Three-dimensional (3D) hybrid organic-inorganic lead halide perovskites (HOIPs) exhibit remarkable optoelectronic properties for solar energy conversion but suffer from long-standing issues of lead-toxicity and environmental stability. Associated two-dimensional (2D) analogs are gaining increasing interest due to superior chemical stability, unprecedented structural diversity and broader property tunability. Towards Pb-free 2D HOIPs, double perovskites (DPs) with mixed valent dual metals are attractive. Translation of mixed-metal DPs to iodides, with their prospectively lower bandgaps, represents an essential target for semiconducting halide perovskites but has so far been thwarted using traditional spacer cations due to either intrinsic instability or formation of non-perovskite competing phases. Here, we present a pathway to an unprecedented 2D Ag-Bi iodide DP by employing a specifically designed conjugated oligothiophene spacer cation (i.e. [bis-aminoethyl] bithiophene) that templates the iodide DP through a collective influence of associated aromatic interactions, hydrogen bonding, bidentate tethering, and structural rigidity. Hybrid density functional theory calculations confirm the direct nature of 2.00(2) eV bandgap observed in the new DP iodide, which is the lowest value reported so far for 2D DPs, and reveal a relatively flat band edges derived principally from Ag-d/I-p (valence band) and Bi-p/I-p (conduction band) states. This work opens up new avenues for exploring specifically designed organic cations to stabilize otherwise inaccessible 2D HOIPs with potential applications for optoelectronics.

Direct-bandgap 2D silver-bismuth iodide double perovskite: Structure-directing influence of an oligothiophene spacer cation

2019 Fall National Meeting

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