Florian Hallermann

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

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.

Localized plasmonic losses at metal back contacts of thin-film silicon solar cells

Investigations of optical losses induced by localized plasmons in protrusions on silver back contacts of thin-film silicon solar cells are presented. The interaction of electromagnetic waves with nanoprotrusions on flat silver layers is simulated with a three-dimensional numerical solver of Maxwells equations. Spatial absorption profiles and spatial electric field profiles as well as the absorption inside the protrusions are calculated. The results presented here show that the absorption of irradiated light at nanorough silver layers can be strongly enhanced by localized plasmonic resonances in Ag nanoprotrusions. Especially, localized plasmons in protrusions with a radius below 60 nm induce strong absorption, which can be several times the energy irradiated on the protrusions cross section. The localized plasmonic resonances in single protrusions on Ag layers are observed to shift to longer wavelengths with increasing refractive index of the surrounding material. At wavelengths above 500 nm localized plasmonic resonances will increase the absorption of nanorough μc-Si:H/Ag interfaces. The localized plasmon induced absorption at nanorough ZnO/Ag interfaces lies at shorter wavelengths due to the lower refractive index of ZnO. For wavelengths above 500 nm, a high reflectivity of the silver back contacts is essential for the light-trapping of thin-film silicon solar cells. Localized-plasmon induced losses at silver back contacts can explain the experimentally observed increase of the solar cell performance when applying a ZnO/Ag back contact in comparison to a μc-Si:H/Ag back contact.

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

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.

Enhanced upconversion quantum yield near spherical gold nanoparticles – a comprehensive simulation based analysis

Photon upconversion is promising for many applications. However, the potential of lanthanide doped upconverter materials is typically limited by low absorption coefficients and low upconversion quantum yields (UCQY) under practical irradiance of the excitation. Modifying the photonic environment can strongly enhance the spontaneous emission and therefore also the upconversion luminescence. Additionally, the non-linear nature of the upconversion processes can be exploited by an increased local optical field introduced by photonic or plasmonic structures. In combination, both processes may lead to a strong enhancement of the UCQY at simultaneously lower incident irradiances. Here, we use a comprehensive 3D computation-based approach to investigate how absorption, upconversion luminescence, and UCQY of an upconverter are altered in the vicinity of spherical gold nanoparticles (GNPs). We use Mie theory and electrodynamic theory to compute the properties of GNPs. The parameters obtained in these calculations were used as input parameters in a rate equation model of the upconverter β-NaYF4: 20% Er3+. We consider different diameters of the GNP and determine the behavior of the system as a function of the incident irradiance. Whether the UCQY is increased or actually decreased depends heavily on the position of the upconverter in respect to the GNP. Whereas the upconversion luminescence enhancement reaches a maximum around a distance of 35 nm to the surface of the GNP, we observe strong quenching of the UCQY for distances <40 nm and a UCQY maximum around 125 to 150 nm, in the case of a 300 nm diameter GNP. Hence, the upconverter material needs to be placed at different positions, depending on whether absorption, upconversion luminescence, or UCQY should be maximized. At the optimum position, we determine a maximum UCQY enhancement of 117% for a 300 nm diameter GNP at a low incident irradiance of 0.01 W/cm2. As the irradiance increases, the maximum UCQY enhancement decreases to 20% at 1 W/cm2. However, this UCQY enhancement translates into a significant improvement of the UCQY from 12.0% to 14.4% absolute.

Increasing upconversion by metal and dielectric nanostructures

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.

Calculation of up-conversion photoluminescence in Er3+ ions near noble-metal nanoparticles

In conventional silicon solar cells, photons with energies lower than the silicon band gap (1.12 eV) are not absorbed in the silicon layer. However, the near-infrared portion of the solar spectrum may still be able to contribute to photocurrent generation if use can be made of up-conversion processes that transform two or more infrared photons into a photon of sufficient energy to be absorbed in silicon. One possible material in which up-conversion processes occur are rare-earth ions such as Er3+. It has recently been shown that up-conversion in such ions could be enhanced by optical near-field coupling to metal nanoparticles in a highly controlled geometry. However, potential photovoltaic applications of the upconversion enhancement will certainly be characterized by different geometric arrangements, with random distances between ions and nanoparticles. Whether or not an overall enhancement of the up-conversion efficiency may be expected under such realistic conditions is an open question. In this work, we address an important aspect of this question, namely the particle-induced enhancement of the optical excitation rate in the rare-earth ions. Our model calculations show that the excitation rate in Er3+ ions can be enhanced using spherical gold nanoparticles. The model includes random distances between ions and nanoparticles, as well as random polarizations of the exciting light. The enhancement of the rate of excitation of the fundamental transition results in increases of the up-conversion rate by up to 20% for an excitation wavelength of 1523 nm, provided that photoluminescence-quenching effects due to nonradiative relaxation in the metal can be neglected.

Gold Nanoparticle-enhanced Upconversion in Erbium Ions

Upconversion of sub-bandgap photons has been suggested to increase the efficiencies of silicon solar cells. Using model calculations we show that gold nanoparticles can enhance the upconversion intensities from erbium ions in a relevant geometry.

Increasing upconversion efficiency by plasmon resonance in metal nano-particles

Upconversion (UC) of sub-band-gap photons has the potential to increase solar cell efficiencies. In this paper, we present investigations of silicon solar cell devices with attached upconverter based on NaYF4:20% Er3+. Such devices showed peak external quantum efficiencies of 0.64% under monochromatic irradiation 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 higher efficiency under broad spectrum illumination is attributed to the parallel resonant excitation of the two most important involved optical transitions. The measured quantum efficiency corresponds to a relative efficiency increase of 0.014% for the used bifacial silicon solar cell with 16.7% overall efficiency. This increase is too small to make upconversion relevant in photovoltaics. Therefore, additional means of increasing the UC efficiency are necessary such as plasmon resonance in metal nano-particles in the proximity of the upconverting material. The higher local field intensities caused by plasmon resonance positively influence upconversion efficiency because of the non-linear nature of UC. Additionally, the metal nano-particles also influence transition probabilities in the upconverter. To investigate the impact of such nano-particles, we used a rate equation model of the up-converting material. It is based on Einstein coefficients of the relevant transitions, which were determined from absorption measurements. The model describes the UC dynamics and considers ground state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multi phonon relaxation. The results show good agreement with photoluminescence measurements. The model was coupled with Mie theory calculations of the changes of the electric field and of the transition probabilities. The calculations suggest that metal nano-particles are able to increase UC efficiency significantly by at least a factor of eight.

Photochromic silver nanoparticles in titania

In this article, we present our recent work on photochromic thin films made of silver nanoparticles embedded in titania. Using optical spectroscopy we measure the photochromic changes induced by laser irradiation at various power densities and irradiation times. It is shown that illumination leads to a reduction of the extinction peak of the photoexcited particles, accompanied by a blueshift of the plasmon line. The intensity dependence of this photochromic behavior indicates that it is due to a one-photon process. Microscopically the photochromic process is explained by a plasmon-assisted electron emission from the Ag nanoparticles into the titania matrix, leading to a structural and/or compositional transformation of the nanoparticles. At large power densities, the plasmon resonance is completely bleached, indicating complete transformation, and absorption in the near-infrared is increased