Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Hybrid Charge-Transfer Semiconductors: (C7H7)SbI4, (C7H7)BiI4, and Their Halide Congeners Iain W. H. Oswald, Eve M. Mozur, Ian P. Moseley, Hyochul Ahn, and James R. Neilson* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
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ABSTRACT: 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 πstacked 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.
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INTRODUCTION Hybrid perovskites and their derivatives have emerged as highly efficient solution-processable semiconducting materials, making them attractive for numerous applications such as lightemitting devices, photovoltaics, photodetectors, and radiation detectors.1−8 These compounds combine many of the desirable properties of traditional inorganic semiconductors such as high carrier mobilities and absorption coefficients with the processability of organic electronics.6,9,10 However, many questions remain as to how the inorganic framework can influence or couple to the electronic behavior of the organic components. In the realm of hybrid perovskites, much focus has been on compounds containing small organic cations such as methylammonium or formamidinium.11−15 Unlike smaller cations, organic cations with delocalized electrons can have electronic states near the valence and conduction band of the inorganic lattice, allowing for charge transfer from the inorganic to organic subunits. The templating of organic molecules within a complex crystal structure by inorganic frameworks remains underexplored. Such templating may give rise to properties from the cooperative behavior (e.g., electrical transport) not found in simple packings of the molecules,16 and spatial charge separation within the bulk of the material (e.g., (Pb2I6)· (H2DPNDI)·(H2O)·(NMP)).17A great diversity of compounds appears in the tetrathiofulvalene (TTF) based charge-transfer salts (e.g., TTF3SnCl6, (TTF)BiI4, (TTF)Pb2I5),18−21 in which the inorganic framework rarely shows spatial connectivity. A recent study has shown that the connectivity of the inorganic subunit can be modified in the © XXXX American Chemical Society
compounds (TMP)[BiBr5], (TMP)[BiCl5], and (TMP)1.5[Bi2I7Cl2] (TMP = N,N,N′,N′-tetramethylpiperazine) by incorporation of multiple halides, showing that structural modification can be achieved, although the organic cation is not optoelectronically active.22 One of the few other reports describing hybrid metal halides with conjugated cations used the N-heterocyclic cations N-methylpyridinium and N-ethylpyridinium in specific molar ratios relative to BiI3 to synthesize new structures with differing degrees of inorganic connectivity, such as the quasi-two-dimensional [Bi3I10]− substructural unit that is bridged by neighboring iodine atoms.23 The layered perovskites, A2PbX4, where X is a halogen and A is an organic cation, host many conjugated organic molecules (e.g., (AEQT)PbI4),24,25 though nearly all compounds rely on an ammonium salt to tether the molecule to the inorganic lattice. The diversity of chemical functionalities in conjugated molecules and the role that the inorganic lattice plays in templating such molecules beg to be expanded. Tropylium, (C 7H7 + ), provides opportunities for the discovery of materials with optoelectronically active cations at the frontier electronic states. The dielectric environment has been shown to have a profound impact on the charge-transfer based optical properties of simple tropylium salts.26 In our previous contribution, (C7H7)PbI3 and (C7H7)2SnI6 both form in solution and show activity of the π*-states derived from tropylium in the overall optical properties.27 It is therefore necessary to investigate other compounds containing cations of Received: January 17, 2019
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DOI: 10.1021/acs.inorgchem.9b00170 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
(C7H7)SbBr4. A 0.200 M Sb3+ stock solution in HBr was first made by dissolving 0.292 g (1.00 mmol) of Sb2O3 into 5 mL of concentrated HBr under magnetic stirring, resulting in a yellow solution. After complete dissolution, tropylium tetrafluoroborate (0.356 g, 2.00 mmol) was dissolved into 1 mL of concentrated HBr and then transferred to the Sb3+ stock solution, resulting in precipitation of an orange microcrystalline powder. 2 mL of acetic acid was added as an antisolvent to increase yield. The solution was then allowed to stir for 15 min. The solid was then filtered and washed with acetic acid and hexanes to remove impurities. Yield: 0.340 g (0.645 mmol); 64.5% based on Sb3+ content. (C7H7)SbI4. A 0.200 M Sb3+ stock solution in stabilized HI (57 wt % in H2O, Oriented Perovskite Sheets. Science 1995, 267, 1473−1476. (5) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. OrganicInorganic Hybrid Materials as Semiconducting Channels in Thin-Film Field-Effect Transistors. Science 1999, 286, 945−947. (6) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316−319. (7) Booker, E. P.; Thomas, T. H.; Quarti, C.; Stanton, M. R.; Dashwood, C. D.; Gillett, A. J.; Richter, J. M.; Pearson, A. J.; Davis, N. J. L. K.; Sirringhaus, H.; Price, M. B.; Greenham, N. C.; Beljonne, D.; Dutton, S. E.; Deschler, F. Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites. J. Am. Chem. Soc. 2017, 139, 18632−18639. (8) Yuan, Z.; Zhou, C.; Tian, Y.; Shu, Y.; Messier, J.; Wang, J. C.; van de Burgt, L. J.; Kountouriotis, K.; Xin, Y.; Holt, E.; Schanze, K.; Clark, R.; Siegrist, T.; Ma, B. One-dimensional organic lead halide perovskites with efficient bluish white-light emission. Nat. Commun. 2017, 8, 14051. (9) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths >175 μm in solution grown CH3NH3PbI3 single crystals. Science 2015, 347, 967−970. (10) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (11) Park, B.-W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo, G.; Johansson, E. M. J. Bismuth Based Hybrid Perovskites A3Bi2I9 (A: Methylammonium or Cesium) for Solar Cell Application. Adv. Mater. 2015, 27, 6806−6813. (12) Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623−3630. (13) Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidinium Lead Trihalide: a Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7, 982−988. (14) Slavney, A. H.; Hu, T.; Lindenberg, A. M.; Karunadasa, H. I. A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications. J. Am. Chem. Soc. 2016, 138, 2138−2141. (15) Lehner, A. J.; Fabini, D. H.; Evans, H. A.; Hébert, C.-A.; Smock, S. R.; Hu, J.; Wang, H.; Zwanziger, J. W.; Chabinyc, M. L.; Seshadri, R. Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics. Chem. Mater. 2015, 27, 7137−7148. (16) Mitzi, D. B. Templating and Structural Engineering in Organic−Inorganic Perovskites. J. Chem. Soc., Dalton Trans. 2001, 1−12. (17) Savory, C. N.; Palgrave, R. G.; Bronstein, H.; Scanlon, D. O. Spatial Electron-hole Separation in a One Dimensional Hybrid Organic−Inorganic Lead Iodide. Sci. Rep. 2016, 6, 20626. H
DOI: 10.1021/acs.inorgchem.9b00170 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.9b00170 Inorg. Chem. XXXX, XXX, XXX−XXX
