Unveiling Double A-Site Cation Perovskite-Inspired Materials: From 0D-Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub> to 2D-Cs<sub>2</sub>AgBi<sub>2</sub>I<sub>9</sub> with Enhanced Charge Transport
Mozakkar Hossain, Kuntal Singh, Ankita Narwal, Md Sariful Sheikh, Sandeep K. Reddy, Kiran Vankayala, Asha Singh, Saleem Khan, S. Khan, Praveen Kumar Velpula, Manohar Chirumamilla, Sharma S. R. K. C. Yamijala, G. Krishnamurthy Grandhi, Paola Vivo, K. D. M. Rao
Abstract
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 Cs 2 AgBi 2 I 9, a two-dimensional perovskite-inspired absorber achieved through partial substitution of Cs + with Ag + at the A-site of Cs 3 Bi 2 I 9 . 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 Cs 3 Bi 2 I 9 . The double A-cation substitution promotes orbital overlap between Ag 5 s and I 6 p orbitals, leading to a narrower band gap of 1.72 eV and a delocalized electronic structure in Cs 2 AgBi 2 I 9 . 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 cm 2 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 Cs 2 AgBi 2 I 9 . Utilizing these exceptional charge transport properties, Cs 2 AgBi 2 I 9 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.