Shedding Light on the Stability and Structure–Property Relationships of Two-Dimensional Hybrid Lead Bromide Perovskites

Two-dimensional
(2D) hybrid lead iodide perovskites have gained
prominence due to their remarkable structural tunability, optoelectronic
features, and moisture stability, which have rendered them as attractive
alternatives to 3D MAPbI3 for optoelectronic devices. 2D
multilayer lead bromide perovskites remain an unfathomed phase space
with the lack of systematic studies to establish the structure, photophysical
properties and stability behavior of this family of 2D halide perovskites.
Herein, we present new members of bilayer lead bromide perovskites
(CmH2m+1NH3)2(CH3NH3)­Pb2Br7 (m = 6–8) that belong to the Ruddlesden–Popper
structure type, incorporating long chain alkyl-monoammonium cations
(CmH2m+1NH3) of hexylammonium (m = 6), heptylammonium
(m = 7), and octylammonium (m =
8). A universal solution synthetic methodology for bulk multilayer
lead bromide perovskites is presented with all structures solved and
refined using single crystal X-ray diffraction. The studied bilayer
lead bromide perovskites demonstrate a decrease in the lattice rigidity
and lattice match of the inorganic perovskite layer–organic
layer, as the alkyl-monoammonium chain length increases. In comparison
to their iodide analogues, the bilayer lead bromide compounds exhibit
elongation of their stacking axis despite the smaller dimensions of
the [PbBr6]4− lattice, while their internal
lattice strain was calculated to be reduced, inferring a greater lattice
match between the inorganic [PbBr6]4− perovskite layer and organic layer. The (CmH2m+1NH3)2(CH3NH3)­Pb2Br7 (m = 4, 6–8) compounds exhibit narrow-band emission
near 2.5 eV. Time-resolved photoluminescence (PL) displays longer
carrier lifetimes on the nanosecond time scale comparing to their
iodide analogues, where electronic structure calculations indicate
that the increase of the alkyl chain length and, thus, lattice softness
enhances nonradiative recombinations. A complete set of air, light,
and heat stability tests on unencapsulated thin films of (CmH2m+1NH3)2(CH3NH3)­Pb2Br7 (m = 4, 6–8) and MAPbBr3 show
they are stable in ambient air for at least 5 months, exhibiting greater
extrinsic stability than the 2D lead iodide congeners. Extraordinarily,
3D MAPbBr3 films prove to be more stable than films of
2D lead bromide perovskites, in contrast to MAPbI3 which
is less stable than the 2D lead iodide perovskites.