Discussion on red phosphor for white LED

White LED Overview As society progresses, energy conservation and environmental protection have become critical concerns worldwide. Lighting electricity constitutes a substantial portion of overall energy consumption in daily life. Traditional lighting methods suffer from high power consumption, short lifespans, low efficiency, and environmental pollution, making them incompatible with modern needs. Hence, the shift towards sustainable lighting solutions like semiconductor white light-emitting diodes (WLEDs) has gained momentum. WLEDs are renowned for their long lifespan, high conversion efficiency, and minimal environmental impact. Compared to conventional lighting, they offer benefits such as reduced energy usage, lower carbon emissions, and smaller physical dimensions. These advantages have led to widespread applications in areas such as transportation, displays, healthcare, and electronics. LEDs are considered one of the most promising light sources of the 21st century. Under identical lighting conditions, WLEDs consume only half the energy of fluorescent lamps and just a fifth of incandescent bulbs. Globally, traditional lighting accounts for approximately 13% of energy consumption. Replacing these with WLEDs could cut energy use by half, yielding significant financial savings. Since the breakthrough in GaN blue LED technology in 1993, researchers have steadily advanced white LED applications. Initially, GaN was used as a blue light source, and phosphor conversion techniques were employed to create white light from single LEDs. This milestone facilitated the integration of LEDs into illumination systems. Household lighting represents the largest application area for WLEDs. Despite progress, challenges remain, including improving luminous efficiency, color rendering, and lifespan. Although LED lighting cannot yet fully replace traditional sources, technological advancements ensure growing popularity. Research on Red Phosphors for White LEDs Current white LED phosphors often exhibit low color rendering indices, high color temperatures, and a bias toward cooler white tones due to insufficient red components. Developing efficient red phosphors is crucial. Based on matrix materials, research primarily focuses on the following systems: 1. Sulfide, Sulfur Oxide, and Lanthanide Oxide Systems Earth alkaline metals serve as cations in sulfide red phosphor matrices, with Eu²⁺ as the activating ion. These phosphors boast high luminous efficiency and broad application. Studies incorporating Ca into SrS:Eu²⁺ matrices via solid-state reactions yielded (Cax,Sr1-x)S:Eu²⁺ phosphors. Research indicates that introducing dopants alters emission peak positions and intensities, shifting peaks toward longer wavelengths as Ca²⁺ concentrations increase. Ruthenium oxide or sulfur oxide-based red phosphors typically utilize Eu³⁺ as the activating ion, with excitation peaks around 350 nm, 380 nm, and 460 nm. Y₂O₂S:xEu³⁺ phosphors have been developed, showing emission peaks shifting rightward with increasing Eu³⁺ concentrations, eventually reaching 626 nm. RF-sputtered Y₂O₂S:Eu phosphor films closely resemble commercial equivalents. While near-ultraviolet and blue LED-excited phosphors exist, their low substrate luminescence efficiency limits versatility. Additionally, sulfide phosphors degrade under high temperatures or moisture exposure, posing environmental risks. 2. Nitride System Nitride red phosphors exhibit superior thermal and chemical stability, making them extensively researched. Eu²⁺ serves as the activating ion, enabling absorption across near-ultraviolet to blue-green wavelengths and emitting yellow, orange, and red wavelengths between 550 nm and 750 nm. Sr₂Si₅N₈:Eu phosphors demonstrate strong emission wavelength shifts with increasing Eu²⁺ concentrations. CaAlSiN₃-doped Eu²⁺ phosphors prepared via high-temperature solid-phase methods showed optimal crystallization at 1700°C, 0.65 MPa for 3 hours, and a 4 mol% doping concentration. Nitride red phosphors surpass sulfides in controllability and physicochemical properties but require stringent preparation conditions—temperatures of 1400°C–2000°C, long-term insulation, and nitrogen protection. 3. Silicate System Silicate phosphors are diverse and perform excellently, making them widely studied. Eu³⁺ is commonly chosen as the activating ion. Previous research experiences guided studies on Eu³⁺-doped silicate-based red phosphors using sol-gel methods, combustion methods, and high-temperature solid-phase techniques. Factors like charge compensators, activator doping concentrations, and flux additives influenced luminescence effects. High-purity NH₄H₂PO₄, CaCO₃, and SiO₂ were used to prepare charge-compensated A⁺-containing halides (A=Na, Li, K). A high-temperature solid-phase method yielded Ca₅(PO₄)₂SiO₄:Eu³⁺ phosphors suitable for near-ultraviolet chip excitation. The phosphor's fluorescence was excellent, with a main peak at 395 nm and a main emission peak at 615 nm. Eu³⁺-activated silicate-based red phosphors were prepared with LaPO₄-5SiO₂ as the composite matrix via the sol-gel method. Tests confirmed excellent performance, with an optimal Eu³⁺ doping concentration of 7 mol% and an excitation wavelength of 395 nm (violet region) and emission wavelength at 612 nm (red light). 4. Tungstate System Tungstates are chemically stable and widely used as phosphor matrix materials. In red phosphor studies, Eu³⁺ is often used as the activating ion. Sr(1-x)MoO₄:xEu³⁺ phosphors were prepared using combustion, sol-gel, and solid-phase methods. Gd₂(MO₄)₃(M=Mo, W):Eu³⁺,Sm³⁺ phosphors were synthesized via the solid-phase method. Testing revealed effective ultraviolet (UV) and blue light excitation, with emission wavelengths close to commercially available GaN-based red phosphors. White LED Implementation Presently, WLED implementation falls into three categories: 1. Multi-Chip Combinations Multi-chip combination WLEDs adjust red, green, and blue light from multiple semiconductor chips to blend into white light at a fixed ratio. This approach offers high efficiency, adjustable color temperature, and a high color rendering index (CRI). However, drawbacks include high production costs, weak domestic manufacturing capabilities, and inconsistencies in chip quantum efficiencies leading to color instability. 2. Fluorescence Conversion Type Fluorescence conversion WLEDs use low-voltage direct current to excite a single semiconductor chip, stimulating a phosphor-coated chip to emit long-wavelength visible light. Adjusting phosphor proportions creates white light. This method includes two types: First Type: Yellow phosphor excited by a blue chip (YAG:Ce³⁺)—blue LEDs excite yellow phosphors, combining emitted yellow and blue light to generate white. Advantages include simple design, low process requirements, and mature technology. Disadvantages include low chip efficiency, significant energy loss, and color temperature drift over time. Incorporating red phosphors improves CRI but requires precise adjustments. Second Type: RGB phosphors excited by UV light chips—near-ultraviolet (360-410 nm) LEDs excite blue, green, and red phosphors to produce white light. Benefits include simple circuitry, low cost, and wide spectral distribution. Challenges include high-power UV LED difficulty, demanding processes, low red phosphor efficiency, and UV leakage risks. 3. Single-Chip Multi-Quantum Well Type Single-chip multi-quantum well WLEDs feature alternating thin-film layers of different materials forming multilayer structures containing quantum wells. Different doping materials produce blue-to-red light emission. Advantages include simple design, high material efficiency, and low energy loss. However, complex preparation techniques and high costs hinder widespread adoption. Current technology remains experimental. In conclusion, WLEDs are primarily produced via three methods, with fluorescence conversion being dominant due to cost-effectiveness and simplicity. Future innovations may focus on single-chip multi-quantum well WLEDs. Red phosphors are critical for fluorescence conversion WLEDs. Finding stable materials excited by ultraviolet/near-ultraviolet light with high CRIs is a key research goal. Material properties like morphology, microstructure, particle size, and agglomeration significantly affect luminescence. Thus, developing highly efficient red phosphors remains challenging. Intern Editor: Liang Jieying

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