Ordered crystal growth of 2D Ruddlesden-Popper perovskites via synergistic fluorination and chlorination for efficient and stable 2D/3D heterostructure perovskite solar cells

Based on epitaxial growth theory, a highly ordered 2D perovskite is necessary to boost the photoelectric performance of 2D/3D perovskite heterostructures. The strong NH...I hydrogen-bonding interaction between spacer cations and inorganic frameworks could be built by widely used dipolar aromatic amine spacer cations with single halogen (F or Cl) substitution, which is crucial to the stabilization of 2D perovskites. However, the effect of synergistic fluorination and chlorination on the octahedral tilt of 2D perovskites is relatively unexplored. Herein, halogen substituted meta, para-aniline iodide (3,4-AnI) salts, including 3-F-4-ClAnI, 3,4-FAnI, 3,4-ClAnI, and 3Cl-4-FAnI, were respectively added into 3D perovskite precursor solution. By contrast, the 3-F-4-ClAnI-treated n = 2 2D Ruddlesden-Popper (RP) perovskite exhibits the highest ordered crystalline structure. We find that 3-F-4ClAn+ spacer cations have both strong NH...I hydrogen interaction and Pb-halide interaction with [PbI6]4octahedron cages. Both are favorable for the stability of 2D perovskites and subsequent orientation growth of 3D perovskite crystals, thereby leading to reduced stack defects as well as improved carrier transport. Consequently, the optimized perovskite solar cell (PSC) with 3-F-4-ClAnI-treated 2D/3D Cs0.05MA0.16FA0.79Pb(I0.83Br0.17)3 perovskite yields a high power conversion efficiency (PCE) of 22.73 % compared with the control device without 2D perovskite (19.54 %). A higher PCE of 24.74 % is delivered for the device assembled with 3-F-4-ClAnI-treated 2D/3D FAPbI3 perovskite. The experimental results are of certain significance in guiding the design of the highly orientated 2D/3D perovskites.