Silver Contact Grid: Inferred Contact Resistivity and Cost Minimization in 19% Silicon Solar Cells

Abstract

The analysis of silicon solar cell contacts having an H-bar front grid pattern is extended by enabling the contact resistivity to be inferred from the measurement of total series resistance and the determination of six individual components of series resistance. Analysis of the contact system was completed for a representative 19% cell fabricated from a 156-mm pseudosquare p-Cz wafer using standard production processes, including phosphorus ion implantation, thermal oxide surface passivation, silicon nitride deposition, and screen-printing and firing of front silver gridlines and busbars, back silver soldering pads, and back aluminum contact. Gridline width was measured to be 80 μm after firing, with an average thickness of 7.4 μm and an effective resistivity of 4.7 μΩ·cm. Contact resistivity to the uniform 91 Ω/ <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\square$</tex></formula> emitter was inferred to be 5.3 mΩ·cm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{2}$</tex></formula> from the total series resistance and its components. Using these values, gridline spacing was optimized for maximum efficiency (1.7 mm, 91 lines, 19.1%, $0.040/W) or minimum silver cost (2.1 mm, 74 lines, 19.0%, $0.038/W). The analysis methodology was further applied to examine the impact of gridline width and silver pricing on cell efficiency and incremental cost. Such analysis can serve as a guideline for future contact system designs when the cost of silver changes or when printing technology allows narrower lines.