Electrical conduction behavior of organic light‐emitting diodes using fluorinated self‐assembled monolayer with molybdenum oxide‐doped hole transporting layer

The electrical conductivity behavior of a fluorinated self‐assembled monolayer (FSAM) of a molybdenum oxide (MoOx)‐doped α‐naphthyl diamine derivative (α‐NPD) in organic light‐emitting diodes (OLEDs) was investigated. The current density of the MoOx‐doped α‐NPD/FSAM device was proportional to its voltage owing to smooth carrier injection through the FSAM and the high carrier density of its bulk. The temperature‐dependent characteristics of this device were investigated. The current density–voltage characteristics at different temperatures were almost the same owing to its very low activation energy. The activation energy of the device was estimated to be 1.056 × 10^?2 [eV] and was very low due to the inelastic electron tunneling of FSAM molecules. Copyright © 2014 John Wiley & Sons, Ltd.     

Blue organic light‐emitting diodes based on fluorene‐bridged quinazoline and quinoxaline derivatives

Two blue emitters based on fluorene‐bridged quinazoline and quinoxaline derivatives were prepared via the Suzuki reaction. Their photoluminescent properties were investigated. Furthermore, theoretical studies on these materials using the density functional theory calculation were conducted. To explore their electroluminescent properties, multilayered organic light‐emitting diodes were fabricated with the following device structure: indium–tin–oxide (180 nm)/4,4′‐bis(N‐(1‐naphthyl)‐N‐phenylamino)biphenyl (50 nm)/blue emitting materials ( 1 and 2 ) (30 nm)/bathophenanthroline (35 nm)/8‐hydroxy‐quinolinato lithium (2 nm)/Al (100 nm). Two devices showed efficient blue emission with the external quantum efficiencies of 1.58% and 1.30%, respectively, at 20 mA/cm², and Commission Internationale dÉclairage coordinates of (0.18, 0.24) and (0.19, 0.27) at 6.0 V. These results suggest that the self‐aggregation properties of emitters would have considerable effects on their photoluminescent and electroluminescent properties.     

Multiple night‐time light‐emitting diode lighting strategies impact grassland invertebrate assemblages

White light‐emitting diodes (LEDs) are rapidly replacing conventional outdoor lighting technologies around the world. Despite rising concerns over their impact on the environment and human health, the flexibility of LEDs has been advocated as a means of mitigating the ecological impacts of globally widespread outdoor night‐time lighting through spectral manipulation, dimming and switching lights off during periods of low demand. We conducted a three‐year field experiment in which each of these lighting strategies was simulated in a previously artificial light naïve grassland ecosystem. White LEDs both increased the total abundance and changed the assemblage composition of adult spiders and beetles. Dimming LEDs by 50% or manipulating their spectra to reduce ecologically damaging wavelengths partially reduced the number of commoner species affected from seven to four. A combination of dimming by 50% and switching lights off between midnight and 04:00 am showed the most promise for reducing the ecological costs of LEDs, but the abundances of two otherwise common species were still affected. The environmental consequences of using alternative lighting technologies are increasingly well established. These results suggest that while management strategies using LEDs can be an effective means of reducing the number of taxa affected, averting the ecological impacts of night‐time lighting may ultimately require avoiding its use altogether.     

Luminescent properties of Ca3SiO4Cl2 co‐doped with Ce3+ and Eu2+ for near‐ultraviolet light‐emitting diodes

Ca₃SiO₄Cl₂ co‐doped with Ce³⁺,Eu²⁺ was prepared by high temperature reaction. The structure, luminescent properties and the energy transfer process of Ca₃SiO₄Cl₂: Ce³⁺,Eu²⁺ were investigated. Eu²⁺ ions can give enhanced green emission through Ce³⁺ → Eu²⁺ energy transfer in these phosphors. The green phosphor Ca_2.9775SiO₄Cl₂:0.0045Ce³⁺,0.018Eu²⁺ showed intense green emission with broader excitation in the near‐ultraviolet light range. A green light‐emitting diode (LED) based on this phosphor was made, and bright green light from this green LED could be observed by the naked eye under 20 mA current excitation. Hence it is considered to be a good candidate for the green component of a three‐band white LED. Copyright © 2015 John Wiley & Sons, Ltd.     

Performance of light‐emitting diode traps for collecting sand flies in entomological surveys in Argentina

The performance of two light‐emitting diode traps with white and black light for capturing phlebotomine sand flies, developed by the Argentinean Leishmaniasis Research Network (REDILA‐WL and REDILA‐BL traps), were compared with the traditional CDC incandescent light trap. Entomological data were obtained from six sand fly surveys conducted in Argentina in different environments. Data analyses were conducted for the presence and the abundance of Lutzomyia longipalpis, Migonemyia migonei, and Nyssomyia whitmani (106 sites). No differences were found in presence/absence among the three types of traps for all sand fly species (p>0.05). The collection mean of Lu. longipalpis from the REDILA‐BL didn´t differ from the CDC trap means, nor were differences seen between the REDILA‐WL and the CDC trap collection means (p>0.05), but collections were larger from the REDILA‐BL trap compared to the REDILA‐WL trap (p<0.05). For Mg. migonei and Ny. whitmani, no differences were found among the three types of traps in the number of individuals captured (p>0.05). These results suggest that both REDILA traps could be used as an alternative capture tool to the original CDC trap for surveillance of these species, and that the REDILA‐BL will also allow a comparable estimation of the abundance of these flies to the CDC light trap captures. In addition, the REDILA‐BL has better performance than the REDILA‐WL, at least for Lu. longipalpis.     

Preparation of balanced trichromatic white phosphors for solid‐state white lighting

High quality white light‐emitting diodes (LEDs) employ multi‐component phosphor mixtures to generate light of a high color rendering index (CRI). The number of distinct components in a typical phosphor mix usually ranges from two to four. Here we describe a systematic experimental technique for starting with phosphors of known chromatic properties and arriving at their respective proportions for creating a blended phosphor to produce light of the desired chromaticity. This method is applicable to both LED pumped and laser diode (LD) pumped white light sources. In this approach, the radiometric power in the down‐converted luminescence of each phosphor is determined and that information is used to estimate the CIE chromaticity coordinate of light generated from the mixed phosphor. A suitable method for mixing multi‐component phosphors is also described. This paper also examines the effect of light scattering particles in phosphors and their use for altering the spectral characteristics of LD‐ and LED‐generated light. This is the only approach available for making high efficiency phosphor‐converted single‐color LEDs that emit light of wide spectral width.     

2‐(4‐Ethoxy phenyl)‐4‐phenyl quinoline organic phosphor for solution processed blue organic light‐emitting diodes

This paper reports the synthesis and characterization of 2‐(4‐ethoxyphenyl)‐4‐phenyl quinoline (OEt‐DPQ) organic phosphor using an acid‐catalyzed Friedlander reaction and the preparation of blended thin films by molecularly doping OEt‐DPQ in poly(methyl methacrylate) (PMMA) at different wt%. The molecular structure of the synthesized phosphor was confirmed by Fourier transform infra‐red (FTIR) spectroscopy and nuclear magnetic resonance spectra (NMR). Surface morphology and percent composition of the elements were assessed by scanning electron microscopy (SEM) and energy dispersive analysis of X‐rays (EDAX). The thermal stability and melting point of OEt‐DPQ and thin films were probed by thermo‐gravimetric analysis (TGA)/differential thermal analysis (DTA) and were found to be 80°C and 113.6°C, respectively. UV–visible optical absorption spectra of OEt‐DPQ in the solid state and blended films produced absorption bands in the range 260–340 nm, while photoluminescence (PL) spectra of OEt‐DPQ in the solid state and blended thin films demonstrated blue emission that was registered at 432 nm when excited at 363–369 nm. However, solvated OEt‐DPQ in chloroform, tetrahydrofuran or dichloromethane showed a blue shift of 31–43 nm. Optical absorption and emission parameters such as molar extinction coefficient (ε), energy gap (E_g), transmittance (T), reflectance (R), refractive index (n), oscillator energy (E₀) and oscillator strength (f), quantum yield (φ_f), oscillator energy (E₀), dispersion energy (E_d), Commission Internationale de l'Éclairage (CIE) co‐ordinates and energy yield fluorescence (E_F) were calculated to assess the phosphor's suitability as a blue emissive material for opto‐electronic applications such as organic light‐emitting diodes (OLEDs), flexible displays and solid‐state lighting technology.     

Effect of blue light‐emitting diode light and antioxidant potential in a somatic cell

Light is an indispensable part of routine laboratory work in which conventional light is generally used. Light‐emitting diodes (LEDs) have come to replace conventional light, and thus could be a potent target in biomedical studies. Since blue light is a major component of visible light wavelength, in this study, using a somatic cell from the African green monkey kidney, we assessed the possible consequences of the blue spectra of LED light in future animal experiments and proposed a potent mitigation against light‐induced damage. COS‐7 cells were exposed to blue LED light (450 nm) and the growth and deoxyribonucleic acid (DNA) damage were assessed at different exposure times. A higher suppression in cell growth and viability was observed under a longer period of blue LED light exposure. The number of apoptotic cells increased as the light exposure time was prolonged. Reactive oxygen species (ROS) generation was also elevated in accordance to the extension of light exposure time. A comparison with dark‐maintained cells revealed that the upregulation of ROS by blue LED light plays a significant role in causing cellular dysfunction in DNA in a time‐dependent manner. In turn, antioxidant treatment has been shown to improve cell growth and viability under blue LED light conditions. This indicates that antioxidants have potential against blue LED light‐induced somatic cell damage. It is expected that this study will contribute to the understanding of the basic mechanism of somatic cell death under visible light and maximize the beneficial use of LED light in future animal experiments.     

Artifact‐free objective‐type multicolor total internal reflection fluorescence microscopy with light‐emitting diode light sources—Part I

Total internal reflection fluorescence excitation (TIRF) microscopy allows the selective observation of fluorescent molecules in immediate proximity to an interface between different refractive indices. Objective‐type or prism‐less TIRF excitation is typically achieved with laser light sources. We here propose a simple, yet optically advantageous light‐emitting diode (LED)‐based implementation of objective‐type TIRF (LED‐TIRF). The proposed LED‐TIRF condenser is affordable and easy to set up at any epifluorescence microscope to perform multicolor TIRF and/or combined TIRF‐epifluorescence imaging with even illumination of the entire field of view. Electrical control of LED light sources replaces mechanical shutters or optical modulators. LED‐TIRF microscopy eliminates safety burdens that are associated with laser sources, offers favorable instrument lifetime and stability without active cooling. The non‐coherent light source and the type of projection eliminate interference fringing and local scattering artifacts that are associated with conventional laser‐TIRF. Unlike azimuthal spinning laser‐TIRF, LED‐TIRF does not require synchronization between beam rotation and the camera and can be monitored with either global or rolling shutter cameras. Typical implementations, such as live cell multicolor imaging in TIRF and epifluorescence of imaging of short‐lived, localized translocation events of a Ca²⁺‐sensitive protein kinase C α fusion protein are demonstrated.     

Tuning the white light spectrum of light emitting diode lamps to reduce attraction of nocturnal arthropods

Artificial lighting allows humans to be active at night, but has many unintended consequences, including interference with ecological processes, disruption of circadian rhythms and increased exposure to insect vectors of diseases. Although ultraviolet and blue light are usually most attractive to arthropods, degree of attraction varies among orders. With a focus on future indoor lighting applications, we manipulated the spectrum of white lamps to investigate the influence of spectral composition on number of arthropods attracted. We compared numbers of arthropods captured at three customizable light-emitting diode (LED) lamps (3510, 2704 and 2728 K), two commercial LED lamps (2700 K), two commercial compact fluorescent lamps (CFLs; 2700 K) and a control. We configured the three custom LEDs to minimize invertebrate attraction based on published attraction curves for honeybees and moths. Lamps were placed with pan traps at an urban and two rural study sites in Los Angeles, California. For all invertebrate orders combined, our custom LED configurations were less attractive than the commercial LED lamps or CFLs of similar colour temperatures. Thus, adjusting spectral composition of white light to minimize attracting nocturnal arthropods is feasible; not all lights with the same colour temperature are equally attractive to arthropods.     

Effects of white light‐emitting diode (LED) exposure on retinal pigment epithelium in vivo

Ageing and alteration of the functions of the retinal pigment epithelium (RPE) are at the origin of lost of vision seen in age‐related macular degeneration (AMD). The RPE is known to be vulnerable to high‐energy blue light. The white light‐emitting diodes (LED) commercially available have relatively high content of blue light, a feature that suggest that they could be deleterious for this retinal cell layer. The aim of our study was to investigate the effects of “white LED” exposure on RPE. For this, commercially available white LEDs were used for exposure experiments on Wistar rats. Immunohistochemical stain on RPE flat mount, transmission electron microscopy and Western blot were used to exam the RPE. LED‐induced RPE damage was evaluated by studying oxidative stress, stress response pathways and cell death pathways as well as the integrity of the outer blood–retinal barrier (BRB). We show that white LED light caused structural alterations leading to the disruption of the outer blood–retinal barrier. We observed an increase in oxidized molecules, disturbance of basal autophagy and cell death by necrosis. We conclude that white LEDs induced strong damages in rat RPE characterized by the breakdown of the BRB and the induction of necrotic cell death.     

Efficient blue organic light‐emitting diodes using N2,N2,N11,N11,5,6,7,8‐octaphenyltriphenylene‐2,11‐diamine derivatives

In this study, we have synthesized phenyl‐substituted triphenylene derivatives, using the Diels–Alder reaction and the Buchwald–Hartwig reaction. To investigate electroluminescence properties of these materials, multilayer organic light‐emitting diode (OLED) devices were fabricated with a structure of indium–tin–oxide (ITO) (180 nm)/4,4′‐bis(N‐(1‐naphthyl)‐N‐phenylamino)biphenyl (NPB) (50 nm)/blue‐emitting materials (1–3) (30 nm)/bathophenanthroline (Bphen) (35 nm)/lithium quinolate (Liq) (2 nm)/Al (100 nm). A device using N²,N²,N¹¹,N¹¹,5,6,7‐heptaphenyltriphenylene‐2,11‐diamine (2) exhibited efficient blue emission with luminous, power, and external quantum efficiencies of 0.92 cd/A, 0.67 lm/W, and 1.17% at 20 mA/cm², respectively. The Commission International de L'Éclairage coordinates of this device were (x = 0.15, y = 0.09) at 6.0 V. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and photophysical properties of 1,2‐diphenylindolizine derivatives: fluorescent blue‐emitting materials for organic light‐emitting device

New blue‐emitting materials based on 1,2‐diphenylindolizine were designed and synthesized through a microwave‐assisted Suzuki coupling reaction. The photophysical, electrochemical, and thermal properties of the 1,2‐diphenylindolizine derivatives were investigated using UV–visible and fluorescence spectroscopy, cyclic voltammetry, thermogravimetric analysis, and differential scanning calorimetry. The 1,2‐diphenylindolizine derivatives had band gaps of 3.1–3.4 eV and indicated proper emission of around 450 nm without significant difference between in solution and thin solid film. The indolizine derivatives show an enhanced thermal stability (?T_m > 100 °C), compared with 1,2‐diphenylindolizine. These results suggest the 1,2‐diphenylindolizine derivatives are suitable for blue‐emitting materials in organic light‐emitting devices. Copyright © 2015 John Wiley & Sons, Ltd.     

Nanocrystalline SrS:Ce3+ system for the generation of white light‐emitting diodes

Nanocrystalline SrS phosphors doped with Ce³⁺ ions at different concentrations (0.5, 1, 1.5 and 2 mol%) are synthesized via the solid‐state diffusion method (SSDM), which is suitable for the large‐scale production of phosphors in industrial applications. The as‐prepared samples are characterized using an X‐ray diffraction (XRD) technique, field emission scanning electron microscopy (FESEM), high‐resolution transmission electron microscopy (HRTEM) and energy‐dispersive X‐ray (EDX) analysis. The optical properties of these phosphors are analyzed using reflectance spectra, photoluminescence spectra and afterglow decay curves. The cubic structure of the SrS phosphor is confirmed by XRD analysis and the crystallite size calculated by Scherer's formula using XRD data shows the nanocrystalline nature of the phosphors. No phase change is observed with increasing concentrations of Ce³⁺ ions. The surface morphology of the prepared phosphors is determined by FESEM, which shows a sphere‐like structure and good connectivity of the grains. The authenticity of the formation of nanocrystalline phosphors is examined by HRTEM analysis. Elemental compositional information for the prepared phosphors is gathered by EDX analysis. Photoluminescence studies reveal that the emission spectra of the prepared phosphor shows broad band emission centered at 458 and 550 nm due to the transition of electrons from the 5d → 4f energy levels. The afterglow decay characteristics of different as‐synthesized SrS:Ce³⁺ nanophosphors are conceptually described. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis and photoluminescence of EuII in barium zinc orthosilicate: a novel green color emitting phosphor for white‐LEDs

A series of Eu²⁺‐activated barium orthosilicates (BaZnSiO₄) were synthesized using a high‐temperature solid‐state reaction. A photoluminescence excitation study of Eu²⁺ shows a broad absorption band in the range of 270–450 nm, with multiple absorption peak maxima (310, 350 and 400 nm) due to 4f–5d electronic transition. The emission spectra of all the compositions show green color emission (in the spectral region 450–550 nm with a peak maximum at 502 nm and a shoulder at ~ 490 nm) with appropriate Comission Internationale de l'Eclairage (CIE) color coordinates. The two emission peaks are due to the presence of Eu²⁺ in two different Ba sites in the BaZnSiO₄ host lattice. The energy transfers between the Eu²⁺ ions in BaZnSiO₄ host are elucidated from the critical concentration quenching data based on the electronic multipolar interaction. All Eu²⁺‐activated BaZnSiO₄ phosphor materials can be efficiently excited in the ultraviolet (UV) to near UV‐region (270–420 nm), making them attractive candidate as a green phosphor for solid state lighting–white light‐emitting diodes.     

Study on constant–step stress accelerated life tests in white organic light‐emitting diodes

In order to obtain reliability information for a white organic light‐emitting diode (OLED), two constant and one step stress tests were conducted with its working current increased. The Weibull function was applied to describe the OLED life distribution, and the maximum likelihood estimation (MLE) and its iterative flow chart were used to calculate shape and scale parameters. Furthermore, the accelerated life equation was determined using the least squares method, a Kolmogorov–Smirnov test was performed to assess if the white OLED life follows a Weibull distribution, and self‐developed software was used to predict the average and the median lifetimes of the OLED. The numerical results indicate that white OLED life conforms to a Weibull distribution, and that the accelerated life equation completely satisfies the inverse power law. The estimated life of a white OLED may provide significant guidelines for its manufacturers and customers. Copyright © 2014 John Wiley & Sons, Ltd.