Hi,
Sometimes, we observe secondary phase though very small (say 5-10 %) in PXRD pattern of synthesized luminescent phosphors wchich could be due to the purity (%) of reactants used for synthesis or the formation of secondary phase. So, my queries are:
1. What is the impact of the secondary phase on LEDs properties and applications?
2. Are the materials having secondary phase preferred for LEDs applications?
Give your suggestions.
Dear Sitakanta,
I fully agree on the previous comments. In some special application areas composite ceramics comprising several luminescent materials could be applied as colour converters onto (In,Ga)N or (Al,Ga)N LEDs.
As far as a single colour conversion material for LEDs, e.g. YAG:Ce, LuAG:Ce, CaAlSiN3:Eu, ß-SiAlON:Eu, SrLiAl3N4:Eu, K2SiF6:Mn and so on is concerned, it is mandatory to keep it single phase, since impurity phases promote PL quenching, afterglow (a problem for emissive displays), reduced colour consistency and lower lifetime.
Moreover, a secondary luminescent phase can competitively absorb the incident light by the LED chip or reabsorb the emitted light by the other phosphors within the package. Finally, secondary phases can destabilise the whole package, as for instance applied fluoride phosphors can be impurified by MnF4 or K2MnF6, while these phases are much more sensitive to photochemical degradation resulting in the release of F2, which is a strong oxidiser.
All the best,
Thomas
Dear Sitakanta,
In general, appearance of secondary phase in a phosphor material often turns out to be most significant PL quenching factor of the primary phosphor material. If the band gap of secondary phase is smaller than primary phase, strong reabsorption of light emitted by the phosphor leads to the reduction of overall QE of the phosphor. Thermal stability of the phosphor is also badly affected by secondary phase. Moreover, if the secondary phase is luminescent, the colour purity of the primary phosphor does not reach up to the mark. So, in my opinion it is always better to keep the system single phase.
Best Regards,
SS
Dear Subhajit Saha,
Thank you very much for your valuable suggestions.
Dear Sitakanta,
I fully agree on the previous comments. In some special application areas composite ceramics comprising several luminescent materials could be applied as colour converters onto (In,Ga)N or (Al,Ga)N LEDs.
As far as a single colour conversion material for LEDs, e.g. YAG:Ce, LuAG:Ce, CaAlSiN3:Eu, ß-SiAlON:Eu, SrLiAl3N4:Eu, K2SiF6:Mn and so on is concerned, it is mandatory to keep it single phase, since impurity phases promote PL quenching, afterglow (a problem for emissive displays), reduced colour consistency and lower lifetime.
Moreover, a secondary luminescent phase can competitively absorb the incident light by the LED chip or reabsorb the emitted light by the other phosphors within the package. Finally, secondary phases can destabilise the whole package, as for instance applied fluoride phosphors can be impurified by MnF4 or K2MnF6, while these phases are much more sensitive to photochemical degradation resulting in the release of F2, which is a strong oxidiser.
All the best,
Thomas
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