Synthesis and Characterization of (FA)3(HEA)2Pb3I11: A Rare Example of <1 1 0>-Oriented Multilayered Halide Perovskites

Metal-halide perovskites have recently demonstrated great potential for a wide variety of optoelectronic applications, with their layered subfamily offering improved stability as compared to their three-dimensional analogues. Among the layered compounds, a subclass of <1 0 0> terminated compounds are currently dominating the field, while the <1 1 0> subclass is comparably much less explored. Here, we report on the synthesis of (FA)3(HEA)2Pb3I11 obtained by room-temperature liquid–gas diffusion involving the intermediate-size cation hydroxyethylammonium (HEA+), formamidinium (FA+), and PbI2 dissolved in an acidic solution. This multilayered hybrid perovskite is a rare example of a m = 3 member of the <1 1 0>-oriented series (A′)2(A)mBmX3m+2. Structural characterization based on X-ray diffraction and nuclear magnetic resonance investigations reveals that the small-size cation FA+ is located both in the inner layer of the perovskite sheet and in the interlayer space, while the intermediate-size cation HEA+ is situated in the outer cavities of the perovskite sheet. (FA)3(HEA)2Pb3I11 exhibits a broad absorption band in the visible region (400–650 nm) leading to an optical band gap of 1.96 eV. Electronic structure calculations confirm the direct nature of the band gap and evidence sizable interlayer electronic coupling related to short I...I distances. These features are reminiscent of those observed for <1 0 0>-oriented Pb3I10 analogues that have shown photovoltaic efficiencies over 18%. These findings should prompt further investigations for the design of other <1 1 0>-oriented multilayered (m = 3, 4,...) metal-halide perovskites.