Yb³⁺ sensitized Mo³⁺ spin-flip luminescence through downconversion and upconversion in halide perovskites

Mo³⁺-activated halide perovskites can exhibit narrowband spin-flip luminescence through downconversion and upconversion. However, because Mo³⁺ absorbs weakly in the near-infrared (NIR) region, NIR excitation generally produces only weak room-temperature Mo³⁺ downconversion emission and Mo³⁺ upconversion emission only at low temperatures. Here, we demonstrate that Yb³⁺ effectively sensitizes Mo³⁺ in Cs₂NaInCl₆ double perovskites. Under 920-nm excitation, Yb³⁺ → Mo³⁺ energy transfer markedly enhances the 1095-nm Mo³⁺ spin-flip emission. This sensitization also enables, to the best of our knowledge, the first room-temperature observations of both 1850-nm NIR emission arising from the ²T_2g → ²E_g/²T_1g inter-excited-state transition and 700-nm Mo³⁺ upconversion emission. Our work establishes Yb³⁺ sensitization as an effective strategy for accessing NIR-excited, room-temperature Mo³⁺-centered emission bands in halide perovskites.