Eckard Wefringhaus

Influence of Surface Topography on the Glass Coverage in the Contact Formation of Silver Screen-Printed Si Solar Cells

For screen-printed Ag-paste metallization, the growth of Ag crystallites into Si is essential for the current conduction from the Si emitter to the silver finger. There are strong indications that for low contact resistances, some of these Ag crystallites need to be in direct contact with the silver finger without a separating glass layer in between. In this paper, we concentrate on the origin of Ag crystallites grown into Si in direct contact with the silver finger. On textured surfaces, we vary the Si pyramid sizes, round the pyramid tips to varying degrees, and fabricate flat smooth surfaces. We observe that the size of the pyramids does not play an important role in the achievement of low specific contact resistivity unless the pyramid heights become smaller than the thickness of the glass layer, but rounding of the pyramid tips with standard heights increases specific contact resistivity significantly. From our microscopic investigations, we conclude that the largest influence on the topography-dependent contact resistance comes from the glass coverage governing the amount of Ag crystallites directly connected with the silver finger bulk. Furthermore, Ag crystallites in direct contact with the silver finger are also observed on c-Si without n+ emitter doping.

Detailed Analysis of Random Pyramid Surfaces With Ray Tracing and Image Processing

For the optimization of alkaline texturing in industrial monocrystalline silicon solar cells, it is of paramount importance to understand the correlation between surface reflectance and surface morphology. In this study, the reflectance is related to the surface geometry using image processing and ray tracing. Surfaces with a variation of random pyramid texturing were scanned with laser scanning microscopy. The resulting height maps were either directly used as input for the ray tracing or used as a template for a synthetic height map. A procedure was developed to extract an effective pyramid base angle from the images. It takes on values from 47.9° to 54.3°, which are below the {111} pyramid base angle of 54.74°. The reflectance decreases linearly with the effective base angle and the pyramid height. Ray tracing of as-measured and reconstructed height maps with ideal pyramid base angle shows that both are a poor representation of the pyramid landscape. The best fit between measured and simulated reflectance is obtained when the effective base angle is used to reconstruct the surface. The surface with lowest reflectance is characterized by pyramids with high base angle and large size.

Impact of Fe and Cu Surface Contamination on High Efficiency Solar Cell Processes

In wafer-based silicon solar cell technology cleaning of silicon surfaces has so far been treated without much interest – the cheapest, simple solution, which means mostly rinsing with or dipping into diluted acids (HCl, HF), was the best. Whatever worked was rarely further investigated. Hence, not much work has been published in the field of contamination control and cleaning process optimization and development. With solar cell efficiencies increasing and processes becoming more and more sensitive, the interest in quality control and optimization of cleaning processes is rising.