Small Cyclic Diammonium Cation Templated (110)-Oriented 2D Halide (X = I, Br, Cl) Perovskites with White-Light Emission

Two-dimensional (2D)
halide perovskites exhibit excellent potential
for optoelectronics because of their outstanding physical properties
and structural diversity. White-light emission is one property of
2D perovskites that originates from self-trapped excitons (STE) in
the highly distorted structures. The so-called (110)-oriented 2D perovskites
are generally distorted and believed to be good candidates for white-light
emitting devices. Here, we report (110)-oriented 2D perovskites, C4N2H12PbX4 (X = I, Br, Cl),
templated by the small cyclic diammonium cation, 3-aminopyrrolidinium
(3APr). Structural characterization by single-crystal X-ray diffraction
reveals that the distortion of the inorganic part of the structures
is influenced by the stereochemical conformation of the cation between
the perovskite layers. The experimental bandgaps follow the trend
I < Br < Cl (2.56 eV, 3.29 eV, 3.85 eV, respectively). Density
functional theory calculations reveal a weak but significant electronic
band dispersion along the stacking axis, suggesting a non-negligible
interlayer electronic coupling caused by the short proximity of adjacent
inorganic layers. The high level of distortion results in the emergence
of white-light emission, rarely seen in iodide perovskites, as well
as the bromide and chloride isostructural analogues, which provides
perfect platform to compare the broad emission mechanism for all three
halides. The bromide and chloride perovskites show longer lifetimes
and higher color rendering index (CRI) (83 and 85), relevant to solid-state
lighting. Temperature-dependent PL measurements confirm that the broad
emission comes from different STE mechanism for different halides,
with the peak broadening persisting even at low temperature for the
chloride compound.