We investigate light-scattering textures for the application in thin-film solar cells which consist of a random texture, as commonly applied in thin-film solar cells, that are superimposed with a two-dimensional grating structure. Those textures are called photonic random texture. A scalar optical model is applied to describe the light-scattering properties of those textures. With this model, we calculate the angular resolved light scattering into silicon in transmission at the front contact and for reflection at the back contact of a microcrystalline silicon solar cell. A quantity to describe the lighttrapping efficiency is derived and verified by rigorous diffraction theory. We show that this quantity is well suitable to predict the short-circuit current density in the light-trapping regime, where the absorptance is low. By varying the period, height and shape of the unit cell, we optimize the grating structure with respect to the total generated current density. The maximal predicted improvement in the spectral range from 600-900 nm is found to be about 3 mA/cm2 compared to the standard random texture and about 6 mA/cm2 compared to a flat solar cell.
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.
The scattering of light by the textured transparent conductive oxide (TCO) in thin-film silicon solar cells is
frequently described by transmission haze and angular intensity distribution (AID) at the interface between the TCO
and air. The scattering is expected to improve the light trapping and, therefore, the absorption of the solar cell. Using
these scattering properties as input parameters for the electrical modeling of thin-film solar cells leads to significant
deviations from the measurements for short circuit current densities. The major disadvantage of the AID measurement
at the TCO/air interface is that in real thin-film silicon solar cells the TCO/Si interface is relevant. We use a
model that is based on scalar scattering theory to calculate the scattering properties at the transition into air and into
silicon. The model takes into account the measured surface topography and the optical constants of the adjacent
media. For a series of μc-Si:H cells on ZnO:Al with different surface topographies, AID and the transmission haze
into a μc-Si:H half space are calculated. From these results, a quantity is derived that describes the scattering
efficiency. This quantity is compared to the short circuit current densities of μc-Si:H solar cells showing good
agreement. It will be shown that for artificially modified textures an increase in the short-circuit current density and
thus, the efficiency of thin-film silicon solar cells can be achieved.