掺Yb3+近红外下转换发光材料的能量传递机制的研究

Recently luminescent materials doped with rare earth ions, especially the Yb3+-doped quantum cutting materials, have attracted a great interest for photovoltaic application to improve solar cell efficiency. Energy transfer from one Tb3+ (Pr3+ or Er3+) ion to two or more Yb3+ ions results in the downconversion of a visible photon into two near infrared (NIR) photons, the quantum efficiency is higher than 100%. However, there are several different mechanisms for the energy transfer process. The judgment of the material properties may be wrong when an uncorrect method is used to calculate the quantum efficiency which depends on what happened during the energy transfer process. The aim of this project is to elucidate the energy transfer mechanisms for the couple ions in different hosts. The energy level positions of both the donor and the acceptor determine the energy transfer mechanism, so we plan to firstly study the location of the ground states of the rare earth ions relative to the valence band and conduction band of the host materials. In addition, emission spectra, excitation spectra and the luminescence decay curves will be investigated. In comparison with the quantum efficiency derived from the experimental evidence, the energy transfer process and the electron transfer process will be analyzed on the basis of both experimental and theoretical results. The energy level positions of rare earth ions relative to their ground states have been widely investigated, but the positions relative to the conduction band and valence band of the hosts are rarely known, so it is necessary to perform correlative investigations to provide information on designing downconversion materials doped with rare earth ions and evaluating their capabilities.

近年来稀土下转换发光材料，特别是掺Yb3+的量子剪裁材料，因有可能提高太阳能电池的效率而成为国内外的研究热点。通过一个Tb3+、Pr3+或Er3+等离子到两个Yb3+离子的能量传递，实现可见光到近红外光的转换，并且量子效率高于100%。但对于能量传递机制目前存在很大争议，而能量传递机制直接决定量子效率的计算方法，影响对材料性能的判断。由于能量传递机制取决于能量给体和受体的能级位置，本申请拟以确定稀土离子的基态相对于基质的价带和导带的能级位置为主，结合光谱信息和衰减特性，从实验和理论两方面深入研究不同基质中双掺离子之间的能量传递或电子转移过程。虽然稀土离子能级结构的理论已经十分成熟，但对于不同的基质中这些能级相对于基质能带的位置尚未形成完整、清晰的认识，因此有必要进行相关研究，为设计稀土下转换发光材料并正确评估其性能，提供实验和理论的依据。

稀土下转换（量子剪裁）发光材料，因理论量子效率可达200%而成为提高发光效率的首选。本项目详细研究了多种可能发生下转换发光的稀土离子掺杂体系的能量传递机制。首先重点研究了Nd3+、Ho3+、Tm3+等离子被激发到高能级后，将能量传递给Yb3+离子，实现了近红外发光。通过对比分析不同掺杂浓度的激发光谱和发射光谱以及不同条件下的不同稀土离子的相关能级的衰减特性，确定了发生的能量传递过程和能量传递效率。同时建立了两步共振能量传递模型，拟合结果给出了传递的能量对Yb3+离子衰减的贡献。其次构建了不同价态稀土离子在不同基质的禁带中的能级结构图，确立了二价和三价稀土离子的基态相对于多种氟化物、铝酸盐和硼酸盐等基质的导带和价带的能级位置。测试分析了不同条件下Pr3+ (Tb3+或Ce3+) 和Yb3+离子掺杂的NaYF4、NaLuF4、Y3Al5O12 (YAG) 和GdBO3等系列样品的光谱信息和衰减特性，表明适当条件下可通过RE4+-Yb2+的电子转移实现近红外发光。再次研究了在NaGdF4和GdB3O6等基质中，Pr3+能量可经由Gd3+传给Dy3+或Tb3+等离子，实现下转换发光和不同波长光的转换；但在K2GdF5中Pr3+和Gd3+-Eu3+的下转换发光均未发生，而是Gd3+作为中间体将Pr3+ 4f5d能级能量传给Eu3+。结合基质的结构特点，计算模拟出不同激发下发光的Eu3+周围Pr3+的概率分布，分析了不同的能量传递过程。另外在含Gd3+的磷酸盐基质中未发生以Tb3+ 4f75d能级为起始的下转换发光，结合光谱和衰减数据建立了物理模型，深入分析了不同激发下Tb3+4f75d态到Gd3+的4f7 态以及Gd3+ 6P7/2 态到Tb3+ 4f8 态的能量传递机制等。上述研究成果对设计和应用稀土下转换发光材料提供了实验和理论依据。