Luminescent properties of narrow emission red phosphor : an ab initio study

Recent experimental discovery of narrow emission red phosphor Sr[Li2Al2O2N2]:Eu2+ enables a potential noticeable leap in the luminous efficacy of phosphor based white LEDs because of its advantageous luminescent properties. Understanding those properties and their temperature dependence from a theoretical perspective is however something that has not been done yet. This master thesis aims at providing such characterization of the temperature dependence of the luminescent properties of SALON:Eu2+. For this purpose, a combination of density functional theory (DFT) calculations and configuration coordinate models (CCM) based on the Franck-Condon overlaps were used. Thermal expansion coefficient was computed within the quasi-harmonic approximation and its effect on the luminescence spectrum was calculated. A complete phonon decomposition of the luminescent spectrum is also obtained with a generating function approach. A new method based on molecular dynamic is finally explored for the theoretical study of luminescence spectrum. The results of this work showed that the use of the 1D-CCM is sufficient to fairly reproduce experimental luminescence spectrum as well as the temperature dependence of the FWHM. The generating function approach allowed us to identify an important phonon mode at 33.7 meV that strongly couples with the optical 4f-5d transition. Results also revealed that the 1D-CCM, greatly used in the field to explain luminescent properties, is not suitable to correctly reproduce the temperature dependence of the emission maximum. It is only by considering all the vibrations modes by means of molecular dynamic simulation that the experimental blue shift was observed. We found also that accounting for the prefactor omega in the computation of the luminescence intensity is crucial to explain the shift (with omega the light frequency).