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 Cs₂NaInCl₆:Yb³⁺,Er³⁺, 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 Cs₂AgInCl₆, Cs₂NaBiCl₆, and Cs₂AgBiCl₆. 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.