Fraunhofer ISE Sets Another Solar Cell Efficiency Record

Originally published on Sustainnovate.

A solar conversion efficiency of 25.1% has been achieved for the first time for a “both sides-contacted silicon solar cell,” by the Fraunhofer Institute for Solar Energy Systems ISE, a 2014 Zayed Future Energy Prize award winner.

The achievement marks the setting of a new world record for the solar cell type, and seems to signal great future potential for efficiency improvements as well.

For those not familiar with the cell type, both sides-contacted silicon solar cells are noted for featuring metal contacts on both the front and the rear of the cells. The new record-holding cells also utilized a full-area passivated back contact.

“To improve the solar cell efficiency, the focus has been on increasingly complex solar cell structures up to now,” stated Dr Martin Hermle, Head of the High Efficiency Solar Cells department at Fraunhofer ISE. “The biggest advantage of our new concept is that we can now contact the entire rear cell surface without patterning. Compared to the high-efficiency solar cell structures presently in use, we offer both a simplified manufacturing process and higher efficiencies at the same time.”

Some further details via a recent press release:

With the so-called TOPCon (Tunnel Oxide Passivated Contact) technology, developed by Fraunhofer ISE, metal contacts are applied to the rear side without patterning. To achieve this, the Fraunhofer researchers developed a selective passivated contact made of tunnel oxide that enables majority charge carriers to pass and prevents the minority carriers from recombining. The thickness of the intermediate passivation layer is reduced to one or two nanometers, allowing the charge carriers to “tunnel” through it. Subsequently, a thin coating of highly doped silicon is deposited over the entire layer of ultra-thin tunnel oxide. This novel combination of layers allows electrical current to flow out of the cell with nearly zero loss.

In the photovoltaics industry, the majority of solar cells have an aluminum-alloyed back contact covering the entire rear side. This type of contact, however, limits the efficiency. Therefore, the industry currently retrofits their production to incorporate the PERC (Passivated Emitter Rear Cell) technology in order to increase the solar cell efficiency. With PERC technology, only a small area on the rear side is contacted in order to reduce recombination. PERC, however, requires additional patterning steps and leads to longer current conduction paths in the silicon wafer. TOPCon, on the other hand, offers a possible approach to reduce these loss mechanisms and increase the efficiency.

“With TOPCon, we have developed a pioneering technology to increase the efficiency of silicon solar cells,” stated Professor Stefan Glunz, Division Director of Solar Cells – Development and Characterization. “At 25.1% efficiency, we are the first research institute to cross the 25% mark with an evolutionary further development for both sides-contacted solar cells and to close the gap on the world record efficiency for back contacted solar cells.”

The research team involved in the concept has reportedly been working on it for around 3 years now, with steady improvements to conversion efficiency being made via the approach.

Image ©Fraunhofer ISE

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