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 study, we present the development of localized p+ SiOx/poly-Si structures, doped with boron through the direct application of boron-ink using FlexTrail printing, combined with wet chemical etching. This approach facilitates the formation of mask-free local TOPCon structures, optimized for high-efficiency tunnel oxide passivated contact (TOPCon) solar cells. We provide a detailed analysis of the factors influencing etch-back selectivity, with particular emphasis on the 1 wt% diluted HF pre-treatment, which is a crucial preparatory step before KOH wet chemical etching. This process optimally removes the native oxide from the intrinsic poly-Si, while preserving the boron silicate glass (BSG) layer on the local p+ poly-Si, thus enabling the selective removal of intrinsic poly-Si to achieve the desired p+ TOPCon structure. For both planar and textured surfaces, line widths between 86.0 - 100.0 µm and 24.0 - 40.0 µm, respectively, were achieved. The test structures incorporating a line grid of local TOPCon features exhibited a maximum open-circuit voltage (iVOC) of 720 mV and the lowest saturation current density (J0SE) of the local p+ SiOx/poly-Si was in the range of 90-120 fA/cm². In future work, the developed local p+ poly-Si structures will be integrated into high-efficiency TOPCon solar cells, positioning the p+ poly-Si beneath the metal contacts to further enhance cell performance.