White-emitting light sources based on light-emitting diodes (white-emitting LEDs) are considered to be the next generation in lighting. A common approach to create a white-emitting LED is to combine a blue-emitting InGaN LED with a yellow-emitting phosphor, but it has a low color rendering index (CRI) value due to the rack of red emission. An alternative approach is to combine a near UV-LED (nUV-LED) with blue-, green-, and red-emitting phosphors. However, the development of phosphors for the white-emitting LEDs is still challenging due to the numerous factors that influence on the luminescent properties. In this research, five approaches have been investigated to design and synthesize phosphors for solid state lighting:
First, the Debye temperature (DT) was used as a descriptor for phosphors with high quantum efficiency (QE) (> 80%). Compositions with DT > 500K were considered. CaMgSi2O6:Eu2+ and Ca7Mg(SiO4)4:Eu2+ were synthesized through a co-precipitation and a sol-gel reactions, respectively, due to the high DT. However, a correlation between high DT and high QE was not found.
Second, a flux (NH4F, NH4Cl, or H3BO3) was introduced during co-precipitation synthesis of CaMgSi2O6:Eu2+ to increase the QE. The flux affected both the crystallite size and the QE of the submicron sized phosphors. It was found that the QE increased 3X compared to no flux.
Third, newly identified phosphors, green-emitting Sr2LiAlO4:Eu2+ and blue-emitting Sr2LiAlO4:Ce3+ were discovered by data mining unexplored chemistries for potentially high QE. The new, Sr2LiAlO4:Eu2+ and Sr2LiAlO4:Ce3+ were successfully predicted and synthesized.
Fourth, color tunable single phase phosphors were developed using the Eu2+ and Ce3+ co-activated Sr2LiAlO4 for improvement of QE. The QE increased by 40% with Eu2+ and Ce3+ co-activation compared to the singly activated phosphors.
Fifth, the synthesis of Na2SiF6 was demonstrated through a green synthetic method without toxic HF that is typically used for fluorides synthesis. The red-emitting phosphor, Na2SiF6:Mn4+ was prepared in a low concentration HF solution using the synthesized Na2SiF6 as a host.
This study provides strategies to design and synthesize phosphors with improved quantum efficiency and thermal stability for white-emitting near-UV LEDs for potential applications in solid state lighting.