Highly Luminescent Rare Earth Metal Halide Solutions Used as Inks for Blue Light‐Emitting Diodes

ABSTRACT Rare earth‐based luminescent materials are renowned for their unique optical properties, including sharp emission spectra, long‐lived excited states, and high quantum efficiency, making them highly desirable for light‐emitting applications. However, these materials typically exhibit characteristic emissions from rare earth elements, making it uncommon for them to produce blue emissions in the 400–500 nm range. Here, we report ytterbium (Yb 3+ )‐ and erbium (Er 3+ )‐based organic–inorganic amorphous metal halides, BDPA‐M‐Cl and DMP‐M‐Cl (BDPA = benzyldimethylphenylammonium, DMP = 2,6‐dimethylpiperazine, M = Yb, Er), which demonstrate unusual blue emission with a decent photoluminescence quantum yield (PLQY) of ≈63%. We attribute the unique blue emission to efficient exciton radiative recombination from Yb 3+ /Er 3+ to Cl − , which suppresses characteristic emissions of these rare earth ions. Moreover, when these materials are dissolved in N,N‐dimethylformamide or dimethyl sulfoxide, the resulting solution still exhibits bright blue emission with an even higher PLQY of ≈96%. Electrically driven LEDs fabricated with BDPA‐Yb‐Cl thin films demonstrate efficient blue emission, with an external quantum efficiency of 0.29%, comparable to that of state‐of‐the‐art, Pb‐free perovskite blue LEDs. Our study represents a significant departure from traditional rare earth emission mechanisms, broadening the scope of rare earth luminescence and laying a foundation for their use in emerging applications.