Persistent luminescence instead of phosphorescence: History, mechanism, and perspective

Great progress has been made in inorganic persistent phosphors, especially in the recent two decades, motivated by the discoveries of the SrAl2O4:Eu2+-Dy3+ in the green and Cr3+ doped spinel compounds in the deep-red to near-infrared (NIR) spectral regions. However, the physical mechanism behind this kind of “self-sustained” luminescence is still the subject of debate, and the improvement of known persistent phosphors and/or the development of new ones are still a matter of trial-and-error. In this review, starting from the introduction of longstanding histories of persistent luminescence (PersL), we provide comprehensive insights into its physical mechanism. Particular focus is put on the state-of-the-art of designing new persistent phosphors via “bandgap engineering”, based on the knowledge about trapping-detrapping mechanisms of charge carriers and energy level locations of emitting/trapping centers. Recent significant works on PersL observed in organic molecules and phosphorescence observed in inorganic phosphors are also highlighted in order to give a clear distinction between these two long-lived luminescence phenomena. Key challenges, feasible improvements and perspectives of PersL working in the ultraviolent (UV), white, red and NIR (over 1000 nm) regions together with new charging concepts by NIR or visible-light lasers are also presented. It is hoped that this review could give new inspiration for the future development of PersL in emerging applications.