Recent advances in solid-state LED phosphors with thermally stable luminescence

J. Rare. Earth., 2019, 37(6), 565-572. https://www.sciencedirect.com/science/article/abs/pii/S1002072118308536

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Phosphor-converted white light-emitting diode (LED) lighting has gained tremendous achievements since the invention of the InGaN blue LED by Nakamura et al., who won the Nobel Physics Prize in 2014. By far, a significant challenge comes from the thermal quenching (TQ) behavior of the present LED phosphors during the high-power LED operation or the updated laser lighting. But systematic research or review on the luminescence quenching character and/or how to realize thermally stable luminescence are lacking. Since TQ is an inherent property of phosphors, it can be diminished by different approaches. This review proceeds from the mechanism of TQ, summarizes previous researches on improving the thermal stability of LED phosphors and also discusses future research opportunities in this field. The developments of the phosphors with properties of high luminance and thermal stability, as well as the improved strategies involved, will benefit the basic researches and applications in high power lighting or high-luminance laser lighting.

Structural Indicator to Characterize the Crystal-Field Splitting of Ce3+ in Garnets

J. Phys. Chem. C., 2020, 124, 1, 870–873. https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.9b09322

The structure–property relationship is a hot research topic in both chemistry and material science. A structural descriptor has significant applications in machine learning and high-throughput screening for rapid estimations of material properties. In this work, we report a simple structural indicator to characterize the crystal-field splitting of Ce3+ ions in the garnet structure. The indicator is the interplanar distance of the octahedron, which is calculated as 2/√3 | x + y + z | · L, where L stands for the cell parameter and (x, y, z) are the oxygen coordinates in a specific form. The indicator value is correlated to the crystal-field splitting of Ce3+ in the lanthanide aluminum/gallium garnets and is able to reproduce the reverse garnet effect. By inspecting the polyhedral competition effect in the garnet structure, this indicator is found to be correlated to the tetragonal distortion of the dodecahedron, which determines the crystal-field splitting.

Lead‐Free Perovskite Derivative Cs2SnCl6−xBrx Single Crystals for Narrowband Photodetectors

Adv. Opt. Mater. 2019, 7(10). https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900139

Lead-free and stable Sn halide perovskites demonstrate tremendous potential in the field of optoelectronic devices. Here, the structure and optical properties of the “defect” perovskites Cs2SnCl6−xBrx are reported, as well as their use as photodetector materials. Millimeter-sized Cs2SnCl6−xBrx single crystals are grown by the hydrothermal method, with the body color continuously changing from transparent to yellow and finally to dark red. Narrowband single-crystal photodetectors using Cs2SnCl6−xBrx crystals are presented, which show a high detectivity of ≈2.71 × 1010 Jones, with narrowband photodetection (full-width at half-maximum ≈45 nm) and high ion diffusion barriers. Moreover, the response spectra are continuously tuned from near violet to orange depending on the variation of the bandgap of the single crystals by changing the halide compositions. The strong surface charge recombination of the excess carriers near the crystal surfaces produced by short wavelength light elucidates the narrowband photodetection behavior. This work provides a new paradigm in the design of lead-free, stable, and high-performance perovskite derivatives for optoelectronics applications.

Emerging ultra-narrow-band cyan-emitting phosphor for white LEDs with enhanced color rendition

Light Sci Appl., 8, 38 (2019). https://www.nature.com/articles/s41377-019-0148-8

Phosphor-converted white LEDs rely on combining a blue-emitting InGaN chip with yellow and red-emitting luminescent materials. The discovery of cyan-emitting (470–500 nm) phosphors is a challenge to compensate for the spectral gap and produce full-spectrum white light. Na0.5K0.5Li3SiO4:Eu2+ (NKLSO:Eu2+) phosphor was developed with impressive properties, providing cyan emission at 486 nm with a narrow full width at half maximum (FWHM) of only 20.7 nm, and good thermal stability with an integrated emission loss of only 7% at 150 °C. The ultra-narrow-band cyan emission results from the high-symmetry cation sites, leading to almost ideal cubic coordination for UCr4C4-type compounds. NKLSO:Eu2+ phosphor allows the valley between the blue and yellow emission peaks in the white LED device to be filled, and the color-rendering index can be enhanced from 86 to 95.2, suggesting great applications in full-spectrum white LEDs.

Design Optimization of Lead-Free Perovskite Cs2AgInCl6:Bi Nanocrystals with 11.4% Photoluminescence Quantum Yield

Chem. Mater. 2019, 31, 9, 3333–3339. https://pubs.acs.org/doi/abs/10.1021/acs.chemmater.9b00410

Lead-free halide double perovskites continue to draw increasing attention in view of their nontoxicity and stability compared to lead-based perovskites. Herein, we optimized the colloidal synthesis of undoped and Bi-doped Cs2AgInCl6 nanocrystals (NCs) via the facile hot-injection process using nontoxic and available precursors. The synthesis conditions including temperature, ligands, and hydrochloric acid are investigated to boost the photoluminescence quantum yield (PLQY) of Cs2AgInCl6:Bi NCs with the trace amount of Bi doping. The broad-band orange emission peaking at 580 nm is observed with the PLQY as high as ∼11.4%, and the related luminescence mechanism has been discussed. This work provides a reliable avenue toward the optimization of optical properties for emerging lead-free halide perovskite NCs with enhanced PLQY.

Manipulation of Bi3+/In3+ Transmutation and Mn2+‐Doping Effect on the Structure and Optical Properties of Double Perovskite Cs2NaBi1‐xInxCl6

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201801435

The halide double perovskite family represented by A2(B+,B3+)X6 can overcome the lead toxicity and enable generally large band gap engineering via B/B sites’ transmutation or exotic dopants to fulfill the emerging applications in the optoelectronic fields. Herein, the design and the experimental synthesis of a new family of Mn2+-doped Cs2NaBi1-xInxCl6 crystals with an intense orange-yellow emission band are reported, and the phase formation stability is discussed via a combined experimental–theoretical approach. Depending on the manipulation of Bi3+/In3+ combination, the band gap increases with In3+ content, and a subsequent evolution from indirect to direct band gap is verified. First-principles calculations and parity analyses indicate a parity forbidden effect on Cs2NaInCl6, and a combination effect of absorption on Cs2NaBi1-xInxCl6 from both Cs2NaBiCl6 and Cs2NaInCl6. The associated Mn2+-doped photoluminescence depending on the Bi3+/In3+ substitution is also addressed from the variation of the different Mn–Cl environment and neighboring-cation effect.

Effects of Neighboring Polyhedron Competition on the 5d Level of Ce3+ in Lanthanide Garnets

https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.9b00395

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Herein, we provide a direct explanation for the reverse garnet effect based on polyhedron competition. On multisite substitution, the deformation of the dodecahedron which will accommodate Ce3+ meets suppression from a neighboring octahedron and tetrahedron. This makes the dodecahedral deformation nonisotropic. Further, it is found that the lowest 5d state of Ce3+ in garnet is closely related to the tetragonal distortion of the dodecahedron, which is characterized by a simplified cuboid model. Crystal-field calculations reveal how the edge-ratio of the cuboid affects energy levels. This model gives a satisfactory explanation for the reverse garnet effect and is helpful for seeking novel garnet-based luminescent materials.

Discovery of New Narrow‐Band Phosphors with the UCr4C4‐Related Type Structure by Alkali Cation Effect

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201801631

Narrow-band luminescence materials used in white light-emitting diodes (WLEDs) have demonstrated a great potential to increase the maximum accessible color gamut, improve the color rendition, or enhance the visual luminous efficacy for illumination and display devices. So far, the discovery of narrow-band rare earth doped phosphors for emerging applications remains challenging owing to the limited systems in use and the broadening effect of 4f–5d transition. Here, two narrow-band cyan-emitting phosphors with the UCr4C4-related type structure, RbNa2K(Li3SiO4)4:Eu2+ (RNKLSO:Eu2+) and CsNa2K(Li3SiO4)4:Eu2+ (CNKLSO:Eu2+), are reported. The narrow-band emission results from the highly condensed network and a cube-like site for the activator (Eu2+). The emission peak assignments are investigated, and the variations of luminescence behavior with compositional changes of alkali cations are elaborated in detail. RNKLSO:8% Eu2+ (95%@250 °C of the integrated emission intensity at 25 °C) shows better thermal stability than that of CNKLSO:8% Eu2+ (79%@250 °C), which can be explained by the thermally activated crossover process represented in the configurational coordinate diagram. The optical properties of the as-fabricated WLEDs are studied and demonstrate potential with tunable properties for the full spectrum phosphor-converted LEDs. These findings in UCr4C4-type phases help shedding light on new avenues for fabricating new and totally unexpected narrow-emitting phosphors with versatile applications.

Synthesis and Luminescence Properties of CsPbX3@Uio-67 Composites toward Stable Photoluminescence Convertors

https://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.8b03295

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All-inorganic halide perovskite (CsPbX3, X = Cl, Br, or I) nanocrystals (NCs) have been widely studied due to their outstanding optoelectronic properties. However, some inevitable factors like light, heat, and moisture affected the stability of CsPbX3 NCs and further limited their practical application. In this work, the stability of all-inorganic halide perovskite NCs can be improved by integrating them in the stable Zr-based metal–organic frameworks (Uio-67). Compared to pristine perovskite NCs, typical CsPbBr3@Uio-67 composites display a stable photoluminescence property that can be maintained for 30 days under ambient atmospheric conditions. Due to the proposed confinement effects of CsPbX3 NCs coordinated with the pore structures of Uio-67, the related structural model of CsPbX3@Uio-67 composites was elucidated. White LED device was further fabricated by combining CsPbBr3@Uio-67 composites and commercial K2SiF6:Mn4+ red phosphors with a blue-emitting chip, which demonstrated a wide color gamut (138% of National Television Standards Committee color space). The strategy on encapsulation of CsPbX3 NCs into Uio-67 will open up a stable platform for optoelectronic applications.

Luminescent perovskites: recent advances in theory and experiments

https://pubs.rsc.org/en/content/articlehtml/2019/qi/c9qi00777f

image file: c9qi00777f-f1.tif

Perovskites form an important and enormous class of inorganic compounds. Recently, perovskite materials have attracted extensive research interest owing to their excellent optoelectronic properties. Deep insights into the relationships between the crystal structure, electronic structure and properties play an important role in the development of new functional materials and high-performance devices. In this review, after a brief introduction, we first discuss the crystal structure and crystal chemistry of perovskites according to their three classes: standard perovskites, low-dimensional perovskites and perovskite-like halides. Next, the electronic structure and luminescence from different physical origins are presented. Then, we present a survey on the design, synthesis and luminescence properties of different perovskites, including halide perovskites, oxide perovskites, and lanthanide- or transition metal-doped perovskites, also including dimension-different perovskites (3D, 2D, 1D and quantum dots). We also summarize the strategies for improving the photoluminescence quantum yield (PLQY) and chemical stability, including by surface passivation, encapsulation and doping. Finally, we review their applications and give a brief outlook.