Highly efficient flexible organic light-emitting devices utilizing F4-TCNQ/m-MTDATA multiple quantum well structures

Flexible organic light-emitting devices (FOLEDs) based on multiple quantum well (MQW) structures, which consist of alternate layers of 2,3,5,6-Tetrafluoro-7,7,8,8,-tetracyano-quinodimethane (F4-TCNQ) and 4,4′,4″-tris-(3-methylphenylphe-nylamino)tripheny-lamine (m-MTDATA) have been fabricated. The Alq₃-based device with double quantum well (DQW) structure exhibits the remarkable electroluminescent (EL) performances for the brightness of 23,500 cd/m² at 14 V and the maximum current efficiency of 7.0 cd/A at 300.3 mA/cm², respectively, which are greatly improved by 114% and 56% compared with the brightness of 10,958 cd/m² at 14 V and the maximum current efficiency of 4.5 cd/A at 174.0 mA/cm² for the conventional device without MQW structures. These results demonstrate that the EL performances of FOLEDs could be greatly improved by utilizing the novel MQW structures, and the reason for this improvement has also been explained by the effect of interfacial dipole and interfacial doping between F4-TCNQ and m-MTDATA in this article.     

A novel blue-emitting Ir(III) complex with short excited state lifetime: Synthesis,structure, photophysical property,and electrophosphorescence performance

In this paper, we synthesize a triphenylamine-derived cyclometalating ligand of (4-benzothiazol-2-yl-phenyl)-diphenyl-amine (referred as BPDA) and its corresponding Ir(III) complex of (BPDA)₂Ir(acac) (acac=acetylacetone). The photophysical property, molecular structure, thermal property and electroluminescence performance of (BPDA)₂Ir(acac) are investigated in detail. It is found that (BPDA)₂Ir(acac) is an efficient emitter with high thermal stability and short excited state lifetime. The emission of (BPDA)₂Ir(acac) changes from deep blue (417 nm) to bluish green (500 nm) upon addition of different solvents. We also investigate its electrophosphorescence performance. A maximum electroluminance of 8820 cd/m² peaking at 494 nm is achieved, with the highest device efficiency of 1.72 cd/A.     

Tetraalkoxylated-PPV derivative: An efficient and pure green electroluminescent polymer with good solution processability

A new solution-processable tetraalkoxy-substituted poly(1,4-phenylenevinylene) derivative, poly{[2-(3′,7′-dimethyloctyloxy)-3,5,6-trimethoxy]-1,4-phenylenevinylene} (TALK-PPV), was synthesized through a dehydrohalogenation polymerization route, and its light-emitting properties were investigated. The TALK-PPV showed highly blue-shifted UV–visible absorption and PL emission spectra compared to the dialkoxy-substituted PPV derivatives. This is because of the disturbance to the π-conjugation caused by a steric hindered structure. The TALK-PPV thin film exhibited an absorption peak at 446 nm, with an onset at 515 nm. Its PL emission maximum was at 554 nm. Cyclic voltammetric analysis showed the HOMO and LUMO energy levels of the TALK-PPV to be 5.77 and 3.36 eV, respectively. Light-emitting devices were fabricated with an ITO (indium-tin oxide)/PEDOT/polymer/Ca/Al configuration. The TALK-PPV component leads to pure green light emission with a CIE 1931 chromaticity of (0.20, 0.74) at 100 cd/m² brightness, which is very close to the standard green (0.21, 0.71) demanded by the NTSC (National Television System Committee). The maximum brightness of this device was 24,900 cd/m² with an efficiency of 1.45 cd/A.     

Synthesis and electroluminescent characterization of a symmetric starburst orange-red light material

A new symmetric starburst orange-red light material, tris(4-(2-(N-butyl-1,8-naphthalimide)ethynyl)phenyl)amine (TNGT), was designed and synthesized. It shows a high fluorescence quantum yield and a slight concentration-quenching effect. A high brightness (6600 cd/m²) and a high current efficiency [4.57 cd/A (at 420 cd/m²)] with CIE (0.59, 0.40) were achieved at a relatively high doping concentration (20 wt%) in a TNGT-based OLED.     

Quantum dot p-i-n structure in an electric field

Time-resolved photoluminescence (PL), steady-state PL, and electroluminescence (EL) techniques have been used to characterize the carrier relaxation processes and carrier escape mechanisms in self-assembled InAs/GaAs quantum dot (SAQD) p-i-n structures under reverse bias. The measurements were performed between 5 K and room temperature on a ring mesa sample as a function of bias. At 100 K, the PL decay time originating from the n  =  1 SAQD decreases with increasing reverse bias from ∼3 ns under flat band condition to∼ 400 ps for a bias of −3 V. The data can be explained by a simple model based on electron recombination in the quantum dots (QDs) or escape out of the dots. The escape can occur by one of three possible routes: direct tunneling out of the distribution of excited electronic levels, thermally assisted tunneling of ground state electrons through the upper excited electronic states or thermionic emission to the wetting layer.     

Synthesis and characterization of PPV-based light-emitting copolymer with alkylsilylphenyloxy pendant group for light-emitting diode applications

A novel light-emitting copolymer with high brightness and luminance efficiency was synthesized using the Gilch polymerization method, and its electro-optical properties were investigated. A polymer light-emitting diode (PLED) was fabricated in ITO/PEDOT/light-emitting copolymer/Ca/Al configuration. The turn-on voltage of the PLED was about 5.0 V with maximum brightness and luminance efficiency up to 1420 cd/m² at 16.2 V and 0.5 cd/A at 6.8 V, respectively.     

Screen-printed white OLED based on polystyrene as a host polymer

We demonstrate the use of screen printing in the fabrication of single-layer organic-light-emitting devices (OLEDs). The organic layer is a single-layer of polystyrene, in which we incorporate rubrene for orange emission and α-NPD, DPVBi for blue emission. An appropriate mixing of the two colors produced white emission by incomplete Förster energy transfer. We showed the role of each constituent, α-NPD, DPVBi and rubrene in the emission characteristics of OLEDs. The turn-on voltage of screen-printed white OLEDs was about 10 V with maximum brightness and luminous efficiency up to 1300 cd/m² and 9 cd/A, respectively.     

Energy transfer induced Eu3+ photoluminescence enhancement in tellurite glass

In this work, structural, thermal and optical properties of Eu³⁺ doped TeO₂–La₂O₃–TiO₂ glass were investigated. The differential scanning calorimetry (DSC) measurements reveal an important stability factor ΔT=143.52 K, which indicates the good thermal and mechanical stabilities of tellurite glass. From the absorption spectrum, the optical band gap was found to be direct with E_g=3.23 eV. The temperature dependences of photoluminescence (PL) properties of Eu-doped and Eu–Tb codoped tellurite glass are investigated. As the temperature increases from 7 to 300 K, both the PL intensity and the PL lifetime relative to the ⁵D₂→⁷F₀ are nearly constant below 230 K and then an enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from charged intrinsic defects states to Eu³⁺ energy levels.By co-doping tellurite glasses with Eu and Tb, a strong Eu³⁺ PL enhancement is shown due to excitation transfer from Tb³⁺ and intrinsic defects to Eu ions.     

Efficient red light phosphorescence emission in simple bi-layered structure organic devices with fluorescent host-phosphorescent guest system

Highly efficient red phosphorescent devices comprising a simple bi-layered structure using tris(1-phenylisoquinoline)iridium (Ir(piq)₃) doped in a narrow band-gap fluorescent host material, bis(10-hydroxybenzo [h] quinolinato)beryllium complex (Bebq₂) are reported. The driving voltage to reach 1000 cd/m² is 3.5 V in Bebq₂:Ir(piq)₃ red phosphorescent device. With a dopant concentration of as low as 4%, the current and power efficiency values of 8.41 cd/A and 7.34 lm/W are obtained in this PHOLEDs, respectively. External quantum efficiency (EQE) of 14.5% is noticed in this red phosphorescent device, promising to high brightness applications.     

Quantum yield of Eu2+ emission in (Ca1−xSrx)S:Eu light emitting diode converter at 20–420 K

A series of red-emitting light converters Ca_1?xSr_xS:Eu²⁺, with tunable composition-dependent emission maxima were synthesized and characterized concerning their photoluminescent (PL) properties. X-ray diffraction patterns, photoluminescence spectra, luminosities and quantum yields were compared for phosphors with strontium concentrations varying from 0 to 100%. The maxima wavelength of emission shifts from 663 down to 619 nm, originating from the dependence of Eu²⁺ 5d state energy on the surrounding crystal field. Upon increasing the temperature from 20 to 420 K, a broadening of emission spectra along with thermal quenching of emission intensity and quantum yield occurs. Satisfying PL properties and their thermal stability demonstrate that the phosphors could be used as light converters in light emitting diodes (LEDs).     

Influence of injection current and temperature on electroluminescence in InGaN/GaN multiple quantum wells

Electroluminescence (EL) spectra of blue InGaN/GaN multiple-quantum-well light-emitting diode (LED) have been investigated over a wide range of injection current (0.001–200 mA) and at various temperatures (6–300 K). Surprisingly, with increasing the injection current the EL peak energy shows an initial blueshift accompanied by a broadening of the EL linewidth at low temperatures (below 30 K). This trend differs from the usual photoluminescence (PL) measurement results, which have shown that with increasing the optical excitation power the PL peak energy gave an initial blueshift accompanied by a narrowing of the PL linewidth at low temperatures. The anomalous current behavior of the EL spectra may be attributed to electron leakage results in the failure of Coulomb screening effect and the relative enhancement of the low-energetic localized state filling at low temperatures and low currents. The electron leakage for the LED is further confirmed by both the current dependence of the EL intensity and the temperature dependence of the EL efficiency.     

Solution-processed white organic light-emitting diodes with mixed-host structures

Efficient white light-emitting diodes (WOLEDs) were fabricated with a solution-processed single emission layer composed of a molecular and polymeric material mixed-host (MH). The main host used was a blue-emitting molecular material of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) and the assisting host used was a hole-transport-type polymer of poly(9-vinylcarbazole) (PVK). By co-doping 4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl and 5,6,11,12-tetraphenylnaphacene into the MH, the performances of the fabricated devices made with different mixing ratio of host materials were investigated, and were to depend on the mixing ratios. Under the optimal PVK:DPVBi ratio (3:7), we achieved a maximum luminance of 14 110 cd/m² and a maximum current efficiency of 9.5 cd/A. These improvements were attributed to the MH structure, which effectively improved the thermal stability of spin-coated film and enhanced the hole-injection/transporting properties of WOLEDs.     

Luminescence properties of InAs quantum dots formed by a modified self-assembled method

The luminescence properties of self-assembled InAs quantum dots (QDs) on GaAs (1 0 0) substrates grown by molecular beam epitaxy have been investigated using temperature-dependent photoluminescence (PL) and time-resolved PL (TRPL). InAs QDs were grown using an In-interruption growth technique, in which the indium flux was periodically interrupted. InAs QDs grown using In-interruption showed reduced PL linewidth, redshifted PL emission energy, increased energy level spacing between the ground state and the first excited state, and reduced decay time, indicating an improvement in the size distribution and size/shape of QDs.     

Highly efficient and stable organic light-emitting diode by balancing drift current of charge

Highly efficient and stable OLED device in which hole-drift current and electron-drift current are balanced was fabricated. Drift current characteristics according to the thickness of organic layer were examined using the device with ITO/m-MTDATA/NPB/Al structure that can only move the hole and the device with Al/LiF/Alq₃/LiF/Al structure that can only move the electron. Using the result of such examination, green device with balanced drift current was produced. Device with the structure of m-MTDATA (80 nm)/NPB (20 nm)/C-545T (3%) doped Alq₃ (5 nm)/Alq₃ (59 nm)/LiF (1 nm)/Al (200 nm) showed color purity of (0.309, 0.643) and high efficiency of 7.0 lm/W (14.4 cd/A). Most of light emission was observed inside the green emitting layer. Through the result of EL spectrum for the device also including red emitting layer, same result could be obtained. The device with balanced drift current also showed half life-time of 175 h for initial luminance of 3000 cd/m², which is more stable in comparison to the device without balanced drift current.     

Organic light sources look forward to optimize the photosynthesis process

We introduce a series of organic LEDs that exploit the monomer and excimer emissions from single phosphor dopant emitters. These organic LEDs were found to be effective in the simultaneous creation of blue and red emission bands essential for plant growth. By varying the concentration of novel phosphorescent dopants selected from a series of newly synthesized platinum complexes [PtL^22–25Cl], we have manufactured the blue-biased LEDs [with the Commission Internationale de L’Eclairage (CIE) coordinates (x, y) (0.27, 0.37)] and the red-biased LEDs [CIE coordinates (0.53, 0.38)], at a high luminance of ≈500 cd/m² and with external electroluminescence (EL) quantum efficiency of 15–18% photon/electron (→ power efficiency 8–12 lm/W). The EL spectrum most suitable for the action spectrum of photosynthesis yield was that of a device incorporating 20 wt.% content of [PtL²³Cl]. This LED yielded photosynthetic photon flux (PPF) approaching 10 μmol s⁻¹ W⁻¹ of the electrical power, a value which significantly exceeds that for the professional lamps used commonly for horticultural lighting.     

Optical properties,luminescence quenching mechanism and radiation hardness of Eu-doped GaN red powder phosphor

We report on the luminescence quenching mechanism of Eu-doped GaN powder phosphor produced with a low-cost, high yield rapid-ammonothermal method. We have studied as-synthesized and acid rinsed Eu-doped GaN powders with the Eu concentration of ～0.5 at.%. The Eu-doped GaN photoluminescence (PL) was investigated with 325 nm excitation wavelength at hydrostatic pressures up to 7.7 GPa in temperature range between 12 K and 300 K. The room temperature integrated Eu³⁺ ion PL intensity from acid rinsed material is a few times stronger than from the as-synthesized material. The temperature dependent PL studies revealed that the thermal quenching of the dominant Eu³⁺ ion transition (⁵D₀ → ⁷F₂) at 622 nm is stronger in the chemically modified phosphor indicating more efficient coupling between the Eu³⁺ ion and passivated GaN powder grains. Furthermore, it was found that thermal quenching of Eu³⁺ ion emission intensity can be completely suppressed in studied materials by applied pressure. This is due to stronger localization of bound exciton on Eu³⁺ ion trap induced by hydrostatic pressure. Furthermore, the effect of 2 MeV oxygen irradiation on the PL properties has been investigated for highly efficient Eu-doped GaN phosphor embedded in KBr–GaN:Eu³⁺ composite. Fairly good radiation damage resistance was obtained for 1.7 × 10¹² to 5 × 10¹³ cm^?2 oxygen fluence. Preliminary data indicate that Eu-doped GaN powder phosphor can be considered for devices in a radiation environment.     

Model and simulation on the efficiencies of microcavity OLEDs

This paper presents simple calculation models of the external quantum efficiency and power efficiency for the microcavity OLEDs. The models take into account the energy spatial distribution of the device and provide a rough estimate of the efficiencies for the planar surface emitting devices, by which the integrating sphere and monochrometer were saved. The external quantum efficiency and luminous current efficiency from the structures of glass/DBR/ITO/NPB/Alq: C545T/Alq/LiF/Al and glass/ITO/NPB/Alq: C545T/Alq/LiF/Al were calculated based on these models and the measured data. Comparing with conventional OLED, the external quantum efficiency and luminous current efficiency of the MOLED were improved 3.1% and 8% at low current density (< 10 mA/cm², corresponding to the display brightness range), respectively.     

Stable green light-emission from poly[9,9-bis(4′-n-octyloxyphenyl)fluorenyl-2,7-vinylene] synthesized via the Gilch polymerization route

A new fluorene-containing poly(arylenevinylene) derivative, poly[9,9-bis(4-octyloxyphenyl)fluorenyl-2,7-vinylene] (PBOPFV), was synthesized via the Gilch polymerization route and its light-emission properties were characterized and compared with those of poly(9,9-di-n-octylfluorenyl-2,7-vinylene) (PFV). As is the case for poly(alkylfluorene)s, PFV exhibits a long-wavelength emission that is additional to its emission in the blue-green region after thermal annealing or the passage of current. We have successfully suppressed this long-wavelength emission by introducing an octyloxyphenyl group at the 9-position of the fluorene group. PBOPFV produces PL emission maxima at 478 and 510 nm and no significant changes were found in its PL emission spectrum even after thermal annealing at 150 °C for 2 h. Light-emitting devices were fabricated with ITO/PEDOT:PSS/polymer/LiF/Al configurations. The EL spectrum of the device constructed using PFV was found to undergo significant changes during device operation, whereas the EL spectrum of the device constructed using PBOPFV was found to be stable.     

Deep blue organic light emitting diode based on anthracene derivative as a dopant

This letter presents a deep blue organic light emitting diode which was fabricated by using 9,10-di(2-naphthyl)anthracene as a dopant and 4,4′-N,N′-dicarbazole-biphenyl as a host. The Commission Internationale de l’Eclairage coordinates of (0.1516, 0.0836) were achieved in the cell, which is very close to the National Television Standards Committee standard of (0.14, 0.08). Meanwhile, maximum luminance over 6500 cd/cm² and maximum current efficiency of 3.5 cd/A were also obtained.     

High-efficiency organic light-emitting diodes (OLEDs) with a mixed layer structure

Improved performance of organic light-emitting diodes (OLEDs) as obtained by a mixed layer was investigated. The OLEDs with a mixed layer which were composed of N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB), tris-(8-hydroxyquinolato) aluminum (Alq₃) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) showed the highest brightness and efficiency, which reached 19048 cd/m² at 17 V and 4.3 cd/A at 10 mA/cm², respectively. The turn-on voltage of the device is 2.6 V. Its Commission Internationale del’Eclairage (CIE) coordinate is (0.497, 0.456) at 17 V, and the CIE coordinates of the device are largely insensitive to the driving voltages, which depicts stabilized yellow color.