  Lead (Pb) halide perovskites have achieved great success in recent years due
to their excellent optoelectronic properties, which is largely attributed to
the lone-pair s orbital-derived antibonding states at the valence band edge.
Guided by the key band-edge orbital character, a series of ns2-containing
(i.e., Sn2+, Sb3+, Bi3+) Pb-free perovskite alternatives have been explored as
potential photovoltaic candidates. On the other hand, based on the band-edge
orbital components (i.e., M2+ s and p/X- p orbitals), a series of strategies
have been proposed to optimize their optoelectronic properties by modifying the
atomic orbitals and orbital interactions. Therefore, understanding the
band-edge electronic features from the recently reported halide perovskites is
essential for future material design and device optimization. Here, this
Perspective first attempts to establish the band-edge orbital-property
relationship using a chemically intuitive approach, and then rationalizes their
superior properties and understands the trends in electronic properties. We
hope that this Perspective will provide atomic-level guidance and insights
toward the rational design of perovskite semiconductors with outstanding
optoelectronic properties.
