The severe back interface recombination remains a major performance limiting factor in Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. In this work, SiO₂ nanoparticles deposited on Mo substrate by spin-coating were used to create discrete local contact points to passivate the back interface. Systematic investigation of nanoparticle size reveals that optimized SiO₂ incorporation enhances absorber crystallinity, reduces interfacial defect density, and suppresses detrimental [2Cu_Zn + Sn_Zn] deep-level clusters. Meanwhile, it can effectively inhibit nanoparticle agglomeration while maintaining optimal hole-transport distances, thereby obtaining a more efficient porous insulator contact (PIC) passivation structure. We propose that the passivation mechanism involves carrier recombination suppression through minimized defective contact area, thus improving the back contact quality. Consequently, the power conversion efficiency of the ultrathin CZTSSe solar cells increases from 9.03% to 10.72%, offering a viable approach for back-contact engineering.