**5.3 Potassium fluorosilicate (KSF)**

Red emitting phosphor is required to compensate for red color deficiency, which increases phosphor-converted SSL performance, such as high CRI and tunable color temperatures. Nowadays, mostly rare-earth-doped nitride red phosphors are used to generate SSL. Red nitride phosphors have some drawbacks, such as high-temperature synthesis process (1500–2000°C), oxygen-free environment [40, 41], broader emission band (FWHM >75–100 nm) [41, 42], low quantum efficiency [41], and emission peak greater than 650 nm (beyond the sensitivity range of human eyes) [43–45]. Therefore, it was necessary to find an alternative narrow-band red-emitting phosphor to enhance the color qualities further. Efficient narrow-band redemitting phosphors such as <sup>4</sup> K SiF : Mn 2 6 <sup>+</sup> (KSF), with an emission peak at 631 nm, were developed to replace wide band nitride red phosphor. Many studies had been

conducted to optimize the environmental stability, quantum efficiency, and synthesis of KSF [46–48].

Recently, a new class of *Mn*<sup>4</sup><sup>+</sup> doped fluoride non-rare-earth red phosphor compounds such as <sup>4</sup> A XF : Mn 2 6 <sup>+</sup> ( = , Na, Rb, *NH*<sup>4</sup> , K; X= , ) has emerged, which has many advantages over red nitride phosphors. Firstly, these components' thermal stability is high and good enough for practical application [49]. Secondly, their internal quantum efficiency is 92–98% [40, 50], and that of nitride red phosphor is 75–80%. Thirdly, they show highly efficient narrow-band red emissions (FWHM <2–10 nm) [51–53], and the red emission peak is generally shorter than 650 nm, which improves color purity and visual colorimetric parameters. The color coordinates of these phosphor components are located deep in the 1931 Commission Internationale de l'Eclairage (CIE) diagram [44]. The excitation and emission spectrum of KSF are shown in **Figure 8**.

The photography of KSF phosphor coated on a glass substrate is shown in **Figure 9(a)**. The acquired emission spectra of generated red light with 445-nm

**Figure 8.** *The excitation and emission spectrum of KSF red phosphor [54].*

#### **Figure 9.**

*(a) Photography of KSF phosphor coated on glass. (b) Emission spectra of KSF as functions of driving bias currents.*

excitation at room temperature under bias currents of 280 mA, 320 mA, and 360 mA are shown in **Figure 9(b)**. The emission peak of KSF is at 631 nm.
