Optimization of multilayered Ruddlesden–Popper perovskite with 4-bromophenylethylamine by ionic liquid for solar cell applications

Two-dimensional (2D) perovskite formed by introducing various organic spacer cations into 3D perovskite is considered to be one of the most effective methods to improve the humidity stability of perovskite materials. In order to solve the phase inhomogeneity and phase distribution nonuniformity of two-dimensional perovskites, additive engineering technique is widely used during their fabrication processes. Although many additives are proved to be effective in improving the quality of perovskite films, the mechanism of additive engineering has not been fully understood. In our work, we prepare 2D Ruddlesden–Popper perovskites based on 4-bromophenylethylammonium (BPEA) using a new additive of ionic liquids MAAc (methylammonium acetate). The (BPEA) 2 (MA) 4 Pb 5 I 16 perovskite prepared with the assistance of MAAc exhibits a stable photoelectric conversion efficiency (PCE) of 8.47%, which is much higher than the one without MAAc additive (4.79%). Besides a nearly tenfold increase in electron mobility, the unencapsulated (BPEA) 2 (MA) 4 Pb 5 I 16 -based device with MAAc additive remains 88% of its initial efficiency after 960 h exposure to air with a humidity of 20 ± 5% at room temperature, while the device without additive decreases to 63% of its initial efficiency. Meanwhile, the interactions between PbI 2 and MAAc at different temperatures and molar ratios are investigated and the formation mechanism of 2D perovskites in the presence of MAAc is proposed. It is found that an intermediate perovskite with distorted [PbI 6− x Ac x ] 4− octahedron is formed with the addition of MAAc, making the quality and photoelectric properties of the resultant films greatly enhanced.