Double A Site Cation Engineering in Perovskite-Inspired Materials—Transforming 0D-Cs₃Bi₂I₉ into 2D-Cs₂AgBi₂I₉ with Enhanced Charge Transport and Photoresponse

Bismuth-based halide perovskite-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 using hot-spin casting method. The crystal structure of the as-obtained crystals was successfully confirmed using Single crystal X-ray diffraction (SC-XRD) analysis. It reveals that silver atoms occupy 1/3rd of the edge sites in the hexagonal lattice, resulting in contracted lattice parameters compared to the parent Cs₃Bi₂I₉. Density functional theory (DFT) calculations describe the effect of the cation mixing at the A-site on the electronic band structures of 2D-Cs₂AgBi₂I₉. The cation mixing 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 with a high responsivity of 1.1 A/W and a remarkable detectivity of 10¹² Jones. 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.