Efficiency improvement and microstructural working principle of LECO on the n-TOPCon rear side of industrial-like TOPCon solar cells

Laser-enhanced contact optimization (LECO) generating current-fired contacts (CFC) is one of the key technologies for production of high-efficiency industrial TOPCon solar cells. Its microstructural working principle on PERC solar cells is well-known and has recently been reported for the diffused boron emitter front side of industrial TOPCon solar cells. However, detailed n-TOPCon rear side specific investigations of the LECO process on contact properties are missing. In this contribution, we close that gap by first providing comprehensive performance data of TOPCon cells with diffused boron emitter front side and n-TOPCon rear side as industrial-like model samples. Second, we were able to identify current-fired contacts on n-TOPCon layers and investigate their nanostructure by employing a contrast etching technique, developing a large-area cross-section preparation approach and an adapted FIB preparation protocol. It becomes apparent that the contact formation significantly differs from non-LECO TOPCon contacts. Thirdly, we show that the efficiency improvement of the cells due to LECO is driven by reduction of Ag/poly-Si contact resistance and that LECO may lead to severe degradation of passivation quality if not optimized properly. Based on our findings, we developed a microstructural model of the working principle of LECO on n-type TOPCon layers.