Material of the Month
October 2011
Improvement of Si Solar Cells by Upconversion of Sub-Band-Gap Photons in b-NaYF4:Er3+

Sub-band-gap photons (l > 1050nm) are unused in silicon solar cells. They amount to 20% of the total power and offer a major potential for improvement of solar cell efficiency. Within the EU-FP7 project Nanospec (NMP4-SL-2010-246200), we investigate and optimize upconversion materials related to b-NaYF4:Er3+. The concept, see Fig.1 [1], involves broad IR absorption by semiconductor nanoparticles (green), energy transfer to b-NaYF4:Er3+ (red), and upconversion across the Si band gap Eg [2].
The 1523nm 4I15/2  4I13/2 absorption of Er3+, followed by the 4I13/2 4I9/2 upconversion, the 4I9/2 4I11/2 relaxation, and the 4I11/2 4I15/2 emission at 1000nm offers a very efficient excitation of Si, see Fig.2. b-NaYF4 is the best host lattice for this process due to its ninefold coordinated M3+ sites and its microscopic disorder [3,4].

This work was carried out in the group of Dr. Karl Krämer.

References:

  1. J. C. Goldschmidt, P. Löper, S. Fischer, S. Janz, M. Peters, S. W. Glunz, G. Willeke, E. Lifshitz, K. Krämer, D. Biner;
    "Advanced upconverter systems with spectral and geometric concentration for high upconversion efficiencies"
    (Proceed. of the IUMRS Int. Conf. Electr. Mat. 2008, Sydney, Australia)
    IEEE, COMMAD 2008, 307 - 311, (2008); doi:10.1109/COMMAD.2008.4802153.
  2. S. Fischer, J. C. Goldschmidt, P. Löper, G. H. Bauer, R. Brüggemann, K. Krämer, D. Biner, M. Hermle, S. W. Glunz;
    "Enhancement of silicon solar cell efficiency by upconversion: Optical and electrical characterization"
    J. Appl. Phys., 108, 044912/1-11, (2010); doi:10.1063/1.3478742.
  3. K. W. Krämer, D. Biner, G. Frei, H. U. Güdel, M. P. Hehlen, S.R. Lüthi;
    "Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors"
    Chem. Mater., 16, 1244-1251, (2004); doi:10.1021/cm031124o.
  4. A. Aebischer, M. Hostettler, J. Hauser, K.W. Krämer, T. Weber, H. U. Güdel, H. B. Bürgi;
    "Structural and Spectroscopic Characterization of Active Sites in a Family of Light-Emitting Sodium Lanthanide Tetrafluorides"
    Angew. Chem. Int. Ed., 45, 2802-2806, (2006); doi:10.1002/anie.200503966.