Red for LED: New narrow band red phosphors for white light emitting diodes

Abstract

White light emitting diodes (LEDs) are nowadays widely applied in general lighting and consumer electronics. Due to their superior energy efficiency and long operation lifetime, white LEDs are considered to be the light sources of the future, and it is anticipated that white LEDs will largely replace incandescent and fluorescent lamps in the coming ten years. Commercial white LEDs are composed of blue-emitting (In,Ga)N LEDs and luminescent materials (‘phosphors’) that convert part of the blue LED emission to yellow and red light. Both yellow and red conversion phosphors are necessary to generate warm white light with a good color quality. The red phosphors that are currently used in white LEDs are Eu2+-doped nitrides. These phosphors have a high efficiency and stability, but a major drawback is that their Eu2+ emission band extends into the deep-red spectral region where the eye sensitivity is low. This causes the efficiency of the white LED to decrease significantly. A worldwide search is therefore aimed at finding efficient narrow band red-emitting materials that can be excited by blue light. The results presented in this thesis provide new insights in the synthesis and luminescence of two types of narrow band red-emitting phosphors: Mn4+-doped phosphors and Eu2+-activated phosphates. We describe the synthesis and optical properties of new Mn4+-doped phosphors and investigate how the Mn4+ emission and energy levels are influenced by the host lattice the Mn4+ ions are situated in. Furthermore, we show which processes are responsible for thermal quenching and concentration quenching of the Mn4+ luminescence in Mn4+-doped phosphors. Both these effects are crucial for the performance of white LEDs containing Mn4+ phosphors. Finally, we study the unusual narrow yellow/red Eu2+ luminescence of Eu2+-activated phosphates through extensive low-temperature spectroscopic measurements. The results show that the unusual narrow yellow/red Eu2+ luminescence can be caused by deformations in the excited state of Eu2+.