Thin–film silicon tandem solar cells consist of an amorphous silicon top cell and a microcrystalline silicon bottom cell stacked in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is essential. In this regard, we present the conceptual design and optical simulations of an intermediate reflector consisting of a stack of microcrystalline silicon oxide layers of different, alternating refractive indices. In contrast to 1–layer intermediate reflectors, the spectral and directional selectivity of these intermediate reflectors result in a gain for the top cell current while simultaneously increasing the charge carrier generation in the bottom cell.
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.
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.
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.
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.
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.
We present the optimization of coatings that can be applied on top of solar cells. The purpose of these coatings
is to reduce the light acceptance cone only in the long wavelength range. This allows for high transmission of
light into the solar cell independent on the angle of incidence in the short wavelength range where the solar cell
material shows high absorption. Furthermore, it allows for enhanced absorption of the direct sunlight in the long
wavelength range by trapping such light using total internal reflection. The coating for this purpose is based on
a combination of various design strategies with well defined impacts on the reflection spectra. It provides a set
of free parameters which describe the index profile of the filter. Realistic numerical procedures for calculating
efficiencies for crystalline silicon solar cells are used to optimize the free parameters of the coating. We show
that these filters can lift the efficiency of 10μm thick solar cells up to 30.1%. This is a factor of 1.05 above the
≈ 28% limit found for unconcentrated illumination by Kerr et al.