(703f) High Efficiency Rare Earth Doped Core-Shell Nanophosphors for Energy Applications

The development of rare-earth (RE) doped phosphors allows for the conversion of photons to energies that are more usable for the desired application. Additionally, these RE phosphors have long lifetimes, on the order of ms, which offer potential in many energy conversion and energy transfer devices. Currently, RE phosphors are used in fiber optics amplifiers, modulated hybrid laser, optical interconnects and switches, optical displays, broad absorption solar cells and various other lighting applications.  Energy transfer mechanisms of the excited RE states, such as defect quenching and sensitizer/emitter interactions, must be understood in order to achieve high efficiency energy conversion and propagation for future applications.

In order to synthesize high efficiency phosphors, trivalent RE ions are being doped into a core-shell metal oxide host lattice. The role of the core-shell structure reduces the effect of the surface quenching sites by increasing the distance between active ions and the surface hydroxyl groups. Secondly, the luminescent fingerprint can be further controlled with proper doping of the shell structure by either increasing the absorption spectrum or adding additional emission peaks. Primarily, this work focuses on the emission of visible photons through upconversion in Y₂O₃:Er³⁺, Yb³⁺ nanophosphors, making them ideal components in broad absorption solar cells. By spatially controlling the position and concentration of the RE ions within the nanostructure, increased luminescence is observed due to energy transfer between the dopant ions within a critical interatomic distance. Passitvation of surface sites with increasing shell thickness was shown to increase luminescent lifetimes up to 53%, with a critical shell thickness of 8 nm, while lowering the theoretical lifetimes extracted from Judd-Ofelt parameters. The effect of the spatially controlled Yb ions was probed through the extraction of the upconversion photon requirement, showing a statistical decrease in photons from 2.16 to 1.43, or ~30 %. Finally, the effective energy transfer distance and energy transfer coefficients were studied by selectively exciting the Yb ⁴F_5/2-⁴F_7/2 transition with a 904 nm laser diode while probing the Er³⁺ emission at 1540 nm as a Y₂O₃ spacer layer is added to the system. Measured results show the occurrence of energy transfer between the ions with a ~3 nm spacer layer, confirming the prediction of the Föester-Dexter theory. Additionally, these results are applied to a RE doped LaPO₄ system, producing white light from a single, high efficiency core-shell phosphor.