German technological research institute Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) has announced achieving 26% power conversion efficiency for both-sides-contacted silicon solar cells, opening the way for even higher efficiencies to be achieved. It terms the achievement as a 'new world record'.
For their research, the institute used its self-developed Tunnel Oxide Passivating Contact (TOPCon) solar cell technology that brings together the advantages of very low surface recombination losses along with efficient charge carrier transport. According to the team, omitting the conductive layers at the front side and still facilitate the lateral charge carrier transport is the 'key' to excellent optoelectrical properties for both-sides-contacted cells.
While standard industrial cells come with a front side p-n junction, Fraunhofer ISE team used a p-n junction at the back surface in the form of a full-area polycrystalline silicon based passivating contact. It calls this a TOPCon Rear Emitter Solar Cell (TOPCoRE) that they believe allows higher voltages and higher fill factors than cells with a collecting emitter on the front side.
"While industrial standard cells have a p-n junction on the front side, the p-n 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," explained the institute.
A detailed power loss analysis for the cell structure revealed, according to the team, that the cell generally compensates for and minimizes both electron and hole transport losses as well as transport and recombination losses.
"Both-sides-contacted solar cells have the potential to reach efficiencies up to 27% and thus surpass the previous world record for silicon solar cells," added Division Director of Photovoltaics Research at Fraunhofer ISE, Prof. Stefan Glunz.
Their research titled Design Rules for High-Efficiency Both-Sides-Contacted Silicon Solar Cell with Balanced Charge Carrier Transport and Recombination Losses has been published in the scientific journal Nature.
At the recent High Efficiency Solar Conference of TaiyangNews, Jochen Retsch from Fraunhofer ISE presented on the Transfer of TOPCon Process to an industrial PECVD Tool