Fraunhofer ISE sets new world efficiency record for both-sides-contacted solar cell

Researchers at Germany’s Fraunhofer Institute for Solar Energy Systems (ISE) have achieved a record conversion efficiency of 26% for both-sides contacted silicon solar cells, the industry standard solar cell and preferred choice in industrial production.

Led by Dr. Armin Richter at Fraunhofer ISE, the researchers demonstrated the structure of their record-breaking cell in a recent article published in the journal Nature Energy, and highlighted fundamental design-related aspects of their research which they hope will lead to even higher efficiencies.

Specifically, the “design of the back-side cell surface as a full-area charge-carrier collecting passivating contact was key to the success,” according to Fraunhofer ISE.

This serves as an important growth opportunity for solar cells made of crystalline silicon, considering that these types of solar cells already account for over 90% of the global solar photovoltaic market.

Further, record efficiencies of around 26% have previously been recorded for solar cells with both metal contacts at the rear – known as interdigitated back contact, or IBC, solar cells – but as Fraunhofer ISE points out, both-sides-contacted solar cells have established themselves as the industry standard and are also the preferred choice in industrial production due to their lower complexity.

The new research from Fraunhofer ISE demonstrates a new approach for developing both-sides contacted cells which the researchers hope will also make possible reaching higher efficiencies for this type of cell.

Based on the TOPCon (Tunnel Oxide Passivating Contact) technology developed at Fraunhofer ISE, the efficiency record-breaking both-sides contacted cells combines the advantages of very low surface recombination losses with efficient charge carrier transport.

As Fraunhofer ISE explain, “industrial standard cells have a pn junction on the front side, the pn junction in the record cell was formed on the back side as a full-surface TOPCon contact.

Thus, the full-surface boron doping on the front side was no longer required so that only a local boron diffusion directly under the front-side contacts was implemented.”

The resulting TOPCoRE (TOPCon Rear Emitter Solar Cell) cell therefore allows higher voltages and higher fill factors than cells with a collecting emitter on the front side.

“Based on a systematic simulation-based analysis, we were able to derive some fundamental design rules for future high-efficiency silicon solar cells above 26% efficiency,” said Professor Stefan Glunz, Division Director of Photovoltaics Research at Fraunhofer ISE.

“Both-sides-contacted solar cells have the potential to reach efficiencies up to 27 percent and thus surpass the previous world record for silicon solar cells.”

The TOPCoRE also demonstrated a very high fill factor of 84.3%, which was “achieved without any full-area conductive layers at the front surface, proves the very effective charge carrier transport in the c-Si bulk to the local front-side contacts,” according to the authors of the article.

“This device performance represents the highest efficiency reported for FBC c-Si solar cells.

This result may open a path to higher-performance FBC c-Si solar cells, as we have shown that no lateral transport layer is necessary for high-performance BJ-FBC solar cells, which means, for instance, that no TCO is needed at the front side of such solar cells.”