Non-volatile free silver paste formulation for front-side metallization of silicon solar cells

Significance 

Climatic changes are pushing world economies away from fossil powered energy production techniques to greener power generation technologies, such as: solar energy, wind energy, geothermal energy and biomass. Today, photovoltaics (PV) i.e. solar cells, is the cheapest energy source available and can basically run for long periods of time maintenance free. Over 90% of the global PV installations are based on silicon wafer technology; metallization and interconnection of cells is achieved through screen printing of silver pastes. Currently, global solar cell production consumes about 7.5% of the global silver output which might become a bottleneck for the highly demanded further strong growth of PV technology on the way to a sustainable energy supply. Technically, typical commercial formulations of screen-printing pastes comprise a mixture of organic solvent containing non-volatile organic binders and additives to control flow properties and to strengthen the adhesion of printed electrodes on the substrate prior to sintering.  Residual carbon from non-volatile additives, however, deteriorate electrical conductivity and prevent further reduction of silver consumption per wafer and enhancement of cell efficiency essential for an expansion of PV.

Development of the screen-printing paste with a focus on silver utilization reduction would come a long way in assisting reduction of metallization costs. In this view, researchers from the Karlsruhe Institute of Technology: Ceren Yüce and Professor Norbert Willenbacher in collaboration with Kuninori Okamoto at the Changzhou Fusion New Material Company, Lindsey Karpowich at the Heraeus Precious Metals North America Conshohocken LLC (Photovoltaic Global Business Unit), and Adrian Adrian at the  International Solar Energy Research Center in Germany proposed a new paste formulation concept coupled with different printing technologies: the screen-printing method with conventional standard screen as well as knotless screen, and the PTP method, for ultra-fine front-side metallization. Their goal was to demonstrate a non-volatile free silver paste formulation based on the capillary suspension concept. Their work is currently published in the research journal, Solar Energy Materials and Solar Cells.

In brief, the research team prepared pastes having different compositions and determined rheological parameters relevant for paste characterization in accordance with specifications from a preceding study. Si-wafers were then successfully metallized with such pastes using conventional screen-printing, knotless screen and Pattern Transfer Printing™. The researchers studied paste spreading via high-speed imaging during screen-printing on glass plates. Morphology of printed lines was analyzed using laser scanning microscopy. Finally, electrical properties of the cells were characterized by employing a solar simulator and electroluminescence spectroscopy.

The authors compared their results with those obtained using commercial pastes including the same silver particles and glass frits. They reported that paste performance strongly depended on the selected secondary fluid. Aspect ratios of about 0.4 – 0.5 could be reached and cell efficiencies of about 21% on Cz- and 18.6% on mc Si-wafers were obtained. Altogether, their paste compared well with commercial pastes and could provide extended shelf-life and distinct flow properties adjustable in a wide range as demanded by the respective printing process, hence yielding residual-free sintered electrodes.

In summary, the study by Professor Norbert Willenbacher and his colleagues demonstrated a versatile, cost-effective formulation platform for highly conductive silver pastes used in front-side metallization of silicon solar cells. Remarkably, electrode morphologies and electrical cell performance data were similar to those obtained with conventional silver pastes. Overall, the results of the study proved that the additive-free paste formulation concept based on the capillary suspension phenomenon is applicable in standard screen, knotless screen as well as PTP technologies and offers opportunities to reduce the silver consumption per wafer significantly.

Reference

Ceren Yüce, Kuninori Okamoto, Lindsey Karpowich, Adrian Adrian, Norbert Willenbacher. Non-volatile free silver paste formulation for front-side metallization of silicon solar cells. Solar Energy Materials and Solar Cells, volume 200 (2019) page 110040.

Go To Solar Energy Materials and Solar Cells

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