Bismuth-based halideperovskite-inspired
materials (PIMs) are gaining
increasing attention as sustainable and stable alternatives to lead
halide perovskites. However, many PIMs have wide band gaps (≥2
eV) and low electronic dimensionality, limiting their utility in optoelectronic
applications. In this study, we introduce Cs2AgBi2I9, a two-dimensional perovskite-inspired absorber achieved
through partial substitution of Cs+ with Ag+ at the A-site of Cs3Bi2I9. Single-crystal
X-ray diffraction analysis reveals that silver atoms occupy the edge
sites in the hexagonal lattice, resulting in contracted lattice parameters
compared to the parent Cs3Bi2I9.
The double A-cation substitution promotes orbital overlap between
Ag 5s and I 6p orbitals, leading
to a narrower band gap of 1.72 eV and a delocalized electronic structure
in Cs2AgBi2I9. Consequently, the
2D-PIM exhibits a three-orders-of-magnitude lower electrical resistivity
and an exceptional carrier mobility-lifetime product (μτ)
of 3.4 × 10–3 cm2 V–1, representing the highest among solution-processed Bi-PIMs. Furthermore,
low-temperature photoluminescence measurements indicate weak electron–phonon
coupling, while transient absorption spectroscopy reveals extended
hot-carrier lifetimes, suggesting efficient exciton transport in Cs2AgBi2I9. Utilizing these exceptional
charge transport properties, Cs2AgBi2I9 photodetectors show a remarkable broad spectral response. This work
demonstrates the potential of a double A-site cation engineering strategy
to develop low-toxicity PIMs with precisely tailored structural and
optoelectronic properties.