Preparation and Photoluminescence Properties of Potential Phosphors for White Light-Emitting Diodes
Date Issued
2010
Date
2010
Author(s)
Hsu, Chia-Hao
Abstract
To overcome the drawbacks of the phosphors for UV-LEDs, such as low thermal stability, short excitation wavelength (< 350 nm) and poor color tunability, the nephelauxetic (covalency) theory, crystal-field theory and energy transfer mechanisms are applied to improve the luminescent properties. In addition, it is known that the thermal stability of the phosphors strongly depends on the covalency of the host materials. In this study, four types of phosphors (oxides, oxynitrides, nitrides and carbonitrides) with increased covalency were prepared, and the luminescent properties were investigated.
Sr2CeO4 phosphors have broad excitation bands in the range of 200 to 450 nm. Sn4+ ions were doped in the host materials to shift the highest excitation peak from 296 to 346 nm. Upon the excitation at around 346 nm, the intensity of the blue emission peak at 483 nm was enhanced 39% as compared to that of undoped Sr2CeO4 phosphors.
In the second section, the energy transfer mechanism of MgY4Si3O13:Ce3+, Mn2+ phosphors from Ce3+ to Mn2+ was revealed to be dipole-quadrupole interaction. As the Mn2+ concentration increased, the Mn2+ emission intensity increased and the Ce3+ emission intensity decreased. This resulted in shifting the chromaticity coordinates of the prepared phosphors from the blue, white to orange region.
In the third section, the oxynitride-based SrSi2O2N2 phosphors with high covalency were selected as the host materials. As Ce3+ and Tb3+ ions were co-doped into SrSi2O2N2, the energy transfer process occurred. With increasing the Tb3+ concentration, the emitting colors of SrSi2O2N2: Ce3+, Tb3+ phosphors shifted from the blue towards green region. The increased temperatures caused the reduction of the emission intensity of the prepared phosphors due to the thermal quenching effects. It is found the prepared SrSi2O2N2-based phosphors have excellent thermal stability.
In the fourth section, oxynitride-based phosphors CaSi2O2N2 were selected as the host materials. Increasing the Eu2+ concentration of the Ce3+ and Eu2+-codoped phosphors led the excitation wavelength to shift from 330 to 352 nm and the excitation intensity to increase. In addition, the Eu2+ emission (550 nm) intensity increased and Ce3+ emission (470 nm) intensity decreased, leading the emitting colors of the prepared phosphors to shift from the blue to yellowish green region.
In the fifth section, the nitridosilicate (CeSi3N5) and carbonitride (Y2Si4N6C) phosphors with high covalency were selected as the host materials. For CeSi3N5 phosphors, the wavelength of the maximum excitation peak was at 353 nm. The emission spectrum exhibited an intense blue emission at 453 nm. When Tb3+ ions were doped, the emitting colors shifted from the blue to greenish blue. For Ce3+-doped Y2Si4N6C phosphors, the incorporation of La3+ ions led the emitting colors of (Y, La)2Si4N6C: Ce3+ phosphors to vary from the yellowish green to blue region. The blue shift in emission bands was due to the variation in the crystal-field strength around the activators. In addition, the excellent thermal stability of Y2Si4N6C-based phosphors was revealed.
Subjects
phosphor
light-emitting diode
enregy transfer
crystal field
Type
thesis
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