Self-trapped
excitons (STEs) have recently been observed in several
metal halide perovskites (MHPs), especially in low-dimensional ones.
Despite studies that have shown that factors like dopant, chemical
composition, lattice distortion, and structural and electronic dimensionality
may all affect the self-trapping of excitons, a general understanding
of their mechanism of formation in MHPs is lacking. Here, we study
the intrinsic and defect-induced self-trapping of excitons in three-,
two-, and one-dimensional MHPs. We find that whether the free excitons
could be trapped is simply determined by the competition of the energy-gap
decrease and deformation-energy increase along with the lattice distortion.
Both introducing halogen defects into the lattice and decreasing the
dimensionality can tip the balance between them and thus facilitate
the self-trapping of free excitons. This general picture of the mechanism
of formation of STEs provides important insights into the design and
development of high-performance white-light devices and solar cells
with MHPs.
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