A Failure Mode Affecting the Reliability of LECO-Treated High-Efficiency TOPCon Solar Cells

Abstract

Laser-enhanced contact optimization (LECO) has become an essential process in enabling the fabrication of >25% efficient tunnel oxide–passivated contact (TOPCon) solar cells, now in use in >100 GW of silicon solar module production. LECO improves the metal–semiconductor interface in silicon solar cells, thus resulting in an excellent trade-off between contact resistance (ρ_c < 1 mΩ.cm²) and surface recombination (J_0met < 160 fA/cm²). This work presents a new failure mode observed at the front p⁺-Ag contact in LECO-treated TOPCon solar cells, which is not observed in standard screen-printed metallization. The bias and temperature stress severely degrade the dark contact resistance in LECO-treated TOPCon, leading to an increase in series resistance of over 100 Ω in a 2 x 2 cm² cell. Unlike standard TOPCon, where degradation has been ascribed to the n⁺-Ag contact, the LECO cells show the most prominent degradation at the p-type contact side. Luminescence measurements on stressed samples show reduced recombination, which could be attributed to improved passivation at the p⁺-Ag interface and/or the enhancement of other recombination-limiting factors such as AlO_x passivation, but negatively impacting conductivity. The temperature and bias stress also deteriorate the light current–voltage characteristics for the samples that underwent the LECO process. These results reveal a potential degradation mode in LECO-treated TOPCon solar cells, indicating the need for further investigation into its impact on efficiency gain, long-term reliability, and bankability.