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Results on upconversion


The University of Bern is located in the middle of Switzerland and has more than 15’000 students in 8 faculties. Within the Department of Chemistry and Biochemistry the solid state chemistry group of Dr. Karl Krämer is active in the synthesis of rare earth halides, crystal growth, and spectroscopic and magnetic investigations.

For the Nanospec project b-NaYF4 : Er based upconversion phosphors are synthesized and characterized. The upconversion phosphor converts two (or more) infra-red photons to one photon of shorter wavelength which is then absorbed by silicon and enhances the efficiency of the solar cell, see Fig. 1.

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Fig. 1. Scheme for the use of infra red light in silicon solar cells by upconversion.


b-NaYF4 is an excellent host lattice for upconversion phosphors. The synthesis of these phosphors was published in Chem. Mater. 16 (2004) 1244 – 1251, see Fig. 2.

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Fig. 2. Strong blue upconversion light is emitted from b-NaYF4:Yb,Tm after Yb excitation at 980 nm.


The structural and spectroscopic characterization of the active sites in this family of sodium lanthanide tetrafluorides is reported in Angew. Chem. Int. Ed. 45 (2006) 2802 – 2806, see Fig. 3.

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Fig. 3. (left) Diffuse scattering originates from the cation disorder in b-NaYF4. (right) The two rare earth sides in b-NaYF4 show microscopic disorder.



Publications on upconversion


Calibrated photoluminescent measurements of the upconverter NaYF4:20% Er3+ and its potential to enhance the efficiency of solar cells in the sub-band-gap spectral region (EUPVSEC 2010)

Enhancement of silicon solar cell efficiency by upconversion: Optical and electrical characterization (Journal of Applied Physics)



Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination (Solar Energy Materials and Solar Cells)

Investigation on the Intensity Dependence of Upconversion Systems (EUPVSEC 2011)

Characterization and simulation of upconversion processes (Quantsol 2011)

Plasmon enhanced upconversion luminescence near gold nanoparticles–simulation and analysis of the interactions (Optics Express)

Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination (Solar Energy Materials and Solar Cells)

Origin of the High Upconversion Green Luminescence Efficiency in beta-NaYF(4):2%Er(3+),20%Yb(3+) (Chemistry of Materials)


Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients (Journal of Applied Physics)

Increasing Upconversion by metal and dielectric Nanostructures (Society of Photo-Optical Instrumentation Engineers -SPIE-)

Effects of photonic structures on upconversion (Society of Photo-Optical Instrumentation Engineers -SPIE-)

Concepts to enhance the efficiency of upconversion for solar applications (Society of Photo-Optical Instrumentation Engineers -SPIE-)

Absolute Upconversion Quantum Yield of β-NaYF4:Er3+ under Broadband Excitation in Dependence on the Erbium Concentration – Measurements and Simulations (Optics Express)

Exploiting Photonic Structures to improve the Efficiency of Upconversion, utilizing both Field Enhancement and modified LDOS (EUPVSEC 2012)

Increasing Upconversion by Plasmon Resonance in Metal Nanoparticles—A Combined Simulation Analysis (IEEE Journal of Photovoltaics)

Broadband excitation of upconversion in lanthanide doped fluorides for enhancement of Si solar cells (Society of Photo-Optical Instrumentation Engineers -SPIE-)



Stark level analysis of the spectral line shape of electronic transitions in rare earth ions embedded in host crystals (New Journal of Physics)




The Nanospec Project is funded by the European Community's Seventh Framework Program (FP7/2007-2013) under grant agreement no. [246200].

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