In 1996, Matsuzawa et al. reported on the extremely long-lasting afterglow of SrAl₂O₄:Eu²⁺ codoped with Dy³⁺ ions, which was more than 10-times brighter than the previously widely used ZnS:Cu,Co. Since then, research for stable and efficient persistent phosphors has continuously gained popularity. However, even today - almost 15 years after the discovery of SrAl₂O₄:Eu²⁺, Dy³⁺ - the number of persistent luminescent materials is still relatively low. Furthermore, the mechanism behind this phenomenon is still unclear. Although most authors agree on the general features, such as the existence of long-lived trap levels, many details are still shrouded in mystery. In this review, we present an overview of the important classes of known persistent luminescent materials based on Eu²⁺-emission and how they were prepared, and we take a closer look at the models and mechanisms that have been suggested to explain bright afterglow in various compounds.

During the past few decades, the research on persistent luminescent materials has focused mainly on Eu²⁺-doped compounds. However, the yearly number of publications on non-Eu²⁺-based materials has also increased steadily. By now, the number of known persistent phosphors has increased to over 200, of which over 80% are not based on Eu²⁺, but rather, on intrinsic host defects, transition metals (manganese, chromium, copper, etc.) or trivalent rare earths (cerium, terbium, dysprosium, etc.). In this review, we present an overview of these non-Eu²⁺-based persistent luminescent materials and their afterglow properties. We also take a closer look at some remaining challenges, such as the excitability with visible light and the possibility of energy transfer between multiple luminescent centers. Finally, we summarize the necessary elements for a complete description of a persistent luminescent material, in order to allow a more objective comparison of these phosphors.

Persistent luminescent materials are able to emit light for hours after being excited. The majority of persistent phosphors emit in the blue or green region of the visible spectrum. Orange- or red-emitting phosphors, strongly desired for emergency signage and medical imaging, are scarce. We prepared the nitrido-silicates Ca₂Si₅N₈:Eu (orange), Sr₂Si₅N₈:Eu (reddish), Ba₂Si₅N₈:Eu (yellowish orange), and their rare-earth codoped variants (R = Nd, Dy, Sm, Tm) through a solid state reaction, and investigated their luminescence and afterglow properties. In this paper, we describe how the persistent luminescence is affected by the type of codopant and the choice and ratio of the starting products. All the materials exhibit some form of persistent luminescence, but for Sr₂Si₅N₈:Eu,R this is very weak. In Ba₂Si₅N₈:Eu the afterglow remains visible for about 400 s, and Ca₂Si₅N₈:Eu,Tm shows the brightest and longest afterglow, lasting about 2,500 s. For optimal persistent luminescence, the dopant and codopant should be added in their fluoride form, in concentrations below 1 mol%. A Ca₃N₂ deficiency of about 5% triples the afterglow intensity. Our results show that Ba₂Si₅N₈:Eu(,R) and Ca₂Si₅N₈:Eu(,R) are promising persistent phosphors for applications requiring orange or red light.