Up-conversion materials for solid-state lighting

Conventional phosphor-converted white LEDs (WLEDs), using (Stokes) down-conversion light emission, suffer from a red deficiency that increases the Color Temperature towards “cool white light” above 5000 K. The present work describes a phosphor-Up-converted WLED approach that wishes to overcome this issue by achieving white light generation (WLG) through Up-conversion photoluminescence (UCPL), without energy losses due to Stokes shift and excited by a 980 nm LED instead of the more expensive blue LEDs. The UC materials include thin films of lanthanide (Ln)-doped aluminosilicate glass multilayers, alternating (Ln)-doped aluminosilicate and titania films in the form of 1-D photonic crystals and Ln ion-implanted materials. Sol-gel (SG) processing has been used as a low-cost high purity processing method to prepare titania, as well as aluminosilicate films containing Er³⁺, Tm³⁺ and Yb³⁺, for WLG and possible application as WLED materials. The total Ln content varied from a high of 22 mol% to as low as 5.8 mol%. The materials prepared have been characterized by UV-Vis-NIR (using a variable angle specular reflection accessory), plus FTIR and UCPL spectroscopies. The UC light emission was analyzed with the help of CIE chromaticity diagrams.

Graphical abstract

Left: UC spectra of bulk aluminosilicate glass with 84.2 SiO₂–10 AlO_1.5–0.3 Tm–0.5 Er–5 Yb, excited at different 980 nm laser power levels. Right: UC spectra of a 84 SiO₂–10 AlO_1.5–0.5 Tm–0.5 Er–5 Yb microcavity excited with 5 W of 980 nm laser power, collected at 10° off-normal (solid black line), together with a reference film with the same number of aluminosilicate glass layers and dopant concentrations (dotted red line).