Thickness-Driven Crystallization and Structural Evolution of Ultra-Thin p⁺ Poly-Si Passivated Contacts

High-efficiency and cost-effective photovoltaic technologies have driven great interest in advanced ultra-thin contact structures and their increasing demands, such as in TOPCon solar cells. The present work aims to develop ultra-thin boron-doped polysilicon (p⁺ poly-Si) passivated contacts to minimize optical losses while maintaining superb electrical performance. Ultra-thin poly-Si layers (<40 nm) were prepared by LPCVD, and a systematic study was conducted to optimize key parameters such as boron diffusion temperature and forming gas annealing profiles, etc. Comprehensive structural and optical characterization revealed a pronounced thickness-dependent behaviour in p⁺ poly-Si layers. Thinner p⁺ poly-Si layers (<10 nm) remain quasi-amorphous and substrate-dominated, intermediate layers (<20 nm) show partial crystallization, and thicker layers (~25 to 40 nm) exhibit well-developed polycrystalline structure with excellent passivation (J₀ ≈ 1.3 to 2 fA/cm²), high carrier lifetime (>2000 µs), and low contact resistivity (~ 1 to 1.5 mΩ.cm²). Results of this work outperform or match the literature despite pushing the p⁺ poly-Si to its thinnest level, and provide a path to developing advanced TOPCon solar cells with higher efficiency. However, this will not be easy, as we have to consider the optical perspective as well as overall stability, whether UV or thermal, for these ultra-thin p⁺ poly-Si passivated contacts to integrate them into prototype TOPCon solar cells.