Upconversion Luminescence of HfO2 Phosphors Embedded into Polyester FILMS

Upconversion (UC) luminescence in powder hafnia (HfO₂) doped with Er³⁺, Yb³⁺ and Li⁺ was studied under continuous wave excitation at 980 nm. Powder samples were prepared by the simple evaporation technique. The simple evaporation technique is an inexpensive and scalable technique to obtain powders with excellent properties. The technique works heating a chemical solution until its evaporation point to achieve a powder. A thermal treatment is practiced in this powder in order to eliminate any kind of remnant from precursor materials and/or achieve an appropriate crystalline structure if it is necessary. This powders were attached into polyester films by spin coating technique; spin coating process consists in supplying a small puddle of a liquid resin onto the center of a substrate and then spinning the substrate at high speed (between 3000 and 6500 rpm), centripetal acceleration will cause the resin to spread and eventually it covers the substrate leaving a thin film of resin on the substrate surface.

This powders and films were optical, structural and luminescent characterized using x-ray diffraction, electron microscopy, IR spectroscopy and UV-Vis. A study of the infrared to green and infrared to red UC processes was performed including the analysis of the spectral and the temporal behavior. The mechanisms contributing to the UC luminescence were identified as excited state absorption and energy transfer among rare earth ions. (x, y) coordinates of CIE diagrams are reported also.

Finally, it is important to sight, that this kind of phosphors attached at polymeric films could be eventually incorporated to solar cells in order to increase its nominal efficiency.

1. François Auzel, Upconversion and Anti-Stokes Processes with f and d Ions in Solids, Chemical Reviews 2004 104 (1), 139-174

2. S. Carmona-Téllez, G. Alarcón-Flores, A. Meza-Rocha, E. Zaleta-Alejandre, M. Aguilar-Futis, H. Murrieta S and C. Falcony, Polyethylene terephthalate thin films; a luminescence study, Optical Materials, 42, 2015, 99–105.