Quantum dots are ideally suited for color conversion in light emitting diodes owing to their spectral tunability, high conversion efficiency and narrow emission bands. These properties are particularly important for display backlights; the highly saturated colors generated by quantum dots justify their higher production cost. Here, we demonstrate the benefits of a hybrid remote phosphor approach that combines a green-emitting europium-doped phosphor with red-emitting CdSe/CdS core/shell quantum dots. Different stacking geometries, including mixed and separate layers of both materials, are studied at the macroscopic and microscopic levels to identify the configuration that achieves maximum device efficiency while minimizing material usage. The influence of reabsorption, optical outcoupling and refractive index-matching between the layers is evaluated in detail with respect to device efficiency and cost. From the findings of this study, general guidelines are derived to optimize both the cost and efficiency of CdSe/CdS and other (potentially cadmium-free) quantum dot systems. When reabsorption of the green and/or red emission is significant compared to the absorption strength for the blue emission of the pumping light emitting diode, the hybrid remote phosphor approach becomes beneficial.

Phosphor-converted white light-emitting diodes (LEDs) are becoming increasingly popular for general lighting. The non-rare-earth phosphor K 2 SiF 6 :Mn 4+ , showing promising saturated red d-d-line emission, was investigated. To evaluate the application potential of this phosphor, the luminescence behavior was studied at high excitation intensities and on the microscopic level. The emission shows a sublinear behavior at excitation powers exceeding 40 W/cm 2 , caused by ground-state depletion due to the ms range luminescence lifetime. The thermal properties of the luminescence in K 2 SiF 6 :Mn 4+ were investigated up to 450 K, with thermal quenching only setting in above 400 K. The luminescence lifetime decreases with increasing temperature, even before thermal quenching sets in, which is favorable to counteract the sublinear response at high excitation intensity. A second, faster, decay component emerges above 295 K, which, according to crystal field calculations, is related to a fraction of the Mn 4+ ions incorporated on tetragonally deformed lattice sites. A combined investigation of structural and luminescence properties in a scanning electron microscope using energy-dispersive X-ray spectroscopy and cathodoluminescence mappings showed both phosphor degradation at high fluxes and a preferential location of the light outcoupling at irregularities in the crystal facets. The use of K 2 SiF 6 :Mn 4+ in a remote phosphor configuration is discussed. Most phosphor-converted white LEDs contain phosphors doped with rare earths such as divalent europium and trivalent cerium. These ions feature relatively broad emission bands based on the parity allowed 5d-4f transition. They are often easily excited with blue light and can show high quantum efficiency, even at elevated temperature. To improve the color rendering of white LEDs, red phosphors are added to the traditional blue LED and yellow Y 3 Al 5 O 12 :Ce (YAG:Ce) phosphor combination. These red phosphors need to be stable and have a high quantum efficiency. The emission spectrum should both be sufficiently red (>600 nm) and well within the eye sensitivity curve, to obtain a high luminous efficacy.1 Sulfide phosphors doped with Eu 2+ , such as (Ca,Sr)S:Eu 2+ are known for their efficient red emission, 2 but they lack stability in humid environments and the eye sensitivity is low for part of their broad emission band.3 Nitride phosphors doped with Eu 2+ are often chemically more stable, but their synthesis at high pressure and temperature is a drawback.4 Most europium-doped nitride phosphors show a relatively broad emission band. 5,6 Cost and supply issues of the rare-earth materials pave the way for transition-metal-doped phosphors. 7 In particular the Mn 4+ ion is a promising alternative for Eu 2+ as it shows line emission from parity and spin-forbidden d-d transitions in the red and near-infrared spectral region. Investigation of the optical properties of the Mn 4+ dopant showed that fluoride hosts are preferred for LED phosphors over oxide hosts, since only the io...

Efficient broadband infrared (IR) light-emitting diodes (LEDs) are needed for emerging applications that exploit near-IR spectroscopy, ranging from hand-held electronics to medicine. Here we report broadband IR luminescence, cooperatively originating from Eu 2+ and Tb 3+ dopants in CaS. This peculiar emission overlaps with the red Eu 2+ emission, ranges up to 1200 nm (full-width-at-half-maximum of 195 nm) and is efficiently excited with visible light. Experimental evidence for metal-to-metal charge transfer (MMCT) luminescence is collected, comprising data from luminescence spectroscopy, microscopy and X-ray spectroscopy. State-of-the-art multiconfigurational ab initio calculations attribute the IR emission to the radiative decay of a metastable MMCT state of a Eu 2+ -Tb 3+ pair. The calculations explain why no MMCT emission is found in the similar compound SrS:Eu,Tb and are used to anticipate how to fine-tune the characteristics of the MMCT luminescence. Finally, a near-IR LED for versatile spectroscopic use is manufactured based on the MMCT emission.

Inspired by their excellent thermal stability and strong fatigue resistance, inorganic photochromic materials have been highlighted as promising candidates in various photonic applications ranging from photoswitches, anti-counterfeiting, and encryption to information storage. However, the lack of suitable inorganic materials with both fast photoresponse and strong coloration contrast heavily restricts their applications. Herein, a new strategy is proposed to achieve high photochromic performance via constructing deep-lying traps in ferroelectric ceramics. The obtained K 0.5 Na 0.5 NbO 3 -Eu (KNN-Eu) ceramic exhibits a reversible yellow-gray color change with high fatigue resistance upon alternating illumination (420 nm) and thermal stimulus (450°C). A fast response time of around 1 s and a large reflectivity difference of 39.2% between the colored and bleached states are simultaneously achieved in KNN-Eu ceramic, which is by far the best performance ever reported in inorganic photochromic materials. Benefiting from these excellent properties, KNN-Eu is the first ferroelectric photochromic ceramic to support an instant and hand-(re)writable information display. The enhanced photochromic performance is expected to facilitate the application of photochromic materials in numerous optical devices and provides a significant guidance to design other inorganic photochromic materials.

Inorganic materials combining photochromism and luminescence modulation characteristics have great potential in dual‐mode rewritable optical storage due to their unique optical features and excellent thermal stability. However, the failure of achieving a large luminescence modulation and a strong photochromic efficiency in photostimulated inorganic photochromic materials limits their applications. Herein, a new strategy for realizing an overlap between the photochromic absorption peak and the photoluminescent emission/excitation peak is proposed for designing high‐performance photochromic materials. The obtained BaMgSiO4: M (M = Ce3+, Mn2+, or Nd3+) ceramics exhibit a reversible white‐pink color change upon alternate 310 nm and 590 nm illumination (or thermal stimulus) accompanied by a high photochromic efficiency (>50%). Benefiting from a perfectly matched photochromic absorption peak and Mn2+ emission peak, a record luminescence modulation of 96.3% with excellent fatigue resistance is obtained in BaMgSiO4: Mn2+ ceramics. These properties are superior to all photochromic materials reported to date, demonstrating great potential in optical information storage applications. The trap‐related photochromic and regulated luminescence behavior is investigated together with a prototype of a dual‐mode information display. This work is expected to promote the practical application of photochromic materials in various optical devices and provides an effective strategy to develop other photochromic materials.

A combination of techniques is used to investigate the incorporation of chromium in the persistent phosphor LiGa5O8:Cr3+. Afterglow can be enhanced by codoping with Si(iv) or Ge(iv) ions.

No abstract

No abstract