Efficient Bulk Defect Suppression Strategy in FASnI₃ Perovskite for Photovoltaic Performance Enhancement

Abstract Despite Sn‐based perovskite solar cells (PSCs) prevailing over lead‐free candidates, the Sn vacancies (V_Sn) and Sn⁴⁺ defects seriously deteriorate device photovoltaic performance. The presently reported methods can only effectively achieve surface defect passivation, and it is of great challenge and fundamental importance to develop efficient strategy to deal with the intrinsic defects located inside the lattice. Herein, a novel bulk defect suppression strategy is proposed, introducing large organic piperazine cations (PZ²⁺) into the lattice of 3D FASnI₃ perovskite to restrain the generation of bulk defects. The incorporation of PZ²⁺ results in forming a FA_1−2yPZ_2ySn_1−yI₃ (0 ≤ y ≤ 0.25) structure with no reduction in dimensionality, which guarantees the continuity of [SnI₆] octahedral structures with unobstructed carrier transport and reduced charged defects. The potent interactions between PZ²⁺ and [SnI₆] structures enhance V_Sn formation energy and effectively suppress bulk defect formation. As a result, the FASnI₃+1%PZ films exhibit optimized crystalline quality, decreased background carrier density, lower p‐type self‐doping, and reduced trap state density. Benefiting from the above advantages, the FASnI₃+1%PZ device achieves an optimal PCE of 9.15% and unencapsulated device maintains over 95% of initial PCE after aging for 1000 h in N₂ golvebox. The bulk defect suppression strategy provides fire‐new building bricks toward high‐performance Sn‐based PSCs.