Bright Light-emitting Diodes Research Articles

Synthesis and photophysical properties of europium(iii)–β-diketonate complexes applied in LEDs

Six β-diketonate ligands were used to prepare the corresponding antenna europium(III) ternary complexes using 1,10-phenanthroline as an ancillary ligand. All the complexes exhibited high decomposition temperatures. Photophysical properties such as FT-IR spectra, UV-Vis absorption spectra, excitation and emission spectra, relative luminescent intensity ratios, luminescence decay curves and quantum yields based on the complexes were systematically studied and compared with each other. The energy-transfer mechanism was proposed as a ligand-sensitized luminescence process. Bright red light-emitting diodes (LEDs) were then fabricated by coating the complexes onto 395 nm-emitting InGaN chips. The light emission from the InGaN chips could be completely absorbed in the spectra of LEDs. The Commission International de I'Eclairage (CIE) chromaticity coordinates are close to the National Television Standard Committee (NTSC) standard value for the red color. All these findings indicate that these Eu(III) complexes are promising red phosphors for fabrication of near UV-based white LEDs.

Efficient and bright organic light-emitting diodes on single-layer graphene electrodes

Organic light-emitting diodes are emerging as leading technologies for both high quality display and lighting. However, the transparent conductive electrode used in the current organic light-emitting diode technologies increases the overall cost and has limited bendability for future flexible applications. Here we use single-layer graphene as an alternative flexible transparent conductor, yielding white organic light-emitting diodes with brightness and efficiency sufficient for general lighting. The performance improvement is attributed to the device structure, which allows direct hole injection from the single-layer graphene anode into the light-emitting layers, reducing carrier trapping induced efficiency roll-off. By employing a light out-coupling structure, phosphorescent green organic light-emitting diodes exhibit external quantum efficiency >60%, while phosphorescent white organic light-emitting diodes exhibit external quantum efficiency >45% at 10,000 cd m(-2) with colour rendering index of 85. The power efficiency of white organic light-emitting diodes reaches 80 lm W(-1) at 3,000 cd m(-2), comparable to the most efficient lighting technologies.

Quasi‐2D Colloidal Semiconductor Nanoplatelets for Narrow Electroluminescence

The first functional light‐emitting diodes (LEDs) based on quasi 2D colloidal core/shell CdSe/CdZnS nanoplatelets (NPLs). The solution‐processed hybrid devices are optimized with respect to their electroluminescent characteristics, first, by improving charge injection through exchanging the as‐synthesized NPL long‐chain ligands to shorter ones such as 3‐mercaptopropionic acid, and second, by comparing different hole‐transporting layers. NPL‐LEDs exhibit a maximum luminance of 4499 cd m‐2 and external quantum efficiencies of 0.63%. In particular, over different applied voltages, systematically narrow electroluminescence of full width at half maximum (FWHM) in the range of 25–30 nm is observed to be independent from the choice of device configuration and NPL ligands. As spectrally narrow electroluminescence is highly attractive in terms of color purity in the context of LED applications, these results emphasize the unique potential of this new class of colloidal core/shell nanoplatelet in achieving bright and functional LEDs of superior color purity.

Bright design

In all likelihood, cheap and bright white organic light-emitting diodes will someday light up our homes. Three-dimensional models can now simulate the dynamics of charges and excitons governing the operation of these light sources and predict their performance with molecular precision.

RbBaPO4:Eu2+: a new alternative blue-emitting phosphor for UV-based white light-emitting diodes

A novel blue-emitting phosphor on a phosphate-based host matrix, RbBaPO4:Eu2+ (RBP), was synthesized by a solid-state reaction. This phosphor could be excited efficiently by ultra violet-visible light in the 220–420 nm range to exhibit a highly efficient emission peak in the range of 420–440 nm. To investigate the possibility of commercial use, it was compared with the commercial blue phosphors of BaMgAl10O17:Eu2+ (BAM) and Sr3MgSi2O8:Eu2+ (SMS) to assess the photoluminescence properties. Compared to commercial phosphors, RBP displayed enhanced photoluminescence properties, in this case a high quantum efficiency and excellent thermal stability under near-Ultra Violet (n-UV) excitation. Furthermore, bright white light-emitting diodes (LEDs) were fabricated by integrating the RBP phosphor, along with commercial green/red phosphors, into n-UV LEDs. These results strongly suggest that the newly discovered RBP phosphor can be commercially utilized in UV-based white LEDs.

Rational Design of Functional Oxide Thin Films with Embedded Magnetic or Plasmonic Metallic Nanoparticles

Getting into films: semiconductor thin films containing magnetic or plasmonic metal nanoparticles are key materials for the development of high-efficiency solar cells, bright light-emitting diodes, and new magnetoelectric devices. The catalytically driven chemical vapor deposition offers a unique way to combine deposition of the metallic nanoparticles with that of functional oxides to produce such films.

DEVELOPMENT OF LED LIDS FOR TISSUE CULTURE LIGHTING

DEVELOPMENT OF LED LIDS FOR TISSUE CULTURE LIGHTING

Highly Efficient and Bright LEDs Overgrown on GaN Nanopillar Substrates

We presented a study of high-performance GaN-based light emitting diodes (LEDs) using a GaN nanopillars (NPs) structure grown on sapphire substrate by integrating RF-plasma molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Nanoscale air voids were clearly observed at the interface between GaN NPs and the overgrown GaN layer by cross-sectional scanning electron microscopy. It can increase the light-extraction efficiency due to additional light scattering. The transmission electron microscopy images suggest the air voids between GaN NPs introduced during nanoscale epitaxial lateral overgrowth of GaN can suppress the threading dislocation density. Moreover, Raman spectrum demonstrated that the strain of the GaN layer grown on GaN NPs was effectively eliminated, resulting in the reduction of quantum-confined Stark effect in InGaN/GaN quantum wells. Consequently, the LEDs fabricated on the GaN NPs template exhibit smaller electroluminescent peak wavelength blue shift and great enhancement of the light output (70% at 20 mA) compared with the conventional LEDs.

Energy Barrier Reduction and Exciton Confinement Using an Intermediate Blocking Layer in Organic Light-Emitting Diodes

The intermediate blocking layer (IBL) was investigated for the development of highly efficient and bright organic light-emitting diodes (OLEDs). The insertion of an IBL between a hole transport layer and an emitting layer (EML) has resulted in the development of highly efficient and bright OLEDs. The quantum efficiency and electrical durability at high voltage were highly dependent on the thickness of the IBL. The maximum external quantum efficiency of the devices with a 1.5-nm-thick IBL was increased by 28% compared with the reference. The enhanced performance of the OLEDs appears to be due to the improvement of carrier balance and exciton confinement.

Aerosol-Synthesized SWCNT Networks with Tunable Conductivity and Transparency by a Dry Transfer Technique

We demonstrate an aerosol CVD process to dry deposit large-area SWCNT networks with tunable conductivity and optical transmittance on a wide range of substrates including flexible polymers. These SWCNT networks can be chemically doped to reach a sheet resistance of as low as 110 Ω/⟨ at 90% optical transmittance. A wide application potential of these networks is demonstrated by fabricating SWCNT network-based devices such as a transparent capacitive touch sensors, thin-film transistors (TFTs), and bright organic light-emitting diodes (OLEDs).

Enhancements of emission rates and efficiencies by surface plasmon coupling

AbstractSurface plasmon (SP) coupling technique was applied to several InGaN/GaN‐based single and multiple quantum wells (QWs) which have various emission wavelengths to understand detail mechanism and consider the possibility of plasmonic light‐emitting diodes (LEDs). The coupling efficiencies from exciton to SP and from SP to photon were estimated by the time‐resolved photoluminescence (TRPL) measurements. We found that the SP enhanced internal quantum efficiencies (IQEs) of emission reached almost 100% at the best matched wavelength. This should be applicable to super bright LEDs at any wavelength by optimizing the best SP coupling condition. Possible device structures of plasmonic LEDs were also proposed and discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

New multinuclear europium(III) complexes as phosphors applied in fabrication of near UV-based light-emitting diodes

Abstract Two new multinuclear europium(III) complexes, [Eu(AFTFBD)3]2(1,4-bmb) and [Eu(AFTFBD)3]3(tmb) were designed and synthesized, where HAFTFBD was 2-acetylfluorene-4,4,4-trifluorobutane-1,3-dione, 1,4-bmb was 1,4-bis[2-(2′-pyridyl)benzimidazolyl]benzene and tmb was 1,3,5-tris[2-(2′-pyridyl)benzimidazolyl]benzene. The complexes were characterized by IR, UV–visible, photoluminescence (PL) spectroscopy and thermogravimetric analysis (TGA). The complexes emit the characteristic red luminescence of Eu3+ ion due to the 5D0–7FJ (J = 0–4) transitions under 395 nm-light excitation. Bright red light-emitting diodes were fabricated by coating the selected complex (Eu(AFTFBD)3)3(tmb) onto ∼395 nm-emitting InGaN chips, and the LEDs show appropriate CIE chromaticity coordinates in red area. All these results suggest that the Eu(III) complexes are promising candidates as red component for application in fabrication of near UV-based white light-emitting diodes.

A highly luminescent dinuclear Eu(III) complex based on 4,4′-bis (4″,4″,4″-trifluoro-1″,3″-dioxobutyl)- o-terphenyl for light-emitting diodes

A dinuclear Eu(III) complex Eu 2(btbt) 3·4H 2O·CH 3CH 2OH·N(CH 2CH 3) 3 was synthesized, where H 2(btbt) was 4,4′-bis (4″,4″,4″,-trifluoro-1″,3″-dioxobutyl)- o-terphenyl. The composition and structure of the complex were characterized by elemental analysis, IR, UV–vis and FAB-MS spectroscopy. The complex emits the characteristic red luminescence of Eu 3+ ion due to the 5 D 0 → 7 F J ( J = 0–4) transitions under ∼395 nm-light excitation with good luminescent quantum efficiency (32%) and exhibits high thermal stability (337 °C). Bright red light-emitting diodes (LEDs) were fabricated by coating the complex onto a ∼395 nm-emitting InGaN chip. When the mass ratio of the red phosphor to the silicone is 1:30, the efficiency of the fabricated LEDs with the europium complex is 0.98 lm w −1. The CIE chromaticity coordinates of the LEDs are x = 0.6057 and y = 0.3137, which are close to the National Television Standard Committee (NTSC) standard values for red color. The results indicate that the complex may act as a red component in the fabrication of near UV InGaN-based white light-emitting diodes with high color-rendering index.

A luminescent quadruple stranded dinuclear Eu(III) complex based on 2,8-bis(4′,4′,4′-trifluoro-1′,3′-dioxobutyl)-dibenzothiophene for light-emitting diodes

A red luminescent quadruple stranded dinuclear Eu(III) complex was synthesized shown as below. Bright red LEDs were successfully fabricated by coating the complex onto InGaN chip. The complex may act as a red component in fabrication of white LED with high color-rendering index. A luminescent quadruple stranded dinuclear Eu(III) complex Na 2 [Eu 2 (dbt) 4 ] was synthesized, where H 2 dbt was 2,8-bis(4′,4′,4′-trifluoro-1′,3′-dioxobutyl)-dibenzothiophene. The complex emits the characteristic red luminescence of Eu 3+ ion due to the 5 D 0 → 7 F J ( J = 0–4) transitions under 395 nm-light excitation with a luminescent quantum efficiency of 23%. Bright red light-emitting diodes (LEDs) were fabricated by coating the complex onto 395 nm-emitting InGaN chip. The results indicate that the complex may act as a red component in the fabrication of near UV-based white light-emitting diodes with high color-rendering index.

Optimization and Temperature-Dependent Luminescence of LiBaPO4 : Eu2 + Phosphor for Near-UV Light-Emitting Diodes

Blue-green phosphor, , was prepared by solid-state reaction and optimized by adding as flux. Temperature-dependent luminescence of phosphor was investigated. Furthermore, the luminescence properties of bright blue-green light-emitting diodes (LEDs) fabricated by incorporating this phosphor with an InGaN-based near-UV chip were also studied based on different forward-bias currents. The results suggest that photoluminescence intensity of can be improved by adding , and luminescence of the prepared phosphor exhibits higher thermal stability than that of commercially available phosphor. It is expected to be a good blue-green phosphor candidate for creating white light in phosphor-converted white LEDs.

White organic light-emitting diodes based on blue fluorescent bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb) 2] doped with DCM dye

Bright white organic light-emitting diodes (WOLEDs) with single active layer has been demonstrated from blue emitting zinc complex bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb) 2] doped with orange luminescent 4-(dicyanomethylene)-2-methyl-6-( p-dimethyl-aminostyryl)-4H-pyran (DCM) dye. White light electroluminescence (EL) spectrum from Zn(hpb) 2 has been achieved by adjusting the concentration of DCM dye. WOLED with a structure of ITO/α-NPD/Zn(hpb) 2:DCM ( x%)/BCP/Alq 3/LiF/Al has been fabricated. The EL spectra covering the whole visible spectra range of 400–700 nm, with two peaks at 446 and 555 nm has been measured. The device emits white light at 10 V with Commission Internationale de I’ Eclairage (CIE) coordinates (0.27, 0.31) and brightness 1083 Cd/m 2. The maximum current efficiency of the device was 1.23 Cd/A at 9.5 V and maximum luminance reaches 2210 Cd/m 2 at 12 V.

Synthesis and Luminescence of Ca3Ln2W2O12 : Eu3 + for NUV-InGaN-Based Red-Emitting LED

A series of -doped tungstate red phosphors, (, Gd, and Y) were prepared by the conventional solid-state reaction. The photoluminescent properties of the phosphors were studied. The phosphors show intensely red emission and superior high-temperature behavior. Among these phosphors, exhibits the strongest red emission under light excitation. Bright red light-emitting diodes (LEDs) were fabricated by combining the tungstate phosphors with emitting indium gallium nitride (InGaN) chips, and the good performances of the LEDs demonstrate that the phosphors may be suitable for application on near-ultraviolet (NUV) InGaN chip-based white-light-emitting diodes.

Liquid cooling of bright LEDs for automotive applications

With the advances in the technology of materials based on GaN, high brightness white light emitting diodes (LEDs) have flourished over the past few years and have shown to be very promising in many new illumination applications such as outdoor illumination, task and decorative lighting as well as aircraft and automobile illuminations. The objective of this paper is to investigate an active liquid cooling solution of such LEDs in an automotive headlights application. The thermal design from device to board to system level has been carried out in this research. Air cooling and passive liquid cooling methods were investigated and excluded as unsuitable, and therefore an active liquid cooling solution was selected. Several configurations of the active liquid cooling system were studied and optimisation work was carried out to find an optimum thermal performance.

Synthesis of new light-emitting donor–acceptor materials: Isomers of phenothiazine–quinoline molecules

Highly fluorescent isomeric donor–acceptor molecules, 2-(4-phenyl-2-quinolyl)-10-methylphenothiazine (2PQMPT) and 3-(4-phenyl-2-quinolyl)-10-methylphenothiazine (3PQMPT), were synthesized in high yields (>85%) and found to exhibit a factor of 2.4 disparity in electroluminescence efficiency. Bright (10,000–25,000 cd/m 2) and efficient (3.6–8.7 cd/A) green organic light-emitting diodes were realized from the two donor–acceptor molecules. The new molecules are good model systems for the study of structure–property relationships of donor–acceptor molecules for organic light-emitting diodes.

Brilliant! Shuji Nakamura and the Revolution in Lighting Technology (review)

Reviewed by: Brilliant! Shuji Nakamura and the Revolution in Lighting Technology Brian Bowers (bio) Brilliant! Shuji Nakamura and the Revolution in Lighting Technology. By Bob Johnstone. Amherst, N.Y.: Prometheus Books, 2007. Pp. 336. $28. The "revolution" in question is the universal replacement of filament and fluorescent lighting by LEDs (light-emitting diodes). That revolution is only just beginning, but, if Bob Johnstone's readable and convincing thesis is correct, then LEDs will soon be used for almost every lighting application and will significantly reduce the demand for electricity. Accounts of scientific developments, business history, and a personal story are intertwined in this book. The first LEDs in 1962 consisted of a layer a few atoms thick of the compound semiconductor material, gallium arsenide. Robust and taking very little power, they were ideal as indicators. They could give only red light, but their high efficiency compared with all other electric lights led people to ask whether a bright white LED could be developed for general illumination. The problem was solved by Shuji Nakamura, born in 1954 on Shikoku, the smallest of Japan's four main islands. Nakamura had studied electrical engineering at a local university, where he became fascinated with solid-state physics and obtained a master's degree. [End Page 479] Unusually for a Japanese, he had married while a student, and he preferred to stay in rural Shikoku rather than take a job with one of the big electrical firms on the main islands. He joined Nichia Chemical Industries, a small firm manufacturing phosphors for fluorescent lamps and television screens. The founder of Nichia, Nobuo Ogawa, had first seen fluorescent lighting in the Philippines during World War II. Impressed, he went into phosphor development and manufacture after the war. The elderly Ogawa must have seen possibilities in Nakamura: he gave him a free hand in his choice of research topics and allowed him to purchase very expensive equipment. Nakamura saw that the key to solving the problem was the development of an efficient bright blue LED. If he could achieve that, he could also make a green LED, and by combining red, green, and blue LEDs produce white. Although blue LEDs based on silicon carbide were being developed by other researchers, their efficiency was very low. Nakamura's studies led him to conclude that the blue LED should be based on gallium nitride, but forming a thin, single-crystal layer of gallium nitride was extremely difficult. Johnstone explains the solid-state physics involved and describes the complex equipment which deposits the semiconductor materials from vapor onto a substrate. Nakamura announced his bright blue LED in 1993. Two years later, at his suggestion, Nichia began manufacturing white LEDs based on the blue LED combined with a yellow phosphor which it was already manufacturing. As well as the physics, Johnstone describes Japanese industrial culture where the expectation is that an employee stays with one employer for life. But when Ogawa died, the new Nichia boss did not support Nakamura's work. By this time Nakamura was becoming known internationally, attending and contributing to physics conferences around the world. Friends urged a move to the United States, and after much soul-searching he resigned from Nichia and became a professor at the University of California at Santa Barbara in 2000. By then Nakamura had obtained numerous patents. The new Nichia management accused him of leaking trade secrets, and Nakamura did not feel that Nichia had rewarded him properly. A series of acrimonious legal battles ended in 2005 when the High Court in Tokyo awarded him 843 million yen (about eight million dollars). Surprisingly, although there is a photograph of a handful of LEDs, there is no simple drawing of one. Johnstone says they are cut from a gallium nitride wafer "the diameter of an Oreo cookie," which is not helpful for non-American readers (though my American friends tell me that an Oreo cookie is about 5 cm in diameter). It is also irritating that, writing of the end of the incandescent lamp, he implies that Edison was the sole inventor. But, since he has given us a comprehensive account of the latest electric lighting, this lapse in the early history...