Small Efficiency Roll-off Research Articles

Molecular design to regulate the photophysical properties of multifunctional TADF emitters towards high-performance TADF-based OLEDs with EQEs up to 22.4% and small efficiency roll-offs.

Simultaneously achieving high efficiency and low efficiency roll-off remains a big challenge for OLEDs based on thermally activated delayed fluorescence (TADF) emitters. To address this issue, we elaborately designed and synthesized a series of new emitters with both TADF and aggregation-induced emission (AIE) properties by introducing 9,9-dimethyl-9,10-dihydroacridine (DMAC) or 10H-phenoxazine (PXZ) as donor units into a quinoxaline framework. By tuning the electron-donating capability of the donor as well as the amount of donor unit, the photophysical properties of the TADF-AIE emitters can be systematically regulated, with emissions ranging from green to red. We demonstrated efficient doped OLEDs with a maximum EQE of 22.4%, a maximum current efficiency (CEmax) of 80.3 cd A-1 and a maximum power efficiency (PEmax) of 64.1 lm W-1 for the green device, and an EQEmax of 14.1%, a CEmax of 36.1 cd A-1 and a PEmax of 28.1 lm W-1 for the orange device. Remarkably, these orange devices rendered small roll-offs of 1.4% and 21.3% respectively at a luminance of 100 and 1000 cd m-2. Attributed to the unique TADF and AIE features, the non-doped devices perform outstandingly with an EQEmax of 12.0%, a CEmax of 41.2 cd A-1 and a PEmax of 45.4 lm W-1.

Precise manipulation of the carrier recombination zone: a universal novel device structure for highly efficient monochrome and white phosphorescent organic light-emitting diodes with extremely small efficiency roll-off

The OLEDs, having mixed hosts sandwiched between hole- and electron-transporting hosts, exhibit an EQE exceeding the theoretical limit and extremely small efficiency roll-off.

Non-Doped Sky-Blue OLEDs Based on Simple Structured AIE Emitters with High Efficiencies at Low Driven Voltages.

Blue organic light-emitting diodes (OLEDs) are necessary for flat-panel display technologies and lighting applications. To make more energy-saving, low-cost and long-lasting OLEDs, efficient materials as well as simple structured devices are in high demand. However, a very limited number of blue OLEDs achieving high stability and color purity have been reported. Herein, three new sky-blue emitters, 1,4,5-triphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEI), 1-(4-methoxyphenyl)-4,5-diphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (TPEMeOPhI) and 1-phenyl-2,4,5-tris(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (3TPEI), with a combination of imidazole and tetraphenylethene groups, have been developed. High photoluminescence quantum yields are obtained for these materials. All derivatives have demonstrated aggregation-induced emission (AIE) behavior, excellent thermal stability with high decomposition and glass transition temperatures. Non-doped sky-blue OLEDs with simple structure have been fabricated employing these materials as emitters and realized high efficiencies of 2.41 % (4.92 cd A-1 , 2.70 lm W-1 ), 2.16 (4.33 cd A-1 , 2.59 lm W-1 ) and 3.13 % (6.97 cd A-1 , 4.74 lm W-1 ) for TPEI, TPEMeOPhI and 3TPEI, with small efficiency roll-off. These are among excellent results for molecules constructed from the combination of imidazole and TPE reported so far. The high performance of a 3TPEI-based device shows the promising potential of the combination of imidazole and AIEgen for synthesizing efficient electroluminescent materials for OLED devices.

Highly Efficient All-Solution-Processed Fluorescent Organic Light-Emitting Diodes Based on a Novel Self-Host Thermally Activated Delayed Fluorescence Emitter.

Here, we conveniently designed and synthesized a self-host thermally activated delayed fluorescence (TADF) emitter, which can not only form a uniform thin film through wet-process, but also allow the subsequently deposition of electron transporting layer (ETL) by orthogonal solvent. By using this self-host material as emitter, the all-solution-processed multilayer TADF organic light emitting diodes (OLEDs) was successfully fabricated. The maximum current, power and external quantum efficiencies of this nondoped device are 46.3 cd A-1, 39.3 lm W1- and 15.5%, respectively, which are much higher than the values of all-solution-processed OLEDs based on tranditional fluorescence and even comparable to the TADF devices with vacuum-deposited ETL. Moreover, the device maintains the high efficiency of 42.9 cd A-1 and 39.0 cd A-1 at the luminance of 100 cd m-2 for display and 1000 cd m-2 for practical lighting. The high efficiency and small efficiency roll-off of the all-solution-processed fluorescent OLEDs can be attributed to the superiority of the newly designed self-host TADF emitter, which possesses the perfect electroluminescent property and sufficient solvent resistance at the same time.

New iridium complexes bearing C^N=N ligand for high efficiency OLEDs

Three iridium complexes, with 3-methyl-6-(2,4-difluoro-phenyl)pyridazine (mdfppya, C^N=N type ligand) as cyclometalated ligands, and picolinate (pic) or acetylacetonate (acac) as ancillary ligand, were synthesized and structurally characterized. Among these complexes,the homoleptic one, which contains two fluorine atoms on the cyclometalated phenyl moiety to lower the LUMO and a methyl group at the cyclometalated pyridazine part to raise the HOMO, possesses the shortest emission wavelength with peak of 506nm. Importing ancillary ligands of picolinate (pic) and acetylacetonate (acac) yielded iridium complexes with emission of longer wavelength. The investigation of photophysical, electrochemical properties, as well as quantum chemistry calculation revealed optoelectrical property differences among them, corresponding principles were discussed. Photophysical, electrochemical features, quantum chemistry simulation and electroluminescence properties were systematically investigated. All complexes were applied to organic light-emitting diodes (OLEDs) as dopant emitters and the influence of ancillary ligands on device performance is significant. Based on pic-complex, maximum device efficiencies of 41.0cdA−1 , 33.9lmW−1 and 11.62% were achieved. Additionally, this device exhibits extremely small efficiency roll-off. The reason for this good performance is mainly attributed to the high photoluminescence quantum yield and fast radiative decay rate of the phosphor, as wells rather matched energy levels in device.

Efficient Near-Infrared-Emitting Cationic Iridium Complexes as Dopants for OLEDs with Small Efficiency Roll-off

Two near-infrared- (NIR-) emitting cationic iridium(III) complexes, [Ir(pbq-g)2(Bphen)]+PF6– (1) and [Ir(mpbqx-g)2(Bphen)]+PF6– (2), were synthesized and characterized, where pbq-g, mpbqx-g, and Bphen represent phenylbenzo[g]quinoline, 2-methyl-3-phenylbenzo[g]quinoxaline, and 4,7-diphenyl-1,10-phenanthroline, respectively. By employing sp2-hybridized N opposite the chelating N atom in the cyclometalated ligand, we succeeded in expanding the emission of iridium complexes with simple structures to the truly NIR region of the spectrum. This subtle structural adjustment significantly lowered the LUMOs of the iridium complexes, leading to a 60–80-nm emission red shift of complex 2 relative to complex 1. Based on these solution-processable phosphors, NIR organic light-emitting devices (OLEDs) were fabricated with the emission covering the range 690–850 nm. Compared with common OLEDs, these NIR-emitting electrophosphorescent devices demonstrate exclusive small efficiency roll-off with increasing current density...

Synthesis, Characterization and Optical Properties Based on a Novel Re(I) Emitter with Bulky Building Block

A novel rhenium complex material, i.e., Re-APPP, (APPP, 2,3-acenaphtho[1,2-b]pyrazino [2,3-f][1,10]phenanthroline) was designed, synthesized in five steps from commercially available starting materials and characterized by FTIR,1H NMR and mass spectroscopy. The photophysical properties of this complex were studied. The absorption bands centered at ca. 326, 366 and 450nm of Re-APPP were attributed to the ligand-centered p→p* transition and the metal-to-ligand charge transfer dπ(Re)→π*(APPP) transition, respectively. Re-APPP was thermally stable with high decomposition temperature of 302°C. The organic light-emitting diodes using Re-APPP as a dopant emitter with the structures of ITO/m-MTDATA(10nm)/NPB(20nm)/CBP: Re-APPP(30nm)/BCP(10nm)/Alq3(30nm)/LiF(1nm)/Al(100nm) were fabricated and a broad electroluminescent peak at 550nm was observed. More importantly, the devices exhibited very small efficiency roll-off of only ca.45% which was probably attributed to its short luminescent lifetime.

Reduced efficiency roll-off in electrophosphorescent devices by a short-living rhenium emitter with well-matched energy levels

We demonstrated reduced efficiency roll-off in electrophosphorescent devices based on a rhenium [Re(I)] complex, {9, 9-Di-[9′-(4″-phenoxybutyl)-9′H-carbazyl]-9-H-4,5-Diazafluorene Re(CO)3Br} (Re-PCDF). The devices based on Re-PCDF exhibited the peak luminance of 7888 cd/m2 and the maximum efficiency of 7.41 cd/A, respectively. Remarkably, the devices exhibited very small efficiency roll-off with only ca. 28%, which is much better than the reported Re(I) complexes based devices. Such excellent performances could be ascribed to the short luminescent lifetime of Re-PCDF and well-matched energy levels between the singlet host and the triplet emitter. The detailed mechanisms of such small efficiency roll-off were also investigated.