Expanding the Cage of 2D Bromide Perovskites by Large A‑Site Cations

Two-dimensional
(2D) halide perovskites have outstanding optoelectronic
properties, and they feature a variety of organic cation spacers and
cage A-site cations that can be incorporated into their structures.
It has recently been reported that the Goldschmidt tolerance factor
can be relaxed and expanded in iodide 2D perovskites. Bromide 2D perovskites,
whose multilayer structures and optical properties are much less studied,
provide a great platform for studying structure–property relationships
for 2D perovskites with large A-site cations. Herein, we report the
synthesis and structure of three new 2D bromide perovskites(BA)2(MHy)2Pb3Br10 (BA, butylammonium;
MHy, methylhydrazinium), (BA)2(EA)2Pb3Br10 (EA is ethylammonium), and (BA)2(DMA)­Pb2Br7 (DMA is dimethylammonium). We compared them
with other 2D perovskites with different A-site cations but with the
same spacer and layer thickness. Single-crystal structures show that
the Pb–Br bonds are elongated to accommodate the large A-site
cations. Additionally, the octahedra in (BA)2(MHy)2Pb3Br10 and (BA)2(EA)2Pb3Br10 are highly distorted, and their
different stacking patterns of the inner and outer layers lead to
the formation of the n = 3 phases. Density functional
theory calculations show that 2D perovskites with larger A-site cations
(e.g., DMA, MHy, and EA) have smaller band dispersions and larger
effective masses than those with Cs+ and MA. (BA)2(MHy)2Pb3Br10 also exhibits one
of the largest Rashba splittings in the literature. Structures with
large cage cations also exhibit high band gaps within the same n number and short photoluminescence (PL) lifetimes. Temperature-
and power-dependent PL measurements reveal that the broad shoulder
in the PL peak originates from the trap states.