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Increased upconversion quantum yield in photonic structures due to local field enhancement and modification of the local density of states--a simulation-based analysis (Optics Express)

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

Highly efficient IR to NIR upconversion in Gd2O2S: Er3+ for photovoltaic applications (Chemistry of materials)

Optimizing infrared to near infrared upconversion quantum yield of β-NaYF4:Er3+ in fluoropolymer matrix for photovoltaic devices (Journal of Applied Physics)

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

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

Tuning of Electronic Properties in IV–VI Colloidal Nanostructures by Alloy Composition and Architecture (Nanoscale Research Letters)

 

 

Increased upconversion quantum yield in photonic structures due to local field enhancement and modification of the local density of states - a simulation-based analysis

Barbara Herter, Sebastian Wolf, Stefan Fischer, Johannes Gutmann, Benedikt Bläsi, Jan Christoph Goldschmidt1 [top]

1Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany

In upconversion processes, two or more low-energy photons are converted into one higher-energy photon. Besides other applications, upconversion has the potential to decrease sub-band-gap losses in silicon solar cells. Unfortunately, upconverting materials known today show quantum yields, which are too low for this application. In order to improve the upconversion quantum yield, two parameters can be tuned using photonic structures: first, the irradiance can be increased within the structure. This is beneficial, as upconversion is a non-linear process. Second, the rates of the radiative transitions between ionic states within the upconverter material can be altered due to a varied local density of photonic states. In this paper, we present a theoretical model of the impact of a photonic structure on upconversion and test this model in a simulation based analysis of the upconverter material β -NaYF(4):20% Er(3+) within a dielectric waveguide structure. The simulation combines a finite-difference time-domain simulation model that describes the variations of the irradiance and the change of the local density of photonic states within a photonic structure, with a rate equation model of the upconversion processes. We find that averaged over the investigated structure the upconversion luminescence is increased by a factor of 3.3, and the upconversion quantum yield can be improved in average by a factor of 1.8 compared to the case without the structure for an initial irradiance of 200 Wm(-2).

 

Stark level analysis of the spectral line shape of electronic transitions in rare earth ions embedded in host crystals

Heiko Steinkemper, Stefan Fischer, Martin Hermle and Jan Christoph Goldschmidt1 [top]

1Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany

Rare earth ions embedded in host crystals are of great interest for many applications. Due to the crystal field of the host material, the energy levels of the rare earth ions split into several Stark levels. The resulting broadening of the spectral line shapes of transitions between those levels determines the upconversion phenomena, especially under broad-spectrum illumination, which are relevant for photovoltaics for instance. In this paper, we present a method to determine the spectral line shape of energy level transitions of rare earth ions from the absorption spectrum of the investigated material. A parameter model is used to describe the structure of the individual energy levels based on a representation of the Stark splitting. The parameters of the model are then determined with an evolutionary optimization algorithm. The described method is applied to the model system of -NaEr0.2Y0.8F4. The results indicate that for illumination with a wavelength around 1523 nm, simple upconversion processes such as two-step absorption or direct energy transfer are less efficient than commonly assumed. Hence a sequence of efficient processes is suggested as an explanation for the high upconversion quantum yield of -NaEr0.2Y0.8F4, which has not yet been reported in the literature.

New Journal of Physics

 

Highly efficient IR to NIR upconversion in Gd2O2S: Er3+ for photovoltaic applications

Rosa Martín-Rodríguez , Stefan Fischer , Aruna Ivaturi §, Benjamin Froehlich , Karl W. Krämer , Jan C. Goldschmidt , Bryce S. Richards §, and Andries Meijerink [top]

Debye Institute for Nanomaterials Science, Department of Chemistry, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands

Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, 79110 Freiburg, Germany

§ Institute of Photonics and Quantum Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, EH14 4AS Edinburgh, United Kingdom

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland

Upconversion (UC) is a promising option to enhance the efficiency of solar cells by conversion of sub-bandgap infrared photons to higher energy photons that can be utilized by the solar cell. The UC quantum yield is a key parameter for a successful application. Here the UC luminescence properties of Er3+-doped Gd2O2S are investigated by means of luminescence spectroscopy, quantum yield measurements, and excited state dynamics experiments. Excitation into the maximum of the 4I15/24I13/2 Er3+ absorption band around 1500 nm induces very efficient UC emission from different Er3+ excited states with energies above the silicon bandgap, in particular, the emission originating from the 4I11/2 state around 1000 nm. Concentration dependent studies reveal that the highest UC quantum yield is realized for a 10% Er3+-doping concentration. The UC luminescence is compared to the well-known Er3+-doped β-NaYF4 UC material for which the highest UC quantum yield has been reported for 25% Er3+. The UC internal quantum yields were measured in this work for Gd2O2S: 10%Er3+ and β-NaYF4: 25%Er3+ to be 12 ± 1% and 8.9 ± 0.7%, respectively, under monochromatic excitation around 1500 nm at a power of 700 W/m2. The UC quantum yield reported here for Gd2O2S: 10%Er3+ is the highest value achieved so far under monochromatic excitation into the 4I13/2 Er3+ level. Power dependence and lifetime measurements were performed to understand the mechanisms responsible for the efficient UC luminescence. We show that the main process yielding 4I11/2 UC emission is energy transfer UC.

Chemistry of materials

 

Optimizing infrared to near infrared upconversion quantum yield of β-NaYF4:Er3+ in fluoropolymer matrix for photovoltaic devices

Aruna Ivaturi1, Sean K. W. MacDougall1, Rosa Martín-Rodríguez2, Marta Quintanilla1, Jose Marques-Hueso1, Karl W. Krämer3, Andries Meijerink2, and Bryce S. Richards1 [top]

1Institute of Photonics and Quantum Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom
2Department of Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
3Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

The present study reports for the first time the optimization of the infrared (1523 nm) to near-infrared (980 nm) upconversion quantum yield (UC-QY) of hexagonal trivalent erbium doped sodium yttrium fluoride (β-NaYF4:Er3+) in a perfluorocyclobutane (PFCB) host matrix under monochromatic excitation. Maximum internal and external UC-QYs of 8.4% ± 0.8% and 6.5% ± 0.7%, respectively, have been achieved for 1523 nm excitation of 970 ± 43 Wm−2 for an optimum Er3+ concentration of 25 mol% and a phosphor concentration of 84.9 w/w% in the matrix. These results correspond to normalized internal and external efficiencies of 0.86 ± 0.12 cm2 W−1 and 0.67 ± 0.10 cm2 W−1, respectively. These are the highest values ever reported for β-NaYF4:Er3+ under monochromatic excitation. The special characteristics of both the UC phosphor β-NaYF4:Er3+ and the PFCB matrix give rise to this outstanding property. Detailed power and time dependent luminescence measurements reveal energy transfer upconversion as the dominant UC mechanism.

Journal of Applied Physics

 

Stark level analysis of the spectral line shape of electronic transitions in rare earth ions embedded in host crystals

Heiko Steinkemper, Stefan Fischer, Martin Hermle and Jan Christoph Goldschmidt [top]

Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, 79110 Freiburg, Germany

Rare earth ions embedded in host crystals are of great interest for many applications. Due to the crystal field of the host material, the energy levels of the rare earth ions split into several Stark levels. The resulting broadening of the spectral line shapes of transitions between those levels determines the upconversion phenomena, especially under broad-spectrum illumination, which are relevant for photovoltaics for instance. In this paper, we present a method to determine the spectral line shape of energy level transitions of rare earth ions from the absorption spectrum of the investigated material. A parameter model is used to describe the structure of the individual energy levels based on a representation of the Stark splitting. The parameters of the model are then determined with an evolutionary optimization algorithm. The described method is applied to the model system of β-NaEr0.2Y0.8F4. The results indicate that for illumination with a wavelength around 1523 nm, simple upconversion processes such as two-step absorption or direct energy transfer are less efficient than commonly assumed. Hence a sequence of efficient processes is suggested as an explanation for the high upconversion quantum yield of β-NaEr0.2Y0.8F4, which has not yet been reported in the literature.

New Journal of Physics

 

Plasmon enhanced upconversion luminescence near gold nanoparticles – simulation and analysis of the interactions: Errata

Stefan Fischer1, Florian Hallermann2, Toni Eichelkraut3Gero von Plessen2, Karl W. Krämer4, Daniel Biner4, Heiko Steinkemper1, Martin Hermle1 and Jan Christoph Goldschmidt1 [top]

1Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, 79110 Freiburg, Germany
2Institute of Physics (1A), RWTH Aachen University, 52056 Aachen, Germany
3Institute of Condensed Matter Theory and Solid State Optics, Abbe Center of Photonics, Friedrich-Schiller-Universität, 07743 Jena, Germany
4Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland

The procedure used in our previous publication (S. Fischer et al., Optics Express 20, 271-282, 2012) to calculate how coupling to a spherical gold nanoparticle changes the upconversion luminescence of Er3+ ions
contained several errors. The errors are corrected here.

Optics Express

 

Tuning of electronic properties in IV-VI colloidal nanostructures by alloy composition and architecture

A. Sashchiuk, D. Yanover, A. Rubin-Brusilovski, GI Maikov, Richard Capek1, R. Vaxenburg, J. Tilchin, G. Zaiats, Efrat Lifshitz1

1 Department of Chemistry and Solid State Institute, Technion, Haifa 3200, Israel

Colloidal lead chalcogenide (IV-VI) quantum dots and rods are of widespread scientific and technological interest, owing to their size tunable energy band gap at the near-infrared optical regime. This article reviews the development and investigation of IV-VI derivatives, consisting of a core (dot or rod) coated with an epitaxial shell, when either the core or the shell (or both) has an alloy composition, so the entire structure has the chemical formula PbSexS1-x/PbSeyS1-y (0 ≤ x(y) ≤ 1). The article describes synthesis procedures and an examination of the structures' chemical and temperature stability. The investigation of the optical properties revealed information about the quantum yield, radiative lifetime, emission's Stokes shift and electron-phonon interaction, on the variation of composition, core-to-shell division, temperature and environment. The study reflected the unique properties of core-shell heterostructures, offering fine electronic tuning (at a fixed size) by changing their architecture. The optical observations are supported by the electronic band structure theoretical model. The challenges related to potential applications of the colloidal lead chalcogenide quantum dots and rods are also briefly addressed in the article.

Nanoscale Research Letters

Funding

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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