Industrial + Specialty Printing

(July 2011) posted on Tue Sep 20, 2011

Keeping up with technology changes and controlling the cost along the way is strategically important in the world of PV manufacturing.

By Lars Wende

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Over the past decade, three screen-printing steps have been used repeatedly in the metallization process of solar cells. The most commonly used manufacturing line configuration is Back-Back-Front (BBF), where the first print step is the silver-bus-bar print on the wafer’s backside, followed by the Al-BSF (Aluminium Back Surface Field), then after flipping the cell, the front-side silver print is applied to form the Ag bus bar and Ag finger grid of the solar cell.

The main advantage of this print order is that the front side of the cell is sunny side up following the last print step; thus, after the subsequent fast-firing step, the cell can be tested in a solar-cell tester directly inline without an additional flip step required. However, there is an inherent risk that dirt and other contaminants, such as silicon debris, can collect on the cell’s sensitive front side during the preceding print steps on the backside, at which time the cell is top down. In such a line configuration, and as long as standard, low-cost screens are being used for all the screen print steps, no higher cost of consumables can be expected.

While a single print step on the front side will always result in similar heights of the Ag bus bar and the Ag finger grid, newer processes now allow the cell manufacturers to print the finger grid higher than the bus bars. Typically, a bus bar height of 10 μm or less is sufficient for the subsequent soldering process in module manufacturing. Solar-cell manufacturers now have the challenge to reduce the finger width while increasing the height of the finger to 25 μm or more to ensure a sufficient conductive path.

Splitting the print step on the front side of the cell into two separate steps can save more than 260 kg of silver paste per 40-MW cell production line per year, which at a silver cost of €800/kg, compares to an annual cost savings of more than €250,000/ production line. At the same time, the smaller finger widths and the taller finger heights of the grid result in increased cell efficiency due to reduced shading of the cell area covered by the finger grid. An associated efficiency increase of 0.25% (absolute) compares to an annual revenue increase of more than €470,000/40-MW line at an average selling price of €0.77 /Watt for 156-mm2-sized cells.