Increase In Electroluminescence Intensity Research Articles

Improved performance of polymer light-emitting diodes using graphene oxide doped with silver nanoparticles as the hole-transport layer

This study successfully demonstrates the fabrication of a thin film of graphene oxide doped with silver nanoparticles (GO:AgNPs) using electrochemical exfoliation and photochemical synthesis. The GO:AgNPs thin film exhibits a higher work function and conductivity compared to poly[3,4-ethylenedioxythiophene] polystyrene sulfonate (PEDOT:PSS), making it a promising candidate for the hole-transport layer in polymer light-emitting diodes (PLEDs). Among various configurations, a PLED incorporating GO:AgNPs as the interlayer, which is prepared under blue-light illumination, exhibits optimal luminescent performance. This configuration achieves the highest luminance of 6341 cd/m2 and the highest average current efficiency of 1.87 cd/A. Notably, this novel PLED demonstrates a remarkable 168 % enhancement in average current efficiency and an impressive 637 % increase in electroluminescence intensity compared to a PLED employing a PEDOT:PSS interlayer.

The dependence of recombination in GaAs solar cells on the number of included GaInAs quantum objects

The experimental characteristics of GaAs p-i-n structures with InGaAs quantum objects (QOs) have been investigated. The study of electroluminescence spectra has shown that an increase of the number of QOs layers leads to relative increase in electroluminescence intensity from QOs and decrease it from the GaAs matrix. The observed increase in QOs recombination has resulted in a drop of open circuit voltage. It has been shown that recombination through deep levels in the QOs begins to dominate over recombination in the matrix at lower number of QOs layers than that through band-to-band recombination.

Electroluminescence in Heterostructures GaSb/AlSb/InAsSb Due to Tunneling Mechanism of Radiative Recombination

We report on the unusually large blue shift of electroluminescence spectrum with increase of the drive current at 77 K in a double-barrier nanoheterostructure with a deep AlSb/InAs0.83Sb0.17/AlSb quantum well grown by MOVPE on n-GaSb:Te substrate. The rise of drive current from 20 to 220 mA led to shift of the electroluminescence spectrum maximum towards higher photon energies by 100 meV. It was shown that this effect is due to indirect (tunneling) radiative transitions between electrons in InAsSb quantum well and heavy holes localized near AlSb/p-GaSb heterointerface. Energy of radiative transition was linearly dependent on applied voltage. In the drive current range of 50–220 mA electroluminescence blue shift was accompanied by the spectrum narrowing by 40 meV and noticeable change of the spectrum shape. With rise in drive current superlinear increase of electroluminescence intensity caused by the nonlinear dependence of tunneling radiative recombination rate on transition energy was observed at 300 and 77 K.

Synthesis, electrochemistry and optical properties with electroluminescence ability of new multisubstituted naphthalene derivatives with thiophene and carbazole motifs

A series of solution-processable tetrasubstituted naphthalene derivatives bearing thiophene or carbazole units were synthesized using the tandem cycloaddition [2+1+2+1] and Diels–Alder reaction[4+2]. Thermal, electrochemical, absorption, and emission properties of synthesized compounds were studied. They can be considered as molecular glasses with glass transition temperature ranging from 37 to 122°C with high thermal stability up to 300 or 400°C. The naphthalene derivatives were electrochemically active and showed low energy band gap between 1.64 and 1.85eV. All derivatives were luminescent and emitted light with maximum emission band located at 379–436nm with photoluminescence quantum yield in the range of 9.5–19.8% in solution. They photo- and electroluminescence ability in solid state as thin film and blends with poly(9-vinylcarbazole) (PVK) and mixture PVK with (2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole) (PBD) were tested. Together with increase of compound content from 2 to 15wt% in binary matrix, the increase of electroluminescence intensity was observed. Diodes with guest-host configuration showed electroluminescence with maximum of emission band from 593 to 637nm. The most intense emission of light under applied voltage characterized devices based on compound bearing carbazole derivatives and one methyl group.

Plasmonic nanograting enhanced quantum dots excitation for cellular imaging on-chip

We present the design and integration of a two-dimensional (2D) plasmonic nanogratings structure on the electrode of colloidal quantum dot-based light-emitting diodes (QDLEDs) as a compact light source towards arrayed on-chip imaging of tumor cells. Colloidal quantum dots (QDs) were used as the emission layer due to their unique capabilities, including multicolor emission, narrow bandwidth, tunable emission wavelengths, and compatibility with silicon fabrication. The nanograting, based on a metal-dielectric-metal plasmonic waveguide, aims to enhance the light intensity through the resonant reflection of surface plasmon (SP) waves. The key parameters of plasmonic nanogratings, including periodicity, slit width, and thicknesses of the metal and dielectric layers, were designed to tailor the frequency bandgap such that it matches the wavelength of operation. We fabricated QDLEDs with the integrated nanogratings and demonstrated an increase in electroluminescence intensity, measured along the direction perpendicular to the metal electrode. We found an increase of 34.72% in QDLED electroluminescence intensity from the area of the pattern and an increase of 32.63% from the photoluminescence of QDs deposited on a metal surface. We performed ex vivo transmission-mode microscopy to evaluate the nucleus–cytoplasm ratios of MDA-MB 231 cultured breast cancer cells using QDLEDs as the light source. We showed wavelength dependent imaging of different cell components and imaging of cells at higher magnification using enhanced emission from QDLEDs with integrated plasmonic nanogratings.

Direct band gap electroluminescence from bulk germanium at room temperature using an asymmetric fin type metal/germanium/metal structure

We demonstrated direct band gap (DBG) electroluminescence (EL) at room temperature from n-type bulk germanium (Ge) using a fin type asymmetric lateral metal/Ge/metal structure with TiN/Ge and HfGe/Ge contacts, which was fabricated using a low temperature (<400 °C) process. Small electron and hole barrier heights were obtained for TiN/Ge and HfGe/Ge contacts, respectively. DBG EL spectrum peaked at 1.55 μm was clearly observed even at a small current density of 2.2 μA/μm. Superlinear increase in EL intensity was also observed with increasing current density, due to superlinear increase in population of elections in direct conduction band. The efficiency of hole injection was also clarified.

Electroluminescence in Organically Capped Cd<sub>1-x</sub>Zn<sub>x</sub>Se Chalcogenide Nanocrystals

Currently there is a great interest in II–VI semiconductor nanoparticles, particularly organically capped soluble particles of cadmium or zinc sulphide and selenide, for their ready to use application in devices. For electroluminescence (EL) devices, it is expected to cover a broad spectrum and to tune various specific colours by preparing Cd1-xZnx Se instead of CdSe and ZnSe. Ternary alloys have composition dependent properties; therefore Cd1-xZnxSe has attracted much attention in the fields of luminescence and optoelectronic devices. It has wide optical band-gap and good stability with respect to environment. In this study, Cd1-xZnxSenanoparticles have been synthesized by using starch as a capping agent through a chemical synthesis route at room temperature. Samples have been prepared varying composition factor ‘x’ in ternary alloy Cd1-xZnxSe. Cubic structure of all has been confirmed by XRD. Crystallite size calculated from XRD was found in the range of 3-5 nm and it was observed that size reduces on increasing Zn content in ternary compound. Optical absorption spectra showed the blue shift in absorption edge with increasing Zn content. Band gap has been obtained by absorption studies and increase in band gap observed on increasing Zn content in the compound. Electroluminescence studies reveal that lower threshold voltage is required for samples with lower ‘x’ value. The Brightness increases on increasing the voltage above threshold voltage and the variation pattern is almost exponential for all samples. The voltage-current curve represents ohmic nature of the EL cell. Impedance was found to increase on increasing ‘x’ value. The increase in EL intensity is faster for higher frequency. EL spectra revealed that light emission is in violet-green region corresponding to band gap for both Cd0.75 Zn 0.25Se and Cd0.5 Zn 0.5Se, with a slight blue shift on increasing Zn content. A ternary system Cd1–xZnxSe, may be engineered better for application purpose by suitably choosing the composition parameter ‘x’.Contents of Paper

Electroluminescence and Transmission Electron Microscopy Characterization of Reverse-Biased AlGaN/GaN Devices

Reverse-bias stress testing has been applied to a large set of more than 50 AlGaN/GaN high electron mobility transistors, which were fabricated using the same process but with different values of the AlN mole fraction and the AlGaN barrier-layer thickness, as well as different substrates (SiC and sapphire). Two sets of devices having different defect types and densities, related to the different growth conditions and the choice of nucleation layer, were also compared. When subjected to gate-drain (or gate-to-drain and source short-circuited) reverse-bias testing, all devices presented the same time-dependent failure mode, consisting of a significant increase in the gate leakage current. This failure mechanism occurred abruptly during step-stress experiments when a certain negative gate voltage, or “critical voltage,” was exceeded or, during constant voltage tests, at a certain time, defined as “time to breakdown.” Electroluminescence (EL) microscopy was systematically used to identify localized damaged areas that induced an increase of gate reverse current. This current increase was correlated with the increase of EL intensity, and significant EL emission during tests occurred only when the critical voltage was exceeded. Focused-ion-beam milling produced cross-sectional samples suitable for electron microscopy observation at the sites of failure points previously identified by EL microscopy. In high-defectivity devices, V-defects were identified that were associated with initially high gate leakage current and corresponding to EL spots already present in untreated devices. Conversely, identification of defects induced by reverse-bias testing proved to be extremely difficult, and only nanometer-size cracks or defect chains, extending vertically from the gate edges through the AlGaN/GaN heterojunction, were found. No signs of metal/semiconductor interdiffusion or extended defective areas were visible. The weak dependence on AlGaN properties, the strong process dependence, the time dependence, and the features of the localized damage identified by EL and electron microscopy suggest a multistep failure mechanism initiated by a process-induced weakness of the gate Schottky junction, which enhances current injection into pre-existing defects. As a result, further defects are generated or activated, eventually resulting in a percolation conductive path and permanent damage. A low-impedance path between the device gate and the channel is formed, increasing gate leakage current and possibly resulting in device burnout.

Optical Improvement of GaN-Based Light Emitting Diodes by Interfacial Si Treatment in InGaN/GaN Quantum Well Structure

We investigated the optical enhancement of GaN-based light-emitting diodes (LEDs) by introducing Si treatment at the interface between the well and the barrier. The results of X-ray diffraction showed that the interfacial quality of InGaN/GaN quantum wells was slightly degraded by interfacial Si treatment. However, the intensities and full width at half-maximums (FWHMs) of photoluminescence (PL) and electroluminescence (EL) were increased by introducing interfacial Si treatment. In addition, the efficiency droop of the reference LED was 34.2% at 50 mA, while that of the Si-treated LED was 26.7% at 50 mA. Despite the increase in EL intensity, the efficiency droop was significantly decreased by interfacial Si treatment. From these results, we believe that the interfacial Si treatment will induce appropriate In localization in the InGaN active region, resulting in the improvement of optical quality of LEDs.

Optical Improvement of GaN-Based Light Emitting Diodes by Interfacial Si Treatment in InGaN/GaN Quantum Well Structure

We investigated the optical enhancement of GaN-based light-emitting diodes (LEDs) by introducing Si treatment at the interface between the well and the barrier. The results of X-ray diffraction showed that the interfacial quality of InGaN/GaN quantum wells was slightly degraded by interfacial Si treatment. However, the intensities and full width at half-maximums (FWHMs) of photoluminescence (PL) and electroluminescence (EL) were increased by introducing interfacial Si treatment. In addition, the efficiency droop of the reference LED was 34.2% at 50 mA, while that of the Si-treated LED was 26.7% at 50 mA. Despite the increase in EL intensity, the efficiency droop was significantly decreased by interfacial Si treatment. From these results, we believe that the interfacial Si treatment will induce appropriate In localization in the InGaN active region, resulting in the improvement of optical quality of LEDs.

Electroluminescence and band structure in p-Al x Ga1−x As/GaAs1−y P y /n-Al x Ga1−x As under uniaxial compression

Band structure calculations in p-Al x Ga1−x As/GaAs1−y P y /n-Al x Ga1−x As heterostructure under uni-axial compression in the 1 1 0 direction indicates an increase in the optical energy gap with dE ph/dP ≈ 85 meV GPa−1, a decrease in the quantum well barriers and light hole–heavy hole crossover at uniaxial stress P ≈ 450–500 MPa. The observed increase in electroluminescence intensity and photon energy shift under uni-axial compression are explained by numerical calculation data.

Optical absorption and emission of fully conjugated heterocyclic aromatic rigid-rod polyelectrolytes containing sulfonated pendants

Fully conjugated and rodlike poly[(1,7-dihydrobenzo[1,2-ḏ:4,5-d′]diimidazole-2,6-diyl)-2-(2-sulfo)-p-phenylene] (sPBI) was synthesized and fabricated for monolayer light emitting diodes showing a threshold voltage of 4.5 V and an emission λmax of 530 nm. Intractable sPBI was derivatized for a fully conjugated water soluble rigid-rod polyelectrolyte sPBI-PS(Li+) which was doped with LiCF3SO3 or LiN(CF3SO2)2 for optical absorption, electrical conductivity, and luminescent emission. sPBI-PS(Li+) light emitting electrochemical cells doped with 0.41 and 1.01 wt % of LiN(CF3SO2)2 showed a threshold voltage of 2.8 V and a tenfold increase in electroluminescence intensity (at λmax=514 nm) which did not increase with its conductivity.

Effect of an Interfacial Oxide Layer on the Electroluminescence Efficiency of Metal–Quantum-Confined Semiconductor Heterostructures

The effect of different surface treatments of GaAs/In(Ga)As/GaAs quantum-confined heterostructures on the electroluminescence efficiency of Schottky diodes based on these structures is investigated. It is ascertained that the largest increase in electroluminescence intensity is observed when the surface is treated in CCl4 at 580°C with subsequent anodic oxidation. It is shown that the interfacial tunnel-thin anodic oxide plays an important role in the injection of minority carriers from the metal into gallium arsenide. The electroluminescence efficiency depends strongly on the anodic-oxide thickness.

Temperature dependence of electroluminescence of Er ions in tunnel diodes based on (111)Si:(Er, O)

Electroluminescence and current-voltage characteristics of tunnel diodes obtained by implantation of Er, O, and B ions into n-Si(111) with the subsequent heat treatment are investigated in a temperature range of 80–300 K in the breakdown mode. The observed increase in electroluminescence intensity with temperature for Er ions is caused by thermal emptying of the traps that captured the holes in the n-region of the diode at low temperatures. This emptying leads to a variation in the breakdown characteristics. It is shown that some of the traps at low temperatures retain the charge captured even after the voltage applied to the diode is switched off. This circumstance gives rise to the peculiar memory effect in the structures investigated.

Effects of Amorphous Carbon Films on the Performance of Porous Silicon Electroluminescence

ABSTRACTEfficient electroluminescence (EL) is obtained at low operating voltages (<3 V) from n+-type silicon- electrochemically oxidized thin nanocrystalline porous silicon (PS)-amorphous carbon-Indium tin oxide (ITO) junctions. The effects of a few nanometer thick amorphous carbon film between PS and ITO on the EL characteristics have been investigated. The carbon film enhances the stability. The EL efficiency is improved due to a reduction of current density and an increase in EL intensity. In addition, the reproducibility from device to device is very much improved by the carbon film. The enhancement in stability should originate from the capping of PS by the carbon film and the high chemical stability of carbon and Si-C bonds, which should prevent PS oxidation. The carbon film acts as an efficient buffer layer between PS and ITO, resulting in enhanced mechanical, electrical and chemical stability of the top contact and providing high reproducibility. The thin carbon film has only positive effects on all the EL characteristics. This is a very important step towards application.

Room- and low-temperature voltage tunable electroluminescence from a single layer of silicon quantum dots in between two thin SiO2 layers

Room- and low-temperature electroluminescence (EL) in the visible range was observed from a single layer of silicon nanocrystals in between two thin SiO2 layers. The EL peak wavelength exhibited tunability from the red (∼800 nm) to the yellow (∼600 nm) depending on the excitation voltage. By decreasing the temperature while keeping the excitation voltage constant, an increase in EL intensity was observed together with a blueshift in EL peak position. This blueshift was much larger than that observed under optical excitation. Nonradiative Auger recombination, Coulomb charging effects, and/or the quantum-confined Stark effect are considered accountable for this behavior.

Effect of Free Carriers on ac-Driven Electroluminescent Devices with Hydrogenated Amorphous Silicon Carbide Thin Films

The benefit of free carriers on ac-driven electroluminescence (EL) in hydrogenated amorphous silicon carbide (a-SiC:H) thin films has been investigated by observing the photoenhanced EL and analyzing of EL in multilayers. The EL intensity was found to be increased with the intensity of the illuminating laser with a sub-band gap energy. This has been correlated with photoluminescence quenching. The EL quenching at higher electric fields observed only with irradiation has been attributed to the effect of a combination of an increased number of free carriers and interaction of impact-generated electron-hole pairs. Moreover, the reduction in the threshold voltage of a-SiC:H/hydrogenated amorphous carbon (a-C:H) multilayers has been found to be related to the a-SiC:H thickness. The slope of the linear increase in EL intensity with ac voltage depends on the multilayer sequence. Both observations can be explained using a microscopic model based on impact excitation by free carriers.