High-efficiency Phosphorescent Organic Light-emitting Diodes Research Articles

High-performance sky-blue phosphorescent organic light-emitting diodes employing wide-bandgap bipolar host materials with thermally activated delayed fluorescence characteristics

Abstract Up to date, several kinds of thermally activated delayed fluorescence (TADF) host molecules have been rapidly developed to fulfill the increasing demand of high-efficiency phosphorescent organic light-emitting diodes (PHOLEDs). Herein, three wide-bandgap TADF materials BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz are newly designed and synthesized as host materials for sky-blue PHOLEDs. By modulation of carbazole-units in the molecular skeleton, we find that the optimized molecules can realize both high triplet energies and balanced carrier transport properties with obvious TADF properties. The sky-blue PHOLEDs with ultralow phosphor doping ratio employing BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz as host materials demonstrate really high performance with alleviated roll-offs and achieve impressive current efficiencies (CEs) of 46.5 cd A−1, 50.0 cd A−1, and 34.5 cd A−1, power efficiencies (PEs) of 40.6 lm W−1, 46.8 lm W−1, and 34.8 lm W−1, and external quantum efficiencies (EQEs) of 22.2%, 24%, and 15.8%, respectively. More encouragingly, even at an applied luminance of 1000 cd m−2, the EQEs could maintain high levels of 21.0%, 21.5% and 12.8%, respectively, which represents the potential utility value of the new family of these molecules in blue PHOLEDs. Our molecular design strategy provides a practical way to construct promising wide-bandgap TADF host materials with an excellent combination of TADF property, high triplet energy, and balanced carrier-transporting ability for high-performance devices.

Preferential Orientation of Tetrahedral Silicon-Based Hosts in Phosphorescent Organic Light-Emitting Diodes.

Vacuum-processed diphenylbis(3-(pyridine-2-yl)phenyl)silane (2PTPS), diphenylbis(3-(pyridine-3-yl)phenyl)silane (3PTPS), and diphenylbis(3-(pyridine-4-yl)phenyl)silane (4PTPS) have been used as electron-transporting host materials combined with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as the hole-transporting host, which induce balanced charge carrier transport for high-efficiency phosphorescent organic light-emitting diodes. The 4PTPS-based organic light-emitting diodes with tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)3] dopant showed highest current efficiency and external quantum efficiency of 53.54 cd/A and 15.61%, compared to 2PTPS (40.75 cd/A, 11.84%) and 3PTPS (29.35 cd/A, 8.54%). These results were attributed to the well-aligned structure with preferential horizontal orientation of the emitting material layer by the diffraction intensity distribution as a function of azimuthal angle in two-dimensional grazing incidence X-ray diffraction analysis. The molecular orientation of TCTA:4PTPS material with a narrow azimuthal intensity distribution had better priority to the horizontal direction than the other TCTA:2PTPS and TCTA:3PTPS materials, which is related to the charge transport as well as the device efficiency. We found that the preferential horizontal orientation of the co-host material with a balanced charge carrier was not affected by Ir(ppy)3 dopant with a homoleptic structure and bis-[2-(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium [Firpic] dopant with a heteroleptic structure in the co-host/dopant system.

Extremely stable graphene electrodes doped with macromolecular acid

Although conventional p-type doping using small molecules on graphene decreases its sheet resistance (Rsh), it increases after exposure to ambient conditions, and this problem has been considered as the biggest impediment to practical application of graphene electrodes. Here, we report an extremely stable graphene electrode doped with macromolecular acid (perfluorinated polymeric sulfonic acid (PFSA)) as a p-type dopant. The PFSA doping on graphene provides not only ultra-high ambient stability for a very long time (> 64 days) but also high chemical/thermal stability, which have been unattainable by doping with conventional small-molecules. PFSA doping also greatly increases the surface potential (~0.8 eV) of graphene, and reduces its Rsh by ~56%, which is very important for practical applications. High-efficiency phosphorescent organic light-emitting diodes are fabricated with the PFSA-doped graphene anode (~98.5 cd A−1 without out-coupling structures). This work lays a solid platform for practical application of thermally-/chemically-/air-stable graphene electrodes in various optoelectronic devices.

Tilted Spiro‐Type Thermally Activated Delayed Fluorescence Host for ≈100% Exciton Harvesting in Red Phosphorescent Electronics with Ultralow Doping Ratio

AbstractDespite promising efficiency, the high fabrication cost due to the required high concentrations of noble metal based phosphors is still problematic for phosphorescent organic light‐emitting diodes (PhOLEDs). This situation becomes even serious considering some practical applications need high luminance, which in turn requires a higher concentration of emitters. A paradigm shift to circumvent these issues can be development of materials that are able to operate efficiently in very low concentrations (<1 wt%). Here, two thermally activated delayed fluorescence hosts (OSTFPCN and OSTFPB) with tilted spiro geometry, small singlet‐triplet splitting (Δ E st), and effective resonance energy transfer are developed. Within expectation, record performances with a power efficiency of 63.6 lm W−1 and an external quantum efficiency (EQE) of 31.2% for the red phosphor Ir(MDQ)2(acac) (2.0 wt%) are achieved with OSTFPCN as host. Additionally, a high power efficiency around 58 lm W−1 is also gained even at an ultralow dopant concentration of 0.5 wt% for a OSTFPB based device. Mechanism studies demonstrate that efficiency roll‐off can be effectively suppressed in such low concentrations. These findings pave a new way to exploit low cost and high efficiency PhOLEDs.

Molecular Design Strategy of Organic Thermally Activated Delayed Fluorescence Emitters

Recently, organic thermally activated delayed fluorescence (TADF) emitters have attracted a great deal of attention because they can theoretically realize 100% internal quantum efficiency. Many TADF emitters have been developed since the first demonstration of close to 20% external quantum efficiency in the devices. Recently developed TADF emitters demonstrated close to 37% external quantum efficiency in blue, above 30% external quantum efficiency in green, and close to 18% external quantum efficiency in red devices. Therefore, TADF organic light-emitting diodes could potentially be substituted for high-efficiency phosphorescent organic light-emitting diodes. In this work, we reviewed molecular design strategies of organic-based TADF emitters by classifying them into several categories depending on the material parameters required for the TADF emitters. In addition, we proposed a future development direction of TADF emitters to make them competitive with phosphorescent emitters.

Stable solution processed hole injection material for organic light-emitting diodes

A new aqueous based polymer, Plexcore® OC AQ1200 (AQ1200) was used as a hole injection layer (HIL) in organic light emitting diodes (OLEDs) and the device results are compared with polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) in terms of injection efficiency and stability. Dark injection transient measurements show a higher injection efficiency in hole-only devices using AQ1200 HIL compared with PEDOT:PSS. Using AQ1200 as an HIL, high efficiency phosphorescent OLEDs are demonstrated to have a long lifetime, with an estimated operational half lifetime (LT 50) of more than 8000h from an initial luminance of 1000cd/m2.

A simple systematic design of phenylcarbazole derivatives for host materials to high-efficiency phosphorescent organic light-emitting diodes

A series of novel blue phosphorescent host materials, namely, CTP-1, CTP-2 and CTP-3 have been designed and synthesized through the Suzuki–Miyaura cross-coupling reaction between N-phenylcarbazole and biphenyl. Their thermal, photophysical and electrochemical properties were systematically investigated. These novel hosts show excellent thermal stability and high glass transition temperatures (Tg) ranging from 113 to 127 °C. The triplet energies of these three materials are significantly higher than that of (4,4′-bis(N-carbazolyl)-2,2′-biphenyl) (CBP, 2.56 eV) and that of the most popular blue phosphorescent material iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2′] picolinate (FIrpic, 2.65 eV). These three novel materials have superior thermal and electronic properties for blue and white phosphorescent OLEDs. Blue emitting devices with an Ir-complex FIrpic as the phosphorescent dopant have been fabricated and show high efficiency with low roll-off. In particular, 40.4 cd A−1 at 100 cd m−2 and 38.2 cd A−1 at 1000 cd m−2 were achieved when CTP-1 was used as the host material. On the basis of this work, an all-phosphor white device with a current efficiency of 64.5 cd A−1 at 100 cd m−2 and 61.9 cd A−1 at 1000 cd m−2 has also been fabricated.

Simple Structure and High Efficiency Phosphorescent OLEDs Using Narrow Band-gap Bipolar Host Material

ABSTRACTWe report high-efficiency phosphorescent blue OLEDs with an organic three stacked structure. Using a high-triplet-energy-hole transporting material of TAPC and a high-triplet-energy-electron transporting material of TmPyPB, the organic three stacked structure has been realized with three new narrow band-gap blue host materials. These host materials have bipolar characteristics and high triplet energy of >2.8 eV. Very low onset voltages of 2.8~3.0 V and driving voltages of 4.2~4.6 V to obtain a brightness of 1000 cd/m2 are achieved in this three stacked device configuration. Maximum external quantum efficiency above 20% is reported.

Managing Charge Balance and Triplet Excitons to Achieve High-Power-Efficiency Phosphorescent Organic Light-Emitting Diodes

High-efficiency phosphorescent organic light-emitting diodes (PhOLEDs) have been achieved by using an ortho-linked triphenylamine/oxadiazole hybrid, namely p-TPA-o-OXD, as host material. The high HOMO level of the compound (5.25 eV) can facilitate efficient hole injection from 1, 4-bis[(1-naphthylphenyl)amino]biphenyl (NPB) layer to the emissive layer. As a result, low driving voltages and high power efficiencies have been attained as compared to the carbazole-based hosts with lower HOMO levels and higher injection barriers under similar device structures. By introducing 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) to replace 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/tris(8-hydroxyquinoline)aluminum (Alq3) as hole blocking/electron transporting layer, followed by tuning the thicknesses of hole-transport NPB layer to manipulate the charge balance, high-efficiency PhOLEDs have been achieved, with maximum current efficiency/power efficiency/external quantum efficiency of 85.7 cd/A/99.7 lm/W/...

Estimation of the mean emission zone in phosphorescent organic light-emitting diodes with a thin emitting layer

We presented an approach to estimate the emission zone (EZ) positions in high efficiency phosphorescent OLEDs with a thin emitting layer. Two devices with different distances between the emitting layer and the cathode (i.e. they are optically different), but exhibiting same current density-voltage characteristics (i.e. they are electrically the same) were used for this purpose. Mean EZ positions in the OLEDs were extracted from the comparison of the experimental luminous intensity ratio vs. the current density with the calculated intensity ratio vs. the EZ position. The validity of the approach was confirmed by the agreement between calculated and experimental spectral changes.

High-Efficiency Phosphorescent OLEDs using Chemically Doped Layers

Organic light-emitting devices (LEDs) were fabricated using wide-energy-gap (over 3.30 eV) materials and fac-tris(2-phenylpyridine) iridium complex (Ir(ppy)3) as the carrier transporting materials and the emitting material, respectively. The organic layers at the electrode interfaces were chemically doped to reduce operating voltage. The p-type and n-type doped layers have specific resistance of 105∼106 ω [SW1]·cm. The device exhibited green emission peaking at approximately 510 nm, and the external quantum efficiency of 19.7% (69 cd/A). A high power efficiency of 68 lm/W was observed at 100 cd/m2 at voltage of 3.22 V.