Importance of Evaluating the Intensity Dependency of the Quantum Efficiency: Impact on LEDs and Persistent Phosphors

The quantum efficiency is a key metric in lighting technology and for the quantification of luminescent processes, indicating how many photons are emitted with respect to the number of absorbed photons. Ideally, this value should approach unity to reduce losses, for instance in the common phosphor converted white LEDs. In this work we demonstrate that in luminescent materials where energy can be stored at defect centers, like in the extreme case of persistent phosphors, the quantum efficiency depends on the excitation intensity. For the green emitting SrAl2O4:Eu2+,Dy3+, which has been proposed for use in AC-LEDs, the internal quantum efficiency drops for increasing excitation intensity from 71% to 54%. At elevated excitation intensities, as encountered in LEDs, the trapped charge carriers can be optically detrapped by the excitation light, leading to this strong reduction of the overall quantum efficiency. Considering that the absorption cross section for this process is 6-29X larger than the absorption cross section for the luminescent ion, the efficiency of LED phosphors can be increased by avoiding the presence of defects acting as trapping centers. Finally, designing persistent phosphors with defects that show a limited optical response for the excitation light could strongly increase their energy storage capacity.