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nanotimes News in Brief

11-04 :: April/May 2011

Solar Cells // New Silicon Solar Cell Concepts for Improved Efficiency

Solar Energy Systems ISE in Freiburg, researchers have been pressing ahead to explore new concepts for several years now. For back-contact silicon solar cells, efficiencies up to 20.2% have been achieved.

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“We are very pleased to present these excellent results upon the fifth anniversary of PV-TEC. The re- sults demonstrate, that silicon photovoltaics is still in the steep part of the learning curve,” says Dr. Daniel Biro, project leader.

For the so-called MWT-PERC solar cells, two dif- ferent approaches for increasing efficiency were applied. In the Metal Wrap Through (MWT) concept for solar cells, the external front-contacts are transfer- red to the backside of the solar cell. As a result, more sunlight is incident on the front surface and the solar cell efficiency increases. In the Passivated Emitter and Rear Cell, or PERC, concept, both the optimized reflection of the solar cell backside as well as passi- vation of the rear surface contribute to higher effici- encies. The screen-printed aluminum back-contact is connected to the p-type silicon material using local laser alloying – the so-called Laser Fired Contact (LFC) process. Just recently, PV-TEC at Fraunhofer ISE manufactured large-format PERC solar cells (edge length 156mm, with solder contacts on the backside) from monocrystalline Czochralski silicon with effi-

t the Photovoltaic Technology Evaluation Center PV-TEC of the Fraunhofer Institute for

ciencies reaching 19.3%. Decisive for even higher efficiencies is the reduced amount of shading that is gained through the MWT concept. For the MWT- PERC solar cells produced, the efficiency increased up to 19.4%. Through the use of high quality float- zone silicon material, the Fraunhofer researchers increased the efficiency even further to 20.2% – the highest efficiency measured up to now for large format solar cells manufactured with cost-effective and industrially applicable screen printing, diffusion and thermal oxidation processes. The edge length of the solar cells corresponds to the original wafer size of 125 x 125 mm2

. Solder contacts of both polarities

are located on the backside and used for connecting the cells in the module. The know-how for manufac- turing MWT-PERC solar cells is already being trans- ferred to several German solar cell manufacturers.

The PV-TEC team also achieved efficiencies of 20.0% for Back-Contact Back-Junction (BC-BJ) so- lar cells based on n-type monocrystalline float-zone silicon material. The development could be substan- tially accelerated by the excellent cooperation with Fraunhofer ISE’s ETAlab. Presently this type of solar cells has an aperture area of 37 x 45 mm², however, all of the used technologies can be transferred to the production of larger formats. BC-BJ solar cells not only have both polarities located on the backside, as in MWT-PERC solar cells, but also the emitter. Thus, the shading losses on the front side can be reduced even further. This results in a very high efficiency