Regulation of Octahedral Distortions and Interlayer Spacing in 2D Perovskite Capping Layer Toward Efficient and Stable Solar Cells

2D perovskites as interfacial modifiers have demonstrated potential for improving the stability of perovskite solar cells (PSCs); however, 2D structures based on flexible long-chain cations often lead to a high degree of octahedral distortions and larger interlayer spacing. These factors hinder efficient charge extraction between the perovskite and charge transport layers and affect the stability of 2D/3D devices. Here, imidazolyl spacers with rigid ring structures are employed as interfacial modifiers. The spacers of different molecular rigidity influence their anisotropic orientation with [PbI₆]^4- octahedral, leading to reduced crystal distortion while maintaining compatible octahedral layer spacing, which facilitates efficient hole extraction from bulk perovskite to the surface layer, and increases structural stability of 2D perovskite. As a result, the optimized 2D/3D PSCs achieve power conversion efficiencies of 26% for unit cells (0.16 cm²) and 22.4% for solar modules (38.9 cm²). Moreover, these spacers enhance the phase stability of the 3D perovskite and effectively mitigate phase degradation, enabling the device to retain 98% of its initial efficiency after 2000 h of continuous operation under 1 sun illumination at 40 °C. These results suggest the potential of imidazolium-based interfacial modifiers in achieving efficient and stable 2D/3D PSCs.