Local p + Poly-Si Passivating Contacts Realized by Direct FlexTrail Printing of Boron Ink and Selective Alkaline Etching for High Efficiency TOPCon Based Solar Cells

In this work, we demonstrate the formation of local boron-doped, SiOₓ/p + poly-Si structures using wet chemical etching by direct printing of boron-ink. FlexTrail printing uses a very hollow (orders of μm for diameter) glass capillary tube filled with boron ink for printing onto silicon substrate. This process represents a mask-free approach for the formation of local TOPCon structures, enabling high-efficiency tunnel oxide passivating contact (TOPCon) solar cells. The factors influencing etch-back selectivity between intrinsic and boron-doped poly-Si were thoroughly investigated. It was determined that pre-treatment with diluted HF (1 wt%) prior to poly-Si removal in a KOH solution is the most critical step to achieve optimal etch selectivity. This treatment effectively removes the native oxide on intrinsic poly-Si while preserving the boron silicate glass (BSG) layer on p + poly-Si, facilitating the selective removal of intrinsic poly-Si and the formation of p + TOPCon structures. Line widths ranging from 24.0 to 100.5 μm on planar surfaces and 40.0-86.0 μm on textured surfaces were achieved. FlexTrail printing allows for significantly lower (and higher) feature sizes, but its fine-line potential was not fully exploited here due to alignment challenges during post-processing. Test structures with a line grid of local TOPCon structures exhibited a maximum open-circuit voltage (𝑖VOC of 720 mV and a lowest saturation current density (𝐽0𝑆𝐸) of ~90-120 fA/cm2. The developed local p + poly-Si will be integrated into high-efficiency TOPCon solar cells, where p + poly-Si will be strategically placed under the metal contact, in the near future.