By Renewable Energy Focus

For back-contact silicon solar cells, Fraunhofer got up to 20.2% efficiency using Metal Wrap Through (MWT) and Passive Emitter and Rear Cell (PERC) technology.

The MWT technology sees the external front-contacts being transferred to the backside of the solar cell, leading to more sunlight on the front surface leading to increased efficiency.

The PERC technology sees optimised reflection of the solar cell backside as well as passivisation of the rear surface contributing to higher efficiencies.

“The screen-printed aluminium back-contact is connected to the p-type silicon materials using local laser alloying – the so-called Laser Fired Contact (LFC) process,” Fraunhofer says.

The reduced amount of shading that is gained through the MWT concept also affects efficiencies. Finally, the use of high quality float-zone silicon enabled the researchers to bring the efficiency up to 20.2%

The screen-printing is industrially applicable, Fraunhofer says.

Fraunhofer also achieved 20% efficiency for Back-Contact-Junction (BC-BJ) solar cells based on n-type monocrystalline float-zone silicon material, where both the polarities and the emitters are placed on the backside.

“For us, it is especially relevant that the A-alloyed has such a high potential. We still see many possibilities here for further increasing the efficiency,” says Robert Wahl, PhD Student at Fraunhofer.

Dr Ralf Preu, Division Director of PV Production Technology and Quality Assurance and Chief Manager of the Photovoltaic Technology Evaluation Centre (PV-TEC), adds:” With these results, an important milestone in cost savings has been reached. Remarkably high solar cell efficiencies of over 20% have been achieved using the established industry-level production technology.

“The public funding, received from the Federal Government and the European Union together with our close cooperation with manufacturers of solar cells, production systems and materials have enabled this path-breaking success.”