Broad Dual Emission by Codoping Cr³⁺ (d→d) and Bi³⁺ (s→p) in Cs₂ Ag_0.6 Na_0.4 InCl₆ Double Perovskite

A phosphor emitting both white light and broad near-infrared (NIR) radiation can simultaneously provide visual inspection and early signs of rotting of food products. The broad NIR emission is absorbed by the vibrational overtones of water molecules present in food items, providing the non-invasive image contrast to assess the food freshness. Here we design a phosphor, namely, Cr³⁺ -Bi³⁺ -codoped Cs₂ Ag_0.6 Na_0.4 InCl₆ , that simultaneously emit warm white light and broad NIR (1000 nm) radiation with quantum yield 27 %. This dual emitter is designed by combining the features of s² -electron (Bi³⁺ ) and d³ -electron (Cr³⁺ ) doping in a weak crystal field of the halide perovskite host. $$ excitation of Bi³⁺ , using a commercial 370 nm ultraviolet light-emitting-diodes (UV-LED), yields both the emissions. A fraction of the excited Bi³⁺ dopants emit the warm white light, and the other fraction transfers its energy non-radiatively to Cr³⁺ . Then the Cr³⁺ de-excites emitting broad NIR emission. Temperature dependent (6.4-300 K) photoluminescence in combination with Tanabe-Sugano diagram show that the Cr³⁺ experiences a weak crystal field ( $$ =2.2), yielding the $$ NIR emission. As a proof of concept, we fabricated a panel containing 122 phosphor-converted LEDs, demonstrating its capability to inspect food products.