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题名	稀土离子掺杂的宽带发射LED用荧光粉的研究
作者	张亮亮
学位类别	博士
答辩日期	2015-05
授予单位	中国科学院大学
导师	张家骅
关键词	荧光粉 白光LED 氮硅铝酸盐 SrAlSi4N7 Li2SrSiO4
其他题名	Rare-earth doped phosphor for LED with broader emission band
学位专业	凝聚态物理
中文摘要	白光发光二极管是新一代的照明光源，具有体积小、全固态、环保、节能等优点，成为照明领域的研究热点。目前产生白光LED的主流方案是在InGaN基蓝光芯片上涂敷Y3Al5O12:Ce3+(YAG:Ce3+)黄色荧光粉，通过管芯的蓝光和荧光粉发射的黄光合成白光。YAG:Ce3+的发射为黄光，缺少红光成分，封装的白光LED显色指数低于80。为解决这一问题，人们提出将黄色（绿色）和红色荧光粉混合的方案。然而由于不同荧光粉之间存在再吸收的问题，白光LED流明效率降低。同时，由于不同荧光粉之间的老化特性和温度特性不一致，白光LED的色彩会使用时间发生漂移。因此，获得光谱成分均衡的LED荧光粉是人们追求的更高目标。 高的显色性要求荧光粉发射光谱中同时包含绿、黄、红三种波段，这就要求荧光粉具有宽的发射谱带。传统的YAG:Ce3+黄色荧光粉正是由于具有宽带发射的特性才在白光LED中得到广泛应用。本论文尝试制备比YAG:Ce3+具有更宽发射谱带的黄色荧光粉，提高白光LED的显色性能。 为实现宽的谱带发射，本论文尝试了两种方案：一种方案是在已知的发射谱带较宽的荧光粉中，改变发光中心离子。另一种方案是在已知的发光材料中再引入一个新的发光中心，直接展宽发射谱带。主要研究结果如下： 1. 首次报道了SrAlSi4N7:Ce3+黄色荧光粉的发光性质。该荧光粉发射谱带位于555nm，发射谱带较YAG:Ce3+宽15nm左右，制备的白光LED，显色指数（CRI）81,色温（CCT）5624K，优于YAG:Ce3+（CRI<80, CCT>6000）。SrAlSi4N7的晶体结构中有两个Sr2+格位，由于两个Sr2+格位配位数相同并且半径相近，因此两个发光中心在发射光谱中很难区分，这导致了SrAlSi4N7:Ce3+的宽带发射。 SrAlSi4N7晶体结构中包含了一个独特的[AlN4]共楞链，这种结构同铝排斥原理相矛盾。通过键价和模型进行计算发现，SrAlSi4N7中[AlN4]四面体通过缩短Al-N键长增加了键价，从而使共楞连接变得合理。但是这种连接方式从能量上讲是亚稳的，最终导致了SrAlSi4N7晶粒的择优取向现象。在材料合成中，发现需要过量的AlN原料才能合成发光性质良好的SrAlSi4N7:Ce3+荧光粉。通过分析发现，过量的AlN一部分进入SrAlSi4N7晶格中，发生Al-O取代Si-N，并导致发射蓝移；另一部分则与SrAlSi4N7共存于最终产物中，增加了荧光粉的结晶性。 2. 将Eu2+引入到SrAlSi4N7:Ce3+黄色荧光粉中，将发射谱带进一步展宽，制备的白光LED显色指数可以达到88。SrAlSi4N7:Ce3+,Eu2+荧光粉的发射谱带可以在550nm-610nm（黄-红）之间连续可调，能够适应不同的应用需求。SrAlSi4N7:Ce3+,Eu2+中存在三个发光中心分别为Ce1、Ce2和Eu2，三个中心的发射分别位于黄绿光、橙黄光、红光波段，三个波段在光谱中的比例可以通过Ce1-Eu2的能量传递过程调节。我们对前期和中后期的衰减曲线分析后获得了Ce1-Ce2能量传递和Ce1-Eu2能量传递速率的关系，并且获得了两个能量传递过程相互影响的临界浓度，并据此解释了发射光谱的调节原理。与此对应的是，当Eu2+掺杂量较少时，发射谱带明显展宽，谱带中同时包含三种波段的光，可以有效提升荧光粉的显色指数，同时Eu2+的引入增加了荧光粉的吸收能力，避免了封装的白光LED的效率损失；当Eu2+掺杂量较多时，发射谱带中黄绿光波段消失，红光波段越来越占据主导低位，制作的LED色温逐渐降低。 3. 观察到Pr3+在Li2SrSiO4中具有较宽发射谱带的现象，并将该现象同格位的扭曲相关联。引入偏心距和球度概念，计算了Li2SrSiO4中[SrO8]配位多面体的扭曲程度，并且同YAG中[YO8]配位多面体做对比。然后进一步将Pr3+红光中心引入Li2SrSiO4:Eu2+黄色荧光粉中，利用其宽带发射的性质来增加红光成分。在Li2SrSiO4:Eu2+，Pr3+中我们发现了Eu-Pr的能量传递过程。Eu2+的5d能级有可能将能量传递给Pr3+的3P2和1D2能级，这样的传递过程将导致Pr3+的1D23H4发射的额外增加，导致Pr3+发射光谱形状的改变。进一步通过Inokuti-Hirayama公式研究能量传递的动力学过程，发现Eu-Pr能量传递的相互作用类型为偶极-四极，临界跃迁距离为3.6 Å，远小于YAG中的Ce-Pr跃迁。
英文摘要	White light emitting diode (WLED) are proposed to take the place of conventional light sources for general illumination, due to their promises of huge energy saving, reducing carbon emission, high efficiency, and long lifetime. The most widespread commercial strategy to achieve (WLED) is using a blue LED chip to excite yellow emitting YAG:Ce3+ phosphor. The color rendering index (CRI) of such a WLED is low (<80) due to deficiency of red fluorescent component in YAG:Ce3+. To achieve high CRIs (> 80), a red phosphor is blended with a yellow or green one. However, some drawbacks such as fluorescence reabsorption, non-uniformity of luminescence, and time dependent shift of color point arise simultaneously. Higher CRI requires both green, yellow, and red component in the spectrum of a phosphor. This indicates broad-band emission property of the phosphor. A broader band means a balance between green and red component in the spectrum. Thus to achieve high CRIs, An ideal solution is to broaden the emission band of single yellow phosphor so as to enrich the red component. In turn, the color rendering is improved without the drawbacks of phosphor blend. Thus, to find a yellow phosphor with an emission band broader than YAG:Ce3+ is our interested topic. In order to achieve broad-band emission phosphor, two schemes are tried in this dissertation: one method is substituting the rare-earth ion of an already known phosphor; another method is introducing new optical center into an already known phosphor. Here is the results: 1. Synthesis and luminescent properties of a new yellow phosphor SrAlSi4N7:Ce3+ are reported. In comparison with YAG: Ce3+, the phosphor exhibits smaller thermal quenching and a broader emission band centering at 555 nm with a bandwidth as large as 115 nm. CRI of the wLED fabricated by such a phosphor is 81, CCT is 5624 K, showing much improvement compared with wLED fabricated by YAG: Ce3+. There are two Sr sites in SrAlSi4N7 lattice. Coordination numbers of both Sr sites are 9. Average Sr1-N and Sr2-N distances are 2.89 Å and 2.91 Å respectively. SrAlSi4N7:Ce3+ appears a broad emission band due to a spectral overlap of the two luminescence centers. SrAlSi4N7 contains spatial infinite chains of edge-sharing [AlN4] tetrahedra running along [0 0 1] in the lattice. Such an edge-sharing type is really unreasonable. Bound valance sum (BVS) calculation is introduced to show the rationality of the infinite chains of edge-sharing [AlN4] tetrahedra. Results show that [AlN4] tetrahedral shorten the Al-N bond length and realized edge-sharing. However, such a connection type is energy metastability and reduces the stability of SrAlSi4N7 lattice. Thus edge-sharing [AlN4] tetrahedra is responsible for the preferred orientation. With increasing excessive AlN in raw materials, SrAlSi4N7:Ce3+ shows better luminescence property. Al3+ substituting for Si4+ is provided with charge compensation by O2- substituting for N3-, as result of which, blue-shift of PL occurs. Since AlN is proved to be oxidated by residual O2, covering method is introduced to decrease the least excessive AlN required in raw materials from 8 mol% to 4 mol%. 2. Eu2+ is introduced into SrAlSi4N7:Ce3+ yellow phosphor to broaden the emission band. CRI of wLED fabricated by SrAlSi4N7:0.05Ce3+,0.01Eu2+ increases into 88 without luminous efficiency loss compared with wLED by SrAlSi4N7:Ce3+. The Ce3+, Eu2+ co-doped SrAlSi4N7 endows a highly color tunable emission from 550 nm (yellow) to 610 nm (red). These properties allow SrAlSi4N7 with a widespread application prospect. There are three subbands that can be ascribed to Ce1, Ce2, and Eu2 in the emission spectrum. Their emission color is green-yellow, orange-yellow, and red respectively. The decrease and increase progress of the three subbands give rise to the red shift of emission. Energy transfer progress between Ce1-Ce2 pair, Ce1-Eu2 pair are observed and proved to be both dipole-dipole interaction. The energy transfer rate of Ce1-Eu2 is proved to be two times of Ce1-Ce2. Thus y=0.035 is calculated to be an important concentration to determine the influence of Ce1-Eu2 energy transfer on Ce1-Ce2 energy transfer. The co-doped Eu2+ ion adjusts the relative quantity of green and red component in emission spectra by Ce1-Eu2 energy transfer progress. Co-doped Eu2+ increases both bandwidth and absorption of the phosphor. Thus wLEDs with SrAlSi4N7:0.05Ce3+,yEu2+ show increase CRI without luminous efficiency loss. Phosphors with low concentration doped Eu2+ are favorable to application in wLEDs requiring high CRI while heavier doped samples are in favor of wLEDs in need of low CCT. 3. Luminescence of Pr3+ in Li2SrSiO4 shows a much broader f-f emission band compared with YAG:Pr3+. Eccentric distance and sphericity are introduced to quantify the distortion polyhedron. Results show that [SrO8] polyhedron in Li2SrSiO4 suffers an off-center of Sr and a larger distorted non-spherical-shape distribution of O compared with [YO8] in YAG. Then, Pr3+ is co-doped with Eu2+ in Li2SrSiO4 to provide a single phosphor for wLEDs. Energy transfer processes between Eu-Pr is observed. There are two routines for the Energy transfer processes: from 5d of Eu2+ to both 3P2 or 1D2 of Pr3+. Such a progress increase the transition between 1D23H4 and change the emission shape of Pr3+. Inokuti-Hirayama model is introduced to study the dynamics of energy transfer. Results show that he electric interaction type of Eu-Pr is dipole-quadrupole and Critical energy transfer distance R0 of Eu-Pr transfer is 3.6 Å, much smaller than that of Ce-Pr transfer in YAG.
公开日期	2015-12-24
内容类型	学位论文
源URL	[http://ir.ciomp.ac.cn/handle/181722/48950]
专题	长春光学精密机械与物理研究所_中科院长春光机所知识产出
推荐引用方式 GB/T 7714	张亮亮. 稀土离子掺杂的宽带发射LED用荧光粉的研究[D]. 中国科学院大学. 2015.

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