(169b) Synthesis of Rare-Earth Doped Core-Shell Nano-Phosphors for Solar Cell Applications

In an effort to build the nation's independence on energy, there is a large research effort to harvest energy from the ultimate energy source, sun. In this area, phosphors can be integrated into photovoltaic devices to help convert the low-energy photons into shorter-wavelength light, via the upconversion process. For example, the use of Er³⁺ or Er³⁺/Yb³⁺ co-doped sodium yttrium fluoride has been shown to be promising in achieving this goal.¹,² Since upconversion is closely related to the separation distance and spatial distribution of the optically active ions, this work focuses on the synthesis of nanostructured phosphors by a hydrothermal process and examine their upconversion properties.

Specifically, we have characterized the upconversion luminescence in post-deposition-annealed Er³⁺ doped Y₂O₃ nano-particles by excitation with NIR radiation (803.29 nm). Green and red luminescence was observed from the ²H_11/2 and ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions for Er³⁺ ions in the Y₂O₃ host between 525 to 575 nm and 640 to 690 nm, respectively. The spectral band shapes and position are very similar to those obtained upon 488 nm excitation, as reported in literature. Moreover, these 5% Er³⁺:Y₂O₃ nanocrystals have a relative green-to-red ratio of almost 1:1, which is close to the reported relative green-to-red ratio of bulk 10% doped Er³⁺:Y₂O₃ material. A mechanism possibly responsible for populating the ⁴F_9/2 → ⁴I_15/2 transition in these Er³⁺:Y₂O₃ nanocrystals is a phonon-assisted ET (PET) process. The efficient PET process suggests that the Er³⁺ to Er³⁺ distances are small in the Er³⁺:Y₂O₃ nanocrystals and there are less surface contaminations such as CO₃^2-, NO₃^- and OH^- after the annealing through exposure to high laser power, as these ionic species result in multiphonon relaxation. The upconversion measurements by exciting the Er³⁺ ions to either ⁴I_13/2 or ⁴I_11/2 levels show similar upconversion responses, making them a viable material to improve the performance of solar cells. In addition, complementary analysis on the various core-shell structures, where the atomic layer deposited shells are luminescent thin film materials containing optical sensitizers, will also be discussed to assess the feasibility of their integration in photovoltaic devices.

¹ Bryce S. Richards and Avi Shalav, ?Enhancing the Near-Infrared Spectral Response of Silicon Optoelectronic Devices via Up-Conversion,? IEEE Transactions on Electron Devices, 54, 10 (2007).

² A. Shalav, B. S. Richards, K. W. Krämer, and H. U. Güdel, ?Application of NaYF₄ : Er³⁺ up-converting phosphors for enhanced near-infrared silicon solar cell response,? Appl. Phys. Lett., 86, 013505 (2005).