Researchers at the Fraunhofer Institute for Solar Energy Systems in Germany achieved a record conversion efficiency of 26.0 percent for double-sided silicon solar cells. Designing the rear cell surface for full area charge carrier collection passivation contact is key to success. The recording unit is based on TOPCon technology (Tunnel oxide passivation contact).
The technology, developed by Fraunhofer ISE, combines the benefits of extremely low surface recombination losses with efficient carrier transport. The industrial standard unit has a PN junction on the front, while the PN junction in the recording unit forms a full-surface TOPCon contact on the back. Therefore, full-surface boron doping on the front side is no longer required, and only local boron diffusion directly below the front contact is achieved. This TOPCoRE cell (TOPCon post-emitter solar cell) has a higher voltage and a higher fill factor than a cell with a collector emitter on the front. This cell design allows better use of the wafer for carrier transport and more efficient passivation of the front side (for which alumina is used).
Detailed power loss analyses show that such cells generally compensate for and minimize electron and hole transport losses as well as transport and compound losses. "Based on a system-based simulation-based analysis, we were able to come up with some basic design rules for future high-efficiency silicon solar cells with efficiencies greater than 26%.
Both solar cells have the potential to achieve an efficiency of 27 percent, surpassing the world record for silicon solar cells." The researchers said.