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) analogues are garnering increasing interest due to superior chemical stability, structural diversity, and broader property tunability. Toward lead-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 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 of bifunctionalized oligothiophene cations, i.e., (bis-aminoethyl)­bithiophene, through a collective influence of aromatic interactions, hydrogen bonding, bidentate tethering, and structural rigidity. Hybrid density functional theory calculations for the new material reveal a direct bandgap, consistent with the experimental value, and 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.

三维（3D）有机-无机杂化卤化铅钙钛矿（hybrid organic–inorganic lead halide perovskites, HOIPs）在太阳能转换领域展现出卓越的光电子特性，但长期存在环境稳定性不足与铅毒性两大棘手问题。与之关联的二维（2D）类似物因化学稳定性更优、结构多样性丰富且性能调控范围更广，正受到学界日益广泛的关注。
针对无铅2D HOIPs，含混合价双金属位点的双钙钛矿（double perovskites, DPs）具有显著研究价值。将混合金属双钙钛矿拓展至碘化物体系，有望获得更低的带隙，这是半导体卤化钙钛矿领域的重要研究目标，但此前受限于传统间隔阳离子易引发本征不稳定性或生成竞争性非钙钛矿相，该目标始终难以实现。
本研究首次报道了以双官能化低聚噻吩阳离子（即双(氨基乙基)联噻吩）为结构模板的2D银-铋碘化物双钙钛矿，其直接带隙为2.00(2) eV，合成过程得益于芳香相互作用、氢键作用、双齿锚定效应与结构刚性的协同调控。
针对该新材料的杂化密度泛函理论（hybrid density functional theory）计算结果显示，其具有直接带隙，与实验测得值高度吻合；其相对平坦的带边主要源自Ag-d/I-p（价带）与Bi-p/I-p（导带）电子态。
本研究为定向设计有机阳离子以稳定此前难以合成的2D HOIPs开辟了全新路径，相关材料在光电子领域具备潜在应用价值。