Structural Engineering of Eu2+-Doped Silicates Phosphors for LED Applications

Phosphor-converted light-emitting diodes (pc-LEDs) are of great importance for their applications in solid-state lighting, backlit display, and near-infrared detection light source. Herein, the main challenges for these emergent pc-LEDs are to achieve full-spectrum lighting, wide color gamut display and broadband high efficiency near-infrared emission, respectively, which depends on the luminescence properties of phosphors used. Owing to the unique 4f-5d transition, Eu2+ is one of the most commonly used activators in luminescent materials for pc-LEDs, and Eu2+-doped earth-abundant silicates phosphors exhibit outstanding luminescence properties, including multicolor emission, adjustable bandwidth, excellent thermal stability as well as high luminescence efficiency. These attributes motivate scientists to find Eu2+-doped silicates phosphors that can practically meet the various LED application requirements. Since the traditional trial and error exploration is time-consuming and not necessarily successful, it is necessary to find reliable structural engineering strategies to discover new phosphor systems and also realize purposeful photoluminescence tuning. The adjustable 4f-5d electronic transitions of Eu2+, the variable crystal structures of the silicate hosts and their coupling effect simultaneously account for the targeted luminescence behaviors and their precise emission color tuning. Thus, we aim at developing Eu2+-doped silicate phosphors that can solve the application challenges through a comprehensive understanding of Eu2+ photoluminescence mechanism and the structure-property relationships. In this Account, we first illustrate the luminescence theory of Eu2+ in inorganic solids and summarize the research results of the effect originated from centroid shift, crystal field splitting, Stokes shift, and emission bandwidth. On the basis of the factors dominating the variation of luminescence characteristics, several structural strategies to manipulate Eu2+ emission in silicates are proposed, including (1) modify the chemical composition and crystal structure by various substitutions, (2) choose or change a suitable crystallographic site for Eu2+ and (3) control crystalline phase transition by external factors. Meanwhile, we briefly introduce the photoluminescence behaviors of Eu2+ in different silicates controlled by these structural engineering strategies. Second, we outline our recent research progress on blue LED pumped Eu2+-doped silicate phosphors with emphasis on the design principle and the relationship between the structure and luminescence. The state-of-the-art LED application including full spectrum solid-state lighting, wide color gamut display and near-infrared night-vision technologies are introduced. Finally, we proposed the future research opportunities and challenges. The development of these Eu2+-doped silicate phosphors exhibiting excellent luminescence performance is highly inspiring, and we expect this Account can be helpful for controlling the photoluminescence by theory-structure-property relationships and guide scientists discover the next generation of Eu2+-doped phosphors for emerging applications.