Phosphorescence:

The energy levels for the activators and co-activators are respectively known as hole and electron traps, which are similar to the acceptor and donor energy levels lie in the energy gap in between the conduction and valence bands.

    The absorption energy of the solid creates an electron-hole pair. Then the holes are quickly trapped holes leads to luminescent emission. It is also possible that the trapping of electrons occur at the donar site before recombination and trapping of a hole by the acceptor level. The electron may recombine with a trapped hole and hence release a luminescent radiation. The time taken by the electron in a trap depends on the depth of the trap and temperature.

    In this material, the emission of light occurs rather slowly due to the electron- hole recombination which takes place indirectly through the impurity levels. Therefore this process is known phosphorescence or slow photo luminescence.