Selectively Fluorinated Benzylammonium-Based Spacer Cation Enables Graded Quasi-2D Perovskites for Efficient and Stable Solar Cells
Guijun Yan, Guomin Sui, Wentao Chen, Kuo Su, Yaqing Feng, Bao Zhang
Abstract
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.