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.


  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.