Multiwavelength Excitation in Ho³⁺-Doped All-Inorganic Double Perovskites Achieved by Codoping Mn²⁺ for Warm-White LEDs and Plant Growth

Doping lanthanide ions is an efficient method to modify the optical properties of lead-free double-perovskite halides. However, most lanthanide-doped double perovskites show a low luminescence efficiency and require a high excitation energy. Here, we have successfully prepared a series of Ho³⁺-doped Cs₂NaBiCl₆ microcrystals through a simple hydrothermal method and obtained strong characteristic emissions of Ho³⁺ at 492 and 657 nm under low-energy excitation (449 nm). After codoping Mn²⁺, apart from the characteristic emissions from Ho³⁺ under 450 nm wavelength excitation, the orangish-red luminescence consisting of the emission band centered at 591 nm from Mn²⁺ and a sharp emission peak at 657 nm from Ho³⁺ is obtained under 355 nm UV light excitation. Photoluminescence (PL) emission and excitation spectra, along with the PL decay curves, confirm the existence of an energy-transfer channel from Cs₂NaBiCl₆ to Mn²⁺ and then from Mn²⁺ to Ho³⁺. The enhanced absorption efficiency (10.5 → 70.7%) suggests that the codoping of Mn²⁺ overcomes the low absorption efficiency caused by f-f forbidden transitions of Ho³⁺. Finally, the diverse luminescent performance within the Cs₂NaBiCl₆:Ho³⁺, Mn²⁺ phosphor is realized by altering the excitation wavelength, thereby enabling its application in warm-white-light-emitting diodes and plant growth in this work.