Near‐Unity Quantum Yield and Thermally Stable NIR–II Emission in Ce ³⁺ /Er ³⁺ Co‐Doped Cs ₂ NaYCl ₆ Double Perovskites via Synergistic Energy Manipulation

The pursuit of photoluminescence quantum yield (PLQY) in the near‐infrared‐II (NIR–II) emission of Er 3+ has been a profoundly challenging goal, primarily due to the inherent inefficiency of Er 3+ emitters caused by parity‐forbidden transitions. In this work, we overcome this fundamental limitation with a Ce 3+ /Er 3+ co‐doped Cs 2 NaYCl 6 double perovskites that simultaneously achieve an absorption efficiency of 63% and an unprecedented NIR PLQY of 96%. We attribute this performance to a dual strategy. Here, Ce 3+ ions efficiently harvest excitation energy via their strongly allowed 4f–5d transitions. The harvested energy is then transferred to Er 3+ through a highly effective Förster resonance energy transfer (FRET) process. Concurrently, a network of cross‐relaxation (CR) processes between Er 3+ –Er 3+ and Ce 3+ –Er 3+ works synergistically to multiply the photon output and populate the 4 I 13/2 emitting state. Additionally, the material exhibits outstanding thermal stability, retaining over 80% of its NIR intensity at 410 K. Due to the excellent properties of Ce 3+ /Er 3+ co‐doped Cs 2 NaYCl 6 , we demonstrate its application potential in NIR–II bioimaging, high‐sensitivity optical thermometry, and X‐ray activated persistent luminescence. This work not only presents an outstanding material candidate for NIR optical application, but also establishes a promising strategy for improving the performance of future rare‐earth‐based optical materials.