Rare Earth Doped Infrared Quantum Cutting Materials And WLED Red Phosphor

The research on renewable energy, especially solar power, attracts great attentions nowadays due to the tremendous demand for energy in modern society. Enhancing the energy conversion efficiency of solar cells is one of the most important issues of concern. Using infrared quantum cutting materials, a special kind of functional material that can split one high energy photon into two low energy infrared photons that can be both absorbed by solar cells, is a possibe method to enhance the energy conversion efficiency. Meanwhile, it is very important to make every reasonable effort to reduce energy comsuption, which sets a high standard in energy saving for electronics devices and optical equipment. White LED (WLED), thanks to its high energy efficiency and environment-friendly character, is considered as the next generation for lighting. Efficienct red phosphor for WLED will offer better redering index and higher lumen efficiency. Based on these analyses, my research focuses on the preparation and luminescent properties of infrared quantum cutting material as well as the design and synthesis of red phosphor for WLED.In chapter one, we first introduced the fundamental knowledge and application of rare earth doped luminescent materials. A brief discussion about the physical picture, including energy levels of rare earth ions, dipole transition selection rules, the crystal field effect, non-radiative transitions involving multiphonon emission and resonant energy transfer, was given in the following paragraph. In this chapter, special attention was paid to the research and development of quantum cutting materials along with the great difficulties and possible solutions.In chapter two, we investigated on rare earth ions double doped infrared quantum cutting materials.First of all, powder samples of LaF3with different Pr3+, Yb3+doping concentration were successfully prepared by copricitation method. X ray diffraction patterns showed that all the samples exhibite the character of single hexagonal phase. In spite of heavy doping, no other inpurity phases were detected. Experimental evidences of energy transfer from Pr3+to Yb3+were presented by the excitation spectra, emission spectra as well as decay curves at room temperature. The energy transfer from3P0level of Pr3+to Yb3+was very efficient and the maximum transfer efficiency reacheed61.6%. From the analysis of temperature-dependent infrared emission spectra, the energy transfer process involved in Pr3+-Yb3+couple was determined to be cross-relaxation Pr3+(3P0)+Yb3+(2F7/2)â†'Pr3+(1G4)+Yb3+(2F5/2) followed by resonant energy transfer Pr3+(1G4)+Yb3+(2F7/2)â†'Pr3+(3H4)+Yb3+(2F5/2), rather than the cooperative energy transfer reported in previous literature.Besides, NaYF4with different Ho3+, Yb3+doping concentration were synthesized by hydrothermal method. X ray diffraction of all the samples shows that they all belong to the hexagonal phase. Adding a reasonable amount of trisodium citrate as chelating agent will lead to well dispersed sample with uniform size (micron). Evidence is provided by spectroscopic measurements to confirm the occurrence of quantum cutting for the first time. Upon excitation of Ho3+5G4level, near-infrared quantum cutting could occur through a two-step resonance energy transfer from Ho3+to Yb3+by cross relaxation. In this process, Ho3+absorded a high photon with energy range in300-400nm and transfered the excited energy to two nearby Yb3+ions, leading to the emission of two low energy photons around1000nm. The quantum efficiency was calculated to be155.2%for sample with optimal condition.In chapter three, Yb3+activated NaY(WO4)2were prepared by solid state method. Efficient emission of the host can be obtained under250-300nm excitation and the emission band located around480nm. Under266nm excitation, samples with Yb3+doping exhibited an intense near infrared emission. The observation of the host absorption bands in the excitation spectra of Yb3+confirmed the presence of energy transfer from the host to Yb3+ions. The position of Yb3+-O2-charge transfer state located at higher energy level than that of the host. Meanwhile,10phonons or more are required to bridge the energy gap between the host and Yb3+. So we attributed the energy transfer process to be cooperative energy transfer. The influence of Yb3+doping concentration on the luminescent properties was also studied. From the decay curves of the host emission, the efficiency of energy transfer from the host to Yb3+can be calculated. An efficiency of81.6%was achieved in sample with40%Yb3+doping. It was also noted that the quenching concentration for Yb3+in this host was very high, which will facilitates the emission intensity. At last, we discussed the possible factors that contribute to the high quenching concentration.In chapter four, the background of white LED and the three solutions were briefly introduced along with the research and development of WLED phosphor. Based on our research experience in molybdate, we had explored the possibility of their use in WLED phosphor. Eu3+ions activated Y2MoO6were synthesized by sol gel method. Samples synthesized at different temperature had various properties in structure and morphology. Increased reaction temperature will lead to increased sample size. We also investigated the influence of reaction temperature and Eu3+doping concentration on the luminescent properties. The intense host absorption band is located in250-440nm region, and its appearance in Eu3+excitation spectra means the existence of energy transfer from the host to Eu3+. Enhanced reaction temperature leads to the red shift of the excitation band of Eu3+ions. However, a further shift was not observed at temperature higher than1200℃. It is found that the quenching concentration for Eu3+in this host is very high (20%). And we discussed the cause of such a phenonmenon from the perspective of the host structure. The overlap of the emission of commercial ultraviolet LED chip and the aborption of the host charge transfer state will be in favor of the emission intensity of the WLED phosphor. Compared with commercial red phosphor in current use, the integrated emission intensity of optimal sample under395nm light excitation was about2.3times higher.