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 introduce a model which allows for the description of scattering properties of randomly textured ZnO
films by evaluating a Fourier surface analysis. The interface is developed into a series of periodic gratings
with well defined diffraction angles. The scattering efficiency is assumed to be the Fourier transform of
the surface profile. This model is applied on different kinds of textures and compared with experimentally
obtained angularly resolved scattering. This Fourier model is extended to obtain the scattering properties
with both spatial and angular resolution which allows the study of the light scattering of individual surface
elements. The identification of structures which scatter light into larger angles is possible. The calculated
scattering properties show a good agreement to the experimentally obtained data. The results are essential
for the further improvement of surface texture to optimize light trapping in thin-film solar cells.
A 3D photonic intermediate reflector for textured micromorph silicon tandem solar cells has been investigated.
In thin-film silicon tandem solar cells consisting of amorphous and microcrystalline silicon with two junctions
of a-Si/c-Si, efficiency enhancements can be achieved by increasing the current density in the a-Si top cell
providing an optimized current matching at high current densities. For an ideal photon-management between
top and bottom cell, a spectrally-selective intermediate reflective layer (IRL) is necessary. We present the
first fully-integrated 3D photonic thin-film IRL device incorporated on a planar substrate. Using a ZnO
inverted opal structure the external quantum efficiency of the top cell in the spectral region of interest could
be enhanced. As an outlook we present the design and the preparation of a 3D self organized photonic crystal
structure in a textured micromorph tandem solar cell.
The realm of nanooptics is usually characterized by the interaction of light with structures having relevant feature sizes
much smaller than the wavelength. To model such problems, a large variety of methods exists. However, most of them
either require a periodic arrangement of a unit cell or can handle only single entities. But there exists a great variety of
functional devices which may have either a spatial extent much larger than the wavelength and which comprise structural
details with sizes in the order of a fraction of the wavelength or they may consist of an amorphous arrangement of
strongly scattering entities. Such structures require large scale simulations where the fine details are retained. In this
contribution we outline our latest research on such devices and detail the computational peculiarities we have to
overcome. Presenting several examples, we show how simulations support the physical understanding of these devices.
Examples are randomly textured surfaces used for solar cells, where guided modes excited in the light absorbing layers
strongly affect the solar cell efficiency, amorphous metamaterials and stochastically arranged nanoantennas. The usage
of computational experiments will be motivated by the unprecedented insight into the functionality of such components.
Proc. SPIE. 7002, Photonics for Solar Energy Systems II
KEYWORDS: Amorphous silicon, Thin films, Finite-difference time-domain method, Near field scanning optical microscopy, Thin film solar cells, Silicon solar cells, Near field, Zinc oxide, Near field optics, Absorption
Randomly textured zinc oxide surfaces with and without amorphous silicon deposited on top are studied by near-field
scanning optical microscopy. By virtue of a three dimensions it allows to access the local light intensity in the entire
spatial domain above the structures. Measurements are compared with large scale finite-difference time-domain
simulations. This study provides new insight into light trapping in thin-film silicon solar cells on a nanoscopic scale.
Light localization on the surface of the textured interface and a focusing of light by the structure further away are
observed as the key features characteristic for such surfaces.