Light-emitting diode (LED)

The rapid manufacture of complex three-dimensional micro-scale components has eluded researchers for decades. Several additive manufacturing options have been limited by either speed or the ability to fabricate true three-dimensional structures. Projection micro-stereolithography (PμSL) is a low cost, high throughput additive fabrication technique capable of generating three-dimensional microstruc- tures in a bottom-up, layer by layer fashion. The PμSL system is reliable and capable of manufactur- ing a variety of highly complex, three-dimensional structures from micro- to meso-scales with micro- scale architecture and submicron precision. Our PμSL system utilizes a reconfigurable digital mask and a 395 nm light-emitting diode (LED) array to polymerize a liquid monomer in a layer-by-layer manufacturing process. This paper discusses the critical process parameters that influence polymer- ization depth and structure quality. Experimental characterization and performance of the LED-based PμSL system for fabricating highly complex three-dimensional structures for a large range of appli- cations is presented. © 2012 American Institute of Physics. [ http://dx.doi.org/10.1063/1.4769050 ]

spin-relaxation process. In addition, it is established that the electron spin polarization reaches its maximum value before the first 700 ps of the EL emission. The spin-polarized light emitting diode structure 关see Fig. 1共a兲 兴 was grown by molecular beam epitaxy 共MBE兲 for the semiconductor part before depositing the tun- nel barrier/ferromagnet contact by sputtering method. The p-i-n LED device has the following structure sequence:

Quantum Dot white LED (QD WLED) technology may be one of the best choices, due to its higher energy efficiency, larger color render in index, better versatility and more[r]

up to 35000 cd.m -2 . We study the low-temperature performance of the LED and find that there is a delay of droop in terms of current density as temperature decreases. In the last part of the paper, we design a large LED (56 mm 2 emitting area) and test its potential for LiFi-like communication. In such approach, the LED is not only a lightning source but also used to transmit a communication signal to a PbS quantum dot solar cell used as a broad band photodetector. Operating conditions compatible with both lighting and information transfer have been identified. This work paves the way toward an all nanocrystal-based communication setup.

Received 11 January 2016; revised 24 March 2016; accepted 24 March 2016 (Doc. ID 257273); published 2 May 2016 High-power light-emitting diodes (LEDs) today are twice as powerful as four years ago while meantime their price has been divided by 4 making them promising sources for laser pumping. However, their irradiance still falls short by one order of mag- nitude of what is needed to efficiently pump solid-state lasers. We demonstrate that an LED-pumped Ce:YAG luminescent concentrator (LC) can increase the irradiance of blue LEDs by a factor of 10, with an optical efficiency of 25%, making them much more suitable to pump solid-state lasers. In our demon- stration, we used 100 Hz pulsed LEDs emitting 190 W∕cm 2 at

for top contacts and another Si 3 N 4 passivation layer were deposited. Finally, a two-pair ZnS/Ge dielectric Bragg mirror was deposited by RF sputtering and patterned by a lift-off process. The reflectivity seen from the cavity is calculated to be around 91%, a value slightly lower than the bottom DBR allowing a better extraction of the light. Figure 5 shows a schematic and a SEM cross section image of the complete RCLED structure at the end of the process flow. The oxide confinement aperture is clearly visible in Figure 5 and is of the same size as the top emission window defined by the contact ring, which ensures an efficient current confinement. In parallel, the IC-LED (without lateral confinement) was processed with mesa diameters of 100 µm and ring contacts width of 20 µm.

Conclusion We have successfully developed luminescent composite films based on the mixture of a single-layered hydroxide and organic luminescent dyes. The organic dyes dicyanomethylene and pyranine were chosen according to their photoluminescent char- acteristics, which are excitation with blue light and an emission spectrum covering a wide range of the visible spectrum. The molecules were each dispersed in an inorganic matrix based on zinc hydroxyacetate single-layered hydroxide. In order to ensure good dispersion and avoid aggregation of the organic dyes, a composite preparation approach that kept the composite in its wet form prior to embedding in silicon second matrix was applied. The resulting films exhibit acceptable absolute quan- tum yields usable in LED devices. These films were placed on a 450 nm commercial LED in a remote-phosphor configuration to determine their photometric characteristics. The best results were obtained with the superposition of the pyranine film over that of dicyanomethylene. Both films were placed in a remote- phosphor configuration on top of a blue LED chip. The photo- metric parameters measured on this system [(CRI of 81, CCT of 5409 K, CIE coordinate of (0.33, 0.28)] were found to be very interesting for display applications. A bright white emission with cool colour temperature was obtained. Studies of the robustness of these luminescent films are underway with the aim to determine their mechanical and thermal stabilities. Furthermore, the stability of their optical properties will be in- vestigated upon thermal and photonic stresses to demonstrate their ability for future applications.

eutectic sample and (b) silver paste sample . ........................................................ 42 Figure I.53: (a) Delamination and (b) curling of phosphor coated LED package . ................................................................................................................................. 43 Figure I.54: Encapsulant yellowing . ..................................................................... 43 Figure I.55: Relative light output from 5–mm indicator lamps and high–power illuminator LEDs . .................................................................................................. 44 Figure I.56: Degradation of phosphor (left: untreated sample, right: after stress at 100 A.cm−2, 120°C) . .......................................................................................... 45 Figure I.57: Lifetime result of LED with (a) remote and (b) die–contact phosphor . ................................................................................................................................ 45 Figure I.58: Creep strain rate vs. tensile stress for different SACxx alloys . ...... 46 Figure I.59: Solder fracture due to creep–fatigue under thermal cyclic load . .... 46 Figure I.60: Solder joint cracks (left: SAC305, right: Innolot) ............................. 47 Figure II.1: Energy loss of carriers as they are captured into the quantum well. ................................................................................................................................. 53 Figure II.2: Current–voltage characteristic of LED showing threshold voltages 2.0 and 1.6 V, at 77°K and 300°K, respectively..................................................... 58 Figure II.3: (a) Pulsed calibration and (b) determination of junction temperature for different DC forward currents. ......................................................................... 58 Figure II.4: (a) Pulsed calibration measurement (small duty cycle 0.1 %) and (b) junction temperature (T j ) versus DC current of AlGaN UV LED, Schubert, 2006

characterization appeared to be slightly lower than the lattice temperature estimated from the temperature dependency of the bandgap energy. We can speculate that locally within the recombination region the temperature is higher than derived from Raman characterization at the junction position. In these hot-spots recombination probability is higher and thus the emission from localized regions with increased temperature could dominate the emission. Considering the lattice temperature from both approaches, Joule heating is slightly increasing the temperature of the lattice with increasing current density. However in this temperature regime we can exclude thermal radiation as a source of emission in the considered range. The peak wavelength and total radiated amount of a thermal emitter vary with temperature according to Wien's displacement law. Emission with a maximum at about 1.5m corresponds thus to a temperature of about 1200K, far above the temperature we determined for our light emitting device and even above the melting temperature of Ge.

* Correspondence: maria.tchernycheva@c2n.upsaclay.fr Received: 15 October 2020; Accepted: 12 November 2020; Published: date Abstract: We analyze the thermal behavior of a flexible nanowire (NW) light-emitting diode (LED) operated under different injection conditions. The LED is based on metal–organic vapor-phase deposition (MOCVD)-grown self-assembled InGaN/GaN NWs in a polydimethylsiloxane (PDMS) matrix. Despite the poor thermal conductivity of the polymer, active nitride NWs effectively dissipate heat to the substrate. Therefore, the flexible LED mounted on a copper heat sink can operate under high injection without significant overheating, while the device mounted on a plastic holder showed a 25% higher temperature for the same injected current. The efficiency of the heat dissipation by nitride NWs was further confirmed with finite-element modeling of the temperature distribution in a NW/polymer composite membrane.

Introduction Organic light-emitting diodes (OLEDs) have attracted signicant attention for broad applications in displays and lighting because of their high electroluminescence (EL) eciency, exibility and low manufacturing cost. 16 In order to improve the EL eciency, various uorescent and phosphorescent materials have been introduced as OLED emitters. 3,79 In practice, it is phosphorescent OLEDs (PhOLEDs) that are utilized to obtain high external quantum eciency (EQE) over 20%. 1013 Because the ratio of singlet and triplet excitons under electrical excitation is 1:3 due to spin statistics, 1416 the internal quantum eciency (IQE) of traditional uorescent OLEDs is limited to 25%. PhOLEDs, on the other hand, can achieve 100% IQE by harvesting both singlet and triplet excitons through strong spin-orbit coupling. 1720

An ultrawide-bandwidth, superluminescent light-emitting diode (SLED) utilizing multiple layers of dots of tuned height is reported. Due to thermal effect, the superluminescent phenomenon is observed only under pulse-mode operation. The device exhibits a 3 dB bandwidth of 190 nm with central wavelength of 1020 nm under continuous- wave (cw) conditions. The maximum corresponding output power achieved in this device under cw and pulsed operation conditions are 0.54 mW and 17 mW, respectively. © 2012 Optical Society of America

9 develop on areas that are frequently exposed to the sun (e.g., face, ears, scalp, neck, forearms, and back of the hands). Studies have shown that if actinic keratosis are untreated, actinic keratosis may regress, or alternatively, may progress to squamous cell carcinoma, with significant morbidity and possible lethal outcome. Predicting which actinic keratosis may progress to squamous cell carcinoma is not possible, nor is the conversion rate for an actinic keratosis to squamous cell carcinoma clear: the transformation rate from an actinic keratosis lesion to squamous cell carcinoma within one year has been reported to be <1:1000. The malignant potential and the fact that it is impossible to predict which actinic keratosis will evolve into squamous cell carcinoma, have led to the common consensus that actinic keratosis have to be treated. Because of the high prevalence of actinic keratosis, their treatment represents a substantial workload, and must therefore be efficacious and easy to perform. Moreover, for patients an ideal treatment should be well tolerated and result in good cosmesis. The most commonly used treatments for actinic keratosis are cryotherapy, topical chemotherapy and, more recently, photodynamic therapy (PDT) [22] However, for this application, PDT is carried out with a wide variety of light sources delivering a broad range of more or less adapted light doses. Due to the complexities of the human anatomy, these light sources do not in fact deliver a uniform light distribution to the skin. For example, in the case of the LED system used usually in Dermatology, Moseley et al demonstrated that the irradiance may be as low as 38% of the central area at a distance of only 2 cm [23].

To evaluate the LED performance, current density–voltage– luminance (J–V–L), EQE-voltage characteristics, and external quantum efficiency spectra are recorded and shown in Fig. 4 . The J–V characteristics [ Fig. 4(a) ] of devices show an electrical turn-on at ∼2.2 V, as evidenced by a sharp rise in the current den- sity. 13 We observe progressive improvement in EQE [ Fig. 4(b) ] with an increase in CsBr content, consistent with the enhancement in the perovskite film PLQY. The most efficient devices containing a 150 nm thick CsPbBr3 emitting layer with a 5:1 precursor ratio have a peak EQE of 1.1% at 5.8 V, which is to our knowledge among the best for perovskite LEDs based on fully evaporated CsPbBr3. The internal quantum efficiency (IQE) of 5.5% can be estimated by divid- ing the peak EQEs with an estimated out-coupling factor (∼0.2). 40,41 This IQE value suggests a near perfect charge injection and charge

voltage curve is not that for a diode. Hence, the first scenario is not the case. Has the Au in the upper contact spiked through the active region? Examination o[r]

__________________ * Corresponding author Indeed, these materials have been widely used in optoelectronics [13] and particularly as emitters in OLEDs by several of us [14-17]. The main goal of our work is turned towards synthetic chemistry of luminescent organic materials belonging to the carbazole family. Unsusbstituted carbazolic members are well-known for their good hole transporting properties and for their blue-light emitting capabilities (see e.g. review ref. [14]).

3.3. Fabrication of the HC patterned OLED Finally, the two HC films (HC1 and HC2) were investigated as light outcoupling layers for OLEDs. Low light outcoupling efficiency is an important limiting factor in the OLED technology, especially for blue devices that inherently su ffer from lower photoluminescence quantum yields compared to the other colors [52 55] . Consequently, insertion of a light outcoupling layer on top of the glass substrate that could help to recover the lost waveguide modes of light is of special signi ficance for blue OLEDs. For this study, a benchmark light emitting material i.e. 4,4′ bis(4 (9H carbazol 9 yl)styryl)biphenyl (BsB4) has been selected as the blue fluorescent emitter [56,57] . To examine the light extraction ability of the honeycomb films deposited on the outer side of the glass substrate the following structures represented in Fig. 4 have been

can not be completely ruled out [24], and dopant excitation could arise from a better electron-hole recombination with the carbazole compounds playing the role of the carrier traps according to the trap mechanism [25]. The electrical characteristics of the 3 host-guest devices (b-type in figure 2), without any doping, 5% PMC-doped and 2% DEC-doped respectively, are shown in figure 6. Every device exhibits a diode behaviour with a threshold value around 8V. Noteworthy, the required voltage for reaching a given current density is higher for the doped devices compared to the nondoped one. This is in agreement with the behaviour of reported devices [24, 25] with dopants working as carrier traps. Indeed, hole mobility is expected to decrease when holes are trapped into dopants, so leading to an increase of the electric resistance of the emitting layer.

Abstract Solid-state deep UV Light emitting diodes (LEDs) based on Al x Ga 1-x N material are nowadays gaining particular attention due to their potential for replacing mercury lamps, currently used for sterilization and water disinfection applications. However, the realization of planar efficient emitting devices is limited by a high density of extended defects and difficult efficient dopant incorporation affecting both optical and electrical properties. As a strategy to alleviate this difficulty, I have focused on the study of nanowire based heterostructure devices, due to their advantage of elastically relaxing the strain during growth, coupled with a higher dopant solubility limit and an eased light extraction coming from their particular morphology. First, correlated experiments of Atom Probe Tomography (APT), Energy Dispersive X-ray Spectroscopy (EDX) or Raman spectroscopy performed on GaN pn junctions grown by plasma assisted molecular beam epitaxy (PA-MBE) have shown that both n-type and p-type dopants, namely Si and Mg, respectively, exhibit an inhomogeneous radial distribution, with dopant incorporation upper limits attaining 10 21 atoms/cm 3 at

1.4.2 Electroluminescence To describe electroluminescence, we can start with various types of processes that happen in the EL phenomenon. The three major components of the EL process are: the use of an electrical bias to accumulate charge carriers of opposite signs (electrons and holes) between two electrodes; the movement of the charge carriers through the electroluminescent material; and the recombination of the charge carriers to generate EL [83]. According to the type of excitation, the EL can be categorized in three different classes: high-field EL, impact EL, and recombination EL. In high- field EL light is generated by using a high electric field to directly excite the atomic or molecular states. The collision of high-energy electrons with the atoms of the emitter molecules causes some covalent bonds to break between atoms of the emissive material, resulting in the release of bound electrons into the conduction band [34]. A large number of minority charge carriers will be produced, resulting in an increase in the reverse current thus increasing the probability of tunneling from the valence band to the conduction band, preventing crystal breakdown. The major problem of this excitation is that the high applied field may breaksdown the crystal before electrons jump to the excited state. The field strength when breakdown occurs for most crystalline luminescent material is around 10 6 V/cm [83].