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.
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 progress of 3D photonic intermediate reflectors for micromorph silicon tandem cells towards a first prototype
cell is presented. Intermediate reflectors enhance the absorption of spectrally-selected light in the top cell
and decrease the current mismatch between both junctions. A numerical method to predict filter properties for
optimal current matching is presented. Our device is an inverted opal structure made of ZnO and fabricated
using self-organized nanoparticles and atomic layer deposition for conformal coating. In particular, the influence
of ZnO-doping and replicated cracks during drying of the opal is discussed with respect to conductivity
and optical properties. A first prototype is compared to a state-of-the-art reference cell.
The concept of a 3D photonic crystal structure as diffractive and spectrally selective intermediate filter within
'micromorphous' (a-Si/μc-Si) tandem solar cells has been investigated numerically and experimentally. Our device aims
for the enhancement of the optical pathway of incident light within the amorphous silicon top cell in its spectral region of
low absorption. From our previous simulations, we expect a significant improvement of the tandem cell efficiency of
about absolutely 1.3%. This increases the efficiency for a typical a-Si / μc-Si tandem cell from 11.1% to 12.4%, as a
result of the optical current-matching of the two junctions. We suggest as wavelength-selective optical element a 3D-structured
optical thin-film, prepared by self-organized artificial opal templates and replicated with atomic layer
deposition. The resulting samples are highly periodic thin-film inverted opals made of conducting and transparent zinc-oxide.
We describe the fabrication processes and compare experimental data on the optical properties in reflection and
transmission with our simulations and photonic band structure calculations.
We suggest three-dimensional photonic crystals as a direction selective filter for ultra-light trapping in solar
cells. 3D photonic crystals allow tailoring of the photonic stop gap in space and energy. We analyzed different
photonic crystal structures concerning their spectral and direction selective properties and defined two figures of
merit for our application: a quality factor and a transmission coefficient. By analyzing different experimentally
feasible 3D photonic crystals, we found that the inverted opal has the best properties. We verified the direction
selective properties of the inverted opal in the microwave spectral range and found a very good agreement
between experiment and simulation.
We suggest an energy selective and diffractive optical element as intermediate layer in thin-film tandem solar cells. By
adjusting the lattice constant of this photonic crystal, we fitted the optical properties to match a silicon tandem pair. Our
device enhances the pathway of incident light within an amorphous silicon top cell in its spectral region of low
absorption. In this spectral overlap region of the tandem-junction's quantum efficiencies, photons are being transferred
towards the amorphous cell, which leads to an increase in the short-circuit current of the limiting top cell. From our
simulations we expect a current increase of 1.44mA/cm2 for an - amorphous/microcrystalline - silicon tandem cell,
corresponding to improvement of the tandem's absolute efficiency of about 1.3%.
Photovoltaic tandem and triple solar cells are currently being developed and produced with reasonable efficiencies at high technological cost. The concept of spectrum splitting has been proposed with the advantage of compatibility to all types of cells. Although additional optical efforts are to be made, external photon management can be achieved to match different solar cell combinations no matter which band gaps involved or how the cells are connected. We present an experimental study comparing optical devices based on either interference or diffraction for tandem and triple cell configurations. Whereas diffractive media such as gratings suffer intrinsically from higher order diffraction losses, devices based on interference such as Bragg filter can yield a significant efficiency increase. For a triple cell configuration consisting of GaInP/GaInAs/GaSb, a net efficiency gain of more than 30% is shown in a solar cell simulator compared to the best cell in direct light.