Unveiling Double A-Site Cation Perovskite-Inspired Materials: From 0D-Cs3Bi2I9 to 2D-Cs2AgBi2I9 with Enhanced Charge Transport

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.