NIR‐II Luminescence With A Recorded 76% EQE Through Energy Extraction From Dark Cr3+

Abstract NIR‐II luminescent materials are the key enablers of smart spectroscopy‐based techniques, offering capabilities for nondestructive analysis and bio‐imaging. Here, a broadband NIR‐II luminescence with a recorded external quantum efficiency (EQE) of 76% in LiGa 5 O 8 are demonstrated, using heavily doping Cr 3+ ion (10%) as a light harvester and a controllable energy transfer (ET) from Cr 3+ to Ni 2+ (0.4%) ions. Even though the Cr 3+ ion produces substantially weak luminescence due to concentration quenching, the introduction of Ni 2+ ions effectively extract the excitation energy to generate NIR‐II luminescence. Complementary investigations using synchrotron‐based EXAFS fittings and structural refinement disclose a significant structural distortion in the LiGa 5 O 8 compound, which facilitates the relaxation of the parity‐selection rules for Cr 3+ and Ni 2+ ions. Additionally, DFT calculations identify specific site occupations, which favor unidirectional ET from Cr 3+ to Ni 2+ ions. As a result, the significant absorption of excitation light by heavy‐doping Cr 3+ ions and high radiative transition probability in Ni 2+ ions synergistically result in a record high EQE. These findings provide pioneering insight into rational NIR‐II light generation by deliberated control of ET pathway in heavily doped systems, thereby with promising implications for NIR spectroscopy applications.