Intrinsic Instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = Halogen) Double

Apr 18, 2017 - Recently, there has been substantial interest in developing double-B-cation halide perovskites, which hold the potential to overcome th...
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Intrinsic Instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = Halogen) Double Perovskites: A Combined Density Functional Theory and Experimental Study Zewen Xiao, ke-zhao Du, Weiwei Meng, Jianbo Wang, David B. Mitzi, and Yanfa Yan J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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Intrinsic Instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = Halogen) Double Perovskites: A Combined Density Functional Theory and Experimental Study Zewen Xiao,†,‡,# Ke-Zhao Du,¶,# Weiwei Meng,†,§ Jianbo Wang,§ David B. Mitzi,¶,* and Yanfa Yan†,* †

Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States ¶ Department of Mechanical Engineering and Materials Science, and Department of Chemistry, Duke University, Durham, NC 27708, United States § School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Micro- and Nanostructures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China

Supporting Information Placeholder ABSTRACT: Recently, there has been substantial interest in developing double-B-cation halide perovskites, which hold the potential to overcome the toxicity and instability issues inherent within emerging lead halide-based solar absorber materials. Among all double perovskites investigated, In(I)-based Cs2InBiCl6 and Cs2InSbCl6 have been proposed as promising thin-film photovoltaic absorber candidates, with computational examination predicting suitable materials properties, including direct bandgap and small effective masses for both electrons and holes. In this study, we report the intrinsic instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = halogen) double perovskites by a combination of density functional theory and experimental study. Our results suggest that the In(I)-based double perovskites are unstable against oxidation into In(III)-based compounds. Further, the results show the need to consider reduction-oxidation (redox) chemistry when predicting stability of new prospective electronic materials, especially when less common oxidation states are involved.

Despite rapid improvement in record power conversion efficiency (PCE) for solar cells based on lead (Pb) halide perovskites (ABX3),1,2 where “A” is a relatively large inorganic or organic cation (e.g., Cs+, CH3NH3+), B = Pb and X = Cl, Br, I, the ultimate commercialization of this emerging technology still faces serious challenges, such as the toxicity of Pb and instability against moisture/air and temperature. Extensive efforts have been undertaken to identify analogous nontoxic or low-toxicity and airstable halide perovskite-based solar cell absorbers. Theoretical studies have shown that the superior photovoltaic properties of Pb halide perovskite absorbers are attributed to the Pb 6s lone-pairinduced strong Pb 6s–I 5p antibonding coupling, the high symmetry of the perovskite structure, and the 3D electronic dimensionality.3–5 Therefore, lone-pair Bi3+/Sb3+ trivalent cations have been employed to replace Pb; however, the resulting A+3B3+2X−9 (B = Bi and Sb) compounds show low electronic as well as structural dimensionalities, leading to large bandgaps (> 2 eV), heavy carrier effective masses, detrimental defect properties, and thus poor photovoltaic performances.6–9 Alternatively, it has been pro-

posed to combine trivalent Bi3+/Sb3+ and monovalent cations on the B-site of the perovskite to form the structurally 3D double perovskites, A2B’B’’X6 (B’ = monovalent cations; B’’ = trivalent cations).10,11 So far, a number of Bi and non-Bi containing halide double perovskites have been synthesized.10–14 While most of these double perovskites exhibit large and indirect bandgaps,10–14 Cs2AgInCl6 shows a direct but parity-forbidden bandgap.14,15 These electronic and optical properties are highly undesirable for efficient thin-film solar cell applications. Density-functional theory (DFT) has been extensively used to screen A2B’B’’X6 double perovskites.16–18 Recently, Zhao et al. have proposed In(I)-based double perovskites Cs2InBiCl6 and Cs2InSbCl6 as promising photovoltaic absorber candidates, primarily because of the suitable direct bandgaps predicted by hybrid functional with spin-orbit coupling (HSE+SOC) calculation (1.02 eV and 0.91 eV, respectively), small effective masses for electrons and holes, and thermodynamic stability against decomposition.18 While the In(I) oxidation state is unusual in inorganic solids, a few In(I)-containing compounds have been reported, including InCl,19 and In3Ti2Br9.20 In contrast, In(III) represents the more typical oxidation state, and In(III)-based double perovskites including Cs2AgInCl6 have already been synthesized.14 Stability of the In(I) state, particularly, against oxidation to In(III), is therefore a major question for the proposed In(I)-based double perovskites. In this work, we use a combination of DFT and experimental study to show that indeed reduction/oxidation (redox) reactions oxidize In(I) to In(III) and reduce Bi(III)/Sb(III) to metal, destabilizing the proposed In(I)-based double perovskites. The DFT calculations suggest that the In(I)-based halide double perovskites will decompose spontaneously into products involving In(III)based ternary compounds such as CsInI4, Cs3In2Br9, Cs3In2Cl9. Solid-state synthesis efforts confirm the theoretically predicted difficulty for preparing these compounds; all attempted reactions do not form the desired double perovskite phases. Instead, they form In(III)-based ternary compounds, with part of the Bi3+/Sb3+ being reduced to metal. We first investigate the possible structure of Cs2In(I)M(III)X6 (M = Sb and Bi, X = halogen) by DFT calculations. It is known that the cubic structure of halide double perovskites is less stable

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than the distorted structures with tilted octahedra at low temperatures (Figure S1a).21 However, at elevate temperatures, the cubic structure can be stabilized by entropy, depending on the energy difference between the cubic and distorted structures at low temperature. Therefore, we calculated the energy differences between the cubic and distorted structures, defined as ∆E = E0-tilt − E3-tilt, for a number of experimentally-synthesized halide double perovskites, as shown in Figure S1b and summarized in Table S1. The double perovskites labeled by squares crystalize in the cubic phase at room temperature (RT), though the cubic phase is unstable against distortion at T = 0 K. Only when the calculated energy difference is large enough (>38 meV/atom), the double perovskites labeled by triangles crystalize into the distorted phase at RT, since the entropy is not enough to stabilize the cubic phase. For Cs2InBiX6 and Cs2InSbX6 studied in this paper, the energy differences are around 10 and 4 meV/atom, respectively. Therefore, based on the available experimental results reported in literature, it is reasonable to consider the cubic phase for Cs2InBiX6 and Cs2InSbX6 at RT, if these hypothetical double perovskites could ever be synthesized. We then explain the fundamental reasons why the prospective In(I)-based double perovskites Cs2InBiCl6 and Cs2InSbCl6 exhibit promising photovoltaic properties—e.g., appropriate bandgap values and small carrier effective masses. We calculated the bandgaps of Cs2In(I)M(III)X6 by the HSE+SOC method. The calculated bandgaps for iodides Cs2InBiI6 and Cs2InSbI6 are rather small, i.e., 0.21 eV and 0.17 eV, respectively. The bromides Cs2InBiBr6 and Cs2InSbBr6 exhibit slightly larger bandgaps of 0.33 eV and 0.41 eV, respectively. The bandgaps of chlorides Cs2InBiCl6 and Cs2InSbCl6 are 0.88 eV and 0.98 eV, respectively. Therefore, while chlorides usually have too large bandgaps for photovoltaic applications, Cs2InBiCl6 and Cs2InSbCl6 are expected to have the most appropriate values for single-junction solar cells and have been proposed as photovoltaic absorber candidates.18 As shown in Figure 1a, Cs2InBiCl6, as a representative Cs2In(I)M(III)X6 compound, exhibits a direct bandgap at the Γ point. The valence band maximum (VBM) consists mainly of antibonding states of In 5s and Cl 3p orbitals (Figure 1b). The conduction band minimum (CBM) is composed mainly of the antibonding states of Bi 6p and Cl 3p orbitals and exhibits a clear SOC-induced splitting (Figure 1b). These results are generally consistent with the those reported by Zhao et al.18

Figure 1. HSE+SOC calculated (a) band structure and (b) total and projected densities of states of Cs2InBiCl6. It should be noted that the antibonding states of Bi 6s and Cl 3p orbitals (marked by the orange arrow in Figure 1b) are located at around 2.8 eV below the VBM and thus have little contribution to the VBM. Instead, the VBM is largely derived from the highenergy-lying In 5s antibonding states (marked by the red arrow in Figure 1b). For a qualitative understanding for these electronic features, it is important to know the relative energy levels of lonepair s states of B cations. For this, we have constructed a hypothetical sextuple perovskite Cs6InTlSnPbSbBiCl18, and calculated

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its density of states, as shown in Figure 2. There are two notable trends. One is that the energy levels of both the occupied ns2 states and the unoccupied np0 states increase in the order Bi→ Sb→ Pb→ Sn→ Tl→ In. The other is that the occupied ns2 of the lighter elements In, Sn and Sb are shallower and more dispersive than those of the heavier elements Tl, Pb and Bi, respectively. Therefore, compared with the heavier lone-pair cations, the lighter ones show stronger coupling with the p states of the halogens. Among the lone-pair cations, In(I) has the highest occupied ns2 states, while Bi(III) and Sb(III) have relatively deep unoccupied np0 states, which combine to yield small bandgaps for the Cs2In(I)M(III)X6 double perovskites.

Figure 2. Crystal structure (left) and PBE+SOC calculated total and projected densities of states of a hypothetical sextuple perovskite Cs6InTlSnPbSbBiCl18. As discussed above, the high-energy-lying In 5s2 states are mainly responsible for the promising photovoltaic properties of In(I)-based double perovskites Cs2InBiCl6 and Cs2InSbCl6. Unfortunately, such high-energy-lying In 5s2 states should also be extremely unstable against decomposition and/or oxidation. Interestingly, Zhao et al. have determined that Cs2InBiCl6 and Cs2InSbCl6 are stable with respect to decomposition.18 However, they did not consider the possibility of oxidation-reduction (redox) decomposition pathways.18 Therefore, in our stability assessments, we have considered experimentally-reported prospective decomposition products including In(III)-based CsInI4, Cs3In2Br9,22 Cs3In2Cl9 (trigonal, R−3c),22 as well as other ternary, binary and elemental phases as possible decomposition products (see Table S2). Figure 3 shows the calculated decomposition energies (∆Hd) of the Cs2In(I)M(III)X6 double perovskites along three representative pathways. First, for the nonredox decompositions into binary compounds (the red bars in Figure 3), all the Cs2In(I)M(III)X6 double perovskites show large positive ∆Hd values, consistent with the results reported by Zhao et al.18 Second, for the nonredox decomposition pathways involving ternary compounds such as Cs3M2X9 (see the green bars in Figure 3), the ∆Hd values are largely reduced as compared with those for the above all-binary-phase pathways. Particularly, for Cs2InBiI6, the ∆Hd value is negative, indicating it may spontaneously decompose even if synthesized. In other words, the attempted reaction for Cs2InBiI6 may result in a mixture of Cs3Bi2I9, CsI, and InI, rather than the desired double perovskite. This is similar to the case of Cs2AgBiI6, which has not been synthesized because of its negative ∆Hd.23

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Journal of the American Chemical Society tion product, we can definitively show the reduction of Bi(III) to Bi metal and nonexistence of the In(I) phase, Cs2InBiCl6, as revealed from the PXRD data (Figure S2a). Even during the process of grinding starting materials together at room temperature, elemental Bi forms, as confirmed by the PXRD (Figure S3a), and transforms the mixed powder to black coloration, as shown in Figures S2b and c. After heating to 320 °C, the Bi crystalline quality becomes much better (Figure 4b). PXRD patterns of the individual InBr, BiBr3 and CsBr starting materials (Figure S3b−d) do not show evidence of Bi peaks, indicating that the elemental Bi doesn’t come from an impurity in the starting materials.

Figure 3. Calculated decomposition energies (∆Hd) of (a) Cs2InBiI6, (b) Cs2InBiBr6, (c) Cs2InBiCl6, (d) Cs2InSbI6, (e) Cs2InSbBr6, and (f) Cs2InSbCl6 along three representative decomposition pathways. The red bars indicate pathways involving only binary compounds. The green and blue bars indicate the lowest∆Hd nonredox and redox pathways involving ternary compounds. Finally, for the redox decompositions involving In(III)-based compounds such as CsInI4, Cs3In2Br9, and Cs3In2Cl9 (see the blue bars in Figure 3), the Bi-and Sb-analogues show rather large differences. For the Bi-analogues, while Cs2InBiI6 shows a positive ∆Hd value of as small as 2.6 meV/atom, Cs2InBiBr6 and Cs2InBiCl6 have negative ∆Hd values of −1.6 and −16.5 meV/atom, respectively. Evidently, the oxidation reaction for In(I)→ In(III) becomes more strongly favored transitioning from iodide to bromide to chloride. Along with the oxidation of In(I) to In(III) states, part of trivalent Bi(III) is reduced to metallic Bi. These results indicate Cs2InBiBr6 and Cs2InBiCl6 may spontaneously decompose into products including Cs3In2X9, Cs3Bi2X9 and Bi, even if synthesized. In other words, the attempted reactions for synthesizing Cs2InBiBr6 and Cs2InBiCl6 should result in mixtures of In(III)-based Cs3In2X9, Cs3Bi2X9 and Bi, rather than the desired double perovskites. Compared to the Bi-analogues, the Sbanalogues show more negative ∆Hd values for the redox decompositions pathways, indicating even lower probability of successful synthesis for the Cs2InSbX6 system. To further evaluate the stability issue for these compounds, we have attempted solid-state synthesis for Cs2InBiBr6 and Cs2InBiCl6, which show positive ∆Hd values for the nonredox decomposition pathways but negative and relatively small ∆Hd values for the redox decompositions. The details of the solid state reaction pathway and characterization are described in the Supporting Information. We did not find any evidence of the formation of the desired double perovskites. After the reaction of the bromide, we obtained colorless hexagonal crystals and black crystals in the quartz reaction tube as shown in Figure 4a. The unit cell parameters of the crystals were determined using singlecrystal X-ray diffraction (SCXRD). The colorless and black crystals have been identified as Cs3In2Br9 and Bi (Table S3), respectively. Powder XRD (PXRD) also confirms the existence of Cs3In2Br9 and Bi phases in the final bromide product (Figure 4b). In the chloride reaction product, black Bi crystals can also be found and identified by SCXRD and PXRD (Figure S2a). While we were not able to identify the other phases in the chloride reac-

Figure 4. (a) Photo of the product in the bromide reaction. Inset shows a black crystal selected for SCXRD. (b) Measured PXRD pattern of the bromide product and simulated PXRD patterns for Cs3In2Br9 (hexagonal, P63/mmc)22 and Bi (trigonal, R−3m). In summary, our results indicate that Cs2In(I)M(III)X6 (M = Bi, Sb; X = halogen) double perovskites are not stable under thermodynamic equilibrium conditions. While the proposed Cs2InBiCl6 and Cs2InSbCl6 systems are among the few double perovskite candidates that are expected to offer promising electronic properties, such as direct bandgaps and small carrier effective masses, they show less practical promise in terms of fabrication of stable semiconducting materials and corresponding solar cells. Even if non-equilibrium conditions might be used to stabilize the In(I)containing double perovskites, the reduction-oxidation potentials for In(I) and Bi(III) will create a driving force for decomposition (i.e., instability). Understanding the impact of redox chemistry is also vital for considering the broader flexibility of the perovskite family to accommodate lower-dimensional structures for functionality beyond photovoltaics.24

ASSOCIATED CONTENT Supporting Information Theoretical methods, stability against structural distortion, calculated decomposition energies, experimental details, single-crystal structural parameters of the bromide products, PXRD pattern of chloride product, photos of the starting chemicals before and after grinding for the chloride reaction. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

Present Addresses ‡

Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama 226-8503, Japan

Author Contributions

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(24)

These authors contributed equally.

Saparov, B.; Mitzi, D. B. Chem. Rev. 2016, 116 (7), 4558.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0006712, and the National Science Foundation under contract no. CHE−1230246 and DMR−1534686. This research used the resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The work at Wuhan University was supported by the National Natural Science Foundation of China (51671148, 51271134, J1210061, 11674251, 51501132, 51601132), and the Hubei Provincial Natural Science Foundation of China (2016CFB446, 2016CFB155).

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