Boosting efficiency and stability of 2D alternating cation perovskite solar cells via rational surface-modification: Marked passivation efficacy of anion

Two-dimensional (2D) alternating cation (ACI) perovskite surface defects, especially dominant iodine vacancies (V_I) and undercoordinated Pb²⁺, limit the performance of perovskite solar cells (PVSCs). To address the issue, 1-butyl-3-methylimidazolium trifluoro-methane-sulfonate (BMIMOTF) and its iodide counterpart (BMIMI) are utilized to modify the perovskite surface respectively. We find that BMIMI can change the perovskite surface, whereas BMIMOTF shows a nondestructive and more effective defect passivation, giving significantly reduced defect density and suppressed charge-carrier nonradiative recombination. This mainly attributes to the marked passivation efficacy of OTF⁻ anion on V_I and undercoordinated Pb²⁺, rather than BMIMI⁺ cation. Benefiting from the rational surface-modification of BMMIMOTF, the films exhibit an optimized energy level alignment, enhanced hydrophobicity and suppressed ion migration. Consequently, the BMIMOTF-modified devices achieve an impressive efficiency of 21.38% with a record open-circuit voltage of 1.195 V, which is among the best efficiencies reported for 2D PVSCs, and display greatly enhanced humidity and thermal stability.