Effect of Defect Energy Level on Improving Cr³⁺-Based Near-Infrared Persistent Luminescence by Codoped Lanthanide Ions

Enhancing the afterglow performance is vital for the near-infrared long persistent luminescence (NIR LPL) material field. Herein, a series of Cr3+–Ln3+ codoped near-infrared long persistent luminescence phosphors Mg2.85Li0.3Ga1.99–xLnxGeO8:0.01Cr3+ (Ln = Pr, Eu, Dy, Tm, Yb, Sm) are studied to optimize their performance. Interestingly, excellent afterglow properties can be obtained even if the lanthanide ions located outside the band gap are codoped. Also, this afterglow duration of Mg2.85Li0.3Ga1.99–xGeO8:0.01Cr3+,xEu3+/Pr3+ can be more than 24 h. We analyze the position of the lanthanide ions relative to band gap based on the establishment of the band gap energy level model. Thermoluminescence (TL) experiments demonstrate that the distribution of traps is different after doping different lanthanide ions. This is consistent with the results of electron spin resonance (ESR) spectroscopy. Our work proves that the shallow defect density has been greatly improved resulting from the strong distortion of the unit cell via doping lanthanide ions located outside the band gap and that the abundant 4f levels of the lanthanide ions located inside the band gap can complement the deeper defects in the material.