Passivating contact solar cells have gradually become the mainstream cell technology due to their excellent performance, and further improving the conversion efficiency has become a focus of subsequent research. Typically, achieving excellent field-effect passivation and low contact resistivity in doped polycrystalline silicon (poly-Si) solar cells requires heavy phosphorus doping. However, this approach can lead to a predicament where excessive phosphorus diffuses into the Si substrate during annealing, causing recombination losses. To address this challenge, a tandem passivation contact structure incorporating an intrinsic amorphous silicon (a-Si ( i)) film within the passivation layers is introduced to retard the diffusion of phosphorus into the Si substrate. Comprehensive characterizations of the tandem structure were carried out to delve into the underlying mechanisms of films with the integrated a-Si ( i) layer, including simulations, surface microscopy, active dopants profiling, crystallographic structure, chemical bonding, elemental distribution, and electrical properties. Simulations revealed that the inserted intrinsic layer effectively counteracts the clustering of phosphorus atoms, leading to a more even distribution during crystal growth. Furthermore, active dopant profiles indicate the potential of the introduced a-Si ( i) layer to tailor the in-diffused dopant profile. Microscopy investigations revealed the occurrence of blistering when the a-Si ( i) thickness exceeds 30 nm. Passivation and contact performances of TOPCon solar cells were assessed as the a-Si ( i) thickness was varied. Notably, optimal electrical properties were achieved with 20 nm a-Si ( i) thickness. At this thickness, the implied open-circuit voltage ( iV _OC) of the hydrogenated lifetime sample was promoted to more than 736.6 mV on the polished wafer, corresponding to the lowest single-side saturation current density ( J ₀) of 4.3 fA/cm ². In addition, a low contact resistivity of 1.4 mΩ·cm ² was achieved. Based on this tandem passivation contact structure, industrial-sized TOPCon solar cells were fabricated, giving an average efficiency of 23.83%, 0.25% higher than that of the baseline counterparts on the production line. The above results demonstrate the role of the a-Si ( i) film as a buffer layer, retarding the diffusion of phosphorus into the Si substrate and obtaining a better passivation effect. This enables us to further tailor the doping profile for high-efficiency solar cells. Our work thus highlights a promising strategy to improve the performance of TOPCon solar cells and showcases its potential for implementation in industrial manufacturing.