Achieving a world record, the Fraunhofer ISE has reached a solar cell efficiency of 25.1% for a both sides-contacted silicon solar cell, using its TOPCon (Tunnel Oxide Passivated Contact) technology.
Unlike other solar cells, the TOPCon cell features metal contacts on both the front and back sides, and removes patterning from the back side a full-area passivated back contact. This has served to both increase efficiencies and simplify manufacturing processes. There is also "great potential" for further efficiency increases, says the institute.
Fraunhofer has been working on its TOPCon technology for the past three years. Explaining the new cell structure, it says, "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."
Currently, PERC (Passivated Emitter Rear Cell) technology is set to become the new benchmark solar technology, with demand spiraling upwards and evidence of bottlenecks emerging. SolarWorld recently broke its own record to achieve a PERC efficiency of 21.7%. However, Fraunhofer says its cells are better.
"With PERC technology, only a small area on the rear side is contacted in order to reduce recombination," it says in a statement released. "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."
Stefan Glunz, division director of Solar Cells Development and Characterization at Fraunhofer will hold a plenary talk on the TOPCon technology today at this years EU PVSEC, which kicked off yesterday in Hamburg, Germany.
pv magazine's September edition included a feature on PERC equipment.