Downconversion Luminescence Properties Of Rare-earth Ions Doped Luminescent Materials

Efficient utilization of solar energy is expected to make a considerable contribution to solving the ongoing energy crisis and global warming problems through lessening the world's dependence on conventional fossil fuels. The conversion from sunlight to electricity using solar-cell devices based on the photovoltaic effect represents a promising approach to green and renewable energy. State-of-the-art commercial crystalline Si (c-Si) solar cells dominate the present photovoltaic technologies, and operate with energy conversion efficiencies around 15%. The best existing Lab-scale c-Si solar cells have photovoltaic efficiencies of 25%, which is very colse to the Shockley-Queisser efficiency limit of 29%. One major issue leading to low energy efficiency of the c-Si solar cells is the mismatch between the incident solar spectrum and the spectral absorption properties of the c-Si semiconductor. The AM1.5G solar spectrum covers photons with energies ranging from about 0.5 to 4.4 eV (280–2500 nm). Unfortunately, commercial single-junction c-Si solar cells can convert only a small part of the incident solar spectrum to electricity according to the bandgap of c-Si (Eg = 1.12 eV,?= 1100 nm). For the incident photons with energies below the bandgap, the energies are not be absorbed (sub-bandgap transmission loss), whereas for the photons with energies above the bandgap, the excess energy is wasted as heat within the solar cell (thermalization loss). These fundamental spectral mismatch losses alone result in over 50% loss of the incident solar energy in solar cell conversion to electricity, whereas they could be minimized by modifying the solar spectrum via luminescence conversion process including upconversion and downconversion. The trivalent rare-earth ions with abundant energy levels arising from the 4fn inner shell configuration offer the ability of photon management and thus are well-suited for spectral conversion in solar cells. If conversion of one incident ultraviolet–visible (300–500 nm) photon into two near-infrared (¹000 nm) photons is realized, the energy loss due to the thermalization in c-Si solar cells could be minimized. In this thesis we focus on research on several rare-earth/metal ion couples doped downconversion materials that may be capable of raising the efficiency beyond the Shockley–Queisser limit.The introduction (Chapter 1) describes the energy level structures and luminescent properties of lanthanide ions and their potential application for spectral conversion in solar cells. The focus has been on the use of these ions in upconversion and downconversion schemes since these mechanisms are capable of raising the efficiency beyond the Shockley–Queisser limit. A passive luminescence converter can be applied and optically coupled, in principle, to any existing solar cell without any modification of the cell itself. Recent theory has predicted that downconversion or upconversion in conjunction with a c-Si solar cell can achieve a conversion efficiency of up to 38.6% or 50%, respectively. However, practical realization of these higher efficiencies is still far away and requires basic research. Chapter 2 introduces several methods for synthesizing rare-earth doped luminescent materials and the related physical and spectroscopic properties measurements.Chapter 3 and Chapter 4 have investigated the downconversion luminescence of Tb³⁺-Yb³⁺ couple in the Zn2SiO4 thin-films and Gd₂(MoO₄)₃ powders, respectively. Both in Zn2SiO4 and Gd2(MoO4)3 downconversion takes place with this couple. Upon excitation in the 5D4 level of Tb³⁺ situated around 20000 cm-1, efficient energy transfer is observed to Yb³⁺, which results in visible emissions (500–700 nm) due to Tb³⁺:⁵D₄?7F0,1,2,3,4,5 transitions and near-infrared emissions (900–1100 nm) due to the Yb³⁺:²F_5/2?²F_7/2 transition. The Tb³⁺:⁷F₆?⁵D₄ excitation peak is obtained by monitoring the emissions of Tb³⁺ and Yb³⁺ ions. Morever, the decay lifetime of Tb³⁺:⁵D₄?⁷F₅ transition significantly decreases with the increase of Yb³⁺ doping concentrations. Energy transfer from Tb³⁺ to two neighbouring Yb³⁺ ions occurs via cooperative dipole–dipole interaction. And the measured maximum near-infrared quantum efficiency is close to 200%.Chapter 5 makes an attempt to enhance the downconversion luminescence of Tb³⁺-Yb³⁺-codoped (Gd,Y)BO3 phosphors by tridoping with Ce³⁺ ion. The luminescence mechanism for Yb³⁺ in Ce³⁺-Tb³⁺-Yb³⁺ tridoped (Gd,Y)BO3 phosphors has been investigated in comparison with Tb³⁺-Yb³⁺ and Ce³⁺-Yb³⁺ codoped YBO3 phosphors. The effects of Tb³⁺ and Yb³⁺ doping concentrations on the Yb³⁺ near-infrared emission intensity have been investigated. Two efficient energy-transfer processes of Ce³⁺?Tb³⁺ direct resonant energy-transfer and subsequent Tb³⁺?Yb³⁺ cooperative energy-transfer were employed to realize the enhanced downconversion luminescence. It is found that Ce³⁺ ion can be an efficient sensitizer harvesting ultraviolet photons of 300–400 nm due to its allowed 4f–5d absorption and then gives about 20³0 times enhancement to the near-infrared emission of 900–1100 nm from Yb³⁺ ion. These phosphors could potentially be incorporated into transparent polymer sheets or be prepared in thin film form as the luminescent downconversion layers in front of c-Si solar cells to enhance the performance of the solar cells.Chapter 6 reports on the photoluminescence and energy-transfer properties of Bi³⁺-Yb³⁺ codoped (Gd,Y)₂O₃ and YVO4 phosphors. In these host lattices, the excitation spectra of Bi³⁺ ions cover the ultraviolet region of 300–400 nm. Upon ultraviolet excitation into Bi³⁺ ions, the Bi³⁺-Yb³⁺ codoped (Gd,Y)2_O3 and YVO4 phosphors give rise to the characteristic near-infrared emission of Yb³⁺ ions due to the ²F_5/2?²F_7/2 transition. The excitation spectra measured by monitoring the visible emission of Bi³⁺ and near-infrared emission of Yb³⁺ ovelap well. The effects of Yb³⁺ doping concentrations and excitation wavelength on the Yb³⁺ near-infrared emission intensity have been investigated. It demonstrates that cooperative downconversion can be realized for Bi³⁺–Yb³⁺ couple and the highest value of the calculated downconversion quantum efficiency approaches 200% based on the integration of decay curves.Dye-sensitized solar cells (DSSCs) based on oxide semiconductors and molecular dyes are attracting wide interest in renewable energy research, and they has been considered as a potential alternative to conventional c-Si cells because of its low cost, high conversion efficiency, good stability and simple preparation procedure. DSSCs can work well in the visible range, whereas they suffer from degradation when exposed to ultraviolet radiation. Chapter 7 discusses the luminescence properties of Bi³⁺–Ln³⁺ co-doped YVO4 (Ln = Dy, Er, Ho, Eu, and Sm) phosphors. These phosphors can effectively harvest the near ultraviolet photons of 250–400 nm through the efficient energy feeding by the Bi³⁺–V⁵⁺ metal-to-metal charge-transfer. Energy-transfer from V⁵⁺-Bi³⁺ charge-transfer-state to Ln³⁺ is observed, giving rise to the characteristic visible emissions from Dy³⁺, Er³⁺, Ho³⁺, Eu³⁺, or Sm³⁺ doped samples. Therefore, YVO4:Bi³⁺,Ln³⁺ (Ln = Dy, Er, Ho, Eu, and Sm) phosphors could be a promising ultraviolet-absorbing spectral converter for DSSCs to enhance their photochemical stability and photovoltaic efficiency.