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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)4Pb5I16 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)4Pb5I16-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 PbI2 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 [PbI6−xAcx]4− octahedron is formed with the addition of MAAc, making the quality and photoelectric properties of the resultant films greatly enhanced. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) organic–inorganic perovskite hybrid materials with enhanced stability have attracted wide attention in optoelectronic field in recent years. Efforts of exploring new 2D perovskites with various organic or inorganic components have been devoted to further improve their solar cell efficiencies. In this work, three structurally similar spacer cations of phenylethylammonium (PEA), 4-bromophenylethylammonium (BPEA) and 4-chlorophenylethylammonium (CPEA) are selected to prepare multilayered 2D perovskites. It is found that the introduction of halogen atoms in the organic cations positively influences the film quality, light absorption, charge transport, device performance and moisture stability of the resulting perovskites. The (BPEA)2(MA)4Pb5I16 and (CPEA)2(MA)4Pb5I16 perovskites exhibit efficiencies of 7.38% and 10.88%, respectively, which are much higher than that of (PEA)2(MA)4Pb5I16 (3.98%). Owning to the larger size and bigger electronegativity of halogen atom in spacer cations, the PCE of unsealed BPEA- and CPEA-based 2D perovskite devices maintains 73% and 63% of their original efficiency after 720 h exposure to 60% humid air, while the PEA-based 2D device decreases to 52% of its initial PCE. Our work demonstrates new insight to modify the quality and stability of 2D perovskite hybrids, highlighting the validity of molecule design of organic cations in improving the efficiency and stability of 2D perovskite solar cells. [ABSTRACT FROM AUTHOR]