Tailoring Organic Cation of 2D Air‐Stable Organometal Halide Perovskites for Highly Efficient Planar Solar Cells

2D perovskites have recently been shown to exhibit significantly improved environmental stability. Derived from their 3D analogues, 2D perovskites are formed by inserting bulky alkylammonium cations in‐between the anionic layers. However, these insulating organic spacer cations also hinder charge transport. Herein, such a 2D perovskite, (iso‐BA) 2 (MA) 3 Pb 4 I 13 , that contains short branched‐chain spacer cations (iso‐BA + ) and shows a remarkable increase of optical absorption and crystallinity in comparison to the conventional linear one, n ‐BA + , is designed. After applying the hot‐casting (HC) technique, all these properties are further improved. The HC (iso‐BA) 2 (MA) 3 Pb 4 I 13 sample exhibits the best ambient stability by maintaining its initial optical absorption after storage of 840 h in an environmental chamber at 20 °C with a relative humidity of 60% without encapsulation. More importantly, the out‐of‐plane crystal orientation of (iso‐BA) 2 (MA) 3 Pb 4 I 13 film is notably enhanced, which increases cross‐plane charge mobility. As a result, the highest power conversion efficiencies (PCEs) measured from for current density versus voltage curves afford 8.82% and 10.63% for room‐temperature and HC‐processed 2D perovskites based planar solar cells, respectively. However, the corresponding steady‐state PCEs are remarkably lower, which is presumably due to the significant hysteresis phenomena caused by low charge extraction efficiency at interfaces of C 60 /2D perovskites.