Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites

We investigate the
origin of the broadband visible emission in
layered hybrid lead-halide perovskites and its connection with structural
and photophysical properties. We study ⟨001⟩ oriented
thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead
iodide, (C12H28N)2PbI4, by combining first-principles simulations with time-resolved photoluminescence,
steady-state absorption and X-ray diffraction measurements on cooling
from 300 to 4 K. Ultrafast transient absorption and photoluminescence
measurements are used to track the formation and recombination of
emissive states. In addition to the excitonic photoluminescence near
the absorption edge, we find a red-shifted, broadband (full-width
at half-maximum of about 0.4 eV), emission band below 200 K, similar
to emission from ⟨110⟩ oriented bromide 2D perovskites
at room temperature. The lifetime of this sub-band-gap emission exceeds
that of the excitonic transition by orders of magnitude. We use X-ray
diffraction measurements to study the changes in crystal lattice with
temperature. We report changes in the octahedral tilt and lattice
spacing in both materials, together with a phase change around 200
K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy
emission is due to interstitial iodide and related Frenkel defects.
Our results demonstrate that white-light emission is not limited to
⟨110⟩ oriented bromide 2D perovskites but a general
property of this class of system, and highlight the importance of
defect control for the formation of low-energy emissive sites, which
can provide a pathway to design tailored white-light emitters.