K_2SiF_6:Mn^4+ as a red phosphor for displays and warm-white LEDs: a review of properties and perspectives

Abstract

Phosphor-converted white light-emitting diodes (pc-WLEDs) are energy efficient and environmentally friendly light sources with a long lifetime, applicable in both display backlights and general lighting. Adding red-emitting phosphors improves the color quality of white LEDs compared to the prototype combination of a blue LED and the yellow Y₃Al₅O₁₂:Ce³⁺. Efficient narrow-band red-emitting phosphors like K₂SiF₆:Mn⁴⁺ can meet the market’s needs. This review recapitulates research since 2008 on K₂SiF₆:Mn⁴⁺ as the first and most discussed fluoride phosphor. The limited nephelauxetic effect, typical for fluorides, allows for the tuning of the Mn⁴⁺ emission in the red part of the spectrum below 650 nm. This is reflected in the spectroscopic parameters of the crystal field theory. Synthesis methods are described, showing the evolution from etching Si wafers to solution synthesis resulting in consistent luminescent and thermal properties. Though important for applications, long-term stability is often neglected, although (in)organic coatings improving stability emerge. This leads not only to warm-white LEDs with high efficacies and good color rendering, but also to efficient displays with a large color gamut.