Papers by Albrecht Winnacker
Physical Review B, 1997
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Springer eBooks, 1997
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Physikalische Blätter, 1990
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MRS Proceedings, 2011
ABSTRACTSpherical monodisperse core/shell-type nanoparticles, comprising an amorphous SiO2 core c... more ABSTRACTSpherical monodisperse core/shell-type nanoparticles, comprising an amorphous SiO2 core coated with a luminescent phosphor layer were synthesized by the modified Pechini processes. The sol-gel method allows covering the 50 – 500 nm core particles with different inorganic phosphor layers of about 10 nm thickness, doped with rare-earth or transition metal ions which determine the luminescent properties. Particles comprising a Zn2SiO4 shell, doped with Mn2+ ions, are not only fluorescent under UV irradiation (260 nm), but store the activation energy by trapping electrons/holes at lattice defects. This energy is released as phosphorescence in the time scale of seconds and minutes, or as photostimulated luminescence under the excitation of red light (650 nm). Traps related to these processes are different, and their concentration is affected by the preparation conditions of the particles.
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SPIE Proceedings, 2012
ABSTRACT Photoluminescence (PL) spectroscopy has been performed in-situ on iridium(III) ionic tra... more ABSTRACT Photoluminescence (PL) spectroscopy has been performed in-situ on iridium(III) ionic transition metal complex (iTMC)- based sandwich-type light-emitting electrochemical cells (LECs) during device operation and after switch-off. It is demonstrated that driving the device leads to a considerable decrease of the PL intensity of the active layer. Two different time regimes for this decrease have been identified. The first one is characterized by a complete recovery of the PL after the device is turned off corroborating the existence of dynamically formed doped regions also in iTMC-based LECs. In the second regime the PL does not completely recover which is attributed to a permanent degradation of the active layer that is the main source for the low lifetime of the devices. Additionally, it is demonstrated how to externally stabilize the dynamic configuration leading to a half lifetime in excess of 1000 hours at simultaneous high brightness of more than 1000 cd/m2 and fast turn-on of less than one second.
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SPIE Proceedings, 2006
OLEDs gain more and more interest in the field of lighting applications. The OLED technology prov... more OLEDs gain more and more interest in the field of lighting applications. The OLED technology provides striking advantages and covers completely new application fields offering a new freedom in design for next generation lighting. Large area OLEDs might act as a 2-dimensional light source which is thin, flat and lightweight generating diffuse, nonglaring illumination. In the first part of our report we investigate small scale inhomogeneities of polymer based OLEDs. Devices were monitored during operation by taking pictures of the active area at constant periods of time. These pictures were analyzed by a software tool with respect to the occurrence and evolution of defects. Initially induced inhomogeneities are growing and dominate the performance with increasing operation time. Within the error margin of the setup no additional spots are generated during operation. The voltage drop inside the ITO anode due to a high resistivity plays an important role for the brightness homogeneity of large area devices. The voltage drop causes a brightness fall-off towards the center of the device. It is maintaining with increasing average current density and luminance, respectively. At a brightness of 1000cd/m2 the deviation at the center exceeds 30%. The homogeneity of luminance is improved by incorporation of additional metal lines on the anode layer. The best results were achieved with 200nm thick aluminum structures with a pitch of 1mm and a width of 60mum of each line. At an average current density of 45mA/cm2 the decay towards the center of the device is only half of the decrease without any additional metallization.
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SPIE Proceedings, 1993
ABSTRACT We report on progress in different domains of n-detection: (1) High gain image intensifi... more ABSTRACT We report on progress in different domains of n-detection: (1) High gain image intensifiers coupled to n-scintillators enable small size n-monitors with high spatial resolution. (2) Standard CCD cameras combined with special frame grabbers provide low cost n-image processing. (3) Progress in diode technology allows direct light detection from n-scintillators with the aim of low effort and very thin n-detectors. (4) New foils with X-ray photography combined with n-converters can be used for efficient large area position-sensitive detectors. The stored information is read out by laser-stimulated emission of light.
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MRS Proceedings, 2011
ABSTRACTA series of Ca1-xSrxS:Eu2+y mol% phosphors were synthesized with solid state reactions an... more ABSTRACTA series of Ca1-xSrxS:Eu2+y mol% phosphors were synthesized with solid state reactions and with various Ca/Sr ratio and Eu2+ doping concentrations. The influences of the lattice composition and the Eu2+ doping level on photoluminescent properties were analyzed. With doping concentrations between 0.1 to 3 mol%, concentration quenching takes place leading to the decrease of luminance; the emission maxima are also red-shifted. Further, this work reports enhanced photosynthetic activities of intact spinach leaves due to spectral modification of simulated solar irradiation by one synthesized phosphor (Ca0.4Sr0.6S:Eu1 mol%). The CO2 assimilation rates of intact spinach leaves were monitored with an effective homemade photosynthesis measurement system with controlled light conditions. The phosphor could efficiently convert the photosynthetically less active green part of the solar spectrum into the red, with a broad-band red emission centered at 650 nm and a halfband-width of 68 nm...
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Materials Chemistry and Physics, 2014
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MRS Proceedings, 2011
ABSTRACTSiO2 nanoparticles (NPs) were coated with Eu3+-doped calcium phosphate (CP) and Mn2+-dope... more ABSTRACTSiO2 nanoparticles (NPs) were coated with Eu3+-doped calcium phosphate (CP) and Mn2+-doped ZnO to give Zn2SiO4 via a modified Pechini sol-gel process. Annealing at high temperatures resulted in NPs with an amorphous core and a crystalline luminescent shell. It was shown that this procedure can be applied to silica cores with diameters below 300 nm. By transmission electron microscopy, elemental analysis and from X-ray diffraction patterns it was determined that shell composition and structure are influenced by the annealing temperature and pH of the coating solution. Measurements of photoluminescence intensities displayed their dependency on the concentration of dopant in the resulting core/shell NPs.
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Aluminum nitride (AlN) is a promising substrate material for epitaxy of Al-rich III-nitrides to b... more Aluminum nitride (AlN) is a promising substrate material for epitaxy of Al-rich III-nitrides to be employed, e.g., in deep-UV optoelectronic and high-power microwave devices. In this context, preparation of bulk AlN crystals by physical vapor transport (PVT) appears to be of most importance. In this work, seeded growth of AlN on (0001)-plane 6H-SiC substrates was investigated. SiC substrates with a diameter of 15 mm were used. AlN layers with thicknesses up to 3 mm were deposited at growth rates in the range of 10 to 40 μm/hour. Such templates provide large-area seeds, but they are often cracked, especially at thicknesses below 1mm. Besides cracks, other defects from the SiC seed propagate into the AlN layer and subsequently into the bulk AlN crystal. That is why, the aim of this work is to assess structural quality and defect content in thick AlN templates grown on (0001) plane SiC substrates. An optimum thickness-quality, the most appropriate growth stage for further use of the AlN template as a seed for subsequent PVT growth of bulk AlN growth, will be provided. We found that low growth rates mitigate crack propagation; slow cooling as well as optimization of the thermal field inside the crucible can prevent formation of new cracks after growth.
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MRS Proceedings, 2008
The thin electron injection layers between the cathode and the light emitting polymer layer in po... more The thin electron injection layers between the cathode and the light emitting polymer layer in polymer light emitting diodes (PLEDs) have been shown to have a big impact on the final device performance. Usually, in PLEDs low work function metals like Ba, Mg or Ca are used to reduce the energy barrier between the cathode and the polymer thus providing a better electron injection from the cathode. Also salts like LiF, NaF, Cs2CO3 and CsF have recently been shown to function as electron injection layers in light emitting devices. From these, especially caesium carbonate (Cs2CO3) results into high efficiency diodes both as a solution processed electron injection layer in PLEDs, as well as an n-dopant in the electron transport layer in vacuum deposited small molecule based OLEDs. The functional mechanism of Cs2CO3 as a pure interlayer is not yet fully understood. The proposed mechanisms include the n-doping of the organic layer with Cs2CO3, the thermal decomposition of Cs2CO3 and followi...
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Radiation Measurements, 2007
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Optical Materials, 2006
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Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2002
Image plates based on storage phosphors are a major application of radiation defects in insulator... more Image plates based on storage phosphors are a major application of radiation defects in insulators. Storage phosphors absorb X-ray quanta creating trapped electron–hole pairs in the material. Optical stimulation of the electron causes recombination leading to light emission. Application of image plates requires an optimal compromise between resolution (represented by the modulation transfer function (MTF)) and sensitivity. In our paper
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Journal of Luminescence, 1998
ABSTRACT
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Journal of Luminescence, 1998
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Journal of Luminescence, 2000
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Journal of Crystal Growth, 1999
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Journal of Crystal Growth, 1998
Abstract The limitation of ZnSe bulk solution growth by the formation of solvent inclusions at hi... more Abstract The limitation of ZnSe bulk solution growth by the formation of solvent inclusions at high growth rates is investigated both theoretically and experimentally in terms of the ratio V/G of growth rate to temperature gradient. For the solvents Se, Zn, ZnCl 2 , PbCl 2 and PbSe the V/G -ratio is calculated using Mullins' stability criterion. THM experiments employing PbCl 2 and PbSe resulted in the formation of inclusions containing other stable phases than ZnSe. THM growth with Se as a solvent encounters experimental difficulties due to the considerable vapor pressures. The results indicate a discrepancy between experiments and calculation, the experimental V/G -ratio for stable growth being at least one order of magnitude smaller. This deviation is explained by the effect of convection and formation of boundary layers on growth stability. Implications for the growth from Se solutions are given.
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Papers by Albrecht Winnacker