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Tolhurst, Thomas M.; Strobel, Philipp; Schmidt, Peter J.; Schnick, Wolfgang; Möwes, Alexander (2017): Direct Measurements of Energy Levels and Correlation with Thermal Quenching Behavior in Nitride Phosphors. In: Chemistry of Materials, Vol. 29, Nr. 18: S. 7976-7983
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Highly efficient narrow-band red emitting (RE) phosphors are the most desired and requested materials for developing illumination grade phosphor-converted light emitting diodes (pcLEDs). This study presents direct measurements of RE energy levels, critical to the color and efficiency of LED phosphors. For the first time, we experimentally determine the energetic separation of the Eu Sd state and the conduction band, which is the key indicator of quantum efficiency. This was achieved for the next-generation pcLED phosphors Li2Ca2[Mg2Si2N6]:Eu2+, Ba[Li-2(Al2Si2)N-6]:Eu2+, and Sr[LiAl3N4]:Eu2+ using resonant inelastic Xray scattering. Band to band and 4f to valence band transitions are directly observed in X-ray excited optical luminescence spectra of Sr[LiAl3N4]:Eu2+ and Sr[Mg3SiN4]:Eu2+. These techniques are widely applicable and create a comprehensive, experimental picture of the Eu2+ energy levels in these compounds, leading to a complete understanding of all pertinent electronic processes. This study forms the base needed for a detailed discussion of the structure property relationships, such as specific atoms, coordination and density of states, underpinning phosphor color and efficiency.