A new generation of polycrystalline silicon thin film solar cells is currently being developed in laboratories, employing a combination of novel laser or electron beam based liquid phase crystallization (LPC) techniques and single side contacting systems. The lateral grain size of these polycrystalline cells is in the millimeter range at an absorber thickness of up to 10 μm. In this contribution we present a comparative simulation study of several 1D, 2D and 3D nano-optical designs for the substrate / illumination side interface to the several micrometer thick back contacted LPC silicon absorber material. The compared geometries comprise multilayer coatings, gratings with step and continuous profiles as well as combinations thereof. Using the transfer matrix method and a finite element method implementation to rigorously solve Maxwell’s equations, we discuss anti-reflection and scattering properties of the different front interface designs in view of the angular distribution of incident light.
KEYWORDS: Pulsed laser operation, Silicon, External quantum efficiency, Transmittance, Tandem solar cells, Air contamination, Solar cells, Solar energy, Picosecond phenomena, Thin film solar cells
We present results on direct pulsed laser interference texturing for the fabrication of diffraction gratings in ZnO:Al layers. Micro gratings of 20 micron diameter with a groove period of 860 nm have been written using single pulses of a 355 nm picosecond laser using a home-built two-beam interference setup. The groove depth depends on the local laser intensity, and reaches up to 120 nm. At too high pulse energies, the grooves vanish due to surface melting of the ZnO. The fast scanning stage and the high repetition rate laser of a laser scribe system have been used to write grating textures of several cm2 in ZnO:Al films with a surface coverage of about 80%. A typical laser written grating texture in a ZnO:Al film showed a haze value of about 9% at 700nm. The total transmission of the film was not lowered compared to the film before texturing, while the sheet resistance increased moderately by 15%. A-Si:H/μc-Si:H solar cells with laser textured ZnO:Al front contact layers so far reach an efficiency of 10% and current densities of 11.0 mA/cm2, and 11.2 mA/cm2 for top and bottom cell, respectively. This is an increase of 16% for the bottom cell current as compared to reference cells on planar ZnO:Al. The voltage of the laser textured cells is not reduced compared to the reference cell when slightly overlapping laser pulses of reduced pulse energy are applied. This method allows to write textures in ZnO:Al films that e.g. have been deposited with strongly varying deposition conditions, or cannot be texture etched in HCl. The method can be improved further by using 2D periodic patterns and optimizing the groove pitch, and may be applicable also to other solar cell technologies.
The benefit of achieving high electron mobilities in transparent conducting oxides (TCOs) is twofold: they first exhibit superior optical properties, especially in the NIR spectral range, and secondly their low resistivity enables the usage of thinner films. Remarkably high mobilities can be obtained in Al-doped zinc oxide by post-deposition annealing under a protective layer. The procedure has not only shown to increase mobility, but also strongly reduces sub-bandgap absorption. Extensive optical, electrical and structural characterization is carried out in the films in order to clarify the microscopic origins of the changes in material properties. While the annealing of defect states, most likely deep acceptors, seems clear, earlier results also suggest some influence of grain boundaries. Tailing, on the contrary, seems to be linked to extended defects. In application to a-Si:H/μc-Si:H thin film solar cells the films have already shown to increase spectral response. When reducing the film thickness, the main challenge is to provide a suitable light trapping scheme. Normally this is achieved by a wet chemical etching step in diluted HCl, which provides a surface structure with suitable light scattering properties. Therefore a TCO-independent light scattering approach using textures glass was applied in conjunction with the high mobility zinc oxide. The substrate enables the use of very thin TCO layers with a strongly reduced parasitic absorption.
TCO films are crucial components of almost all thin-film solar cells and a-Si:H/c-Si heterojunction solar cells. As they are used as front contacts, the requirements for electrical conductivity and optical trnamission are generally very high. Further restrictions are imposed onto the deposition process by the cell manufacturing process, in which e.g. the maximum substrate temperature can be limited. In this paper the optimization of ZnO:Al layer deposited by magnetron sputtering to different solar cells is discussed. For a-Si:H/c-Si heterojunction solar cells the advantages and limitations of different variations of magnetron sputtering of ZnO:Al are discussed and compared to standard ITO deposition. For a-Si:H/μc-Si:H the usage of post-deposition treatments to improve the optical and electrical performance is briefly discussed.
Optical methods are powerful and non-destructive means to characterize highly doped transparent conducting oxide thin
films. In order to describe the optical properties of high-mobility ZnO films we present a dielectric function composed of
different analytic expressions to describe the different contributions to the dielectric function of the films. This allows for
the correct description of measured optical spectra and reduces the complex functions to a set of fitting parameters. In a
second step we compare the obtained parameters to theoretical models. The basic theories are nicely reproduced and the
basic link between optical and electrical properties can be understood. The findings can help on the route to a complete
presiction of optical properties from the basic material properties or vice versa.
KEYWORDS: Monte Carlo methods, Silicon, Absorption, Transparent conductors, Interfaces, Solar cells, Glasses, Chemical elements, Silicon films, Optical simulations
We apply a hybrid finite element / transfer matrix solver to calculate generation rate spectra of thin film
silicon solar cells with textured interfaces. Our focus lies on interfaces with statistical rough textures. Due to
limited computational domain size the treatment of such textures requires a Monte Carlo sampling of texture
representations to obtain a statistical average of integral target quantities. This contribution discusses our choice
of synthetic rough interface generation, the Monte Carlo sampling and the need for an incorporation of the cell's
substrate into optical simulation when illumination of the cell happens through the substrate. We present results
of the numerical characterization and generation rates for a single junction cell layout.
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