Furfurylammonium as a Spacer for Efficient 2D Ruddlesden–Popper Perovskite Solar Cells

2D perovskites have recently emerged as promising materials for solar cells due to their appealing ambient stability and structural diversity. The organic spacer cation plays an important role in the stability and properties of the 2D perovskites. Herein, a new organic ammonium cation, 2‐furfurylammonium (FuMA + ), is developed as the spacer cation for 2D Ruddlesden–Popper perovskites. Thin films of (FuMA) 2 (MA) 4 Pb 5 I 16 with increased crystal size and enhanced vertical orientation are obtained via an additive‐assisted film forming technique, which dramatically improves the efficiency of 2D perovskite solar cells from 4.90% to 15.66%. Moreover, the perovskite films based on FuMA + can be prepared in ambient air, and the devices based on the perovskite films prepared in ambient air with 30% relative humidity can perform a power conversion efficiency (PCE) of 15.24%.