0D and 2D: The Cases of Phenylethylammonium Tin Bromide Hybrids

Tin
halide perovskites and perovskite-related materials have emerged
as promising lead-free hybrid materials for various optoelectronic
applications. While remarkable progress has been achieved in the development
of organic tin halide hybrids with diverse structures and controlled
dimensionalities at the molecular level, some controversial results
that have been reported recently need to be addressed. For instance,
different photophysical properties have been reported for two-dimensional
(2D) (PEA)2SnBr4 (PEA = phenylethylammonium)
by several groups with distinct emission peaks at around 468 and 550
nm. Here we report our efforts in the synthesis of phenylethylammonium
tin bromide hybrids with zero-dimensional (0D) and 2D structures,
and characterizations of their structural and photophysical properties.
0D [(PEA)4SnBr6]­[(PEA)­Br]2[CCl2H2]2 was found to exhibit strong yellow
emission peak at 566 nm with a photoluminescence quantum efficiency
(PLQE) of ∼90%, while 2D (PEA)2SnBr4 had
weak emission peak at 470 nm with a PLQE of <0.1%. Interestingly,
0D [(PEA)4SnBr6]­[(PEA)­Br]2[CCl2H2]2 can be converted into 2D (PEA)2SnBr4 upon drying, which would return to 0D [(PEA)4SnBr6]­[(PEA)­Br]2[CCl2H2]2 upon addition of dichloromethane. Powder X-ray
diffraction results confirmed the reversible transformation between
0D and 2D structures. Density functional theory calculations showed
that excitons in 0D [(PEA)4SnBr6]­[(PEA)­Br]2[CCl2H2]2 are highly localized,
resulting in a strongly Stokes shifted broadband emission, while delocalized
electronic states in 2D (PEA)2SnBr4 result in
weaker exciton binding, a higher exciton mobility, and a higher nonradiative
decay.