Upconversion (UC) presents a possibility to exploit sub-bandgap photons for current generation in solar cells by creating
one high-energy photon out of at least two lower-energy photons. Photonic structures can enhance UC by two effects: a
locally increased irradiance and a modified local density of photon states (LDOS). Bragg stacks are promising photonic
structures for this application, because they are straightforward to optimize and overall absorption can be increased by
adding more layers. In this work, we present a comprehensive simulation-based analysis of the photonic effects of a
Bragg stack on UC luminescence. The investigated organic-inorganic hybrid Bragg stack consists of alternating layers of
Poly(methylmethacrylate) (PMMA), containing purpose-built β-NaYF4:25% Er3+ core-shell nanoparticles and titanium
dioxide (TiO2). From optical characterization of single thin layers, input parameters for simulations of the photonic
effects are generated. The local irradiance enhancement and modulated LDOS are first simulated separately.
Subsequently they are coupled in a rate equation model of the upconversion dynamics. Using the integrated model, UC
luminescence is maximized by adapting the Bragg stack design. For a Bragg stack of only 5 bilayers, UC luminescence
is enhanced by a factor of 3.8 at an incident irradiance of 2000 W/m2. Our results identify the Bragg stack as promising
for enhancing UC, especially in the low-irradiance regime, relevant for the application in photovoltaics. Therefore, we
experimentally realized optimized Bragg stack designs. The PMMA layers, containing UC nanoparticles, are produced
via spin-coating from a toluene based solution. The TiO2 layers are produced by atomic layer deposition from molecular
precursors. The reflectance measurements show that the realized Bragg stacks are in good agreement with predictions
from simulation.
Upconversion of sub-band-gap photons is a promising approach to increase the efficiency of solar cells. In this paper, we review the recent progress in upconverter material development and realization of efficient upconverter silicon solar cell devices. Current published record values for the increase in the short-circuit current density due to upconversion are 13.1 mA/cm2 at a solar concentration of 210 suns determined in a sun simulator measurement. This increase is equivalent to a relative efficiency enhancement of 0.19% for the silicon solar cell. Although this is a considerable enhancement by more than one order of magnitude from values published only a few years ago, further enhancement of the upconversion performance is necessary. To this end, we investigate theoretically the application of resonant cavity and grating photonic structures. Our simulation based analysis considers irradiance enhancement and modified density of photon states due to the photonic structures and their impact on the upconversion dynamics in β-NaYF4: 20%Er3+. It shows that an optimized grating can increase upconversion luminescence by a factor of 3 averaged over the whole structure in comparison to an unstructured reference with the same amount of upconverter material.
For high band gap solar cells, organic molecule based upconverter materials are promising to reduce transmission losses of photons with energies below the absorption threshold. We investigate the approach of embedding the organic upconverter DPA:PtOEP directly into each second layer of a Bragg stack to achieve an enhancement of upconversion performance. The two major effects that influence the upconversion process within the Bragg stack are simulated based on experimentally determined input parameters. The locally increased irradiance is simulated using the scattering matrix method. The variation of the density of photon states is obtained from calculations of the eigenmodes of the photonic crystal using the plane wave expansion method. A relative irradiance enhancement of 3.23 has been found for a Bragg stack of 31 layers including λ/8-layers on both sides. For suppressing the loss mechanism of direct sensitizer triplet decay via variations of the density of photon states, a different design of the Bragg stack is necessary than for maximum irradiance enhancement. In order to find the optimum design to increase upconversion quantum yield, both simulation results need to be coupled in a rate-equation model. The irradiance enhancement found in our simulation is significantly higher than the one found in the simulation of a grating-waveguide structure, which achieved an increase of upconversion quantum yield by a factor of 1.8. Thus, the Bragg structure is very promising for upconversion quantum yield enhancement.
Upconverter materials and upconverter solar devices were recently investigated with broad-band excitation revealing the great potential of upconversion to enhance the efficiency of solar cell at comparatively low solar concentration factors. In this work first attempts are made to simulate the behavior of the upconverter β-NaYF4 doped with Er3+ under broad-band excitation. An existing model was adapted to account for the lower absorption of broader excitation spectra. While the same trends as observed for the experiments were found in the simulation, the absolute values are fairly different. This makes an upconversion model that specifically considers the line shape function of the ground state absorption indispensable to achieve accurate simulations of upconverter materials and upconverter solar cell devices with broadband excitations, such as the solar radiation.
Upconversion of low-energy photons presents a possibility to overcome the Shockley-Queisser efficiency limit for solar cells. In silicon 20% of the incident energy is lost due to transmission of these photons with energies below the band gap. Unfortunately, upconversion materials known today show pretty low absorption and quantum yields which are too low for this application. One possibility to boost the upconversion luminescence and even the quantum yield could be the embedding of the material in a suitable photonic structure environment. This influences the local irradiance onto the upconverter and the local density of states at the transition wavelengths. Thus, the radiative recombination from a specific energy level can be influenced. Hence, this approach has the potential to beneficially influence the upconversion quantum yield. For the buried grating structure shown here, a luminescence enhancement by a factor of 1.85 could be achieved, averaged over the grating.
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 β-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.
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.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.