First-principle study of phosphors for white-LED applications : Stokes shift, emission linewidth and thermal quenching.

Despite current impact of white-LED technology, available blue-to-red phosphors exhibit a too wide emission linewidth or some other drawback, such as strong thermal quenching behavior. In view of large-scale high-throughput search for better phosphors, an accurate but fast computational methodology is needed, not restricted to the ground state (formation energy), but covering excitation and emission energies, Stokes shift, emission linewidth, and emission intensity reduction through thermal quenching. We will present results from a constrained density functional theory methodology for about 30 Ce- and Eu- doped materials [1,2] with 4f → 5d transitions. It matches experimental data within 0.3 eV for both absorption and emission energies (ranging between 2.0 eV and 5.0 eV) and provides Stokes shifts usually within 30%. By contrast, the largely used semi-empirical Dorenbos approach [3] does not perform as well, although it provides physical insights to explain trends among the materials, including the different Stokes shifts. This first-principles approach also delivers emission linewidth and assessment of mechanisms for thermal quenching, based on a simple 1-dimensional configuration coordinate approach. For our representative set of Eu-doped materials, we find that the 4f-5d crossover model cannot be the dominant thermal quenching mechanism: the predicted barrier at the 4f-5d crossing is always higher than 1.5 eV. Also, it will be shown how to waive the one-dimensional restriction in the search for the lowest 4f-5d crossing energy barrier. [1] Y. Jia, S. Poncé, A. Miglio, M. Mikami & X. Gonze, Adv. Opt. Mat. 5, 1600997 (2017). [2] Y. Jia, A. Miglio, S. Poncé, M. Mikami & X. Gonze, Phys. Rev. B 96, 125132 (2017). [3] P. Dorenbos, J. Lumin. 91, 155 (2000); Phys. Rev. B 62, 15640 (2000).