Directional Management Self‐Additive Spacer Cations for Stable 2D Ruddlesden–Popper Perovskite Solar Cells with Efficiency over 21%

Abstract 2D Ruddlesden–Popper perovskites are highly regarded materials for improving the stability of perovskite solar cells (PSCs). Wherein, self‐additive 2D perovskites have recently been proposed to provide substantial strategies for managing the crystallization kinetics and bulk defects. For a profound understanding of the formation mechanisms, herein, with selecting three self‐additive 2D perovskites as demonstrations, a comprehensive analysis of the self‐additive behavior combing experimental and theoretical calculations is conducted. Self‐additive 2D perovskites exhibit more suitable formation energies and strong interaction, which is conducive to realize the self‐additive effect to form more stable structure. As demonstrated, glycine (Gly)‐based spacer cations played a pivotal role in the nucleation and growth of 2D perovskites by adjusting the aggregation state of colloids precursor, resulting in excellent‐quality films with large average grain size (≈3 µm). Meanwhile, theoretical analysis of electronic distribution and binding energies (E_b) revealed that glycine ethyl ester (Gly‐E) perovskite possesses highly robust internal interactions, which will effectively mitigate defect formation and enhance device stability. Endowing with the above outstanding feature, Gly‐E devices exhibited an optimized PCE of 21.60%, one of the highest PCEs among all 2D RP PSCs (n ≤ 6). The findings provide a basis for the rational design of self‐additive 2D perovskites and achieving highly‐performance 2D RP PSCs.