Green Phosphorescent Organic Light-emitting Diodes Research Articles

Green Phosphorescent Organic Light-Emitting Diode Based on Interlayer Emitting Layer Blend of Hole- and Electron-Transporting Materials as a Co-Host of the Three Emitting Layers

Highly efficient green phosphorescent organic light-emitting diodes (PHOLEDs) are achieved by using a three emitting layers structure in which the interlayer composes a blend of a hole- and an electron-transporting materials as the co-host. Both the efficiency and operational lifetime of the devices are improved with such a three emitting layers structure. The optimized device shows a maximum current efficiency and luminance of 65.8 cd/A and 127435 cd/m2, respectively, which are nearly two folds over the conventional structure device. The three emitting layers structure improves the charge carriers injection and transport balance and confinement in the emitting layers, which lead to increasing charge carriers recombination probability and decreasing exciton annihilation by the hole transporting and electron transporting materials. Such factors are critical to improve the efficiency and operational lifetime of the green PHOLEDs.

GO:PEDOT:PSS for High-Performance Green Phosphorescent Organic Light-Emitting Diode

A high-performance green phosphorescent organic light-emitting diode (GPhOLED) based on easily available graphene oxide (GO)-doped poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) as anode buffer layer and simple device fabricating process has been demonstrated. The GO:PEDOT:PSS-based GPhOLEDs show a better performance compared to the PEDOT:PSS only GPhOLEDs with current and power efficiencies of 52 and 41 cd/A and 36 and 27 lm/W at 1000 cd/m2, respectively. These findings shed new light on the development of high-performance GPhOLEDs.

Efficient Green Phosphorescent Organic Light-Emitting Diodes Depending on Concentration of Lithium Quinolate in Electron Transport Layer

Systematic studies on carrier injection and transport are very important for achieving high efficiency in OLEDs. We demonstrate excellent green phosphorescent organic light-emitting diodes (OLED) with lithium quinolate (Liq) doped in 1,3,5-tris(N-phenylbenzimidazole-2-yl) benzene (TPBi) as the electron transport layer (ETL). The doping concentration of Liq was varied from 0% to 10%. The optimized green phosphorescent OLED with 5% Liq in the ETL showed the best efficiencies, which were maximum luminous efficiency, power efficiency, and quantum efficiency of 65.76 cd/A, 57.39 Im/W, and 20.03%, respectively. Moreover, high triplet energy states of TCTA and TPBi as a triplet exciton-blocking layer (TEBL) played a role in efficient exciton confinement.

Effect of gold nanoparticles on the performances of the phosphorescent organic light-emitting devices

Gold nanoparticles (GNPs) on the performance of the phosphorescent organic light-emitting devices (OLEDs) were investigated. The green phosphorescent OLEDs with GNPs incorporated in hole transporting layer (HTL) or hole blocking layer (HBL) were fabricated using thermal evaporation technique. The results indicated that the performance of the OLEDs with GNPs were dependent on the position of the GNPs. The optimized device with GNPs in HBL shows enhanced current efficiency and reduced efficiency roll-off. However, the efficiency of the device with GNPs in HTL was decreased. The detailed physical mechanism is investigated in order to unveil such difference.

Molecular design of hole-transporting material for efficient and stable green phosphorescent organic light-emitting diodes

We examined the hole-transporting-material (HTM)-dependent device characteristics of green phosphorescent organic light-emitting diodes (PHOLEDs) using an electron-transporting host. The emission efficiency of the PHOLEDs was proportional to the optical band gap and the triplet energy of the HTMs. On the other hand, the operational stability of the PHOLEDs was not proportional to the emission efficiency. By analyzing the device characteristics in relation to the molecular structure of HTMs, amine derivative with dibenzothiophene was found to be effective HTMs suitable for highly efficient and stable green PHOLEDs.

A novel, bipolar host based on triazine for efficient solution-processed single-layer green phosphorescent organic light-emitting diodes

A novel N-phenyl carbazole substituted 2, 4,6-trisphenyl-triazine host material(TPCPZ) for solution processed green phosphorescent organic light-emitting devices (PhOLEDs) was synthesized by a Suzuki-cross coupling reaction. The optical, electrochemical and thermal properties of TPCPZ have been characterized. TPCPZ exhibits a high glass transition temperature of 165 °C and a triplet energy of 2.63 eV. The appropriate HOMO energy level (−5.39 eV) and LUMO energy level (−2.16 eV) matching with the HOMO energy level of PEDOT:PSS(−5.35 eV) and the LUMO energy level of Cs2CO3/Al bilayer cathode (−2.2 eV), facilitate the transfer of holes and electrons. The solution-processed single-layer device using TPCPZ as the host for fac-tris(2-(4-phenylpyridine)iridium (Ir(ppy)3) exhibited a low turn-on voltage of 3.5 V, a maximum current efficiency of 20.8 cd A−1 and a maximum luminance of 18,000 cd m−2. These results demonstrated that TPCPZ as a host material is advantageous for fabrication of highly efficient single-layer green PhOLEDs.

Al Complex as a Host Material for High Efficiency Green Phosphorescent Organic Light-Emitting Diodes

An Al based complex, tris((2-(1H-pyrazol-1-yl)pyridin-3-yl)oxy)aluminum (AlOx), was synthesized as the host material for green phosphorescent organic light-emitting diodes (PHOLEDs). The AlOx host material showed a high triplet energy of 2.75 eV for efficient energy transfer to green-emitting iridium(III) tris(2-phenylpyridine) (Ir(ppy)3) triplet emitter. A high quantum efficiency of 22.5% was obtained in the green PHOLEDs fabricated using the AlOx host material at a Ir(ppy)3 doping concentration of 3%.

High hole mobility hole transport material for organic light-emitting devices

A new hole-transporting material, 5,10,15-triphenyl-5H-diindolo[3,2-a:3′,2′-c]carbazole (TPDI) is reported for organic light-emitting device (OLED) applications. It shows excellent hole mobility (6.14×10−3cm2/Vs), one order higher than that of NPB (4,4′-bis(N-phenyl-1-naphthylamino)biphenyl), and a good HOMO level of 5.3eV. Fabricated fluorescent blue OLEDs exhibit about 1.0V voltage reduction and 18% external quantum efficiency (EQE) improvement by replacing TPDI instead of NPB as a hole transport layer. In the green phosphorescent OLEDs, the driving voltage improves about 1.8V and EQE increases about 65%. This TPDI will be applicable to not only in fluorescent OLEDs but also in phosphorescent OLEDs.

High efficiency green phosphorescent organic light-emitting diodes with a low roll-off at high brightness

Highly efficient green phosphorescent organic light-emitting diodes (PHOLEDs) with low efficiency roll-off at high brightness have been demonstrated with a novel iridium complex. The host material 1,3-bis(carbazol-9-yl)benzene (mCP) with high triplet energy is also used as the hole transporting layer to avoid carrier accumulation near the exciton formation interface and reduce exciton quenching. It provides a new approach for easily fabricating PHOLED with high triplet energy emitter. Moreover, the hole blocking layer is extended into the light emitting layer to form a co-host, realizing better control of the carrier balance and broader recombination zone. As a consequence, a maximum external quantum efficiency of 20.8% and current efficiency of 72.9cd/A have been achieved, and maintain to 17.4% and 60.7cd/A even at 10,000cd/m2, respectively.

Light‐Emitting Diodes: Organic Light‐Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter (Adv. Funct. Mater. 31/2013)

Green phosphorescent organic light-emitting diodes (OLEDs) based on a horizontally oriented emitter are analysed by J.-J. Kim and co-workers on page 3896. They demonstrate that theoretical analysis based on the orientation factor (the ratio of horizontal dipoles to total dipoles) and the quantum yield of the emitter predicts that the maximum EQE of OLEDs with this emitter is about 30%, matching experimental data, indicating that the electrical loss of the OLEDs is negligible and the device structure can be utilized as a platform to demonstrate the validity of optical modeling.

Degradation of phosphorescent organic light-emitting diodes under pulsed current stressing

The electrical and optical degradation of green phosphorescent organic light-emitting diodes (OLEDs) stressed under 50mA/cm2 pulsed currents with 10–50% duty cycles was studied. The stressing resulted in significant increases in low-bias leakage current and operational voltage. The luminance evolution comprised an initial rapid decay regime and a subsequent slow decay regime, and only the latter was governed predominantly by electrical excitation. Compared to continuous-wave stressing, pulsed stressing with 10% duty cycle improved the effective half life by only ∼15%, indicating that self-heating plays a minor role in the performance degradation process. Adding a reverse bias component to the pulsed current led to suppressed low-bias leakage and current-induced luminance decay due to defect removal and alleviated charge build-up.

A Spiro [Fluorene-9, 9’-Xanthene]-Based Host Material for Efficient Green and Blue Phosphorescent OLED

A spiro [fluorene-9,9-xanthen-based host material SFX-PF, without possessing conventional hole-or electron-transporting units, has been demonstrated to be host material for efficient and low voltage green phosphorescent organic light-emitting device (PHOLED). This compound showed good thermal stability with high glass transition temperatures (Tg) at 172 °C. The green device exhibited a low turn-on voltage of 3 V, and maximum current efficiency, power efficiency, and external quantum efficiency (EQE) up to 47.9 cd/A, 45.4 lm/W, and 13.2%, respectively. At the luminance level of 1000 cd/m2, the driving voltages are still lower than 4 V with 15.9% roll-off in the EQE. In addition, SFX-PF can also host Fir6 with the blue device exhibiting a low turn-on voltage of 2.8 V and maximum current efficiency, power efficiency, and EQE up to 12.8 cd/A, 11.3 lm/W, and 7.5%, respectively. Based on our present and previous studies, it can be concluded that SFX could serve as a new building block for designing blue and green phosphorescent host materials.

New host materials based on fluorene and benzimidazole units for efficient solution-processed green phosphorescent OLEDs

New green host materials 1-(9,9-diphenyl-9H-fluorene-2-yl)-2-phenyl-1H-benzimidazole and 1-(9,9′-spirobifluorene-2-yl)-2-phenyl-1H-benzimidazole for solution-processed green phosphorescent organic light-emitting devices have been designed and synthesized by attaching the electron transporting benzimidazole units to the rigid fluorene units. Owing to the non-planar structures, which decrease the π conjugation length of fluorene and benzimidazole rings, these fluorene derived derivatives show high triplet energy. The high triplet energy of newly host materials ensures efficient energy transfer from the host to the triplet emitter tris(2-phenylpyridine)iridium. Furthermore, the thermal, photophysical, electrochemical properties and crystal structures of 1-(9,9-diphenyl-9H-fluorene-2-yl)-2-phenyl-1H-benzimidazole and 1-(9,9′-spirobifluorene-2-yl)-2-phenyl-1H-benzimidazole were investigated. The solution-processed single-layer green device using 1-(9,9-diphenyl-9H-fluorene-2-yl)-2-phenyl-1H-benzimidazole as the host for the phosphorescence emitter tris(2-phenylpyridine)iridium showed the maximum luminance efficiencies of 10.1cd/A. This result demonstrated that the newly synthesized, fluorene-based rigid host materials are advantageous for fabrication of highly efficient green phosphorescent organic light-emitting diodes.

High triplet energy Zn complexes as host materials for green and blue phosphorescent organic light-emitting diodes

Two Zn complexes, bis(2-(oxazol-2-yl)phenolate) zinc (ZnOx2) and bis(2-(1-methyl-1H-imidazol-2-yl)phenolate) zinc (ZnIm2), were synthesized as high triplet energy host materials for green and blue phosphorescent organic light-emitting diodes. High triplet energies of 2.71 eV and 2.84 eV were obtained for ZnOx2 and ZnIm2, respectively. Green and blue phosphorescent devices were fabricated using the ZnOx2 and ZnIm2 host materials and high quantum efficiencies of 23.5% and 20.1% were achieved in green and blue devices, respectively.

Synthesis and device application of 3- position modified benzothieno[3,2-c]pyridine derivative

A new electron deficient core, benzothieno[3,2-c]pyridine (3BTP) functionalized at 3- position, was developed as the electron transport unit and a derivative of 3BTP, 3-(3-(9H-carbazol-9-yl)phenyl)benzothieno[3,2-c]pyridine (Cz3BTP), was synthesized as the triplet host material for green phosphorescent organic light-emitting diodes (PHOLEDs). The Cz3BTP host showed a triplet energy of 2.63 eV and bipolar charge transport properties. Green PHOLEDs were fabricated using the Cz3BTP host and a quantum efficiency of 11.6% was achieved.

P.107: New Emissive Materials for Mixed Host Architecture to Achieve Longer Lifetime for Green to Red Phosphorescent OLED Display and Lighting Applications

AbstractIncorporation of dibenzothiophene units into emissive materials (CH1) increased the charge carrier mobility in the order of 10−3 cm2 V−1s−1. Mixing the hole‐rich CH1 with electron transporting PH3 in the emissive layers in appropriate proportions enhanced efficiency, lowered the driving voltage and prolonged the lifetime of the green and red phosphorescent OLED devices. Green phosphorescent OLED achieved an efficiency of 44.6 lm/W at a driving voltage of 3.67V and exhibited a lifetime of 115Kh (T50) at an initial luminance of 1000 nits.

High-efficiency green phosphorescent organic light-emitting diodes with double-emission layer and thick N-doped electron transport layer

We have developed green phosphorescent organic light-emitting diodes (OLEDs) with high external quantum efficiency of 59.7% and power efficiency of 243lm/W at 2.73V at 0.053mA/cm2. A double emission layer and a thick n-doped electron transport layer were adopted to improve the exciton recombination factor. A high refractive index hemispherical lens was attached to a high refractive index substrate for extracting light trapped inside the substrate and the multiple-layers of OLEDs to air. Additionally, we analyzed an energy loss mechanism to clarify room for the improvement of our OLEDs including the charge balance factor.

Efficient Solution-Processed Green Phosphorescent Organic Light-Emitting Diodes Using Bipolar Host Material

Solution-based processing was applied to fabricate green phosphorescent organic light-emitting diodes (OLEDs). EPH31 was used as a phosphorescent host, doped with guest dopant green phosphorescent Ir(ppy)3, and dissolved in chlorobenzene solvent to form the emitting layer. Device structural parameters were controlled by changing the spin coating speed of the emitting layer and hole injection layer [poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), PDOT:PSS] to adjust the thickness of the electron transport layer [tris(8-hydroxyquinolinato)aluminum, Alq3]. In addition, the differences in using CsF and LiF materials as the electron injection layer were investigated. A maximum current efficiency of 13.6 cd·A-1 was obtained at a high emitting layer spin coating speed. Despite the close resemblance in both the luminance intensity and current efficiency when using CsF and LiF as the electron injection layer, CsF devices had a low driving voltage. Smooth and stable films resulting from the spin coated hole injection layer, along with the control of the thickness of the electron transport layer (Alq3) and electron injection layer (CsF), effectively improved the performance of green OLEDs. The emitting layer host material (CBP) and three guest dopants [Firpic, Ir(ppy)3, and Ir(piq)2] were dissolved in toluene solvent during solution preparation to fabricate white OLEDs. The properties of the resulting solution-processed white PHOLEDs are a current efficiency of 2.4 cd·A-1 at 20 mA·cm-2 and CIE coordinates of (0.33, 0.33) at 9 V. Results of these experiments demonstrate that solution processing can be used as an alternative to and in conjunction with thermal evaporation.

9-(Pyridin-3-yl)-9H-carbazole derivatives as host materials for green phosphorescent organic light-emitting diodes

9-(Pyridin-3-yl)-9H-carbazole (PyCz) derivatives were synthesized as bipolar host materials for green phosphorescent organic light-emitting diodes (PHOLEDs) and the device performance was investigated. The PyCz core was modified with carbazole and diphenylamine to prepare the bipolar host materials. Bipolar charge transport properties were observed in the PyCz derivative with the carbazole unit and a high quantum efficiency of 21.3% was obtained in green PHOLEDs.

Novel dibenzothiophene based host materials incorporating spirobifluorene for high-efficiency white phosphorescent organic light-emitting diodes

Two host materials, DBTSF2 and DBTSF4, were designed and synthesized, incorporating dibenzothiophene (DBT) and spirobifluorene (SF) blocks. Their thermal, electrochemical and photo-physical properties were fully characterized. DBTSF4, which adopted an ortho-linkage between DBT and SF moieties, showed a significantly higher T1 energy of 2.82eV as compared to its para-linkage analogue DBTSF2 (2.49eV). Their applications as host for green, blue and white phosphorescent organic light-emitting diodes (PHOLEDs) were explored. The DBTSF4 based blue PHOLED has a highest current efficiency of 23.5cdA−1. And using DBTSF4 as a single host, two-color based white PHOLEDs were achieved from cold white emission with CIE coordinate of (0.31, 0.43) to yellowish warm white emission (0.44, 0.49) with maximum current efficiencies varying from 35.8 to 52.3cdA−1 and maximum external quantum efficiencies from 13.1% to 16.9% respectively. The white PHOLED devices also showed a low efficiency roll-off even at 10,000cdm−2.