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Increasing upconversion by metal and dielectric nanostructures (SPIE)

Concepts to enhance the efficiency of upconversion for solar applications (SPIE Conference)

Effects of photonic structures on upconversion (SPIE Conference)

Optical Simulation of Bifacial Solar Cells (2nd SiliconPV Leuven 2012)

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

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

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

 

 

Increasing upconversion by metal and dielectric nanostructures

J.C. Goldschmidta, S. Fischera, H. Steinkempera, B. Hertera, T. Rista, S. Wolfa, B. Bläsia, F. Hallermannb, G. von Plessenb, K.W. Krämerc, D. Biner and M. Hermlea, [top]
a Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany
b Institute of Physics (1A), RWTH Aachen University, 52056 Aachen, Germany
c Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland

Upconversion (UC) of sub-band-gap photons can increase solar cell efficiencies. Up to now, the achieved efficiencies are too low, to make UC relevant for photovoltaics. Therefore, additional means of increasing UC efficiency are necessary. In this paper, we investigate both metal and dielectric photonic nanostructures for this purpose. The theoretical analysis is based on a rate equation model that describes the UC dynamics in β-NaYF4 : 20% Er3+. The model considers ground state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multi phonon relaxation. For one, this model is coupled with results of Mie theory and exact electrodynamic theory calculations of plasmon resonance in gold nanoparticles. The effects of a 200 nm gold nanoparticle on the local field density and on the transition rates within in the upconverter are considered. Calculations are performed in high resolution for a three dimensional simulation volume. Furthermore, the effect of changed local fields in the proximity of grating waveguide dielectric nanostructure is investigated. For this purpose FDTD simulation models of such structures are coupled with the rate equation model of the upconverter. The results suggest that both metal nanoparticles and dielectric nanostructures can increase UC efficiency. [top]

doi:10.1117/12.910915

 

Concepts to enhance the efficiency of upconversion for solar applications

S. Fischera*, H. Steinkempera, K. W. Krämerb and J. C. Goldschmidta [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

Upconversion of otherwise lost sub-band-gap photons is a promising approach for more efficient solar cells. We investigate upconverter materials based on lanthanides, especially trivalent erbium. They are known for high upconversion efficiency of infrared photons under laser excitation at a wavelength around 1520 nm. However, the achieved upconversion efficiency is still not large enough and the absorption range of these materials is too narrow for an application in photovoltaics. Herein, we present an overview of different possibilities to enhance the efficiency of upconversion for silicon solar cells. The concepts discussed can be divided into two groups. The first group comprises internal concepts, e.g., the host material itself, size effects and dopant concentration. The second group consists of external methods, which change the physical environment around the upconverter to improve the absorption properties and enhance the upconversion quantum yield. By considering the different effects in a sophisticated rate equation model of the upconverting material b-NaY0.8Er0.2F4, and comparing the results with experimental data, we show that there is a big potential to improve the upconversion properties for solar applications. Furthermore we show variety opportunities to increase the upconversion quantum yield are. [top]

SPIE Conference Programm [pdf]

 

Effects of photonic structures on upconversion

B. Herter, S. Wolf, S. Fischer, M. Peters, B. Bläsi, J.C. Goldschmidt [top]
Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany

Photonic crystals modify the local density of photon states. These variations influence the emission properties of a dipole embedded within the photonic crystal. Furthermore, field enhancement can be observed within photonic crystals. In this paper, we investigate how these effects influence upconversion processes in β-NaYF4:Er3+. For this purpose we use finite-difference time-domain (FDTD) simulations of a grating-waveguide-structure in combination with a rate equation model of the upconversion processes in β-NaYF4:Er3+. The grating parameters are optimized to achieve large field enhancements within the structure for the combination of s- and p-polarized light. Furthermore, the variation of the spontaneous emission rates for dipole emitters within the structure is simulated. The varied transition rates, as well as the field enhancement, serve as input parameters for the rate equation model for upconversion. Using this approach, the influence of the structure on the upconversion quantum yield is calculated. For a simulated initial irradiance of 1000 W/m², we find enhancement factors of up to four for the field enhancement in the upconverter region and up to a factor of three for the upconversion quantum yield. In consequence, the incorporation of upconverting material in photonic structures in very promising to increase upconversion efficiencies. [top]

SPIE Conference Programm [pdf]

 

Optical Simulation of Bifacial Solar Cells

Judith Frank, Marc Rüdiger, Stefan Fischer, Jan Christoph Goldschmidt and Martin Hermle [top]

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

Bifacial solar cells are used in several specialized photovoltaic system concepts, such as albedo collecting modules or in conjunction with upconverter materials. We present a simulation model for bifacial solar cells that allows the investigation of the rear side illumination of solar cells with great flexibility. We apply this model to bifacial test structures with different rear side reflectors and compare them to structures without rear side reflector. We consider the optical properties of planar cells, random pyramids only on the front side and random pyramids on front and rear side. Thereby, the optical simulation results show good qualitative agreement with test samples featuring the simulated reflection properties. Samples with texturing on both sides together with white paint as a rear side reflector show very good light trapping properties, but also other rear reflector configurations with diffuse scatterers, like e.g. PTFE, and specular reflectors like Ag mirrors show a comparable high increase in the photogenerated current density. [top]

SiliconPV Program [pdf]

 

Increasing Upconversion by Plasmon Resonance in Metal Nanoparticles - A Combined Simulation Analysis

J.C. Goldschmidt, S. Fischer, H. Steinkemper, F. Hallermann, G. von Plessen, K.W. Krämer, D. Biner and M. Hermle [top]

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

Upconversion (UC) of subbandgap photons has the potential to increase solar cell efficiencies. In this paper, we first review our recent investigations of silicon solar cell devices with an attached upconverter based on β-NaYF4 :20%Er3+. Such devices showed peak external quantum efficiencies of 0.64% under monochromatic excitation at 1523 nm and an irradiance of 2305 Wm -2. Under broad spectrum illumination, an average UC efficiency of 1.07 ± 0.13% in the spectral range from 1460 to 1600 nm was achieved. The measured quantum efficiency corresponds to a relative efficiency increase of 0.014% for the used bifacial silicon solar cell with 16.70% overall efficiency. This increase is too small to make UC relevant in photovoltaics. Therefore, additional means of increasing the UC efficiency are necessary. In this paper, we investigate plasmon resonance in metal nanoparticles in the proximity of the UC material, with the aim of increasing UC efficiency. The local field enhancement by the plasmon resonance positively influences UC efficiency because of the nonlinear nature of UC. Additionally, the metal nanoparticles also influence the transition probabilities in the upconverter. To investigate the effects, we combine different simulation models. We use a rate equation model to describe the UC dynamics in β-NaYF4 :20%Er3+. The model considers ground state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multiphonon decay. The rate equation model is coupled with Mie theory calculations of the changed optical field in the proximity of a gold nanoparticle. The changes of the transition rates both for radiative and nonradiative processes are calculated with exact electrodynamic theory. Calculations are performed in high resolution for a 3-D simulation volume. The results suggest that metal nanoparticles can increase UC efficiency.

doi: 10.1109/JPHOTOV.2011.2182179

 

Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients

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

Upconversion of infrared photons is a promising possibility to enhance solar cell efficiency by producing electricity from otherwise unused sub-band-gap photons. We present a rate equation model, and the relevant processes, in order to describe upconversion of near-infrared photons. The model considers stimulated and spontaneous processes, multi-phonon relaxation and energy transfer between neighboring ions. The input parameters for the model are experimentally determined for the material system \beta-NaEr0.2Y0.8F4. The determination of the transition probabilities, also known as the Einstein coefficients, is in the focus of the parameterization. The influence of multi-phonon relaxation and energy transfer on the upconversion are evaluated and discussed in detail. Since upconversion is a non-linear process, the irradiance dependence of the simulations is investigated and compared to experimental data of quantum efficiency measurements. The results are very promising and indicate that upconversion is physically reasonably described by the rate equations. Therefore, the presented model will be the basis for further simulations concerning various applications of upconversion, such as in combination with plasmon resonances in metal nanoparticles. [top]

 

doi: 10.1063/1.3674319


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

Stefan Fischer, Florian Hallermann, Toni Eichelkraut, Gero von Plessen, Karl W. Krämer, Daniel Biner, Heiko Steinkemper, Martin Hermle, and Jan C. Goldschmidt [top]

We investigate plasmon resonances in gold nanoparticles to enhance the quantum yield of upconverting materials. For this purpose, we use a rate equation model that describes the upconversion of trivalent erbium based upconverters. Changes of the optical field acting on the upconverter and the changes to the transition probabilities of the upconverter in the proximity of a gold nanoparticle are calculated using Mie theory and exact electrodynamic theory respectively. With this data, the influence on the luminescence of the upconverter is determined using the rate equation model. The results show that upconversion luminescence can be increased in the proximity of a spherical gold nanoparticle due to the change in the optical field and the modification of the transition rates. [top]

doi:10.1364/OE.20.000271

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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