Solid-state Light-emitting Diodes Research Articles

Ionic liquid alkyl chain regulated highly stable and luminescent silica-coated perovskite quantum dots for bright white light-emitting diodes

Silica-coated perovskite quantum dots (QDs), which exhibit exceptional photophysical properties, hold great promise for various optoelectronic applications. Nonetheless, the issue of insufficient stability poses a significant challenge to their continued advancement and widespread adoption. Herein, we present a novel approach for the synthesis of silica-coated CH3NH3PbBr3 (CH3NH3 = MA) QDs with exceptional stability and luminescence, which utilized hydrophobic ionic liquids (ILs) with unlike alkyl chain lengths as surface ligands. As the alkyl chain length of the hydrophobic ILs increased, we observed a blue-shift in the wavelength and a reduction in the time required to reach maximum fluorescence for the silica-coated perovskite QDs. It not only effectively prevents QD aggregation but also accelerates the formation rate of the SiO2 shell. The introduction of ILs generated the enhancement of photoluminescence quantum yield (PLQY) from 85% for the pristine sample to 92.83% for ILs treated C16-QDs@SiO2. Furthermore, the stability of the ILs-treated C16-QDs@SiO2 was notably enhanced, with the fluorescence intensity retaining 80.5% after 40 days of storage. The as-prepared white light emitting diode (LED) displayed a high color rendering index (CRI) of 75.3 and a correlated color temperature (CCT) of 7529 K. This work provides an alternative strategy to solve the challenging stability issue of silica-coated perovskite QDs and promote the silica-coated perovskite QDs to the practical application in solid-state LEDs.

Zwitterionic Carbazole Ligands Enhance the Stability and Performance of Perovskite Nanocrystals in Light-Emitting Diodes.

We introduce a new carbazole-based zwitterionic ligand (DCzGPC) synthesized via Yamaguchi esterification which enhances the efficiency of lead halide perovskite (LHP) nanocrystals (NCs) in light-emitting diodes (LED). A facile ligand exchange of the native ligand shell, monitored by nuclear magnetic resonance (NMR), ultraviolet-visible (UV-vis), and photoluminescence (PL) spectroscopy, enables more stable and efficient LHP NCs. The improved stability is demonstrated in solution and solid-state LEDs, where the NCs exhibit prolonged luminescence lifetimes and improved luminance, respectively. These results represent a promising strategy to enhance the stability of LHP NCs and to tune their optoelectronic properties for further application in LEDs or solar cells.

Bridging the Green Gap: Monochromatic InP-Based Quantum-Dot-on-Chip LEDs with over 50% Color Conversion Efficiency.

Solid-state light-emitting diodes (LEDs) emit nearly monochromatic light, yet seamless tuning of emission color throughout the visible region remains elusive. Color-converting powder phosphors are therefore used for making LEDs with a bespoke emission spectrum, yet broad emission lines and low absorption coefficients compromise the formation of small-footprint monochromatic LEDs. Color conversion by quantum dots (QDs) can address these issues, but high-performance monochromatic LEDs made using QDs free of restricted, hazardous elements remain to be demonstrated. Here, we show green, amber, and red LEDs formed using InP-based QDs as on-chip color convertor for blue LEDs. Implementing QDs with near-unity photoluminescence efficiency yields a color conversion efficiency over 50% with little intensity roll-off and nearly complete blue light rejection. Moreover, as the conversion efficiency is mostly limited by package losses, we conclude that on-chip color conversion using InP-based QDs can provide spectrum-on-demand LEDs, including monochromatic LEDs that bridge the green gap.

Mn Doping in Low-Dimensional Perovskites: A Switch for Controlling Dopant and Host Emission

The growth of solid-state photonics relies on the design of highly efficient devices with tunable photoluminescence (PL) based on thermally stable phosphors. In this study, a room-temperature, all-ambient synthesis of Mn-doped hybrid lead halide perovskite phosphors is presented. By applying temperature-dependent (4.2–300 K) steady-state and transient PL measurements, the photophysical pathways of the hybrid system were identified and the carrier recombination processes at play were defined. At higher Mn-doping concentrations (>4%), the excitonic white light host emission nearly disappears, and a bright Mn transition appears at ∼650 nm with a record PL quantum yield of ∼87.4%. The intense sensitized Mn luminescence results from rapid and efficient exciton-to-dopant energy transfer, as evidenced by time-gated and femtosecond fluorescence upconversion measurements. A millisecond PL decay time attributed to the Mn dopant and a microsecond PL decay time associated with the excitonic emission of the host were found, indicating energy and reverse energy transfer in the system. These findings provide unique insights into the chemical tailoring of the Mn-dopant emission by investigating the coupling between the host semiconductor and the dopant ion, a stepping stone toward the development of high-performance solid-state light-emitting diodes and scintillators.

Fabrication of transparent ultrathin films with ordered solid luminescence by LBL assembly of CdTe quantum dots with exfoliated vermiculite

Quantum dots luminescent films have great potential for application in lasers, optoelectronics, solid-state light-emitting diodes, and optical devices. This work describes multilayer ordered vermiculite-based luminescent thin films (VLFs) with 2D architecture via layer-by-layer assembly of PDDA-modified CdTe QDs on exfoliated vermiculite (VMT). The assembly of negative CdTe QDs and negative VMT was achieved, overcoming the nonluminance of solid CdTe QDs. The ordered VLFs possess periodic layered structures involving heterogeneous superlattices associated with individual VMT layers. The rational combination of luminescent components affords precise control of the emission wavelength and intensity, resulting in a gradual change in luminescence intensity in the selected light-emitting spectral region upon increasing the number of deposition cycles. Compared to the CdTe QDs, the fluorescence lifetime of VLFs is improved owing to the ordered structure. This work provides a facile approach to fabricate heterogeneous VLFs with high luminous properties and optical stability, which is expected to prevent easy aggregation of luminescent materials and can be potentially applied in light displays and multicolor optical and optoelectronic devices.

First Estimation of Global Trends in Nocturnal Power Emissions Reveals Acceleration of Light Pollution

The global spread of artificial light is eroding the natural night-time environment. The estimation of the pattern and rate of growth of light pollution on multi-decadal scales has nonetheless proven challenging. Here we show that the power of global satellite observable light emissions increased from 1992 to 2017 by at least 49%. We estimate the hidden impact of the transition to solid-state light-emitting diode (LED) technology, which increases emissions at visible wavelengths undetectable to existing satellite sensors, suggesting that the true increase in radiance in the visible spectrum may be as high as globally 270% and 400% on specific regions. These dynamics vary by region, but there is limited evidence that advances in lighting technology have led to decreased emissions.

Estimating the quantum requirements for plant growth and related electricity demand for LED lighting systems

Indoor plant production systems with artificial lighting are considered an emerging technology contributing to biomass-based value webs. The viability of this concept greatly relies on the energy requirements (ER, Watt) for lighting. We estimated the ER for plant growth by calculating the conversion efficiency of electricity to light of solid-state light-emitting diodes (LED) and the quantum requirements for plant growth of a fictional plant stand producing 2500 g of dry weight per m2 of ground during 100 days, representing a high productivity benchmark of field crops. The quantum output (µmol s−1 W−1) of eight LEDs of different colours varied between 0.78 for green and 2.54 for deep red. Uncertainty in the H+ demand for ATP synthesis during photosynthesis, the relative portion of photorespiration and the fraction of light intercepted by plant canopies (fabs) were considered in a pessimistic (PA) and optimistic (OA) approach of calculation of ER. Cumulative ER were 606 and 265 kWh m−2 for the PA and OA scenarios. The energy conversion efficiencies in the PA and OA scenarios were 2.07 and 4.72%. Estimates of energy savings by suppressing photorespiration and increasing fabs vary between 24 and 38%. The peak daily ER were 9.44 and 4.14 kWh in the PA and OA scenarios. Results are discussed in the context of the design of lighting in indoor plant production systems and commercial greenhouses where natural fluctuation in solar radiation could be balanced by dimmable LED panels.

Highly Thermotolerant Metal Halide Perovskite Solids.

By virtue of their narrow emission bands, near-unity quantum yield, and low fabrication cost, metal halide perovskites hold great promise in numerous aspects of optoelectronic applications, including solid-state lighting, lasing, and displays. Despite such promise, the poor temperature tolerance and suboptimal quantum yield of the existing metal halide perovskites in their solid state have severely limited their practical applications. Here, a straightforward heterogeneous interfacial method to develop superior thermotolerant and highly emissive solid-state metal halide perovskites is reported and their use as long-lasting high-temperature and high-input-power durable solid-state light-emitting diodes is illustrated. It is found that the resultant materials can well maintain their superior quantum efficiency after heating at a temperature over 150 °C for up to 22 h. A white light-emitting diode (w-LED) constructed from the metal halide perovskite solid exhibits superior temperature sustainable lifetime over 1100 h. The w-LED also displays a highly durable high-power-driving capability, and its working current can go up to 300 mA. It is believed that such highly thermotolerant metal halide perovskites will unleash the possibility of a wide variety of high-power and high-temperature solid-state lighting, lasing, and display devices that have been limited by existing methods.

Characterization of the Interdependence Between the Light Output and Self-Heating of Gallium Nitride Light-Emitting Diodes

Abstract With the advent of gallium nitride (GaN) as an enabling material system for the solid-state lighting industry, high-power and high-brightness light-emitting diodes (LEDs) with wavelengths ranging from near ultraviolet to blue are being manufactured as part of a tremendously large and ever-increasing market. However, device self-heating and the environment temperature significantly deteriorate the LED's optical performance. Hence, it is important to accurately quantify the LED's temperature and correlate its impact on optical performance. In this work, three different characterization methods and thermal simulation were used to measure and calculate the temperature rise of an InGaN/GaN LED, as a result of self-heating. Nanoparticle-assisted Raman thermometry was used to measure the LED mesa surface temperature. A transient Raman thermometry technique was utilized to investigate the transient thermal response of the LED. It was found that under a 300 mW input power condition, self-heating is negligible for an input current pulse width of 1 ms or less. The temperature measured using nanoparticle-assisted Raman thermometry was compared with data obtained by using the forward voltage method (FVM) and infrared (IR) thermal microscopy. The IR and Raman measurement results were in close agreement whereas the data obtained from the widely accepted FVM underestimated the LED temperature by 5–10%. It was also observed that an increase in environment temperature from 25 °C to 100 °C would degrade the LED optical power output by 12%.

Monodisperse Bismuth-Halide Double Perovskite Nanocrystals Confined in Mesoporous Silica Templates

Metal halide perovskites have fascinating electronic properties and have already been implemented in various devices. Although the behavior of the properties of lead halide perovskite nanocrystals has been studied, the properties of lead-free perovskite nanocrystals are less well-understood because synthesizing them is still very challenging. Here, a simple and popularizable method has been demonstrated to grow monodisperse bismuth-halide double perovskite nanocrystals, Cs2AgBiBr6 (1), inside three kinds of mesoporous silica templates. The size and morphology of nanocrystals depend on the structure and pore size of the template. Structural analysis shows that the nanocrystals of various sizes and morphologies retain the crystal structure of bimetallic perovskite. 1 exhibits different morphologies in the silicon channels of three templates: square nanoparticles in KIT-6, spherical and rodlike particles in SBA-15, and nanowires in MCM-41. UV-vis-NIR and photoluminescence measurements show us the variation of band gap and carrier recombination time due to quantum confinement of nanocrystals in mesoporous silicon materials. The band gaps of nanocrystals in the template exhibit an obvious blue shift compared with that of the bulk sample, and the carrier recombination time is significantly shortened. We show that mesoporous silicon templates can be used to prepare lead-free perovskite nanocrystals, and the controllable preparation of nanocrystals can be achieved by the template's own characteristics. This provides a new idea for us to find new functional materials of lead-free metal halide solid-state light-emitting diodes.

Fluorescence microscope light source stability.

The process of fluorescence starts with the efficient generation of light that is required for the excitation of fluorophores. As such, light sources are a crucial component of a fluorescence microscope. Choosing the right illumination tool can not only improve the quality of experimental results, but also the microscope's economic and environmental footprint. While arc lamps have historically proven to be a reliable light source for widefield fluorescence microscopy, solid-state light-emitting diodes (LEDs) have become the light source of choice for new fluorescence microscopy systems. In this paper, we demonstrate that LEDs have superior light stability on all timescales tested and use less electrical power than traditional light sources when used at lower power outputs. They can be readily switched on and off electronically, have a longer lifetime and they do not contain mercury, and thus are better for the environment. We demonstrate that it is important to measure light source power output during warm-up and switching, as a light source's responsiveness (in terms of power) can be quite variable. Several general protocols for testing light source stability are presented. A detailed life cycle analysis shows that an LED light source can have a fourfold lower environmental impact when compared to a metal halide source.

Efficacy of lurasidone versus placebo in schizophrenia: a systematic review and meta-analysis of randomized controlled studies

Radiometry relates to the physical aspects of radiant energy and the impact on cultural heritage, i.e. degradation. Radiation is considered in terms of thermal emission from bodies (infrared radiation), in the visible band, or as ultraviolet radiation. All sources of light, either natural or artificial light, are considered. Solar light is related to astronomic motions of the sun and irradiation on vertical, horizontal, or tilted surfaces, at any orientation. Sunlight penetrating through windows and its changeable colour is also considered. Exhibition lighting, types of electrical light sources (e.g. incandescent, halogen, metal halides, fluorescent, solid-state light-emitting diode), and their use for exhibition and restoration purposes are discussed within the conservation and biological colonization. Finally, how to improve the quality of light using optical filters for lamps and windows and optical fibres is also discussed.The degradation mechanisms caused by light, including overheating and drying, colour fading, convective motions, surface deposition and blackening, biological colonization development, and control of algae in damp environments are also considered.

Highly Stable Silica-Wrapped Mn-Doped CsPbCl3 Quantum Dots for Bright White Light-Emitting Devices.

As an outstanding less-Pb candidate, doping Mn2+ ions into perovskite quantum dots (QDs) has received significant interest in the application of light-emitting diodes (LEDs). However, their further applications are impeded by poor chemical instability. Here, the silica-wrapped Mn-doped CsPbCl3 QDs are fabricated via hydrolyzing (3-aminopropyl) triethoxysilane with improved operational stability. Also, the photoluminescence quantum yield as high as 55.4% for the CsPbMnCl3@SiO2 composite is achieved. Silica wrapping can protect the perovskite QDs from damage by temperature and humidity as well as anion exchange. Furthermore, white LED devices are prepared by employing the mixture of green CsPbBr3 QDs and orange-red CsPbMnCl3@SiO2 composites. The as-obtained white LED device operated at a forward current of 20 mA exhibits bright natural light with a high luminous efficiency of 77.59 lm/W, and the corresponding color rendering index of 82 and color temperature (CCT) of 3950 K are obtained. Additionally, the electroluminescence spectrum shows nearly no variation after 24 h operation. This work will promote the Mn-doped CsPbCl3 QDs material to the practical application in solid-state LEDs.

Observation of Quantum Confinement in Monodisperse Methylammonium Lead Halide Perovskite Nanocrystals Embedded in Mesoporous Silica.

Hybrid organic-inorganic metal halide perovskites have fascinating electronic properties and have already been implemented in various devices. Although the behavior of bulk metal halide perovskites has been widely studied, the properties of perovskite nanocrystals are less well-understood because synthesizing them is still very challenging, in part because of stability. Here we demonstrate a simple and versatile method to grow monodisperse CH3NH3PbBrxIx-3 perovskite nanocrystals inside mesoporous silica templates. The size of the nanocrystal is governed by the pore size of the templates (3.3, 3.7, 4.2, 6.2, and 7.1 nm). In-depth structural analysis shows that the nanocrystals maintain the perovskite crystal structure, but it is slightly distorted. Quantum confinement was observed by tuning the size of the particles via the template. This approach provides an additional route to tune the optical bandgap of the nanocrystal. The level of quantum confinement was modeled taking into account the dimensions of the rod-shaped nanocrystals and their close packing inside the channels of the template. Photoluminescence measurements on CH3NH3PbBr clearly show a shift from green to blue as the pore size is decreased. Synthesizing perovskite nanostructures in templates improves their stability and enables tunable electronic properties via quantum confinement. These structures may be useful as reference materials for comparison with other perovskites, or as functional materials in all solid-state light-emitting diodes.

Multicolor fluorescent light-emitting diodes based on cesium lead halide perovskite quantum dots

High quantum yield, narrow full width at half-maximum and tunable emission color of perovskite quantum dots (QDs) make this kind of material good prospects for light-emitting diodes (LEDs). However, the relatively poor stability under high temperature and air condition limits the device performance. To overcome this issue, the liquid-type packaging structure in combination with blue LED chip was employed to fabricate the fluorescent perovskite quantum dot-based LEDs. A variety of monochromatic LEDs with green, yellow, reddish-orange, and red emission were fabricated by utilizing the inorganic cesium lead halide perovskite quantum dots as the color-conversion layer, which exhibited the narrow full width at half-maximum (<35 nm), the relatively high luminous efficiency (reaching 75.5 lm/W), and the relatively high external quantum efficiency (14.6%), making it the best-performing perovskite LEDs so far. Compared to the solid state LED device, the liquid-type LED devices exhibited excellent color stability against the various working currents. Furthermore, we demonstrated the potential prospects of all-inorganic perovskite QDs for the liquid-type warm white LEDs.

TCAD design of InGaN-based monolithic multi-wavelength LED with controlled Power spectral distributions

Voltage-controlled tricolor novel solid state LED has been presented in this paper. The monolithic structure of indium gallium nitride (InGaN)-based double Quantum Well (DQW) light-emitting diode (LED) has been designed and simulated with SILVACO Technology Computer Aided Design (TCAD). Each quantum well and decoupling region is carefully designed to control illumination colors with applied potentials. Electron–hole recombination and illumination in each quantum well dominate at specific applied potentials. The spectral power distribution, electrons and holes concentrations in the active regions, energy band diagrams and power conversion efficiency for the solid-state lighting (SSL) structure are investigated. Effects of doping concentration in decoupling region, indium content in the InGaN alloy and active layers thicknesses, on the emission spectra are also discussed. The simulated results indicate that an LED with an optimized structure can be produced from blue to cyan and then green light emissions with voltage tuning.

Light: An underestimated environmental factor in sleep medicine

Sleep researchers and clinicians working in sleep medicine know that the environmental light-dark cycle is crucial for entraining human circadian rhythms to the solar day. Despite this knowledge, the importance of light-dark exposure is still fairly underestimated. In fact, if the sleep-wake cycle of their study participants or patients shows no major deviation from the 24-h day, as indexed by sleep logs or actigraphy, they commonly assume normal circadian entrainment and thus no aberrant light-dark exposure. Only a few sleep clinicians measure the dim light melatonin onset (DLMO), a reliable circadian marker, to actually prove whether their “circadian diagnosis” is correct. In fact, DLMO should become common clinical practice for quantifying circadian phase position in every sleep-disturbed person. Many people working in the sleep field are not aware about light’s non-circadian action. Besides photic resetting of the circadian clock, artificial light striking the retina between dusk and dawn directly or indirectly impacts on neurons regulating sleep-wake cycles, by inhibiting sleep-promoting neurons and activating arousal-promoting orexin neurons in the hypothalamus, and by suppressing the release of the soporific hormone melatonin at night. These effects can outlast the duration of light exposure without necessarily affecting circadian phase. There is a clear doseand wavelength dependency with a strong “blue-shift” most probably also involving the novel photoreceptor pigment melanopsin as a mediator of the direct effects of light in humans. Blue-enriched light as low as at 40 lux, and light from solid-state light-emitting diodes (LED)-backlit computer screens in the evening is strong enough to elicit significant alerting responses as indexed by subjective and objective correlates of sleepiness. Furthermore, subjective well-being and performance in different cognitive domains are enhanced during a 2-hour evening exposure to the above-mentioned light sources and affects subsequent sleep. Energy efficient LEDs are now widely used in televisions and computer screens, laptops, tablets and hand-held devices, and used very often between dusk and dawn, particularly in the late evening. Checking messages etc. on their hand-held devices before going to sleep has become common practice in teenagers, but also in adults, with the consequence of reducing sleepiness, increasing alertness and interfering with sleep. Thus, “light hygiene” should be become an integral part of sleep hygiene. In fact, “light hygiene” could advise us on how much daily natural light-dark exposure we need, and, equally important, on what we “should” and “should not” do when using artificial light, thus enhancing its pros and minimizing its cons on alertness, performance and sleep. The discoveries of light’s repercussions on the brain’s sleep and wake-promoting regions, along with the recent advances in solid-state LED technology, will help to design and implement potentially successful novel light devices and light exposure schedules for at home and in the work place environment to promote beneficial effects of light beyond vision. bs_bs_banner

The effects of LED illumination spectra and intensity on carotenoid content in Brassicaceae microgreens

The objective of this study was to evaluate the effects of irradiance levels and spectra produced by solid-state light-emitting diodes (LEDs) on carotenoid content and composition changes in Brassicaceae microgreens. A system of five high-power, solid-state lighting modules with standard 447-, 638-, 665-, and 731-nm LEDs was used in the experiments. Two experiments were performed: (1) evaluation of LED irradiance levels of 545, 440, 330, 220, and 110μmolm−2s−1 photosynthetically active flux density (PPFD) and (2) evaluation of the effects of 520-, 595-, and 622-nm LEDs supplemental to the standard set of LEDs. Concentrations of various carotenoids in red pak choi and tatsoi were higher under illumination of 330–440μmolm−2s−1 and at 110–220μmolm−2s−1 in mustard. All supplemental wavelengths increased total carotenoid content in mustard but decreased it in red pak choi. Carotenoid content increased in tatsoi under supplemental yellow light.

Analyzing Thermal Module Developments and Trends in High-Power LED

The solid-state light emitting diode (SSLED) has been verified as consumer-electronic products and attracts attention to indoor and outdoor lighting lamp, which has a great benefit in saving energy and environmental protection. However, LED junction temperature will influence the luminous efficiency, spectral color, life cycle, and stability. This study utilizes thermal performance experiments with the illumination-analysis method and window program (vapour chamber thermal module, VCTM V1.0) to investigate and analyze the high-power LED (Hi-LED) lighting thermal module, in order to achieve the best solution of the fin parameters under the natural convection. The computing core of the VCTM program employs the theoretical thermal resistance analytical approach with iterative convergence stated in this study to obtain a numerical solution. Results showed that the best geometry of thermal module is 4.4 mm fin thickness, 9.4 mm fin pitch, and 37 mm fin height with the LED junction temperature of 58.8°C. And the experimental thermal resistances are in good agreement with the theoretical thermal resistances; calculating error between measured data and simulation results is no more than ±7%. Thus, the Hi-LED illumination lamp has high life cycle and reliability.

LED irradiance level affects growth and nutritional quality of Brassica microgreens

AbstractThis study examines the effect of irradiance level produced by solid-state light-emitting diodes (LEDs) on the growth, nutritional quality and antioxidant properties of Brassicaceae family microgreens. Kohlrabi (Brassica oleracea var. gongylodes, ‘Delicacy Purple’) mustard (Brassica juncea L., ‘Red Lion’), red pak choi (Brassica rapa var. chinensis, ‘Rubi F1’) and tatsoi (Brassica rapa var. rosularis) were grown using peat substrate in controlled-environment chambers until harvest time (10 days, 21/17°C, 16 h). A system of five lighting modules with 455, 638, 665 and 731 nm LEDs at a total photosynthetic photon flux densities (PPFD) of 545, 440, 330, 220 and 110 µmol m−2s−1 respectively were used. Insufficient levels of photosynthetically active photon flux (110 µmol m−2 s−1) suppressed normal growth and diminished the nutritional value of the Brassica microgreens studied. In general, the most suitable conditions for growth and nutritional quality of the microgreens was 330–440 µmol m−2 s−1 irradiation, which resulted in a larger leaf surface area, lower content of nitrates and higher total anthocyanins, total phenols and 2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical scavenging capacity. High light levels (545 µmol m−2 s−1), which was expected to induce mild photostress, had no significant positive impact for most of investigated parameters.