Chromium Ion Pairs Induce Highly Efficient and Thermostable Broadband NIR Phosphor for Plant Lighting and Night Vision

Near-infrared (NIR) light has a high application value in various fields due to its spectral characteristics. Typically, the broadband NIR emission of Cr³⁺ is based on the transition of ⁴T₂→⁴A₂, while the ²E level emits narrowband emission. This work discovered broadband NIR emission from the [⁴A₂, ⁴T₂] coupling excited state energy level in the spinel structure. The dense and adjacent Al1O₆ as the occupied sites of Cr³⁺ provides a structural basis for the exchange coupling effect under high Cr³⁺ concentration, as confirmed by the analysis of site occupancy, formation energy, bond valence sum, and effective coordination number. Spectral information and EPR signals indicate that the [⁴A₂, ⁴T₂] coupling excited state in the emission spectrum corresponds to the downward shift of the T₂ energy level caused by the Cr³⁺-Cr³⁺ exchange coupling interaction in the strong crystal field of LAO:Cr³⁺. The internal and external quantum efficiencies of LAO:0.15Cr³⁺ are 80.6% and 30.3%, respectively. The luminescence intensity at 370 and 423 K is approximately 92% and 85% respectively of that at room temperature. The excellent luminescence thermal stability is attributed to the activation energy of 0.21 eV and the Huang-Rhys factor S = 3.95. Finally, the material was fabricated into pc-LEDs and its applications in night vision and plant lighting were explored.