The buried interface between perovskite absorbers and electron-transport layers plays a critical role not only in charge extraction but also in governing the early stages of perovskite crystallization. This interfacial influence is particularly important in two-step fabrication, where precursor coordination and subsequent conversion determine the structural and electronic quality of the resulting films. However, conventional interfacial treatments have primarily focused on suppressing electronic defects, while their potential to regulate precursor organization and nucleation remains relatively unexplored. Developing an interfacial strategy that simultaneously addresses these aspects is therefore essential for advancing high-performance perovskite solar cells (PSCs).

Here, we introduce an alkali-metal-based ionic surface modifier onto chemical bath-deposited SnO₂ to simultaneously regulate interfacial defects and perovskite crystallization. Interestingly, the treatment exhibits distinct concentration-dependent behavior, which enables the buried interface to function not merely as an electronic contact but also as an active platform for directing crystallization.

The underlying mechanism is associated with modified precursor interactions at the treated interface. Density functional theory calculations reveal that the ionic modifier strengthens precursor association while weakening PbI₂–DMSO coordination, facilitating the formation of the photoactive perovskite phase. Consistent with these findings, in situ analyses demonstrate accelerated crystallization even before conventional thermal annealing. Beyond nucleation control, partial diffusion of alkali-metal species into the perovskite contributes to residual lattice strain relaxation, while counter anion species preferentially remain near the buried interface, supporting interfacial defect passivation. These complementary effects improve perovskite crystallinity, facilitate charge extraction, and suppress nonradiative recombination.

Consequently, the optimized PSCs achieve a champion power conversion efficiency of 27.01% (26.66% certified), while wider-bandgap devices deliver a certified efficiency of 24.81%. The treated devices also retain over 90.8% of their initial efficiency after 1000 h of continuous illumination. This work highlights the importance of interfacial precursor pre-organization and establishes ionic surface engineering as an effective strategy for simultaneously controlling perovskite crystallization and interfacial electronic properties.