基于白光LED应用的氮化物荧光粉的热致光衰机理研究

Nitride phosphors are attracting considerable attention from both academic and industrial communities, which are well known as one of most suitable wavelength-conversion luminescent materials for fabricating white LEDs with high color rendition, wide color gamut, tunable color temperature and high efficiency. Our previous studies indicated that, thermal degradation occurred seriously in Sr2Si5N8 and SrSi2O2N2-based phosphors which showed excellent photoluminescent properties, leading to the fast degradation of white LEDs. Unfortunately, the thermal degradation mechanism and hence the solutions are not clear yet. In this project, we will clarify the thermal degradation mechanism of Sr2Si5N8 and SrSi2O2N2 phosphors by investigating the temperature effect as well as the synergistic effect of moisture and irradiation stresses on photoluminescent properties (i.e., spectrum, quantum efficiency, absorption, lifetime, etc.), and understanding the changes in surface, chemical composition, valence and concentration of activators, and local structure through surface and structure analyses. Also, we will attempt to establish the methods for evaluating thermal degradation of phosphors in white LEDs. Finally, we will try to realize the control synthesis of high efficiency and low degradation of nitride phosphors through composition tailoring, surface modification of particles and synthetic processing optimization.

氮化物荧光粉被公认为是制造高显色、广色域、色温可调以及高光效白光LED最合适的波长变换材料之一，倍受学术界和产业界的青睐和关注。前期研究表明：发光性能优异的Sr2Si5N8:Eu红粉和SrSi2O2N2:Eu绿粉存在着严重的可导致白光LED迅速老化失效的热致光衰问题，但热致光衰的机理和解决方法尚不清楚。本项目以白光LED用氮化物荧光粉为研究对象，研究温度对荧光粉的发光性能(光谱、强度、量子效率、吸收率、发光寿命)的影响，考察一定温度条件下湿度和辐照强度的协同作用效应，掌握荧光粉热致光衰的变化规律；综合利用表面和微观结构等分析技术研究荧光粉的表面结构、化学组成、发光中心离子价态/浓度、局域结构的变化，揭示热致光衰的形成机制；初步建立正确评价热致光衰的方法；通过材料组成设计、荧光粉颗粒表面修饰、以及合成工艺优化达到实现高效、低光衰氮化物荧光粉的可控制备。

白光LED是公认的一种既环保又节能的绿色光源，已经广泛应用于家庭照明、液晶背光源和汽车车灯等方面。荧光粉结合蓝光芯片是目前白光LED封装技术的主流技术，作为关键核心材料之一的荧光粉扮演着波长转换的重要作用。荧光粉在外加载荷（如温度、湿度和激发强度等）的作用下发生发光性能的衰减，如热致光衰，是导致白光LED失效的主要原因之一。因此，研究荧光粉的热致光衰及其机制不仅可以择优选择荧光粉、提高白光LED性能，而且可以优化荧光粉的组成和结构设计，制备高可靠性发光材料。本项目主要（1）开展了氮化物荧光粉（SrSi2O2N2:Eu2+、Sr2Si5N8:Eu2+和CaAlSiN3:Eu2+）在温度或者湿度作用下的光衰现象，通过显微结构、化学组成、物相和光谱分析揭示了氮化物荧光粉的热致光衰机制，即发光中心离子和基质的氧化；（2）利用溶胶凝胶方法实现了SrSi2O2N2:Eu2+荧光粉表面均匀的TiO2包覆（厚度50-70 nm），600oC处理后的光衰减小了48%，显著改善了其热致光衰；（3）通过碳掺杂、固溶等手段改变基质的化学组成和发光中心离子周围的局域结构，提高了AlN:Eu2+和CaAlSiN3:Eu2+的高温发光性能，制备了高光效（100 lm/W）、高显色（Ra > 90，R9 > 90）的白光LED。此外，本项目提出并实践了高温压力锅评测手段，能有效快速地评价荧光粉高温稳定性；并且联合开发了小型评测仪器。