Spacer Cation Tuning Enables Vertically Oriented and Graded Quasi‐2D Perovskites for Efficient Solar Cells

Abstract Halide substitution in phenethylammonium spacer cations (X‐PEA + , X = F, Cl, Br) is a facile strategy to improve the performance of PEA based perovskite solar cells (PSCs). However, the power conversion efficiency (PCE) of X‐PEA based quasi‐2D (Q‐2D) PSCs is still unsatisfactory and the underlying mechanisms are in debate. Here, the in‐depth study on the impact of halide substitution on the crystal orientation and multi‐phase distribution in PEA based perovskite films are reported. The halide substitution eliminates n = 1 2D perovskite and thus leads to the perpendicular crystal orientation. Furthermore, nucleation competition exists between small‐ n and large‐ n phases in PEA and X‐PEA based perovskites. This gives rise to the orderly distribution of different n ‐phases in the PEA and F‐PEA based films, and random distribution in Cl‐PEA and Br‐PEA based films. As a result, (F‐PEA) 2 MA 3 Pb 4 I 12 (MA = CH 3 NH 3 + , n = 4) based PSCs achieve a PCE of 18.10%, significantly higher than those of PEA (12.23%), Cl‐PEA (7.93%) and Br‐PEA (6.08%) based PSCs. Moreover, the F‐PEA based devices exhibit remarkably improved stability compared to their 3D counterparts.