Increased Series Resistance in the Laser-Enhanced Contacts of Polysilicon-Based Solar Cells from an Additional Light Anneal

In the fabrication of silicon solar cells, a light anneal step, performed at moderate temperatures under controlled illumination, is commonly employed after the firing step to promote hydrogen in-diffusion. This process effectively passivates both bulk and surface defects, improving the open-circuit voltage and power conversion efficiency of modern silicon solar cells, such as tunneling oxide passivating contact (TOPCon) cells. With the emergence of laser-enhanced contact technologies, the timing of this light anneal becomes critical to fully harness its benefits. In this study, the impact of an additional light anneal applied after the laser-enhanced contact process is investigated. This additional light anneal is found to cause a significant increase in series resistance, traced via luminescence imaging and contact resistance measurements to the interface between the metal and p⁺ emitter region. Electrical measurements under varying bias and temperature conditions suggest that the series resistance increase is likely due to excessive hydrogen accumulation at the metal/silicon interface. The possible formation of a thicker glass layer is not observed by microscale characterization, although this possibility can not be ruled out either. These findings underscore the importance of positioning the laser-enhanced contact process as the final step in the fabrication sequence for high-efficiency TOPCon solar cells.