Energy-level-dominated luminescence tuning of Mn2+-doped Cs3Cu2I5 perovskites for single-component white LEDs

Few researchers examined different red light amounts added in white light-emitting diodes (LEDs) with varied daily light integrals (DLIs) for hydroponic lettuce (Lactuca sativa L.). In this study, effects of DLI and LED light quality (LQ) on growth, nutritional quality, and energy use efficiency of hydroponic lettuce were investigated in a plant factory with artificial lighting (PFAL). Hydroponic lettuce plants (cv. Ziwei) were grown for 20 days under 20 combinations of five levels of DLIs at 5.04, 7.56, 10.08, 12.60, and 15.12 mol·m−2·d−1 and four LQs: two kinds of white LEDs with red to blue ratio (R:B ratio) of 0.9 and 1.8, and two white LEDs plus red chips with R:B ratio of 2.7 and 3.6, respectively. Results showed that leaf and root weights and power consumption based on fresh and dry weights increased linearly with increasing DLI, and light and electrical energy use efficiency (LUE and EUE) decreased linearly as DLI increased. However, no statistically significant differences were found in leaf fresh and dry weights and nitrate and vitamin C contents between DLI at 12.60 and 15.12 mol·m−2·d−1. Also, no effects of LQ on leaf dry weight of hydroponic lettuce were observed at a DLI of 5.04 mol·m−2·d−1. White plus red LEDs with an R:B ratio of 2.7 resulted in higher leaf fresh weight than the two white LEDs. LUE increased by more than 20% when red light fraction increased from 24.2% to 48.6%. In summary, white plus red LEDs with an R:B ratio of 2.7 at DLI at 12.60 mol·m−2·d−1 were recommended for commercial hydroponic lettuce (cv. Ziwei) production in PFALs.

Optically pumped white-light emitting diodes (WLEDs), including down conversion phosphors and blue/ultraviolet chips have attracted considerable attention in the solid-state lighting. However, commercial WLEDs contain massive rare-earth elements, which may suffer issues of unsustainability, potential price increasing due to insufficient supply. Thus, it is important to explore rare-earth-free light emitters with a broadband emission, a high photoluminescence quantum yield (PLQY) and an excellent stability. Recently, low-dimensional hybrid metal halides have received remarkable progress in WLEDs due to their high PLQY, ultra-broadband emission and easy synthetic procedures. In this review, the synthesis methods of low-dimensional hybrid metal halides are given followed by the discussions of their photoluminescence mechanisms. After that, low-dimensional hybrid metal halides with diversity colors including blue/blue-violet, green, yellow/orange, red/near-infrared are summarized. Specially, white light-emitting diodes based on low-dimensional metal hybrid halides will be reviewed. Finally, the perspective of the evolutions and challenges, the current limitations of the materials WLEDs are discussed, aiming to point of the inspirational outlook of their future development directions.

Blue-light-excitable broadband yellow-emitting CaGd2HfSc(AlO4)3:Ce3+ garnet phosphors for white light-emitting diode devices with improved color rendering index

Powder synthesis and luminescence of a novel yellow-emitting Ba5Si11Al7N25: Eu2+ phosphor discovered by a single-particle-diagnosis approach for warm w-LEDs

Recently, white light-emitting devices (WLEDs) based on halide perovskites has been extensively studied. However, the lead toxicity and poor stability of conventional lead halide perovskites severely hinder their commercial applications. In this study, lead-free double perovskite Cs2AgInCl6 with a broadband emission was fabricated by a heat-assisted solution evaporation method, in which a compositional engineering by sodium (Na+) alloying and bismuth (Bi3+) doping was performed. The photoluminescence quantum yield was promoted from ∼1.1 to 46.4% and then to 87.2% by Na+ alloying and subsequent Bi3+ doping. In addition, the theory calculation reveals that the diffusion barrier of Cl- vacancy in Cs2AgInCl6 can be increased by Na+ alloying, which would contribute to the stability of the material. Experimentally, the resulting Cs2Ag0.7Na0.3InCl6:Bi products demonstrate a remarkable stability under heat, ultraviolet light, and moisture conditions. The above advantages make it possible for this material to be used as solid-state phosphors for WLED applications, and the Commission International de I'Eclairage color coordinates at (0.38, 0.44), correlated color temperature of 4347 K, and high color rendering index of 87.8 were achieved. More importantly, the WLED demonstrates a remarkable operation stability in air ambient, and only 4.5% emission decay occurs after a long working time for 1000 h, the longest lifetime for perovskite-based WLEDs as far as we know.

Yttrium borate phosphor co-doping Ce3+, Tb3+ ions (YBO3: Ce3+, Tb3+) is fabricated using solid state reaction, and then its luminescence is investigated through the computational energy transfer process. Under excited near-UV light, this YBO3: Ce3+, Tb3+ phosphor exhibits strong absorption with broad and sharp emission bands due to the 4f – 5d and 5d – 4f transitions of Ce3+ ions and the 4f – 4f transition of Tb3+ ions, respectively. The phosphor's emission chromaticity could be tunable by adjusting the concentration of doping ions. With 15% Tb3+ and 3% Ce3+ in the composition, the phosphor can gain maximum 76.7% external quantum efficacy. The phosphor is proposed for utilization in the phosphor package of white light-emitting diodes (WLEDs) to enhance their lighting performances. The findings point out that by modifying YBO3: Ce3+, Tb3+ concentration (5% – 10%), improvements in luminous intensities, color consistency, and color rendering indices can be observed. The higher concentration (10%) of YBO3: Ce3+, Tb3+ is more advantageous to the luminous flux and chromatic uniformity in cases of 4000 K and 5000 K WLEDs, while lower (5%) concentration greatly benefits those properties in the case of 3000 K WLED. Regardless of CCTs, the WLEDs show a reduction in chromatic reproduction efficiency with the increasing concentration of YBO3: Ce3+, Tb3+. Hence, this green phosphor could be a good material for high-luminescence WLED, yet the modification of phosphor concentration is advisable if the simultaneous good chromaticity is desired.

Yellow phosphors of high quantum yields excitable by both ultraviolet and blue light with Gd3BWO9 as the host

Low-dimension perovskite materials have attracted wide attention due to their excellent optical properties and stability. Herein, Sb3+-doped Cs2ZrCl6 crystals are synthesized by a coprecipitation method in which Sb3+ ions partially replace Zr4+ ions. The Cs2ZrCl6:xSb3+ powder shows blue and orange-red emissions under a 254 and 365 nm light, respectively, due to the [ZrCl6]2- octahedron and [SbCl6]3- octahedron. The photoluminescence quantum yield (PLQY) of Cs2ZrCl6:xSb3+ (x = 0.1) crystals is up to 52.5%. According to experimental and computational results, the emission mechanism of the Cs2ZrCl6:xSb3+ crystals is proposed. On the one hand, a wide blue emission with a large Stokes shift is caused by the self-trapping excitons of [ZrCl6]2- octahedra under a 260 nm excitation. On the other hand, the luminescence mechanism of [SbCl6]3- octahedron is divided into two parts: 1P1 → 1S0 (490 nm) and 3P1 → 1S0 (625 nm). The broad-band emission, high PLQY, and excellent stability endow the Cs2ZrCl6:xSb3+ powders with the potential for the fabrication of white light-emitting diodes (WLEDs). A WLED device is fabricated using a commercial 310 nm NUV chip, which shows a high color rendering index of 89.7 and a correlated color temperature of 5333 K. In addition, the synthesized Cs2ZrCl6:xSb3+ crystals can be also successfully used for information encryption. Our work will provide a deep understanding of the photophysical properties of Sb3+-doped perovskites and facilitate the development of Cs2ZrCl6:xSb3+ crystals in encrypting multilevel optical codes and WLEDs.

In a line with most recent trends in developing non-toxic fluorescent nanomaterials, we combined blue emissive carbon dots with green and red emissive zinc copper indium sulfide (ZCIS) core/shell quantum dots (QDs) to achieve white light-emitting diodes (WLEDs) with a high color rendering index of 93. This indicates that ZCIS QDs, with their broad emission bands, can be employed to effectively make up the emission of carbon dots in the yellow and red regions to produce WLEDs in the wide region of color temperature by tuning the volume ratio of these constituting luminophores. Their electroluminescence characteristics including color rendering index, Commission Internationale de l'Eclairage (CIE) color coordinates, and color temperatures were evaluated as a function of forward current. The CIE-1931 chromaticity coordinates of the as-prepared WLEDs, exhibiting good stability, were slightly shifted from (0.321, 0.312) at 10 mA to (0.351, 0.322) at 30 mA, which was mainly caused by the different thermal quenching coefficients of carbon dots and ZCIS QDs.

Design of a broadband cyan-emitting phosphor with robust thermal stability for high-power WLED application

Recently, the investigation of quantum dots (QDs) as color converters for white light-emitting diodes (WLEDs) has attracted a significant amount of attention, because the emission wavelength and broad excitation band of QDs can be controlled by their particle sizes and compositions. However, owing to the thermal effect, the long-term stability of QDWLEDs is one of the important tasks for their application. Moreover, the commercial WLEDs have low color rendering index (CRI) and the reabsorption effect. Therefore, in order to increase the device stability and CRI, in this study, we have used easy one-pot process to synthesize the colloidal ternary Zn 0.8 Cd 0.2 S (ZnCdS) QDs with wide emission, a quantum yield (QY) of 42 % and a particle size of 3.2 ± 0.5 nm. Three methods have been applied to encapsulate the QDs and their average CRIs are 78. We have demonstrated that the QDs-based WLEDs can achieve color converter robust against photooxidation, thermal quenching, and scattering, and can be applied to fabricate reliable P-QD1 device with an air gap between the emission layers. This encapsulation setup can protect the color converter and maintain the best stability over 750 hours operation time, and the Commission International d’Eclairage (CIE) chromaticity coordinates and luminous efficacy can be tuned from (0.36, 0.40) to (0.32, 0.33) and 4.5 to 3.2, respectively.

The roles of zinc selenide sandwiched between organic layers and its applications in white light-emitting diodes

Bright and Blue Nanocrystal Emitters

White light emitting diodes (WLEDs) have attracted the attention of numerous scholars because of its high photoelectric conversion efficiency, low energy consumption and high reliability. They are known as the new generation of lighting energy in the twenty first century. Currently, the most common WLEDs are combined by blue LED chip and ce-doped yttrium aluminum garnet (YAG: Ce3+) yellow phosphors. The advantages are simple production, low cost and high luminous efficiency. However, WLEDs made of blue LED chip and yellow phosphors lacks red spectrum and exhibits luminescence, and its color rendering index (CRI) is low, usually only 70. At the same time, quantum dots (QDs) are considered to be the most potential materials for WLEDs color conversion layers due to their wide excitation spectrum, narrow emission spectrum, adjustable band gap and high quantum yield. In this paper, we fabricated green graphene QDs (GQDs) and red GQDs, measured and analyzed their morphology, structure and photoluminescence. The green GQDs, red GQDs and yellow phosphors were used to fabricate the WLEDs. The produced WLEDs showed excellent optical performance and working performance, among which Commission International de L 'Eclairage (CIE) chromaticity coordinates are (0.3332, 0.3359), and the correlated color temperature (CCT) is 5464 K (driving current is 30 mA). Electroluminescence (EL) spectra of the device at different currents were also measured.

The efficient broad-band emission from low-dimensional metal halides has garnered significant interest. However, most of these materials exhibit poor stability at the operating temperature of light-emitting diodes. In this study, using the solution method (temperature lower than 90 °C), a new compound (NH4)3In0.95Sb0.05Cl6 was obtained with the structure in the Pnma space group featuring unit-cell parameters of a = 12.3871(4) Å, b = 24.9895(9) Å, and c = 7.7844(3) Å. (NH4)3In0.95Sb0.05Cl6 can be prepared by doping (NH4)2InCl5·H2O when the Sb3+ feeding ratio is in the range of 30-80%. Thermal analysis reveals that (NH4)3In0.95Sb0.05Cl6 is stable up to 320 °C. (NH4)3In0.95Sb0.05Cl6 exhibits broad-band yellow-white emission with extremely high internal and external photoluminescence quantum yields of 93 and 77%, respectively. Interestingly, (NH4)3In0.95Sb0.05Cl6 displays remarkable resistance to thermal quenching, retaining 83% of its initial photoluminescence intensity at 80 °C. A white light-emitting diode is fabricated by combining (NH4)3In0.95Sb0.05Cl6 with a commercial phosphor, and a high color rendering of 92.8 was obtained. This work presents an environmentally friendly, efficient, stable UV-excited broad-band emission material for potential solid-state lighting applications.