Electroluminescence Enhancement Research Articles

Optimization of electroluminescence from n-ZnO/AlN/p-GaN light-emitting diodes by tailoring Ag localized surface plasmon

The localized surface plasmon (LSP)-enhanced n-ZnO/AlN/p-GaN light-emitting diodes (LEDs) were fabricated by inserting Ag nanoparticles (NPs) into the ZnO/AlN interface. To investigate the effects of morphology of Ag NPs on the electroluminescence (EL) of device, the Ag NPs with various sizes were prepared by annealing Ag thin films with different deposition times. It is found that the insertion of Ag NPs with suitable size and surface coverage is favorable for the effective resonant coupling between excitons in ZnO and LSP of Ag NPs, and thereby significantly improve the EL performance of the device. For the n-ZnO/AlN/p-GaN LED with 10 nm Ag NPs, a maximum EL enhancement factor of 3.7 was observed at 420 nm at an injection current of 10 mA. For the device with the smaller Ag NPs, only the weaker enhancement is observed due to the smaller scattering cross section. On the other hand, in the case of the larger Ag NPs, the energy mismatch between the LSP of Ag NPs and the near band-edge emission of ZnO, as well as the poor crystalline quality of the ZnO film, leads to degradation in device performance.

Improved electroluminescence from silicon nitride light emitting devices by localized surface plasmons

Enhanced electroluminescence (EL) from SiNx light emitting devices (LEDs) with an ITO/SiO2/SiNx/Ag/p/p+-Si/Al structure was observed. Comparing to SiNx LEDs without Ag islands layer, those with Ag islands layer could conduct a higher injection current and extract light more efficiently due to the roughness of Ag islands film. Moreover, the localized surface plasmons induced by Ag islands enhanced the radiative efficiency of LEDs, resulting in the EL enhancement of ~14. By the combination enhancement on light extraction efficiency, radiative efficiency, and current-injection efficiency, the external quantum efficiency of EL from SiNx LEDs was improved by at least one order of magnitude.

Microcavity enhanced single photon emission from an electrically driven site-controlled quantum dot

In this work we report on the integration of single site-controlled quantum dots (SCQDs) into electrically driven micropillar cavities. The electroluminescence of these devices features emission of single SCQDs with inhomogeneous broadenings down to 170 µeV. The enhancement of electroluminescence by quantum dot-cavity coupling is demonstrated by temperature dependent investigations. Single photon emission from a spatially and spectrally coupled SCQD-resonator system is confirmed by photon autocorrelation measurements under electrical excitation yielding a g(2)(0) value of 0.42.

Surface plasmon-enhanced electroluminescence in organic light-emitting diodes incorporating Au nanoparticles

Surface plasmon-enhanced electroluminescence (EL) in an organic light-emitting diode is demonstrated by incorporating the synthesized Au nanoparticles (NPs) in the hole injection layer of poly(3,4-ethylene dioxythiophene):polystyrene sulfonic acid. An increase of ∼25% in the EL intensity and efficiency are achieved for devices with Au NPs, whereas the spectral and electrical properties remain almost identical to the control device. Time-resolved photoluminescence spectroscopy reveals that the EL enhancement is ascribed to the increase in spontaneous emission rate due to the plasmonic near-field effect induced by Au NPs.

GaN light-emitting diodes on glass substrates with enhanced electroluminescence

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates by controlling GaN crystal morphology, crystallinity, and device fabrication. Depending on the degree of epitaxy, we studied three different GaN morphologies; randomly oriented GaN polycrystals, nearly single-crystalline pyramid arrays, and fully single-crystalline pyramid arrays, which were fabricated by controlling the epitaxial relationship with the substrates. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. All the different GaN morphologies were further fabricated into LEDs to investigate their EL characteristics. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of about 2700 and 1150 cd m−2 at peak wavelengths of 537 and 480 nm, respectively.

Grating amplitude effect on electroluminescence enhancement of corrugated organic light-emitting devices

We report grating amplitude dependence of electroluminescence (EL) in organic light-emitting devices with one-dimensional corrugated structure. Our proposed devices can emit light from both the top silver and bottom quartz side, and both exhibit amplitude-dependence EL enhancement. The effect of grating amplitude on the EL intensity has been studied experimentally and numerically to find out the optimal grating amplitude for the greatest EL enhancement. We deduce from the numerical simulations and experimental results that surface plasmon-polariton modes and waveguide modes are coupled out of the corrugated devices efficiently at the optimal amplitude; therefore, higher efficiency of light extraction could be realized.

Enhanced luminescence efficiency by surface plasmon coupling of Ag nanoparticles in a polymer light-emitting diode

This study achieved a substantial enhancement in electroluminescence by coupling localized surface plasmons in a single layer of Ag nanoparticles. Thermal evaporation was used to fabricate 20-nm Ag particles sandwiched between a gallium-doped zinc oxide film and a glass substrate to form novel window materials for use in polymer light-emitting diodes (PLEDs). The PLEDs discussed herein are single-layer devices based on a poly(9,9-di-n-octyl-2,7-fluorene) (PFO) emissive layer. In addition to low cost, this novel fabrication method can effectively prevent interruption or degradation of the charge transport properties of the active layer to meet the high performance requirements of PLEDs. Due to the surface-plasmon-enhanced emission, the electroluminescence intensity was increased by nearly 1-fold, compared to that of the same PLED without the interlayer of Ag nanoparticles.

Electroluminescence Enhancement in Polymer Light-Emitting Diodes through Inelastic Scattering of Oppositely Charged Bipolarons

Combining the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model and the extended Hubbard model (EHM), the scattering and combination of oppositely charged bipolarons in conjugated polymers are investigated using a nonadiabatic evolution method. On the basis of this physical picture, bipolarons can scatter into a singlet biexcitonic state, in which two electrons and two holes are trapped together by a lattice distortion. This biexcitonic state can emit one photon to an exciton state, which can subsequently decay to the ground state. The results indicate that the scattering and combination of oppositely charged bipolarons may provide guidance for improving the internal quantum efficiency for electroluminescence to as high as 75% in polymer light-emitting diodes.

Improved Hole-Blocking and Electron Injection Using a TPBI Interlayer at the Cathode Interface of OLEDs

The effect of a benzimidazole derivative (TPBI) electron injection layer (EIL) on the performance of Alq3 based organic light-emitting devices (OLEDs) with a Cs2CO3/Al cathode is investigated. An increasing current density from 71.9 mA/cm2too 188.3 mA/cm2, and an enhanced electroluminescence (EL) efficiency from 3.2 cd/A to 3.64 cd/A at 9 V are found when a thin TPBI layer (5 nm) is inserted at the Alq3/Cs2CO3 interface. After further increasing the TPBI thickness to 10 nm, OLEDs display a further increase in EL efficiency to 4.-53 cd/A. Our experiment suggests that the TPBI thin layer at the Alq3/Cs2CO3 interface facilitates the electron injection and is also involved with hole-blocking and exciton confinement.

Enhancement of Luminance via Blending F8BT with Tetraphenyldiaminobiphenyl-Containing Hole Transport Polymer

We demonstrated polymer light-emitting diodes (PLEDs) based on poly(9,9'-dioctylfluorene-alt-benzothiadiazole) (F8BT) doped with a nonpolyfluorene tetraphenyldiaminobiphenyl-containing hole transport material, poly(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl) benzidine) (PPB). The devices have a configuration of ITO/PEDOT:PSS/emissive layer/Al. The emissive layers consisted of F8BT blended with PPB at varied weight ratio. The brightness of the blend devices was significantly increased in comparison with pure F8BT. For the 7% PPB blend, the luminance reached 1078 cd/m2 at 47.5 mA (528 mA/cm2), improved by 3 fold. We propose that increased charge carrier injection and transport is responsible for the enhanced electroluminescence (EL) performance. EL spectrum was hardly influenced by blending.

Electroluminescence Enhancement by Modifying Poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl

We report on the fabrication and characterization of light emitting diodes based on pristine or modified poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl]. The modified polyfluorene was formed by blending with the hole transport material poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N′-bis(4-butylphenyl-1,1′-biphenylene-4,4′-diamine))], or end-capping with dimethylphenyl. The addition of the hole-transporting polymer to the host polymer or the attachment of the end-capper to the polymer chain improved device properties. Phase-separated domains in blend thin film morphology were observed in scanning electron microscope pictures. These domains may play a role in the improved device performance. The introduction of the hole-transporting polymer or the end-capper also improved blue emission.

Enhanced Electroluminescence of CdSe/ZnS Quantum Dot Light–emitting Diodes with Phosphorescent Donors

ABSTRACTWe demonstrated the enhancement of electroluminescence (EL) from green CdSe/ZnS QDs in hybrid QD/organic light-emitting diodes (QD-LEDs) by employing blue phosphorescent dyes Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) as efficient exciton harvesters and energy transfer donors. Precise control of the concentration of the FIrpic donors doped in a 4,4’-N, N’-dicarbazole-biphenyl (CBP) host and their distance from the QD layer led to complete triplet exciton energy transfer and EL enhancement by a factor of 2.5. The Förster distance between FIrpic molecules and green CdSe/ZnS QDs was determined to be ∼ 8 nm, which is in a good agreement with the value calculated using the Förster model. Our study shows that integrating colloidal QDs with phosphorescent organic dyes provides an effective means for improving the quantum efficiency of QD-based hybrid LEDs.

Electroluminescence of green CdSe/ZnS quantum dots enhanced by harvesting excitons from phosphorescent molecules

We demonstrated the enhancement of electroluminescence from green CdSe/ZnS quantum dots (QDs) in hybrid QD/organic light-emitting diodes (LEDs) by employing blue phosphorescent dyes bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) as efficient exciton harvesters and energy transfer donors. Precise control over the position and concentration of the donors doped in a fluorescent host led to complete exciton energy transfer from FIrpic molecules located within the Förster distance of ∼8 nm from the QD layer, and a 2.5-fold increase in the quantum efficiency of the QD-LEDs.

Enhancement of electroluminescence in GaN-based light-emitting diodes by metallic nanoparticles

The enhanced electroluminescence of GaN-based light-emitting diodes (LEDs) with noble metallic nanoparticles (MNPs) is demonstrated. The sample with well-designed Ag MNPs has shown the best performance enhancement of 126% in electroluminescent intensity compared with a conventional LED sample, even though the MNPs are placed at least 200 nm away from the quantum-well active layer. The MNPs provide enhanced photon scattering and coupling between localized surface plasmon resonance (LSPR) modes and photon modes internally trapped in a device. To investigate this effect, the peculiarities of the LSPR and the corresponding structural properties of the MNPs are discussed through the effective medium approach.

Enhancement of ultraviolet electroluminescence based on n-ZnO/n-GaN isotype heterojunction with low threshold voltage

Ultraviolet light-emitting diodes based on simple n-ZnO/n-GaN isotype heterojunction have been fabricated using a radio frequency magnetron sputtering system. Ultraviolet emission peaking around ∼368 nm with a full-width at half maximum of ∼7 nm was observed at room temperature when the devices were under sufficient forward bias. With the presence of an i-MgO layer inserted between the ZnO and GaN layers, the ultraviolet emission intensity and output power have been much enhanced, while the threshold voltage drops down to 2.5 V. The electroluminescence mechanisms in these devices were discussed in terms of the band diagrams of the heterojunctions.

Effects of dilution and charge trapping on the performance of a light‐emitting diode of poly(9‐vinylcarbazole) doped with poly[2‐methoxy‐5‐(2′‐ethyl hexyloxy)– 1,4‐phenylene vinylene

AbstractWe report the electroluminescence (EL) enhancement of polymer light‐emitting diodes made by the dispersion of poly[2‐methoxy‐5‐(2′‐ethyl hexyloxy)–1,4‐phenylene vinylene] (MEH–PPV) into poly(9‐vinylcarbazole) (PVK). From the photoluminescence and EL spectra of the polymer blend films, we found that, because of the dilution effect, interchain interactions in MEH–PPV were reduced markedly. The EL spectrum indicated that the charge‐trapping effect contributed to the formation of excitons at the MEH–PPV centers and, thus, strongly enhanced EL. Because of the balance between increasing trapping centers and suppressing interchain interactions, the best device performance was achieved when the polymer blend contained 12% MEH–PPV and 88% PVK (weight ratio), and this also led to improved color purity of the polymer blend devices. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Distributed Bragg reflector enhancement of electroluminescence from a silicon nanocrystal light emitting device

We demonstrate distributed Bragg reflector (DBR) enhanced electroluminescence from a silicon nanocrystal-based light emitting device. An a-Si/SiO 2 superlattice containing silicon nanocrystals serves as the intrinsic layer in an n–i–n device that is embedded in a DBR cavity consisting of alternating layers of silicon and silicon dioxide. The entire structure, including DBR, superlattice and contact layers, is deposited by plasma-enhanced chemical vapor deposition. The photoluminescence, electroluminescence (EL) and optical output power are measured and compared to a reference device. The DBR is found to enhance the peak EL intensity by a factor of 25 and the external quantum and power conversion efficiencies by a factor of 2.

Low turn-on and high efficient oxidized amorphous silicon nitride light-emitting devices induced by high density amorphous silicon nanoparticles

The role of amorphous silicon nanoparticles (a-Si NPs) in electroluminescent characteristics of oxidized amorphous silicon nitride (a-SiN x :O) light-emitting devices (LEDs) has been studied. A-Si NPs with a high density of 1 × 10 12 cm − 2 are formed in the a-SiN x :O films after rapid thermal annealing at 900 °C for 40 s. A notably enhanced electroluminescence (EL) is obtained from the a-Si-in-SiN x :O devices and the EL peak position can be tuned from red to green-yellow by controlling the forward voltage. Compared to EL of the a-SiN x :O device, the turn-on voltage can be reduced to 3 V and the EL power conversion efficiency can be almost six times higher. The improved performance of the LEDs is ascribed to the effective carrier injection due to introduction of high density a-Si NPs.

Enhancement of electroluminescence from TiO2/p+-Si heterostructure-based devices through engineering of oxygen vacancies in TiO2

We report that electroluminescence (EL) from TiO2/p+-Si heterostructure-based devices can be significantly enhanced through a prior treatment of TiO2 films in argon (Ar) plasma. It is found that the Ar-plasma treatment introduces excess oxygen vacancies within a certain depth of TiO2 films. The increase in the concentration of oxygen vacancies leads to the enhancement of EL from TiO2/p+-Si heterostructure-based devices because oxygen vacancies are the light-emitting centers. This work demonstrates the use of defect engineering to improve the performance of oxide-based optoelectronic devices.

Enhanced electroluminescence from nanoscale siliconp+–n junctions made with an anodic aluminum oxide pattern

An enhancement of the electroluminescence (EL) from nanoscale siliconp+–n junctions made with an anodic aluminum oxide (AAO) pattern wasdemonstrated. The nanoporous AAO pattern with a pore density of1.4 × 1010 cm−2 and a porediameter of 50 ± 10 nm was fabricated by the two-step anodic oxidation methodon a n-type silicon wafer. The nanoscale AAO patterned Sip+–n junctions achieved an EL enhancement factor up to about 5 compared to the unpatterned Sip+–n junctions. The enhancement may originate from a reduction of nonradiativerecombination due to partial passivation of the Si surface by the AAO pattern andimprovement of the light extraction due to surface nanotextures.