214 research outputs found
Scalable fabrication of nanostructured p-Si/n-ZnO heterojunctions by femtosecond-laser processing
No full textWe present a versatile, large-scale fabrication method for nanostructured semiconducting junctions. Silicon substrates were processed by femtosecond laser pulses in methanol and a quasi-ordered distribution of columnar nanospikes was formed on the surface of the substrates. A thin (80 nm) layer of ZnO was deposited on the laser-processed silicon surface by pulsed laser deposition, forming a nanostructured p-Si/n-ZnO heterojunction. We characterized the structural, optical, and electrical properties of the heterojunction. Electrical I-V measurements on the nanostructured p-Si/n-ZnO device show non-linear electric characteristics with a diode-like behavior. Electrical I-V measurements on a flat p-Si/n-ZnO reference sample show similar characteristics, however the forward current and rectification ratio are improved by orders of magnitude in the nanostructured device owing to its increased surface area. The fabrication method employed in this work can be extended to other homojunctions or heterojunctions for electronic and optoelectronic devices with large surface area.</p
A combined experimental and simulation study on thickness dependence of the emission characteristics in multicolor single layer organic light-emitting diodes
No full textThe impact of the active layer thickness on the emission characteristics of multicolor single layer organic light-emitting diodes based on poly(9-vinylcarbazole) is examined by combining experimental results with model simulations. We compare experimental electroluminescence spectra with simulations using photon-emitting point dipoles and find a very good agreement. We also simulate the location of the recombination zone, considering that the emission probability distribution has a peak located 25 nm from the cathode, which decays exponentially above and below that point. Simulated radiation patterns show that microcavity effects dominate the thickness dependent emitting properties of these devices.</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
Optimized surface silylation of chemically amplified epoxidized photoresists for micromachining applications
No full textWe explored the selective wet silylation of noncrosslinked areas of epoxidized photoresists using chlorosilanes. Emphasis was placed on the Si uptake of the epoxy films when controlled low levels of water were incorporated into the silylation solution. Fourier transform infrared measurements and oxygen-plasma resistance data with in situ laser interferometry and multiwavelength ellipsometry are presented. The fine tuning of the moisture level was found to be crucial for the generation of satisfactory and reproducible structures. The optimized version of the process was shown to be useful for epoxy-based dry micromachining. Overall, an attractive positive-tone process is presented as an alternative to the usual negative-tone process for commercial epoxy resists
Energy transfer processes among emitters dispersed in a single polymer layer for colour tuning in OLEDs
No full textThe energy transfer processes taking place in a single polymeric layer that enable the definition of the three primary colours (red, green and blue) in selected areas via photochemically induced emission tuning are discussed. The polymers used as hosts are two wide band gap polymers, PVK and a polyfluorenyl derivative. In the polymer matrix are dispersed the green emitter, 1-(4'-dimethyl-aminophenyl)-6-phenyl-1,3,5-hexatriene (DMA-DPH), the red emitter, 4-dimethylamino-4'-nitrostilbene (DANS) and a photoacid generator (PAG). Upon irradiation, protons are released from the PAG and they react gradually with the two emitters, causing the blue shift of the green emitter fluorescence and the extinction of the red emitter fluorescence. Depending on the protonation extent, the relative concentrations of the emitters and the exposure dose the energy transfer processes occurring inside the matrix result in definition of different colour emitting areas. The understanding of the energy transfer processes with photoluminescence experiments is a necessary first step in order to rationalize the selection of suitable components enabling the definition of the three primary colours in OLEDs.</p
Theoretical investigation on the effect of protonation on the absorption and emission spectra of two amine-group-bearing, red push'pull emitters, 4-dimethylamino-4′-nitrostilbene and 4-(dicyanomethylene)-2-methyl-6- p -(dimethylamino) styryl-4H-pyran, by DFT and TDDFT Calculations
No full textA theoretical investigation on the electronic structure of 4-dimethylamino-4′-nitrostilbene (DANS), 4-(dicyanomethylene)-2-methyl-6- p-(dimethylamino) styryl-4H-pyran (DCM), and their protonated forms is presented in an effort to rationalize recent experimental results on the tuning of the emitted color of organic light-emitting diodes through photochemically induced protonation. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been carried out on the neutral and protonated forms of DANS and DCM, employing both the B3LYP and the CAM-B3LYP functionals. It was found that the CAM-B3LYP functional leads to better agreement than the B3LYP of the calculated with the experimental absorption λmax for DANS, whereas B3LYP is more appropriate than CAM-B3LYP for DCM. The results of the calculations aid in a rationalization of the observed differences of the spectra of DANS and DCM upon protonation, and in particular those differences that make DANS a more attractive system for absorbance and emission tuning.</p
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
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
Flexible organic light emitting diodes (OLEDs) based on a blue emitting polyfluorene
No full textFlexible OLEDs were demonstrated using a highly efficient blue electroluminescent polyfluorene derivative. The flexible devices were fabricated on indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates with a sheet resistance of 35 Ω per sq. The emitting layer was poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF). A significant improvement of the luminance and device efficiency was achieved by confining the exciton formation zone within PF by two wide band-gap materials, namely PVK as a hole transport layer (HTL) and an inorganic oxide layer (IOL) as an electron transport and hole blocking layer. In order to achieve full-color LEDs based on a common host material, we probed the use of suitable dye emitters dispersed in PF at appropriate concentrations. The selection of the emitters is based on their capability to be effective energy transfer acceptors from the blue emitting PF. In particular, energy transfer was demonstrated from blue to green for PF-doped with the green dye emitter 1-[4-(dimethylamino)phenyl]-6- phenylhexa-1,3,5,-triene (DMA-DPH), and from blue to red for PF-doped with the red dye emitter (4-dimethylamino-4′-nitrostilbene) (DANS). This demonstration paves the way for developing highly efficient blue, green and red flexible OLEDs based on a common blue emitting PF host.</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
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