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Relation between excitation power density and Er3+ doping yielding the highest absolute upconversion quantum yield

 
: Fischer, S.; Fröhlich, B.; Krämer, K.W.; Goldschmidt, J.C.

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Journal of physical chemistry. C, Nanomaterials and interfaces 118 (2014), No.51, pp.30106-30114
ISSN: 1932-7447
ISSN: 1932-7455
English
Journal Article
Fraunhofer ISE ()
Solarzellen - Entwicklung und Charakterisierung; Farbstoff; Organische und Neuartige Solarzellen; Photonenmanagement; yield; upconversion; energy transfer; quenching

Abstract
The upconversion quantum yield (UCQY) is one of the most significant parameters for upconverter materials. A high UCQY is essential for a succesful integration of upconversion in many applications, such as harvesting of the solar radiation. However, little is known about which doping level of the rare-earth ions yields the highest UCQY in the different host lattices and what are the underlying causes. Here, we investigate which Er3+ doping yields the highest UCQY in the host lattices β-NaYF4 and Gd2O2S under 4I15/2 → 4I13/2 excitation. We show for both host lattices that the optimum Er3+ doping is not fixed and it actually decreases as the irradiance of the excitation increases. To find the optimum Er3+ doping for a given irradiance, we determined the peak position of the internal UCQY as a function of the average Er–Er distance. For this purpose, we used a fit on experimental data, where the average Er–Er distance was calculated from the Er3+ doping of the upconverter samples and the lattice parameters of the host materials. We observe optimum average Er–Er distances for the host lattices β-NaYF4 and Gd2O2S with differences <14% at the same irradiance levels, whereas the optimum Er3+ doping are around 2× higher for β-NaYF4 than for Gd2O2S. Estimations by extrapolation to higher irradiances indicate that the optimum average Er–Er distance converges to values around 0.88 and 0.83 nm for β-NaYF4 and Gd2O2S, respectively. Our findings point to a fundamental relationship and focusing on the average distance between the active rare-earth ions might be a very efficient way to optimize the doping of rare-earth ions with regard to the highest achievable UCQY.

: http://publica.fraunhofer.de/documents/N-324125.html