Ultrahigh Quantum Efficiency Near-Infrared-II Emission Achieved by Cr³⁺ Clusters to Ni²⁺ Energy Transfer

Increasing demand for near-infrared-II (NIR-II) light sources requires improved NIR-II phosphors. We present a series of phosphors codoped with Cr3+ and Ni2+ that possess NIR-II emission with an unprecedented internal quantum efficiency (IQE) of 97.4%. Our study reveals an energy transfer mechanism involving clusters of Cr3+ where luminescent centers are closely matched in energy and where the Ni2+ emission intensity can be tuned through sintering temperature. The profound electron paramagnetic resonance (EPR) studies disclose the interaction between Cr3+ clusters and Cr3+-Ni2+ pairs, further proving the cause of such a high IQE and the significance of Cr3+ clusters. This work provides promising pathways for the development of NIR-II light-emitting diodes with outstanding efficiency by suggesting a new energy transfer source of Cr3+