A phosphor commonly used on TV screens is ZnS doped with Cu

A phosphor commonly used on TV screens is ZnS doped with Cu^+. ZnS is a semiconductor. Why is it efficient at transferring energy to impurity states for emission?

Solution

This happens because native defects are present in these semiconductors which either have
a donor or acceptor characterstic, and they act as compensating centers. These defects react with
dopant impurities to form complexes and thus they end up conducting. In this way it is efficient
at transferring energy to impurity states for emission.