In their prototype production line, scientists at the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) have successfully completed a multi-crystalline solar cell with rear side contacts. The Freiburg, Germany-based researchers will be presenting their findings in September at the 22nd European Photovoltaic Solar Energy Conference and Exhibition, September 3-7, in Milan, Italy.

"The additional expense is most definitely worth it because the minimization of metallization on the front side enables us to obtain reduced shading and thus increased energy production, which leads to a significant increase in efficiency."

In comparison to the production of standard solar cells, there are three process stages of Metal Wrap Through (MWT) solar cells, which partially shift the front side contacts over to the rear side, thus reducing the front side metallization almost in half.

First, a laser is used to drill holes into the cells. The through-connection of the cells is simultaneously achieved through the subsequent silkscreen printing process for the production of the rear side contacts. To this effect, the silk-screen printing paste is used to fill the recently created holes, thus establishing the electrical connection to the front side. Contrary to typical processes, the isolation of the contacts reduces additional costs. The rear side contact of the MWT solar cells demands a small modification of the standard process.

According to Dr. Ralf Preu, Head of the PV Production Technology and Quality Assurance Department at the Fraunhofer ISE, "The additional expense is most definitely worth it because the minimization of metallization on the front side enables us to obtain reduced shading and thus increased energy production, which leads to a significant increase in efficiency."

The first MWT cells from the prototype production line obtained more than 16% efficiency. This is an increase of up to a half percent over comparable standard silkscreen printed solar cells made up of the same multi-crystalline material.

"Because we no longer have a problem with front side shading, it is now possible to use wider cell connectors. As a consequence, we obtain substantially lower losses during energy conduction," explained Dr. Harry Wirth, Head of the Photovoltaics Modules Group. "At the same time, we can now place the cells more closely together because the cell connector no longer has to be conducted from one side to the other."