Overview of White LEDs
With the advancement of society, energy conservation and environmental protection have become increasingly important concerns in today's world. As a result, there is a growing emphasis on finding new lighting solutions that are both energy-efficient and environmentally friendly. Traditional lighting methods, while widespread, suffer from high energy consumption, short lifespans, and low conversion efficiency, making them less sustainable. The transition to more efficient lighting technologies like white LEDs (WLEDs) is crucial for addressing these issues.
Researchers have made significant strides in developing semiconductor-based WLEDs, which offer a longer lifespan, higher efficiency, and reduced environmental impact. Compared to conventional lighting methods, WLEDs boast several advantages, including low carbon emissions, minimal mercury contamination, compact size, and energy savings. These features make WLEDs highly applicable in various sectors such as transportation, healthcare, and consumer electronics.
Currently, WLEDs are seen as one of the most promising innovations in lighting technology. Under comparable 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 total energy consumption. By replacing traditional lighting with WLEDs, energy usage could potentially be cut in half, leading to substantial energy savings and economic benefits.
Known as the fourth-generation lighting device, the white LED has gained significant attention due to its exceptional performance. Ongoing research and development efforts have broadened its applications in areas like display technology and general illumination. A major breakthrough in LED technology occurred in 1993 when gallium nitride-based blue LEDs were successfully developed, paving the way for the integration of LEDs into lighting systems.
The largest application of WLEDs lies in household lighting. Despite their potential, current WLEDs face challenges such as lower color rendering indices and shorter lifespans. Improving these aspects remains a priority to ensure their widespread adoption. While LED lighting has yet to fully replace traditional sources, technological advancements suggest that LEDs will continue to gain popularity.
Research on Red Phosphors for White LEDs
Current white LED phosphors often exhibit issues such as low color rendering indices and high color temperatures, favoring colder tones. This is primarily due to a lack of sufficient red light components. Developing high-efficiency red phosphors is thus critical. Based on the matrix material, phosphors can be categorized into several systems:
1. Sulfide, Sulfate, and Lanthanide Oxide Systems
Researchers have explored alkaline earth metals as cations for sulfide red phosphors, with europium (Eu²âº) serving as the activating ion. These phosphors demonstrate high luminous efficiency and are widely used in white light applications. Studies have shown that introducing dopants like calcium into strontium sulfide (SrS:Eu²âº) matrices leads to changes in emission peak positions and intensities as concentrations increase.
Phosphors based on ruthenium oxides or sulfur oxides typically utilize Eu³⺠as the activating ion, with excitation peaks around 350 nm, 380 nm, and 460 nm. Y₂O₂S:xEu³⺠red phosphors have been developed, showing emission peaks shifting rightward with increasing Eu³⺠concentrations, eventually reaching 626 nm.
RF-sputtered Yâ‚‚Oâ‚‚S:Eu luminescent films closely mimic commercial phosphors. Although these phosphors can be excited by near-ultraviolet light and blue LED chips, their low substrate luminescence efficiency limits their versatility. Additionally, sulfide phosphors are chemically unstable, decomposing under high temperatures or moisture exposure, posing environmental risks.
2. Nitride System
Nitride-based red phosphors exhibit excellent thermal and chemical stability, making them a focal point of research. Eu²⺠is commonly used as the activating ion. Sr₂Si₅N₈:Eu red phosphors can absorb wavelengths from near-ultraviolet to blue-green and emit yellow, orange, and red light from 550 nm to 750 nm, with high efficiency. Emission wavelengths shift redward with increasing Eu²⺠doping concentration.
CaAlSiN₃:Eu²⺠phosphor powders prepared via high-temperature solid-phase methods showed optimal crystallization at 1700°C, 0.65 MPa for 3 hours, and 4 mol% doping. Compared to commercial sulfide red phosphors, nitride red phosphors offer better color control and superior physicochemical properties but require stringent preparation conditions—high temperatures (1400°C–2000°C), prolonged insulation, and nitrogen protection, which increases costs and resource consumption.
3. Silicate System
Silicate-based phosphors are numerous and perform exceptionally well, making them popular in phosphor research. Activating ions such as Eu³⺠are commonly used to create red phosphors. Studies of Eu³âº-doped silicate-based red phosphors have built upon previous experiences, employing methods like the sol-gel approach, combustion techniques, and high-temperature solid-phase processes to investigate the transition of Eu³⺠ions at 5D₀→7Fâ‚‚. These studies also explore the effects of different charge compensators, activator doping concentrations, and flux additions on luminescence.
Using high-purity NH₄H₂PO₄, CaCO₃, and SiO₂ as raw materials, charge-compensated ions derived from alkali-metal-containing halides were obtained. A high-temperature solid-phase method was used to prepare Ca₅(PO₄)₂SiO₄:Eu³⺠red phosphor materials suitable for near-ultraviolet chip excitation. The resulting phosphor material demonstrated excellent fluorescence, with a main peak at 395 nm in the near-ultraviolet region and a dominant emission peak at 615 nm. Its fluorescence intensity and color coordinates closely matched those of commercial phosphors.
A red phosphor with Eu³⺠as the activator and LaPO₄-5SiO₂ as the composite matrix was prepared using the sol-gel method. Test results indicated that the prepared silicate-based red phosphor performed excellently, with an optimal Eu³⺠doping concentration of 7 mol%, an optimal excitation wavelength of 395 nm (in the violet region), and an emission wavelength of 612 nm (red light).
4. Tungstate System
Tungstates are chemically stable inorganic materials, making them ideal matrix materials for phosphors. In red phosphor research, Eu³⺠is often used as the activating ion. Gdâ‚‚(MOâ‚„)₃(M=Mo, W):Eu³âº,Sm³⺠red phosphors were synthesized using solid-phase methods, demonstrating effective excitation by ultraviolet (UV) and blue light, with emission wavelengths close to those of commercially available GaN-based red phosphors.
Implementation of White LEDs
WLEDs can currently be implemented through three main methods:
1. Multiple Chip Combinations
Multiple chip combination type WLEDs adjust red, green, and blue light emitted from several semiconductor chips and combine them into white light at a specific ratio. This method offers high efficiency, a high color rendering index, adjustable color temperature, and low photoelectric losses. However, it suffers from high production costs due to the large number of chips required and weak domestic independent production capabilities. Additionally, differing quantum efficiencies among chips lead to varying attenuation rates under inconsistent operating conditions, causing color instability and limiting practical applications.
2. Fluorescence Conversion Type
Fluorescence conversion type WLEDs work by exciting a single-substrate semiconductor chip with low-voltage direct current, causing the chip’s emitted light to excite phosphors coated on it, emitting long-wavelength visible light. White light is achieved by adjusting the proportion of phosphors.
This method can be further divided into two types based on excitation methods:
The first type involves exciting a yellow phosphor with a blue chip (YAG:Ce³âº): A blue LED chip serves as the excitation source, exciting a yellow phosphor that matches the chip’s emission wavelength. The combination of emitted yellow light and the blue excitation light produces white light. Advantages include simple structural design, low production process requirements, and mature technology. However, the method has significant drawbacks, including low luminous efficiency of the excitation source, significant energy loss during light conversion, and color temperature drift over time and environmental changes. The lack of red light in the structural design results in a low color rendering index. To address this, an appropriate amount of red phosphor should be incorporated into the YAG:Ce³⺠system to enhance the color rendering index.
The second type involves exciting red, green, and blue phosphors with an ultraviolet light chip: A near-ultraviolet (360-410 nm) LED chip acts as the excitation source, exciting three types of phosphors capable of emitting blue, green, and red primary colors. Various colors are combined to produce white light. Key advantages include simple circuit design, low production costs, and easy control. Excitation and emission spectrum peaks can be precisely controlled, offering wide spectral distribution and readily available luminescent materials. However, high-power near-ultraviolet LEDs are difficult to manufacture, and the process requirements are high. Current red phosphors have low luminous efficiency and unstable color temperatures. Using near-ultraviolet chips as excitation sources also poses risks of UV damage, aging, shortened lifespan, and UV leakage.
3. Single-Chip Multi-Quantum Well Type
Single-chip multi-quantum well type WLEDs feature alternating layers of different materials grown on a substrate to form a multilayer structure containing numerous separated quantum wells. Different quantum materials are formed by changing doping materials, enabling emission from blue to red light to achieve white light emission. These WLEDs offer advantages such as simple structural design, high material luminous efficiency, and low energy loss during light conversion. However, preparing single-chip multi-quantum well WLEDs requires advanced process technology and results in high production costs, making them difficult to mass-produce. Currently, this technology remains in the laboratory testing and development stage, with immature technology and applications.
In summary, WLEDs are mainly produced through three methods, with the first two being the primary focus of current applied research. Fluorescence-converted WLEDs are currently the simplest and most effective method considering production processes, technical requirements, and costs. Next come multiple chip combination type WLEDs. From a future development and innovation perspective, single-chip multi-quantum well type WLEDs hold significant potential for growth.
Conclusion
In conclusion, the red phosphor plays a crucial role in fluorescence conversion type white LEDs. Developing red phosphors with stable physicochemical properties that can be excited by ultraviolet/near-ultraviolet light and achieve high color rendering indices has become a vital research direction. Since material properties are influenced by factors such as morphology, microstructure, particle size, and aggregation, luminescence properties are significantly affected. Thus, preparing highly efficient red phosphors remains a challenging task.
Intern Editor: Liang Jieying
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