Luminescent Properties and Energy Transfer in Pr3+ Doped and Pr3+-Yb3+ Co-doped ZnO Thin Films

Embedding luminescent rare earth ions into transparent oxides such as ZnO is a well-known approach to functionalize the material by adding photon-management properties. In this paper we present a detailed study of the luminescence properties and energy level structure of Pr3+ and Yb3+ ions embedded in ZnO thin films deposited by magnetron reactive sputtering. Careful study of the photoluminescence and excitation spectra allowed identifying and locating almost all excited levels of Pr3+ and Yb3+ ions. Thus, an almost complete electronic energy level diagram of these ions in the ZnO crystal lattice can be drawn for the first time. In particular, we show that the crystal field of ZnO strongly modifies the energy level structure of the Pr3+ and Yb3+ ions creating energy mismatches between the transitions useful for cooperative down conversion. Finally, we demonstrate that light emission from Pr3+ ions can be induced both by direct excitation of the ions and indirectly by energy transfer from the ZnO matrix.