Simultaneous Improvement and Tailoring Upconversion and Downshift Luminescence of Cs₂NaErCl₆ via Yb³⁺ Alloying for Versatile Photoelectric Applications

Lead-free double perovskite (DP) materials have garnered growing interest because of their outstanding optoelectronic attributes. Nevertheless, realizing efficient, multimodal photoluminescence (PL) with adjustable emission spectra within single-host DP materials still poses a formidable hurdle. Herein, Er-based lead-free DPs (Cs₂NaErCl₆) were developed, which achieves downshift (DS) emissions from visible to near-infrared (NIR) and multicolor upconversion (UC) emissions, resulting from the abundant energy levels of Er³⁺ ions. Alloying with Yb³⁺ introduces new Yb³⁺-Er³⁺ cross-relaxation (CR) pathways, leading to a 308.5-fold increase in DS red emission and achieving a maximum color purity of 98.63%. Additionally, Yb³⁺ alloying enhances UC luminescence by 10.8-fold due to its strong absorption cross section around 980 nm. The colors of the UC emission can be adjusted by altering the doping concentration of Yb³⁺ and laser power density. Spectral analysis and theoretical calculations reveal the high compatibility of Yb³⁺ ions with Cs₂NaErCl₆, and the critical role of CR and energy-transfer processes in achieving efficient, tunable, and multimodal PL. Furthermore, the developed DP materials have been utilized in anticounterfeiting, information encoding, and narrow-band NIR photodetectors, expanding the versatile uses of rare-earth (RE)-based DP materials in the optoelectronics field.