No abstract

Traditional light sources, e.g., incandescent and fluorescent lamps, are currently being replaced by white light-emitting diodes (wLEDs) because of their improved efficiency, prolonged lifetime, and environmental friendliness. Much effort has recently been spent to the development of Mn-doped fluoride phosphors that can enhance the color gamut in displays and improve the color rendering index, luminous efficacy of the radiation, and correlated color temperature of wLEDs used for lighting. Purity, stability, and degradation of fluoride phosphors are, however, rarely discussed. Nevertheless, the typical wet chemical synthesis routes (involving hydrogen fluoride (HF)) and the large variety of possible Mn valence states often lead to impurities that drastically influence the performance and stability of these phosphors. In this article, the origins and consequences of impurities formed during synthesis and aging of KSiF:Mn are revealed. Both crystalline impurities such as KHF and ionic impurities such as Mn are found to affect the phosphor performance. While Mn mainly influences the optical absorption behavior, KHF can affect both the optical performance and chemical stability of the phosphor. Moisture leads to decomposition of KHF, forming HF and amorphous hydrated potassium fluoride. As a consequence of hydrate formation, significant amounts of water can be absorbed in impure phosphor powders containing KHF, facilitating the hydrolysis of [MnF] complexes and affecting the optical absorption of the phosphors. Strategies are discussed to identify impurities and to achieve pure and stable phosphors with internal quantum efficiencies of more than 90%.

The use of an Al 2 O 3 -or TiO 2 -coated layer, deposited by using atomic layer deposition (ALD), was explored to improve the adhesion properties of fluoride phosphor particles. K 2 SiF 6 :Mn 4+ was investigated as a benchmark fluoride phosphor. Mn 4+ -doped fluoride phosphors show a narrow red emission band centered at around 630 nm, making them suitable candidates for the red component in white light-emitting diodes. In lighting applications, red fluoride phosphors can lower the correlated color temperature without sacrificing the luminous efficacy of the radiation. In displays, their saturated red emission enlarges the color gamut. Although attractive optical properties are provided by fluoride phosphors, a remaining hurdle is their sensitivity to moisture, which leads to phosphor degradation. Protective hydrophobic shells can be used to enhance the moisture stability of phosphor materials. For fluoride phosphors, the surface fluorine inhibits a good adhesion between the core and shell, potentially leading to delamination of the shell with time. Ideally, the thin ALD-deposited seed layers provide an OH-saturated particle surface, offering an enhanced adhesion to outer moisture barrier layers or hydrophobic encapsulants. The results obtained indicate that the Al 2 O 3 seed layers suffered from delamination and blistering after deposition, related to surface reactions during ALD. In contrast to Al 2 O 3 , the TiO 2 layer could be grown with high uniformity and conformality. In a final step, the ALD-coated fluoride powders were successfully made hydrophobic using a nonchlorinated hydrophobic precursor.

Degradation pathways of K2MnF6, an essential precursor for the synthesis of red fluoride phosphors, are revealed.

Correction for ‘K2MnF6 as a precursor for saturated red fluoride phosphors: the struggle for structural stability’ by Reinert Verstraete et al., J. Mater. Chem. C, 2017, 5, 10761–10769.