  Based on first-principles calculations, we show that chemically active metal
ns2 lone pairs play an important role in exciton relaxation and dissociation in
low-dimensional halide perovskites. We studied excited-state properties of
several recently discovered luminescent all-inorganic and hybrid
organic-inorganic zero-dimensional (0D) Sn and Pb halides. The results show
that, despite the similarity in ground-state electronic structure between Sn
and Pb halide perovskites, the chemically more active Sn2+ lone pair leads to
stronger excited-state structural distortion and larger Stokes shift in Sn
halides. The enhanced Stokes shift hinders excitation energy transport, which
reduces energy loss to defects and increases the photoluminescence quantum
efficiency (PLQE). The presence of the ns2 metal cations in the 0D halide
perovskites also promotes the exciton dissociation into electron and hole
polarons especially in all-inorganic compounds, in which the coupling between
metal-halide clusters is significant.
