Electroluminescence Enhancement Research Articles

Enhancement of optical properties of nanoscaled objects by metal nanoparticles

We provide a simple analytical model for the modification of optical properties of active molecules and other objects when they are placed in the vicinity of metal nanoparticles of subwavelength dimensions. Specifically, we study the enhancement of optical radiation, electroluminescence, and photoluminescence absorbed or emitted by these objects. The theory takes into account the radiative decay of the surface plasmon mode supported by the metal nanospheres--a basic phenomenon that has been ignored in electrostatic treatment. Using the example of Ag nanospheres embedded in a GaN dielectric, we show that enhancement for each case depends strongly on the nanoparticle size-enabling optimization for each combination of absorption cross section, original radiative efficiency, and separation between the object and metal sphere. The enhancement effect is most significant for relatively weak and diluted absorbers and rather inefficient emitters that are placed in close proximity to the metal nanoparticles.

Blue electroluminescence of ytterbium clusters in SiO2 by co-operative up-conversion

Metal-oxynitride-oxide-silicon (MONOS) structures containing a SiO2 layer implanted with different concentrations of ytterbium atoms were investigated. The electroluminescence (EL) measurements show that the MONOS:Yb devices can operate as blue or near infrared light emitters depending on the Yb concentration and annealing time. For an Yb concentration of up to 1.5% and annealing times of 30 min the near infrared EL with a peak at 975 nm corresponding to the 2F5/2→2F7/2 transition dominates. The short time annealed (6 s) sample containing 3% of Yb atoms exhibits mainly the blue EL due to co-operative up-conversion emission in the Yb3+–Yb3+ system. An enhancement of the red EL at 650 nm due to up-conversion energy transfer from the 2F5/2 excited state to the nonbridging oxygen hole center (NBOHC) is also presented.

Carbon nanotube induced enhancement of electroluminescence of phosphor

Electroluminescence (EL) was observed on conventional cathodoluminescent (CL) phosphor with the incorporation of carbon nanotube (CNT) at ambient air. The role of CNT can be understood as enhancing the local electrical field, which allows electron injection to the active center of phosphor at relatively low operating voltages. In this EL device, the brightness of CL phosphor was significantly improved from no light emission in the case of no addition of CNT to 35 cd/m2 with 1 wt % CNT at 10 kHz of ac 300 V.

Studies on organic light-emitting diodes based on rubrene-doped zinc quinolate

We report on the fabrication and performance of organic light-emitting diodes (OLEDs) employing rubrene-doped metal chelate namely, zinc quinolate (Znq2) as emissive layer. Different OLED architectures were carried out to improve the device performance by doping varying concentrations (by wt%) of rubrene dye into Znq2 host. Enhanced electroluminescence (EL) intensity was achieved using oxygen-plasma-treated indium tin oxide as anode, N,N′-di-1-naphthyl-N,N′-diphenyl-1,1′-biphenyl-4,4′diamine (α-NPD) as hole-transport layer, 10 weight percent rubrene-doped Znq2 as emission layer, Znq2 as electron-transport layer, LiF as electron-injection layer and aluminum as cathode. It has been demonstrated that by using Znq2 as electron-transport layer and host for dopant (rubrene) is the most suitable device structure for achieving low drive voltage and high efficiency. Electroluminescence spectra of rubrene-doped OLED devices were also studied. The EL spectral peak was found to be shifted to 565 nm and spectral full-width at half maximum (FWHM) of rubrene-doped device was found to be 58 nm, while it was 95 nm in the case of a pure Znq2-based OLED device. No further significant changes in EL spectra, such as, spectral broadening, narrowing or peak shifting were observed on changing the concentration of the rubrene dye. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Copolyfluorenes Containing Bipolar Groups: Synthesis and Application To Enhance Electroluminescence of MEH−PPV

Enhancing electroluminescence of conventional MEH−PPV {poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]} is desirable due to its popularity in polymeric emitting materials. The enhancement is much readily attained by simple blending with functional polymers instead of tedious structural modification. In response to this, three copolyfluorenes (P1−P3) containing bipolar groups (3.1−11.2 mol %), directly linked hole-transporting triphenylamine and electron-transporting 1,2,4-triazole, are synthesized by Suzuki coupling reaction. The bipolar groups not only suppress undesirable green emission of polyfluorene under thermal annealing but also increase hole and electron affinity of the resulting copolyfluorenes. Blending the bipolar copolyfluorenes with MEH−PPV effectively improves the emission efficiency of its electroluminescent devices [ITO/PEDOT:PSS/polymer blend/Ca(50 nm)/Al(100 nm)]. The maximum luminance and maximum luminance efficiency are significantly enhanced from 3120 cd/m2 and 0.49 cd/A (...

Electroluminescence and magnetoresistance of the organic light-emitting diode with a La0.7Sr0.3MnO3 anode

Electroluminescence (EL) with brightness up to 300cd∕m2 is observed from organic light-emitting diodes fabricated on oxygen-treated La0.7Sr0.3MnO3 anodes. An external magnetic field of 150mT applied parallel to the device surface can enhance the EL intensity by 10%, accompanied by a raised current efficiency. In-plane magnetization of the ferromagnetic anode is found to be the main origin of increase in the current contributable to EL, though magnetoresistance of the organic functional materials also plays a role in the EL enhancement observed in the magnetic field.

Photon modulated electroluminescence of GaInN/GaN multiple quantum well light emitting diodes

AbstractUnder external laser excitation, we studied the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy. The EL intensity was found to be greatly enhanced under the photon bias. This enhancement strongly supersedes the photoluminescence (PL) signal. The EL enhancement varied with injection carrier density. The relative EL enhancement under 325‐nm laser bias starts with a high ratio and decreases monotonically. This increase of the EL intensity is tentatively attributed to a balance between photocarrier screening and carrier drift within the active region. These findings elucidate the transition from highly efficient recombination at low current density to the region of efficiency droop at high current densities. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electroluminescence of ZnO nanocrystalline particles annealed from mesoporous precursors

Enhancing electroluminescence of ZnO nanocrystalline particles annealed from mesoporous precursors were reported. Special structured ZnO nanocrystalline particles was prepared from the as-prepared mesoporous ZnS by using octadcylamine (ODA) and pluronic F-127 (F-127) as template reagents, respectively. The nitrogen adsorption–desorption isotherms showed that the two as-prepared mesoporous ZnS samples possessed a narrow pore distribution and a large surface area. ZnO nanocrystalline particles were obtained by annealing these precursors at 500 °C for 6 h. All these ZnO samples revealed a green emission at 510 nm. And the emission intensity of samples annealed from the precursors dealt with ODA and F-127 were about 1.5 and 6.5 times high of that of the sample dealt with no surfactants, respectively. The green emission and the enhancement of emission intensity of these ZnO samples are coincident with those prepared by sol–gel method, but more efficient in enhancing the emission intensity.

Dual enhancing properties of LiF with varying positions inside organic light-emitting devices

A multilayer organic light-emitting device (OLED) has been fabricated with a thin (0.3nm) lithium fluoride (LiF) layer inserted inside an electron transport layer (ETL), aluminum tris(8-hydroxyquinoline) (Alq3). The LiF electron injection layer (EIL) has not been used at an Al/Alq3 interface in the device on purpose to observe properties of LiF. The electron injection-limited OLED with the LiF layer inside 50nm Alq3 at a one forth, a half or a three forth position assures two different enhancing properties of LiF. When the LiF layer is positioned closer to the Al cathode, the injection-limited OLED shows enhanced injection by Al interdiffusion. The Al interdiffusion at least up to 12.5nm inside Alq3 rules out the possible insulating buffer model in a small molecule bottom-emission (BE) OLED with a thin, less than one nanometer, electron injection layer (EIL). If the position is further away from the Al cathode, the Al diffusion reaches the LiF layer no longer and the device shows the electroluminescence (EL) enhancement without an enhanced injection. The suggested mechanism of LiF EL efficiency enhancer is that the thin LiF layer induces carrier trap sites and the trapped charges alters the distribution of the field inside the OLED and, consequently, gives a better recombination of the device. By substituting the Alq3 ETL region with copper phthalocyanine (CuPc), all of the electron injection from the cathode of Al/CuPc interface, the induced recombination at the Alq3 emitting layer (EML) by the LiF EL efficiency enhancer, and the operating voltage reduction from high conductive CuPc can be achieved. The enhanced property reaches 100mA/cm2 of current density and 1000cd/m2 of luminance at 5V with its turn-on slightly larger than 2V. The enhanced device is as good as our previously reported non-injection limited LiF EIL device [Yeonjin Yi, Seong Jun Kang, Kwanghee Cho, Jong Mo Koo, Kyul Han, Kyongjin Park, Myungkeun Noh, Chung Nam Whang, Kwangho Jeong, Appl. Phys. Lett. 86 (2005) 213502].

Enhanced electroluminescence in polymer-nanotube composites

Enhanced electroluminescence (EL) was observed with increasing carbon nanotube concentration in the MEH-PPV/SWCNT composites, where MEH-PPV and SWCNTs correspond to poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] and single-walled carbon nanotubes, respectively. Spatially separated conduction paths of the two distinct charge carriers, induced by blending SWCNTs with MEH-PPV, may explain the enhanced EL.

Spin injection from ferromagnetic Co nanoclusters into organic semiconducting polymers

We report a spin-polarized hole injection into organic semiconductor poly[2-methoxy-5-(${2}^{\ensuremath{'}}$-ethylhexyloxy)-1,4-phenylenevinylene (MEHPPV) layer from cobalt (Co) nanoscale clusters based on electroluminescence (EL) studies at liquid nitrogen temperature. The magnetic field effect on EL intensity indicates that the spin injection essentially increases the singlet-to-triplet exciton ratio in organic semiconductors. The Co size-dependent EL shows that the spin-injection efficiency decreases rapidly when the Co clusters percolate to a continuous film. The thickness-dependent EL enhancement of magnetic field suggests that the diffusion length of spin-polarized holes is about $60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in the MEHPPV.

Properties of InAsSbN quantum well laser diodes operating at 2 μm wavelength region grown on InP substrates

Effects of rapid thermal annealing (RTA) on emission properties of InAsSbN quantum well laser diodes grown on InP substrates were studied. It was found that a marked enhancement of electroluminescence (EL) intensity about one order of magnitude and a blue-shift of the EL peak energy was observed at low temperature upon RTA. On the other hand, no blue-shift of the EL peak energy was observed near at room temperature. The observed EL peak blue-shift at low temperature can be explained by decrease in localized levels formed by nitrogen introduction in the InAsSbN quantum wells. Laser operation was obtained for both diodes with and without RTA. It was found that threshold current density J th of the InAsSbN laser diode with RTA reduces compared to that without RTA. The lasing wavelength of the diode before RTA is 2.31 μm at 190 K, while that of the diode after RTA is 2.28 μm at 210 K. The T 0 values are 60 K for both diodes.

Enhancing electroluminescence performance of MEH-PPV based polymer light emitting device via blending with organosoluble polyhedral oligomeric silsesquioxanes

Organosoluble polyhedral oligomeric silsesquioxanes (POSS) blending effect on electroluminescence properties of MEH-PPV based polymer light emitting device was investigated. Excellent compatibility and surface morphology of organosoluble POSS and MEH-PPV based composite films were observed using AFM spectroscopy. The surface roughness of POSS:MEH-PPV composite film increased with increasing POSS content. Interfacial area between the light-emitting layer and cathode was favorably enhanced for cathode electron-injection. MEH-PPV blended with POSS would create a better balance between the electron and hole fluxes for POSS:MEH-PPV composite film based devices. This led to greater current efficiency of the POSS:MEH-PPV composite film based device as compared to one with a light emitting layer of MEH-PPV. Organosoluble POSS concentration effects on the PL spectra and EL performances were also studied for the POSS:MEH-PPV composite film based polymer light emitting devices.

Green Light Emitting Diodes under Photon Modulation

AbstractWith an external laser excitation, the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy has been investigated. The EL was found significantly enhanced under this bias and the difference can not merely be attributed to additional photoluminescence (PL). Under 325 nm photon bias, the EL enhancement starts at its highest value and decreases along with an increase of the LED current. Under 408 nm and 488 nm bias, it increases first to a value smaller than that of the 325 nm bias and then decreases with a much lower rate along the current increase. The EL enhancement is attributed to the more efficient carrier injection into quantum wells (QWs) resulting from the screening of the QW polarization by photon bias. Therefore, an enhanced balance of majority and minority carriers was obtained resulting in a better radiative recombination rate. Meanwhile, the current-voltage characteristics show a negative current first and then a voltage reduction as forward voltage increases. The reverse photocurrent indicates carrier loss due to the solar cell effect in our LED device while at high current region the carrier loss is attributed to another effect controlled by the external electrical field. At the balance point of those two effects, the EL enhancement is the highest. These findings clarify the transition from highly efficient radiative recombination at low current density to the region of efficiency droop at high current densities.

Enhanced light emission from one-layered organic light-emitting devices doped with organic salt by simultaneous thermal and electrical annealing

The authors studied the effect of thermal and electrical annealing on light emission of fluorescent one-layered organic light-emitting devices (OLEDs) doped with organic salts. From the annealed OLEDs, we clearly observed homogeneous and enhanced electroluminescent (EL) emission over the whole active area with fast responses. Moreover, improved efficiency was also observed from annealed phosphorescent OLEDs. These improved EL characteristics indicate that simultaneous annealing can induce proper adsorption of charged salt ions at the electrode surfaces, leading to enhanced electroluminescence of one-layered OLEDs due to increased and balanced injection of carriers.

Synthesis of Si nanopyramids at SiOx∕Si interface for enhancing electroluminescence of Si-rich SiOx

Enhanced electroluminescence (EL) of ITO∕SiOx∕Si-nanopyramid/p-Si∕Al diode is investigated. By using low-power plasma enhanced chemical vapor deposition at high substrate temperature, anomalous (100)-oriented Si nanopyramids with a surface density of 1.6×1010cm−2 are synthesized at SiOx∕Si interface prior to grow Si-rich SiOx film. Si nanopyramids greatly improve Fowler-Nordheim tunneling based carrier transport and benefit from less damaged oxide structure at lower biases. The turn-on voltage and threshold current density of the diode are reduced to 50V and 0.2mA∕cm2, respectively. Defect-related blue-green EL are suppressed to enhance stable near-infrared EL at 30nW with a lifetime >10h.

Luminescence properties of MEH-PPV and its crosslinked polymer: Effect of crosslink on photoluminescence and electroluminescence

Photoluminescence (PL) and electroluminescence (EL) of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) and its crosslinked polymer have been investigated in this work. EL and PL of MEH-PPV were affected in both their spectral shape and intensity after introducing a small degree of crosslink between the polymer chains. It is suggested that the EL enhancement can be attributed to the chemical structural variation by the crosslink in the network structure.

Enhanced emission from Si-based light-emitting diodes using surface plasmons

Excitation of surface plasmons on metallic nanoparticles has potential for increasing the absorption and emission from thin Si devices. We report an eight-fold enhancement in electroluminescence from silicon-on-insulator light-emitting diodes at 900nm via excitation of surface plasmon resonance in silver nanoparticles, along with a redshift in the electroluminescence by 70nm by overcoating the nanoparticles with ZnS. The enhancement is due to coupling between the electromagnetic excitations of the silver nanoparticles and the waveguide modes.

Enhancement of Green Electroluminescence from Nanocrystalline Silicon by Wet and Dry Processes

Correlation between defects and luminescence property from electroluminescent (EL) device composed of nanocrystalline silicon (nc-Si) prepared by wet and dry processes such as hydrofluoric (HF) acid solution treatment and annealing have investigated using electron spin resonance and EL measurements. The EL device using HF-treated nc-Si emitted strong red light, because of existence of only P'ce-centers (radiative recombination centers) on the surface vicinity. On the other hand, the EL device using annealed nc-Si above 400 degrees C exhibited green luminescence by the reduction of particle size due to surface oxidation. When the annealing temperature was risen from 400 degrees C up to 600 degrees C, the green luminescence strengthened with increasing the P'ce-centers. These results indicate that the formation of many radiative recombination centers onto the nc-Si surface vicinity lead to the enhancement of green luminescence from the nc-Si based EL device.

Carbon nanotube effects on electroluminescence and photovoltaic response in conjugated polymers

This letter reports the experimental results of enhanced electroluminescence (EL) and photovoltaic (PV) response upon doping single-wall carbon nanotubes (SWNTs) into conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylenevinylene] (MEHPPV) based on single-layer light-emitting diodes. We found that the dispersed SWNTs result in two processes: charge transport and exciton dissociation at the tube-chain interface in the SWNT/polymer composites. The detailed EL and PV studies indicate that low SWNT doping concentrations mainly improve the bipolar charge injection, leading to enhanced both reverse and forward EL with reduced threshold voltage. As the SWNT doping concentration continues to increase, the interfacial exciton dissociation becomes dominated, giving rise to an increased PV response. This SWNT concentration-dependent charge transport and exciton dissociation present a pathway to individually address the dual EL and PV functionalities of SWNT-doped polymer composites by controlling the doping level of the SWNTs.