Hybrid remote quantum dot/powder phosphor designs for display backlights

Encapsulation of Cadmium-Free InP-based Quantum Dots in Cross-Linked Core-Shell Microparticles via Coaxial Electrospraying

Quantum dots (QDs) are inorganic semiconductor nanocrystals capable of emitting light. The current major challenge lies in the use of heavy metals, which are known to be highly toxic to humans and pose significant environmental risks. Researchers have turned to indium (In) as a promising option for more environmentally benign QDs, specifically indium phosphide (InP). A significant obstacle remains in sustaining the long-term photostability of InP-based QDs when exposed to the environment. To tackle this, electrospraying is used in this work to protect indium phosphide/zinc selenide/zinc sulfide (InP/ZnSe/ZnS) QDs by embedding them within polymer core-shell microparticles of poly[(lauryl methacrylate)-co-(ethylene glycol dimethacrylate)]/poly(methyl methacrylate) (poly(LMA-co-EGDMA)/PMMA). During the flight of droplets, the liquid monomer core of LMA and EGDMA with QDs is encapsulated by the solid shell of PMMA formed due to solvent evaporation, resulting in a liquid-core/solid-shell particle structure. After that, the captured core of monomers is polymerized into a cross-linked polymer with the embedded QDs via a thermal initiation. They demonstrate how a successful core-shell particle formation is achieved to produce structures for initially liquid monomer systems via coaxial electrospraying that are used for cross-linked polymers, which are of major interest for the encapsulation of InP-based QDs for generally improved photostability over pristine QDs.

Luminescence center modulation for the synthesis of a narrow-band green phosphor: mechanism and backlighting display application

A highly efficient and stable green phosphor with a narrow emission-band in a hexagonal aluminate was synthesized based on the energy transfer between Eu2+ and Mn2+ luminescence centers. The related mechanism was elucidated from the viewpoints of the crystal structure and energy level, providing insights for designing novel phosphors with high performance.

The broadband emission of Cr3+-doped CaY2Mg2Ge3O12 and its applications for NIR detectors

A phosphor-converted light-emitting diode (pc-LED) is a prime light source in smart broadband near-infrared (NIR) spectroscopy. The performance of NIR pc-LEDs crucially depends on the employed NIR luminescent materials. In this study, we synthesized a novel high-efficiency broadband NIR phosphor, CaY2Mg2Ge3O12:Cr3+ (CYMG:Cr3+). Under 450 nm excitation, CYMG:Cr3+ exhibited remarkable broadband NIR emission from 650 to 900 nm with a full width at half maximum (FWHM) of 115 nm. Within the CYMG lattice, the Cr3+ ion occupies Ca/Y sites in the dodecahedron Ca/YO8 and Mg sites in the octahedron MgO6, giving rise to two distinct Cr3+ luminescence centers. Remarkably, the emission at 100 °C remained at 92% of its room temperature intensity and 81% at 150 °C, showcasing its exceptional thermal stability. The internal quantum efficiency (IQE) reached an impressive 81.1%, with an activation energy ΔE of 0.324 eV. Furthermore, we integrated the CYMG:Cr3+ phosphor with a commercial 450 nm blue chip to fabricate a micro NIR pc-LED, which exhibited stable NIR emission across different driving currents, with a NIR output power of 49.65 mW@400 mA and a photoelectric conversion efficiency of 10.52% at 20 mA. All findings highlight CYMG:Cr3+ as a stable and efficient broadband luminescent material for high-performance NIR LEDs.

Color Rendering Index over 95 Achieved by Using Light Recycling Process Based on Hybrid Remote-Type Red Quantum-Dot Components Applied to Conventional LED Lighting Devices

Red color conversion materials have often been used in conventional white LEDs (light-emitting diodes) to enhance the insufficient deep-red component and thus improve the color-rendering property. Quantum dots (QDs) are one of the candidates for this due to their flexibility in controlling the emission wavelength, which is attributed to the quantum confinement effect. Two types of remote QD components, i.e., QD films and QD caps, were prepared and applied to conventional white LED illumination to improve the color-rendering properties. Thanks to the red component near 630 nm caused by the QD components, the color rendering indices (CRIs) of both Ra and R9 could be increased to over 95. It was found that both the diffusing nature of the reflector and the light recycling process in the vertical cavity between the bottom reflector and the top optical films play important roles in improving the color conversion efficiency of remote QD components. The present study showed that the proper application of remote QDs combined with a suitable optical cavity can control the correlated color temperature of the illumination over a wide range, thus realizing different color appearances of white LED illumination. In addition, a high CRI of over 95 could be achieved due to the sufficient excitation from fewer QDs, due to the strong optical cavity effect.

Advancing Red-Emitting Fluoride Phosphors for Highly Stable White Light-Emitting Diodes: Crystal Reconstruction and Covalence Enhancement Strategy

Mn⁴⁺-activated fluoride red phosphors exhibit excellent luminescence properties. However, a persistent technical challenge lies in their poor moisture resistance. Current strategies primarily focus on surface modifications to effectively shield the [MnF₆]^2- species from water molecules while neglecting the underlying structure of the fluoride matrix. In this study, we introduce Si⁴⁺ and Ge⁴⁺ ions into the K₂TiF₆:Mn⁴⁺ crystal to create covalent fluoride solid solutions, namely, K₂Ti_1-xSi_xF₆:Mn⁴⁺ and K₂Ti_1-yGe_yF₆:Mn⁴⁺, through crystal reconstruction. The findings reveal that the incorporation of Si⁴⁺ leads to increased particle size, enhanced luminescence intensity (by 40%), and improved moisture resistance. Furthermore, after undergoing 1000 h of aging at high temperature and high humidity conditions, the white LED featuring the K₂Ti_0.97Si_0.03F₆:Mn⁴⁺ phosphor demonstrates remarkable durability by retaining 90% of its initial luminous efficacy. This performance surpasses that of the device utilizing the K₂TiF₆:Mn⁴⁺ phosphor, which only retains 74% of its original efficacy. The crystal reconstruction method and covalent enhancement strategy proposed in this work contribute to enhancing the luminescence efficiency and moisture resistance of fluoride phosphors, thereby offering new insights for advancing the development of high-efficiency and highly stable white light LED devices.

Encapsulation of Cadmium-Free InP/ZnSe/ZnS Quantum Dots in Poly(LMA-co-EGDMA) Microparticles via Co-flow Droplet Microfluidics

Quantum dots (QDs) are semiconductor nanocrystals that are used in optoelectronic applications. Most modern QDs are based on toxic metals, for example Cd, and do not comply with the European Restriction of Hazardous Substances regulation of the European Union. Latest promising developments focus on safer QD alternatives based on elements from the III-V group. However, the InP-based QDs lack an overall photostability under environmental influences. One design path of achieving stability is through encapsulation in cross-linked polymer matrices with the possibility to covalently link the matrix to surface ligands of modified core-shell QDs. The work focuses on the formation of polymer microbeads suitable for InP-based QD encapsulation, allowing for an individual protection of QDs and an improved processibility via this particle-based approach. For this, a microfluidic based method in the co-flow regime is used that consists of an oil-in-water droplet system in a glass capillary environment. The generated monomer droplets are polymerized in-flow into poly(LMA-co-EGDMA) microparticles with embedded InP/ZnSe/ZnS QDs using a UV initiation. They demonstrate how a successful polymer microparticle formation via droplet microfluidics produces optimized matrix structures leading to a distinct photostability improvement of InP-based QDs compared to nonprotected QDs.

Full-color micro-LED display with photo-patternable and highly ambient-stable perovskite quantum dot/siloxane composite as color conversion layers

In this paper, we successfully fabricated color conversion layers (CCLs) for full-color-mico-LED display using a perovskite quantum dot (PQD)/siloxane composite by ligand exchanged PQD with silane composite followed by surface activation by an addition of halide-anion containing salt. Due to this surface activation, it was possible to construct the PQD surface with a silane ligand using a non-polar organic solvent that does not damage the PQD. As a result, the ligand-exchanged PQD with a silane compound exhibited high dispersibility in the siloxane matrix and excellent atmospheric stability due to sol-gel condensation. Based on highly ambient stable PQD/siloxane composite based CCLs, full-color micro-LED display has a 1 mm pixel pitch, about 25.4 pixels per inch (PPI) resolution was achieved. In addition, due to the thin thickness of the black matrix to prevent blue light interference, the possibility of a flexible display that can be operated without damage even with a bending radius of 5 mm was demonstrated.

CuInS2 Nanocrystals Embedded PMMA Composite Films: Adjustment of Polymer Molecule Weights and Application in Remote-Type White LEDs

The commercial application of colloidal semiconductor nanocrystals has been realized owing to the development of composite film technology. Here, we demonstrated the fabrication of green and red emissive CuInS₂ nanocrystals embedded polymer composite films of equal thickness by using a precise solution casting method. The impacts of polymer molecular weight on the dispersibility of CuInS₂ nanocrystals were then systematically studied through evaluating the decrease in transmittance and red shift of emission wavelength. The composite films made from PMMA of small molecular weights exhibited higher transmittance. Applications of these green and red emissive composite films as color converters in remote-type light-emitting devices were further demonstrated.

Optical and thermal properties of rare earth-doped K4 Ca(PO4 )2 phosphor

The luminescent properties and energy transfer (ET) mechanism in the Ln³⁺ pair of the RE³⁺ (RE = Eu³⁺ , Ce³⁺ , Dy³⁺ and Sm³⁺ ) doped K₄ Ca(PO₄ )₂ phosphor were successfully investigated using a conventional high-temperature solid-state reaction. In the near infrared (NIR) range, Ce³⁺ -doped K₄ Ca(PO₄ )₂ phosphor exhibited a UV-Vis. emission band, whereas K₄ Ca(PO₄ )₂ :Dy³⁺ exhibited characteristic emission bands centred at 481 and 576 nm in the near-ultraviolet excitation range. The possibility of ET from Ce³⁺ to Dy³⁺ in K₄ Ca(PO₄ )₂ phosphor was confirmed by a significant increase in the photoluminescence intensity of the Dy³⁺ ion based on the spectral overlap of acceptor and donor ions. X-ray diffraction, Fourier-transform infrared and thermogravimetric analysis/differential thermal analysis TGA/DTA were carried out to study phase purity, presence of functional groups and amount of weight loss under different temperature regimes. Therefore, the RE³⁺ -doped K₄ Ca(PO₄ )₂ phosphor may be a stable phosphor host for light-emitting diode applications.

Nanoantennas Patterned by Colloidal Lithography for Enhanced Nanophosphor Light Emission

Transparent coatings made of rare-earth doped nanocrystals, also known as nanophosphors, feature efficient photoluminescence and excellent thermal and optical stability. Herein, we demonstrate that the optical antennas prepared by colloidal lithography render thin nanophosphor films with a brighter emission. In particular, we fabricate gold nanostructures in the proximity of GdVO₄:Eu³⁺ nanophosphors by metal evaporation using a mask made of a monolayer of polymer beads arranged in a triangular lattice. Optical modes supported by the antennas can be controlled by tuning the diameter of the polymer spheres in the colloidal mask, which determines the shape of the gold nanostructure, as confirmed by numerical simulations. Confocal microscopy reveals that metallic antennas induce brighter photoluminescence at specific spatial regions of the nanophosphor film at targeted frequencies as a result of the coupling between gold nanostructures and nanophosphors. Patterning of nanophosphor thin layers with arrays of metallic antennas offers an inexpensive nanophotonic solution to develop bright emitting coatings of interest for color conversion, labeling, or anti-counterfeiting.

Ultrahigh Stability of Perovskite Nanocrystals by Using Semiconducting Molecular Species for Displays

The instability of perovskite nanocrystals (NCs) to moisture, heat, and blue light severely hinders their commercial applications in quantum dot displays. Here, organic semiconducting molecules are introduced onto CsPbBr₃ NCs, and the as-obtained CsPbBr₃ NCs have a high photoluminescent quantum yield (PLQY) of 82% and extremely high stability in harsh commercial accelerated operational stability tests (such as high temperature (85 °C) and high humidity (85%)). The products can survive and maintain more than 80% of the initial PL intensity value under high temperature, high humidity, and long-term blue light irradiation for hundreds to thousands of hours. They are among the most stable perovskite NCs and even superior to those encapsulated by inert shells and commercial green-emissive CdSe@ZnS quantum dots (QDs). The mechanism of the exceptional stability has been proposed, mainly including the strong interaction and moderate photocarrier transfer between the quasi type II heterostructure formed by the molecule and CsPbBr₃. By using these stable CsPbBr₃ NCs, a QD-enhanced liquid crystal display prototype has been successfully fabricated with a wide color gamut. This work provides understandings on the functionality of ligands in perovskite fields and a promising prospect in perovskite-based display technologies.

Structural Optimization of Vertically-Stacked White LEDs with a Yellow Phosphor Plate and a Red Quantum-Dot Film

A remote-type white light-emitting diode (LED) consisting of a red quantum-dot (QD) film and a yellow phosphor plate was studied by both experiment and optical simulation. The sequence of the two color-conversion films had a substantial effect on the color-rendering properties of the vertically-stacked white LED, and the optimized configuration exhibited a high color rendering index of more than 90 thanks to the enhanced red component via the QD film. For the design of high-power white LED devices of a remote type, it was necessary to locate the color-conversion films below the diffuser plate to remove the substantial color dispersion depending on the viewing angle. The present study shows that high power and high color-rendering white LED devices can be realized in terms of two vertically-stacked color-conversion materials, which would provide long-term stability due to the remote design.

Substantial Improvement of Color-Rendering Properties of Conventional White LEDs Using Remote-Type Red Quantum-Dot Caps

A new type of remote red quantum-dot (QD) component was designed and fabricated to improve the color-rendering properties of conventional white LED (light-emitting diode) lightings. Based on an optical simulation, the rectangular cavity-type QD cap was designed with an opening window on the top surface. Red QD caps were fabricated using a typical injection molding technique and CdSe/ZnS QDs with a core/shell structure whose average size was ~6 nm. Red QD caps were applied to conventional 6-inch, 15-W white LED downlighting consisting of 72 LEDs arrayed concentrically. The red QD caps placed over white LEDs enhanced the red components in the long-wavelength range resulting in the increase of the color rendering index (CRI) from 82.9 to 94.5. The correlated color temperature was tuned easily in a wide range by adopting various configurations consisting of different QD caps. The spatial and angular homogeneities were secured on the emitting area because QD caps placed over the white LEDs did not exhibit any substantial optical path length difference. The present study demonstrates that adopting QD caps in conventional LED lightings provides a flexible and efficient method to realize a high color-rendering property and to adjust correlated color temperature appropriately for a specific application.

K+-doping-induced highly efficient red emission in CsPb(Br,I)3 quantum dot glass toward Rec. 2020 displays

It is demonstrated that the incorporation of K⁺ into CsPb(Br,I)₃ perovskite quantum dot glass leads to the simultaneous increases of quantum efficiency and phase stability. The latent mechanism is analyzed via the microstructural and spectroscopic studies. The constructed prototype white-light-emitting diode device yields an ultra-wide color gamut attaining 96% Rec. 2020 standard.

Luminous efficacy enhancement for LED lamps using highly reflective quantum dot-based photoluminescent films

In this study, a strongly reflective and photoluminescent (PL) poly(lactic-co-glycolic acid) quantum dot (QD) hybrid nanofiber (PQHN) structure is introduced to enhance the luminous efficacy of QD-phosphor hybrid white light-emitting diodes (QD-WLEDs). As the thickness of PQHN film increases, the PL is found to continuously increase, exhibiting a maximum peak intensity at 120 μm, which is 1.92 times that at 12 μm, and showing the highest diffuse reflectance of 94.4% at 640 nm. Consequently, while using the QD-WLEDs, the PQHN structure achieves a 53.8% improvement in luminous flux compared with the traditional structure under a similar correlated color temperature (CCT) of 3,540 K, achieving a high luminous efficacy of 202.11 lm W^-1 for QD-WLEDs. In addition, the maximum deviation of the CCT is only 11 K when the current is changed from 50 to 950 mA, demonstrating good stability. Therefore, the PQHN films have great potential in lighting systems as a hybrid functional film including light conversion and reflectance.

Dual Role of Quantum Dots as Color Conversion Layer and Suppression of Input Light for Full-Color Micro-LED Displays

In micro-light-emitting diode (micro-LED) displays with color-conversion layers, a facile and efficient technology getting rid of the use of the color filters leads to a big technical leap in cost-effective fabrication. In this study, it is demonstrated that quantum dot (QD) color conversion layers can significantly suppress residual blue excitation light because of the high extinction coefficients of QDs, ∼0.1% transmittance of blue light for green and red core/shell CdSe/ZnS QD film with thickness of less than 17 μm, and produce green and red colors. Incorporation of TiO₂ nanoparticles into QD solutions enhances more than 10% of the luminous intensity by the scattering effect. It is found that the suppression of QD reabsorption is essential to achieve a high color-conversion efficiency. Our results provide a clear path to a cost-effective fabrication of QD conversion layer micro-LED displays over the full range of their applications.

Synthesis and low-order optical nonlinearities of colloidal HgSe quantum dots in the visible and near infrared ranges

We synthesize colloidal HgSe quantum dots and characterize their nonlinear refraction and nonlinear absorption using a Nd:YAG laser and its second harmonic. The 7.5 nm quantum dots were synthesized using the hot-injection method. The nonlinear absorption (β = 9×10^-7 cm W^-1) and negative nonlinear refraction (γ = -5×10^-12 cm² W^-1) coefficients of colloidal quantum dots were determined using the 10 ns, 532 nm laser radiation. The joint influence of above processes was realized at a higher intensity of probe pulses. In the case of 10 ns, 1064 nm radiation, only negative nonlinear refraction dominated during z-scans of these quantum dots. The studies of optical limiting using two laser sources demonstrated the effectiveness of this process at 532 nm. The role of nonlinear scattering is analyzed. We discuss the mechanisms responsible for the nonlinear refraction processes in colloidal HgSe quantum dots.

Toward 200 Lumens per Watt of Quantum-Dot White-Light-Emitting Diodes by Reducing Reabsorption Loss

In this study, we analyze the influence of the pore structure of an SBA-15 particle on the light emission from its inner adsorbed quantum dots (QDs) and outer light-emitting diode (LED) chips. It is found that the particle features of a high refractive index, comparable feature size of pore structure, and lower amount of QD adsorption help with QD light extraction, demonstrating a mechanism to suppress QD light propagating through pores and thus reducing the reabsorption loss. We consequently developed highly efficient QD white LEDs with wet-mixing QD/SBA-15 nanocomposite particles (NPs) by further optimizing the packaging methods and the introduced NP mass ratio. The LEDs demonstrated a record luminous efficacy (the ratio of luminous flux to electrical power) of 206.8 (entrusted test efficiency of 205.8 lm W^-1 certificated by China National Accreditation Service) and 137.6 lm W^-1 at 20 mA for white LEDs integrating only green QDs and green-red QD color convertors, respectively, with improved operating stability. These results are comparable to conventional phosphor-based white LEDs, which can be a starting point for white LEDs only using QDs as convertors toward commercialization in the near future.

InP Quantum Dots: Synthesis and Lighting Applications

InP quantum dots (QDs) are typical III-V group semiconductor nanocrystals that feature large excitonic Bohr radius and high carrier mobility. The merits of InP QDs include large absorption coefficient, broad color tunability, and low toxicity, which render them promising alternatives to classic Cd/Pb-based QDs for applications in practical settings. Over the past two decades, the advances in wet-chemistry methods have enabled the synthesis of small-sized colloidal InP QDs with the assistance of organic ligands. By proper selection of synthetic protocols and precursor materials coupled with surface passivation, the QYs of InP QDs are pushed to near unity with modest color purity. The state-of-the-art InP QDs with appealing optical and electronic properties have excelled in many applications with the potential for commercialization. This work focuses on the recent development of wet-chemistry protocols and various precursor materials for the synthesis and surface modification of InP QDs. Current methods for constructing light-emitting diodes using novel InP-based QDs are also summarized.

Advances in Quantum-Dot-Based Displays

In terms of their use in displays, quantum dots (QDs) exhibit several advantages, including high illumination efficiency and color rendering, low-cost, and capacity for mass production. Furthermore, they are environmentally friendly. Excellent luminescence and charge transport properties of QDs led to their application in QD-based light-emitting diodes (LEDs), which have attracted considerable attention in display and solid-state lighting applications. In this review, we discuss the applications of QDs which are used on color conversion filter that exhibit high efficiency in white LEDs, full-color micro-LED devices, and liquid-type structure devices, among others. Furthermore, we discuss different QD printing processes and coating methods to achieve the full-color micro-LED. With the rise in popularity of wearable and see-through red, green, and blue (RGB) full-color displays, the flexible substrate is considered as a good potential candidate. The anisotropic conductive film method provides a small controllable linewidth of electrically conductive particles. Finally, we discuss the advanced application for flexible full-color and highly efficient QD micro-LEDs. The general conclusion of this study also involves the demand for a more straightforward QD deposition technique, whose breakthrough is expected.

Micro-light-emitting diodes with quantum dots in display technology

Micro-light-emitting diodes (μ-LEDs) are regarded as the cornerstone of next-generation display technology to meet the personalised demands of advanced applications, such as mobile phones, wearable watches, virtual/augmented reality, micro-projectors and ultrahigh-definition TVs. However, as the LED chip size shrinks to below 20 μm, conventional phosphor colour conversion cannot present sufficient luminance and yield to support high-resolution displays due to the low absorption cross-section. The emergence of quantum dot (QD) materials is expected to fill this gap due to their remarkable photoluminescence, narrow bandwidth emission, colour tuneability, high quantum yield and nanoscale size, providing a powerful full-colour solution for μ-LED displays. Here, we comprehensively review the latest progress concerning the implementation of μ-LEDs and QDs in display technology, including μ-LED design and fabrication, large-scale μ-LED transfer and QD full-colour strategy. Outlooks on QD stability, patterning and deposition and challenges of μ-LED displays are also provided. Finally, we discuss the advanced applications of QD-based μ-LED displays, showing the bright future of this technology.

White electroluminescent devices based on hybrid structure with quantum dot color convertors

In this work, a hybrid structure of white electroluminescent (EL) devices has been designed. CdSe/ZnS core-shell quantum dots (QDs) are introduced as color convertors. The photometric and colorimetric characteristics of this device are further experimentally investigated with varying frequency and voltage. Compared with that of conventional white EL devices, this novel device achieves a twice luminance and more stable white light. Moreover, the concept of "color conversion index" is proposed to measure the color conversion ability of QDs in EL devices in consideration of human visual effect.

Efficiency enhancement of quantum dot-phosphor hybrid white-light-emitting diodes using a centrifugation-based quasi-horizontal separation structure

In this paper, a centrifugation-based quasi-horizontal separation (c-HS) structure is proposed to enhance the QD light extraction of QD-phosphor hybrid white LEDs (WLEDs), effectively suppressing the backscattered loss from phosphor at the top region of the QD layer. Results indicate that a large centrifugation speed and dispensing mass of the QD layer is more beneficial to reducing the local density of phosphor at the top region, realizing quasi-horizontal separation between phosphor and QDs. Moreover, WLEDs with c-HS structure and conventional vertically layered packaging reference structure were compared at different correlated color temperatures (CCT). The radiant power and luminous flux achieved by the c-HS structure were 13.6% and 10.8%, respectively, higher than the reference structure at a typical warm white color of ∼4000 K. Consequently, this study can provide a new perspective on designing the separation structure for QD-phosphor hybrid WLEDs considering the backscattering loss of QD light.

Long-living and highly efficient bio-hybrid light-emitting diodes with zero-thermal-quenching biophosphors

Bio-hybrid light-emitting diodes (Bio-HLEDs) based on color down-converting filters with fluorescent proteins (FPs) have achieved moderate efficiencies (50 lm/W) and stabilities (300 h) due to both thermal- and photo-degradation. Here, we present a significant enhancement in efficiency (~130 lm/W) and stability (>150 days) using a zero-thermal-quenching bio-phosphor design. This is achieved shielding the FP surface with a hydrophilic polymer allowing their homogenous integration into the network of a light-guiding and hydrophobic host polymer. We rationalize how the control of the mechanical and optical features of this bio-phosphor is paramount towards highly stable and efficient Bio-HLEDs, regardless of the operation conditions. This is validated by the relationships between the stiffness of the FP-polymer phosphor and the maximum temperature reached under device operation as well as the transmittance of the filters and device efficiency.

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.

Insights into the complexity of the excited states of Eu-doped luminescent materials

Multiconfigurational ab initio embedded-cluster methods give a detailed view of the excited states of Eu-doped luminescent materials, improving the understanding of their structure and how it is affected by changing the host's chemical composition.

A K3ScSi2O7:Eu2+ based phosphor with broad-band NIR emission and robust thermal stability for NIR pc-LEDs

A novel K₃ScSi₂O₇:Eu²⁺ based phosphor with broad-band NIR emission and robust thermal stability has been obtained.

Highly Efficient and Stable White Light-Emitting Diodes Using Perovskite Quantum Dot Paper

Perovskite quantum dots (PQDs) are a competitive candidate for next-generation display technologies as a result of their superior photoluminescence, narrow emission, high quantum yield, and color tunability. However, due to poor thermal resistance and instability under high energy radiation, most PQD-based white light-emitting diodes (LEDs) show only modest luminous efficiency of ≈50 lm W-1 and a short lifetime of <100 h. In this study, by incorporating cellulose nanocrystals, a new type of QD film is fabricated: CH3NH3PbBr3 PQD paper that features 91% optical absorption, intense green light emission (518 nm), and excellent stability attributed to the complexation effect between the nanocellulose and PQDs. The PQD paper is combined with red K2SiF6:Mn4+ phosphor and blue GaN LED chips to fabricate a high-performance white LED demonstrating ultrahigh luminous efficiency (124 lm W-1), wide color gamut (123% of National Television System Committee), and long operation lifetime (240 h), which paves the way for advanced lighting technology.

The Physics of Light Emission in Halide Perovskite Devices

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light-emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state-of-the-art metal-halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

Self-Powered Perovskite/CdS Heterostructure Photodetectors

Methylammonium lead halide perovskites have gained a lot of attention because of their remarkable physical properties and potential for numerous (opto)electronic applications. Here, high-performance photodetectors based on CH₃NH₃PbI₃ (MAPbI₃)/CdS heterostructures are demonstrated. The resulting self-powered MAPbI₃/CdS photodetectors show excellent operating characteristics including a maximum detectivity of 2.3 × 10¹¹ Jones with a responsivity of 0.43 A/W measured at 730 nm. A temporal response time of less than 14 ms was achieved. The mechanisms of charge separation and transport at the interface of the MAPbI₃/CdS junction were investigated via conductive atomic force microscopy (AFM) and photoconductive AFM. Obtained results show that grain boundaries exhibit higher photocurrent than flat regions of the top perovskite layer, which indicates that excitons preferentially separate at the grain boundaries of the perovskite thin film, that is, at the edges of the MAPbI₃ crystals. The study of the photoelectric mechanism at the nanoscale suggests the device performance could potentially be fine-tuned through grain boundary engineering, which provides essential insights for the fabrication of the high-performance photodetector. The demonstrated self-powered photodetector is promising for numerous applications in low-energy consumption optoelectronic devices.

Microcarrier-Assisted Inorganic Shelling of Lead Halide Perovskite Nanocrystals

The conventional strategy of synthetic colloidal chemistry for bright and stable quantum dots has been the production of epitaxially matched core/shell heterostructures to mitigate the presence of deep trap states. This mindset has been shown to be incompatible with lead halide perovskite nanocrystals (LHP NCs) due to their dynamic surface and low melting point. Nevertheless, enhancements to their chemical stability are still in great demand for the deployment of LHP NCs in light-emitting devices. Rather than contend with their attributes, we propose a method in which we can utilize their dynamic, ionic lattice and uniquely defect-tolerant band structure to prepare non-epitaxial salt-shelled heterostructures that are able to stabilize these materials against their environment, while maintaining their excellent optical properties and increasing scattering to improve out-coupling efficiency. To do so, anchored LHP NCs are first synthesized through the heterogeneous nucleation of LHPs onto the surface of microcrystalline carriers, such as alkali halides. This first step stabilizes the LHP NCs against further merging, and this allows them to be coated with an additional inorganic shell through the surface-mediated reaction of amphiphilic Na and Br precursors in apolar media. These inorganically shelled NC@carrier composites offer significantly improved chemical stability toward polar organic solvents, such as γ-butyrolactone, acetonitrile, N-methylpyrrolidone, and trimethylamine, demonstrate high thermal stability with photoluminescence intensity reversibly dropping by no more than 40% at temperatures up to 120 °C, and improve compatibility with various UV-curable resins. This mindset for LHP NCs creates opportunities for their successful integration into next-generation light-emitting devices.

Enhancement of efficiency and CCT uniformity for red phosphor thin films, red LEDs and laminated white LEDs based on near-ultraviolet LEDs using MgO nanoparticles

Red phosphor thin films (PTFs) with different MgO nanoparticle concentrations for near-ultraviolet (NUV) LEDs were prepared based on their strong scattering effect; red LEDs and laminated white LEDs were packaged further. SEM and XRD showed that CaAlSiN3:Eu2+ (CASN) and MgO nanoparticles were uniformly distributed in silicone resin and their crystal structure remained unchanged. The phosphor conversion efficiency (PCE) had a maximum value of 83.15% when the MgO nanoparticle concentration was 15 wt%. An increase of concentration can improve the spatial distribution uniformity of photonics for 410 nm, 627 nm and 660 nm. Fluorescence lifetime showed that the value is positively correlated with concentration change. The packaged red LED luminous flux reaches a maximum of 20.337 lm at a concentration of 15 wt%. The laminated white LED showed that the MgO nanoparticle concentration can be used to adjust the correlated color temperature (CCT) from 4322 K to 1987 K. Under similar CCT, the red phosphor concentration is only 1.83 wt%, the dosage is reduced by 56.12%, and the corresponding luminous efficiency of radiation (LER) and luminous efficiency (LE) are 296.03 lm W-1 and 73.72 lm W-1 respectively. The increase was 11.42%, the decrease was 10.14%, the color rendering index (CRI) increased from 90.6 to 91.8, and CCT uniformity increased from 82.04% to 89.27% with an increase of 8.81%. Research shows that MgO nanoparticles have potential application value in the preparation of high-quality white LEDs.

In Situ Photoluminescence of Colloidal Quantum Dots During Gas Exposure-The Role of Water and Reactive Atomic Layer Deposition Precursors

Colloidal quantum dots (QDs) are a promising material for optoelectronic applications. Typically, device integration requires QDs to be embedded in a host material. Atomic layer deposition (ALD) is often considered as a deposition technique for such purposes. However, it is known that ALD and vacuum processes often influence the optical properties of QDs in a negative way. Here, we describe an in situ photoluminescence (PL) measurement setup and use it to monitor the PL of QDs under vacuum and during ALD. For CdSe-based core/shell QDs, a reduction in the QD PL was observed upon exposure to vacuum. Water was identified as crucial for maintaining a high PL as evidenced by re-exposure to different gases. Furthermore, we addressed the influence of vacuum, different plasmas (O₂, H₂O, H₂, H₂S/Ar, and Ar), precursors (trimethylaluminum, diethylzinc, tetrakis(dimethylamido)titanium, and tetrakis(ethylmethylamido)hafnium), reactants (H₂O, H₂S, and O₃), and ALD processes (Al₂O₃, TiO₂, HfO₂, and ZnS) on QDs. We observed a PL reduction by up to 90% upon plasma treatments. Furthermore, we found that trimethylaluminum and diethylzinc reduced the PL efficiency by more than 70% while exposure to tetrakis(dimethylamido)titanium and tetrakis(ethylmethylamido)hafnium lowered the PL by only 10-20%. Surprisingly, tetrakis(dimethylamido)titanium and H₂O, which by themselves had only a minor influence on the QD PL, still caused an 80% drop of the PL efficiency when combined as an ALD process. On the other hand, ALD growth of HfO₂ by combining tetrakis(ethylmethylamido)hafnium and O₃ preserved 80% of the initial PL quantum yield, making it a promising process for QD embedding. These results put forward in situ PL measurements as a versatile technique to identify suitable precursors, reactants and ALD processes for QD embedding and investigate the interaction between QDs and reactive gaseous species in general.

White Emissive Carbon Dots Actuated by the H-/J-Aggregates and Förster Resonance Energy Transfer

Carbon dots (CDs) have been demonstrated to be fluorescent materials for the new phosphor-free white light-emitting diodes (WLEDs). Herein, we synthesized a novel white CDs (WCDs). The spectrum highly matches the solar light spectrum (AM 1.5), which is a potentially high-color-quality lighting source material. The CDs contain blue, green, and red emissive centers produced from catechol, o-phenylenediamine, and their complexes, respectively. In addition, the photoluminescence mechanism had been revealed; three emission centers could be excited by a single UV source actuated by the formation of H- and J-aggregates and FRET between the CDs. Then the phosphor-free WLEDs were fabricated with a UV chip encapsulated with silicon resin containing the as-obtained CDs, which exhibit CIE coordinates of (0.33,0.33), a color rendering index (CRI) of 93, and a correlated color temperature (CCT) of 5453 K. The WLEDs show super stability and a high solar spectrum matching degree of 85-114%, protecting the eyesight. This provides a new way to design healthy lighting materials.

Improving the Stability of Metal Halide Perovskite Quantum Dots by Encapsulation

Metal halide perovskite quantum dots (PQDs), with excellent optical properties and spectacular characteristics of direct and tunable bandgaps, strong light-absorption coefficients, high defect tolerance, and low nonradiative recombination rates, are highly attractive for modern optoelectronic devices. However, the stability issue of PQDs remains a critical challenge of this newly emerged material despite the recent rapid progress. Here, the encapsulation strategies to improve the stability of PQDs are comprehensively reviewed. A special emphasis is put on the effects of encapsulation, ranging from the improvement of chemical stability, to the inhibition of light-induced decomposition, to the enhancement of thermal stability. Particular attention is devoted to summarizing the encapsulation approaches, including the sol-gel method, the template method, physical blending, and microencapsulation. The selection principles of encapsulation materials, including the rigid lattice or porous structure of inorganic compounds, the low penetration rate of oxygen or water, as well as the swelling-deswelling process of polymers, are addressed systematically. Special interest is put on the applications of the encapsulated PQDs with improved stability in white light-emitting diodes, lasers, and biological applications. Finally, the main challenges in encapsulating PQDs and further investigation directions are discussed for future research to promote the development of stable metal halide perovskite materials.

Stretchable Organometal-Halide-Perovskite Quantum-Dot Light-Emitting Diodes

Stretchable light-emitting diodes (LEDs) and electroluminescent capacitors have been reported to potentially bring new opportunities to wearable electronics; however, these devices lack in efficiency and/or stretchability. Here, a stretchable organometal-halide-perovskite quantum-dot LED with both high efficiency and mechanical compliancy is demonstrated. The hybrid device employs an ultrathin (<3 µm) LED structure conformed on a surface-wrinkled elastomer substrate. Its luminescent efficiency is up to 9.2 cd A^-1 , which is 70% higher than a control diode fabricated on the rigid indium tin oxide/glass substrate. Mechanical deformations up to 50% tensile strain do not induce significant loss of the electroluminescent property. The device can survive 1000 stretch-release cycles of 20% tensile strain with small fluctuations in electroluminescent performance.

Coordination-induced emission enhancement in gold-nanoclusters with solid-state quantum yields up to 40% for eco-friendly, low-reabsorption nano-phosphors

Colloidal quantum dots (CQDs) have gained much attention as light-emitting materials for light-conversion nano-phosphors and luminescent solar concentrators. Unfortunately, those CQDs involve toxic heavy metals and frequently need to be synthesized in the hazardous organic solvent. In addition, they suffer from severe solid-state aggregation-induced self-quenching and reabsorption losses. To address these issues, here we prepare Zn-coordinated glutathione-stabilized gold-nanocluster (Zn-GSH-AuNCs) assemblies without involving heavy metals and organic solvent. Unlike GSH-AuNCs dispersed in an aqueous solution with poor photoluminescence quantum yields (PL-QYs, typically ~1%), those Zn-GSH-AuNCs powders hold high solid-state PL-QYs up to 40 ± 5% in the aggregated state. Such Zn-induced coordination-enhanced emission (CEE) is attributed to the combined effects of suppressed non-radiative relaxation and enhanced charge-transfer interaction. In addition, they also exhibit a large Stokes shift, thus mitigating both aggregation-induced self-quenching and reabsorption losses. Motivated by these photophysical properties, we demonstrated white-light emission from all non-toxic, aqueous-synthesis nano-materials.

Configuration- and concentration-dependent hybrid white light generation using red, green, and blue quantum dots embedded in DNA thin films

Artificial white-light production from primary red, green, and blue (RGB) colours is in high demand in the lighting, information technology, and wearable display industries, and it requires a simple device structure and efficient templates with stable luminophores. To realize efficient and stable optoelectronic devices, relevant materials and device structures need to be identified. Therefore, we report the construction of a simple hybrid white-light optoelectronic device with a single excitation source with efficient RGB colours on a stable optical platform. Emission wavelength-tunable R, G, and B quantum dots (QDs) with specific ligands and cetyltrimethylammonium chloride (CTMA)-modified DNA (CDNA) are prepared for the fabrication of QDs embedded in CDNA thin films with mixed and orthogonally stacked configurations. Fourier transform infrared, photoluminescence quantum yield (PLQY), ultraviolet (UV)-vis absorbance, photoluminescence, and electroluminescence (excited by blue and UV LEDs) spectra of the QDs embedded in CDNA thin films are analyzed to investigate their ligand attachment, luminescence efficiency, optical excitation, spectral emission, and hybrid white-light properties. In addition, the dispersion and photostability of QDs in the CDNA matrix were analyzed. The colour rendering index (CRI) values and colour gamut of the QDs embedded in CDNA thin films are studied for evaluating the light quality. The results show that the ligands on the QDs enhance PLQY up to 95 and 25% in liquid and solid phases, respectively. The optical properties of the QDs in the CDNA thin films are not significantly affected by phase changes, which implies the effective hosting of QDs within CDNA. The CRI values of the mixed and stacked configuration-dependent QDs embedded in CDNA thin films are 21 and 80%, respectively, which suggest the relatively stronger self-absorption of R QDs in the mixed configuration than in the stacked configuration. In addition, CRI values and colour gamut are affected by different R, G, and B QD concentrations in CDNA. These findings are important for solid-state lighting, information display systems, flexible displays, and wearable displays.

Color-Tunable Upconversion in Er3+/Yb3+-Codoped KLaF4 Nanophosphors by Incorporation of Tm3+ Ions for Biological Applications

Heavily doped nanocrystals of host KLaF4 with rare earth (RE3+ = Er3+, Tm3+, and Yb3+) ions prepared by a simple one-step template-free wet-chemical route have been reported. Prepared KLaF4 nanocrystals reveal phase-pure cubic structures (lattice constant a = 5.931Å) with space group Fm3m. Precisely defined molar ratios of heavily dopant RE3+ ions allow us to achieve wide color upconversion (UC) emission tunability (blue, green to yellow-orange-red) and white light, without any morphology and structure changes. The enhanced red emission by a factor of ∼120 has been achieved in 20% Yb3+ and 5% Tm3+ ions in KLaF4:1% Er3+ nanocrystals, which is due to an efficient sensitizer-acceptor (Yb3+ to Er3+ and Tm3+ ions) energy transfer and interexchange energy process between acceptors. For the first time, the key role of sensitizer (Yb3+) for UC emission energy transfer to Er3+ and/or Tm3+ is experimentally demonstrated. The evidence of upconversion photoluminescence excitation spectra reveals a broad safe biological excitation window (690-1040 nm), which can be well demonstrated by low-cost NIR diode lasers/LEDs. The applicability of these cubic nanophosphors is demonstrated as light-emitting polymer composite coatings and blocks for LEDs and solar cell panels. These well-dispersed UC nanocrystals can also be found to have greater use in bioimaging and spectral studies.

Greener Luminescent Solar Concentrators with High Loading Contents Based on in Situ Cross-Linked Carbon Nanodots for Enhancing Solar Energy Harvesting and Resisting Concentration-Induced Quenching

A luminescent solar concentrator (LSC) is composed of loaded luminophores and a waveguide that can be employed to harvest and concentrate both direct and diffused sunlight for promising applications in solar windows. Thus far, most of efficient LSCs still relied on the heavy-metal-containing colloidal quantum dots (CQDs) dispersed into a polymer matrix with a very low loading (typically <1 wt %). Such low-loading constraint is required to mitigate the concentration-induced quenching (CIQ) and maintain high optical quality and film uniformity, but this would strongly reduce the light-absorbing efficiency. To address all issues, greener LSCs with high loading concentration were prepared by in situ cross-linking organosilane-functionalized carbon nanodots (Si-CNDs), and their photophysical properties relevant to LSC operation were studied. The PL emission is stable and does not suffer from the severe CIQ effect for cross-linked Si-CNDs even with 25 wt % loadings, thus exhibiting high solid-state quantum yields (QYs) up to 45 ± 5% after the calibration of the reabsorption losses. Furthermore, such LSCs can still hold high optical quality and film uniformity, leading to low scattering losses and high internal quantum efficiency of ∼22%. However, the reabsorption losses need to be further addressed to realize large-area LSCs based on earth-abundant, cost-effective CNDs.

Microwave-Assisted Heating Method toward Multicolor Quantum Dot-Based Phosphors with Much Improved Luminescence

Solid-state highly photoluminescent quantum dot (QD)-based phosphors attract great scientific interests as color converters because of an increasing demand for white-light-emitting devices. Herein, a microwave-assisted heating method is presented to fabricate multicolor QD-based phosphors within 30 s through microwave-assisted heating of the mixture of QDs and sodium silicate aqueous solution. In the composites, the formed cross-linked networks not only play as a matrix to prevent QD aggregation in solid state but also cause the variation of the refractive index around QDs and the QD surface optimization, which contributes to good stabilities and twice enhancement in photoluminescence quantum yields (69%) compared with the initial QD aqueous solution (33%). Using the QD-based phosphors as color conversion layers, white-light-emitting diodes were realized with controllable color temperature, high color purity, and high color-rendering index (90.3), which show a great potential in display and illumination. Furthermore, the luminescence lifetime of the QD-based phosphors is less than 25 ns. The potential application of the QD-based phosphors in visible light communication was also demonstrated, with the modulation bandwidth achieving 42 MHz.

The Role of Metal Halide Perovskites in Next-Generation Lighting Devices

The development of smart illumination sources represents a central challenge for current technology. In this context, the quest for novel materials that enable efficient light generation is essential. Metal halide compounds with perovskite crystalline structure (ABX₃) have gained tremendous interest in the last five years since they come as easy-to-prepare high performance semiconductors. Perovskite absorbers are driving the power-conversion-efficiencies of thin film photovoltaics to unprecedented values. Nowadays, mixed-cation, mixed-halide lead perovskite solar cells reach efficiencies consistently over 20% and promise to get close to 30% in multijunction devices when combined with silicon cells at no surcharge. Nonetheless, perovskites' fame extends further since extensive research on these novel semiconductors has also revealed their brightest side. Soon after their irruption in the photovoltaic scenario, demonstration of efficient color tunable-with high color purity-perovskite emitters has opened new avenues for light generation applications that are timely to discuss herein.

Red Mn4+-Doped Fluoride Phosphors: Why Purity Matters

Traditional light sources, e.g., incandescent and fluorescent lamps, are currently being replaced by white light-emitting diodes (wLEDs) because of their improved efficiency, prolonged lifetime, and environmental friendliness. Much effort has recently been spent to the development of Mn⁴⁺-doped fluoride phosphors that can enhance the color gamut in displays and improve the color rendering index, luminous efficacy of the radiation, and correlated color temperature of wLEDs used for lighting. Purity, stability, and degradation of fluoride phosphors are, however, rarely discussed. Nevertheless, the typical wet chemical synthesis routes (involving hydrogen fluoride (HF)) and the large variety of possible Mn valence states often lead to impurities that drastically influence the performance and stability of these phosphors. In this article, the origins and consequences of impurities formed during synthesis and aging of K₂SiF₆:Mn⁴⁺ are revealed. Both crystalline impurities such as KHF₂ and ionic impurities such as Mn³⁺ are found to affect the phosphor performance. While Mn³⁺ mainly influences the optical absorption behavior, KHF₂ can affect both the optical performance and chemical stability of the phosphor. Moisture leads to decomposition of KHF₂, forming HF and amorphous hydrated potassium fluoride. As a consequence of hydrate formation, significant amounts of water can be absorbed in impure phosphor powders containing KHF₂, facilitating the hydrolysis of [MnF₆]^2- complexes and affecting the optical absorption of the phosphors. Strategies are discussed to identify impurities and to achieve pure and stable phosphors with internal quantum efficiencies of more than 90%.

Red carbon dots-based phosphors for white light-emitting diodes with color rendering index of 92

Exploration of solid-state efficient red emissive carbon dots (CDs) phosphors is strongly desired for the development of high performance CDs-based white light-emitting diodes (WLEDs). In this work, enhanced red emissive CDs-based phosphors with photoluminescence quantum yields (PLQYs) of 25% were prepared by embedding red emissive CDs (PLQYs of 23%) into polyvinyl pyrrolidone (PVP). Because of the protection of PVP, the phosphors could preserve strong luminescence under long-term UV excitation or being mixed with conventional packaging materials. By applying the red emissive phosphors as the color conversion layer, WLEDs with high color rendering index of 92 and color coordinate of (0.33, 0.33) are fabricated.

Toward Highly Luminescent and Stabilized Silica-Coated Perovskite Quantum Dots through Simply Mixing and Stirring under Room Temperature in Air

Methylammonium (MA) lead halide (MAPbX₃, X = Cl, Br, I) perovskite quantum dots (PQDs) are very sensitive to environment (moisture, oxygen, and temperature), suffering from poor stability. To improve the stability, we synthesized silica-coated PQDs (SPQDs) by an improved ligand-assisted reprecipitation method through simply mixing and stirring under room temperature in air without adding water and catalyst, the whole process took only a few seconds. The photoluminescence (PL) spectra of the SPQDs can be tuned continuously from 460 to 662 nm via adjusting the composition proportion of precursors. The highest PL quantum yields (PLQYs) of blue-, green-, and red-emissive SPQDs are 56, 95, and 70%, respectively. The SPQDs show remarkably improved environmental and thermal stability compared to the naked PQDs because of effective barrier created by the coated silica between the core materials and the ambience. Furthermore, it is found that different light-emitting SPQDs can maintain their original PL properties after mixing of them and anion-exchange reactions have not happened. These attributes were then used to mix green- and yellow-emissive SPQDs with polystyrene (PS) to form color-converting layers for the fabrication of white light-emitting devices (WLEDs). The WLEDs exhibit excellent white light characteristics with CIE 1931 color coordinates of (0.31, 0.34) and color rendering index (CRI) of 85, demonstrating promising applications of SPQDs in lighting and displays.

Photonic Crystal Phosphors Integrated on a Blue LED Chip for Efficient White Light Generation

Following the proof-of-concept experiment in the unit structure level, photonic crystal (PhC) phosphors-structurally engineered phosphor materials based on the nanophotonics principles-are integrated with a blue light-emitting diode (LED) chip to demonstrate a compact and efficient white light source. Red- or green-emitting CdSe-based colloidal quantum dots (CQDs) are coated on a Si₃ N₄ thin-film grating to fabricate PhC phosphors. The underlying PhC structure is designed such that the photonic band-edge modes at the zone center (k_∣∣ = 0) are tuned to the energy of the blue excitation photons. By progressively stacking the PhC phosphor plates on a blue LED chip, the blue, green, and red emission intensities can be tightly controlled to obtain white light with the desired properties. The chromaticity coordinates, (0.332, 0.341), and correlated color temperature, 5500 K, are obtained from a stack of 3 red and 11 green PhC phosphor plates; in contrast, a stack of 5 red and 16 green reference phosphor plates are required to generate a similar white light. Overall, the PhC phosphors produce 8% higher total emission intensity out of 33% less amount of CQDs than the reference phosphors.

Selecting the optimal synthesis parameters of InP/CdxZn1-xSe quantum dots for a hybrid remote phosphor white LED for general lighting applications

Quantum dots can be used in white LEDs for lighting applications to fill the spectral gaps in the combined emission spectrum of the blue pumping LED and a broad band phosphor, in order to improve the source color rendering properties. Because quantum dots are low scattering materials, their use can also reduce the amount of backscattered light which can increase the overall efficiency of the white LED. The absorption spectrum and narrow emission spectrum of quantum dots can be easily tuned by altering their synthesis parameters. Due to the re-absorption events between the different luminescent materials and the light interaction with the LED package, determining the optimal quantum dot properties is a highly non-trivial task. In this paper we propose a methodology to select the optimal quantum dot to be combined with a broad band phosphor in order to realize a white LED with optimal luminous efficacy and CRI. The methodology is based on accurate and efficient simulations using the extended adding-doubling approach that take into account all the optical interactions. The method is elaborated for the specific case of a hybrid, remote phosphor white LED with YAG:Ce phosphor in combination with InP/CdxZn_1-xSe type quantum dots. The absorption and emission spectrum of the quantum dots are generated in function of three synthesis parameters (core size, shell size and cadmium fraction) by a semi-empirical 'quantum dot model' to include the continuous tunability of these spectra. The sufficiently fast simulations allow to scan the full parameter space consisting of these synthesis parameters and luminescent material concentrations in terms of CRI and efficacy. A conclusive visualization of the final performance allows to make a well-considered trade-off between these performance parameters. For the hybrid white remote phosphor LED with YAG:Ce and InP/CdxZn_1-xSe quantum dots a CRI Ra = 90 (with R9>50) and an overall efficacy of 110 lm/W is found.