Regioisomer‐Induced In Situ Construction of 1D/3D Perovskite Heterojunctions for Efficient and Stable Solar Cells

Perovskite solar cells (PSCs) are limited by poor operational stability arising from undercoordinated Pb/I defects that induce severe non‐radiative recombination. Here, a regioisomer‐guided molecular engineering strategy is proposed to modulate interfacial coordination and structural reconstruction. Pyridinecarboxamide isomers with distinct substitution positions are introduced, among which para‐configured isonicotinamide (INA) exhibits a larger dipole moment and enhanced charge redistribution, enabling stronger coordination with undercoordinated Pb 2+ and more efficient interfacial charge transfer. The cooperative interaction between the amide group and pyridine nitrogen further directs ordered molecular stacking and anisotropic growth, inducing the in situ formation of a 1D (INA)PbI 3 layer on the three‐dimensional perovskite surface and constructing a robust 1D/3D heterojunction. This architecture simultaneously suppresses defect‐assisted recombination and enhances environmental tolerance. As a result, INA‐modified devices achieve a high power conversion efficiency of 26.57%, representing one of the highest efficiencies reported for 1D/3D PSCs. Moreover, the unencapsulated devices retain 91% of their initial efficiency after 600 h under harsh damp heat conditions (85°C/85% RH) and exhibit excellent operational stability under cyclic illumination. This work provides a structure–property correlation for isomer engineering and offers an effective route toward highly efficient and stable PSCs.