All-Solution-Processed Red and Orange-Red Organic Light-Emitting Diodes with High-Efficiencies: The Effect of Mixed-Host Emissive Layers and Thermal Annealing Treatment.

The instability of the organic light-emitting diodes (OLEDs) during operation can be attributed to the existence of point defects on the organic layers. In this work, the effect of mixed-host emissive layer and the thermal annealing treatment were investigated to eliminate defects and to boost the device performance. The mixed-host system includes 4,4',4''-tri (9-carbazoyl) triphenylamine (TCTA) and 2,7-bis(diphenylphosphoryl)-9, 9'-spirobi[fluorene] (SPPO13). The mixed-host emissive layer with thermal annealing treatment showed low roughness and few pinholes, and the devices fabricated from this emissive layer exhibited high efficiencies, high stabilities, and long lifetimes. The red and orange-red OLEDs exhibited efficiencies of 13.9 cd/A and 24.35 cd/A, respectively. The longest half-lifetime (L0 =500 cd/m2 ) of the red and orange-red OLEDs were 158 h and 180 h, respectively. Efforts were made to solve problems in large-area coating and to reduce the number of defects on in organic layer. Large-active-area (active area=3 cm×4 cm) red phosphorescent OLEDs (PhOLEDs) devices were realized with very high current efficiency up to 9 cd/A.

Pt(II) Phosphors Featuring Both Dicarbene and Functional Biazolate Chelates: Synthesis, Luminescent Properties, and Applications in Organic Light-Emitting Diodes.

Pt(II) metal complexes [Pt(C^C)(X^X)] comprising three functional dianionic azolate chelates (X^XH2: bipzH2 = 5,5'-di(trifluoromethyl)-3,3'-bipyrazole, bitzH2 = 5,5'-di(trifluoromethyl)-3,3'-bi-1,2,4-triazole, and phpzH2 = 3-(trifluoromethyl)-5-(4-(trifluoromethyl)phenyl)-1H-pyrazole), together with three different charge-neutral dicarbene chelates (i.e., C^C = 1,1'-methylene bis(3-methyl-imidazol-2-ylidene), 1,1'-methylene bis(3-isopropyl-imidazol-2-ylidene), and 1,1'-(propane-1,3-diyl) bis(3-isopropyl-imidazol-2-ylidene), were synthesized and found to show bright solid-state emission depending on the associated X^X and C^C chelates. Pt(II) complexes 1a, 2, and 6 were examined by X-ray diffraction studies, confirming the square-planar skeleton. These Pt(II) metal complexes are found to be nonemissive in degassed solution at RT. The photophysical measurements as neat powder reveals emission maxima ranging from purple to sky blue emission and with high quantum yields for the majority of them. (Time-dependent) density functional theory (DFT/TD-DFT) calculations were executed to elucidate the emission process that was predominated by the combined3LLCT/(3)LMCT/(3)IL character, where LLCT and LMCT and IL stand for ligand-to-ligand charge transfer, ligand-to-metal charge transfer, and intraligand ππ* transition processes. Organic light-emitting devices comprising complex 5a achieved high efficiency (8.9%, 19.4 cd·A(-1), 22.5 lm·W(-1)) with a sky blue emission showing CIEx,y coordinates of (0.18, 0.32).

Ir(III)-Based Phosphors with Bipyrazolate Ancillaries; Rational Design, Photophysics, and Applications in Organic Light-Emitting Diodes.

A series of three charge-neutral Ir(III) complexes bearing both neutral chelating ligands 4,4'-di-t-butyl-2,2'-bipyridine (dtbbpy) and monoanionic cyclometalated ligands derived from 2-phenylpyridine (ppyH), together with either two monoanionic ligands (i.e., chloride and monodentate pyrazolate) or a single dianionic chelate derived from 5,5'-di(trifluoromethyl)-3,3'-bipyrazole (bipzH2) or 5,5'-(1-methylethylidene)-bis-(3-trifluoromethyl-1H-pyrazole) (mepzH2), was successfully synthesized. These complexes are derived from a common, structurally characterized, Ir(III) intermediate complex [Ir(dtbbpy) (ppy)Cl2] (1), from treatment of IrCl3·3H2O with equal amount of the diimine (N^N) and precursor of the cyclometalated (C^N) ligands in a form of one-pot reaction. Treatment of 1 with various functional pyrazoles afforded [Ir(dtbbpy) (ppy) (pz)Cl] (2), [Ir(dtbbpy) (ppy) (bipz)] (3), and [Ir(dtbbpy) (ppy) (mepz)] (4), which display intense room-temperature emission with λmax spanning the region between 532 and 593 nm in both fluid and solid states. The Ir(III) complexes, 3 and 4, showcase rare examples of three distinctive chelates (i.e., neutral, anionic, and dianionic) assembled around the central Ir(III) cation. Hybrid density functional theory (DFT; B3LYP) electronic structure calculations on 1-4 reveal the lowest unoccupied molecular orbital to be π*(bpy) in character for all complexes and highest occupied molecular orbital (HOMO) offering d(Ir)-π(phenyl) character for 1, 2, and 4 and π(bipz) character for 3. The different HOMO composition of 3 and 4 is also predicted by calculations using pure DFT (BLYP) and wave function (MP2) methods. On the basis of time-dependent DFT calculations, the emissive processes are dominated by the phenyl group-to-bipyridine, ligand(ppy)-to-ligand(bpy) charge transfer admixed with metal-to-ligand transition for all Ir(III) complexes. Organic light emitting diodes were successfully fabricated. A double emitting layer design was adopted in the device architecture using Ir(III) metal complexes 3 and 4, attaining peak external quantum efficiencies, luminance efficiencies, and power efficiencies of 18.1% (59.0 cd/A and 38.6 lm/W) and 16.6% (53.3 cd/A and 33.5 lm/W), respectively.

Os(II) phosphors with near-infrared emission induced by ligand-to-ligand charge transfer transition.

Heating of Os3(CO)12 with 6 equiv of 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl) pyridine (fptzH) in refluxing diethylene glycol monomethyl ether, followed by sequential treatment with stoichiometric Me3NO and addition of PPhMe2, afforded two isomeric mixtures of red-emitting [Os(fptz)2(PPhMe2)2] (1T and 1C), for which the notations T and C stand for the trans and cis-oriented fptz chelates, respectively. Alternatively, preparation of Os(II) complex using a 1:1 mixture of 5,5'-di(trifluoromethyl)-3,3'-di-1,2,4-triazole (dttzH2) and 2,2'-bipyridine (bpy), instead of fptzH, gave isolation of a mononuclear Os(II) complex [Os(bpy)(dttz)(CO)2] (2) in moderate yield. Replacement of CO with PPhMe2 on 2 afforded near-infrared (NIR)-emitting Os(II) complex [Os(bpy)(dttz)(PPhMe2)2] (3). The single-crystal X-ray structural analyses were executed on 1C, 2, and 3 to reveal the structural influence imposed by the various chelates. The photophysical and electrochemical properties were measured and discussed using the results of density functional theory (DFT) and time-dependent DFT calculations. Complex 3 is selected as the dopant to probe its electroluminescent properties by fabrication of the NIR emitting organic light-emitting diodes.

A new class of sky-blue-emitting Ir(III) phosphors assembled using fluorine-free pyridyl pyrimidine cyclometalates: application toward high-performance sky-blue- and white-emitting OLEDs.

Two pyrimidine chelates with the pyridin-2-yl group residing at either the 5- or 4-positions are synthesized. These chelates are then utilized in synthesizing of a new class of heteroleptic Ir(III) metal complexes, namely [Ir(b5ppm)2(fppz)] (1), [Ir(b5bpm)2(fppz)] (2), [Ir(b4bpm)2(fppz)] (3), and [Ir(b5bpm)(fppz)2] (4), for which the abbreviations b5ppm, b5bpm, b4bpm, and fppz represent chelates derived from 2-t-butyl-5-(pyridin-2-yl)pyrimidine, 2-t-butyl-5-(4-t-butylpyridin-2-yl)pyrimidine, 2-t-butyl-4-(4-t-butylpyridin-2-yl)pyrimidine, and 3-trifluoromethyl-5-(pyridin-2-yl) pyrazole, respectively. The single crystal X-ray structural analyses were executed on 1 to reveal their coordination arrangement around the Ir(III) metal element. The 5-substituted pyrimidine complexes 1, 2, and 4 exhibited the first emission peak wavelength (λmax) located in the range 452-457 nm with high quantum yields, whereas the emission of 3 with 4-substituted pyrimidine was red-shifted substantially to longer wavelength with λmax = 535 nm. These photophysical properties were discussed under the basis of computational approaches, particularly the relationship between emission color and the relative position of nitrogen atoms of pyrimidine fragment. For application, organic light-emitting diodes (OLEDs) were also fabricated using 2 and 4 as dopants, attaining the peak external quantum, luminance, and power efficiencies of 17.9% (38.0 cd/A and 35.8 lm/W) and 15.8% (30.6 cd/A and 24.8 lm/W), respectively. Combining sky blue-emitting 2 and red-emitting [Os(bpftz)2(PPh2Me)2] (5), the phosphorescent white OLEDs were demonstrated with stable pure-white emission at CIE coordinate of (0.33, 0.34), and peak luminance efficiency of 35.3 cd/A, power efficiency of 30.4 lm/W, and external quantum efficiency up to 17.3%.