Imidazolium Ionic Liquid as Organic Spacer for Tuning the Excitonic Structure of 2D Perovskite Materials

Even though two-dimensional (2D) perovskites have outstanding atmospheric durability, the quantum well (QW) structure of the materials considerably limit their photovoltaic efficiencies far below the three-dimensional (3D) analogues. Herein in this work, we adopt the ionic liquid (IL) 1-butyl-3-methylimidazolium iodide (BMI) as an organic spacer in 2D perovskite to fundamentally alter the QW structures. In comparison to the traditional Ruddlesden–Popper (RP) type perovskites, the new RP-2D perovskite shows drastically decreased interlayer distance to 3.5 Å, which endows stronger interactions between the inorganic slabs and a weaker quantum confinement effect of the material. We further unveil that the combination of BM+ and BA+ cations enables a facile regulation of the excitonic structures and optical properties of 2D perovskite. By this design, the hybrid (BM)2–x(BA)xMAn–1PbnI3n+1 based PSCs achieve an optimized PCE of 17.3%, which is among the highest reported values for 2D PSCs to date. Due to the outstanding heat resilience of IL, the PSCs also demonstrate extraordinary thermal stability under aging at 85 °C.