The screen printed 'fingers' and busbars of a solar cell. Light is absorbed and causes charge carriers to flow along the fingers to the busbars, which serve as endpoints for the soldered connections between the cells within a module.
Common screen printing technology results in fingers which are about 135 μm wide and have an electrical resistance of around 180 mΩ/cm. The best, but unaffordable methods achieve 20 μm and 1 Ω/cm.

"Metallization is well in hand, or so the thinking went. Sufficiently understood, predictable, hard to improve. But these processes are drawing renewed attention now," says Jaap Hoornstra of ECN Solar Energy. "Because large gains can still be made."

Most solar cells are made of silicon. Chemical treatments separate the material into layers, usually an n-type emitter on a p-type base. This creates a diode with the rear side of the cell as its positive terminal, the front being negative. Metallization is needed to provide low-resistance conductors between the cells within a module (solar panel) and the external load.
Where money is no object – like in the space industry – excellent results have been achieved for decades. But when aiming at an alternative for power stations which burn coal or natural gas all existing solutions are too expensive, because the amount of energy produced is so very small; a modern solar cell measuring 156 by 156 millimeter will supply only about 2.6 kilowatt-hour a year, at our latitude. The industrial society is used to kWh which cost much less than ten eurocents to produce. So during its entire life span of about 25 years, a solar cell will earn no more than a few euros.

Silver and glass
Much emphasis must therefore be put on development of faster and cheaper production methods, like screen printing. The desired pattern is transferred using a photographic emulsion on a fine wire mesh; the screen. A printing machine then forces a paste of silver particles and glass frit through the open areas in the pattern, on to the solar cell. Finally the cell is fired to melt the glass and sinter the silver, resulting in good contact between metal and silicon. About 85 percent of the current production lines use a process of this kind.
Screen printing is cheap, but reduces the efficiency of a solar cell by 3.5 to 4 percent when compared with the best (and too expensive) methods. ECN's research is aimed at possible ways to reduce that difference, without losing the price advantage. One approach is to Improve the print resolution.

Aspect ratio
Ideally, the total area of a solar cell is available to catch light. The silver tracks on the front side must therefore be very narrow, yet have a low electrical resistance. "Industrial screen printing demands a high processing speed, which means that the paste must flow rather easily," says Hoornstra. "Otherwise the mesh becomes too much of an obstacle." The disadvantage is that the silver tracks sag out of shape as soon as the screen is pulled away. Seen in cross section, they look like flattish droplets.
Whether a track is wide and flat or high and narrow, the electrical resistance remains the same. But wide tracks cover a larger part of the surface area. Together with solar cell manufacturer Solland, Hoornstra went looking for solutions. Better printing technology is one option. "A metal stencil instead of an emulsion on a wire mesh. It allows us to reduce the width by almost half, and gives us a lower resistance as a bonus."
In recent tests, the aspect ratio (height / width) of a stencil printed silver track was increased to 0.37, much higher than the 0.13 achieved by a standard screen print. This increased the overall efficiency of the cell from 16.2 to 16.6 percent.

The graph shows the result of a normal screen print (black) in comparison with the much higher and narrower track achieved by stencil printing when using an optimized paste.
The photo shows that a screen (left) is more of an obstruction to the flow of the paste than the stencil next to it.

SunLab
The remaining loss of efficiency is mostly caused by the interaction between the emitter, the paste and the melting process. A relatively deep emitter is needed to enable good contact, while ensuring that the mixture of glass and silver doesn't melt through, causing shunts. However, that requires a low concentration of the substance (phosphorus) which gives the emitter its n-type properties. Otherwise the efficiency suffers, partly because the solar cell becomes less sensitive to light at the blue, energy-rich end of the spectrum. But a good electrical contact between the emitter and the printed metal track can only be achieved with a rather high phosphorus concentration.
Hoornstra: "It boils down to a good emitter and good contact not going well together. Something may well be done about that; in recent years, much has been achieved with different paste mixtures. Better control over the temperature and the speed of the melt and sinter process may also help quite a bit. Our group is working on processes with improved emitters (a joined project with Tempress BV), and on instruments to measure various properties of the results. One spin-off is selling such equipment to the industry, through ECN subsidiary SunLab BV."
Another promising development is screen printing in two stages. The first makes use of a paste optimized for good contact. A second layer printed on top of the first takes care of reducing the track's resistance and increases the aspect ratio. "And the second printing machine is so cheap, that the total costs are left almost unchanged. That makes double screen printing almost as good as the stencil method, at least for the time being."

Synergy
Hoornstra expects near-term progress to arise mostly from the integrated development of metallization and other processing steps, and from solar cells designed together with the method for combining them into modules. "Metal-wrap-through, taking the emitter contacts down metallized holes in the cell to the rear side was a real breakthrough," he explains. "It reduced the frontal area covered by silver tracks, and allowed us to do away with the soldered joints between cells within a module. The cells are simply glued to a backplane using electrically conducting glue, patterned to make the required connections; a much faster and cheaper method. We are pretty good at realizing such ideas; ECN was the first institute of its kind to have its own module group."

Contact
Jaap Hoornstra
ECN Solar Energy
Tel.: +31 (0)22 456 46 97
Email: Jaap Hoornstra  

Text: Steven Bolt

Info
Stencil print applications and progress for crystalline silicon solar cells an ECN-report
Website of 2nd Workshop on Metallization for Crystalline Silicon Solar Cells  
SunLab, meetinstrumenten voor zonneceltechnologie

This ECN Newsletter article may be published without permission provided reference is made to the source: www.ecn.nl/nl/nieuws/newsletter-en/