Blue Phosphorescence Research Articles

A host material with a small singlet–triplet exchange energy for phosphorescent organic light-emitting diodes: Guest, host, and exciplex emission

A host material containing a triazine core and three phenylcarbazole arms, called 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was developed for phosphorescent organic light-emitting diodes (OLEDs). Ultra-low driving voltages were achieved by utilizing TCPZ as the host due to its decreased singlet–triplet exchange energy (ΔEST) and low-lying lowest unoccupied molecular orbital (LUMO) energy level. Interaction between the RGB triplet emitters and TCPZ were studied in both photoluminescent and electroluminescent processes. Transient photoluminescence (PL) measurement of the co-deposited film of fac-tris(2-phenylpyridine) iridium (Ir(PPy)3):TCPZ exhibits a shoulder at 565nm whose lifetime is about two times longer than that of the Ir(PPy)3 triplet excitons and can be attributed to the triplet exciplex formed between Ir(PPy)3 and TCPZ. Such exciplex was also found for the green phosphorescent OLED, giving the most efficient phosphorescent OLED with triplet exciplex emission hitherto. Different from the PL process, a broad featureless band with a maximum at 535nm was found for the OLED based on an EML of iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2′)picolinate (FIrpic):TCPZ, which can be attributed to the emission from the singlet excited state of TCPZ formed by direct hole-electron recombination. A multi-emitting-layer white OLED was also fabricated by utilizing FIrpic and tris(1-phenylisoquinolinolato-C2,N)iridium(III) (Ir(piq)3) as the complementary triplet emitters and TCPZ as the host. Different from most of ever reported white OLEDs fabricated with blue/red complementary triplet emitters that exhibit color rendering index (CRI) lower than 70, a high CRI of 82 is achieved due to the combination of blue and red phosphorescence emissions from FIrpic and Ir(piq)3, and the emerging green fluorescence emission from TCPZ.

(Invited) Blue Phosphorescence in Oxidized Nano-Porous Silicon

Heavily oxidized porous silicon treated by high-pressure water vapor annealing exhibits a strong blue light emission band which includes a long-lived phosphorescence (Ph) component. This Ph is particularly strong below 200 K. Above 200 K, it experiences a progressive thermally activated quenching. The intensity of the Ph and the peak of its spectrum depend on the excitation energy, and a large Stokes shift was observed. The Ph spectrum included six components exhibiting similar optical properties and likely due to vibronic features of molecular-like species. The Ph was modeled as the result of recombinations from triplet states to ground levels. Its origin is related to silicon nanostructures in an oxide matrix.

Strong Ligand Field Effects of Blue Phosphorescent Ir(III) Complexes with Phenylpyrazole and Phosphines

In the paper, we describe new Ir complexes for achieving efficient blue phosphorescence. New blue-emitting mixed-ligand Ir complexes comprising one cyclometalating, two phosphines trans to each other such as Ir(dppz)(PPh3)2(H)(L) (Ll= Cl, NCMe+, CN), [dppz = 3,5-Diphenylpyrazole] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. To gain insight into the factors responsible for the emission color change and the variation of luminescence efficiency, we investigate the electron-withdrawing capabilities of ancillary ligands using DFT and TD-DFT calculations on the ground and excited states of the complexes. To achieve deep blue emission and increase the emission efficiency, (1) we substitute the phenyl group on the 3-position of the pyrazole ring that lowers the triplet energy enough that the quenching channel is not thermally accessible and (2) change the ancillary ligands coordinated to iridium atom to phosphine and cyano groups known as very strong field ligands. Their inclusion in the coordination sphere can increase the HOMO-LUMO gap to achieve the hypsochromic shift in emission color and lower the HOMO and LUMO energy level, which causes a large d-orbital energy splitting and avoids the quenching effect to improve the luminescence efficiency. The maximum emission spectra of Ir(dppz)(PPh3)2(H)(CI) and Ir(dppz)(PPh3)2(H)(CN) were in the ranges of 439, 432 nm, respectively.

(Invited) Blue Phosphorescence in Oxidized Nano-Porous Silicon and Related Functions

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Bright Blue Phosphorescence from Cationic Bis-Cyclometalated Iridium(III) Isocyanide Complexes

We report new bis-cyclometalated cationic iridium(III) complexes [(C(^)N)(2)Ir(CN-tert-Bu)(2)](CF(3)SO(3)) that have tert-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4'-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-tert-butyl, or5-trifluoromethyl) as C(^)N ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58-3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440-459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46-82 μs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky tert-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid.

Synthesis and Crystal Structure of Blue Phosphorescent mer-Tris(2',6'-difluoro-2,3'-bipyridinato-N,C4') Iridium(III)

. Inaddition, the former is readily oxidized than the latter in electrochemical behavior.Key Words : Blue phosphorescence, Facial/meridional isomer, Thermal property, Crystal structure, Photo-physical propertyIntroductionIr(III) complexes with C^N chelated ligand such asphenylpyridine(ppy) have attracted much attention becauseof their applicability to Phosphorescence Organic Light-Emitting Diodes (PHOLEDs).

Soluble processed low-voltage and high efficiency blue phosphorescent organic light-emitting devices using small molecule host systems

We report low voltage driving and highly efficient blue phosphorescence organic light emitting diodes (PHOLEDs) fabricated by soluble process. A soluble small molecule mixed host system consisting of hole transporting 4,4’,4’’ tris(N-carbazolyl)triphenylamine (TCTA) and bipolar carrier transporting 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) exhibits high solubility with smooth surface properties. Moreover, this small molecule host shows the smoothest morphological property similar to a vacuum deposited amorphous film. A low driving voltage of 5.4V at 1000cd/m2 and maximum external quantum efficiency 14.6% obtained in the solution processed blue PHOLEDs are useful for large area low cost manufacturing.

Blue Phosphorescence in Oxidized Nano-Porous Silicon

Heavily oxidized porous silicon treated by high-pressure water vapor annealing exhibits a strong blue light emission band which includes a long-lived phosphorescence (Ph) component. This Ph is particularly strong below 200 K. Above 200 K, it experiences a progressive thermally activated quenching. The intensity of the Ph and the peak of its spectrum depend on the excitation energy, and a large Stokes shift was observed. The Ph spectrum included six components exhibiting similar optical properties and likely due to vibronic features of molecular-like species. The Ph was modeled as the result of recombinations from triplet states to ground levels. Its origin is related to silicon nanostructures in an oxide matrix.

Efficient solid-state white light-emitting electrochemical cells based on phosphorescent sensitization

Efficient phosphorescent sensitized white light-emitting electrochemical cells (LECs) based on a blue-emitting phosphorescent cationic transition metal complex (CTMC) doped with a red-emitting fluorescent dye are demonstrated. Blue phosphorescence and phosphorescence-sensitized red fluorescence contribute to white emission. Furthermore, the microcavity effect of the device structure is utilized to suppress the green part of the electroluminescence (EL) spectrum of the blue-emitting phosphorescent metal complex (1) via destructive interference. Hence, more saturated blue EL emission can be obtained. When combined with saturated red emission, white EL emission with Commission Internationale de l'Eclairage coordinates approaching (0.33, 0.33) can be obtained without the need for saturated deep-blue emitting CTMCs. Peak external quantum efficiency and power efficiency of the phosphorescence sensitized white LECs are up to 7.9% and 15.6 lm W−1, respectively. These efficiencies are among the highest reported for white LECs and reveal that phosphorescent sensitization is useful for improving device efficiencies of white LECs.

New blue phosphorescent iridium complexes for OLEDs

Abstract— A new class of ligands for complexation with Ir(III) has been developed. Tris‐homoleptic complexes derived from these ligands have been found to exhibit highly efficient blue phosphorescence with photoluminescent quantum yields in solution at room temperature of >0.9. These complexes have been applied as the emissive materials in OLEDs to give devices with efficiencies of up to 26 cd/A and an E.Q.E. of 1 7.4% at 1 mA/cm2.

Theoretical Studies of Mono-Cyclometalated Ir(III) Complexes with Phenylpyrazole Based Ligands and Phosphines

New blue emitting mixed ligand iridium(III) complexes comprising one cyclometalating, two phosphines trans to each other such as Ir(dppzc)(PPhMe2)2(H)(L) (L = Cl, CN) [dppzc = 1,3-Diphenyl-1H-pyrazole-4-carboxaldehyde] were 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 complexes. From these results, we discussed how the ancillary ligand influences the emission peak as well as the metal to ligand charge transfer (MLCT) transition efficiency. The calculated energies of triplets are 3.09 and 3.16 eV, respectively, which are all higher than that of Ir(dfppy)2(acac) and FIrpic known as blue phosphorescence materials with 2.69 and 2.73 eV.

Properties of Blue Polymer Light Emitting Diodes According to the Doping Concentrations of FIrpic Phosphorescence

PFO [poly(9,9-dioctylfluorene-2-7-diyl) end capped with N,N-Bis(4-methylphenyl)-4-aniline] based blue polymer light emitting diodes (PLEDs) that use a FIrpic [iridium(III) bis[4,6-(difluorophenyl) pyridinato-N,C20 picolinate] phosphorescent dopant as an emission layer were fabricated using a host-dopant system in order to achieve high luminance and efficiency properties. To find the optimum doping concentration, the FIrpic was doped at various concentrations, ranging from 1 to 4 wt%, into the PFO host. Blue phosphorescence PLEDs with the structure of ITO/PEDOT:PSS/PFO:FIrpic/TPBI/LiF/Al were prepared. The electrical and optical properties were investigated and compared in the prepared PLED devices with and without the FIrpic dopant. The luminance and the current efficiency were improved by introducing the FIrpic dopant. For the PLED device with a 2 wt% FIrpic concentration, the maximum luminance and current efficiency was found to be 4128 cd/m2 and 0.92 cd/A, respectively. The CIE color coordinates was (x, y = 0.19, 0.28), exhibiting a sky-blue color.

The Effects of Mn2+ Doping on the Luminescence Properties of 12CaO·7Al2O3:Eu2+ Nanocrystal Phosphor

The long lasting blue phosphorescence (LLP) and photostimulated luminescence (PSL) after ultraviolet light irradiation at room temperature in 12CaO 7Al2O3:xEu2+, yMn2+ (x = 0, 0.001; y = 0, 0.01) prepared by the chemical co-precipitation method were observed. It was shown that novel oxide 12CaO 7Al2O3:Eu2+, Mn2+ (C12A7:Eu2+, Mn2+) with unique nanocage structure can store energy when irradiated with 365 nm photons. And photon energy can be subsequently released by exposed to 980 nm light. The codopant Mn2+ enhances the intensity of the persistent phosphorescence and PSL due to the existence of more shallow and new deeper electron traps in C12A7: Eu2+, Mn2+. A model for energy storing and recovering and the detailed mechanism of PSL are presented through comparing with the luminescence properties of the co-doped C12A7:Eu2+, Mn2+ and C12A7:Eu2+.

Effects of the Introduced Single-Wall Carbon Nanotubes on the Performance of Blue Phosphorescence Organic Light-Emitting Diodes

In this paper, the effects of single-walled carbon nanotubes (SWCNTs) on the performance of blue phosphorescence organic light-emitting diodes (OLEDs) are investigated by altering the place of SWCNTs in prepared devices. Three kinds of OLEDs in which SWCNTs are spin-coated between the organic layer and the cathode or doped into the organic emitting layer or into the hole-injection layer are prepared. It is found that the SWCNTs doped into the hole-injection layer can enhance the current efficiency because the SWCNTs can reduce hole transport ability and balance two kinds of carriers. Even though SWCNTs doped into the organic emitting layer can improve the transport characteristics of polymer, excess holes lead to an imbalance of holes and electrons in the organic layer, and a lower current efficiency is obtained. SWCNTs located between the organic layer and the cathode can enhance the electron injection from the cathode to the organic layer, and the luminance and current efficiency characteristics are effectively improved.

P‐221L: Late‐News Poster: New Blue Phosphorescent Host for High‐efficiency White OLED

AbstractA new blue phosphorescent host with an electron transporting dibenzothiophene molecular skeleton connected to hole transporting carbazolyl moiety has been developed. Using this material, an all‐phosphorescent white OLED with a high power efficiency of 60 lm/W, a low driving voltage of 3.6V and a color‐rendering index of 75 at practical brightness of 1000 cd/m2 can be achieved.

Dislocations and impurities introduced from etch-pits at the epitaxial growth resumption of diamond

The fabrication of diamond-based electronic devices requires that several active layers with different doping concentrations are grown in different reactors. In this paper, we have investigated the effect of interrupting and resuming the epitaxial growth of a homoepitaxial diamond film using high-power plasma CVD. It was found that long lifetime blue phosphorescence which is localised on regions with a high dislocation density is generated. Such phosphorescence is related to a higher uptake of impurities at the interface between two subsequent films, due to an increased surface roughness from etching at the epitaxial growth resumption. Etching was found to occur preferentially on threading dislocations leaving typical etch-pits. Cathodoluminescence helps identify boron as the main background impurity. Besides, the formation of new dislocations was observed on the facets of these etch-pits. The continuation of epitaxy on a roughened surface thus comes with a substantial decrease in crystal quality.

Two novel lineal d10 metal-organic frameworks with a ditopic flexible linker: Structures, blue luminescence and thermal stability

Abstract Two novel supramolecular Ag(I) complexes, {[AgL]NO3}n (1), and {[AgL]CF3SO3}n (2), (L) = ethane-1,2-diyl bis(pyridine-3-carboxylate) have been prepared by self-assembly of Ag(I) salts with ethane-1,2-diyl bis(pyridine-3-carboxylate) (L) in THF/H2O system. The IR, TGA and elemental analysis have been recorded and both complexes were structurally characterized by X-ray crystallography confirming that complexes (1) and (2) are one-dimensional coordination polymers with lineal and helical chain motifs respectively. Solid emission spectra at room temperature of (1) and (2) show interesting blue phosphorescence with maximum intensity at 414 and 411 nm respectively, which are assigned to (LLCT) transition.

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs

Functional Device Applications of Nanosilicon

The particular physical functions of quantum-sized silicon have been investigated, along with exploration of their potential device applications. A strong confinement effect fully modifies the original optical, electrical, and thermal properties of bulk silicon. A discussion regarding their control and applications is presented, which addresses blue phosphorescence, enhanced photoconduction, operation of a ballistic electron emitter in solutions, and digital drive of a thermo-acoustic sound emitter.