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Advanced Inorganic Materials for Solid State Lighting
The bright future of solid-state lighting
In 1907, Henry Round described the first electroluminescent device based on silicon carbide (Product No. S104650) 1,2. Since then, lighting based on solid-state devices has started a long research road. The development of bright "candela-class" green, blue, and near-ultraviolet diodes by Nakamura 3 in the 1990s was a major breakthrough, completing the spectral color gamut of white light and enabling the development of solid-state lighting (SSL) into a viable alternative plan. It can replace other less efficient light sources such as incandescent light bulbs or even compact fluorescent lamps (CFL). SSL has higher efficiency, longer lifetime (50,000 to 100,000 hours, compared to 15,000 hours for CFLs and 1,000 to 2,000 hours for incandescent lamps) and It does not contain toxic elements such as mercury, and will soon become the standard technology for artificial light sources 4-6.
White solid-state light can be produced in three different ways: The first, using three diodes that emit red, green, and blue light, respectively. The second, using a near-violet LED, excites several phosphors that emit light across the entire spectrum. Or the third and most widely used alternative, which involves down-converting a portion of the blue LED light to longer wavelengths, producing white light in this way. Other strategies are less used due to some inherent difficulties. For example, it is currently impossible to produce LEDs that emit efficiently in the green region of the visible spectrum, which hinders the three LED strategies, and NUV light with a full down conversion strategy is inherently inefficient due to the large Stokes shift involved, and inorganic, inorganic Phosphors play a key role in implementing the widely adopted partial down conversion strategy 7.
In addition to the blue light initially emitted, the phosphor also provides the required green/yellow/red light for the white light source. Inorganic phosphors typically consist of host crystals, which can be oxides, oxynitrides, nitrides, halides, or oxyhalides, chosen for their wide bandgap and other key properties, while doped with small amounts of rare earths and/or transition metal ions, as luminescent centers 8-12 . As shown in the CIE (Commission Inter nationale de Illumination) chart in Figure 1, almost every color can be rendered by using phosphors in combination with near-ultraviolet or blue light sources.
Rare earth ions such as Eu3+, Tb3+ or Sm3+ can be used to prepare phosphors. However, in these systems, the emission relies on ff transitions, which are forbidden transitions and are therefore rather inefficient. In addition, the lower f orbital well shields the coordination environment of the ion, so the sharp emission from these ff transitions is not suitable to cover a large region of the visible spectrum. To avoid these problems of narrow emission and low efficiency, the most common strategy is to dope the phosphors with ions with broad emission, such as Mn2+, Ce3+ or Eu2+ (see also the example in Figure 1). In Ce3+ and Eu2+ luminescence is due to the 4f to 5d transition within the ion. In free Ce3+ ions, the 5d state is degenerate and its energy is much higher than the two 4f ground states of the ion. However, the d-orbital interacts significantly with the lattice, as shown in the case of Ce3+ in Figure 2. Once introduced into the host lattice, the energy levels of the d orbitals are lowered due to their interactions with surrounding ligands (center of mass shift due to nephelauxetic effects). Crystal field splitting promotes degeneracy due to different interactions of ligands with different d orbitals (xy, xz, yz, x2-y2, z2) and, in most structures, leads to up to five Different states are in energy. The energy difference between the d state of the free ion and the lowest d state in the crystal host is called D(A), or spectral redshift 13.
Figure 1. Color coordinates for some phosphor materials, blue and near
Figure 2. Electronic states of free cerium ions and crystal field splitting observed in the matrix 5
The quest for improved materials
Due to the great significance to improve the performance of SSL, a lot of research work has been done in the development of phosphor materials. A schematic diagram of this type of solid-state white light lighting device is shown in Figure 3. It consists of a blue-emitting LED chip with phosphors directly above the chip, dispersed in transparent silicone12or covered on its surface, as shown in Figure 3. Blue light passes through this phosphor layer, converting part of the blue light to yellow, producing (cool) white light. There are many issues of efficiency (many phosphors do not have near 100% quantum efficiency), proper color rendering, and color temperature that drive phosphor research. Equally important is the efficiency loss at high temperatures, which is increasingly relevant for higher-power LED white light sources, such as those used in automotive headlamps.
Figure 3. The LED emits blue-generated white light that is down converted by a yellow phosphor encapsulated in a silicone cap
To achieve optimal device efficiency, phosphors need to meet certain requirements, including but not limited to:
1.Very high quantum efficiency to maximize the number of photons (re)emitted by the phosphor.
2.suitable excitation and emission spectra. The excitation spectrum of the phosphor should have sufficient spectral overlap with the emission spectrum of the LED for efficient pumping. An example is given in Figure 4, showing the emission and excitation spectra of cerium - doped Y3Al5O12 (YAG). The emission of the phosphor itself should be broad, thus reproducing the sun's wide spectral color gamut well. Measured in the Color Rendering Index, CRI or Ra, this property compares the color rendering of any light source to a reference blackbody source at temperatures approaching 5000K. A value of 90 is called the ideal value.
3.High-performance devices require chemical and thermodynamic adaptability. This is important for the long-term prospects of solid-state lighting. It is also important to use phosphors on high-power LED chips, whose operating temperatures can reach nearly 200°C.
Figure 4. Two-dimensional excitation/emission spectra of Y3Al5O12:Ce3+ . The excitation band shows a maximum around 460 nm, while the emission maximum is located around 550 nm. One-dimensional excitation/emission spectra are shown on the left and top of the figure and were taken at the emission/excitation wavelength corresponding to the maximum intensity in the spectrum
Often, some of these requirements are contradictory. This means, for example, that broad-emitting phosphors may have poor efficiency, while materials with adequate quantum efficiency may not cover the emission spectrum in the desired wavelength range. Some of these properties are strongly dependent on dopant ions, while others are easily influenced by the matrix. Therefore, the challenge for materials scientists is to finally prepare stable, inexpensive and efficient materials, finding the optimal combination of host lattice and activator ions.
Phosphorus Preparation
Various preparation methods have been developed to obtain phase-pure and high-quality phosphor materials. Typically, high temperature solid state reactions are the preferred method, which means thorough mixing and homogenization of the starting compounds (usually oxides, carbonates or nitrates), followed by under heating. A reducing atmosphere, such as a mixture of H and N or CO gas, is typically used to convert the dopant ions to the desired valence state (eg, Ce3+ instead of Ce4+, and Eu2+ instead of Eu3+). Alternative reactions include solution-based methods such as hydrothermal or solvothermal preparations, or sol-gel and spray pyrolysis methods.
More exotic synthetic routes include combustion synthesis, as well as microwave-assisted solid-state preparation14, offering unparalleled reaction rates but much less control over the properties of the final product. The preparation of oxynitrides and nitrides often requires more severe conditions, such as very high temperatures (sometimes over 2000°C) and high N2 partial pressures, to ensure the incorporation of nitrogen into the crystal lattice. Typically, air-sensitive precursors require preparation steps under inert conditions, as do certain oxide materials.
Phosphorus oxide
Among the many different phosphorus compounds that have been studied, oxides occupy the largest share due to their ease of preparation and low-cost production, usually combined with the excellent stability of the compounds produced. So far, the most widely used material in solid-state lighting is yttrium aluminum garnet, Y3Al5O12, doped with a small amount of cerium, abbreviated as YAG:Ce . The most efficient materials are prepared at temperatures above 1500°C, typically doped with 2 mol-% to 3 mol-% cerium. An image of the cell is shown in Figure 5A. YAG crystallizes in cubic space group Ia-3d and consists of AlO tetrahedra and AlO octahedra, fully angularly connected, forming a rigid, highly connected 3D network.
Figure 5. Schematic diagram of the unit cell of widely used phosphors: A) showing garnet Y3Al5O12, B) orthosilicate Ba2SiO4, C) oxynitride CaSi2O2N2 and D) Nitride Sr2Si5N8. Gray spheres represent Y, Ba, Ca, and Sr atoms, while light blue, red, orange, and dark blue spheres represent Al, Si, O, and N atoms, respectively
Y3+ ions occupy the voids in this network, and they are coordinated by a total of eight oxygen ions, creating a twisted coordination environment. The Y3+ ions are also connected in three dimensions through their polyhedral edges, forming an interwoven network with the connected AlOn polyhedra in a manner known from the double helix structure in block copolymers. YAG:Ce , first prepared by Blase and Bril in 196715 , has become a standard phosphor material in solid-state white lighting applications. The reasons are manifold, such as it is composed of only relatively cheap and abundant elements and can be prepared on a large scale at low cost. Optical properties also make it a very good phosphor material. YAG:Ce shows a broad excitation band around 450 nm, making it well suited for emission from blue InGaN LEDs. The broad emission band of YAG:Ce is centered around 550 nm but reaches 650 nm (Figures 1 and 4), which together with blue LEDs produces the aforementioned cool white light 16 .
In addition, it possesses other highly desirable properties, such as very good chemical and temperature stability. This is an important issue in phosphor-converted solid-state lighting. Although more electrical energy is converted into visible light than other lighting devices, the LED chips that emit the radiation that ultimately excite the phosphors can reach temperatures of hundreds of degrees Celsius. As shown in Figure 6, the emission wavelength of YAG:Ce does not change significantly with increasing temperature, and the quantum efficiency is only slightly lower than the value at room temperature, thus enabling long-term lighting applications.
Likewise, the emission wavelength of cerium -doped YAG can be slightly tuned by changing its chemical composition (eg - replacing the combination of Mg2+and Ge4+/Si4+with Gd3+,Lu3+andAl3+). Changes in chemical composition, due to changes in bond length or the strength and type of those bonds, lead to differences in the coordination environment of the emitting ions, thereby altering the crystal field splitting. Of course, the anion composition can also be varied, as is the case with (oxy)nitrides later in this article. Chemical tuning of this phosphor emission wavelength is an important tool for materials scientists.
Changes in composition play a very important role in another large class of oxide phosphor materials (orthosilicates). Some silicates have a very rich chemical composition (almost 90% of the earth's crust consists of silicates), and if doped with broad-emitting ions (Ce3+, Eu2+), they have high quantum efficiency and good temperature stability Excellent phosphor material with excellent performance. This article briefly discusses the structure and properties of one of the simplest members of the phosphosilicate family, barium orthosilicate Ba2SiO4. As indicated by its systematic name, the structure of Ba2SiO4 (orthogonal space group Pnma) consists of mutually isolated [SiO4]4- tetrahedra (hence orthosilicates), as shown in Figure 5B. The Ba ions in this structure occupy two different crystallographic sites and are 9- or 10-fold coordinated. Phosphors from the orthosilicate family, especially barium compounds, have been the focus of many research efforts due to their synthetic simplicity (high temperature route in reducing atmosphere) and due to their ability to form solid solutions with strontium and calcium terminal members.
introduction of a small amount of europium ions produces a strong green emission (maximum concentrated around 505 nm) under near-ultraviolet (~395 nm) excitation (see also the color coordinates in Fig. 1)17. This very intense green emission spectrum makes it a suitable candidate for white light generation that relies on multi-color phosphors or phosphor mixtures.
Figure 6. Temperature dependence of emission properties and photoluminescence quantum yields of Y3Al5O12:Ce3+
(Oxy)phosphorus nitride
As mentioned before, one possibility to tune the emission color (as well as other properties) is through chemical substitution of cations or anions. Another class of phosphors related to orthosilicates are the oxynitrides from the MSi2O2N2:Ln family, where M is Ca, Sr or Ba and Ln is Ce3+or Eu2+ 18,19. Figure 5C shows a description of the CaSi2O2N2 unit cell. It consists of alternating layers of Ca ions and layers formed by a network of SiON tetrahedra, which are connected by three nitrogen-terminated corners. The structures of Sr and Ba MSi2O2N2:Ln oxynitrides are similar, but the difference in composition results in slightly different unit cell sizes. These compounds and their solid solutions exhibit high quantum efficiencies (up to 93% at room temperature) and very good temperature stability (quenching temperatures up to 600 K for SrSi2O2N2 and a Si2O2N2:Eu2+) It is rapidly being fully investigated and widely used as an alternative down conversion material for white light emitting devices. If doped with divalent europium, the emission maxima of the pure compounds (ie, CaSi2O2N2, SrSi2O2N2, BaSi2O2N2) are centered at 558 nm, 538 nm and 495 nm, respectively. Compositional tuning between these end members results in very efficient yellow-green phosphors that may be alternatives to YAG:Ce .
A fourth class of phosphors are nitrides, which typically have a red-shifted emission color compared to oxides. The red shift is due to the presence of a larger nephelauxetic effect in nitrides, which reduces the Racah interelectron repulsion parameter of the activator ion , resulting in larger crystal field splitting. The red emission of lighting is critical for reducing the color temperature of lighting, making solid-state light sources more pleasing to the eye and suitable for residential lighting applications. A well-known example of nitrided phosphors is the M2Si5N8 :Ln family, where M is Sr or Ba and Ln is Ce3+ or Eu2+?20,21 . The structure is orthogonal (steric group Pmn21), as shown by Sr2Si5N8 in Figure 5D. The unit cell consists of a fully connected corner-sharing SiN tetrahedral network that extends in all three dimensions. Sr ions are located in the voids created by the SiN network, resulting in two distinct sites with 6-fold and 7-fold coordination, respectively. Replacing Sr or Ba with 100% Eu2+can form a complete solid solution. Substitution of Eu2+ results in red emission from Sr2Si5N8 and yellow emission from Ba2Si5N8, while increasing the amount of Eu2+ red-shifts the emission to a maximum of 680 nm. The highly interconnected lattice of Sr2Si5N8 results in high quantum efficiency (up to 80 % at room temperature) and very good thermal stability of red-emitting phosphors. As shown in Fig. 7 22, the excitation band of Sr2Si5N8:Eu2+ ranges from 370 nm to 460 nm, besides the efficient and stable red emission, this compound also becomes an attractive the choice of force can add red spectral components to InGaN -based warm-white LEDs.
Figure 7. The relative intensities of the Sr2Si5N8:Eu2+ phosphor and the LED decrease with increasing temperature, indicating the high thermal stability of this phosphor, which is ineffective in other red-emitting phosphors Not common in powder. The inset shows a photo of a fluorescent encapsulated silicone cap illuminated by blue LEDs22
Summary
Solid-state lighting has great potential for energy savings. The performance of solid-state lighting devices is largely determined by the down-conversion phosphor or combination of phosphors used in the device. As demonstrated herein, advanced inorganic materials, including but not limited to oxides, oxynitrides, and nitrides doped with small amounts of rare earth and/or transition metal elements, can be used as phosphors to efficiently generate white light in solid-state devices.
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