Phosphor-free White Light-emitting Diodes Research Articles

White Emissive Carbon Dots Actuated by the H-/J-Aggregates and Förster Resonance Energy Transfer.

Carbon dots (CDs) have been demonstrated to be fluorescent materials for the new phosphor-free white light-emitting diodes (WLEDs). Herein, we synthesized a novel white CDs (WCDs). The spectrum highly matches the solar light spectrum (AM 1.5), which is a potentially high-color-quality lighting source material. The CDs contain blue, green, and red emissive centers produced from catechol, o-phenylenediamine, and their complexes, respectively. In addition, the photoluminescence mechanism had been revealed; three emission centers could be excited by a single UV source actuated by the formation of H- and J-aggregates and FRET between the CDs. Then the phosphor-free WLEDs were fabricated with a UV chip encapsulated with silicon resin containing the as-obtained CDs, which exhibit CIE coordinates of (0.33,0.33), a color rendering index (CRI) of 93, and a correlated color temperature (CCT) of 5453 K. The WLEDs show super stability and a high solar spectrum matching degree of 85-114%, protecting the eyesight. This provides a new way to design healthy lighting materials.

Hybrid White Light-Emitting Diodes Utilizing Radiative or Nonradiative Energy Transfer for Wavelength Conversion

Hybrid light-emitting diodes (LEDs) were fabricated by spin coating poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) on blue InGaN/GaN multiple-quantum-well (MQW) LED chips as a downconverter. As the F8BT film thickness was in the range of 235–290 nm, the hybrid LEDs emitted cool white light with a downconversion efficiency of 28.8%. The F8BT film quality was markedly improved by postannealing at 200 °C, raising the downconversion efficiency to 82.6%. To obtain more efficient white light emission, we proposed to insert Ag nanoparticles (NPs) with a diameter of 13 nm at the inorganic–organic interface to form an MQWs/NPs/F8BT structure. Simulations showed that nonradiative energy transfer from the MQWs to F8BT can be mediated by localized surface plasmons, leading to more efficient downconversion and overall white light emission. The proof of concept of such a hybrid LED will pave the way toward high-performance phosphor-free white LED sources for microdisplay and general lighting applications.

Effect of co-doped Tb3+ ions on electroluminescence of ZnO:Eu3+ LED

Rare earth (RE) -doped ZnO electroluminescence is worthy of investigation for phosphor-free white light-emitting diodes (LEDs) due to their pure and sharp emissions. Whereas, the low solubility of RE ions in ZnO films is found to hinder the performance of RE-doped ZnO devices. Herein, ZnO:Eu and ZnO:Eu/Tb LEDs were synthesized and the electroluminescence properties were tested. The results show that the emission intensity of ZnO: Eu/Tb LED is 8 times higher than that of ZnO: Eu LED while the input power is smaller when the concentration of terbium is proper. Furthermore, we discussed the excitation mechanism and found that the ratio of the EL intensity of the 5D1 → 7F1 to 5D0 → 7FJ (J=0 − 4) transition increases with increasing Tb doping concentration, which may indicate the possibility of energy transfer from Tb3+ to Eu3+. The results are believed to be an effective strategy to improve the electroluminescence of RE-doped semiconductor for white LEDs.

Flower-Like Internal Emission Distribution of LEDs with Monolithic Integration of InGaN-based Quantum Wells Emitting Narrow Blue, Green, and Red Spectra

We report a phosphor-free white light-emitting diodes (LED) realized by the monolithic integration of In0.18Ga0.82N/GaN (438 nm, blue), In0.26Ga0.74N/GaN (513 nm, green), and In0.45Ga0.55N/In0.13Ga0.87N (602 nm, red) quantum wells (QWs) as an active medium. The QWs corresponding to blue and green light were grown using a conventional growth mode. For the red spectral emission, five-stacked In0.45Ga0.55N/In0.13Ga0.87N QWs were realized by the so-called Ga-flow-interruption (Ga-FI) technique, wherein the Ga supply was periodically interrupted during the deposition of In0.3Ga0.7N to form an In0.45Ga0.55N well. The vertical and lateral distributions of the three different light emissions were investigated by fluorescence microscope (FM) images. The FM image measured at a focal point in the middle of the n-GaN cladding layer for the red-emitting LED shows that light emissions with flower-like patterns with six petals are periodically observed. The chromaticity coordinates of the electroluminescence spectrum for the white LEDs at an injection current of 80 mA are measured to be (0.316, 0.312), which is close to ideal white light. In contrast with phosphor-free white-light-emitting devices based on nanostructures, our white light device exhibits a mixture of three independent wavelengths by monolithically grown InGaN-based QWs, thus demonstrating a more facile technique to obtain white LEDs.

Colour-crafted phosphor-free white light emitters via in-situ nanostructure engineering

Colour-temperature (Tc) is a crucial specification of white light-emitting diodes (WLEDs) used in a variety of smart-lighting applications. Commonly, Tc is controlled by distributing various phosphors on top of the blue or ultra violet LED chip in conventional phosphor-conversion WLEDs (PC-WLEDs). Unfortunately, the high cost of phosphors, additional packaging processes required, and phosphor degradation by internal thermal damage must be resolved to obtain higher-quality PC-WLEDs. Here, we suggest a practical in-situ nanostructure engineering strategy for fabricating Tc-controlled phosphor-free white light-emitting diodes (PF-WLEDs) using metal-organic chemical vapour deposition. The dimension controls of in-situ nanofacets on gallium nitride nanostructures, and the growth temperature of quantum wells on these materials, were key factors for Tc control. Warm, true, and cold white emissions were successfully demonstrated in this study without any external processing.

InGaN/GaN multiple quantum wells on selectively grown GaN microfacets and the applications for phosphor-free white light-emitting diodes

Abstract Phosphor-free InGaN/GaN multiple quantum well (MQW) white light-emitting diodes (LEDs) have the advantages of simpler device process and potentially higher efficiency, and have attracted much attention in recent years. A host of technologies are emerging for implementing such white-light LEDs. Among them, the key issue is the color tuning of different emission wavelengths from InGaN/GaN MQWs with different indium (In) content. However, owing to the limited growth technology for long-wavelength InGaN/GaN MQWs with high In content, it is very attractive to study selective area epitaxy (SAE) of InGaN/GaN MQWs on GaN microstructures with non- or semipolar microfacets combined with (0001) c -plane. In this paper, we briefly review the previous developments of InGaN/GaN MQW based phosphor-free white light LEDs, then the particular technology for the growth of InGaN/GaN MQWs on the regrown GaN microfacets using SAE has been introduced, and related mechanisms for the formation of different non- or semipolar GaN microfacets fabricated by various mask patterns are discussed in detail. Furthermore, sophisticated approaches made use of the InGaN/GaN MQWs on GaN microfacets to fabricated phosphor-free white light LEDs with polychromatic emissions are reviewed.

(Invited) Phosphor-Free III-Nitride Nanowire White Light Emitting Diodes: Challenges and Prospects

Tremendous progress has been made in GaN-based nanowire light-emitting diodes (LEDs), which can exhibit drastically reduced dislocation densities and polarization fields, compared to the conventional quantum well devices. However, the performance of nanowire LEDs is limited due to the poor hole transport, the electron overflow, and the large surface nonradiative recombination. In this context, we have investigated the molecular beam epitaxial growth, fabrication and characterization of full-color InGaN/(Al)GaN dot-in-a-wire LEDs on Si substrates, wherein the emission characteristics are controlled by the dot properties in a single epitaxial step. With the use of p-type modulation doping, electron blocking layer (EBL), and core-shell heterostructures, we have further demonstrated superior performance phosphor-free white LEDs that can exhibit record high output power of ~ 5.2 mW and unprecedentedly high color rendering index (CRI) of > 95.

Growth and characterization of phosphor-free white light-emitting diodes based on InGaN blue quantum wells and green–yellow quantum dots

Phosphor-free white light-emitting diodes consisting of 4 layers of InGaN/GaN quantum dots and 4 layers of quantum wells have been grown by metal organic chemical vapor deposition. A white emission was demonstrated under electrical injection by mixing the green–yellow light from quantum dots and the blue light from quantum wells. At the injection current of 5mA, the electroluminescence peak wavelengths of quantum dots and quantum wells were 548nm and 450nm, respectively, resulting in the color-rendering index Ra of 62. As the injection current increased, a faster emission enhancement of quantum well and an emission blue shift of the quantum dots were observed, which led to the decrease of Ra.

Controlling Electron Overflow in Phosphor-Free InGaN/GaN Nanowire White Light-Emitting Diodes

We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes.

P-Type Modulation Doped InGaN/GaN Dot-in-a-Wire White-Light-Emitting Diodes Monolithically Grown on Si(111)

Full-color, catalyst-free InGaN/GaN dot-in-a-wire light-emitting diodes (LEDs) were monolithically grown on Si(111) by molecular beam epitaxy, with the emission characteristics controlled by the dot properties in a single epitaxial growth step. With the use of p-type modulation doping in the dot-in-a-wire heterostructures, we have demonstrated the most efficient phosphor-free white LEDs ever reported, which exhibit an internal quantum efficiency of ∼56.8%, nearly unaltered CIE chromaticity coordinates with increasing injection current, and virtually zero efficiency droop at current densities up to ∼640 A/cm(2). The remarkable performance is attributed to the superior three-dimensional carrier confinement provided by the electronically coupled dot-in-a-wire heterostructures, the nearly defect- and strain-free GaN nanowires, and the significantly enhanced hole transport due to the p-type modulation doping.

Invariable optical properties of phosphor-free white light-emitting diode under electrical stress

This paper reports that a dual-wavelength white light-emitting diode is fabricated by using a metal-organic chemical vapor deposition method. Through a 200-hours' current stress, the reverse leakage current of this light-emitting diode increases with the aging time, but the optical properties remained unchanged despite the enhanced reverse leakage current. Transmission electron microscopy and cathodeluminescence images show that indium atoms were assembled in and around V-shape pits with various compositions, which can be ascribed to the emitted white light. Evolution of cathodeluminescence intensities under electron irradiation is also performed. Combining cathodeluminescence intensities under electron irradiation and above results, the increase of leakage channels and crystalline quality degradation are realized. Although leakage channels increase with aging, potential fluctuation caused by indium aggregation can effectively avoid the impact of leakage channels. Indium aggregation can be attributed to the mechanism of preventing optical degradation in phosphor-free white light-emitting diode.

Recent Progress in GaN‐Based Light‐Emitting Diodes

AbstractIn the last few years the GaN‐based white light‐emitting diode (LED) has been remarkable as a commercially available solid‐state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V‐grooved c‐plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light‐emitting efficiency (because of the enhanced reflection from the V‐grooved plane). Furthermore, anomalous tunneling‐assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to ∼72%. Finally, a single‐chip phosphor‐free white LED is fabricated, a stable white light is emitted for currents from 20 to 60 mA, which makes the LED chip suitable for lighting applications.

Phosphor-free white light-emitting diode with laterally distributed multiple quantum wells

A phosphor-free white light-emitting diode (LED) was fabricated with laterally distributed blue and green InGaN∕GaN multiple quantum wells (MQWs) grown by a selective area growth method. Photoluminescence and electroluminescence (EL) spectra of the LED showed emission peaks corresponding to the individual blue and green MQWs. The integrated EL intensity ratio of green to blue emission varied from 2.5 to 6.5 with the injection current below 300mA, but remained constant at high injection currents above 300mA. The stability of the emission color at high currents is attributed to parallel carrier injection into both MQWs.

Cascade single-chip phosphor-free white light-emitting diodes

In order to resolve the problems existing in the conventional phosphor-converted light-emitting-diodes (LEDs) and red-green-blue LEDs, the cascade single-chip phosphor-free white LED was proposed with GaAs∕GaN heterojunction direct wafer bonding. Corresponding to the color-matching calculation, the white LED demonstrated the CIE chromaticity coordinates of about (0.3, 0.3) at 20mA, which was very close to the ideal white light position (1∕3, 1∕3) on the chromaticity diagram. The fabrication and the electrical and optical performances of such white LEDs were described.