Cost-effective, fast, and non-destructive, on-site photovoltaic (PV) characterization methods are of interest to PV operators to determine countermeasures against defects causing power loss or against safety problems. Combining the advantages of both methods electroluminescence (EL) and thermography is photoluminescence (PL). With our PL setup, we achieved high resolution luminescence images of large area PV modules without any physical and electrical contact. Defects are recognizable with a high rate compared to indoor EL images at controlled conditions. We analyzed inactive areas, cracks, potential induced degradation, snail trails, EVA degradation, and interconnection failures and compared the PL images with different characterization methods.
Energy yield is a key metric for evaluating the performance of photovoltaic systems. It describes the total amount of energy generated by a photovoltaic (PV) installation over a given period, typically a year, and depends on physical properties of the solar cell like efficiency, band gap and temperature coefficient, as well as the operating conditions in a given location. Because the response of a solar cell to these conditions varies, two photovoltaic technologies may have a different energy yield, even if their lab efficiency is identical. Predicting energy yield accurately is important to system operators and installers to estimate the technical and economic performance of a PV installation. In this paper, we summarize our findings about satellite based energy yield predictions of solar cells with various technologies.
A large variety of optical systems incorporate multiple textured surfaces for reflectance reduction, light redirection or absorptance enhancement. One example for such a system is a textured silicon wafer solar cell. We introduce the OPTOS (Optical Properties of Textured Optical Sheets) formalism for the modelling of light propagation and absorption in optically thick sheets with two arbitrary surface textures at the front and rear side, and demonstrate applications.
In contrast to many optical simulation techniques, which are tailored to specific surface morphologies, the OPTOS formalism is a matrix-based method that allows including textures that are described by different optical modelling techniques (e.g. ray optical or wave optical) within one simulation tool. It offers the computationally efficient simulation of light redistribution and non-coherent propagation inside thick sheets. After calculating redistribution matrices for each individual surface texture with the most appropriate technique, optical properties of the complete textured sheet, like e. g. angle dependent reflectance, transmittance or depth resolved absorptance, can be determined via iterative matrix multiplications (for propagation and redistribution) with low computational effort.
In this work, we focus on textured wafer-based silicon solar cells as application examples for the OPTOS formalism. The simulation enables us to investigate and optimize combinations of front and rear textures on solar cells in order to increase the photocurrent generation. A solar cell with inverted pyramid front side and a diffractive grating at the rear is found to show similar light trapping properties as one with Lambertian scattering at the rear.
KEYWORDS: Air contamination, Light scattering, Scattering, Glasses, Interfaces, Solar cells, Thin film solar cells, Thin films, Silicon solar cells, Spatial frequencies
Optimization of light scattering by designing proper randomly textured surfaces is one of the important issues when
designing thin-film silicon solar cell structures. The wavelength region that needs to be scattered depends on the
absorber material and the thickness of the solar cell. The optimum morphology of the textured substrate can be defined
regarding the wavelength range intended for scattering. Good scattering is experimentally achieved by optimizing the
fabrication process of the randomly textured substrate. However, optimum morphological parameters have not been
analytically formulated. In this work we develop the morphological criteria for optimum light scattering in a-Si:H solar
cells using Aluminum Induced Texture (AIT) glass superstrates. Transmission haze is widely used as an evaluating
factor for scattering properties. Haze can be easily measured for the substrate/air interface. However, the relevant
scattering properties are those in the absorber material. These properties cannot be measured directly, but can be
predicted by an appropriate model. The simple model for haze calculation based on scalar scattering theory cannot
correctly estimate the haze value because it only considers the root mean square (RMS) roughness of the textured
surface, which does not contain information about lateral feature size. In addition, the opening angel of the haze
measurement is not considered in the equation. In this work, we demonstrate that the power spectral density (PSD)
function of the randomly textured surface can provide the missing information in the haze equation. A general
formulation for calculating the lateral feature size based on the PSD function is presented. We use this calculated haze
value based on PSD to find the optimum lateral feature size for scattering a specific wavelength into the desired material.
The optimum lateral feature size for scattering 620-nm light, which is weakly absorbed in a-Si:H, is shown to be 100 nm.
To achieve higher efficiencies in solar cells one possibility is to integrate angular selective filters, with the aim of
decreasing losses caused by radiative recombination. In fact, thermodynamically, angular selectivity is equivalent to
concentration. In both cases the Shockley-Queisser-Limit of solar cells is overcome by manipulating the ratio of
incoming and outgoing radiation represented by the angles of incidence and emission. In concentrating systems the angle
of incidence is increased, whereas in systems with an angular confinement the angle of emission can be decreased.
Another possibility to achieve highest efficiencies is to combine both, concentration and angular confinement. Starting
with a given concentrating system, photonic angularly selective filters such as thin film stacks are investigated and
optimized for the use in this system. We present results of wave optical simulations of these filters and show some of
their characteristics. The goal of this study is, however, not only to optimize optical filters but also to consider the whole
system. One approach is to use results from optical simulations as input values for detailed balance simulations of the
solar cell. So, the main advantage is, that in fact not the optical characteristics are optimized separately, but rather the
whole system is taken into account, which allows predictions of theoretical efficiency enhancement.
In conventional luminescent solar concentrators (LSC) incident light is absorbed by luminophores and emitted
isotropically. Most of the emitted light is trapped inside the LSC by total internal reflection and guided to solar
cells at the edges. Light emitted towards the surfaces, however, is lost in the escape cone. Furthermore, when
the luminophore emits light in its absorption range, light is lost due to reabsorption. To overcome these losses,
we embed the luminescent material in photonic structures to influence the emission characteristics. Directional
and spectral redistribution of emission is supposed to enhance the light guiding in LSCs and reduce reabsorption
losses. For this purpose, we prepared opal films from PMMA colloids, in which Rhodamine B was embedded
during the polymerization process. In direction-resolved luminescence measurements a strong dependence of the
emitted spectrum on the detection direction was observed. Further, the light collection efficiency of the samples
was determined with optical measurements and light guiding due to the intended absorption and emission process
was observed. The overall performance, however, suffers from cracks and defects in the photonic crystal.
KEYWORDS: Absorption, Scattering, Solar cells, External quantum efficiency, Light scattering, Silicon solar cells, Thin film solar cells, Silicon, Atomic force microscopy, Thin films
Random structures are typically used for light trapping in thin-film silicon solar cells. However, theoretically periodic
structures can outperform random structures in such applications. In this paper we compare random and periodic
structures of similar shape. Both types of structure are based on atomic force microscopy (AFM) scans of a sputtered and
etched ZnO layer. The absorption in a solar cell on both structures was calculated and compared to external quantum
efficiency (EQE) measurements of samples fabricated on the random texture. Measured and simulated currents were
found to be comparable. A scalar scattering approach was used to simulate random structures, the rigorous coupled wave
analysis (RCWA) to simulate periodic structures. The length and height of random and periodic structures were scaled
and changes in the photocurrent were investigated. A high height/length ratio seems beneficial for periodic and random
structures. Very high currents were found for random structures with very high roughness. For periodic structures,
current maxima were found for specific periods and heights. An optimized periodic structure had a period of Λ = 534 nm
and a depth of d = 277 nm. The photocurrent of this structure was increased by 1.6 mA/cm2 or 15% relative compared to
the initial (random) structure in the spectral range between 600 nm and 900 nm.
We present a process chain to generate periodically arranged metallic nanostructures supporting plasmons for the use in
solar cells. As proof-of-concept, platinum and silver nanoparticles with a period of 1 μm and a diameter of 600 nm were
fabricated. For the platinum particles, an absorption enhancement for light with a wavelength of 3.6 μm was observed in
silicon. By decreasing structure sizes the active spectral region can be shifted to wavelength relevant for solar cell
applications. Therefore, in a second step a silver grating with a smaller period and also smaller diameter of
approximately 200 nm was realized.
Benedikt Bläsi, Hubert Hauser, Christian Walk, Bernhard Michl, Aron Guttowski, Alexander Mellor, Jan Benick, Marius Peters, Sabrina Jüchter, Christine Wellens, Volker Kübler, Martin Hermle, Andreas Wolf
Since micro- and nanostructures for photon management are of increasing importance in novel high-efficiency solar cell
concepts, structuring techniques with up-scaling potential play a key role in their realization. Interference lithography
and nanoimprint processes are presented as technologies for origination and replication of fine-tailored photonic
structures on large areas.
At first, these structure origination and replication technologies are presented in detail: With the interference pattern of
two or more coherent waves, a wide variety of structures with feature sizes ranging from 100 nm to 100 μm can be
generated in photoresist by interference lithography. Examples are linear gratings, crossed gratings, hexagonal structures,
three dimensional photonic crystals or surface-relief diffusers. The strength of this technology is that homogeneous
structures can be originated on areas of up to 1.2 x 1.2 m2.
The structures in photoresist, the so-called master structures, can serve as an etching mask for a pattern transfer, as a
template for infiltration with different materials or they can be replicated via electroplating and subsequent replication
processes. Especially in combination with replication steps, the industrially feasible production of elaborate structures is
possible. As a particularly interesting process, nanoimprint lithography (NIL) is described in detail. As a way towards
industrial production, a roller NIL tool is presented.
After the description of the basic technologies, three application examples for solar cells are presented with details about
the design of the structures, the structuring processes, sample characterization and evaluation: (1) honeycomb structures
for the front side texturization of multicrystalline silicon wafer solar cells, (2) diffractive rear side gratings for absorption
enhancement in the spectral region near the band gap of silicon, and (3) plasmonic metal nanoparticle arrays
manufactured by combined imprint and lift off processes.
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.
Luminescent solar concentrators (LSC) concentrate both diffuse and direct radiation with no need for tracking.
They consist of transparent plates doped with luminescent materials that absorb incident light. Most of the
emitted light is trapped inside the plate by total internal reflection, where it is guided to solar cells at the
plate's edge faces. We investigate the concept of a photonic LSC (PLSC) that mitigates the major LSC loss
mechanisms, namely the escape cone and reabsorption of emitted light. Embedding the luminescent material in
a photonic crystal allows highly efficient light guiding and can reduce reabsorption through inhibited emission at
unwanted wavelengths. We present FDTD simulations that show how the emission characteristic is influenced
by the surrounding structure due to an altered photon density of states. Further, enhanced light guiding in a
broad spectral range was obtained with efficiencies of up to 99.7%. We also report on our progress in fabrication
of PLSC devices for experimental investigation of the concept: polymer thin films with and without luminescent
doping were spin coated and characterized to estimate the number of dye layers needed in PLSCs to achieve
sufficiently high absorption.
Light management in single and tandem solar cells is becoming increasingly important to optimize the optical
and electro-optical properties of solar cells. After a short introduction to state-of-the-art light management
approaches, different applications of photonic crystals for photon management in solar cells are reviewed
and discussed concerning their applicability. Results on direction- and energy-selective filters for ultra-light-trapping,
intermediate reflectors for optimal current matching in tandem cells, and photonic crystal coating
for fluorescence collectors will be presented and discussed.
Fluorescent concentrators concentrate both diffuse and direct radiation without requiring tracking of the sun. In
fluorescent concentrators, luminescent materials embedded in a transparent matrix absorb sunlight and emit radiation
with a different wavelength. Total internal reflection traps the emitted light and guides it to solar cells attached to the
concentrator's edges. The escape cone of total internal reflection, however, limits the light collection efficiency.
Spectrally selective photonic structures, which transmit light in the absorption range of the luminescent material and
reflect the emitted light, reduce these losses. In this paper, we review different realizations of such structures and show
that they increase collection efficiency by 20%. However, light emitted into steep angles in respect to the front surface,
which would be lost without the photonic structures, has a very long effective path inside the concentrator until it reaches
a solar cell. Therefore it suffers from path length dependent losses. We discuss how emission into the unfavorable
directions can be suppressed by integrating the luminescent material into photonic structures, thus reducing these losses.
We present possible realizations both for the concentrator design and for the solar cells used in such systems.
Diffractive effects have the potential to greatly increase light trapping in solar cells. The simulation of solar cells with
diffractive elements is, however, difficult. The reason for this difficulty is that wave optical considerations are required.
Typically, for solar cell simulations a ray tracing approach complemented by a transfer matrix algorithm is sufficient to
simulate the optical properties. In this paper we present a more fundamental method to consider wave optical effects for
solar cells. The optical characteristics of a solar cell are externally simulated using a wave optical approach and are used
as input parameters for a simulation of the electrical characteristics. This coupled method is tested on an exemplary
system; a solar cell with a backside diffractive grating. In a first step we show that substituting the optical parameters by
externally simulated ones is expedient. Furthermore we show that diffractive effects, which hitherto could not be
considered, can be made accessible for solar cell simulations. Additionally, the potential for a grating within a solar cell
was investigated, resulting in an increase of 1% efficiency absolute for a crystalline silicon solar cell.
An effective light trapping system is required in silicon solar cells in order to collect a large amount of photons. That is
why we focus our investigation on the fabrication and evaluation of two types of optical systems introduced on the back
side of solar cells. The aim of these structures is to enhance the light trapping of the long wavelength photons (above
1000 nm). On the one hand, we evaluated a Si/SiO2 linear nanograting; on the other hand, hexagonal nanostructures
fabricated with SiO2 nanoparticles and a filling matrix are under investigation. In this paper, we describe the fabrication
processes developed for both approaches and we present the solar cell results and characterisation. For the first approach,
we show a reflectance reduction on test structures, which occurs at the same wavelength as the increase of absorption
induced by the simulated gratings. Moreover, we demonstrate the feasibility of the fabrication of silicon solar cells with the
hexagonal nanostructures as a diffractive back reflector. Although no short circuit current increase has been observed due to
a poor rear side passivation, a current gain up to 0.3 mA/cm2 is possible in the wavelength range of 1050-1150 nm due to
these nanostructures. Finally, we also comment on the advantages and drawbacks of each approach and on the feasibility
to introduce these systems in the solar cell process flow.
The subject of this work is the development of an electrically conductive Rugate filter for photovoltaic applications. We
think that the optical as well as the electrical performance of the filter can be adapted especially to the requirements of
crystalline Si thin-film and amorphous/crystalline silicon tandem solar cells. We have deposited amorphous
hydrogenated Silicon Carbide layers (a-SixC1-x:H) with the precursor gases methane (CH4), silane (SiH4) and diborane
(B2H6) applying Plasma Enhanced Chemical Vapour Deposition (PECVD). Through changing just the precursor flows a
floating refractive index n from 1.9 to 3.5 (at 633 nm) could be achieved quite accurately. Different complex layer stacks
(up to 200 layers) with a sinusoidal refractive index variation normal to the incident light were deposited in just 80 min
on 100x100 mm2. Transmission measurements show good agreement between simulation and experiment which proofs
our ability to control the deposition process, the good knowledge of the optical behaviour of the different SiC single
layers and the advanced stage of our simulation model. The doped single layers show lateral conductivities which were
extremely dependent on the Si/C ratio.
Optical absorption losses limit the efficiency of thin-film solar cells. We demonstrate how to increase the absorption in
hydrogenated amorphous silicon solar cells by using a directionally selective optical multilayer filter covering the front
glass. The filter transmits perpendicularly incident photons in the wavelength range 350 nm - 770 nm. In the regime of
low absorptance, i.e. large optical absorption lengths, however, it blocks those photons impinging under oblique angles.
Thus, the incoming radiation is transmitted with almost no loss while the emitted radiation is mostly blocked due to its
wider angle distribution. We determine the enhancement in the optical path length from reflectivity measurements. In the
weakly absorbing high wavelength range (650 nm - 770 nm) we observe a peak optical path length enhancement of
κ ~ 3.5. The effective path length enhancement κ ~ calculated from the external quantum efficiency of the solar cell with
filter, however, peaks at a lower value of only κ ~ 1.5 in the same wavelength range. Parasitic absorption in the layers
adjacent to the photovoltaic absorber limit the increase in the effective light path enhancement. Nonetheless we
determine an increase of 0.2 mAcm-2 in the total short circuit current density.
We investigated a three dimensional inverted opal having the potential to notably increase light-trapping
in solar cells. The 3D photonic crystal top layer is an angle- and direction-selective filter, which decreases
the acceptance cone of the solar cell. Numerical optimisation methods are used to verify the optical and
electrical properties for a large angluar and energy spectrum for a system consisting of an inverted opal on
top of a thin crystalline silicon solar cell. It is numerically shown that an inverted opal grown in the Τ - Xdirection
might fulfill the requirement for such a filter. An estimate for the theoretically achievable efficiency
for nonconcentrated light is presented that do show an enchanced efficiency near the electronic band edge of
the absorber. The fabrication of first opals grown in Τ - Xdirection is presented and discussed with respect
to the quality and large scale fabrication.
The Yablonovitch limit for light trapping in solar cells with Lambertian surfaces can be increased using angle selective
absorbers thereby exploiting the limited incidence angle of solar radiation. We simulate the efficiency gain or loss caused
by an angular and energy selective filter on top of the absorber, compared to a Lambertian and a flat absorber.
Additionally, we introduce two possible implementations of such a filter, a Rugate stack and inverted opal layers.
Photonic structures can be used to eliminate the main loss mechanism in fluorescent concentrators. Simulation routines
have been established to investigate the optical characteristic of different photonic crystals. Especially two kinds of
structures with an appropriate characteristic have been examined closely. The first is the rugate filter, a one-dimensional
photonic structure. In the rugate filter the refractive index is varied sinusoidally over the thickness of the filter. The
second is the opal, a three-dimensional photonic crystals made of spheres that are arranged in a self organization process.
Filters from these structures have been designed and optimized for the application and fluorescent concentrators and
have been optimized. Additional aspects of the structures like angular effects have been examined.
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