荧光粉应用小知识+新型荧光粉开发

LED Datasheet

参考：OSRAM Opto Semiconductors视频

Six main criteria for selecting phopshor for WLE?

[Philipe_JES_2011]

• emission spectra (shape and position)
• excitation spectra (shape and position, match pumping LED and large absorption strength)
• thermal quenching behavior (PL/PLE/QE unchanged at elevated temperature) 
• quantum efficiency (electrical-to-optical, whole package)
• stability (chemical/ temperature)
• saturation effect
  

 

Saturation effect /Ground state depletion / Bottleneck effect

【saturation effects】(饱和效应)：加大激发功率，发射光强度不怎么变化。如果发光中心的寿命比较长，那么往往就有比较明显的饱和效应。[荆西平_合金-2014]

These saturation effects manifest themselves at high excitation fluxes as a sublinear increase in the emission intensity and a decrease in the overall absorption of the phosphor. This can lead to color variations with varying excitation intensities, which is clearly an unwanted effect. [David-Acs Photonics-2017]

Saturation effect就是a sublinear luminescent output with increasing excitation density. 饱和效应一般发生在寿命比较长的离子上，而像Ce³⁺这种寿命很短的很难有saturation effect。提高LED的drive current无法区分这两种的影响：(1) elevated LED package temperature and then thermal quenching. (2)saturation effect(incident fluxes >\(10 \mathrm{W} / \mathrm{cm^2}\))

Eu²⁺-Mn²⁺共掺杂体系中的ground state depletion，以Ca₅(PO₄)₃Cl为例，由于Mn²⁺的寿命比较长，那么在Eu²⁺快速向Mn²⁺传递能量的时候，越来越多的Mn²⁺处于激发态，那么基态Mn²⁺的数量越来越少，也就是出现ground state depletion，最终反向限制ET过程，这就是bottleneck effect，也可能出现ET增大的过程，这是因为激发态的Mn²⁺再次吸收ET过来的能量(ESA)，跃迁到更高的激发态，然后又无辐射弛豫到先前的那个激发态，于是ET传递的效率增加了，但是Mn²⁺的发光强度并没有增加。 [Eu²⁺–Mn²⁺ phosphor saturation in 5mm light emitting diode lamps-APL-2008-A. A. Setlur] 注意：一般情况下，我们是可以认为发光强度和入射光的强度成正比的，虽然我们这里是粉末，不那么严谨，但是这样近似处理也说得通，也符合一般实验现象。

 

Clustering Effect

碱土金属的Eu²⁺掺杂：Often, Eu²⁺ ions are substituting for divalent alkaline earth ions, such as Mg²⁺,Ca²⁺,Sr²⁺ and Ba²⁺, thus requiring no charge compensation. As the ionic radius of Eu²⁺ is very similar to the one for Sr²⁺, high dopant concentrations can be substitutionally doped into Sr-based host lattices without the formation of europium-rich precipitates. For alkaline earth ions with a smaller ionic radius, such as Ca²⁺, Eu-clustering has been observed at higher concentrations. In combination with this large solubility, the shape and position of the emission spectrum is roughly independent of the dopant concentration for most of the Eu²⁺ doped compounds, provided that the crystallographic phase of the undoped host is maintained upon doping. Depending on the value of the Stokes shift, reabsorption can become prominent at elevated doping concentrations, leading to a red-shift of the emission spectrum. [Philipe_JES_2011]

Ce³⁺占据二价氧离子格位：Ce³⁺ can show luminescence when substituted at both a divalent or trivalent cation site. At the former site, charge compensation is required. This can be provided by intrinsic defects, such as vacancies, but this often leads to more significant concentration quenching, with clustering of Ce³⁺ ions and correlated red shift of the emission spectrum. Codoping with monovalent cations can improve the incorporation in this case, by restoring charge balance. [Philipe_JES_2011]

CaS:Eu,Sm：However, with more and more Eu being doped in CaS, they become closer and closer to each other. Those closely positioned Eu²⁺ will change their local electronic field and tend to aggregate with each other. Finally, a cluster of Eu²⁺ is formed in CaS which prevents Sm³⁺ replacing adjacent Ca²⁺. Therefore, both PL and ISL drop down at high level of Eu. [Study on relationship of luminescence in CaS:Eu,Sm and dopants concentration-JL-2002]

 

Remote-type white light LED

传统的方式是黄色荧光粉和树脂混在一起包裹在LED蓝光芯片上。而remote的方式，荧光粉不直接和LED芯片接触，好处是better thermal stability(chip上的热量很容易散出去)，而且不容易发生saturation effect，因为单位面积上的光照更少了；它的缺点是需要更多的荧光粉。[Philipe_JES_2011] 目前蓝光芯片的效率提升了不少，发热不那么严重，荧光粉的heat production反而是主要问题[Jonas_NM_2017]。

Q：如何评价entire LED package(chip+phosphor)的conversion efficiency? 也就是electrical-to-optical的转换效率，不考虑eye sensitivity) ?
答：以blue chip + yellow phosphor为例$$\eta_{\mathrm{e}-\mathrm{o}, \mathrm{B}}=\eta_{\mathrm{L}, \mathrm{B}}\left[f+(1-f) Q_{\mathrm{P}} \frac{\bar{\lambda}_{\mathrm{L}, \mathrm{B}}}{\bar{\lambda}_{\mathrm{P}}}\right]$$其中\(\eta_{\mathrm{L}, \mathrm{B}}\)为the electrical to optical power conversion efficiency of the LED chip alone, \(f\)为芯片发出的蓝光直接发射出来的比例，\( (1-f)\)为蓝光被黄色荧光粉吸收的比例(相当于吸收系数，\(Q_{\mathrm{P}}\)为荧光粉的内量子效率，即\( (1-f)Q_{\mathrm{P}}\)为荧光粉的外量子效率。\(\bar{\lambda}=\displaystyle\frac{\int I(\lambda) \lambda d \lambda}{\int I(\lambda) d \lambda}\)代表的是barycenter of the emission spectrum。

Q：既然\(\displaystyle\frac{\bar{\lambda}_{\mathrm{L}, \mathrm{B}}}{\bar{\lambda}_{\mathrm{P}}}\)很影响最终的electrical-to-optical的转换效率，那么为什么还有人选择用UV芯片+多种荧光粉呢？(公式与上面类似，不一样)？[Philipe_JES_2011]
答：(1) UV chip的电光转换效率可能更高，或者更匹配荧光粉的激发峰；(2) UV chip + 双(三)色荧光粉，宽带的话，可以得到更好的CRI(覆盖500 nm前后)，而且光谱更稳定 with respect to the driving current and temperature of the LED cip(no spectral shift).

Q：既然f-f跃迁的是narrow-emission peaks，不能得到好的color rendering，那么就完全没用吗?
答：不是，比如Eu³⁺，相比于氮化物荧光粉在红光区的宽带发射，Eu³⁺的发射峰集中在613 nm附近，该波长(频率)人眼更敏感，即视见函数此时的值比较大，即the luminous efficiency of the radiation比较高，其定义为$$\operatorname{LER}(\text{l m / W})=683 \operatorname{lm} / W \cdot \frac{\displaystyle\int_{360 n m}^{830 n m} I(\lambda) V(\lambda) d \lambda}{\displaystyle\int_{360 n m}^{830 n m} I(\lambda) d \lambda}$$也就是说氮化物红色荧光粉看起来似乎量子效率高，但是发出那些光有相当一部分是人眼不怎么敏感的，浪费了。因此，narrow-emission peaks can be an ideal choice for display application, such as LCDs, where LEDs can be used as backlights. 而我们日常的warm white led是不能作为LCD背光源的。

注：LCD中，有三个filter，其中两个分别用于蓝光和绿光的带通，另一个用于红光的长通。

 

Phosphor-in-Polymer / Glass / Ceramic

(1) phosphor-in-polymer，phosphor一般high refractive index，polymer一般low refractive index，这种区别会影响outcoupling和scattering(影响激发的主要因素)。scattering决定了有多少荧光粉可以被can be exposed to the excitation light。[David_Materials-2017]

冯昂的polymer对300 nm以下的光不吸收，在不混入任何其他荧光粉的情况下，是透明的(可见光区)。Resin为Epikote resin MGS LR 135，Harder为 MGS LR 135。

(1-2) phosphor-in-polymer在文献中的例子：
PMMA

PDMS

(2) phosphor-in-glass，决定PIG发光效率的两个重要因素：(a) excellent heat resistence of phosphor particles against glass melting to keep their original properties. (b) refractive index matching between phosphor and glass matrix to reduce adverse light scattering to keep PIG transparent.

(3) phosphor-in-ceramic，[Thomas Juestel-PPT]   [谢荣军-2019-JMCC]

LED flicker

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参考资料：Defining phosphor luminescence property requirements for white AC LED flicker reduction—JL-2015

Trial and error (Edisonian fashion)

至今，新材料主要是在实验上经大量实验、或偶然被发现的，此即人们所熟知的“炒菜法”、或“尝试法”(Trail and Error，或Serendipity/Edisonian 方法)

wiki的解释：The Edisonian approach to innovation is characterized by trial and error discovery rather than a systematic theoretical approach. This may be a convenient term, but it is an inaccurate and misleading description of the method of invention actually used by Thomas Edison. An often quoted example of the Edisonian approach is the successful but protracted process he is said to have used to invent a practical incandescent light bulb. Trial and error alone cannot account for Edison's success with electric lighting when so many others failed (Friedel and Israel (1987) list 23 others) or his remarkable record of almost 1100 patents (see the list of Edison patents).

Exploratory crystal growth, combinatorial chemistry screening, single-particle diagnosis本质上都是以Edisonian fashion去寻找新材料(基质)。

 

Exploratory crystal growth

参考文献：
(1) Narrow-band red-emitting Sr[LiAl₃N₄]:Eu²⁺ as a next-generation LEDphosphor material
(2) Single Crystal Growth and Crystal Structure Analysis of Novel Orange-Red Emission Pure Nitride CaAl₂Si₄N₈:Eu²⁺ Phosphor

 

Combinatorial Chemistry

【组合化学】(Combinatorial Chemistry)是一门将化学合成、组合理论、计算机辅助设计及机械手结合一体，并在短时间内将不同构建模块用巧妙构思，根据组合原理，系统反复连接，从而产生大批的分子多样性群体，形成【化合物库】(compound library)，然后，运用组合原理，以巧妙的手段对库成分进行筛选、优化，得到可能的有目标性能的化合物结构的科学。

比如传统的有机化学反应：A + B → C。其中A可以是A1，A2，A3，A4等等，具有相同的官能团，但是是不同的分子，B也类似可以有很多种B，于是通过变换A和B可以得到不同类型的C，这里就有点像是做笛卡尔积。

drug design也可以利用上述方法，上面的图，先给出药物的结构框架，然后分别在三个位置插入官能团(根据compound library)，即可得到不同的产物，然后再分别测试其药效。

 

基于此方法的荧光粉筛选，可能涉及一些优化算法(参考AFM-2012)，比如heuristic optimization techniques such as the nondominated-sorting genetic algorithm (NSGA) and particle swarm optimization (PSO).

南策文：相比于试错法，组合化学是材料的快速实验筛选方法，但仍然属“尝试法”的一种“特殊高级版”。随着材料理论、计算方法（包括计算机）的发展，新材料的计算设计已受到越来越多的广泛的关注，已发展成为“计算材料科学”(Computational Materials Science)，即在计算机虚拟环境中进行“实验”，以模拟、理解、预示材料的性质（性能）。这种完全从理论产生新材料（the creation of new materials from theory）是最高境界，但目前仍是非常的困难的。实际上，最有效的方法是实验与计算的有机结合。

参考文献：
(1) Discovery of a Phosphor for Light Emitting Diode Applications and Its Structural Determination, Ba(Si,Al)₅(O,N)₈:Eu²⁺
(2) Investigation of strontium silicate yellow phosphors for white light emitting diodes from a combinatorial chemistry
(3) 如何理解组合化学，以及在药物研发领域的作用？—知乎

 

Single-Particle-Diagnosis 

Schematics of home-built single-particle fluorescence microspectroscopy setups for measuring (a) photoluminescence spectra, and (b) quantum efficiency.

Single-Particle-Diagnosis 其实也是试错法的一种，就是各种配比一混高温烧制，然后在光学显微镜下配合光纤和辐照光进行筛选，选出发光好的，然后进行X射线单晶解析以及发光性能的测试。

参考文献：
(1) Discovery of New Nitridosilicate Phosphors for Solid State Lighting by the Single-Particle-Diagnosis Approach
(2) Blue-Emitting Sr₃Si_8–xAl_xO_7+xN_8–x:Eu²⁺ Discovered by a Single-Particle-Diagnosis Approach: Crystal Structure, Luminescence, Scale-Up Synthesis, and Its Abnormal Thermal Quenching Behavior
(3) Cyan-Emitting Sialon-Polytypoid Phosphor Discovered by a Single-Particle-Diagnosis Approach
(4) Structure and luminescence of a novel orange-yellow-emitting Ca_1.62Eu_0.38Si₅O₃N₆ phosphor for warm white LEDs, discovered by a single-particle-diagnosis approach
(5) Screening and discovery of phosphors by the single-particle-diagnosis approach

 

Mineral-Inspired

Xia's group recently proposed a mineral-inspired phosphor discovery strategy, which originated from the design of new phosphors based on the existed structural models. In this strategy, the typical mineral compounds with the specific chemical formula can be regarded as the initial prototype compounds, which inspire people to design and discover the new phosphors via the substitution or structural evolution. This method gives the scientists many opportunities to find new phosphors, which demonstrates great potential in a scientific and practical value.

参考[固态照明用稀土荧光粉和无机量子点：机遇与挑战]，主要的可用于荧光粉的矿物类型：

• garnet
• Si3N4
• apatite
• melilite
• A3MX5   (Sr3SiO5)
• UCr4C4
• β-K2SO4
• β-Ca3(PO4)2

 

高通量DFT计算

High-throughput density functional theory (DFT) calculations

在合成新基质荧光粉的时候，或者利用大数据(机器学习)等预测新材料的时候，要寻找热力学和动力学稳定的荧光粉，thermodynamically stable和dynamically stable的基质。We demonstrate that Sr₂LiAlO₄ is predicted by DFT calculations to be thermodynamically and dynamically stable and to have the necessary bandgap, structural rigidity, and near-UV excitation wavelength when activated with either Eu²⁺ or Ce³⁺. 简单来说，动力学稳定关注的是反应速度，稳定的物质，即使会参与反应，但反应过慢以至于没有明显表现；热力学稳定是能量上的稳定，放热反应容易进行，吸热反应难以进行，也就是说能量越低的物质越稳定。玻璃热力学不稳定(含有过剩的内能)，动力学稳定(粘度大)。

参考文献：
(1) Mining Unexplored Chemistries for Phosphors for High-Color-Quality White-Light-Emitting Diodes
(2) Electronic Structure Descriptor for the Discovery of Narrow-Band Red-Emitting Phosphors

 

Phenomenological Approach

Dorenbos的研究很多都是phenomenological approach，比如[Dorenbos_CM_2005]这篇文章的主要目的不是深入解释镧系稀土离子价态的稳定性(在特定基质中)。目的是提供一个包含不同类别化合物的overview。采用phenomenological approach的方法来indetify价态的稳定性，研究的参数包括阴离子的种类，格位的大小，以及材料的band gap。

氟化物荧光粉的合成

要点参考文献：[Luminescence of Eu²⁺ in some fluorides prepared by reactive atmosphere processing—JAC-2008]