Study on ZnGa2O4:Cr a.c. powder electroluminescent device

An a.c. powder electroluminescent (EL) device using ZnGa₂O₄:Cr³⁺ phosphor was fabricated by the screen printing method. Optical and electrical properties of the device were investigated. The fabricated device shows a red emission at 695 nm driven by the a.c. voltage. The emission is attributed to the energy transfer from hot electrons to Cr³⁺ centers via self-activated Ga-O groups. Luminance (L) versus voltage (V) matches the well-known equation of L = L₀exp(− bV ^− 1 / 2) and luminance increases proportionally with frequency due to the increase of excitation probability of host lattice or Cr³⁺ centers. The diagram of the charge density (Q) versus applied voltage (V) is based on a conventional Sawyer-Tower circuit. At 280 V and 1000 Hz, the luminance and the luminous efficiency of the fabricated powder EL device are about 1.0 cd/m² and 13 lm/W, respectively. And under the high field, the device fabricated with the oxide-based phosphor of ZnGa₂O₄:Cr³⁺ shows excellent stability in comparison with the conventional sulfide powder EL device.     

Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes

The blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes containing the F₂-ppy (2,4-difluorophenylpyridine) and ppy (2-phenylpyridine) ligands were fabricated. Ir(ppy)₃ has been known to have a high phosphorescence efficiency in electroluminescence owing to its strong metal-to-ligand-charge transfer (MLCT) excited state, whereas the luminous efficiency of Ir(F₂-ppy)₃ was found to be low due to weak MLCT. Herein, we report two heteroleptic phosphorescent blue-green emitters, Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂, that exhibit emission peaks at 502 nm and 495 nm, respectively. The maximum luminous efficiencies of the devices with Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂ were 8.93 cd/A and 13.80 cd/A, respectively. The quantum efficiency of the device containing Ir(ppy)(F₂-ppy)₂ was 3.63% at J = 10 mA/cm².     

Efficient red electrophosphorescent organic light-emitting diodes based on the new sensitized heteroleptic tris-cyclometalated Ir(III) complexes

Organic light-emitting device (OLED) was fabricated using the novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, bis(2-phenylpyridine)iridium(III)[2(5′-methylphenyl)-4-diphenylquinoline] [Ir(ppy)₂(dpq-5CH₃)], based on 2-phenylpyridine (ppy) and 2(5′-methylphenyl)-4-diphenylquinoline (dpq-5CH₃) ligand. Generally, the ppy ligand in heteroleptic iridium complexes plays an important role as “sensitizer” in the efficient energy transfer from the host (CBP; 4,4,N,N′-dicarbazolebiphenyl) to the luminescent ligand (dpq-5CH₃). We demonstrated that high efficiency through the “sensitizer” can be obtained, when the T₁ of the emitting ligand is close to T₁ of the sensitizing ligand. The device containing Ir(ppy)₂(dpq-5CH₃) produced red light emission of 614 nm with maximum luminescence efficiency and power efficiency of 8.29 cd/A (at 0.09 mA/cm²) and 5.79 lm/W (at 0.09 mA/cm²), respectively.     

Effect of fabrication process on characteristics of phosphorescence organic light emitting diodes with methoxy-substituted starburst low-molecule as a host

The effect of dry process and wet process on the characteristics of phosphorescence organic light-emitting devices (OLEDs) employing a phosphorescent dye fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)₃) doped into a methoxy-substituted starburst low-molecule material methoxy-substituted 1,3,5-tris[4-(diphenylamino) phenyl]benzene (TDAPB) are investigated. The FT-IR and absorption spectra of TDAPB films fabricated by a dry process, and a wet process are almost same, and the PL spectra of those films are different. The carrier transport capability of TDAPB by a dry process is lower than that by a wet process. The photoluminescence intensity of Ir(ppy)₃ doped in TDAPB fabricated by a wet process is higher than that by a dry process. A maximum external current efficiency of more than 20 cd/A and luminance of more than 10,000 cd/m² were obtained. Maximum luminance of devices monotonously decreases with increasing the thickness of a dry-processed emitting layer. The main emission zone of the OLED was located in almost at the center of the emitting layer. The improvement of device performance in the OLED fabricated by a wet process was achieved due to the high efficient energy transfer from TDAPB to Ir(ppy)₃, high carrier transporting capability and the formation of homogeneous film, compared with that fabricated by a dry process.     

White polymer light-emitting diodes based on poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) doped with a poly(p-phenylene vinylene) derivative

Efficient white polymer light-emitting diodes based on the polymer blend of poly(2-(4′-(diphenylamino)phenylenevinyl)-1,4-phenylene-alt-9,9-n-dihexylfluorene-2,7-diyl) doped with poly{2-[3′,5′-bis(2?-ethylhexyloxy) benzyloxy]-1,4-phenylenevinylene}-co-poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) were fabricated. The electroluminescence (EL) spectrum is easily controlled by changing the dopant concentration. A white light emission was realized on the device with the dopant concentration of 0.194‰ and the emission light is less sensitive to the applied voltage in a wide voltage range. The maximum luminance and the maximum EL efficiency of the single-layer device were 2330 cd/m² and 0.29 cd/A, respectively. By introducing an Alq₃ layer as an electron transporting and hole blocking layer, the overall performance of the double layer device was dramatically improved, the maximum luminance and the maximum EL efficiency reached 3300 cd/m² and 2.37 cd/A, respectively.     

White OLEDs based on novel emissive materials such as Zn(HPB)2 and Zn(HPB)q

Organic light emitting diodes (OLEDs) show a lot of advantages for display purposes. Because OLEDs provide white light emission with high efficiency and stability, it is desirable to apply OLEDs as an illumination light source and backlight in LCD displays. We synthesized new emissive materials, namely [2-(2-hydroxyphenyl)benzoxazole] (Zn(HPB)₂) and [(2-(2-hydroxyphenyl)benzoxazole)(8-hydoxyquinoline)] (Zn(HPB)q), which have a low molecular compound and thermal stability. We studied white OLEDs using Zn(HPB)₂ and Zn(HPB)q. The fundamental structures of the white OLEDs were ITO/PEDOT:PSS (23 nm)/NPB (40 nm)/Zn(HPB)₂ (40 nm)/Zn(HPB)q (20, 30 or 40 nm)/Alq₃ (10 nm)/LiAl (120 nm). As a result, when the thickness of the Zn(HPB)q layer was 20 nm, white emission is achieved. We obtained a maximum luminance of 15325 cd/m² at a current density of 997 mA/cm². The CIE (Commission International de l'Eclairage) coordinates are (0.28, 0.35) at an applied voltage of 9.75 V.     

Improving light efficiency of white polymer light emitting diodes by introducing the TPBi exciton protection layer

We fabricated and evaluated the efficient white polymer light emitting diode (WPLED) by introducing TPBi exciton protection layer with ITO/PEDOT:PSS/PFO:MEH-PPV/TPBi/LiF/Al structure. PFO and MEH-PPV were prepared by the spin coating as the light emitting host and guest materials. TPBi was used as exciton protection material. The dependences of the MEH-PPV concentrations into the PFO (PFO:MEH-PPV) on the optical and electrical properties of the WPLEDs were investigated. The effect of the introduction of TPBi layer was studied by means of the property comparison between the samples with and without TPBi layer. The maximum luminance with 1480 cd/m² was obtained at the MEH-PPV concentration of 1.0 wt.% for the ITO/PEDOT:PSS/PFO:MEH-PPV/LiF/Al structure. In addition, the maximum luminance and current efficiency of the WPLED with TPBi layer were 7560 cd/m² at 12 V and 7.8 cd/A at 10 cd/m², respectively. The CIE color coordinates for WPLED with 1.0 wt.% MEH-PPV concentration was found to be (x, y) = (0.36, 0.33), showing pure white color.     

Impedance spectroscopy measurements of charge carrier mobility in 4,4'-N,N'-dicarbazole-biphenyl thin films doped with tris(2-phenylpyridine) iridium

The charge carrier mobility of green phosphorescent emissive layers, tris(2-phenylpyridine) iridium [Ir(ppy)₃]-doped 4,4'-N,N'-dicarbazole-biphenyl (CBP) thin films, has been determined using impedance spectroscopy (IS) measurements. The theoretical basis of mobility measurement by IS rests on a theory for single-injection space-charge limited current. The hole mobilities of the Ir(ppy)₃-doped CBP thin films were measured to be 10^− 10–10^− 8 cm²V^− 1 s^− 1 in the 2–7 wt.% Ir(ppy)₃-doped CBP from the frequency dependence of both conductance and capacitance. These hole mobility values are much lower than those of the undoped CBP thin films (~ 10^− 3 cm²V^− 1 s^− 1) because the Ir(ppy)₃ molecules act as trapping centers in the CBP host matrix. These mobility measurements in the Ir(ppy)₃-doped CBP thin films provide insight into the hole injection process.     

Sr3.5Mg0.5Si3O8Cl4: Eu bluish-green-emitting phosphor for NUV-based LED

Sr₄Si₃O₈Cl₄:Eu²⁺ and Sr_3.5Mg_0.5Si₃O₈Cl₄:Eu²⁺ phosphors were prepared by a conventional solid state reaction (SS). Excited by 370 nm near-ultraviolet light, the phosphors show an efficient bluish-green wide-band emission centering at 484 nm, which originates from the 4f5d¹ → 4f⁷ transition of Eu²⁺ ion. The excitation spectra of the phosphors are a broad band extending from 250 nm to 400 nm. Mg²⁺-codoping greatly enhances the bluish-green emission of the phosphors. An LED was fabricated by coating the Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ phosphor onto an ~ 370 nm-emitting InGaN chip. The LED exhibits bright bluish-green emission under a forward bias of 20 mA. The results indicate that Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ is a candidate as a bluish-green component for fabrication of NUV-based white LEDs.     

Heteroleptic tris-cyclometalated iridium(III) complexes with phenylpridine and diphenylquinoline derivative ligands

The synthesis and photophysical study of efficient phosphorescent heteroleptic tris-cyclometalated iridium(III) complexes having two different (C^N) ligands are reported. In order to improve the luminescence efficiency by avoiding triplet-triplet (T-T) annihilation, new heteroleptic tris-cyclometalated iridium complexes, Ir(ppy)₂(dpq), Ir(ppy)₂(dpq-3-F) and Ir(ppy)₂(dpq-CF₃), are designed and prepared where ppy, dpq, dpq-3-F and dpq-CF₃ represent 2-phenylpyridine, 2,4-diphenylquinoline, 2-(3-fluorophenyl)-4-phenylquinoline, and 4-phenyl-2-(4-(trifluoromethyl)phenyl)quinoline, respectively. Ppy ligands and dpq derivatives can act as a source of energy supply. When new heteroleptic tris-cyclometalated iridium complex, Ir(ppy)₂(dpq-3-F) is placed in the lowest excited state, the excitation energy is neither quenched nor deactivated but quickly intermolecularly transferred from two ppy ligands to one luminescent dpq-3-F ligand. Such transfer can occur because the triplet energy level of Ir(ppy)₃ is higher than that of Ir(dpq-3-F)₃ and because Ir(dpq-3-F)₃ was known to have a shorter lifetime than that of Ir(ppy)₃. As a result, Ir(ppy)₂(dpq-3-F) shows strong emission band at 620 nm from dpq-3-F ligand in the end. Thus it allows more reddish luminescent color and improves the luminescence by the decrease of quenching or energy deactivation by decreasing the number of the luminescent ligand. To analyze luminescent mechanism, we calculated these complexes theoretically by using computational method.     

Iridium-based light-emitting electrochemical cells containing ionic liquids in the luminous layer

To fabricate transition metal complex-based LECs (light-emitting electrochemical cells), ([Ir(ppy)₂(5,6-dime-1,10-phenthroline)]PF₆ was synthesized and used as a luminous material and ILs (ionic liquids) were incorporated into a luminous layer, in which two types of ionic liquid were used; 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄). ILs were added to a [Ir(ppy)₂(5,6-dime-1,10-phenthroline)]PF₆ luminous layer to improve ionic conductivity and light intensity. Both ILs significantly increased the current density and luminance. Due to the small molecule of BF₄^?, turn-on time was reduced and ionic conductivity was increased. However, the device stability was sacrificed. High current efficiency of 34.5 cd/A was investigated at 7 V of BMIMPF₆-doped luminous layer. The LECs based on [Ir(ppy)₂(5,6-dime-1,10-phenthroline)]PF₆ gave yellow emission color when ILs were added into light-emitting layer, and no significant change of color has been found in this study.     

Observation of phosphorescence from fluorescent organic material Bebq2 using phosphorescent sensitizer

Absorption, emission and excitation spectra of bis(10-hydroxybenzo [h] quinolinato)-beryllium (Bebq₂) were studied using polystyrene film doped with 5 wt% Bebq₂, N,N-di(naphthalene-1-yl)-N,N-diphenyl-benzidene (NPB) film doped with 60 wt% Bebq₂, and neat film. The monomer and aggregate of Bebq₂ give fluorescence at 492 and 511 nm at 12 K, respectively. A strong T₁ emission with a vibronic structure was observed from Bebq₂ below 70 K by heavily doping with phosphorescent tris(2-phenylpyridine) iridium [Ir(ppy)₃]. The T₁ energy of Bebq₂ was estimated to be 2.26 eV from the onset of the 573 nm 0–0 vibronic emission band. The energy transfer mechanism from Ir(ppy)₃ to the T₁ state of Bebq₂ is discussed.     

Improved dielectric properties of CaCu3Ti4O12 thin films on oxide bottom electrode of La0.5Sr0.5CoO3

We report on the effect of La_0.5Sr_0.5CoO₃ (LSCO) bottom electrode to the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) thin films grown on Ir/Ti/SiO₂/Si substrates. Compared with the films grown directly on Ir/Ti/SiO₂/Si substrates, the dielectric constant has been increased greatly about 100%, and the dielectric loss decreased to lower than 0.2 in the frequency range of 1-100 kHz. The origin has been discussed in details based on the analysis of the X-ray diffraction and impedance spectra measurements. Results of the impedance spectra suggest that the absence of undesired interfacial layer between Ir/CCTO thin films might be one of the major reasons of the improvement of the dielectric properties when the LSCO was introduced as the bottom electrode.     

Single dopant white electrophosphorescent light emitting diodes using heteroleptic tris-cyclometalated Iridium(III) complexes

We report single dopant single emissive layer white organic electroluminescent (EL) device based on the heteroleptic tris-cyclometalated iridium(III) complex, Ir(dfppy)₂(pq), as the guest, where dfppy and pq are 2-(2,4-difluorophenyl) pyridine and 2-phenylquinoline, respectively, and 1,4-phenylenesis(triphenylsilane) (UGH2) as the host. The maximum luminous and power efficiencies of the device were 11.00 cd/A (J = 0.05 mA/cm²) and 5.60 lm/W (J = 0.001 mA/cm²), respectively. The CIE coordinates of the device with Ir(dfppy)₂(pq) are (0.443, 0.473) and the EL spectrum of device shows emission band at 473 and 544 nm, at the applied voltage of 12 V. The similar phosphorescent decay rate of two ligands can lead to emit luminescence in two ligands at the same time.     

Enhanced performance in organic light-emitting diodes by sputtering TiO2 ultra-thin film as the hole buffer layer

Organic light-emitting diodes were prepared using titanium oxide (TiO₂) ultra-thin film by RF magnetron sputtering as the hole buffer layer. The device configuration is ITO/TiO₂/N-N′-diphenyl-N-N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine/tris(8-quinolinolato)-aluminum/LiF/Al. The maximum luminous efficiency for the 1.2 nm TiO₂ device is increased by approximately 46% (6.0 cd/A), in comparison with that of the control device (4.1 cd/A). The atomic force microscopy shows that with the insertion of TiO₂ buffer layer, the roughness of ITO surface decreases, which is favorable to improve the device luminance and increase the device lifetime. The mechanism behind the enhanced performance is that the TiO₂ layer enhances most of the holes injected from the anode and improves the balance of the hole and electron injections.     

Piezoelectric, ferroelectric and dielectric properties of La2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

La₂O₃ (0–0.8 wt.%)-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction. The influences of La₂O₃ on the microstructure, the dielectric, ferroelectric and piezoelectric properties of the composites were investigated. X-ray diffraction (XRD) patterns indicate that 0.2-0.8 wt.% of La₂O₃ has diffused into the lattice of BNBT6 ceramics. Consequently, a pure perovskite phase is formed. SEM images show that the microstructure of the ceramics is changed with the addition of a small amount of La₂O₃. The temperature dependence of the relative dielectric constant shows that Curie point decreases with the increase of La₂O₃. At room temperature, the ceramics doped with 0.6 wt.% La₂O₃ show superior performance with high piezoelectric constant (d₃₃ = 167 pC/N), high planar electromechanical coupling factor (k_p = 0.30), high mechanical quality factor (Q_m = 118), high relative dielectric constant (ε_r = 1470) and lower dissipation factor (tanδ = 0.056) at a frequency of 10 kHz.     

The effects of quenching treatment and AlF3 coating on LiNi0.5Mn0.5O2 cathode materials for lithium-ion battery

Submicron layered LiNi_0.5Mn_0.5O₂ was synthesized via a co-precipitation and solid-state reaction method together with a quenching process. The crystal structure and morphology of the materials were investigated by X-ray diffraction (XRD), Brunauer–Emmett and Teller (BET) surface area and scanning electron microscopy (SEM) techniques. It is found that LiNi_0.5Mn_0.5O₂ material prepared with quenching methods has smooth and regular structure in submicron scale with surface area of 0.43 m² g⁻¹. The initial discharge capacities are 175.8 mAh g⁻¹ at 0.1 C (28 mA g⁻¹) and 120.3 mAh g⁻¹ at 5.0 C (1400 mA g⁻¹), respectively, for the quenched samples between 2.5 and 4.5 V. It is demonstrated that quenching method is a useful approach for the preparation of submicron layered LiNi_0.5Mn_0.5O₂ cathode materials with excellent rate performance. In addition, the cycling performance of quenched-LiNi_0.5Mn_0.5O₂ material was also greatly improved by AlF₃ coating technique.     

Efficiency optimization of green phosphorescent organic light-emitting device

Using a narrow band gap host of bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp₂) and green phosphorescent Ir(ppy)₃ [fac-tris(2-phenylpyridine) iridium III] guest concentration as low as 2%, high efficiency phosphorescent organic light-emitting diode (PHOLED) is realized. Current and power efficiencies of 62.5 cd/A (max.), 51.0 lm/W (max.), and external quantum efficiency (max.) of 19.8% are reported in this green PHOLED. A low current efficiency roll-off value of 10% over the brightness of 10,000 cd/m² is noticed in this Bepp₂ single host device. Such a high efficiency is obtained by the optimization of the doping concentration with the knowledge of the hole trapping and the emission zone situations in this host-guest system. It is suggested that the reported device performance is suitable for applications in high brightness displays and lighting.     

Visible to near-infrared organic light-emitting diodes using phosphorescent materials by solution process

The characteristics of visible to near-infrared OLEDs with co-doping three phosphorescent dyes, iridium (III) bis(2-(4,6-difluorephenyl)pyridinato-N,C²′) (FIrpic), tris(1-phenylisoquinoline)iridium(III) (Ir(piq)₃), Pt-tetraphenyltetrabenzoporphyrin (Pt(tpbp)) in poly(N-vinylcarbazole) host as blue, red and near-infrared emitters are investigated. Visible to near-infrared OLEDs covering the wavelength range from 450 to 850 nm were achieved. The device with 11.7 wt.% FIrpic, 0.3 wt.% Ir(piq)₃ and 0.1 wt.% Pt(tpbp) showed white light emission of CIE (0.34, 0.39). The co-doping results in efficient cascade energy transfer from host through Ir complexes. For 0.1 wt.% Pt(tpbp), the optimal device exhibited the maximum output power of 3 mW/cm², maximum luminance of 2900 cd/m² and the maximum efficiency of 7 cd/A.     

Diode-end-pumped passively Q-switched 1.34 μm Nd:Gd0.5Y0.5VO4 laser with Co:LMA saturable absorber

In this paper, the output performances at 1.34 μm in continuous wave operation and passive Q-switching regime of a diode-end-pumped Nd:Gd_0.5Y_0.5VO₄ laser have been investigated. The passive Q-switching regime was achieved with Co²⁺:LaMgAl₁₁O₁₉ (Co²⁺:LMA) saturable absorbers crystals. A maximum average output power of 230 mW was recorded with a Co²⁺:LMA with initial transmission of 81%. The minimum pulse duration was 116 ns, which corresponded to a repetition rate of 360 kHz, the single pulse energy of 2.1 μJ and the pulse peak power of 5.5 W.