Selectively Fluorinated Benzylammonium-Based Spacer Cation Enables Graded Quasi-2D Perovskites for Efficient and Stable Solar Cells

It is an effective strategy to improve the performance of quasi-two-dimensional (Q-2D) perovskite solar cells (PSCs) by introducing fluoro-substituted aromatic alkylammonium spacer cations into the active layer. Herein, the influence of the benzylammonium (BA) and the x-fluorobenzylammonium (xFBA) cation, in which x stands for the substitution position of the benzene ring (o, ortho; m, meta; p, para), on the crystal orientation, phase distribution, film morphology of the Q-2D (n = 5) perovskite films, and the corresponding device performance is systematically evaluated. The result suggests that compared with BA and oFBA, mFBA and pFBA have a larger dipole moment, forming a dense perovskite film with gradient structures where the n = 1 2D perovskite mainly exists at the top of the film and the large n-phase perovskite exists at the bottom of the film. Encouragingly, the (pFBA)₂MA₄Pb₅I₁₆ (MA = CH₃NH₃⁺)-based perovskite solar cells achieve the highest efficiency of 17.12%, with an open-circuit voltage of 1.175 V, a short-circuit current density of 18.50 mA cm^–2, and a fill factor of 78.78%, which is significantly higher than those of BA (14.07%)-, oFBA (12.89%)-, and mFBA (14.67%)-based PSCs. Furthermore, pFBA-based devices also exhibit the best stability compared to the other three devices.