Pure Green Upconversion from a Multicolor Downshifting Perovskite Crystal

Abstract Lanthanide‐doped halide perovskite has emerged as a promising phosphor for multimode luminescence. Here, a chloride‐based perovskite single crystal is reported, namely Cs 2 NaInCl 6 :Yb 3+ ,Er 3+ , that is grown in a hydrothermal reactor. The as‐grown crystals feature two distinct luminescence behaviors under UV and 980 nm laser excitation, respectively. Under UV lamp excitation, the crystal shows a multicolor emission of sharp bands at 448, 550, and 660 nm, covering a wide color gamut in CIE diagram. In stark contrast, the crystal under 980 nm excitation exhibits a pure green upconversion with a green‐to‐red ratio up to 211. The universal green dominance is further demonstrated in many chlorides, including Cs 2 AgInCl 6 , Cs 2 NaBiCl 6 , and Cs 2 AgBiCl 6 . The strong green upconversion is used for optical thermometry, which demonstrates an unprecedented sensitivity up to 9.3% K −1 at the low temperature end. The work introduces a kind of new hosts for single‐band upconversion luminescence, opening up many possibilities in areas such as low‐threshold laser and pure‐green color converter.