Electroluminescence Wavelength Research Articles

Effect of Residual Stress and Sidewall Emission of InGaN-Based LED by Varying Sapphire Substrate Thickness

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The effect of residual stress and the sidewall emission in InGaN–GaN films with different thickness of sapphire substrate were investigated. The peak wavelength of electroluminescence was blue-shifted as thinning the sapphire substrate, because the wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN–GaN multiple quantum-well active region of the light-emitting diode (LED). A sideview LED with 170-<formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex> </formula>-thick sapphire exhibited the highest output power of 14.04 mW at a forward current of 20 mA, improved by 7% compared to that with 80-<formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula>-thick sapphire. The maximum output power can be obtained by considering both the photon escaping probability from the edges of the sapphire and the photon absorption probability in the sapphire as well as the residual mechanical stress induced by the wafer bowing. </para>

Near-white and tunable electrophosphorescence from bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II)-based organic light emitting diodes

A new phosphorescent platinum(II)-pyridyltriazolate complex, bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II), Pt(ptp) 2, was synthesized and incorporated into organic light emitting diodes for evaluation as electrophosphorescent dopant. Single crystal X-ray diffraction shows that the complex forms columnar stacks stabilized via strong intermolecular Pt(II) ⋯ Pt(II) interactions (3.289 Å), making it amenable to form excimers. Three types of phosphorescent emissions are seen for neat and doped thin films of Pt(ptp) 2 into 4,4′-bis(carbazol-9-yl)triphenylamine (CBP): structured monomer emission in the blue–green region ( λ max ∼ 480 nm), unstructured excimer emission in the yellow region ( λ max ∼ 550 nm), and rather broad unstructured extended excimer emission in the orange–red region ( λ max ∼ 600 nm). By varying the doping level in x% Pt(ptp) 2:CBP thin films, the ratio of monomer to excimer emission could be adjusted leading to tuning both the photoluminescence (PL) and electroluminescence (EL) wavelengths. Doping levels of ∼5–10%v were found to be optimal for both EL efficiency and white color coordinates resulting from simultaneous monomer and excimer emissions. Peak power and luminous efficiencies obtained were 9.8 lm/W and 14 cd/A, respectively, while the peak external quantum efficiency was 6.6% in a standard OLED device structure. The emissions for the 7.5–15%v Pt(ptp) 2:CBP devices are characterized by CIE coordinates of ∼(0.3, 0.5), rendering near-white EL with green hue. The CRI increased with dopant concentration, reaching 56 at the 45% doping level. More optimal white EL color could not be attained by fine-tuning the doping level. The devices exhibit good stability at higher current density and brightness levels, which is atypical of phosphorescent organic light emitting diodes. Strategies to attain higher efficiencies and CRI are proposed based on the results of this work.

Simple and Efficient Near‐Infrared Organic Chromophores for Light‐Emitting Diodes with Single Electroluminescent Emission above 1000 nm

A series of NIR organic chromophores with donor–π–acceptor–π–donor structure are synthesized. Good thermal stability and strong photoluminescence in solid state render them suitable for application in light-emitting diodes. Exclusive near-infrared emission at 1080 nm with external quantum efficiency of 0.28% is obtained from the nondoped OLEDs. The longest electroluminescence wavelength is 1220 nm.

Influence of segregation effects on the electroluminescence spectra of InGaAs/GaAs quantum well heterostructures grown by H-MOVPE

Surface segregation during epitaxial growth of stressed InGaAs/GaAs quantum-well heterostructures significantly distorts the nominal concentration profile of quantum wells. The consideration of the effect for growth conditions and elastic stresses appearing during epitaxy on segregation made it possible to simulate the concentration profile with a high accuracy and to calculate the electroluminescence wavelength of actual InGaAs/GaAs heterostructures with quantum wells. It was shown that the observed effect of the long-wave-length shift for the interband transition wavelength in the important case of heterostructures with two neighboring quantum wells is caused by the influence of elastic stresses during growth.

InGaN-based Light Emitting Diodes with Photoelectrochemical Lateral Etching Process on InGaN Active Layers

The InGaN-based Light Emitting Diodes (LEDs) with a wider lateral etched InGaN active layer were fabricated through the photoelectrochemical (PEC) etching process with an 800W illuminated Hg lamp, an unintentionally doped GaN filter, and immersed in a 2.2 M potassium hydroxide (KOH) solution. The lateral etched LED structures were achieved by using a GaN filter and InGaN as a sacrificial layer with a 2.57μm lateral etching width. The electroluminescence (EL) wavelengths of InGaN/GaN multi-quantum-well (MQW) active layer were observed at 460nm for standard LED and 455.7 nm for lateral etching LED with 20mA DC injection current. The peak EL emission wavelength had 4.3 nm-blueshift phenomenon in the lateral etched LED compared to standard one that was caused by partial reducing the compress strain in InGaN/GaN active layers. The light output power of the lateral etched LED had 54% enhancement compared to the standard LED. By using the PEC lateral etching process, the selective lateral etched LED structure have higher light scattering process and higher carrier recombination efficiency with lower piezoelectric field for higher efficiency optical device application

Band Gap Tunable, Donor−Acceptor−Donor Charge-Transfer Heteroquinoid-Based Chromophores: Near Infrared Photoluminescence and Electroluminescence

A series of D-π-A-π-D type of near-infrared (NIR) fluorescent compounds based on benzobis(thiadiazole) and its selenium analogues were synthesized and fully characterized by 1H and 13C NMR, high-resolution mass spectrometry, and elemental analysis. The absorption, fluorescence, and electrochemical properties were also studied. Photoluminescence of these chromophores ranges from 900 to 1600 nm and their band gaps are between 1.19 and 0.56 eV. Replacing the sulfur by selenium can lead to a red shift for emission and reduce the band gaps further. Interestingly, compound 1 exhibits aggregation-induced emission enhancement effect in the solid state. All-organic light-emitting diodes based on M1 and M2 were made and exclusive NIR emissions above 1 μm with external quantum efficiency of 0.05% and maximum radiance of 60 mW Sr−1 m−2 were observed. The longest electroluminescence wavelength reaches 1115 nm.

Effect of p–n Junction Location on Characteristics of InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with different p–n junction locations have been investigated by current–voltage (I–V), electroluminescence (EL), and capacitance–voltage (C–V) measurements. At 20 mA injection current, the operating voltage of the LEDs decreases by 0.15 V, the EL intensity increases by 14% and the EL wavelength shifts from 465 to 455 nm owing to the shortening of the distance between n-InGaN/GaN MQW and p-type materials. C–V measurements indicate that only the quantum well immediately adjacent to the p-type region still remains inside the depletion region when LEDs are forward biased. Analysis reveals that the characteristics of the LEDs can be improved by optimizing the p–n junction location through the improvement of the holes' tunneling injection and weakening of the piezoelectric field in InGaN quantum wells.

Wideband wavelength electroluminescence from InAs/InP QDs using double-cap procedure by MOVPE selective area growth

Wideband wavelength electroluminescence (EL) more than 400 nm was obtained in self-assembled Stranski–Krastanov (S–K) InAs/InP quantum dots (QDs) grown by selective area low-pressure metal-organic vapor-phase epitaxy (MOVPE). We used two techniques to control the EL peak spectrum in order to obtain the wideband emission spectrum of the device. One is selective area growth using a SiO 2 narrow 16-stripe mask array pattern, and the other is a double-cap procedure in the growth of the QDs layer to change the height of QDs layer by layer.

First all‐HVPE grown InGaN/InGaN MQW LED structures for 460‐510 nm

AbstractIn this paper we report on first InGaN‐based light emitting structures grown by hydride vapour phase epitaxy (HVPE). InGaN layers and multi layer InGaN/InGaN epitaxial structures were grown on GaN/sapphire template substrates and characterized. InN content in the InGaN layers was varied from 5 to 35 mol.%. Thickness of InGaN layers was controlled from 10 nm to 2 microns. Density of treading dislocations in the InGaN layers was estimated to be in the 109 cm–2 range. X‐ray diffraction measurements and transmission electron microscopy data confirmed a formation of InGaN/InGaN superlattice structures. Light emitting diode epitaxial wafers were fabricated by HVPE deposition of n‐type InGaN layers and multi layer structures on p‐type GaN template substrates. Depending on InN content in the InGaN light emitting regions, peak electroluminescence wavelength varied from 450 to 510 nm. Results of material characterization are reported. Advantages of the proposed upside down LED configuration and future applications of HVPE to grow InGaN layers and structures are discussed. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Alkyl side chain driven tunable red-yellow-green emission: Investigation on the new π-conjugated polymers comprising of 2,7-carbazole unit and 2,1,3-benzo-thiadiazole units with different side chains

Four new soluble polymers containing a 2,7-carbazole unit and a 2,1,3-benzothiadiazole unit in the main chain were synthesized by Suzuki polycondensation. Variation of the substituent groups (R) at 5-position of 2,1,3-benzothiadiazole unit resulted in different color emission of the copolymers. Thus, when R was CH3 (or H), the polymer showed yellow–green (or red) emission; whereas the polymers showed the emission from green to yellow–green, when R was CH2(CH2)5CH3 or CH2OCH(CH3)2. To investigate the nature of the color change, a Gaussian 03 program was used for estimation of the dihedral angles between a 5-R-2,1,3-benzothiadiazole unit and a 2,7-carbazole unit. The results showed that the different substituents at 5-position of 2,1,3-benzothiadiazole brought about different the dihedral angles, which gave the different conjugation levels to the polymers. Hence, the tunablity of emission color may be attributed to the different conjugation levels between 2,7-carbazole units and 5-R-2,1,3-benzothiadiazole units induced by simply changing substituent groups at 5-position of benzothiadiazole unit. Electrochemically, the copolymers exhibited a higher oxidation potential as well as the reversible reduction behavior bearing from 2,1,3-benzothiadiazole unit. To investigate the electroluminescent properties of the polymers, the nonoptimized devices were fabricated and the results showed that the electroluminescent emission wavelength was basically similar to that of the photoluminescent. All polymers showed good thermal stability with 5 wt % loss temperature of more than 296 °C. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1376–1387, 2008

Synthesis and characterization of efficient orange-red emitting poly( p-phenylenevinylene) derivatives with 1,3,4-oxadiazole unit

A series of the statistical copolymers of tetradecyloxy-3,5-bis-{2-(4-bromo-phenyl)-[1,3,4]oxadiazole}-benzene ( 4), 1,4-didecyloxy-2,5-diiodobenzene ( 5) and 1,4-dodecyloxy-2,5-divinylbenzene ( 6), for efficient light emitting diode were synthesized. As the feed ratio of compound 4 decreased, the absorption band at 320–360 nm decreased in the UV–vis spectra. In addition, as the composition of compound 4 decreased, the absorption intensity ratio at 1541 cm −1 decreased in the FT-IR spectra. The photoluminescence and electroluminescence emission color of P showed green-yellow color ( λ max = 552 nm). However, the maximum wavelength of photoluminescence and electroluminescence of P1 and P2 were 581 and 583 nm, respectively, which were dramatically red-shifted than that of P. Moreover, these values were almost the same as the values of P3 (580 nm). It is concluded that the energy transfer from high energy oxadiazole segments to low energy alkoxy-PPV segments occurred completely. The efficiency of ITO/ P2/Al was 0.0181%, which is the best efficiency among the device based on the copolymers. This is presumably due to that P2 has the best balanced hole- and electron-injection/transporting properties among the copolymers.

High power and high efficiency blue light emitting diode on freestanding semipolar (101¯1¯) bulk GaN substrate

Blue InGaN∕GaN multiple-quantum-well light emitting diodes with a peak emission wavelength of 444nm were grown on low extended defect density semipolar (101¯1¯) bulk GaN substrates by conventional metal-organic chemical vapor deposition. The calculated external quantum efficiency and output power at a drive current of 20mA under pulsed operations (10% duty cycle) were 29% and 16.21mW, respectively. The device exhibited virtually no peak electroluminescence wavelength shift with increasing drive currents, indicating a significant reduction of polarization-related internal electric fields.

Demonstration of high power blue-green light emitting diode on semipolar (112̄2) bulk GaN substrate

Blue-green InGaN/GaN multiple-quantum-well light emitting diodes with peak emission wavelength of 480 nm were grown on low extended defect density semipolar (112̄2) bulk GaN substrates by conventional metal organic chemical vapour deposition. The calculated external quantum efficiency and output power at a drive current of 20 mA under pulsed operations (10% duty cycle) were 18% and 9 mW, respectively. The device exhibited small electroluminescence wavelength shift (4.5 nm) with drive currents ranging from 30 to 100 mA, indicating significant reduction of polarisation-related internal electric fields.

Synthesis, structure and luminescence properties of zinc (II) complexes with terpyridine derivatives as ligands

Five zinc (II) complexes ( 1– 5) with 4′-phenyl-2,2′:6′,2″-terpyridine (ptpy) derivatives as ligands have been synthesized and fully characterized. The para-position of phenyl in ptpy is substituted by the group (R), i.e. tert-butyl ( t-Bu), hexyloxy (OHex), carbazole-9-yl (Cz), naphthalen-1-yl-phenyl-amine- N-yl (NPA) and diphenyl amine- N-yl (DPA), with different electron-donating ability. With increasing donor ability of the R, the emission color of the complexes in film was modulated from violet (392 nm) to reddish orange (604 nm). The photoexcited luminescence exhibits significant solvatochromism because the emission of the complexes involves the intra-ligand charge transfer (ILCT) excited state. The electrochemical investigations show that the complexes with stronger electro-donating substituent have lower oxidation potential and then higher HOMO level. The electroluminescence (EL) properties of these zinc (II) complexes were studied with the device structure of ITO/PEDOT/Zn (II) complex: PBD:PMMA/BCP/AlQ/LiF/Al. Complexes 3, 4 and 5 exhibit EL wavelength at 552, 600 and 609 nm with maximum current efficiency of 5.28, 2.83 and 2.00 cd/A, respectively.

Coherent tuning of semiconductor laser wavelength

Coherent laser light was tuned by cumulative refraction of light waves in a semiconductor laser cavity, while the wavelength of electroluminescence was not tuned by any noticeable amount. This effect is explained by the fact that the laser light has perfect spatial coherence, while the electroluminescence has a spatial coherence of 0.7 µm, smaller than the width of the wavefront. The refraction angle of 2.4 deg means that more than 16 triangular electrodes are involved in the refraction. This observation of coherent tuning may lead to a new optoelectronic device, a coherent filter, which filters out only coherent light from a coherent-incoherent mixture.

1.3–1.5 µm electroluminescence from Schottky diodes made on Au-InAs/GaAs quantum-size heterostructures

This communication reports the results of experimental studies of the electroluminescence (EL) of forward-biased Schottky barrier (SB) diodes fabricated on heterostructures where an InAs/GaAs quantum dot (QD) layer was placed in the space charge region. In order to reach a higher EL wavelength, the QD layer was overgrown by a thin combined GaAs/InGaAs cladding layer. The heterostructures with Au SBs were found to show pronounced EL originating from several confined exciton states in InAs QDs which is higher at 300 K than at low temperatures. It is shown that the EL wavelength can be tuned within the 1.3–1.57 µm range by changing the thickness and composition of the combined GaAs/InGaAs capping layer. We discuss possible mechanisms of the injection of holes needed for the radiative recombination in the QDs and possibilities of improving the EL quantum yield.

Efficient poly( p-phenylenevinylene) derivative with 1,2-diphenyl-2′-cyanoethene for single layer light-emitting diodes

A novel luminescent polymer, poly(2-methoxy-5-[6′-{1-cyano-1′-phenyl-2-(4-phenoxy)-ethene}]-hexyloxy-1,4-phenylenevinylene- alt-2,5-bis-(dodecyloxy)-1,4-phenylenevinylene) (MCN-PPV), was synthesized by the Heck coupling reaction. Electron withdrawing pendant, 1,2-diphenyl-2′-cyanoethene (CNST), is separated from the main chain via linear 1,6-hexamethylenedioxy chain. The band gap of the polymer figured out from the UV-Vis spectrum was 2.10 eV and the maximum of photoluminescence (PL) appeared at 582 nm. These values are similar to those of poly(2-methoxy-5-ethylhexyloxy-1,4-pnenylenevinylene) (MEH-PPV) (2.12 eV and 580 nm). The maximum wavelength of electroluminescence (EL) of the device based on single layer structure (ITO/MCN-PPV/Al) appeared at 582 nm, which is similar to that of MEH-PPV (583 nm). Relative PL quantum yield of MCN-PPV film is 2.5 times higher than that of MEH-PPV film. In the PL and EL spectra, emission from CNST pendants was not observed. The presence of CNST pendant did not affect the EL and PL maximum of the main chain of MCN-PPV. The HOMO and LUMO energy levels of the main chain figured out from the cyclic voltammogram and the UV-Vis spectrum are −4.99 and −2.89 eV, respectively, which are similar to those of MEH-PPV (−4.94, −2.82 eV). The estimated HOMO and LUMO energy levels of the pendant are −6.23 and −3.07 eV, respectively. A single layer EL device based on MCN-PPV has an efficiency of 0.114 cd/A at a current density of 63.0 mA/cm 2, which is significantly higher than that of MEH-PPV (5.93×10 −3 cd/A at a current density of 65.8 mA/cm 2) measured under the same conditions. The energy levels figured out from optical and electrochemical data strongly supports that the CNST pendants have hole-blocking as well as electron-transporting properties.

Novel Red Electroluminescent Polymers Derived from Carbazole and 3,6-Bis(2-thienyl)-1,2,7-benzothiadiazole

A series of novel carbazole-based soluble copolymers of different composition was synthesized by Suzuki coupling reaction. Efficient energy transfer from the wide-band-gap segment polycarbazole due to exciton trapping on narrow-band-gap bis(thienyl)ben-zothiadiazole sites has been observed. Maximum electro-luminescence wavelengths of the copolymers varied between 668 nm and 716 nm, and the external quantum efficiency for copolymer devices is 0.3% for copolymers with 20% benzothiadiazole content.

Improved electroluminescence of InAs quantum dots with strain reducing layer

The effects of the arrangement between InAs quantum dots and InGaAs strain-reducing layer (SRL) on the optical properties of quantum dot light emitting diodes are investigated. Electroluminescence wavelength longer than 1.3 μm is obtained as InAs quantum dots (QDs) are covered with a thin InGaAs SRL. For the same sample, the full-width at half-maximum of the ground state emission peak is as narrow as 19 meV at low injection current, and less than 40 meV even at saturation condition. It is also found that the slope efficiency of the diode is higher than that of the other samples in the linear region and its light output saturation level is higher because of the higher density of QDs.

Electroluminescent property and charge separation state of bis-naphthalimides

Electroluminescence (EL) of some bis-naphthalimide compounds has been studied. The EL spectra show that the peak wavelength of EL is in excellent agreement with the energy of the charge separation states of these donor–acceptor bis-naphthalimide compounds. This indicates that EL of these donor–acceptor compounds is derived from the recombination for their respective charge separation states.