Wet Etching Pathways for TOPCon Solar Cell Plating Preparation: Etchant Effectiveness and Pinholes Formation

Due to material scarcity, studies are focusing on replacing the Ag metallization of TOPCon with Cu deposited via electroplating, with the outer passivation layers selectively removed to obtain conductive surfaces. Wet etching is being studied as an alternative to the usual laser contact opening (LCO), to reduce the thermal stress on the precursors, and avoid laser damage introduction. Previous tests of photolithography and wet etching with buffered hydrofluoric acid (BHF) showed slower etch rate than expected and formation of pinholes in the poly-Si layer, negatively affecting the cell performance. This work evaluates 10% HF as alternative etchant and aims to determine the mechanism of the pinhole formation. Optical microscope investigation and cross-section SEM images show that 10% HF reduces the etching time for complete passivation removal from 36 to 12 min, when compared to BHF. Moreover, the pinholes originate from pre-existing trenches in the poly-Si, where the passivation is thinner and is completely removed earlier than in the rest of the sample. Additionally, BHF locally over-etches the poly-Si around the exposed trenches, likely due to grain boundary reactivity, while 10% HF leaves the surface intact.