Efficient Blue Light-emitting Diodes Research Articles

Efficient blue light-emitting diodes based on a classical “push–pull” architecture molecule 4,4′-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,2′-bipyridine

A novel, highly blue luminescent molecule containing donor and acceptor groups, 4,4′-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,2′-bipyridine, that shows photoluminescence emission at 450 nm with 43% quantum yield is designed and synthesized. Time dependent-DFT calculations show an excellent correlation between theoretical and experimental absorption spectra, thus allowing for a detailed description of the electronic structure and assignment of the main absorption features. An optimized organic light-emitting diode based on a 4,4′-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,2′-bipyridine blue emitting layer, with a 4,4′,4″-tris(carbazol-9-yl)-triphenylamine) hole transporting layer, a 2,9-dimethyl-4.7-diphenyl-phenatroline hole blocking layer, and a tris(8-hydroxyquinoline)aluminium electron transport layer exhibited 2.1% quantum efficiency.

Efficient blue electroluminescent device using tetra(β-naphthyl)silane as a hole-blocking material

We report an efficient blue light-emitting diode (LED) using N, N′-bis(1-naphthyl)-N, N′-diphenylbenzidine as an emitting layer and tetra(β-naphthyl)silane (TNS) as a hole-blocking layer (HBL). We find that the hole-blocking performance, thermal stability, and film-forming ability of TNS are improved over those of the prototypical hole-blocking material 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). The device that used TNS as the HBL exhibits a narrower light emission and higher current efficiency (2.5cd∕A) as compared with the device containing BCP as the HBL. TNS should be promising as an excellent hole-blocking layer in LEDs.

Enhancement in external quantum efficiency of blue light-emitting diode by photonic crystal surface grating

Blue photonic crystal light-emitting diodes were evaluated with photonic crystal hole patterns. The diameter of the holes, lattice constant, and emission wavelength were selected to be 110, 220 and 450 nm, respectively, to attain a maximum enhancement in the wall-plug efficiency. The improvement in external quantum efficiency was 30–50%. Changes in voltage characteristics caused by the etched holes were found to be correlated with increased leakage current.

Efficient and stable blue light-emitting diodes based on an anthracene derivative doped poly(N-vinylcarbazole)

Single-layer blue light-emitting diodes (LEDs) are fabricated by spin coating a blend of 9,10-bis(3′,5′-diaryl)phenyl anthracene in poly(N-vinylcarbazole) (PVK) or in the mixture of PVK and an electron-transporting molecule, 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole. The Commission Internationale de I’Eclairage coordinates of the resulting LEDs are very close to that of the blue standard from the National Television Standards Committee. These devices also show excellent color stability when operated at a voltage span from 6to22V. High external quantum efficiency (>1.5%) and brightness (>3000cd∕m2) can be obtained in these devices.

Efficient blue light-emitting diodes with InGaN/GaN triangular shaped multiple quantum wells

InGaN/GaN triangular shaped multiple quantum wells (QWs) grown by grading In composition with time were adopted as an active layer of blue light-emitting diodes (LEDs). Compared to the LEDs with conventional rectangular QW structures, the triangular QW LEDs showed a higher intensity and a narrower linewidth of electrical luminescence (EL), a lower operation voltage, and a stronger light-output power. EL spectra of the triangular-QW-based LEDs also showed that the peak energy is nearly independent of the injection current and temperature, indicating that the triangular QW LED is more efficient and stable than the rectangular one.

Bright and efficient blue and green light-emitting diodes based on conjugated polymer blends

We report the use of green and blue fluorene conjugated polymers doped with hole transport materials consisting of triarylamine copolymers to fabricate bright and efficient blue and green single-layer light-emitting diodes (LEDs). These blends show enhanced quantum and power efficiency, much higher brightness and current densities and lower turn on and operating voltages compared with undoped devices. Optimised blue emission devices exhibited a maximum brightness of 1550 cd/m 2, a maximum external electroluminescence quantum efficiency of 0.9 cd/A or 0.4% and a maximum power efficiency of 0.3 lm/W. Optimised green emission devices showed a maximum brightness of 7400 cd/m 2, a maximum external electroluminescence quantum yield of 0.9% or 2.75 cd/A and a maximum power efficiency of 0.64 lm/W at high brightness.

High brightness and efficiency blue light-emitting polymer diodes

Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness. A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer. The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 0.25 cd/A and a luminosity of 0.04 lm/W. These values are optimized at a critical emissive layer thickness.

Copolymers with fragments of meta-pyridylvinylene and para-phenylenevinylene: synthesis, quaternization reaction and photophysical properties

Conjugated copolymers with fragments of 2,6-pyridylvinylene and 1,4-phenylenevinylene have been prepared by the Wittig reaction between the diphosphonium salt of 2,6-bis(chloromethyl)pyridine and terephthaldehyde or 2,5-dialkoxyterephthaldehydes. The protonation reaction of poly(1,4-phenylenevinylene)- co-(2,6-pyridylvinylene) with hydrochloric acid and the quaternization reaction of poly(2,5-dioctyloxy-1,4-phenylenevinylene)- co-(2,6-pyridylvinylene) with methyl triflate have been investigated by spectrophotometric and fluorescent methods. The absorption, excitation and fluorescence spectra of these copolymers as well as their corresponding model compounds were studied and compared. The photophysical properties of the investigated polymers suggest that these materials could be good candidates for the fabrication of efficient blue light-emitting diodes.

Aromatic copolyamides and copolyesters with vinylenarylene and terthiophene fragments in the polymer chain: synthesis and photophysical properties

A series of copoly( m-phenyleneisophthalamides) and copoly( m-phenyleneisopropylidenedi-4,4′-dicarboxylates) with fragments of 2,6-distyrylnaphthalene, 9,10-distyrylanthracene, 5,5″-distyryl-2,2′ :5′,2rd-terthiophene and 3′-dodecyl-2,2′:5′,2″-terthiophene has been prepared. The absorption, excitation and fluorescence spectra of these copolymers as well as corresponding model compounds were studied and compared. The actual molar ratios of luminophoric and non-luminophoric moieties in the copolymer structure were measured by comparison to model compounds. The photophysical properties of the investigated polymers suggest that these materials could be good candidates for the fabrication of efficient blue and green light-emitting diodes.

Efficient blue light-emitting diodes from a soluble poly ( para-phenylene) internal field emission measurement of the energy gap in semiconducting polymers

Polymer light-emitting diodes with poly (2-decyloxy-1,4-phenylene), DO-PPP, as the luminescent polymer emit blue light with external quantum efficieny of ∼3%. Using internal field emission (Fowler-Nordheim tunneling) in single carrier devices, the top of the π band and the bottom of the π * band have been determined for DO-PPP, and for poly (2-methoxy-5-(2'-ethyl-hexyloxy)-1,4phenylene vinylene), MEH-PPV. For DO-PPP, the single particle π-π * energy gap is 3.4–3.6 eV; for MEH-PPV, the π-π * energy gap is 2.2–2.3 eV. Comparison with optical absorption (the optical gap) implies that the exciton binding energy is small; for both polymers, E exc ≤ 0.2 eV. Such small residual values are probably not significant in the presence of disorder broadening which is also 0.1–0.2 eV and in the presence of lattice relaxation with self-localization energies of approximately 0.2 eV.

Effects of oxygen impurity on the surface in ZnS crystals and blue light-emitting diodes

Rutherford backscattering (RBS)/channelling and X-ray photoelectron spectroscopy (XPS) measurements have been performed to elucidate surface states of cubic-structure ZnS crystals doped with iodine. The XPS studies combined with an Ar+-ion sputtering technique revealed that adsorption of oxygen and chemical oxidation at the cleaved (110) faces resulted in significant changes of the surface properties, and that a large oxygen concentration, above 1019 cm-3, was detectable at the surface and in the bulk of the crystal prepared under various oxidation conditions. It is tentatively suggested that the chemically oxidised layer consists of ZnO and ZnS mixed alloys. The formation of oxide layers was applied to low-resistive n-ZnS and allowed the fabrication of an efficient blue light-emitting diode (LED) with the M pi S structure. It is demonstrated that the blue LED, with an effective interfacial layer of about 300 AA, exhibit a high-efficiency forward-bias injection blue luminescence around 2.76 eV at room temperature (an external quantum efficiency of about 8*10-4 photons/electron) as a result of the involvement of two kinds of donor-acceptor pair recombination centres.

Electrical conduction and low voltage blue electroluminescence in vacuum-deposited organic films

Bright blue (400–500 nm) recombination electroluminescence (EL) has often been observed from anthracene crystals with solid electrodes; external quantum efficiencies of 1%–8% have been seen and the theoretical efficiency is approximately 40%. The power efficiency has always been greatly degraded by the high voltages necessary to obtain reasonable currents through the comparatively thick crystals used. We report the preparation, and the structural, electrical and EL properties of approximately 0.6 μm vacuum-deposited anthracene films through which large steady state current densities can be passed at rather low applied voltages. The films showed the critical optimization effect, various structural and electrical properties exhibiting sharp singularities when the films were prepared at substrate temperatures within a narrow range near −60 °C. EL visible in normal room lighting was obtained at applied voltages of only about 30 V and up; in thinner films, EL could be seen in a darkened room at only about 12 V. This is apparently the first time that clearly visible EL has been reported from an organic material at voltages significantly less than 100 V. Broadly similar results were obtained for perylene and 1,12-benzperylene films; EL does not appear to have been observed before from these substances in any form. We discuss in some detail the prospects for practical EL devices based on organic films. In view of our results, and of the great improvements which are now probably feasible, we conclude that the prospects are promising; if these improvements can be realized, such devices could include high efficiency blue light-emitting diodes, and large-area broader wavelength systems for illumination applications.