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On the Crystal Structure of “AgBiI” Thin Films Zewen Xiao, Weiwei Meng, David B. Mitzi, and Yanfa Yan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01834 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 16, 2016
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On the Crystal Structure of “AgBi2I7” Thin Films Zewen Xiao,†,# Weiwei Meng,†,# David B. Mitzi,‡,* and Yanfa Yan†,*
†
Department of Physics and Astronomy, and Wright Center for Photovoltaic Innovation and
Commercialization, The University of Toledo, Toledo, Ohio 43607, United States ‡
Department of Mechanical Engineering and Materials Science, and Department of Chemistry,
Duke University, Durham, North Carolina 27708, United States AUTHOR INFORMATION Corresponding Author *D.M.: Email:
[email protected] *Y.Y.: Email:
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ABSTRACT: Synthesis of cubic-phase AgBi2I7 iodobismuthate thin films and fabrication of airstable Pb-free solar cells using the AgBi2I7 absorber have recently been reported. Based on X-ray diffraction (XRD) analysis and nominal composition, it was suggested that the synthesized films have a cubic ThZr2H7 crystal structure with AgBi2I7 stoichiometry. Through careful examination of the proposed structure and computational evaluation of the phase stability and bandgap, we find that the reported “AgBi2I7” films cannot be forming with the ThZr2H7-type structure, but rather more likely adopt an Ag-deficient AgBiI4 type. Both the experimental X-ray diffraction pattern and band gap can be better explained by the AgBiI4 structure. Additionally, the proposed AgBiI4 structure, with octahedral bismuth coordination, removes unphysically short Bi–I bonding within the BiI8 hexahedra of the ThZr2I7 model. Our results provide critical insights for assessing the photovoltaic properties of AgBi2I7 iodobismuthate materials.
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Organic-inorganic lead (Pb) halide perovskites have been demonstrated as promising absorbers for fabricating efficient thin-film solar cells, with a current record power conversion efficiency (PCE) of more than 22%.1,2 However, the inclusion of toxic Pb and the instability against moisture and temperature have raised serious concerns regarding the commercialization of this new technology. Developing non- or low-toxicity and air stable halide perovskite-based solar cells has attracted extensive attention. Recently, bismuth halide compounds including A3Bi2I9 (A = CH3NH3, NH4, alkali metal;)3–7 and Cs2AgBiX6 (X = Br, Cl)8–11 have been reported to be much more stable than Pb halide perovskites and do not contain any toxic elements. Sargent and co-workers have also reported the synthesis of cubic-phase AgBi2I7 iodobismuthate thin films and fabrication of air-stable Pb-free solar cells using the AgBi2I7 absorber, with PCEs up to 1.22%.12 AgBi2I7 belongs to the AgI–BiI3 phase diagram, which has attracted significant attention in the search for lead-free photovoltaic absorbers. However, the reports in literature on the ternary compounds of the AgI–BiI3 system are controversial (see Table 1). Dzeranova et al.13 reported that the system contains two ternary compounds, Ag3BiI6 and AgBiI4, the crystal structures of which have been determined by Oldag et al.14 Ag3BiI6 crystallizes in space group R–3m with a = 4.3537(6) Å, c = 20.810(4) Å and Z = 1, whereas AgBiI4 is reported in space group Fd–3m with a = 12.223(1) Å and Z = 8. In both Ag3BiI6 and AgBiI4, bismuth and silver share octahedral sites. It is useful to note that Oldag et al.14 prepared the crystals for the Ag3BiI6 and AgBiI4 structural determination using a solvothermal approach from non-stoichiometric mixtures and no independent determination of the final crystal stoichiometry was performed beyond the structure refinement. Therefore, while the structure type and connectivity are wellestablished from these crystal structure refinements, the exact stoichiometry of the “Ag3BiI6” and
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“AgBiI4” phases is not well established. In contrast, Mashadieva et al.15 reported that the Ag– BiI3 system contains two ternary compounds with completely different nominal compositions of Ag2BiI5 and AgBi2I7, which crystallize in the space group R–3m, with a = 4.350 Å and c = 20.820 Å and the space group Fd–3m with a = 12.216 Å, respectively. These reported lattice parameters of Ag2BiI5 and AgBi2I7 are surprisingly almost the same as those of Ag3BiI6 and AgBiI4, respectively, and the authors attribute this different stoichiometry to cationic disordering and possibility for non-stoichiometry in these phases. However, Mashadieva et al. did not determine the full crystal structures (atomic positions) for the Ag2BiI5 and AgBi2I7 samples. Based on the film X-ray diffraction (XRD) and the nominal composition, Sargent and coworkers have proposed that the synthesized iodobismuthate thin films in their study crystallize in the space group Fd–3m, with a = 12.223 Å, Z = 8 and AgBi2I7 stoichiometry.12 The crystal structure is described as being similar to that of ThZr2H7, consisting of [AgI6] octahedra and [BiI8] hexahedra.12 However, the crystal symmetry and lattice constant are also essentially identical to that reported for the AgBiI4 structure. Additionally, eight-fold coordination of iodine around bismuth, as would be the case for the ThZr2H7-type structure, is unusual. Therefore, to appropriately assess the potential for solar cell applications of this new compound, it is useful to reexamine the structural and electronic properties of the reported AgBi2I7 films. In this work, through careful examination of the proposed structure and computational evaluation of the phase stability and bandgap, we find that the reported “AgBi2I7” cannot be forming in the ThZr2H7type structure with [BiI8] hexahedra, but more likely these films adopt a Ag-deficient AgBiI4 structure, which consists of edge-sharing [(Ag/Bi)I6] octahedra. The ThZr2H7-type structure is found to be energetically unstable for the AgBi2I7 system. The XRD pattern and optical
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bandgaps of the AgBi2I7 thin films can also be explained better by the thermodynamically stable “AgBiI4” structure type. Figure 1a shows the proposed ThZr2H7-type crystal structure of AgBi2I7,12 which has some unusual features that are energetically highly unfavorable. First, the [BiI8] hexahedron is a very unusual configuration. In most known Bi–I compounds, due to the large radius of the I– ion, the highest coordination for Bi is six, generally leading to [BiI6] octahedra (see Table 1). Second, the Ag–I and Bi–I bond lengths in the reported structure are 3.06 and 2.65 Å, respectively. Comparing to the Ag–I and Bi–I bond lengths in known Ag–Bi–I compounds (see Table 1), while the Ag–I bond length is reasonable, the Bi–I bond length is unphysically too short given that the Shannon ionic radii for six- and eight-coordinated Bi3+ are 1.03 Å and 1.17 Å, respectively, and the radius for I– is 2.2 Å.16 As a result, the reported ThZr2H7-type AgBi2I7 structure has an unreasonably large mass density of 10.29 g/cm3, almost twice that of other known Ag–Bi–I compounds.
Figure 1. (a) Proposed ThZr2H7-type crystal structure of AgBi2I7. (b) Total energy of AgBi2I7 as a function of the molecular dynamics simulation time. Inset of (b) shows a typical equilibrated structure of AgBi2I7 at 300K after starting from the ThZr2H7 configuration.
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Table 1. Literature structural parameters of compounds in the AgI–BiI3 phase diagram. Compound System
Space group Z Lattice constants (Å)
β-AgI
Hexagonal P63mc 17
HP-AgIb
Cubic
Ag3BiI6
ρ (g/cm3) Ag–I (Å)a Bi–I (Å)a
2 a = 4.595(4), c = 7.511(6)
5.68
2.81 [4]
Fm–3m 18
4 a =6.068
6.98
3.03 [6]
Trigonal
R–3m 14
1 a = 4.3537(6), c = 20.810(4)
6.29
3.08 [6]
3.08 [6]
Ag2BiI5
Trigonal
R–3m 15
AgBiI4
Cubic
Fd–3m 14
8 a = 12.2223(1)
6.00
3.09 [6]
3.09 [6]
AgBi2I7
Cubic
Fd–3m 12,15
8 a = 12.223,12 12.21615
10.29c
3.06 [6]c
2.65 [8]c
BiI3
Trigonal
R–3 19
6 a = 7.516(3), c = 20.7180(20)
5.80
a
a = 4.350, c =20.820
3.12 [6]
The Ag–I and Bi–I coordination numbers are given in brackets. b“HP” indicates high pressure
phase. cThe ThZr2H7–type structure reported in Ref. 12.
The unphysically short Bi–I bond length and too high mass density for the ThZr2H7-type structure is confirmed by our density functional theory (DFT) structural relaxations of the AgBi2I7 cells with fixed symmetry. As summarized in Table 2, the lattice constant, cell volume, and Bi–I bond length increase significantly after the structural relaxation. As a result, the relaxed density becomes comparable with the other known Ag–Bi–I compounds. It should be noted that
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local density approximation (LDA) usually underestimates the lattice parameters, while Perdew– Burke–Ernzerhof (PBE)20 generalized gradient Approximation (GGA) overestimates the lattice parameters, typically within 5%. However, for AgBi2I7 with the ThZr2H7-type structural model,12 the lattice constants predicted by LDA and PBE are both overestimated from the experimental value by as much as 15.1% and 24.5%, respectively. Compared with LDA and PBE, the Heyd– Scuseria–Ernzerhof (HSE)21,22 hybrid functional generally predicts more accurate lattice constants. However, HSE still overestimates the lattice constant by 23.4%. These large overestimations are apparently beyond the typical DFT errors, but actually suggest that the formula of AgBi2I7 contains too many atoms for the experimentally determined unit cell, assuming space group Fd–3m with a = 12.223 Å and Z = 8.
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Table 2. Experimental and calculated lattice parameters and bandgaps for AgBi2I7 (in the ThZr2H7 structure type) and AgBiI4.a Compound Method a (Å)
AgBi2I7
ρ (g/cm3)
Ag–I (Å)
b Bi–I (Å) Eg (eV)
Exp. 12
12.223
1826.16
10.29
3.06
2.65
1.66i, 1.87d
LDA
14.063
2781.21
6.75
3.03
3.28
