Abstract
Hybrid bismuth-containing halides are emerging as alternative candidates to lead-containing perovskites for light-harvesting applications, as Bi3+ is isoelectronic with Pb2+ and the presence of an active lone pair of electrons is expected to result in outstanding charge-carrier transport properties. Here, we report a family of one binary and three ternary iodobismuthates containing 1,4-diazabicyclo[2.2.2]octane (DABCO). These materials have been prepared solvothermally and their crystal structures, thermal stability, and optical properties determined. Reactions carried out in the presence of bismuth iodide and DABCO produced (C6H12N2)BiI3 (1), which consists of hybrid ribbons in which pairs of edge-sharing bismuth octahedra are linked by DABCO ligands. Short I···I contacts give rise to a three-dimensional network. Similar reactions in the presence of copper iodide produced (C8H17N2)2Bi2Cu2I10(2) and [(C6H13N2)2BiCu2I7](C2H5OH) (3) in which either ethylated DABCO cations (EtDABCO)+ or monoprotonated DABCO cations (DABCOH)+ are coordinated to copper in discrete tetranuclear and trinuclear clusters, respectively. In the presence of potassium iodide, a unique three-dimensional framework, (C6H14N2)[(C6H12N2)KBiI6] (4), was formed, which contains one-dimensional hexagonal channels approximately 6 Å in diameter. The optical band gaps of these materials, which are semiconductors, range between 1.82 and 2.27 eV, with the lowest values found for the copper-containing discrete clusters. Preliminary results on the preparation of thin films are presented.
Highlights
Lead and bismuth halides have long been investigated due to their optical and electronic properties.1. Interest in these materials has grown exponentially since the discovery in 2009 of the potential use of methylammonium lead triiodide (MAPI) as a photovoltaic material,2 which has been incorporated into solar cells with conversion efficiencies exceeding 20%
The hybrid ribbons found in 1 contrast sharply with the crystal structures of previously reported iodobismuthates containing DABCO,34 which consist of discrete iodobismuthate anions, with protonated (DABCOH)+ and (DABCOH2)2+ acting as countercations
We have shown that in solvothermal reactions carried out in the absence of hydriodic acid, DABCO can act as a ligand instead of a countercation
Summary
Lead and bismuth halides have long been investigated due to their optical and electronic properties. Interest in these materials has grown exponentially since the discovery in 2009 of the potential use of methylammonium lead triiodide (MAPI) as a photovoltaic material, which has been incorporated into solar cells with conversion efficiencies exceeding 20%.3 The exploitation of MAPI in commercial solar cells may be limited by the toxicity of lead and the intrinsic instability of these materials. As the presence of an active lone pair of electrons in Pb2+ is believed to be key for the exceptional properties of lead-containing halides, the search for alternative materials is focused on halides containing post-transition-metal cations with an ns electronic configuration. A number of onedimensional polymeric units are known, such as edge-sharing [BiI4]− chains, while structures of higher dimensionalities are extremely rare, and examples appear to be limited to a 2dimensional metal-deficient perovskite, (H2AEQT)Bi2/3I4 (AEQT = 5,5′′′′-bis(aminoethyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene), the layered A3Bi2I9 (A = K, Rb) phases, and (Me2C = NMe2)Bi2I7.13 One approach adopted to increase the dimensionality of iodobismuthates to produce lead-free materials that are structurally related to the photovoltaic lead perovskites is the heterovalent substitution of Pb2+ by Bi3+ together with the incorporation of a monovalent cation, such as Ag+ or Cu+.
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