Full-Wafer Roller-NIL Processes for Silicon Solar Cell Texturisation

The highest solar cell efficiencies both for c-Si and mc-Si were reached using template based texturing processes. Especially for mc-Si the benefit of a defined texture, the so called honeycomb texture, was demonstrated impressively. However, up until now, no industrially feasible process has been available to pattern the necessary etching masks with the sufficient resolution. Roller-Nanoimprint Lithography (Roller-NIL) has the potential to overcome these limitations and to allow high quality pattern transfers, even in the sub-micron regime, in continuous in-line processes. Therefore, this etch-mask patterning technique is a suitable solution to bring such elaborate features like the honeycomb texture to an industrial realization. Beyond that, this fast printing-like technology opens up new possibilities to introduce promising concepts like photonic structures into solar cells

The moth-eye effect - from fundamentals to commercial exploitation

The historical developments of artificial "moth-eye" structures as anti-reflective surfaces are described. In theory, "moth-eye" structures are regarded as zero-order diffraction gratings as the relation of the grating period to the wavelength is such that only zero-order diffracted waves can propagate. The optical properties of the grating can be described by effective medium theories. It is shown that propagation of zero-order diffracted waves alone is very difficult to achieve for visible wavelengths in general due to limitations concerning the manufacturability on large areas. Therefore, nonideal anti-reflective moth-eye structures had to be optimized with respect to the grating periods depending on the grating type and of course on the specific application. The results of the optimization methods are presented. Then the manufacturing methods especially suited for the origination and replication on large areas are described in detail. Finally, the applications of "moth-eye" structures and developments beyond the mimicry of nature are presented

Impact of Front Side Pyramid Size on the Light Trapping Performance of Wafer Based Silicon Solar Cells and Modules

Smaller pyramid sizes for Silicon solar cell front side textures attract more and more interest. At the same time a very good optical performance of the front side texture is crucial to achieve high PV module efficiencies. In this paper an optical study of the impact of periodically arranged front side pyramids on the useful absorption in a Silicon solar cell is presented. Also a way to account for random pyramids is described. Results for solar cells facing semi-infinite EVA/glass are compared to the modeling results of a full solar module stack with the help of the OPTOS formalism. We found that the impact of different pyramids differs depending on the quality of the planar rear side reflector. It is shown that the impact of the module case is crucial and that it has to be accounted for when predicting the optimal pyramid sizes that are relevant for cell and module manufacturers. We also demonstrate that the use of simple parameters such as a single pass light path enhancement factor for the structured surface can lead to wrong conclusions, and a full modeling is required to predict the real module performance. The results indicate that the overall optical performance of a solar module does almost not vary for pyramid sizes above 600 nm

Theoretical study of pyramid sizes and scattering effects in silicon photovoltaic module stacks

Front side pyramids are the industrial standard for wafer based monocrystalline silicon solar cells. These pyramids fulfill two tasks: They act as anti-reflective structure on the one hand and as a light-trapping structure on the other hand. In recent development smaller pyramids with sizes below 1 µm attract more and more interest. In this paper an optical analysis of periodically arranged front side pyramids is performed. The impact on the reflectance as well as on the useful absorption within the solar cell is investigated depending on the pyramids size, the amount of additional scattering in the system and the quality of the rear side reflector. In contrast to other investigations not only the solar cell, but the full photovoltaic (PV) module stack is considered. This can strongly influence results, as we show in this paper. The results indicate that in a PV module stack with realistic assumptions for the amount of scattering as well as for the rear side reflectance only small differences for pyramids with sizes above 600 nm occur. Preliminary conclusions for random pyramids deduced from these results for periodically arranged pyramids indicate that these differences could become even smaller