Warm white LEDs based on Lumogen® Red and Yellow

One of the most widespread solutions for the production of white LEDs is the frequency downconversion of a part of the light, coming from a blue source, by exciting one or more materials (typically Ce:YAG) that emit at longer wavelength [1]. In this work we report ona simple and less expensive method to fabricate warmwhite-light LEDsusingthe photoluminescence of Lumogen®, a perylene-based polymer dyecommercialized by BASF,that has already beenprovedto be a good substitute for conventi onal inorganic colour conversion [2],[3]. Standard InGaN-based blue LEDs (~ 450 nm) were fabricated on a sapphire substrate by metal organic chemical vapour deposition. Both Lumogen® Yellow, and Red dyes wered…

Intersubband Transitions in Nitride Based Quantum Wells

AlInN/GaN quantum wells for intersubband transitions

Blue lasing at room temperature in high quality factor GaN/AlInN microdisks with InGaN quantum wells

The authors report on the achievement of optically pumped III-V nitride blue microdisk lasers operating at room temperature. Controlled wet chemical etching of an AlInN interlayer lattice matched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed in the photoluminescence spectra of InGaN/GaN quantum wells embedded in the GaN microdisks. Typical quality factors of several thousands are found (Q>4000). Laser action at similar to 420 nm is achieved under pulsed excitation at room temperature for a peak power density of 400 kW/cm(2). The lasing emission linewidth is down to 0.033 nm.

Blue-violet heterojunction LEDs based on hydrothermally synthesized ZnO nanorods

Zinc oxide nanorods have great potential for the realization of high efficiency heterostructure LEDs based on p-doped GaN. Well-aligned vertical nanorods are desirable to enhance the LED outcoupling performances due to a better confinement of the light [1]. However, due to the lack of reproducible p-type ZnO, a p-GaN substrate is still needed. This work reports on the fabrication of n-ZnO/p-GaN heterojunction LEDs based on vertical ZnO nanorods grown by hydrothermal method. The chemical reaction in the hydrothermal growth of ZnO is based on the decomposition of zinc nitrate, (Zn(NO3)2) with resulting Zn2+ ions reacting with the hydroxyl ions obtained by the thermal degradation of hexamethyl…

Stability improvement of PMMA and Lumogen® coatings for hybrid white LEDs

Hybrid white LEDs employing perylene-based dyes for the frequency down-conversion of blue light, generated by a standard inorganic source, suffer from colour rendering variations due to the degradation of the organic molecule under prolonged irradiation. To avoid such inconvenient, proper encapsulation of the dyes in resins or other polymer matrices can prevent their accelerated ageing; nevertheless, embedding polymers can also exhibit significant bleaching caused by chemico-physical agents. Among all, polymethyl methacrilate (PMMA) is one of the most used materials for the fabrication of hybrid LEDs' colour conversion coatings, therefore its stability needs to be investigated.

Al(In)N/GaN Heterostructures for Intersubband Transitions

White LED light obtained by frequency down-conversion of perylene-based dyes

Influence of electrodes layout on hydrothermally-grown GaN/ZnO LEDs

Light-emitting diodes (LEDs) based on zinc oxide, with wide direct band gap, have drawn much attention in the last years. ZnO, which is natural n-type, has excellent physical and chemical properties, is inexpensive, abundant and nontoxic. Unfortunately, the preparation of reproducible and high quality p-type ZnO film is comparatively difficult, due to the low solubility of acceptor dopants, self-compensating effects, and acceptor level energy height. Although many groups have reported on ZnO-based homostructure LEDs, the results are controversial as recently reviewed in [1]. Hence, heterojunction LEDs based on ZnO as the nside and GaN as the p-side of the junction have been put forward [2].…

Warm white LED light by frequency down- conversion of mixed perylene-based dyes

The growing demand of the solid-state lighting market for the development of sources for illumination has led to the fabrication of the first white LED in 1997, which employed a blue LED coated by a Ce:YAG phosphor to mix the down-converted yellow light with the blue one. The white light appears cold due to the weakness of red components in the emission spectrum. In order to obtain a warmer white, one possible solution is to add a red phosphor to the yellow one to move the chromatic coordinates properly, though the luminous efficiency drastically decreases due to the increased light absorption of the coating layer. It is generally believed that the low efficacy of warm white LEDs is the mai…

Hybrid LEDs pave way to new lighting applications

Many analysts agree that the global lighting market is close to a real revolution: the LED revolution. One of the most widespread solutions for the production of white LEDs is the conversion of a part of the light, coming from a known source, by exciting one or more materials that emit at a longer wavelength. The result is an emission spectrum given by the superposition of the single source and the photoexcited material; the big advantage is that the phenomenon of photoluminescence replaces the further integration of other solid-state devices of different colors. The photoluminescence of a perylene-based polymer dye turned out to be a good substitute for conventional inorganic color convers…

AlInN Based Quantum Wells for Intersubband Transitions