Defect-induced Ce3+ sites preferences and multilevel concentration quenching of a high-efficiency cyan phosphor for high-quality full-visible-spectrum wLED

Currently, the efficient way to synthesize white light-emitting diodes (WLEDs) is combining a near-ultraviolet (n-UV, 380-420 nm) emitting LED chip with tricolor (red, green, and blue) emitting phosphors. However, further improving the color rendering index (CRI) for WLEDs is hindered by the absence of cyan components. Hence, a series of high-efficiency and continuously tunable Ce³⁺,Gd³⁺-doped CaScBO₄ (CSBO) blue-cyan phosphors with an orthorhombic structure were successfully developed by a high-temperature solid-state reaction method. Based on density-functional theory (DFT) calculation, a vacancy was produced along with inequivalent replacement (3Ca²⁺ → 2Ce³⁺/Gd³⁺ + V''_Ca) when just adding the trivalent cations, meanwhile causing the local environment of the lattice to relax so Ce³⁺/Gd³⁺ ions find it easier to enter into Sc³⁺ sites at a higher doping concentration. Under the excitation of n-UV, the emission peak position moves from 443 nm to 480 nm and two concentration quenching points appear with an increase in Ce³⁺ ions by defect-induced site-selective occupation. The two samples at concentration quenching points both have high quantum efficiencies of 88.6% and 86.2% and an acceptable thermal quenching performance. The property performance and internal mechanism are illuminated by the excitation and emission spectra and theoretical analysis. Finally, by combining CSBO:Ce³⁺, commercial green and red phosphors and an n-UV LED chip, an as-fabricated WLED with a great CRI value of 93.2 and a low CCT (4291K) was obtained. This work demonstrates the potential of CSBO:Ce³⁺ as a blue-cyan phosphor for use in high-quality full-visible-spectrum WLEDs in the future. The investigation of the mechanism for the defect-induced site preferences provides a reference for developing new photoluminescent materials.

Mechanism of upconversion luminescence enhancement in Yb3+/Er3+ co-doped Y2O3 through Li+ incorporation

Li⁺ doping is a well-known, simple, yet efficient strategy to optimize the properties of upconverting materials. Nonetheless, the position of Li⁺ in the lattice and the mechanism of upconversion enhancement are still controversial, especially in Yb³⁺/Er³⁺ co-doped Y₂O₃. This paper presents a comprehensive investigation of the above issues (i.e. the position occupied by Li⁺ in the lattice and the mechanism of luminescence enhancement, in terms of decreased defects) by studying (Y_0.78-XYb_0.20Er_0.02Li_X)₂O₃ powders. Neutron powder diffraction was employed for the first time in the literature to show that Li⁺ ions are accommodated in Y sites of YO₆ octahedra, confirmed also by the content of oxygen defects, which was increased with the increase of Li⁺ concentration. FT-IR showed that there was a small change in the amount and the type of the surface-absorbed groups with the increase in the Li⁺ content, thus not supporting the prevailing conclusion that the quenching groups are decreased by doping Li⁺. Positron annihilation lifetime (PLAS) experiments showed that the total defect concentration and the large defect clusters, which are considered as quenching centers, are decreased with increasing Li⁺-content, resulting in the enhancement of the emission intensity in Yb³⁺/Er³⁺ co-doped Y₂O₃.