High-efficiency White Light-emitting Diodes Research Articles

Vacancy-ordered Te4+-doped Rb2ZrCl6 double perovskite microcrystals for solid-state lighting and non-contact optical thermometry

In this work, we prepared the highly efficient yellow light-emitting vacancy-ordered Te4+-doped Rb2ZrCl6 microcrystals (RZCTs) through a hydrothermal method. RZCT with a photoluminescence quantum yield of 34.6% was prepared at Te4+ feeding ratio of 5%. RZCT is used to fabricate high-efficiency white light-emitting diodes, and adjusting the current can realize the transition of cold and warm white light with CIE color coordinates of (0.31, 0.28) and (0.36, 0.37), respectively. It is also applied to non-contact thermometry using photoluminescence lifetime, of which maximum relative and absolute sensitivities were obtained as 0.89% and 4.76 × 10−3 K−1, respectively. The study shows that the lead-free Te4+-doped perovskite microcrystals exist great potential in next-generation solid-state lighting and non-contact optical thermometry.

Liquid Diffused Separation Induced Crystallization to efficiently grow centimeter-sized diamond-like Cs4PbBr6 crystals for light-emitting applications

Large-size single crystal (SCs) is an excellent platform for studying the optoelectronic properties of materials. However, growing high-quality crystals has always been a challenge. Here we develop a liquid-induced diffusion (LDSC) method for high-efficiency and low-cost growth of centimeter-sized Cs4PbBr6 crystals. All-sized crystals with a special diamond-like appearance exhibit strong green photoluminescence (PL) and good stability. In addition, X-ray diffraction (XRD) reveals that the diamond-like Cs4PbBr6 crystal exhibits a different crystal surface orientation from the rhombic SCs. More importantly, different from the CsPbBr3 impurity-induced photoluminescence mechanism, the strong green PL of the as-prepared diamond-like Cs4PbBr6 crystal originates from its defect state: Br vacancies (VBr). Furthermore, high-efficiency white light-emitting diodes (WLED) devices are prepared by using high-luminous intensity Cs4PbBr6 crystals as green light emitters. The optimized devices generate high-quality white light with luminous efficiency of ∼54 lm W−1, which is much better than that of conventional perovskite nanocrystals-based LED devices. Our work demonstrates that the Cs4PbBr6 crystal devices possess higher luminous efficiency, better stability and great application potential in display backlighting.

Stable and large-scale organic-inorganic halide perovskite nanocrystal/polymer nanofiber films prepared via a green in situ fiber spinning chemistry method.

Organic-inorganic halide perovskite nanocrystals (PNCs) have shown great advantages in recent years due to their tunable emission wavelengths, narrow full-width at half-maximum (FWHM) and high photoluminescence quantum yield (PLQY). However, PNCs still face the challenges of poor stability, difficulty in processing and generation of heavy metal wastes; therefore, it is necessary to develop a green synthetic method to prepare PNCs. Here, we present for the first time a facile fiber spinning chemistry (FSC) method for the rapid preparation of organic-inorganic halide PAN/MAPbX3 (MA = CH3NH3, X = Cl, Br and I) nanofiber films at room temperature. The FSC process utilizes spinning fibers as the reactor, and polymer solidification and the in situ generation of PNCs occur simultaneously with solvent evaporation during the spinning process. This method not only achieves a continuous large-scale preparation of PNC/polymer nanofiber films but also avoids the generation of heavy metal waste. The organic-inorganic halide PAN/MAPbX3 nanofiber films fabricated by FSC demonstrated tunable emission in the range of 464-612 nm and PLQY of up to 58%, and the fluorescence intensity remained essentially unchanged after 90 days of storage in the atmospheric environment. Interestingly, we successfully prepared high-efficiency white light-emitting diodes (WLEDs) and wide color gamut liquid crystal displays (LCDs) with a color gamut of 116.1% using PAN/MAPbBr3 nanofiber films as fluorescence conversion materials. This study provides a novel way to construct high-performance PNC/polymer fiber composites on a large scale.

One-Step Green Solvothermal Synthesis of Full-Color Carbon Quantum Dots Based on a Doping Strategy

Proposing a simple strategy for developing full-color carbon quantum dots (CQDs) and exploring how the luminescence can be tuned and improved is attractive and encouraging. Herein, blue, green, yellow-green, and orange-red CQDs doped with heteroatoms were synthesized in one pot and separated by column chromatography, with emission peaks of 435 nm, 495 nm [photoluminescence quantum yield (PLQY) of 88.9%], 525 nm, and 595 nm (full width at half-maximum of 31 nm), respectively. The abundant C-O/C-O-C electron donor groups greatly improve the PLQY of green CQDs, and the expended effective conjugated domains (particle size, doped chlorine, and conjugated nitrogen) of CQDs boost the red-shifts of emission spectra. Energy transfer (ET) in a concentrated mixed solution of CQDs was discovered, and possible ET mechanisms are proposed. Furthermore, a high-efficiency white light-emitting diode with Commission Internationale de L'Eclairage coordinates of (0.361, 0.369), a correlated color temperature of 4534 K, and a high color rendering index of 90.8 was fabricated.

Choice of low thermal quenching phosphors based on high lattice energy for light-emitting application

High efficient white light-emitting diodes, near-ultraviolet chips with trichromatic phosphors, have been widely studied in recent years. Herein, we report novel blue-/red-emitting Bi3+/Eu3+ doped Ca3Sc2Si3O12 (CSSO) phosphors with superior thermal stability. Comparing to traditional fluorescent materials Y3Al5O12 (YAG) and Y2O3, photoluminescence (PL) emission intensities of Bi3+ or Eu3+ doped CSSO phosphors maintain 92.4% and 98.6% of initial intensity at 300 K when measured at 423 K. More importantly, it is found that the lattice energy (Ucal) of host can determine thermal quenching behavior of phosphors. By chemical bond theory for complex crystals, the Ucalof CSSO, YAG and Y2O3 are calculated to be 143,060, 58,331 and 12,484 kJ/mol, indicating that CSSO with weak thermal quenching property is due to high Ucal. Subsequently, superior thermal stability of CSSO phosphors is further certified by fingerprints identification and anti-counterfeiting ink at high temperature. This work clearly exhibits a new and convenient strategy to predict the luminescent phosphors with good thermal stability before doing experiments.

Tunable dual emission in Mn2+-doped CsPbX3 (X = Cl, Br) quantum dots for high efficiency white light-emitting diodes

Doping of Mn2+ into semiconductor nanocrystals has been demonstrated to endow them with novel electronic, optical and magnetic functionalities. In this paper, Mn-doped CsPbX3 (X = Br, Cl) quantum dots (QDs) were synthesized at room temperature via a facile strategy by introducing dimethyl sulfoxide (DMSO)-MnBr2/PbX2 composite as a precursor. The excitonic emission spectra of the as-obtained Mn-doped CsPbX3 QDs can be tuned from 517 nm to 418 nm by adjusting the ratio of PbBr2/PbCl2 precisely, and the luminescence mechanism of the doped QDs is discussed in detail. Moreover, the highest photoluminescence quantum yield of the Mn2+ emission achieves 36.7%, which is comparable with QDs prepared by the conventional hot-injection method. Depending on the ratios of PbPb2/PbCl2, the energy transfer rate from the band-edge to Mn2+ excited state is in the range of 0.006–20.42 × 107 s−1. Furthermore, white light-emitting diodes (LEDs) were successfully fabricated by combining the as-prepared Mn-doped CsPbX3 QDs with commercial UV GaN chips, and the high luminous efficiency of the as-prepared white LEDs was developed to 55.9 lm W−1. This work strongly supports the fact that Mn-doped CsPbX3 QDs are promising materials for application in lighting and displaying fields.

Spatial-Coded Phosphor Coating for High-Efficiency White LEDs

In this paper, a new structure of spatial-coded coating phosphor is proposed and effectively benefits the efficiency enhancement in the white light-emitting diodes (LEDs). The spatial-coded phosphor plate contains two subregions. One subregion is of the yellow phosphor for providing the greatest extracted yellow light, and the other subregion is the open windows for preventing the blue light from the absorption by the yellow phosphor and the Stoke loss in the wavelength conversion. In the design process, the thickness and the concentration of the phosphor and photon recycling in a high-reflectivity cavity are all simulated and optimized. Finally, the predicting spectra of the resultant white LEDs with the spatial-coded phosphors by the simulation model are all in very good agreement with the corresponding spectra from the practical measurements. Moreover, as the correlated color temperature (CCT) is around 6500 K, the maximal packaging efficiency of the white LEDs with the spatial-coded phosphors is as high as 68.0% in the measurement, and the corresponding simulation is 71.8%. It is much higher even than the optimal packaging efficiency of the conventional conformal or remote-dome phosphor packaging structure.

Two in one: switchable ion conductivity and white light emission integrated in an iodoplumbate-based twin chain hybrid crystal.

An iodoplumbate-based hybrid, [C7-Apy][PbI3] (1), where C7-Apy(+) = 1-heptyl-4-aminopyridinium, was prepared using a simple solution process. Three sequential phase transitions occur in the range of 402-443 K. In both the lowest and highest temperature phases, hybrid crystal 1 is composed of discrete [Pb2I6]∞ twin chains surrounded by C7-Apy(+) cations. The connectivity between PbI6 octahedra within a [Pb2I6]∞ twin chain and the arrangement of cations are quite difference between the lowest and highest temperature phases. Hybrid crystal 1 shows switchable ion conductivity due to the structural phase transition and white light emission attributed to the broad band semiconductor emission of the twin chain. The former functionality has potential application in ion conductor devices; the single-phase white light emitter is a useful material in low-cost, easily-made, high-efficiency white light-emitting diodes.

Self-Adaptive Structure of Remote Phosphor for Phosphor-Converted White LEDs

High-efficiency white light-emitting diodes (LEDS) with superior spatial color homogeneity (SCH) have been investigated. In this paper, a new method of packaging of phosphor-converted white LEDs (pc-WLEDs) that employs a self-exposure technique as well as a remote phosphor configuration is presented. It is suggested that this remote phosphor configuration reduces the backscattering of emission light of LEDs as well as the operating temperature of the phosphor, which improve the luminous efficiency and reliability of pc-WLEDs. The slurry method with self-exposure process was applied to achieve a self-adaptive remote phosphor coating layer surrounding the LED chips which could map the angular intensity distribution of blue light emitted from LED chips every direction and therefore improve the spatial color homogeneity. The experimental results show that pc-WLEDs with self-adaptive remote phosphor coating has remarkable advantages over conventional dispensing coating and conformal coating in terms of efficiency and color homogeneity.

Transparent Electrodes Based on Three-Dimensional Nanohelixes Light Harvesting Applications

We present transparent electrodes based on 3-dimensional nanohelix array for light harvesting applications. Arrays of nanohelixes fabricated by electron-beam oblique-angle deposition (OAD) of various metal oxides including TiO2, WO3 and ITO have been used for improving the performance of dye-sensitized solar cells (DSSCs), Photoelectrochemical (PEC) cells for water splitting, and organic photovoltaics (OPVs), respectively. Such light harvesting devices with transparent electrodes based on metal-oxides nanohelixes array show enhanced performances such as higher light absorption and power conversion efficiency than devices with conventional thin-film and sintered nanoparticle electrodes. This is attributed to the unique optical, microstructural, and electrical properties of metal-oxides nanohelixes such as enhanced light scattering and large specific area enabled by 3-dimensional geometry, and improved separation and transportation of photo-generated carriers by the near-single-crystallinity of metal-oxides nanohelixes. In addition, a number of applications including dichroic filters for high efficiency white light-emitting diodes, near-perfect anti-reflection coatings for solar cells, and high performance chemical sensors will be discussed.

High-Efficiency White LED Packaging With Reduced Phosphor Concentration

In this letter, important factors for white light emitting diodes (LED) packaging, including lens encapsulation, reflectivity of cavity reflectors, and phosphor concentration are studied. Simulation as well as the corresponding experiment shows that reflectivity of the reflectors in packaging cavity is more effective than lens encapsulation to reach high efficiency. Besides, a white LED packaging without lens encapsulation but with high reflectivity of cavity reflectors may increase interaction possibility between blue photons and the phosphor so that the phosphor concentration can be reduced to meet the requested correlated color temperature.

Patterned structure of REMOTE PHOSPHOR for phosphor-converted white LEDs

High efficiency white light-emitting diodes with superior color-mixing have been investigated. It is suggested that the patterned remote phosphor structure could improve the uniformity of angular-dependent correlated color temperature (CCT) and achieve high chromatic stability in wider operating current range, as compared to the conventional remote phosphor coating structure. In this experiment, we employed a pulse spray coating method to place the patterned phosphor on the package and to leave a window region. The window area, a clear space without coating of the phosphor not only increases the extraction efficiency of blue rays at large angle, but also improves the stability of angular-dependent CCT. Moreover, the CCT deviation could be reduced from 1320 K to 266 K by this patterned remote phosphor method, and the stray blue/yellow light within the package can be effectively reduced and controlled. The design was verified both experimentally and theoretically.

Current Development and Patents on High-Brightness White LED for Illumination

In this paper, we reviewed the current development and patents for the application of high-brightness and high-efficiency white light-emitting diode (LED). The high-efficiency GaN nanostructures, such as disk, pyramid, and rod were grown on LiAlO(2) substrate by plasma-assisted molecular-beam epitaxy, and a model was developed to demonstrate the growth of the GaN nanostructures. Based on the results, the GaN disk p-n junction was designed for the application of high brightness and high efficiency white LED.

Recent progress of high efficiency white LEDs

AbstractWe fabricated three types of white light emitting diodes (LEDs). The first is the white LED, which has a high general color rendering index (Ra) of 97 and CRI‐No. 9 of 96. The CRI‐No. 9 denotes the color reproduction in the red region. These values are higher than those of a tri‐phosphor fluorescent lamp (Ra = 85 and CRI‐No. 9 = 8). The second is the high efficiency white LED fabricated from the small‐size high efficiency blue LED chip. The output power (Po), the external quantum efficiency (ηex) and wall‐plug efficiency (WPE) of the small‐size blue LED were 35.0 mW, 63.3% and 56.3%, respectively, at a forward‐bias current of 20 mA. The luminous flux (Φ), luminous efficiency (ηL) and WPE of the second white LED are 8.6 lm, 138 lm/W and 41.7%, respectively. The luminous efficiency is 1.5 times greater than that of a tri‐phosphor fluorescent lamp (90 lm/W). The third is the high power white LED fabricated from the larger‐size blue LED chip. Po, ηex and W.P.E. are 458 mW, 47.2% and 39.7%, respectively, at 350 mA. Φ, ηL and WPE of the third white LED are 106 lm, 91.7 lm/W and 27.7% at 350 mA, respectively. Moreover, Φ of 247 lm and 402 lm at 1 A and 2 A are obtained, respectively. Φ at 2 A is equivalent to the total flux of a 30 W incandescent lamp. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)