Recent progress in quantum cutting phosphors

Luminescent materials with the quantum efficiency (QE) higher than unity could be playing a significant role in the progress of lighting industry and certain electronic display systems. The recent demonstration of an efficient visible quantum cutting (QC) in vacuum ultraviolet (VUV)-excited LiGdF₄:Eu phosphors Wegh RT, Donker H, Oskam KD, Meijerink A. Visible quantum cutting in LiGdF₄:Eu³⁺ through downconversion. Science 1999; 283: 663-6] has provided an exciting and interesting trends in the development of several potentially important luminescent materials and devices. The possibility of the higher QE depends on the principle of QC in phosphors which could generate two or more low-energy photons for every incident high-energy photon that is being absorbed by phosphors. Investigation on QC systems has started on single ions doped-fluorides capable of a cascade emission from ions such as Pr³⁺, Tm³⁺, Er³⁺ and Gd³⁺. The focus has now been shifted to the combination of two ions, where the energy of the donor ion could be transferred stepwise to two acceptor ions via a downconversion. A well-known example is the Gd³⁺-Eu³⁺ dual ions. QC via downconversion has now been widely witnessed in many rare earths (RE)-based phosphors, the interesting and appreciable QE in the visible spectral region has earlier been reported from LiGdF₄:Eu (190%) and BaF₂:Gd,Eu (194%) phosphors. QC materials could also be used in solar cells, if conversion of one UV-visible photon into two near-infrared (NIR) photons is realized, and energy loss due to thermalization of electron-hole pairs is minimized. The present article reviews on the recent progress made on: (a) materials and developments in the fields of UV-visible QC phosphors and the mechanism involved, including QC in single RE ion activated fluorides- and oxides-based phosphors, energy transfer and downconversion, QC in dual/ternary ions activated phosphors; and (b) NIR QC in RE³⁺-Yb³⁺ (RE = Tb, Tm, and Pr) dual ions doped phosphors via cooperative energy transfer. Appropriate discussions have been made on materials, materials synthesis and characterization, the structural and luminescence properties of various QC luminescent materials via different synthesis techniques. In addition, applications, challenge and future advances of the visible- and NIR-QC phosphors have also been dealt with.