Structural Diversity in White-Light-Emitting Hybrid Lead Bromide Perovskites

Hybrid
organic–inorganic halide perovskites are under intense
investigations because of their astounding physical properties and
promises for optoelectronics. Lead bromide and chloride perovskites
exhibit intrinsic white-light emission believed to arise from self-trapped
excitons (STEs). Here, we report a series of new structurally diverse
hybrid lead bromide perovskites that have broad-band emission at room
temperature. They feature Pb/Br structures which vary from 1D face-sharing
structures to 3D corner- and edge-sharing structures. Through single-crystal
X-ray diffraction and low-frequency Raman spectroscopy, we have identified
the local distortion level of the octahedral environments of Pb2+ within the structures. The band gaps of these compounds
range from 2.92 to 3.50 eV, following the trend of “corner-sharing
< edge-sharing < face-sharing”. Density functional theory
calculations suggest that the electronic structure is highly dependent
on the connectivity mode of the PbBr6 octahedra, where
the edge- and corner-sharing 1D structure of (2,6-dmpz)3Pb2Br10 exhibits more disperse bands and smaller
band gap (2.49 eV) than the face-sharing 1D structure of (hep)­PbBr3 (3.10 eV). Using photoemission spectroscopy, we measured
the energies of the valence band of these compounds and found them
to remain almost constant, while the energy of conduction bands varies.
Temperature-dependent PL measurements reveal that the 2D and 3D compounds
have narrower PL emission at low temperature (∼5 K), whereas
the 1D compounds have both free exciton emission and STE emission.
The 1D compound (2,6-dmpz)3Pb2Br10 has the highest photoluminescence quantum yield of 12%, owing to
its unique structure that allows efficient charge carrier relaxation
and light emission.