Suppressing Multi‐Dimensional Defects in Cs 0.05 FA 0.95 PbI ₃ Single Crystals Enables Efficient and Stable Back‐Contacted Perovskite Photovoltaics

Back‐contacted architectures offer cost and stability advantages for perovskite solar cells (PSCs), yet their efficiencies have plateaued at ∼12% due to defect‐induced recombination and limited carrier diffusion in thin single crystals. Herein, a multi‐dimensional defect suppression strategy is reported to overcome this bottleneck by incorporating N‐methylformamidinium (MFA + ) into Cs 0.05 FA 0.95 PbI 3 (FA = CH(NH 2 ) 2 + ) crystals. MFA + strengthens interaction between A‐site cations with iodide ions, thereby suppressing iodide vacancies (point defects), relieving tensile microstrain, and eliminating dislocations and surface wrinkles (line and plane defects). This approach yields high‐quality crystals with extended electron diffusion lengths (∼400 µm). As a result, an impressive efficiency of 17.35% is obtained, representing a substantial advance over reported back‐contacted PSCs. Moreover, the devices exhibit excellent operational stability with no performance degradation after 1350 h of continuous light illumination. This work highlights the importance of suppressing multi‐dimensional defects for enhancing carrier transport, which is instructive for developing efficient back‐contacted PSCs.