A Direct-Bandgap 2D Silver-Bismuth Iodide Double Perovskite: The Structure-Directing Influence of an Oligothiophene Spacer Cation

Three-dimensional (3D) hybrid organic-inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from long-standing issues of environmental stability and lead-toxicity. Associated two-dimensional (2D) analogs are garnering increasing interest due to superior chemical stability, structural diversity and broader property tunability. Towards lead-free 2D HOIPs, double perovskites (DPs) with mixed valent dual metals are attractive but remain unexplored beyond a recent 2D Ag-Bi bromide DP with n-butylammonium spacer cations. Translation of mixed-metal DPs to iodides, with their prospectively lower bandgaps, represents an important target for semiconducting halide perovskites, but has so far proven inaccessible using traditional spacer cations due to either intrinsic instability or formation of competing non-perovskite phases. Here, we demonstrate the first example of a 2D Ag-Bi iodide DP with a direct bandgap of 2.00(2) eV, templated by a layer...