (III) blue emitting layer

Blue phosphorescent organic light-emitting devices based on fac-tris (mesityl-2-phenyl-1H-imidazole) iridium (III) (fac-Ir(mpim)3) doped in different host materials are fabricated. The device with a 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine host shows an excellent performance. Based on this blue emitting device, white organic light-emitting devices with high performance are achieved. The optimized device shows a color rendering index of 92 and a color temperature of 2572 K, which are comparable to the incandescent bulb. Besides, it also presents a high current efficiency of 23.1 cd A−1 and a power efficiency of 15.8 lm W−1 at brightness of 1000 cd m−2. This candlelight style white organic light-emitting device has the potential as a comfortable lighting source.

Interfacial engineering of graphene for highly efficient blue and white organic light-emitting devices

Graphene as anodes of flexible organic light-emitting devices (OLEDs) has intrinsic drawbacks of a low work function and a high sheet resistance although it can eliminate the brittle feature of ITO. Chemical doping as a conventional approach is universally used to decrease the sheet resistance and adjust the work function of graphene electrodes, but it suffers from instability problems due to the volatility of chemical species. Here, an insulated poly(4-styrenesulphonate) (PSS) modification layer is firstly coated on the graphene surface along with improved air-stability and hole-injection ability via interfacial dipoles. Besides, the utilization of PSS is beneficial to reduce the leakage current of OLEDs. Then a gradient injection layer of poly(3,4-ethylenedioxythiophene):PSS (PEDOT:PSS)/tetrafluoroethyleneperfluoro-3,6-dioxa-4-methyl-7-octenesulphonic acid copolymer-doped PEDOT:PSS is covered onto the PSS-modified graphene to further promote hole injection and improve carrier balance inside OLEDs. With above interfacial modification technique, very high efficiencies of 201.9 cd A−1 (76.1 lm W−1, 45.2%) and 326.5 cd A−1 (128.2 lm W−1, 99.5%) for blue and white emissions are obtained, which are comparable to the most efficient display and lighting technologies so far.

Efficient non-doped blue phosphorescent organic light-emitting devices by incorporating Ag-island nanostructures

In this work, vacuum-evaporated Ag-island nanostructures were incorporated into the electron transporting layer (ETL) and their effects on the performance of non-doped blue phosphorescent organic light-emitting devices (PhOLEDs) using an ultrathin phosphorescent dye as the emitting layer (EML) were investigated systematically. By changing the thickness and deposition rate of Ag layer, the surface morphologies and resonance absorption spectra of the Ag-island nanostructures can be fine modulated. The device performance is significantly dependent on the structure and position of Ag layer. The experimental results indicate that 0.5 nm Ag layer deposited at a rate of 0.06 Å/s presents excellent spectral overlap between the resonance absorption spectra of Ag-island layer and the emission spectra of EML that impacts the electrical properties. Compared to the reference device without Ag-island layer, the current efficiency of the optimized device was enhanced by 54% from 15.1 cd/A to 23.2 cd/A when 0.5 nm Ag-island layer was incorporated into ETL with 20 nm away from EML. The performance enhancement is attributed to the synergistic effects for the improved electron transport properties induced by the Ag-island layer and the effective couplings between excitons and localized surface plasmons (LSPs).