607 research outputs found
A water soluble inorganic molecular oxide as a novel efficient electron injection layer for hybrid light-emitting diodes (HyLEDs)
No full textWe demonstrate that electron injection in single-layer polyfluorene based polymer light-emitting diodes (PLEDs) can be significantly enhanced by inserting a thin (<10 nm) inorganic polyoxometalate (POM) molecular oxide layer between the polymer layer and an aluminum cathode. Hydrophilic POM was spin-cast from methanol, an orthogonal solvent with regard to the hydrophobic polymer layer underneath, to form the thin cathode interfacial/electron injection layer. A lower turn-on and operating voltage and a higher luminance and current density was obtained in the POM-modified hybrid LEDs (HyLEDs) which are associated with the electron injection barrier reduction in the modified polymer/Al interface, evidenced by the increased open circuit voltage from photovoltaic measurements. These results demonstrate the potential of polyoxometalates as novel, stable cathode interfacial layers for efficient electron injection/transport in high performance HyLEDs.</p
Tuning the emitting color of organic light-emitting diodes through photochemically induced transformations: Towards single-layer, patterned, full-color displays and white-lighting applications
No full textPhotochemically induced emission tuning for the definition of pixels emitting the three primary colors, red, green, blue (RGB), in a single conducting polymeric layer is investigated. The approach proposed is based on an acid-induced emission shift of the (1-[4-(dimethylamino)phenyl]-6- phenylhexatriene) (DMA-DPH) green emitter and acid-induced quenching of the red fluorescent emitter (4-dimethylamino-4′-nitrostilbene) (DANS). The two emitters are dispersed in the wide bandgap conducting polymer poly(9-vinylcarbazole) (PVK), along with a photoacid generator (PAG). In the unexposed film areas, red emission is observed because of efficient energy transfer from PVK and DMA-DPH to DANS. Exposure of selected areas of the film at different doses results in quenching of the red emitter's fluorescence and the formation of green, blue, or even other color-emitting pixels, depending on the exposure dose and the relative concentrations of the different compounds in the film. Organic light-emitting diodes having the PVK polymer containing the appropriate amounts of DMA-DPH, DANS, and PAG as the emitting layer are fabricated and electroluminescence spectra are recorded. The time stability of induced emission spectrum changes and the color stability during device operation are also examined, and the first encouraging results are obtained.</p
Emergence of ambient temperature ferroelectricity in meso-tetrakis(1- methylpyridinium-4-yl)porphyrin chloride thin films
No full textHere, we demonstrate that the meso-tetrakis(1-methylpyridinium-4-yl) porphyrin chloride, [H2TMPyP]4+Cl4, with a face-to-face orientation directed along a single direction displays ferroelectric properties at room temperature. This is attributed to its spontaneous polarization, due to an extensive hydrogen-bonded network. From C-V measurements, a remnant polarization of approximately 0.5 μC cm-2 was estimated for pristine porphyrin film, which increases linearly up to about 1.7 μC cm-2 after applying 2 V at the top electrode and further to 9.6 μC cm-2 after 5 V positive poling. This large - for practical utilization - level of remnant polarization of [H2TMPyP] 4+Cl4 makes it promising for future applications.</p
Incorporating triphenyl sulfonium salts in polyfluorene PLEDs: An all-organic approach to improved charge injection
No full textAll-organic sulfonium salts are introduced as a class of ionic compounds that show high compatibility with conjugated polymers and may form blends with attractive luminescent properties leading to significant improvement in single-layer polymer light emitting diodes' (PLEDs') performance. We demonstrate that triphenylsulfonium (TPS) triflate:polyfluorene-co-benzothiadiazole (F8BT)-blend based PLEDs show a lower turn-on voltage, an increased luminous efficiency and higher peak luminance values. These results are being rationalized in terms of anionic accumulation and space charge formation at the anode side, which facilitates hole injection, leading to more balanced injection and subsequently to a higher recombination rate. Moreover, we find that the salt anion size plays a critical role in the device operating characteristics. The judicious choice of both the salt and the emitting polymer by considering relative energy level alignment, salt electrochemical stability and acquired thermodynamic stability of blend morphology is important for the achievement of high performance PLEDs without requiring elaborate device architectures.</p
Barrierless hole injection through sub-bandgap occupied states in organic light emitting diodes using substoichiometric MoO<sub>x</sub> anode interfacial layer
No full textIn this letter, highly efficient hole injection was demonstrated in hole only devices based on organic semiconductors with different highest occupied molecular orbital level and transport properties. The barrierless hole injection was achieved by using a substoichiometric MoOx thin film (consisting of 65% Mo+6 and 35% Mo+5) as a higly effective anode interfacial layer. The current in these devices was found to be space charge limited, achieved due to the formation of highly efficient anode ohmic contact via the excellent band alignment through occupied gap states at the ITO/MoOx and MoOx/organic semiconductor modified interface. Quite remarkably, the efficiency of hole injection was found to be almost independent of the MoOx thickness, which is indicative of perfect band alignment at the anode interface.</p
Effect of triphenylsulfonium triflate addition in wide band-gap polymer light-emitting diodes: improved charge injection, transport and electroplex-induced emission tuning
No full textThe presence of mobile anions in the emitting layer of polymer-based OLEDs has been proven to influence substantially the injection characteristics of the diode. In this work we report on the improvement of both injection and transport of charge carriers in blue emitting poly[2-(6-cyano-6-methyl-heptyloxy)-1,4- phenylene] (CN-PPP) based OLEDs upon insertion of the all-organic triphenylsulfonium (TPS) triflate salt in the emitting layer. On one hand, the anion displacement influences the energetics at the polymer/anode interface facilitating hole injection, whereas, on the other hand, the triphenylsulfonium cations act as electron transporting sites. The OLEDs exhibit significantly reduced turn-on voltage to half their initial value and increased luminance at low operating voltage. Moreover, the large energetic mismatch of the polymer and the triphenylsulfonium salt as well as the polarity induced by the ions result in simultaneous dual emission originating from the polymer exciton and from an electroplex, which is proposed to be formed at the triphenylsulfonium salt/polymer interfaces in the bulk. These results show that triphenylsulfonium salts represent an attractive class of materials that can be blended with conjugated polymers and can modify their electrical and/or emissive characteristics.</p
Theoretical study on the electronic structure of triphenyl sulfonium salts: electronic excitation and electron transfer processes
No full textDensity functional theory (DFT) and Time Dependent DFT calculations on triphenyl sulfonium cation (TPS) and the salts of TPS with triflate, nonaflate, perfluoro-1-octanesulfonate and hexafluoro antimonate anions are presented. These systems are widely used as cationic photoinitiators and as electron ejection layer for polymer light-emitting diodes. While some differences exist in the electronic structure of the different salts, their lowest energy intense absorption maxima are calculated at nearly the same energy for all systems. The first excited state of TPS and of the TPS salts is dissociating. Electron addition to the TPS salts lowers their energy by 1.0-1.33 eV.</p
Influence of the anion on the optoelectronic characteristics of triphenylsulfonium salts modified polymer light emitting devices
No full textTriphenylsulfonium salts addition in the emitting layer of polymer light emitting diodes (PLEDs) has been shown to be beneficial for charge injection and transport due to both ionic effects and π-conjugation in the phenyl rings of the cation. In some cases the emission profile can be also modified through an electroplex formation. Herein we investigate the effect of four TPS-salts with different counter anions on the overall PLED performance upon blending each salt with the conjugated polymer poly[2-(6-cyano-6-methyl-heptyloxy)-1,4-phenylene] (CN-PPP). In particular, three perfluoroalkanesulfonate organic anions of increasing size (triflate, nonaflate, PFOS) and a perfluorinated inorganic anion (SbF6) are compared. It is shown that the anion size affects primarily the turn-on and operational voltage, whereas its chemical nature is crucial for achieving high luminance values. The counteranion exerts also a direct impact on the dispersion properties of the salt in the polymer matrix, and thus, the film morphology, which in turn influences the emission colour and efficiency of an electroplex that is proposed to be formed at the sulfonium salt/polymer interfaces in the bulk. This study highlights the importance of properly selecting the counterions of the salts added in the emitting layer of PLEDs, which, in addition to their various functionalities, significantly influence device performance.</p
Environmental ethics: values in and duties to the natural world (summarized with commentary by Panagiotis Perros)
No full textSummarized with commentary in Greek by Panagiotis Perros.Environmental ethics stands on a frontier, as radically theoretical as it is applied. Alone, it asks whether there can be nonhuman objects of duty. Animals, plants, endangered species, ecosystems, and even Earth are progressively unfamiliar as objects of duty, and puzzles arise both for theory and practice. Answers to such questions are as urgent as any humans face, and intimately related to the four principal issues on the world agenda: peace, population, development, and environment
Immobilization of lipid substrates: application on phospholipase A2 determination
No full textThe purpose of the study was to assess a fluorimetric assay for the determination of total phospholipase A2 (PLA2) activity in biological samples introducing the innovation of immobilized substrates on crosslinked polymeric membranes. The immobilized C12-NBD-PtdCho, a fluorescent analogue of phosphatidylcholine, exhibited excellent stability for 3 months at 4 °C and was not desorbed in the aqueous reaction mixture during analysis. The limit of detection was 0.5 pmol FA (0.2 pg) and the linear part of the response curve extended from 1 up to 190 nmol FA/h/mL sample. The intra- and inter-day relative standard deviations (%RSD), were ≤6 and ≤9 %, respectively. Statistical comparison with other fluorescent methods showed excellent correlation and agreement. Semiempirical calculations showed a fair amount of electrostatic interaction between the NBD-labeled substrate and the crosslinked polyvinyl alcohol with the styryl pyridinium residues (PVA-SbQ) material, from the plane of which, the sn-2 acyl chain of the phospholipid stands out and is accessible by PLA2. Atomic Force Microscopy revealed morphological alterations of the immobilized substrate after the reaction with PLA2. Mass spectrometry showed that only C12-NBD-FA, the PLA2 hydrolysis product, was detected in the reaction mixture, indicating that PLA2 recognizes PVA-SbQ/C12-NBD-PtdCho as a surface to perform catalysis.</p
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