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.