A guide to use fluorinated aromatic bulky cations for stable and high-performance 2D/3D perovskite solar cells: The more fluorination the better?

While serious stability issues impede the commercialization of perovskite solar cells (PSCs), two-dimensional (2D) perovskites based on fluorinated bulky cations have emerged as more intrinsically stable materials. However, the influence of fluorination degree of the bulky aromatic cation on the performance of resulting PSCs has not been scrutinized. Here, 2D perovskites (FxPEA)2PbI4 (x = 1, 2, 3, 5) are grown in situ on the surface of the three-dimensional (3D) perovskite and demonstrate effective passivation of the surface defects of 3D perovskite. The power conversion efficiency (PCE) of the optimized devices were boosted from 20.75% for the control device to 21.09%, 22.06%, 22.74% and 21.86% for 2D/3D devices treated with 4-fluorophenethylamine iodide, 3,5-difluorophenylethylamine iodide, 2,4,5-trifluoroethylphenylethylamine iodide, and 1,2,3,4,5-pentafluorophenylethylamine iodide, respectively. We firstly reported two unexplored RP-type layered perovskites with F2PEAI and F3PEAI as bulky cations. The combined experimental and theoretical analysis revealed the reasons behind the various morphology, device performances, dynamic behavior, and humidity stability. The best performing F5PEAI-treated device retaining 95.0% of its initial PCE under ambient atmosphere (with RH of 60% ± 5%) without encapsulation for 300 h storage. This work provides useful guidance for selecting fluorinated bulky cations with different molecular electronic properties, which will play an essential role in further improving the performance/stability of PSCs for the sake of further commercialization.