Green Color Purity Control of Dual-Excitation Upconversion Display by Using Polymer/NaYF₄:Er³⁺ Crystal Transparent Composite

Abstract

Invisible near-infrared (NIR) to visible (VIS) upconversion (UC) emission of rare-earth doped ceramics is receiving much attention for transparent display applications. In this study, a new dual-wavelength (DW) NIR excitation scheme for the VIS-UC emission of Er³⁺ doped NaYF₄ (NaYF₄:Er³⁺) was proposed to achieve a high green color purity and was applied to non-projection-type transparent displays. For the optimization of green color purity, various rare-earth ions (RE³⁺) were co-doped into NaYF₄:Er³⁺ (NaYF₄:Er³⁺, RE³⁺). Among those, NaYF₄ co-doped with Er³⁺ and Ce³⁺ (NaYF₄:Er³⁺, Ce³⁺) showed the highest green color purity under DW-NIR excitation. Furthermore, the excitation wavelengths of DW-UC were also investigated. Moreover, a NaYF₄:Er³⁺, Ce³⁺ embedded transparent and flexible polymer composite was prepared, which emitted a strong "point emission" at the cross point of the DW-NIR excitation lights that were introduced from the side surface of the composite sheet. Therefore, the obtained polymer/NaYF₄:Er³⁺, Ce³⁺ composite is an attractive candidate material for non-projection-type transparent and flexible displays.