Sebastian Neubert

Hybrid Organic/Inorganic Thin-Film Multijunction Solar Cells Exceeding 11% Power Conversion Efficiency

Hybrid multijunction solar cells comprising hydrogenated amorphous silicon and an organic bulk heterojunction are presented, reaching 11.7% power conversion efficiency. The benefits of merging inorganic and organic subcells are pointed out, the optimization of the cells, including optical modeling predictions and tuning of the recombination contact are described, and an outlook of this technique is given.

Quadruple-junction solar cells and modules based on amorphous and microcrystalline silicon with high stable efficiencies

Quadruple junction solar cells and modules are presented, which consist of hydrogenated amorphous (a-Si:H) and microcrystalline silicon (µc-Si:H) in the a-Si:H/a-Si:H/µc-Si:H/µc-Si:H configuration. The highest measured conversion efficiency of a mini-module with an aperture area of 61.44 cm2 was 13.4% before and 12.0% after more than 1000 h of light soaking, respectively. In this paper, we discuss the advantages of the quadruple junction design over the common tandem design, which is ascribed mainly to the fact that the total absorber thickness can be increased while electronic properties and stability are maintained or even improved. The role of the µc-SiOx:H intermediate reflector is highlighted and an optimization of the doping concentration in this layer is presented. Furthermore, the advantage of the high maximum power voltage for the monolithic cell interconnection laser design of modules is shown.

Implications of TCO Topography on Intermediate Reflector Design for a-Si/μc-Si Tandem Solar Cells—Experiments and Rigorous Optical Simulations

The influence of the transparent conducting oxide (TCO) topography was studied on the performance of a silicon oxide intermediate reflector layer (IRL) in a-Si/μc-Si tandem cells, both experimentally and by 3-D optical simulations. Therefore, cells with varying IRL thickness were deposited on three different types of TCOs. Clear differences were observed regarding the performance of the IRL as well as its ideal thickness, both experimentally and in the simulations. Optical modeling suggests that a small autocorrelation length is essential for a good performance. Design rules for both the TCO topography and the IRL thickness can be derived from this interplay.

ZnO:Al with tuned properties for photovoltaic applications: thin layers and high mobility material

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.

Material properties of high-mobility TCOs and application to solar cells

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.

Direct pulsed laser interference texturing for light trapping in a-Si:H/μc-Si:H tandem 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.

Detailed comparison of transparent front contacts for thin film silicon solar cells

In this contribution we compare the optical, electronic and surface morphological properties of various textured transparent conducting oxides (TCO) based on zinc oxide and fluorine doped tin oxide. Since the TCO material properties tend to be interrelated, comparison of such films is a rather complex exercise. The TCO films were characterised by atomic force microscopy, scanning electron microscopy, spectrophotometry, angle resolved scattering, four point probe and Hall Effect measurements. Thereafter, an attempt was made to correlate the TCO material properties with the corresponding current-voltage characteristics and quantum efficiency in actual solar cells. It was found that the solar cells were more sensitive to changes in the optical properties of the TCO substrates than to those in the sheet resistance. Based on these results, we recommend that when optimising TCO films for thin film silicon production, the first priority should be to obtain the highest transmittance possible and then to tune the electrical properties accordingly.