Thulium‐Doped Sodium‐Ytterbium Double Perovskite Single Crystals: A Novel and Efficient Upconverter for Achieving Highly Sensitive Narrowband Near‐Infrared Perovskite Photodetectors

Abstract Developing fluorescent materials that can achieve efficient upconversion (UC) emission under low power density excitation has always been a great challenge, especially for Tm 3+ ions. Here, Tm 3+ ‐doped Cs 2 NaYbCl 6 double perovskite single‐crystals (DPSCs) are prepared successfully. The DPSCs exhibit efficient UC emission under 980‐nm laser excitation, and the optimum UC efficiency reaches 9.48%. It is a recorded value. More importantly, its efficient UC emission can be achieved under low‐power density of 0.117 W cm −2 excitation. Moreover, the luminescence color of the DPSCs can be regulated from blue to red by increasing the doping concentration of Tm 3+ ions. This is because the severe cross‐relaxation occurs between Tm 3+ ions at high doping concentrations. The Cs 2 NaYb 0.98 Tm 0.02 Cl 6 DPSCs with efficient UC emission are coupled to the CsPbI 3 perovskite photodetector, achieving good visible and near‐infrared (NIR) detection without an external bias. It is worth emphasizing that the detectivity for 980 nm light can reach 1.26 × 10 11 Jones, which is the highest value for self‐powered NIR photodetectors. Finally, given the dual functional characteristics of the device's response to broadband visible and narrowband NIR light, the dual encryption function is implemented during information transmission. This work will open new chances for applications in advanced integrated photonics devices.