“Phosphorescent materials store energy from absorbed sunlight by trapping excited electrons in lattice defects, and they emit light as the electrons slowly escape and return to lower-energy states.”

“The gradual release of excited electrons can be classified as a form of energy storage. This particular phenomenon involves the presence of energy traps, such as electron or hole traps, within a material, which are occupied during the excitation process.” “Once the external stimulus is removed, the trapped charges are gradually released, returning to the ground state and emitting stored energy in the form of luminescence.”

For diamond, these point defects include missing carbon atoms (vacancies), displaced atoms (interstitials), and impurity complexes based on elements including nitrogen, boron, nickel, silicon, and hydrogen. Their presence disturbs the host lattice, introducing additional energy levels within the band gap. These defects open up the possibility that incident light can be absorbed and subsequently emitted if the photon energy lies within a defect’s absorption band.

“Samples treated between 200 and 400 °C exhibit remarkable phosphorescence in the visible range… The afterglow results from a trapping and detrapping process, which delays the recombination at the active optical centers.” “Our findings suggest that the characteristic blue phosphorescence of treated boric acid is correlated to trapped charged carriers, whose shallow levels are estimated to be ≈0.57 eV beneath the conduction band… Two traps are located at 0.57 and 0.70 eV from the bottom of the conduction band. Those levels can be filled up by light excitation and eventually emptied by thermal energy.”

The review states: “Persistent luminescence in many inorganic phosphors is generally explained by the trapping of charge carriers (electrons and/or holes) in defect levels situated in the band gap of the host lattice.” “Upon thermal or optical stimulation at ambient conditions, the trapped carriers are gradually released and recombine at luminescent centres, giving rise to long-lasting afterglow emission long after the excitation source has been removed.”

A portion of electrons can be effectively captured by energy traps, which are primarily lattice intrinsic defects. The trapped charges are then thermally or optically released, producing persistent luminescence after the excitation source is removed.

The phosphors investigated here exhibit strong long-lasting phosphorescence after exposure to fluorescent or sunlight. The mechanism is attributed to the trapping of charge carriers at crystal imperfections or co-dopant levels introduced into the host lattice. Gradual thermal release of these carriers and their subsequent recombination with luminescent centers accounts for the observed afterglow emission.

Discussing SrAl2O4:Eu2+,Dy3+ phosphors, the paper notes: “The long persistent phosphorescence is commonly explained by the trapping of electrons in lattice defects created by co-dopants and intrinsic imperfections.” “It is suggested that irradiation excites electrons into the conduction band, from where they are captured by traps; gradual thermal release of the electrons followed by recombination at Eu2+ centers accounts for the observed afterglow.”

“In long afterglow phosphors, the excitation energy from incident light is stored in traps formed by defects or impurities in the crystal lattice.” “After the removal of the excitation source, the trapped charge carriers are gradually released and recombine with centers of opposite charge, emitting photons during this process. The slow emptying of traps leads to persistent luminescence over long times.”

“In fluorescence, the emission is basically immediate… while phosphorescent material can store the absorbed light energy for some time and release light later, resulting in an afterglow that persists after the light has been switched off… Depending on the material, this afterglow can last anywhere from a few seconds to hours.” “Triplet excited states are characterized by parallel spin of both electrons and are metastable… The return to relaxed singlet ground state (S0) might occur after considerable delay (10−3 to >100 sec). Additionally, more energy is dissipated by non-radiative processes during phosphorescent relaxation than in fluorescence.”

“Phosphorescence is a type of photoluminescence where light is emitted by a substance without combustion or noticeable heat… This occurs when molecules absorb photons, transition to an excited state, and then slowly return to the ground state, emitting light over an extended period. Unlike fluorescence, which emits light in nanoseconds, phosphorescence features longer lifetimes ranging from microseconds to hours.” “Defect Centers: Defect centers in phosphorescent materials can trap and later release excited carriers, contributing to light emission. Carefully controlling these defects can enhance both the brightness and efficiency of the phosphorescent material.”

“Unlike fluorescence, which ceases immediately when the lighting source is switched off, phosphorescence lasts for a more or less variable period… Phosphorescence is similar to fluorescence, but in this case the emission of brightness continues for some time, even after the lighting has been switched off.” “The excited electrons remain in high energy states for some time before returning to their ground states and emitting light… Depending on the nature of the phosphorescent object, the relaxation process can last from a few minutes to several hours. This delay is due to the more complex electronic transitions in the molecule… This is why phosphorescent objects can glow in the dark and emit light for some time after being exposed to light.”

Describing phosphorescence in solids, the article notes that after excitation “these free electrons and holes can migrate through the solid until they become trapped at defect sites in the crystal lattice. Over time, the electrons and holes are released from their traps and recombine, emitting photons.” It further explains that these trapping processes lead to delayed emission: “The presence of traps in the forbidden gap is responsible for the afterglow observed in many phosphorescent materials.”

He told me that glow-in-the-dark things use a type of phosphorescence. That means they absorb energy—like light—and then glow. The tiny flaws in a phosphor—like missing atoms—make little nooks or holes. When electrons zoom up to higher orbitals, they can get trapped in those holes. “Sooner or later—sometimes minutes, sometimes hours or even days—the trapped electrons come back down,” Eilers said. “That’s what you’re seeing in the dark.”

In many commercial afterglow phosphors (such as SrAl2O4:Eu,Dy and ZnS:Cu-based materials), ultraviolet and high-energy visible components of sunlight are sufficient to excite electrons into the conduction band or higher-energy states. A portion of these carriers are captured by defect-related traps and later thermally released, producing visible afterglow. In practice, these materials ‘charge’ under sunlight or artificial light and then emit a slowly decaying phosphorescent glow as the trapped electrons return to lower-energy states.

In phosphorescent materials the excited electrons once they get pumped up to a higher energy level, they get sort of trapped in that higher energy level for a little while before gradually decaying and relaxing down to their ground state. This trapping effect is what allows the phosphorescent materials to actually continue to glow even after you turn the light source off. Now it's often mixed with copper to create that characteristic green glow… the copper atoms are what actually create the trapping states that hold the electrons in the excited state for a longer period of time.

In the explanation aimed at beginners, the presenter contrasts fluorescence and phosphorescence: “Fluorescence means emission of light… when you switch off the UV light, the visible light will switch off automatically; this is called fluorescence… while if it remains for some time, this is called phosphorescence.” He attributes the longer wavelength and delay in phosphorescence to additional relaxation steps: “In phosphorescence there are so many phenomena… the electron lost most of energy because it has done a vibrational relaxation, internal conversion and additional intersystem crossing, so [the] electron loses a lot of energy and finally it has very long wavelength.”

In phosphorescence, the excited electrons become trapped at higher energy states due to certain defects or impurities in the material's crystal structure. The return of the electrons to their ground state occurs through a slower process, and this transition is usually accompanied by the emission of light.