Deep-blue Organic Light-emitting Devices Research Articles

Fluorescent pyrene-imidazole material for deep-blue organic light-emitting devices

Numerous endeavors have been exerted to devise deep-blue light-emitting materials (LEMs) for organic light-emitting devices (OLEDs). Nevertheless, deep-blue light-emitting materials exhibiting high efficiency for simple-structured OLEDs are still scarce. Herein, a donor-acceptor based LEM, 4'-(4, 5-diphenyl-1-(3-(pyren-1-yl)phenyl)-1H-imidazole-2-yl)-N,N-diphenylbiphenyl-4-amine (TPA-PyI) is developed by merging imidazole, triphenyl-amine and pyrene units for OLEDs. TPA-PyI is used as the emitting layer in OLED device, which emits a deep-blue fluorescence at 444 nm. With a current efficiency (CE) of 3.05 cd/A and a power efficiency (PE) of 2.95 lm/W, an EQE of 2.97% is achieved. High thermal-stability is achieved with decomposition-temperature (Td) of 516 °C.

Highly Efficient Deep-Blue Electroluminescence from a Charge-Transfer Emitter with Stable Donor Skeleton.

Organic materials containing arylamines have been widely used as hole-transporting materials as well as emitters in organic light-emitting devices (OLEDs). However, it has been pointed out that the C-N bonds in these arylamines can easily suffer from degradation in excited states, especially in deep-blue OLEDs. In this work, phenanthro[9,10-d]imidazole (PI) is proposed as a potential donor with higher stability than those of arylamines. Using PI as the donor, a donor-acceptor type deep-blue fluorophore 1-phenyl-2-(4″-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1':4',1″-terphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole (BITPI) is designed and synthesized. Results from UV-aging test on neat films of BITPI and other three arylamine compounds demonstrate that PI is indeed a more stable donor comparing to common arylamines. An OLED using BITPI as an emitter exhibits good device performances (EQE over 7%) with stable deep-blue emission (color index: (0.15, 0.13)) and longer operation lifetime than the similarly structured device using arylamine-based emitter. Single-organic layer device based on BITPI also shows superior performances, which are comparable to the best results from the arylamine-based donor-acceptor emitters, suggesting that PI is a stable donor with good hole transport/injection capability.

Deep blue organic light-emitting devices enabled by bipolar phenanthro[9,10-d]imidazole derivatives

Novel phenanthroimidazole derivatives with D–π–A structures have been successfully designed and prepared. Non-doped organic light emitting diodes (OLEDs) were fabricated by employing the compounds, which display deep blue emission.

A Deep-Blue Emitter with Electron Transporting Property to Improve Charge Balance for Organic Light-Emitting Device

Highly efficient deep-blue organic light-emitting devices (OLEDs) have been fabricated using 2,7-di(2,2':6',2″-terpyridin-4-yl)-9,9-dioctyl-9H-fluorene (DTPF) as the emitter, which has a wide energy gap, high emission quantum yield (Φf = 0.88), and high electron transporting property to improve the charge balance. A high efficiency of 2.55 cd/A and 2.67 lm/W are obtained in OLED. The device also exhibits a low turn-on voltage of 3.0 V and Commission Internationale de l'Éclairage (CIE) coordinates of (0.16, 0.09).

Synthesis and properties of n-type triphenylpyridine derivatives and applications in deep-blue organic light-emitting devices as electron-transporting layer

Two series of n-type triphenylpyridine derivatives with good thermal properties and efficient deep-blue emissions were designed, synthesized and systematically characterized. Most of them show high glass transition temperatures (Tg > 100 °C), relatively high electron mobilities, large ionization potentials (IP > 6.31 eV) and suitable electron affinities (EA > 2.93 eV) for facilitating efficient electron-injection. These attributes of the n-type triphenylpyridine derivatives favours their applications in organic light-emitting devices (OLEDs) as electron-transporting and hole-blocking materials (ETMs and HBMs). With these new materials, deep-blue OLEDs with a configuration of indium-tin oxide (ITO)/α-napthylphenylbiphenyl diamine (NPB)/9,10-di(2-naphthyl)anthracene (ADN)/triphenylpyridine derivative/LiF/MgAg were fabricated to investigate the properties of these triphenylpyridine derivatives as ETMs and HBMs. The devices show higher efficiency (2.54 cd A−1), and better color purity (0.15, 0.10) compared to those of similarly-structured blue OLEDs using state-of-the-art ETMs. The large IP and deep-blue emission of the triphenylpyridine derivatives are considered to be key factors for the higher efficiencies and better color purity. Optical and other properties of the compounds are discussed in terms of their molecular structures.

Blue Phosphorescent Mono-cyclometalated Iridium(III) Complexes

New deep blue phosphorescent iridium(III) complexes comprised of one cyclometalate, two phosphines trans to each other and two cis-ancillary ligands, such as Ir(F2Meppy)(PPh2Me)2(H)(Cl), Ir(F2Meppy)(PPh2Me)2(H)(NCMe) , and Ir(F2Meppy)(PPh2Me)2(H)(CN), [F2Meppy = 2-(2’, 4’difluorophenyl)-4-methyl-pyridine] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. We investigated the strong field effects of ancillary ligands to gain insight into the factors responsible for the emission color change and the different luminescence efficiency. Reducing the molecular weight of the phosphine ligand with PPh2Me leads to more efficient deep-blue organic light-emitting devices (OLED) by thermal processing instead of through solution processing. The electron-withdrawing difluoro group substituted on the phenyl ring, the electron-donating methyl group on the pyridyl ring, and the cyano strong field ancillary ligand increased the HOMO-LUMO gap and achieved a hypsochromic shift in the emission color. As a result, the maximum emission spectra of Ir(F2Meppy)(PPh2Me)2(H)(Cl), Ir(F2Meppy)(PPh2Me)2(H)(NCMe) , and Ir(F2Meppy)(PPh2Me)2-(H)(CN) were in the ranges of 440.5, 437, 436 nm, respectively.

Dumbbell-Shaped Spirocyclic Aromatic Hydrocarbon to Control Intermolecular π−π Stacking Interaction for High-Performance Nondoped Deep-Blue Organic Light-Emitting Devices

Dumbbell-shaped spirocyclic aromatic hydrocarbons on the basis of molecular modeling exhibit no π−π stacking interaction among chromophores in the molecular packing diagrams. A model compound of 1,6-di(spiro[fluorene-9,9′-xanthene]-2-yl)pyrene (DSFXPy) with the pyrene−pyrene distance of up to 10.4 A has been synthesized for high-performance nondoped deep-blue organic light-emitting devices. The proof-of-concept nondoped OLEDs have been fabricated with the configuration of ITO/MoOx(2 nm)/NPB(30 nm)/DSFXPy(30 nm)/TPBi(40 nm)/LiF(1 nm)/Al. DSFXPy-based nondoped OLED exhibits a maximum current efficiency of 7.4 cd/A (4.6% at 260 cd m−2) and excellent CIE coordinates (0.16, 0.15) (at 6500 cd m−2), surpassing most reported nondoped deep-blue OLEDs. Dumbbell-shaped spirocyclic arenes will be promising candidates for OLEDs.

Strong ligand field effects of blue phosphorescent mono-cyclometalated iridium(III) complexes

A series of mono-cyclometalated blue phosphorescent iridium(III) complexes with two phosphines trans to each other and two cis-ancillary ligands, such as Ir(F 2Meppy)(PPhMe 2) 2(H)(Cl), [Ir(F 2Meppy)(PPhMe 2) 2(H)(NCMe)] + and Ir(F 2Meppy)(PPhMe 2) 2-(H)(CN), [F 2Meppy = 2-(2′,4′-difluorophenyl)-4-methyl-pyridine] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. We investigate the electron-withdrawing capabilities of ancillary ligands using the DFT and TD-DFT calculations on the ground and excited states of the three complexes to gain insight into the factors responsible for the emission color change and the different luminescence efficiency. Reducing the molecular weight of phosphine ligand with PPhMe 2 leads to a strategy of the efficient deep blue organic light-emitting devices (OLED) by thermal processing instead of the solution processing. The electron-withdrawing difluoro group substituted on the phenyl ring and the cyano strong field ancillary ligand in the trans position to the carbon atom of phenyl ring increased HOMO-LUMO gap and achieved the hypsochromic shift in emission color. As a result, the maximum emission spectra of Ir(F 2Meppy)(PPhMe 2) 2(H)(Cl), [Ir(F 2Meppy)(PPhMe 2) 2(H)-(NCMe)] + and Ir(F 2Meppy)(PPh-Me 2) 2 (H)(CN) were in the ranges of 446, 440, 439 nm, respectively.

Strong Ligand Field Effects of Blue Phosphorescent Iridium(III) Complexes

In this study, new deep blue emitting mixed ligand iridium(III) complexes such as Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy)(PPhMe2)2(H)(NCMe)]+ and Ir(dFppy)(PPhMe2)2(H)(CN), [dFppy = 2-(2′,4′-difluoro-phenyl)pyridine] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. These iridium complexes comprise one cyclometalating, two phosphines trans to each other and two cis-ancillary ligands. We investigate the electron-withdrawing capabilities of ancillary ligands using the DFT and TD-DFT calculations on the ground and excited states of the three complexes to gain insight into the factors responsible for the emission color change and the different luminescence efficiency. Reducing the molecular weight of phosphine ligand with PPhMe2 leads to a strategy of the efficient deep blue organic light-emitting devices (OLED) by thermal processing instead of the solution processing. The electron-withdrawing difluoro group substituted on the phenyl ring and the cyano strong field ancillary ligand in the trans-position to the carbon atom of phenyl ring increased HOMO-LUMO gap and achieved the hypsochromic shift in emission color. As a result, the maximum emission spectra of Ir(dFppy)(PPhMe2)2(H)(Cl), [Ir(dFppy) (PPhMe2)2(H)(NCMe)]+ and Ir(dFppy)(PPhMe2)2(H)(CN) were in the ranges of 452, 443, 442 nm, respectively.

Controlling Supramolecular Microstructure to Realize Highly Efficient Nondoped Deep Blue Organic Light-Emitting Devices: The Role of Diphenyl Substituents in Distyrylbenzene Derivatives

trans-Distyrylbenzene (trans-DSB), the most typical model compound for poly(p-phenylenevinylene) (PPV) derivatives, has highly efficient and pure blue emission in dilute solution; while in solid or film state, its photoluminescence efficiency decreases dramatically with red-shifted emission, because of the formation of H-aggregate. Herein, we introduce two phenyl substituent groups to the central phenyl ring of trans-DSB, which restrains the formation of H-aggregate efficiently in the condensed state and then induces highly luminescent molecular aggregate modes (X-aggregate and J-aggregate). Most important, the introduction of the phenyl substituent groups makes the resulting compounds form high quality films easily under vacuum deposition conditions. High performance nondoped deep blue organic light-emitting device with CIE (0.15, 0.10), maximum luminous efficiency of 4.2 cd/A (corresponding external quantum efficiency of 3.9%), is achieved on the basis of this kind of material.

Efficiency Stabilized Deep Blue Organic Light-Emitting Devices with a DPVBi/CBP Step Emitting Layer Operating at Low Voltages

The electrical and the optical properties of deep blue organic light-emitting devices (OLED) utilizing a step emitting layer (SEL) consisting of 4,4′-Bis(2,2′-diphenyl-ethen-1-yl)biphenyl (DPVBi) and 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) were investigated. The turn-on voltage of the OLEDs with a SEL was smaller than those of the OLEDs with a CBP or a DPVBi emitting layer. While the efficiency of the OLEDs with a SEL was very stable, regardless of variations in the applied voltage, that of the OLEDs with a CBP or a DPVBi emitting layer varied with the applied voltage. The dominant peak in the electroluminescence spectrum for the OLEDs with a SEL appeared at 455 nm. The Commission Internationale de l'Eclairage chromaticity (CIE) coordinates of the OLEDs with a SEL at 11 V were (0.160, 0.168), which is similar to the CIE coordinates of the CBP layer.

Efficiency stabilization in deep-blue organic light-emitting devices with a double emitting layer acting as electron and hole trapping layers

The influence of the presence of a double emitting layer (DEL) consisting of 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) and 4,4′-Bis(2,2-diphenyl-ethen-1-yl)diphenyl (DPVBi) acting as electron and hole trapping layers in the organic light-emitting devices (OLEDs) was studied. While the luminance efficiency of the OLEDs with a DEL was very stable, regardless of variations in the applied voltage, that of the OLEDs with a CBP or a DPVBi emitting layer varied with the applied voltage. The rate of decrease is smaller than the single emitting layer, about 30%. The dominant peak corresponding to the CBP layer, in the electroluminescence spectrum for the OLEDs with a DEL appeared at 451 nm, which is in the deep-blue region. The Commission Internationale de l'Eclairage chromaticity coordinates of the OLEDs with a DEL at 11 V were (0.150, 0.137), indicative of a deep, stabilized blue color. These results indicate that luminance efficiency-stabilized blue OLEDs can be fabricated using a CBP/DPVBi DEL acting as electron and hole trapping layers.

High-efficiency deep blue host for organic light-emitting devices

High-efficiency deep blue organic light-emitting devices have been fabricated using asymmetrically substituted 9-(α-naphthyl)-10-(β-naphthyl)-anthracene as blue host material and diphenyl-[4-(2-[1,1′;4′,1″]terphenyl-4-yl-vinyl)-phenyl]amine as dopant emitter. With an optimized device structure and doping concentration, the deep blue electroluminescent (EL) device achieved an EL efficiency of 4.9cd∕A and an external quantum efficiency of 4.4% at a current density of 20mA∕cm2 at saturated blue Commission Internationale de l’Eclairage coordinates of (0.15, 0.13). The deep blue device also has a current efficiency which is invariable with driving conditions ranging from 10to100mA∕cm2.

P-133: Distinguished Student Paper: Highly Efficient Deep Blue Organic Light-Emitting Device

We report a highly efficient deep blue organic light-emitting device (OLED) incorporating a novel composite hole transport layer (c-HTL) and an emitter based on the new fluorescent dopant BD1 in the stable host MADN which achieved a luminance efficiency of 5.4 cd/A with a Commission Internationale d'Eclairage (CIEx, y) of (0.14, 0.13) and an external quantum efficiency of 5.1% at 20 mA/cm2 and 6.8 V. We attribute the increased device efficiency to an improved balance between hole and electron currents arriving at the recombination zone.