Greater photovoltaic efficiency may require a step back.

Moving contact points to the back of the cell isn’t the only area where efficiencies can be gained with metal wrap through (MWT): Combining multiple available technologies could improve conversion efficiency exponentially. For example, print-on-print processes are also viable with MWT cells, making the collection fingers taller and narrower and achieving similar improvements in cell efficiency. Add to this selective emitter technology (reference previous column, “Which Way for Selective Emitter,” January 2011, CIRCUITS ASSEMBLY), which provides for different levels of phosphorus doping underneath and between the collector fingers, and the MWT cell becomes even more appealing from an efficiency point of view. I’m not suggesting that merging all these available technologies would be an easy feat, as this would be challenging. But, with highly accurate and repeatable metallization systems capable of precise deposition (exactly on the highly doped collector areas for selective emitter and then again for print-on-print), this is certainly achievable.

I must admit that I believe MWT is really an intermediate stage on the way to emitter wrap through (EWT), which is more difficult but more rewarding technically. Conceptually, EWT is similar to MWT in that there are multiple through holes in the wafer, and cell connection occurs on the backside only. But, with EWT, there are many, many times more holes in the cell than with MWT. To give a sense for how many holes there are, I recently went to an institute in Germany where I saw an EWT wafer with 60,000 holes in it – all done in the span of about 2.5 sec. on a laser machine! With EWT, the phosphorus doping is not only done on the top side, it is also done through the holes as well. So, similar to other cell types, the bulk layer of the EWT cell is still p-type silicon, and then the phosphorus, or n-type, is on the top surface and through the holes. Then, all that is required is printing of the two different metallizations; silver for the n-type and aluminum for the p-type.

The rear side of an EWT cell has two sets of gridlines. One set is for the silver, which makes contact with the wrapped through emitter, and the second set is for the aluminum, which makes contact with the p-type bulk. Paste deposition accuracy is critical, as space between the gridlines is minimal and any cross-contamination would result in a local short-circuit, thus reducing overall efficiency. Because the phosphorus doping of the hole provides the conductivity, no metal is required on the top surface of the cell, leaving all of the area unoccupied by holes available for capturing sunlight. As one can imagine, this has a profound impact on conversion efficiency, with EWT cells producing efficiencies in excess of 19% for screen-printed EWT cells. Medium-term roadmaps forecast over 20% cell efficiency for rear-passivated EWT cells.

The winning back contact technology remains to be determined. While I’m of the opinion that ultimately EWT will win out, it is a bit harder to produce at the moment, and MWT is further down the line in terms of its development and ability to be easily implemented from a manufacturing point of view. Time will tell. What is clear, however, is that whether it’s traditional front side cells or backside cells, high wph metallization systems will be required to accommodate the accuracy and speed needed for high-volume, lower cost production.

References

1. “SunPower Announces New World Record Solar Cell Efficiency,” June 23, 2010, www.sunpowercorp.com.
2. T. Adcock and A. Henckens, “Lower Cost, Greater Efficiency Drive Development of Back-contact Solar Modules,” PV World, May/June 2011.

Tom Falcon is a senior process development specialist at DEK Solar (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .