Efficient Intramolecular Charge-Transfer Fluorophores Based on Substituted Triphenylphosphine Donors.

Triphenylphosphine (TPP)-based luminescent compounds are rarely investigated because of the low photoluminescence quantum yield (PLQY). Here, we demonstrate that introducing steric hindrance groups to the TPP moiety and separating the orbitals involved in the transition can drastically suppress the non-radiative decay induced by structural distortion of TPP in the excited state. High PLQY up to 0.89 as well as thermally activated delayed fluorescence are observed from the intramolecular charge-transfer (ICT) molecules with substituted TPP donors (sTPPs) in doped films. The red organic light-emitting diodes employing these emitters achieve comparable external quantum efficiencies to the control device containing a classical phosphorescent dye, revealing the great potential of the ICT emitters based on electrochemically stable sTPPs.

Photoluminescence properties of Eu(3+)-doped glaserite-type orthovanadates CsK(2)Gd[VO(4)](2).

Undoped and Eu(3+)-doped glaserite-type orthovanadates CsK2Gd1-xEux[VO4]2 with various Eu(3+) concentrations of x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 were synthesized via the solid-state reaction. The formation of a single phase compound was verified through the X-ray diffraction studies. The photoluminescence (PL) and PL excitation (PLE) spectra, PL decay curves, and absolute quantum efficiency (QE) were investigated. Unlike the conventional Eu(3+)-doped vanadates, these Eu(3+)-doped samples showed not only several sharp emission lines due to Eu(3+) but also a broad emission band with a maximum at 530 nm due to the [VO4](3-) host. The intensities of the host and Eu(3+) emissions increased when the Eu(3+) concentration was increased from x = 0 to x = 0.6 and decreased above x = 0.6. Similar concentration dependence was observed for QE. The host emission, even if in the Eu(3+)-condensed host of CsK2Eu(VO4), was never quenched indicating inefficient energy transfer from the host [VO4](3-) to Eu(3+). This inefficient energy transfer is understood by suppression of the energy transfer by the V-O-Eu bond angle deviated from 180° and the separation of Eu(3+) ions at the Gd(3+) site from [VO4](3-). Like the 530 nm charge transfer [VO4](3-) emission, two broad and intense PLE bands with maxima at 330 and 312 nm were observed for the Eu(3+) emission. A maximum QE of 38.5% was obtained from CsK2Gd1-xEux[VO4]2 (x = 0.6). A white-colored emission was obtained by the combination of the broad 530 nm emission band and the intense sharp lines due to Eu(3+) at 590-620 nm.

Novel yellow-emitting phosphors of Ca5M4(VO4)6 (M=Mg, Zn) with isolated VO4 tetrahedra.

The self-activated yellow-emitting phosphors of vanadates Ca5M4(VO4)6 (M = Mg, Zn) were synthesized via the solid-state reaction route. The formation of single phase compound with garnet structure was verified through X-ray diffraction (XRD) studies. The excitation and emission spectra and the thermal quenching of luminescence intensities were measured. The different luminescence properties of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 phosphors were presented, e.g., the spectra shift, the luminescence lifetimes, the absolute quantum efficiency, the color coordinates and the Stokes shift. This deference was discussed on the base of the relationship between the micro-structure and the charge transfer transitions in [VO4]3- groups in the lattices. Ca5Mg4(VO4)6 could be suggested to be a potential yellow-emitting phosphor for the application on near-UV excited white LEDs.

Tandem organic light-emitting diodes with KBH4 doped 9,10-bis(3-(pyridin-3-yl)phenyl) anthracene connected to the charge generation layer.

Tandem organic light emitting diodes (OLEDs) are ideal for lighting applications due to their low working current density at high brightness. In this work, we have studied an efficient electron transporting layer of KBH(4) doped 9,10-bis(3-(pyridin-3-yl)phenyl)anthracene (DPyPA) which is located adjacent to charge generation layer of MoO(3)/NPB. The excellent transporting property of the DPyPA:KBH(4) layer helps the tandem OLED to achieve a lower voltage than the tandem device with the widely used tris-(8-hydroxyquinoline)aluminum:Li. For the tandem white OLED with a fluorescent blue unit and a phosphorescent yellow unit, we've achieved a high current efficiency of 75 cd/A, which can be further improved to 120 cd/A by attaching a diffuser layer.

Effect of ITO surface treatment on organic light emitting diodes.

Performance of organic light emitting diodes with structure of ITO/NPB/NPB:Ir(piq)3/Alq3/Al was studied by inserting MoO3 hole injection layer (HIL) and using differently cleaned indium tin oxides (ITOs). High luminance and quantum efficiencies were obtained using HIL and highly cleaned ITO. High power efficiency was obtained using HIL and conventionally cleaned ITO, while low driving voltage was obtained using highly cleaned ITO without HIL.

Zero-Zero Energy-Dominated Degradation in Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence.

Blue-emitting organic light-emitting diodes (OLEDs) fall significantly behind other OLEDs in operational stability. To better understand the key factors governing the stability of blue OLEDs employing thermally activated delayed fluorescence (TADF), nine efficient sky-blue to green TADF emitters with different frontier orbital energy levels and different TADF lifetimes have been designed and synthesized on the basis of charge-transfer (CT) acridine/phenyltriazine derivatives. Among them, ToDMAC-TRZ, a molecule composed of a 9,9-dimethyl-2,7-di-o-tolyl-9,10-dihydroacridine donor and a 2,4,6-triphenyl-1,3,5-triazine acceptor, shows a quantum yield of nearly 1 and a TADF lifetime as short as 0.59 µs in thin film. However, the stability of OLEDs is independent of the frontier orbital energy levels and TADF lifetime of the emitter. In contrast, the device half-life is found to decrease by five-sixths as the 0-0 energy of the singlet excitons increases by about 0.06 eV, which can be well-explained by the Arrhenius equation employing a photoreaction model. Whether in photoluminescence or electroluminescence, the contribution of long-lifetime triplet excitons to degradation is much lower than expected, which can be accounted for by how the solid-state solvation effect reduces the energy of the 3CT state and how most molecules have a low-lying locally excited triplet state.

Toward a BT.2020 green emitter through a combined multiple resonance effect and multi-lock strategy.

Color-saturated green-emitting molecules with high Commission Internationale de L'Eclairage (CIE) y values have great potential applications for displays and imaging. Here, we linked the outer phenyl groups in multiple-resonance (MR)-type blue-emitting B (boron)-N (nitrogen) molecules through bonding and spiro-carbon bridges, resulting in rigid green emitters with thermally activated delayed fluorescence. The MR effect and multiple interlocking strategy greatly suppressed the high-frequency vibrations in the molecules, which emit green light with a full-width at half-maximum of 14 nm and a CIE y value of 0.77 in cyclohexane. These were the purest green molecules with quantum efficiency and color purity that were comparable with current best quantum dots. Doping these emitters into a traditional green-emitting phosphorescence organic light-emitting diode (OLED) endowed the device with a Broadcast Service Television 2020 color-gamut, 50% improved external quantum efficiency, and an extremely high luminescence of 5.1 × 105 cd/m2, making it the greenest and brightest OLED ever reported.

The new red-emitting phosphor of oxyfluoride Ca2RF4PO4:Eu3+ (R=Gd, Y) for solid state lighting applications.

The novel red-emitting phosphors of Eu3+-activated Ca2RF4PO4:Eu3+ (R=Gd, Y) prepared by a solid-state reaction have been evaluated as a candidate for white solid state lighting. The detailed luminescence properties, e.g., the excitation spectra, the luminescence spectra and quantum efficiency under the excitation of near-UV, and decay lifetimes were reported. The phosphors can be efficiently excited by near UV light and exhibit a dominant emission peaked at 611 nm (5D0-7F2) with CIE coordinates of (x=0.661, y=0.333). The thermal stabilities were investigated from the luminescence intensities, color purity and the decay curves by increasing temperature. The luminescence parameters related to white LEDs applications were compared to some red phosphors and discussed in details. The red-emitting Ca2RF4PO4:Eu3+ (R=Gd, Y) may be potentially useful in the fabrication of white LEDs.

Conjugated asymmetric donor-substituted 1,3,5-triazines: new host materials for blue phosphorescent organic light-emitting diodes.

Conjugated asymmetric donor-substituted 1,3,5-triazines (ADTs) have been synthesized by nucleophilic substitution of organolithium catalyzed by [Pd(PPh(3))(4)]. Theoretical and experimental investigations show that ADTs possess high solubility and thermostability, high fluorescent quantum yield (35%), low HOMO (-6.0 eV) and LUMO (-2.8 eV), and high triplet energy (E(T), 3.0 eV) according to the different substitution pattern of triazine. The application as host materials for blue PHOLEDs yielded a maximum current efficiency of 20.9 cd A(-1), a maximum external quantum efficiency of 9.8%, and a brightness of 9671 cd m(-2) at 5.4 V, making ADTs good candidates for optoelectronic devices.

The thermal stabilities of luminescence and microstructures of Eu2+-doped KBaPO4 and NaSrPO4 with ß-K2SO4 type structure.

Eu(2+)-doped monophosphates NaSrPO(4) and KBaPO(4) with the ß-K(2)SO(4) structure were synthesized using the conventional high temperature solid state reaction. The X-ray powder diffraction, photoluminescence excitation, and emission spectra and decay curves were measured. The phosphors can be efficiently excited by UV-visible light from 220 to 430 nm to realize emission in the visible range. The natures of the Eu(2+) emission, e.g., the chromaticity coordinates, the Stokes shifts, and the luminescence absolute quantum efficiencies, were reported. The luminescence quenching temperatures and the thermal activation energies for NaSrPO(4):Eu(2+) and KBaPO(4):Eu(2+) were obtained from the temperature dependent (10-435 K) luminescence intensities and decay curves. KBaPO(4):Eu(2+) presents only one emission center; however, Eu(2+) ions have a "disordered environment" in NaSrPO(4) lattices. The relationship between the luminescence thermal stabilities and the crystal structures was discussed. The crystallographic occupations of rare earth ions doped in these hosts were analyzed by the site-selective emission spectra and the excitation spectra of Eu(3+) ions in the (7)F(0)→(5)D(0) transitions using a pulsed, tunable, and narrow-band dye laser. In KBaPO(4), the Eu(3+) ions could be distributed in the host with a high "ordered state" in only one site in the lattices. However, the multiple site structure of Eu(3+) ions with highly disordered distributions in NaSrPO(4) lattices was suggested.

High Fluorescence Rate of Thermally Activated Delayed Fluorescence Emitters for Efficient and Stable Blue OLEDs.

A lack of an efficient and stable blue device is a critical factor restricting the development of organic light-emitting diode (OLED) technology that is currently expected to be overcome by employing thermally activated delayed fluorescence (TADF). Here, we investigate the TADF and electroluminescence (EL) performance of six carbazole/triphenyltriazine derivatives in different hosts. A good linearity between lg(LT50/kF2) and the EL emission wavelength is found, where LT50 is the half-life of the devices and kF is the fluorescence rate of the emitters, suggesting the dominance of the singlet exciton energy and lifetime in device stability. An indolylcarbazole/triphenyltriazine derivative (ICz-TRZ) with the capability to suppress solid-state solvation exhibits blue-shifted emission and an increased kF (1.5 × 108 s-1) in comparison to the control emitters in doped films. ICz-TRZ-based devices achieve a maximum external quantum efficiency (EQE) of 18% and an EQE of 5.5% at a very high luminance of 7 × 104 cd/m2. Ignoring the poor electrochemical stability of ICz-TRZ, the device offers an LT50 approaching 100 h under an initial luminance of 1000 cd/m2 and CIE coordinates of (0.14, 0.19). The findings in this work suggest that computer-aided design of high kF TADF emitters can be an approach to realize efficient and stable blue OLEDs.

Weakly Conjugated Phosphine Oxide Hosts for Efficient Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes.

The development of host materials with high first-triplet state (T1) energy and high charge mobility is a key to achieve efficient true-blue organic light-emitting diodes (OLEDs), employing phosphorescence and thermally activated delayed fluorescence (TADF). An ether-bridged double triphenylphosphine oxide (TPPO) compound, bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), was reported to have a very high T1 energy of 3.3 eV but suffers from poor charge mobility. Here, five bridge-controlled multi-TPPO derivatives were studied through a combination of experiments and theory. We demonstrate that the push-pull electron capability of the bridge group governs the T1 energy and electron mobility of these materials. Replacing the ether bridge by a bis(trifluoromethyl)methylene group can reduce the energy barrier for intramolecular electron exchange and consequently enhance the electron mobility by two orders of magnitude without lowering the T1 energy. A blue TADF OLED employing this bis(trifluoromethyl)methylene-bridged compound achieves the same high external quantum efficiency but a much higher current density compared to the control device employing DPEPO. In contrast, a bridge group with strong electron-withdrawing capability, such as phosphine oxygen or sulfone, lowers the T1 energy of the compound by enhancing the electronic coupling between TPPO subunits and inhibits intermolecular electron transfer by trapping the electron charge around the bridge.

Very broad white-emission spectrum based organic light-emitting diodes by four exciplex emission bands.

White light is emitted by an organic light-emitting diode by inserting two blend layers of m-MTDATA:Al(DBM)(3) and TPD:Bphen between an m-MTDATA hole-transporting layer and a Bphen electron-transporting layer, where m-MTDATA, TPD, Al(DBM)(3), and Bphen are 4,4('),4('')-tris[methylpheny(phenyl)amino]-triphenylamine, N,N(')-bis(3-methylphenyl)-N,N(')-diphenylbenzidine, tris(dibenzoyl methane)-aluminum, and 4,7-diphenyl-1,10-phenanthroline molecules, respectively. The white-light spectrum consists of four broad bands that arise from blue-emitting TPD/Bphen, green-emitting m-MTDATA/Bphen, orange-emitting TPD/Al(DBM)(3), and red-emitting m-MTDATA/Al(DBM)(3) exciplexes, respectively, and strongly overlap at 400-760 nm. Any monomer emission is not generated. A high-color rendering index of 94.1, Commission Internationale de l'Eclairage-1931(x,y) coordinates of (0.33, 0.35), and correlated color temperature of 5477 K were obtained at 10 V. Discussion is given for the formation mechanism of the four exciplexes.

Spectroscopic properties of Pr3+ ions in a PbWO4 single crystal.

Site-selective fluorescence laser spectroscopy of Pr (3+) ions in lead tungstate single crystal were investigated at temperatures from 10 to 300 K. The site-selective emission spectra and fluorescence decays from the (3)P J ( J = 0, 1, 2) and (1)D 2 states were analyzed. The (3)P J ( J = 0, 1, 2) level shows its predominantly radioactive character with the typical greenish-blue luminescence ascribed to (3)P J transition. The emission from the (1)D 2 level is only observed when this level is directly excited. The decay kinetic of the (1)D 2 level was measured under site-selective excitation and discussed in terms of cross-relaxation. The up-conversion emission from levels (3)P 1 and (3)P 0 following excitation of the (1)D 2 state was observed in the PbWO 4 crystal between 10 and 300 K. The main up-conversion mechanism, together with the understanding the quenching of the (1)D 2 fluorescence in this Pr (3+) heavily doped PbWO 4 were discussed. The presence of the complex structures of the emission spectra and different decay profiles indicate that several processes contribute to the quenching of the (1)D 2 fluorescence of Pr (3+) ions. It was found that the up-conversion fluorescence intensity had a quadratic dependence on the laser input power. The temporal behavior of the up converted emission indicates that an energy-transfer up-conversion is the dominant process.

Recent Progress in Ionic Iridium(III) Complexes for Organic Electronic Devices.

Ionic iridium(III) complexes are emerging with great promise for organic electronic devices, owing to their unique features such as ease of molecular design and synthesis, excellent photophysical properties, superior redox stability, and highly efficient emissions of virtually all colors. Here, recent progress on new material design, regarding photo- and electroluminescence is highlighted, including several interesting topics such as: i) color-tuning strategies of cationic iridium(III) complexes, ii) widespread utilization in phosphorescent light-emitting devices fabricated by not only solution processes but also vacuum evaporation deposition, and iii) potential applications in data record, storage, and sercurity. Results on anionic iridium(III) complexes and "soft salts" are also discussed, indicating a new related subject. Finally, a brief outlook is suggested, pointing out that ionic iridium(III) complexes should play a more significant role in future organic electronic materials technology.

Optical study of Yb(3+)/Yb(2+) conversion in CaF(2) crystals.

Yb(3+) ions with various site symmetries have been observed in the absorption and emission spectra of Yb(3+):CaF(2) crystals, both γ-irradiated and annealed in hydrogen. The absorption intensity value is found to be much higher for the γ-irradiated crystal and strongly dependent on the gamma dose. The UV absorption spectra of γ-irradiated and H(2)-annealed CaF(2):5 at.% Yb(3+) crystals are quite similar. Yb(2+) absorption bands are observed at 360, 315, 271, 260, 227 and 214 nm, which are called A, B, C, D, F and G bands, respectively. For γ-irradiated CaF(2):30 at.% Yb(3+), an additional band at 234 nm can be seen. It is suggested that only a negligible amount of Yb(3+) ions are converted into Yb(2+) under the γ-irradiation. The presence of Yb(2+) is confirmed by the 565 and 540 nm luminescence under 357 nm excitation. It is also suggested that the excitation in the A, C, D and F absorption bands of Yb(2+) gives rise to photo-ionization of Yb(2+) ions and electrons in the conduction band to form the excited Yb(3+) ions which emit IR Yb(3+) luminescence.The UV absorption and emission spectra obtained for γ-irradiated and H(2)-annealed crystals have different structures. This suggests that different mechanisms are responsible for the creation of Yb(2+) ions. γ-irradiation favours Yb(2+) isolated centres by reduction of Yb(3+) ions located at Ca(2+) lattice sites, whereas annealing in hydrogen favours Yb(2+) centres neighbouring Yb(3+) ions when a Yb(3+) ion pair captures a Compton electron. Also, γ-irradiation does not change the position of Yb(3+) ions converted into Yb(2+) in the CaF(2) lattice. In the case of H(2) annealing, a Yb(3+) ion converted to Yb(2+) is shifted to the Ca(2+) position in the lattice.

Concentration quenching of praseodymium ions Pr(3+) in BaGd2(MoO4)4 crystals.

The concentration effect on the photoluminescence (PL) of the praseodymium Pr(3+) ion is studied at 298-12 K for barium gadolinium molybdate (BaGd2(MoO4)4, called BGM) crystals with a wide Pr(3+) concentration range of 0.05-25.0 mol%. Three types of concentration dependences are observed for the emissions although all types show PL quenching at high concentrations. The first type (Type A) has the maximum PL intensity at about 10 mol% with a non-zero intensity at high concentrations, which is observed for the (3)P0 emissions except for emission at 621 nm. The second and third types (Type B-1 and B-2) have the maximum at about 1 mol% with a finite residual intensity and nearly zero intensity at high concentrations, respectively, which are observed for the 621 nm emission and all the (1)D2 emissions. It is suggested that the energy migration mechanism is responsible for Type A, while the non-resonant cross-relaxation is responsible for Type B-1 and the resonant cross-relaxation for Type B-2.

Lighting object-based nondoped-type white organic light-emitting diode with N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-benzidine)-4,4'-diamine as the chromaticity-tuning layer.

We demonstrate a nondoped white organic light-emitting diode in which the blue, green, and red emissions are generated from 4,4(')-bis(2,2(')-diphenylvinyl)-1,1(')-biphenyl, tris(8-hydroxyquinoline)aluminum, and a submonolayer of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7- tetramethyl-julolidyl 9-enyl)-4H-pyran layers, respectively. A thin layer of N,N(')-diphenyl-N,N(')-bis(1-naphthyl)(1,1(')-benzidine)-4,4(')-diamine (NPB), which differed from the traditional hole-transporting layer, was introduced into the device. The thickness of this thin NPB layer was changed to tune the chromaticity and optimize the white color quality. The white device with a 3 nm chromaticity-tuning NPB layer gives the Commission Internationale de l'Eclairage-1931 xy coordinate of (0.327, 0.336), a color rendering index of 90.2, a maximum luminance of 19,096 cd/m(2), and a maximum current efficiency of 4.12 cd/A. The electroluminescence mechanism of the white device was also discussed.