2014
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Absolute upconversion quantum yield of β-NaYF4 doped with Er3+ and external quantum efficiency of upconverter solar cell devices under broad-band excitation considering spectral mismatch corrections (Solar Energy Materials and Solar Cells)
Upconverter Silicon Solar Cell Devices for Efficient Utilization of Sub-Band-Gap Photons Under Concentrated Solar Radiation (IEEE Journal of Photovoltaics)
Absolute upconversion quantum yield of β-NaYF4 doped with Er3+ and external quantum efficiency of upconverter solar cell devices under broad-band excitation considering spectral mismatch corrections
a, a, a, b, a, c [top]
a Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany
b Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
c Department of Physics, Imperial College, South Kensington Campus, London SW7 2AZ, United Kingdom
For applications to harvest solar energy, it is essential to characterize upconverter materials under broad-band excitation at reasonable irradiance levels achievable by concentration of solar radiation. We present a method to determine the absolute upconversion quantum yield (UCQY) under broad-band excitation by photoluminescence measurements. We introduce a spectral mismatch correction that allows calculating the UCQY that can be expected under illumination with the solar spectrum with a certain solar concentration. Applying these methods to β-NaYF4 doped with 25%Er3+, we determine an external UCQY of 2.0% in the spectral range from 1450 nm to 1600 nm under a comparatively low solar concentration of only 50 suns. This value corresponds to a potential increase of the short-circuit current density of 3.89 mA/cm2. Subsequently, we measure the external quantum efficiency due to upconversion of sub-band-gap photons for a bifacial silicon solar cell with upconverter attached to its rear side under the same broad-band excitation. We determine an additional short-circuit current density due to upconversion of 4.03 mA/cm2 for a solar concentration of only 77 suns. This value agrees very well, with the expected additional short-circuit current density due to upconversion, calculated from the external UCQY values as determined by the photoluminescence measurements, by considering the transmittance of the solar cell for sub-band-gap photons and the external quantum efficiency of the solar cell for photons emitted by the upconverter. Finally, a comparison with different literature values based on a set of figures of merit shows that our results currently constitute the largest enhancement of the short-circuit current density due to upconversion. [top]
Upconverter Silicon Solar Cell Devices for Efficient Utilization of Sub-Band-Gap Photons Under Concentrated Solar Radiation
Upconversion (UC) of sub-band-gap photons has the potential to increase the efficiency of solar cells significantly. We realized an upconverter solar cell device, by attaching an upconverter layer of β-NaYF4 doped with 25% Er3+ embedded in the polymer perfluorocyclobutyl to the rear side of a bifacial silicon solar cell. We determined the external quantum efficiency of such upconverter solar cell devices under broad-band sub-band-gap excitation. Under consideration of spectral mismatch, we calculated the expected increase of the short-circuit current density due to UC under the air mass 1.5 global illumination. We determined an enhancement of 2.2 mA/cm2 for a spectral excitation band ranging from 1450 to 1600 nm and a comparatively low solar concentration of 78 suns. Subsequently, a system of concentrator lens and upconverter solar cell device was characterized with a solar simulator. We determined an increase of the short-circuit current density due to UC of sub-band-gap photons of 13.1 mA/cm2 under a concentration of 210 suns. This corresponds to a potential relative increase of the solar cell efficiency of 0.19%. [top]
