Enhanced charge transport in A-site ordered perovskite derivatives A₂A′Bi₂I₉ (A = Cs; A′ = Ag, Cu): a first-principles study

Recent experiments have synthesized Cs₂AgBi₂I₉ by partially substituting Cs⁺ with Ag⁺ at the A-site of Cs₃Bi₂I₉, resulting in enhanced charge transport properties compared to Cs₃Bi₂I₉. However, the atomic-scale mechanisms behind this enhancement remain unclear. In this work, we investigate the carrier transport mechanisms in Cs₂A'Bi₂I₉ (A' = Ag, Cu) using first-principles calculations and Boltzmann transport equation. Our results reveal that A-site ordered Cs₂A'Bi₂I₉ exhibits carrier mobilities that are 3-4 times higher than those of Cs₃Bi₂I₉ within the 100-500 K temperature range. We identify polar phonon scattering as the dominant mechanism limiting mobility. Furthermore, the enhanced out-of-plane carrier mobility in Cs₂A'Bi₂I₉, particularly between 100 and 200 K, leads to reduced mobility anisotropy. These improvements are mainly due to the shorter A'-I bond lengths and increased Ag⁺/Cu⁺ s-I^- p orbital coupling. Notably, substitution with Cu⁺ results in a further reduction in the band gap and enhanced hole mobility compared to Ag⁺ substitution in Cs₃Bi₂I₉. Further analysis reveals that the significant increase in carrier mobility in Cs₂A'Bi₂I₉ can be largely explained by the smaller carrier effective masses (<i>m</i>*) and weaker Fröhlich coupling strengths (<i>α</i>), resulting in a lower polar mass <i>α</i>(<i>m</i>*/<i>m</i>_e), compared to Cs₃Bi₂I₉. Our study provides valuable insights into the transport properties of Bi-based perovskite derivatives, paving the way for their future applications in optoelectronic devices.