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c-Si Metallization Paste Rheology and Print Metrology Techniques for Achieving and Quantifying Improved High Aspect Ratio Finger Topography
R. Lathrop, B. Smith
c-Si, Front Contact, High Efficiency, Aspect Ratio, Metallization, Metallisation
Wafer-Based Silicon Solar Cells and Materials Technology
Subtopic: Manufacturing Issues and Processing
Event: 27th European Photovoltaic Solar Energy Conference and Exhibition
Session: 2AV.5.31
1145 - 1149
ISBN: 3-936338-28-0
Paper DOI: 10.4229/27thEUPVSEC2012-2AV.5.31
0,00 EUR
Document(s): paper


For crystalline silicon applications, one method for increasing cell efficiency is to reduce shadowing by printing narrower lines. To maintain low line resistance, a higher fired contact finger height is the general approach. High aspect ratio, defined as the ratio of the line height to the line width, can be achieved by front contact paste formulation, the imaging tool (screen/stencil) and printing parameters. However, simply measuring aspect ratio can be misleading. The regularity or smoothness of the line geometry over the relatively long distance of the contact fingers is of equal importance. Mesh marks and/or peaks and valleys can be a typical byproduct of high viscosity paste. These conductor lines, while yielding high average aspect ratio, can effectively be a series of high and low spots which act as resistors in extreme cases defeating the benefits of the reduced shadowing. Furthermore, the peaks do not enhance the conductivity of the line, but only serve to increase silver usage. The best solution is to maximize conductivity and simultaneously minimize material usage. This can be achieved by creating very smooth, very narrow lines. These properties are typically controlled by the rheology of the paste. Paste suppliers provide viscosity data measured at a single point using a specific technique. Front contact pastes are very viscous in order to print narrow lines, however, this usually conflicts with the leveling properties necessary to achieve smooth lines. This paper will look at the entire viscosity profile and demonstrate how the rheology can be engineered to produce tall, narrow lines with improved topography. The anti-reflective coating typical on today’s PV solar cells is optimized to minimize the reflectance of the incident sun light. This leads to significant challenges for optical metrology systems that are based on the typical light sources, i.e. halogen, xenon or red lasers, as the majority of the incident spectrum is absorbed. Any resulting signal reflected back into the sensor is too weak to provide a sufficient signal-to-noise ratio for metrology. This paper will cover the 3D measurement techniques required to quantify line aspect ratio smoothness using a wavelength optimized confocal system tuned to the anti-reflective coatings typical of today’s crystalline silicon cells.