From Dopant to Host: Solution Synthesis and Light‐Emitting Applications of Organic‐Inorganic Lanthanide‐Based Metal Halides

The rich and unique energy level structure arising from 4fⁿ inner shell configuration of trivalent lanthanide ions (Ln³⁺) renders them highly attractive for light‐emitting applications. Currently, research primarily focuses on Ln³⁺ doping in either traditional garnets or the recently developed perovskite phosphors. However, there have been few reports on stable phosphors crystallized with pure lanthanide elements. Herein, a universal solution‐based route to eight Ln³⁺‐based metal halides from the organic‐inorganic A₄LnX₇ family is described, where A⁺ = 4,4‐difluoropiperidinium (DFPD⁺), Ln³⁺ = Nd³⁺, Eu³⁺, Ho³⁺, Sm³⁺, Tm³⁺, Tb³⁺, Yb³⁺, Er³⁺, and X⁻ = Cl⁻, Br⁻. Visible photoluminescence (PL) is observed from Tb³⁺‐, Eu³⁺‐, Ho³⁺‐, and Sm³⁺‐based compounds with Tb and Eu compositions exhibiting high PL quantum yields of 90–100%; Nd³⁺‐, Tm³⁺‐, Yb³⁺‐, and Er³⁺‐based crystals show fascinating near‐infrared emission. Light‐emitting diodes (LEDs) fabricated with (DFPD)₄TbCl₇ yield characteristic emission of Tb³⁺, representing the first demonstration of electroluminescence from these organic‐inorganic Ln³⁺‐based metal halides. Moreover, these materials exhibit distinct excitation wavelength‐dependent emission after alloying with different Ln³⁺ ions, making them interesting for multicolor display and multilevel information encryption applications. It is foreseen that this study will open up the way to a possible design of robust optoelectronic devices based on lanthanide metal halides.