Cation Engineering in Two-Dimensional Ruddlesden–Popper Lead Iodide Perovskites with Mixed Large A‑Site Cations in the Cages

The
Goldschmidt tolerance factor in halide perovskites limits the
number of cations that can enter their cages without destabilizing
their overall structure. Here, we have explored the limits of this
geometric factor and found that the ethylammonium (EA) cations which
lie outside the tolerance factor range can still enter the cages of
the 2D halide perovskites by stretching them. The new perovskites
allow us to study how these large cations occupying the perovskite
cages affect the structural, optical, and electronic properties. We
report a series of cation engineered 2D Ruddlesden–Popper lead
iodide perovskites (BA)2(EAxMA1–x)2Pb3I10 (x = 0–1, BA is n-butylammonium, MA is methylammonium) by the incorporation of a large
EA cation in the cage. An analysis of the single-crystal structures
reveals that the incorporation of EA in the cage significantly stretches
Pb–I bonds, expands the cage, and induces a larger octahedral
distortion in the inorganic framework. Spectroscopic and theoretical
studies show that such structural deformation leads to a blue-shifted
bandgap, sub-bandgap trap states with wider energetic distribution,
and stronger photoluminescence quenching. These results enrich the
family of 2D perovskites and provide new insights for understanding
the structure–property relationship in perovskite materials.