Protonated organic cations enhance the luminescence performance and thermal stability of fluoride red phosphor K2TiF6:Mn4+

The application of red luminescent phosphors composed of Mn4+-activated fluoride in WLEDs has aroused great interest. However, most of the fluoride phosphors doped with Mn4+ have poor luminescent thermostability. In order to improve their luminescent thermostability, a new type of organic-inorganic hybrid red luminescent phosphor K2TiF6:PIPH22+,Mn4+ (PIPH22+ = protonated piperazine) was synthesized by a simple cation exchange method in this work. Protonated piperazine was incorporated into K2TiF6 lattice, and the strong hydrogen bond between H- and F- enabled the organic group to exist stably in the inorganic lattice. Compared with K2TiF6:Mn4+, the addition of PIPH22+ increases distance between adjacent [TiF6]2- and inhibits the energy migration of Mn4+ ions between [MnF6]2- to reduce nonradiative transitions. Therefore, the hybrid sample K2TiF6:PIPH22+,Mn4+ has higher emission intensity and quantum yield. Under the excitation of 467 nm blue light, K2TiF6:PIPH22+, Mn4+ exhibits sharp red emission near 631 nm, and the luminescence intensity is 2.14 times that of K2TiF6:Mn4+. K2TiF6:PIPH22+,Mn4+ has significant negative thermal quenching and higher luminescence thermostability, which can be attributed to electron-phonon interaction mechanism at high temperature. The prototype WLED prepared with YAG:Ce3+, K2TiF6:PIPH22+,Mn4+ and InGaN blue LED chip can emit warm white light (CCT = 2795 K, CRI = 91.1) under 20 mA current, and the luminescence efficiency was 105.8 Lm/W. The prototype WLED was tested under 20–120 mA current, and the results showed that the changes of CCT and CRI were stable. These results indicate the potential of K2TiF6:PIPH22+,Mn4+ in WLED applications.