| Over the past decades, the upconversion and the downconversion of the rare earths doped glass and glass-ceramics materials have been extensively investigated due to their potential applications to many fields, such as color display, high density memories, optical data storage, sensor and energy solar cell, etc. The downconversion (or quantum cutting) of the glass and glass-ceramics on the cooperative energy transfer process could greatly benefit the development of Si solar cells because their emission energy is just above the band gap of Si. According to the research results of Shockley W and Queisser H J, the theoretical maximum conversion efficiency of solar cells is 30%. A large part of the energy losses that affect to the efficiency of solar cells is due to the spectral mismatch:the photons with energy smaller than the band gap (Eg) will not be absorbed sub band gap transmission and a large part of the energy of photons with energy larger than the band gap is lost as heat thermalization losses. To reduce the energy losses, there are many ways to adapt the solar spectrum before it is absorbed by solar cell.The first option is to add two lower energy photons that are otherwise transmitted to obtain one higher energy photon that can be absorbed by solar cell. This process is known as upconversion and is especially useful for solar cells with a large band gap where transmission losses dominate. The upconversion mechanism has been reported since the 1960s. The whole field of upconversion in ion-doped systems can be traced back to an idea of Bloembergen’N. However, the role of energy transfer in the upconversion processes was not recognized until 1966, when it was suggested by Auzel F that energy transfer between the rare earth ions could take place between two ions, both of them being in an excited state of the energy transfer initial step. Until then, all energy transfer was assumed to take place from a first ion in an excited state to a second one in its ground state. The energy transfer among doped luminescent ions in the solid materials is very important in the enhancing luminescent emission and excitation efficiency. The energy transfer between the rare earth ions and transition metals plays an important role in the optical materials; the investigation on the energy transfer has been attracting much attention for its practical utility in optical devices. The first successful synthesis of glass-ceramics with high upconversion efficiency can date back to (Pb, Cd)F2 system by Wang Y and Ohwaki J in 1993. Then, there have been many reported on this field. Subsequently, oxyfluoride glass-ceramics doped with rare earth ions had been researched widely in the past few decades. The fluoride usually has lower phonon energy than oxide, while oxide usually has an advantage superior to fluoride due to their chemical durability, thermal stability and mechanical strength. The transparent oxyfluoride glass-ceramics with the advantages from both fluoride and oxide are considered to be a good choice as hosts for rare earth ions and thus have turned to be one of the most promising optical materials. When compared to organic phosphors and semiconductor quantum dots, the rare earth ions compound nanocrystals offer higher chemical and thermodynamic stability in addition to sharper emission bandwidth. Many series of them, such as fluorides, oxyfluorides, alkali metal’s rare earth fluorides and phosphates were studied for their high performance of optical multifunction as well as the high performance in applications.The second option is to split one higher energy photon to obtain two photons with a smaller energy. Each of these photons can subsequently be absorbed by solar cell and generate an electron hole pair. This is known as downconversion and is most beneficial for solar cells with a smaller band gap where thermalization losses are the major loss factor. This process is also known as quantum cutting because one photon is cut into two smaller energy photons. The effects of using either upconversion or downconversion materials in combination with solar cells has been modeled by Trupke T et al.Therefore, the issue of enhancement the downconversion/upconversion emission intensity has been interested in investigation to meet the development of the optical applications in this field. There are ways to enhancement the downconversion/upconversion emission intensity of the rare earth ions doped in the glass and glass-ceramics for recent studies in the field optical materials have been reported. Among them, effects of the energy transfer process between the rare earth ions and the heat treatment process of glass-ceramics are two ways have been used in recent studies.However, until now, the issue of enhancement the downconversion/upconversion emission intensity of the rare earth ions doped in the glass-ceramics containing nanocrystals is still interested studied extensively to find the best efficiency.In this Ph. D. thesis, we chose the downconversion/upconversion optical properties of the rare earth co-doped in the transparent glass-ceramics to research subjects. Our purpose is to find the materials that can perform for the optical applications, such as color display, optical data storage, sensor and energy solar cell. At the same time, through the study of the mechanism of energy transfer between the rare earth ions (Er3+, Tm3+, Tb3+, Pr3+, Nd3+ and Yb3+ ions) and metal ions (Mn2+, Cu+, Au+ and Ag+ ions) to achieve enhanced downconversion/upconversion emission. For the illustration of the above works, this Ph. D. thesis has been divided in seven chapters.In chapter 1, we mainly presented the basic theory for the field of our research, such as: Downconversion mechanisms, upconversion mechanisms, energy transfer mechanisms, cross-relaxation, applications of the downconversion/upconversion, Judd-Ofelt theory (J-O theory), purposes of research, research methods, introduction of the materials experimental, methods experimental, performance testing of experimental samples, etc.In chapter 2, the firstly, we prepared the glass materials with composition of 50SiO2-10AlF3-5TiO2-30BaF2-5LaF3 (SABTL) using the conventional quenching techniques. By differential thermal analysis, we found proper temperatures for heat treatment process. Then, we carried out heat treated SABTL glass, under the changing of temperatures and times heat treatment to become the transparent glass-ceramics containing Ba2LaF7 nanocrystals. The structural investigation carried out by X-ray diffraction (XRD) and the transmission electron microscopy (TEM) evidenced the formation of the cubic Ba2LaF7 nanocrystals with the crystal size about 14 nm. Subsequently, we were investigated the upconversion emission of the RE/Yb3+ (RE:Er3+, Tm3+ and Tb3+ ions) co-doped transparent glass-ceramics containing Ba2LaF7 nanocrystals under 980 nm laser diode excitation by heat treatment process. The result of these studies:the upconversion luminescence of the RE/Yb3+(RE:Er3+, Tm3+ and Tb3+ ions) co-doped in the glass-ceramics has significantly enhanced in comparison with glass before heat treatment. At the same time, the reasons for the highly efficient upconversion luminescence and energy transfer mechanisms between the rare earth ions were discussed.In chapter 3, we have investigated the upconversion emission of the Er3+/Tb3+/Yb3+ Tm3+/Tb3+/Yb3+ and Er3+/Tm3+/Yb3+ tri-doped transparent glass-ceramics containing Ba2LaF7 nanocrystals under 980 nm laser diode excitation by heat treatment process. By changed the mixing ratio of the Er3+, Tb3+, Tm3+ and Yb3+ ions in the transparent glass-ceramics, we found mixing ratio between the rare earth ions for upconversion emissions intensity are optimality. At the same time, the reasons for the highly efficient upconversion emissions intensity and energy transfer mechanisms between the Er3+, Tb3+, Tm3+ and Yb3+ ions were proposed and discussed.In chapter 4, we have investigated effect of the Mn2+ ions on the enhancement upconversion emission intensity of the RE/Yb3+(RE:Er3+, Tm3+ and Tb3+ ions) co-doped transparent glass-ceramics containing Ba2LaF7 nanocrystals under 980 nm laser diode excitation. By changed the mixing ratio of the Mn2+, Er3*, Tb3+, Tm3+ and Yb3+ ions in the transparent glass-ceramics, we found mixing ratio between the Mn2+ ions and rare earth ions for upconversion emissions intensity are optimality. At the same time, the energy transfer mechanisms between the(Mn2+, Yb3+) dimer and the RE ions (RE:Er3+, Tm3+ and Tb3+ ions) were proposed.In chapter 5, the firstly, we also prepared the glass materials with composition of 45SiO2-25AlF3-18BaF2-6TiO2-6CaCO3 (SABTC) using the conventional quenching techniques. By differential thermal analysis, we found proper temperatures for heat treatment process. Then, similarly in chapter 2, we carried out heat treated SABTC glass, under the changing of temperatures and times heat treatment to become the transparent glass-ceramics containing BaF2 nanocrystals. The structural investigation carried out by X-ray diffraction (XRD) and the transmission electron microscopy (TEM) evidenced the formation of cubic BaF2 nanocrystals with crystal size about 13 nm. Subsequently, we have investigated effect of precious metal ions (Cu+, Au+ and Ag+ ions) on the upconversion emission intensity of the RE/Yb3+(RE:Er3+, Tm3+ and Tb3+ ions) co-doped transparent silicate glass-ceramics containing BaF2 nanocrystals under 980 nm laser diode excitation with the broadband about from 500 to 900 nm.In chapter 6, we have investigated the downconversion emission of the RE/Yb3+(RE: Er3+, Nd3+ and Pr3+ ions) co-doped transparent silicate glass-ceramics. At the same time, the effect of the metal ions on the enhancement downconversion emission intensity of the RE/Yb3+(RE:Er3+, Nd3+ and Pr3+ ions) co-doped transparent silicate glass-ceramics also investigated.Finally, in chapter 7, we mainly concluded the main research results of this thesis and we also proposed research directions in future. |
PDF Full Text Request