Efficient 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.

Donor engineering to regulate fluorescence of a symmetrical structure based on a fluorene bridge for white light emission.

A white organic light-emitting device (WOLED) obtained using blue and yellow complementary colors possesses extremely high optical efficiency. We designed and prepared a completely symmetric D-π-D type efficient blue light small molecule FFA based on octylfluorene as a π bridge, where the undoped device showed efficient blue organic light-emitting device (OLED) performance with a maximum emission wavelength of 428 nm, Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.11) and one of the narrowest full width at half maximum (FWHM) of 35 nm. To improve the matching measure of complementary color materials for achieving white light emission, a yellow light small molecule FCzA was prepared by adjusting the conjugation degree of peripheral electron-donating groups based on the same fluorene-based π bridge with FFA. Undoped devices based on FCzA demonstrated an electroluminescence (EL) emission peak at 576 nm with CIE coordinates of (0.43, 0.49) and a relatively wide FWHM of 130 nm. Ultimately, the white OLED device was modulated with CIE coordinates located at (0.33, 0.38) via proportional regulation with a mixture of FFA and FCzA in a ratio of 10 : 3 as the light-emitting layer.

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%.

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 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.

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.

Color-stable WRGB emission from blue OLEDs with quantum dots-based patterned down-conversion layer

For full-color displays, white organic light-emitting devices (OLEDs) combining with color filters are a promising technology. It avoids the precise shadow masking that is often used to separate RGB pixels. But the spectra of traditional white OLEDs are changeable, which would make the full-color displays show poor color stability. In this work, in order to realize color-stable white, red, green and blue emission, blue bottom-emitting OLEDs are integrated with color filters and two types of patterned down-conversion films containing red and green quantum dots (QDs). Besides, by solution-based transfer printing, micro-pillar arrays are formed on the surface of down-conversion films to improve the light extraction property. Based on this principle, color-stable white, red, green and blue emission are achieved from efficient blue OLEDs. Due to the highly saturated color emission of QDs, wider color gamut can be anticipated by employing these devices as three primary colors, nearly 15% wider than that based on commercial white LEDs or white OLEDs with the same color filters.

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).

A novel spiro-annulated benzimidazole host for highly efficient blue phosphorescent organic light-emitting devices

A novel host material featuring a spiro-annulated benzimidazole configuration is exploited for blue phosphorescent organic light-emitting devices (PhOLEDs). The new material exhibits a high triplet energy (3.07 eV) and a bipolar characteristic and is effective as the host for FIrpic-, FIr6- and FK306-based blue PhOLEDs with high performances.

Anomalously Long-Lasting Blue PhOLED Featuring Phenyl-Pyrimidine Cyclometalated Iridium Emitter

Summary Simply replacing pyridine in cyclometalating ligands in FIrpic with pyrimidine affords the complex MS 2 , which has a slight red-shifted emission but shorter excited-state lifetime and the capability to form a highly efficient blue phosphorescent organic light-emitting device (OLED) with long operation lifetime ( T 50 > 2,200 hr). Further replacement of the picolinate-based ancillary ligand of MS 2 with strong-field CF 3 -containing pyridine-azole ancillary ligands leads to bluer emitters ( MS 17 and MS 19 ) with nearly unitary photoluminescence quantum yield and preferential horizontal emitting dipole orientations (with horizontal dipole ratios of 75%–77%) beneficial for OLED optical out-coupling. OLEDs using MS 17 and MS 19 as emitters gave bluer emission and very high external quantum efficiency exceeding 31% yet with comparably short device lifetimes as FIrpic -based devices. Theoretical analysis indicates that the distinct stability of the MS 2 -based device might be attributed to its relatively large energy difference between 3 MC (metal center d-d transition) and 3 MLCT states.

Suppression of efficiency roll-off in highly efficient blue phosphorescent organic light-emitting devices using novel iridium phosphors with good electron mobility

High-efficiency blue organic light-emitting diodes were reported by adopting two novel iridium phosphors. Due to phosphoryl moiety in ancillary ligands, both complexes (dfppy)2Ir(ppp) and (dfppy)2Ir(dpp) (dyppy = 2-(2,4-difluorophenyl)pyridine, ppp = phenyl(pyridin-2-yl)phosphinate, dpp = dipyridinylphosphinate) own high electron mobility which can balance the injection and transport of carriers. Furthermore, the double light-emitting layers with TcTa (4,4′,4″-tris(carbazol-9-yl)triphenylamine) and 26DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine) hosts broaden the exciton formation zone and suppress efficiency roll-off. The optimized double light-emitting layers devices exhibited decent performances with peak current efficiency near 50 cd/A and external quantum efficiency above 20% as well as negligible efficiency roll-off.

Adjusting Nitrogen Atom Orientations of Pyridine Ring in Tetraphenylsilane-Based Hosts for Highly Efficient Blue Phosphorescent Organic Light-Emitting Devices.

Four wide bandgap host materials, namely, 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silyl-phenyl)-9H-carbazole (CSmP), 9-(4-diphenyl(4-(pyridin-2-yl)phenyl)silylphenyl)-9H-carbazole (CSoP), 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silylphenyl)-9H-3,9'-bicarbazole (DCSmP), and 9-(4-(diphenyl(4-(pyridin-2-yl)phenyl)silyl)phenyl)-9H-3,9'-bicarbazole (DCSoP), have developed by incorporation of pyridine with varied N atom orientation and carbazole/dimer carbazole units into the tetraphenylsilane skeleton for blue phosphorescent light-emitting diodes. These host materials all possess wide bandgap (3.54-3.64 eV) and high triplet energies (2.77-2.95 eV). As revealed by the absorption and emission spectra, theoretical calculations, and CV measurements, the N atom orientation exerts a strong influence on the LUMO energy level and electron-transportation behaviors without deterioring the photophysical properties. Among them, DCSmP with 3-pyridyl substituent manifests the best electron-transporting capability. The FIrpic-doped blue phosphorescent device using DCSmP as host material exhibits excellent electroluminescence performance with a maximum current efficiency of 40.1 cd A(-1) and a maximum external quantum efficiency of 20.0%. The current efficiency and external quantum efficiency are improved 3-fold, higher than those fabricated from DCSpP with 4-pyridyl as substituent, demonstrating an effective strategy for large improvement in device performance by a subtle change in molecular structure.

Sulfonyl-Substituted Heteroleptic Cyclometalated Iridium(III) Complexes as Blue Emitters for Solution-Processable Phosphorescent Organic Light-Emitting Diodes.

The synthesis is reported of a series of blue-emitting heteroleptic iridium complexes with phenylpyridine (ppy) ligands substituted with sulfonyl, fluorine, and/or methoxy substituents on the phenyl ring and a picolinate (pic) ancillary ligand. Some derivatives are additionally substituted with a mesityl substituent on the pyridyl ring of ppy to increase solubility. Analogues with two ppy and one 2-(2′-oxyphenyl)pyridyl (oppy) ancillary ligand were obtained by an unusual in situ nucleophilic displacement of a fluorine substituent on one of the ppy ligands by water followed by N^O chelation to iridium. The X-ray crystal structures of seven of the complexes are reported. The photophysical and electrochemical properties of the complexes are supported by density functional theory (DFT) and time-dependent DFT calculations. Efficient blue phosphorescent organic light-emitting devices (PhOLEDs) were fabricated using a selection of the complexes in a simple device architecture using a solution-processed single-em...

Efficient blue fluorescent electroluminescence based on a tert-butylated 9,9′-bianthracene derivative with a twisted intramolecular charge-transfer excited state

An orthogonally twisting tert-butylated 9,9′-bianthracene derivative, namely 10,10′-bis(4-tert-butylphenyl)-9,9′-bianthracene (BAn-(4)-tBu), possessing high thermal stability and high luminescence efficiency was synthesized. The BAn-(4)-tBu emitter with a twisted intramolecular charge transfer state characteristics, originating from its twisting 9,9′-bianthracene moiety, enhanced the occurrence of singlet excitons in the fluorescent emitter. The BAn-(4)-tBu device doped with sky blue fluorescent material 4,4-bis[4-(di-p-tolylamino)styryl]biphenyl displayed a maximum singlet generation fraction of >25%, maximum current and external quantum efficiencies of 6.00cdA−1 and 3.22%, respectively. Remarkably, a low efficiency roll-off was observed. These results demonstrated that BAn-(4)-tBu as a host material is advantageous for fabrication of highly efficient blue fluorescent organic light-emitting devices.

Study on blue organic light-emitting diodes doped with 4,4’‐bis (9‐ethyl‐3carbazovinylene)‐1,1’‐biphenyl in various host materials

We have fabricated efficient blue organic light-emitting devices (OLEDs) with 4,4′-bis (9-ethyl-3carbazovinylene)-1,1′-biphenyl(BCzVBi) as the fluorescent emitter doped into 4,4′-bis(carbazol-9-yl)biphenyl(CBP) and 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi), respectively, and the results show that luminance and luminous efficiency are greatly enhanced in the doped devices. Particularly, the optimized blue CBP-host device with a well-designed structure has a significantly higher luminous efficiency of 4.45cd/A. The energy level structure of the BCzVBi molecules is obtained, which yields useful information on the light emission processes. We carry out a spectroscopic analysis based on Gaussian multi-peak fit for the electroluminescence (EL) emission spectra and present a theoretical explanation of the energy transfer mechanism in the host–guest system. These are expected to provide an effective strategy in enhancing high-efficiency blue OLEDs.

Efficient blue organic light-emitting devices based on solution-processed starburst macromolecular electron injection layer

Highly efficient blue organic light-emitting devices using an alcohol-soluble conjugated starburst macromolecule TrOH as electron injection layer were investigated. Multilayered OLEDs based on a blue polymer poly(9,9-dioctylfluorene) (PFO) and a blue small molecule 1,6-bis(3′,6′-bis(octyloxy)spiro [fluorene-9,9′- xanthene]-2-yl)pyrene (DOSFXPy) were fabricated by solution-processing. The devices exhibit low turn-on voltages of 3.8V (PFO) and 3.5V (DOSFXPy), and high current efficiencies of 4.3cd/A (PFO) and 3.1cd/A (DOSFXPy), respectively, which are better than those of the devices using TPBi/LiF as electron injection layer. These improved properties are attributed to the reduced barrier for electron injection by inserting a TrOH electron injection layer.

Efficient single light-emitting layer pure blue phosphorescent organic light-emitting devices with wide gap host and matched interlayer

In this work, we report the highly efficient pure blue electroluminescent (EL) device based on bis[(3,5-difluoro-4-cyanophenyl)pyridine]picolinate iridium(III) (FCNIrpic) doped 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) film. The matched energy levels of FCNIrpic and CzSi are helpful in facilitating the trapping of carriers, while the high triplet energy of CzSi can well avoid the undesired reverse energy transfer. More importantly, the injection of holes was further accelerated by inserting 5nm 4,4′,4″-Tri(9-carbazoyl)triphenylamine (TcTa) film between hole transport layer and lighting-emitting layer (EML) as interlayer. Consequently, EL performances were significantly enhanced attributed to wider recombination zone and better balance of holes and electrons. Interestingly, single-EML device displayed higher performances than those of double-EMLs device. Finally, pure blue EL device with the structure of ITO/MoO3 (3nm)/TAPC (40nm)/TcTa (5nm)/FCNIrpic (20%): CzSi (30nm)/TmPyPB (40nm)/LiF (1nm)/Al (100nm) realized the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 12,505cd/m2, 36.20cd/A, 28.42lm/W and 16.9%, respectively. Even at the high brightness of 1000cd/m2, current efficiency and external quantum efficiency up to 17.40cd/A and 8.1%, respectively, can be retained by the same device.