B. Henke

Eu-activated fluorochlorozirconate glass-ceramic scintillators

Rare-earth-doped fluorochlorozirconate (FCZ) glass-ceramic materials have been developed as scintillators and their properties investigated as a function of dopant level. The paper presents the relative scintillation efficiency in comparison to single-crystal cadmium tungstate, the scintillation intensity as a function of x-ray intensity and x-ray energy, and the spatial resolution (modulation transfer function). Images obtained with the FCZ glass-ceramic scintillator and with cadmium tungstate are also presented. Comparison shows that the image quality obtained using the glass ceramic is close to that from cadmium tungstate. Therefore, the glass-ceramic scintillator could be used as an alternative material for image formation resulting from scintillation. Other inorganic scintillators such as single crystals or polycrystalline films have limitations in resolution or size, but the transparent glass-ceramic can be scaled to any shape or size with excellent resolution.

Eu oxidation state in fluorozirconate-based glass ceramics

The influence of InF(3) doping and remelting on Eu-doped fluorozirconate-based glass ceramics was investigated using near-edge x-ray absorption and optical spectroscopy. It was found that the addition of InF(3) to the melt decreases the Eu(2+)Eu(3+) mole ratio, while remelting leads to a significant change in the Eu(2+)Eu(3+) ratio in favor of Eu(2+). Photoluminescence spectroscopy shows that additional annealing steps lead to the formation of BaCl(2) nanoparticles in the glass. In as-made glass ceramics containing InF(3), a phase transition of the nanoparticles from hexagonal to orthorhombic structure is observed. This phase transition is not observed in the remelted glasses studied here.

The effect of x-ray, γ-ray, and UV radiations on the optical properties of RbCdF3:Mn2+

We report the results of optical transmission, photoluminescence (PL), and radio-luminescence measurements on x ray, γ ray, and UV irradiated RbCdF3:Mn2+ single crystals with different Mn2+ concentrations. There is a radiation-induced increase in the optical absorption and PL intensity that correlates with the Mn2+ concentration. This suggests that the defect center that gives rise to the observed effect is associated with the Mn2+ ions. We propose that the defect center is an F-center where an electron is trapped at a fluorine vacancy situated near but not adjacent to the Mn2+ site. The PL intensity for UV (240–270nm), x-ray (<40keV), or γ-ray (60keV) irradiation saturates at high irradiation intensities, which can be attributed to a fixed concentration of defect sites determined by the Mn2+ concentration. We show that the radiation-induced PL intensity is correlated with the UV, x-ray, and Am241 γ-ray dose. This indicates that RbCdF3:Mn2+ has potential as a radiation dosimeter material.

PV module defect detection by combination of mechanical and electrical analysis methods

The lifetime and reliability of photovoltaic modules (PV modules) is influenced by defects which have their origin either in manufacturing processes or in operation exposure. Characterization of PV modules is necessary for manufacturers to assure their warranty and to observe process difficulties during production process and for improving their modules during development processes. For customers PV module characterization is important to observe output performance of their PV system and to proof intactness of single modules. For this purpose reliable and nondestructive testing methods are desirable.

Characterization of PV modules by combining results of mechanical and electrical analysis methods

Photovoltaic modules (PV modules) are supposed to have a lifetime of more than 20 years under various environmental conditions like temperature changes, mechanical loads, etc. Common outdoor exposure may influence efficiency and lifetime which necessitates assessment of PV module performance and detection of output deficits. For this purpose reliable and nondestructive testing methods are desirable. Commercially available PV modules were tested by different analysis methods. The PV modules electrical properties were investigated by thermography and electroluminescence measurements. The combination of these two techniques is well-suited to detect many cell and module defects. A crystalline module showed significant cell breakage after temperature cycle test. To observe the mechanisms of this specific defect type laminated test specimens on smaller scales were produced and analyzed over production process and during temperature cycles derived from the international standards IEC 61215 and IEC 61646. The defect study on small scales allows conclusions about the defects influence on larger PV modules. Further methods capable for mechanical characterization like Laser Doppler vibrometry, surface geometry scan and digital image correlation are presented briefly. The combination of the methods mentioned above allows a very precise assessment of the mechanical and electrical capability which is essential for reliability and lifetime concepts.

Enhanced up-converted fluorescence in fluorozirconate based glass ceramics for high efficiency solar cells

Thermal processing of as-made fluorozirconate glasses which were additionally doped with neodymium and chlorine ions leads to enhanced up-conversion fluorescence intensities in these glass ceramics. The samples were annealed between 240°C and 290°C while the optimum value was found for the 270°C sample. We investigated the power dependence of the infrared fluorescence, the 2-photon up-conversion, and the 3-photon up-conversion fluorescence intensities as well as the corresponding radiative lifetimes. In analogy to the up-conversion intensity, the radiative lifetime of the Nd3+ fluorescence at about 880 nm depends significantly on the annealing temperature: the longest lifetime was observed for the 270°C sample.

Progress on up- and down-converted fluorescence in rare-doped fluorozirconate-based glass ceramics for high efficiency solar cells

Transparent glasses as up- or down-converters are attractive systems to increase the efficiency of solar cells. Er-doped fluorozirconate (FZ) glasses show an intense up-conversion upon excitation at 1540 nm. Transmission spectra show that the absorbance at 1540 nm grows linearly with the Er-doping level. In Eu-doped FZ glasses, which were additionally doped with chlorine ions, the growth of BaCl2 nanocrystals can be observed upon thermal annealing. For high annealing temperatures a phase change from hexagonal to orthorhombic phase BaCl2 can be seen. Upon excitation in the ultraviolet (UV) spectral range these glass ceramics emit an intense blue emission. A combination of a silicon solar cell and an Eu-doped FZ glass ceramic as a down-converting top layer shows an increase in the short circuit current in the UV spectral range compared to a solar cell without a down-converting top layer.

Advances in up- and down-converted fluorescence for high efficiency solar cells using rare-earth doped fluorozirconate-based glasses and glass ceramics

Transparent, rare-earth doped fluorozirconate-based glasses and glass ceramics are attractive systems as up- and downconverters to increase solar cell efficiency. For down-conversion applications, the efficiency of a silicon solar cell could be significantly increased in the ultraviolet spectral range by placing a europium-doped glass ceramic on top. High transparency is a key issue here to avoid scattering losses and to obtain high light output. Transmission spectra of fluorozirconate glasses, which were additionally doped with chlorine ions to initiate the growth of BaCl2 nanoparticles therein upon thermal annealing, show that the absorbance in the visible spectral depends significantly on the annealing conditions. For up-conversion applications, erbium-doped fluorozironate glasses have been investigated. 2-dimensional intensity mapping of the up-converted fluorescence yielded information on the homogeneity of the glass sample and the erbium distribution therein. Depth scan experiments showed that the position of the focus of the excitation laser beam plays a crucial role since saturation of the 2-photon up-conversion occurs for high excitation power.

Zr and Ba edge phenomena in the scintillation intensity of fluorozirconate-based glass-ceramic X-ray detectors

The energy-dependent scintillation intensity of Eu-doped fluorozirconate glass-ceramic X-ray detectors has been investigated in the energy range from 10 to 40 keV. The experiments were performed at the Advanced Photon Source, Argonne National Laboratory, USA. The glass ceramics are based on Eu-doped fluorozirconate glasses, which were additionally doped with chlorine to initiate the nucleation of BaCl(2) nanocrystals therein. The X-ray excited scintillation is mainly due to the 5d-4f transition of Eu(2+) embedded in the BaCl(2) nanocrystals; Eu(2+) in the glass does not luminesce. Upon appropriate annealing the nanocrystals grow and undergo a phase transition from a hexagonal to an orthorhombic phase of BaCl(2). The scintillation intensity is investigated as a function of the X-ray energy, particle size and structure of the embedded nanocrystals. The scintillation intensity versus X-ray energy dependence shows that the intensity is inversely proportional to the photoelectric absorption of the material, i.e. the more photoelectric absorption the less scintillation. At 18 and 37.4 keV a significant decrease in the scintillation intensity can be observed; this energy corresponds to the K-edge of Zr and Ba, respectively. The glass matrix as well as the structure and size of the embedded nanocrystals have an influence on the scintillation properties of the glass ceramics.

Energy-dependent scintillation intensity of fluorozirconate-based glass-ceramic x-ray detectors

We investigated the energy-dependent scintillation intensity of Eu-doped fluorozirconate glass-ceramic x-ray detectors in the energy range from 6 to 20 keV. The experiments were performed at the Advanced Photon Source, Argonne National Laboratory. The glass ceramics are based on Eu-doped fluorozirconate glasses, which were additionally doped with chlorine to initiate the nucleation of BaCl2 nanocrystals therein. The x-ray excited scintillation is mainly due to the 5d-4f transition of Eu2+ embedded in the BaCl2 nanocrystals; Eu2+ in the glass does not luminesce. Upon appropriate annealing the nanocrystals grow and undergo a phase transition from a hexagonal to an orthorhombic phase of BaCl2. The scintillation intensity is investigated as a function of the x-ray energy as well as of the particle size and structure of the embedded nanoparticles. The scintillation intensity versus x-ray energy dependence shows that the intensity is inversely proportional to the photoelectric absorption of the material, i.e. the more photoelectric absorption the less scintillation.