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Hybrid Charge-Transfer Semiconductors: (C7H7)SbI4, (C7H7)BiI4, and Their Halide Congeners

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journal contribution
posted on 22.03.2019, 00:00 by Iain W. H. Oswald, Eve M. Mozur, Ian P. Moseley, Hyochul Ahn, James R. Neilson
Hybrid metal halides yield highly desirable optoelectronic properties and offer significant opportunity due to their solution processability. This contribution reports a new series of hybrid semiconductors, (C7H7)­MX4 (M = Bi3+, Sb3+; X = Cl, Br, I), that are composed of edge-sharing MX6 chains separated in space by π-stacked tropylium (C7H7+) cations; the inorganic chains resemble the connectivity of BiI3. The Bi3+ compounds have blue-shifted optical absorptions relative to the Sb3+ compounds that span the visible and near-IR region. Consistent with observations, DFT calculations reveal that the conduction band is composed of the tropylium cation and valence band primarily the inorganic chain: a charge-transfer semiconductor. The band gaps for both Bi3+ and Sb3+ compounds decrease systematically as a function of increasing halide size. These compounds are a rare example of charge-transfer semiconductors that also exhibit efficient crystal packing of the organic cations, thus providing an opportunity to study how structural packing affects optoelectronic properties.