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Title: Quantum cutting downconversion phosphors based on oxides for solar cell application
Authors: Lau, Mei Kwan
Degree: M.Phil.
Issue Date: 2012
Abstract: The concept of quantum cutting (QC) to generate two or more near-infrared (NIR) photons per incident high-energy photon has recently attracted growing attention in rare-earth (RE) materials. Attempts have been made to fully utilize the energy in the terrestrial solar spectrum in order to enhance solar cell energy conversion efficiency. If spectroscopic downconversion is realized, the major energy loss due to thermalization of hot charge carriers after the absorption of a high-energy photon in commercial photovoltaic cells can be restrained. The main objective of the present work is to study the properties of RE doped oxide-based phosphor with NIR quantum cutting effect. The photoluminescence (PL) is used to investigate these promising phosphors for the potential applications in the field of photovoltaic cells. In this thesis, the RE (Eu³⁺, Ce³⁺, Tb³⁺, Yb³⁺) doped yttrium aluminium garnet (YAG) with a nominal composition formula of Y₃Al₅O₁₂ was fabricated by conventional sol-gel method. Structural and luminescent characterizations were performed by X-ray diffraction (XRD), field-emission scanning electron microscopy, PL spectra and lifetime. Red and green emission presented upon ultra-violet (UV) excitation for the Eu³⁺-Yb³⁺ and Tb³⁺-Yb³⁺ co-doped samples, respectively, were ascribed to the 4f configurational transitions in Eu³⁺ and Tb³⁺. Both of them exhibited NIR emission which is owed to the energy transfer (ET) between ²F₅/₂ - ²F₇/₂ levels of Yb³⁺. When excited by blue light, green and NIR emissions were observed for the Ce³⁺-Yb³⁺ co-doped specimen. The excitation band is attributed to the 4f-5d transition of Ce³⁺ which covers a broad spectral range. More importantly, the energy of the 4f-5d transition of Ce³⁺ can be tuned by changing the crystal field strength. By varying the mole ratio of the doping pairs, an optimum condition is found. Lifetime measurement is done to evaluate the energy transfer efficiency and hence the maximum quantum efficiency as high as 192% is deduced. The adverse light scattering of powder makes it not applicable to practical solar cell device. Therefore, the phosphor in form of thin films were successfully grown on silicon wafers and fused quartz by pulsed laser deposition (PLD), while maintaining the broadband downconversion characteristics. Polycrystalline YAG films grown on fused silica displayed satisfactory optical transmittance of over 80% for the spectrum ranged from 300 nm to 850 nm. In addition, different morphologies of zinc oxide (ZnO) were synthesized by hydrothermal method. Yb³⁺ singly doped ZnO showed desirable broad excitation bandwidth and the NIR emission is also approximately in agreement with the energy bandgap (~1.0 - 1.7 eV) of copper indium gallium diselenide (CuIn1-xGaxSe₂, abbreviated CIGS) based solar cells. In conclusion, YAG co-doped with RE³⁺-Yb³⁺ (RE = Ce, Eu, Tb) is suitable as solar spectrum downconverter from the viewpoint of spectral characteristics of the PL emission and excitation. Deposition of oxide-based thin films offers a convenient approach compatible with thin film solar cells. ZnO: Yb³⁺ could also be considered to act as a light-absorbing layer for enhancing the spectral response of CIGS solar cell.
Subjects: Phosphors.
Metallic oxides.
Rare earth metal compounds.
Hong Kong Polytechnic University -- Dissertations
Pages: xv, 103 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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