Nowadays, a stack of heavily doped polysilicon (poly-Si) and tunnel oxide (SiO_x) is widely employed to improve the passivation performance in n-type tunnel oxide passivated contact (TOPCon) silicon solar cells. In this case, it is critical to develop an in-line advanced fabrication process capable of producing high-quality tunnel SiO_x. Herein, an in-line ozone-gas oxidation (OGO) process to prepare the tunnel SiO_x is proposed to be applied in n-type TOPCon solar cell fabrication, which has obtained better performance compared with previously reported in-line plasma-assisted N₂O oxidation (PANO) process. In order to explore the underlying mechanism, the electrical properties of the OGO and PANO tunnel SiO_x are analyzed by deep-level transient spectroscopy technology. Notably, continuous interface states in the band gap are detected for OGO tunnel SiO_x, with the interface state densities (D_it) of 1.2 × 10¹²–3.6 × 10¹² cm⁻² eV⁻¹ distributed in E_v + (0.15–0.40) eV, which is significantly lower than PANO tunnel SiO_x. Furthermore, X-ray photoelectron spectroscopy analysis indicate that the percentage of SiO₂ (Si⁴⁺) in OGO tunnel SiO_x is higher than which in PANO tunnel SiO_x. Therefore, we ascribe the lower D_it to the good inhibitory effects on the formation of low-valent silicon oxides during the OGO process. In a nutshell, OGO tunnel SiO_x has a great potential to be applied in n-type TOPCon silicon solar cell, which may be available for global photovoltaics industry.

Nanoscale Kirkendall effect has been widely used for rationally fabricating high-quality hollow nanocrystals, but often requires the intrinsic diffusion coefficient of out-diffusion materials higher than that of in-diffusion components. Here we demonstrate an unexpected Kirkendall effect that occurs in diffusing intrinsically faster Cu atoms into Pd icosahedra, leading to the formation of PdCu alloyed hollow nanocrystals. The control experiment with Pd octahedra replacing icosahedra indicates the critical role of twin boundaries in facilitating such unexpected Kirkendall effect. In addition, geometric phase analysis and density functional theory calculation show that out-diffusion of Pd atoms in the icosahedra is faster than in-diffusion of Cu atoms, particularly through the twin boundaries, upon the strain gradient with an inward distribution from tensile to compressive strains. The unexpected Kirkendall effect is also found in the interdiffusion of Ag and Pd atoms in Pd icosahedra. Our finds break the limitation of the intrinsic diffusion coefficient for the synthesis of hollow nanocrystals through Kirkendall effect and are expected to enormously enrich the family of hollow nanocrystals which have shown great potential in broad areas, such as fine chemical production, energy storage and conversion, and environmental protection. This work also provides a deep understanding in the diffusion behavior of atoms upon the strain gradient.