Solid-state White Light Sources Research Articles

Biocompatible Perovskite Nanocrystals with Enhanced Stability for White Light-Emitting Diodes.

Recently emerged lead halide perovskite CsPbX3 (X = Cl, Br, and I) nanocrystals (PNCs) have attracted tremendous attention due to their excellent optical properties. However, the poor water stability, unsatisfactory luminescence efficiency, disappointing lead leakage, and toxicity have restricted their practical applications in photoelectronics and biomedical fields. Herein, a controllable encapsulated strategy is investigated to realize CsPbX3 PNCs/PVP @PMMA composites with superior luminescence properties and excellent biocompatibility. Additionally, the synthesized CsPbBr3 and CsPbBr0.6I2.4 PNCs/PVP@PMMA structures exhibit green and red emissions with a maximal photoluminescence quantum yield (PLQY) of about 70.24% and 98.26%, respectively. These CsPbX3 PNCs/PVP@PMMA structures show high emission efficiency, excellent stability after water storage for 18 months, and low cytotoxicity at the PNC concentration at 500 μg mL-1. Moreover, white light-emitting diode (WLED) devices based on mixtures of CsPbBr3 and CsPbBr0.6I2.4 PNCs/PVP@PMMA perovskite structures are investigated, which exhibit excellent warm-white light emissions at room temperature. A flexible manipulation method is used to fabricate the white light emitters based on these perovskite composites, providing a fantastic platform for fabricating solid-state white light sources and full-color displays.

Room-Temperature Broad-Wavelength-Tunable Single-Mode Lasing from Alloyed CdS1-xSex Nanotripods.

Wavelength-tunable semiconductor nanolasers have attracted tremendous attention for their tunable emissions and robust stability, bringing possibilities for various applications, including nanophotonic circuits, solid-state white-light sources, wavelength-converted devices, and on-chip optical communications. Here, we report on the demonstration of broadband-tunable, single-mode nanolasers based on high-quality alloyed single crystalline CdS1-xSex (x = 0-1) nanotripods with well-formed facets fabricated using a conventional CVD approach. Microstructural characterization and optical investigations reveal that these structures are crystalline with composition-tunable CdS1-xSex alloys. Microphotoluminescence spectra and mapping of these nanotripods exhibit emissions with continuous wavelengths from 509 to 712 nm, further demonstrating that the CdS1-xSex alloys have tunable bandgaps due to the composition gradient. Additionally, under a pulse laser illumination, room-temperature single-mode lasing is clearly observed from these nanotripods cavities, which shows almost identical emission lines with a high-quality factor of ∼1231. More importantly, wavelength continuously tunable nanolasers from 520 to 738 nm are successfully constructed using these bandgap gradient nanotripods. The capability to fabricate high-quality tunable nanolasers represents a significant step toward high-integration optical circuits and photonics communications.

Enhancement of Laser Multiplexing Efficiency for Solid State Lighting Through Photon Recycling

An optical design of a novel structure has been proposed and demonstrated to perform photon recycling in a transmission-type laser-pumping white light. The structure includes a hemisphere reflector and a phosphor plate located at the center of a hemisphere reflector. The hemisphere reflector is not only used for photon recycling to increase the energy efficiency but also to keep the energy efficiency nearly unchanged when the number of the multiplexing laser increases. The experimental measurement shows that the photon recycling with the hemisphere reflector can increase the energy efficiency from 36.2% to 53.3% for on-axis incidence, and 35.2% to 55.7% for off-axis incidence. Besides, the efficiency keeps the same up to three-laser multiplexing in the experiment, and potentially can be extended to more than 10 in the simulation for various hole structures. The result is not only on the efficiency enhancement but also on the enhanced exitance of the laser white light and the unchanged etendue of the light source. This property is an important advantage for lasers rather than LEDs in a solid-state white light source.

TemperatureDependent Luminescence of RedEmitting Ba2Y5B5O17: Eu3+ Phosphors with Efficiencies Close to Unity for NearUV LEDs.

Solid state white light sources based on a near-UV LED chip are gaining more and more attention. This is due to the increasing efficiency of near-UV-emitting LED chips and wider phosphors selection if compared to devices based on blue LED chips. Here, a brief overview is given of the concepts of generating white light employing near-UV LED and some optical properties of the available phosphors are discussed. Finally, the synthesis and optical properties of very efficient red-emitting Ba2Y5B5O17:Eu3+ phosphor powder and ceramics is reported and discussed in terms of possible application as a red component in near-UV LED-based white light sources.

Holistic opto-thermal simulation framework for high-brightness light sources based on fluorescent conversion

Multi-physics approaches are increasingly adopted in the development of efficient, high brightness solid-state light sources, in particular for the realistic modelling of the fluorescent colour conversion element that is typically used to create white light. When a fluorescent material is excited by a high-power laser diode, it will self-heat and reach high temperatures. The efficiency or quantum yield of fluorescent materials lowers as their temperature increases, an effect called thermal quenching. The lower efficiency further increases the amount of phosphor self-heating which can lead to thermal runaway. This effect has been considered by different researchers when modelling the opto-thermal behaviour of the fluorescent colour conversion elements. However, other key fluorescent properties such as the absorption and emission spectrum also depend on temperature, and often also on the radiant flux density. This gives rise to a complex set of interplays between optical and thermal properties which are not considered in the current opto-thermal models but that significantly influence the performance of fluorescent material based solid-state light sources. In this work, we present a holistic opto-thermal simulation framework: a novel comprehensive simulation tool that includes all relevant multi-physics considerations. We show that the framework allows for an accurate and realistic prediction of the performance of high-luminance solid-state white light sources by comparing simulation results to experimentalmeasurements of a laser-based configuration, thereby validating the framework.

Broadband White-Light Emission from Alumina Nitride Bulk Single Crystals

Alumina nitride bulk single crystals (AlN BSCs) were grown using a two-heater Physical Vapor Transport (th-PVT) method. The crystal contains massive lattice defects including aluminum vacancy (VAl) and oxygen substitution (ON). The photoluminescence (PL) spectrum of the crystal demonstrated a broad emission covering from 250 to 1000 nm. By study the PL spectrum, abundant midgap states in the wide band gap of AlN were nailed down. Based on the crystals, metal-AlN-metal Schottky devices were fabricated. These devices emitted high quality white light with color rendering index (CRI) over 90 under a bias of 60 V. Moreover, it was found that the white light emitting property of AlN BSCs was adjustable through changing impurity density and device structure. This research aims to pave a new way for solid-state white light source.

Nearly white light photoluminescence from ZnO/rGO nanocomposite prepared by a one-step hydrothermal method

A zinc oxide (ZnO)/reduced graphene oxide (rGO) nanocomposite was prepared by a facile one-step hydrothermal method, i. e., the reduction from graphene oxide (GO) to rGO and the growing of ZnO nanocrystals on rGO were accomplished simultaneously. ZnO/rGO nanocomposite was proved to be made of flower-like ZnO nanocrystals enwrapped by rGO voiles. Under the excitation of ultraviolet light, the photoluminescence (PL) from ZnO/rGO exhibits a broad emission band spanned from ∼350 nm to ∼800 nm, and as an integral a nearly white light PL with a chromaticity coordinate of (∼0.25, ∼0.23) was obtained. Through Gaussian fitting process, the broad PL band was decomposed into six PL peaks, which were attributed to the radiative recombinations from ZnO and rGO, respectively. An analysis on the PL mechanism verified that the synergistic effect between the two constituent parts plays an important role in modulating the PL peak positions and intensities through enhancing or restraining the transport and transition process. Our experiments might provide an alternative path for preparing high-quality solid-state white light sources using ZnO/rGO as ultraviolet-activated phosphors.

Laser-Scanned Programmable Color Temperature of Electroluminescence from White Light-Emitting Electrochemical Cells.

Recently, the control of correlated color temperature (CCT) of artificial solid-state white-light sources starts to attract more attention since CTs affect human physiology and health profoundly. In this work, we proposed and demonstrated a method that can widely tune the CCTs of electroluminescence (EL) from white-light-emitting electrochemical cells (LECs) by employing plasmonic filters. These integrated on-chip plasmonic filters are composed of semicontinuous thin Ag film or Ag nanoparticles (NPs) both included in the indium tin oxide anode contact, which have different characteristics of plasmonic resonant absorptions that can tune the EL spectra of white LECs. The CCTs of EL from white LECs integrated with semicontinuous thin Ag film and randomly distributed Ag NPs are 5778 and 2350 K, respectively. A commercially available laser scanning system was used to locally thermal anneal the semicontinuous thin Ag film to form the randomly distributed Ag NPs on the scanned areas. Hence, these two kinds of filters can be integrated on the same chip of white LEC, giving more freedom to control the CCTs of white EL and more potential applications. In addition, the laser scanning system used here is quite often used in display manufactures so that our proposed method can be immediately adopted by the light-emitting diode industry.

Tunable color temperature solid state white light source using flux grown phosphor crystals of Eu3+, Dy3+ and Tb3+ activated calcium sodium molybdenum oxide

Solid state light sources with dynamically tunable color temperature in the range of 3000–6000K with chromaticity coordinates lying on the Planckian black body curve has been designed using mixtures of narrow emissions at 615nm, 575nm and 550nm. These respective emissions lines were generated by individual phosphor crystals of trivalent rare earth (RE3+) species, europium (Eu3+), dysprosium (Dy3+) and terbium (Tb3+) activated calcium sodium molybdenum oxide (Ca1−2xNaxMoO4:RE3+x), when excited by near-ultra-violet (NUV) light emitting diode (LED) with emission wavelength of 380nm. Highly luminescent crystals of these compounds have been grown from molten solutions (flux) of molybdenum (VI) oxide. The flux grown crystals exhibit emission intensity 2–4 times more than phosphor powders of the same compounds synthesized by traditional solid-state reactions. An optimum flux to solute ratio of 2.5 and solute dissolution temperature of 1100°C resulted in the largest size crystals.

Multiband photoluminescence from carbon nanoflakes synthesized by hot filament CVD: towards solid-state white light sources

Carbon nanoflakes (CNFLs) are synthesized on silicon substrates deposited with carbon islands in a methane environment using hot filament chemical vapor deposition.

Efficient and stable laser-driven white lighting

Laser-based white lighting offers a viable option as an efficient and color-stable high-power solid-state white light source. We show that white light generation is possible using blue or near-UV laser diodes in combination with yellow-emitting cerium-substituted yttrium aluminum garnet (YAG:Ce) or a mixture of red-, green-, and blue-emitting phosphors. A variety of correlated color temperatures (CCT) are achieved, ranging from cool white light with a CCT of 4400 K using a blue laser diode to a warm white light with a CCT of 2700 K using a near-UV laser diode, with respective color rendering indices of 57 and 95. The luminous flux of these devices are measured to be 252 lm and 53 lm with luminous efficacies of 76 lm/W and 19 lm/W, respectively. An estimation of the maximum efficacy of a device comprising a blue laser diode in combination with YAG:Ce is calculated and the results are used to optimize the device.

LED-based metameric light sources: Rendering the colours of objects and other colour quality criteria

Solid-state white light sources based on multiple light emitting diodes (LEDs) have three features that make them radically different from other sources of electrically generated light. These are the narrowband spectra of the individual LEDs, the possibility of controlling the light output of the individual LEDs and the limited stability of the LED’s colour. These features make it possible to achieve a set of solid-state white light sources that are metameric to a selected reference light source. A computational method to optimise the intensities of narrowband LEDs to obtain these metameric light sources is proposed. The appearance of object colours under these metameric light sources differs and can be used to satisfy users’ requests according to the various aspects of colour quality required, without altering the visual appearance of the light source.

Nano-Order Structural Analysis of White Light-Emitting Silicon Oxide Prepared by Successive Thermal Carbonization/Oxidation of the Porous Silicon

Recently the present authors’ group found that porous silicon showed strong and stable white/white-blue light emission after successive thermal carbonization and oxidation by water vapor. This material can be considered as a price-competitive solid-state white-light source. We examined these layers by electron energy-loss spectroscopy (EELS), energy-filtering transmission electron microscopy (EFTEM). The EEL spectra indicated that the silicon skeleton in the porous layer was completely oxidized by the thermal treatment in wet argon ambient and multi-types of carbon phases were present in the 1073 K oxidized sample of stronger emission, while carbon complexes including Si and/or O were formed in the 1223 K oxidized sample of weaker light emission. EF-TEM images showed that carbon/oxygen were more uniformly distributed in the 1223 K oxidized sample. It is assumed that the strong light-emitting properties are controlled by the size and internal chemical bonding states of carbon clusters incorporated.