Layered Halide Double Perovskites Cs3+nM(II)nSb2X9+3n (M

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Letter 3+n

n

2

9+3n

Layered Halide Double Perovskites Cs M(II)SbX (M = Sn, Ge) for Photovoltaic Applications

Gang Tang, Zewen Xiao, Hideo Hosono, Toshio Kamiya, Daining Fang, and Jiawang Hong J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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The Journal of Physical Chemistry Letters

Layered Halide Double Perovskites Cs3+nM(II)nSb2X9+3n (M = Sn, Ge) for Photovoltaic Applications Gang Tang,a, # Zewen Xiao,b, # Hideo Hosono,b, c Toshio Kamiya,b, c Daining Fang,d and Jiawang Hong a, * a

School of Aerospace Engineering, Beijing Institute of Technology, Beijing, 100081, China

b

Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama,

226-8503, Japan c

Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo Institute of

Technology, Yokohama, 226-8503, Japan d

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081,

China Corresponding Author *E-mail: [email protected]. #

These authors contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT Over the past few years, the development of lead-free and stabile perovskite absorbers with excellent performance has attracted extensive attention. Much effort has been devoted to screening and synthesizing this type of solar cell absorbers. Here, we present a general design strategy for designing the layered halide double perovskites Cs 3+nM(II)nSb2X9+3n (M = Sn, Ge) with desired photovoltaic-relevant properties by inserting [MX6] octahedral layers, based on the principles of increased electronic dimensionality. Compared to Cs3Sb2I9, the more suitable bandgaps, smaller carrier effective masses, larger dielectric constants, lower exciton binding energies, and higher optical absorption can be achieved by inserting variable [SnI6] or [GeI6] octahedral layers into the [Sb2I9] bilayers. Moreover, our results show that adjusting the thickness of inserted octahedral layers is an effective approach to tune the bandgaps and carrier effective masses in a large range. Our work provides useful guidance for designing the promising layered antimony halide double perovskite absorbers for photovoltaic applications.

TOC GRAPHICS


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Despite the rapid improvement in the record power conversion efficiency (PCE) of solar cells based on lead (Pb) halide perovskites,1,

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the ultimate implementation of this emerging

technology still faces obstacles, such as poor long-term stability against moisture/air and toxicity of Pb.3 Substantial efforts have been devoted to the search for Pb-free and stable perovskitebased photovoltaic absorbers.4 Theoretical studies have shown that the superior photovoltaic properties of Pb halide perovskites are largely attributed to the high electronic dimensionality associated with the perovskite structure and some unique electronic features correlated with the Pb 6s pseudo-closed shell configuration.4-6 Therefore, ns2 type cations such as Sn2+, Ge2+, Bi3+, and Sb3+ have been employed to replace Pb to form Pb-free halide perovskites. Sn halide perovskites have produced solar cells with reasonable PCEs up to ~9%, however, they are unstable against oxidation into Sn4+-containing compounds.7, 8 Ge halide perovskites have not produced efficient solar cells (PCEs < 1%), which may be attributed to the anomalously large bandgaps caused by the structural distortion as well as the detrimental defect properties.9,

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Alternatively, trivalent Bi3+ and Sb3+-based compounds A3B2X9 (A = monovalent cation; B = Bi3+, Sb3+; X = halogen) have been examined as photovoltaic absorbers.11, 12 However, these compounds crystallize with low dimensional structures such as layered perovskite and the nonperovskites with isolated [B2X9] bioctahedra and thus exhibit low electronic dimensionality that is undesired for photovoltaic properties.6 Unlike the cases of divalent Pb2+ or Sn2+-based layered perovskites such as (BA)2(MA)n-1PbnI3n+1 (BA = C4H9NH3+, MA = CH3NH3+),13 where the optoelectronic properties can be tuned by controlling the layer number of the divalent cations-based octahedra, the A3B2X9 perovskites have rigid [B2X9] bioctahedra layers, making it difficult to tune the optoelectronic properties of layered Bi and Sb halide perovskites by changing the layer number of trivalent cations containing bioctahedra.


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Over the past one year, halide double perovskites A2M(I)M(III)X6,14, 15 where M(I) and M(III) are a monovalent cation and a trivalent cation, respectively, have been proposed as stable and nontoxic substitutions. Those double perovskites have a three-dimensional (3D) structure, but not necessarily a high electronic dimensionality,4 which is quite important for efficient photovoltaic absorbers. So far, the newly-synthesized double perovskites including Cs2AgBiX6 (X = Br, Cl) and Cs2AgInCl6 usually exhibit large bandgaps and carrier effective masses that are unsuitable for single-junction solar cells.15-17 Recently, In(I)- and TI(I)-based double perovskites have been reported to exhibit electronic structures similar to those of Pb halide perovskites; however, the former are thermodynamically unstable and the latter are toxic.18-20 Therefore, it seems to be challenging for halide double perovskites to achieve desired photovoltaic properties. Recently, Vargas et al.21 firstly combined divalent Cu2+ and trivalent Sb3+ cations to form triple-layered perovskite Cs4CuSb2Cl12, which exhibits a bandgap of 1.0 eV and high photo- and thermal-stability. Despite these preferred properties, a recent theoretical study22 has revealed that the conduction band of Cs4CuSb2Cl12 derives from the antibonding states of unoccupied d orbital of Cu(II) and Cl 3p orbital, which are fairly localized, leading to a large electron effective mass and weak optical absorption in the visible light region. Obviously, unlike Cu(I),23 the Cu(II) states are not favoured for designing the promising photovoltaic absorbers. Nevertheless, the discovery of Cs4CuSb2Cl12 has provided a strategy for simultaneously incorporating divalent and trivalent metal cations to form layered double perovskites. More importantly, it is expected that the photovoltaic properties of such layered double perovskites can be tuned by controlling the thickness of the divalent cation-based layers, similar to the typical layered Pb iodide perovskites (BA)2(MA)n-1PbnI3n+1.


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In this work, we propose a general strategy for designing the layered halide double perovskites Cs3+nM(II)nSb2X9+3n (M = Sn, Ge) for photovoltaic applications based on the principles of increased electronic dimensionality. Our first-principles calculations show that incorporating [MI6] octahedral layers into Cs3Sb2I9 results in the desired photovoltaic-related properties, i.e., more suitable bandgap, smaller carrier effective masses (me), larger dielectric constants, lower exciton binding energies, and higher optical absorption, than the original layered perovskite Cs3Sb2I9. Our results further indicate that the bandgaps and carrier effective masses of Cs3+nM(II)nSb2X9+3n can be tuned to be very close to those of the traditional MAPbI3 by increasing the layer number of inserted octahedra. Finally, the stability of as-designed layered halide double perovskites Cs4M(II)Sb2X12 will be discussed. As shown in Figure 1, the first step in our strategy is to select the appropriate MII cation embedded into Cs3Sb2I9 to form the layered halide double perovskites Cs4M(II)Sb2I12. Sn2+ and Ge2+ are firstly considered since they are low toxicity and isoelectronic with Sb3+, which can have good orbital match and overlap with Sb3+. The next step is to adjust the layer number of incorporated [MIII6] octahedra to further tune the photovoltaic-related properties of Cs3+nM(II)nSb2X9+3n (n = 1~ ∞). Compared to the limited performance in Cs3Sb2I9, by combing these two steps, it is expected to increase the electronic dimensionality of the system and greatly improve the photovoltaic performances of the Sb-based layered perovskites.


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Figure 1. Illustration of the general design principle for the layered halide double perovskites Cs3+nM(II)nSb2X9+3n (M = Sn, Ge) by inserting [MIII6] octahedral layers between [SbI6] layers in Cs3Sb2I9, left and right panels show the crystal structures of Cs3Sb2I9 and hypothetical Cs3+nM(II)nSb2X9+3n with n = 3, respectively. Cs and I atoms are shown in green and orange spheres, respectively; Sb and Sn(Ge) atoms coordination polyhedra are displayed in purple and cyan , respectively. Figure 2 shows the band structures of the hypothetical layered double perovskites Cs4M(II)Sb2I12 (MII = Cu, Sn, Ge), along with their parent compound Cs3Sb2I9 for comparison. For Cs3Sb2I9, the indirect and direct bandgaps from HSE06 are 2.03 and 2.04 eV, respectively, which are close to the experimental value of 2.05 eV (see Table 1).11 The bandgap from HSE06+SOC (spin-orbit coupling interaction at the semi-relativistic level) is 1.87 eV. The SOCinduced bandgap reduction is as small as 0.16 eV, unlike the Pb halide perovskites, for which the SOC-induced bandgap reductions are ~1.0 eV.24 Therefore, we use the HSE06 results for the following discussions. As shown in Figure 2a and Figure S1 in the Supporting Information, the valence band maximum (VBM) consists of antibonding states of Sb 5s and I 5p orbitals, while the conduction band minimum (CBM) is mainly composed of Sb 5p states. For the hypothetical


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Cs4CuSb2I12, the ground state is antiferromagnetic (see Figure S2a in the Supporting Information),22 with the bandgap of 0.56 eV from HSE06, which is much smaller compared to Cs3Sb2I9. As Cu is in the +2 oxidation state with the 3d9 configuration, the partially unoccupied d states hybridize with I 5p states, forming a localized conduction band below the Sb 5p states (see Figure 2b). Based on our calculations and a recent study,22 it can be seen that such localized conduction band leads to a large electron effective mass (i.e., me = 6.97 m0 for Cs4CuSb2Cl12) and weak optical absorption. Therefore, although the Cs4CuSb2X12 (X = I, Br, Cl) layered double perovskites possess smaller bandgaps, they show less promise for photovoltaic applications. Table 1. Electronic properties of Cs3Sb2I9, Cs4SnSb2I12, Cs4GeSb2I12 and Cs4CuSb2Cl12. Bandgap is denoted by Eg, hole and electron effective masses are given by mh and me, the static dielectric constant is indicated by εstd, and exciton binding energy is showed by Eb. Note that m0 is the electron static mass. Ega (eV) Exp. Cs3Sb2I9d

SOC

2.05 2.03ind (2.04d) 1.87ind (1.87d)

Dielectric Constantc Eb (meV)

nonSOC

SOC

xx εstd

yy εstd

zz εstd

0.78/0.68

0.72/1.81

20.29

20.20

16.13

266

Cs4SnSb2I12d

-

1.66d

1.50d

0.52/3.48

0.59/3.19

21.68

20.26

20.88

204

Cs4GeSb2I12d

-

1.76d

1.60d

0.50/2.99

0.53/2.81

38.03

37.11

58.09

175

1.27ind

-

6.97/1.09

-

-

-

-

-

Cs4CuSb2Cl12e 1.0 a

nonSOC

me/mhb (m0)

The superscript “ind” and “d” indicate indirect and direct bandgaps. bThe detailed effective

masses are shown in the Table S1 (Supporting Information). cThe static dielectric constants are derived from the contributions of the electronic (ε∞) and the ionic (εion) that are given in the Table S2 (Supporting Information).

d

Electronic properties of Cs3Sb2I9, Cs4SnSb2I12, and

Cs4GeSb2I12 were calculated using HSE06 method. eElectronic properties of Cs4CuSb2Cl12 was


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calculated using PBE+U method. The experimental values of Cs3Sb2I9 and Cs4CuSb2Cl12 are taken from Refs. 11 and 21, respectively.

Figure 2. Band structures of (a) Cs3Sb2I9, (b) Cs4CuSb2I12, (c) Cs4SnSb2I12, and (d) Cs4GeSb2I12 from HSE06 calculations. Note that the energy scales are aligned with respect to the Cs 5s states, and the VBM of Cs3Sb2I9 is set to 0 eV. The band structures from HSE06+SOC are also shown in Figure S4 (Supporting Information) for comparison. An effective approach to improve the photovoltaic-relevant properties is to replace Cu(II) by other divalent cations with ns2 configuration, for example, Sn(II) and Ge(II). The resulting Cs4SnSb2I12 and Cs4GeSb2I12 layered double perovskites exhibit direct bandgaps of 1.66 and 1.76 eV, respectively, which are also smaller than their parent compound Cs3Sb2I9. As expected, unlike the Cu(II) analogue, these Sn(II) and Ge(II) analogues exhibit rather dispersive CBMs that mainly derived from Sn 5p or Ge 4p and Sb 5p orbitals (see Figure 2c, d). As summarized in Table 1, the estimated electron effective masses me for the Cs4SnSb2I12 and Cs4GeSb2I12 are smaller than those of Cs3Sb2I9 and Cs4CuSb2Cl12. Although hole effective masses mh are still relatively large, as discussed later, we can continue to reduce mh by increasing the layer number of [SnI6] or [GeI6] octahedra. Herein we explain the reasons for the bandgap reductions resulted from the Sn and Ge incorporations. As compared to Cs3Sb2I9, the VBM for Cs4SnSb2I12 shifts up by 0.44 eV and that for Cs4GeSb2I12 by 0.19 eV, resulting from the higher energy levels of Sn 5s


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and Ge 4s orbitals (than the Sb 5s orbitals) (see Figure S3 in the Supporting Information ) and the higher electronic dimensionality for thicker octahedral layers. On the other hand, while CBM of Cs4SnSb2I12 slightly shifts up by 0.07 eV, that for Cs4GeSb2I12 slightly down by 0.08 eV. Therefore, the narrowed bandgaps are mainly attributed to the higher-energy-lying Sn 5s and Ge 4s states raising the VBMs. It is known that large Born effective charges and dielectric constants can effectively screen defects and impurities, which benefits the carrier transport properties in halide perovskite absorbers.25 In MAPbI3, charge carrier screening is very effective owing to the striking large dielectric constant (> 27) across many orders of magnitude in frequency that confirmed by the latest experimental research.26 Here, we calculated the static dielectric constants and Born effective charges for Cs3Sb2I9 and Cs4M(II)Sb2I12, as shown in Table 1 and Table S2-S3 (Supporting Information). Compared to the static dielectric tensors for the parent phase Cs3Sb2I9 zz yy xx = 20.29, εstd = 20.20, and εstd = 16.13), those for the layered double perovskites are larger; i.e., ( εstd

zz yy yy xx xx εstd = 21.68, εstd = 20.26, and εstd = 20.88 for Cs4SnSb2I12 and εstd = 38.03, εstd = 37.11,

zz = 58.09 for Cs4GeSb2I12. These observations are consistent with the trend in the calculated and εstd

Born effective charges (see Table S3). The stronger cross-bandgap hybridization between the cation-p and anion-p bands in Cs4SnSb2I12 and Cs4GeSb2I12 (see Figure 2) is responsible for the enhanced Born effective charges and dielectric constants, in line with previous report in the BiI3.27 Furthermore, based on the calculated effective masses and dielectric constants, the exciton binding energies Eb are estimated to be 266 meV, 204 meV, and 175 meV for Cs3Sb2I9, Cs4SnSb2I12 and Cs4GeSb2I12, respectively. The smaller Eb indicates the faster photon-induced carrier dissociation and therefore the small Eb is considerably important for the solar cell


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absorbers. Overall, incorporating the [SnI6] or [GeI6] octahedra into Cs3Sb2I9 can effectively increase the static dielectric constant and reduce the exciton binding energies. The optical absorption can also be greatly improved by inserting [SbI6] or [GeI6] octahedra into the [Sb2I9] bilayers of Cs3Sb2I9. Figure 3 shows the calculated optical absorption spectra of Cs3Sb2I9, Cs4SnSb2I12, and Cs4GeSb2I12, along with that reported for Cs4CuSb2Cl12 for comparison.22 It can be clearly seen that the optical absorptions for Cs4SnSb2I12 and Cs4GeSb2I12 are both higher than that of Cs3Sb2I9 due to the strong p-p transition, and significantly red-shifted. Although Cs4CuSb2Cl12 has a smaller bandgap, its optical absorption spectrum shows a small peak above the bandgap and low absorption in the range of 2.0-3.0 eV, which is caused by the isolated and extremely localized conduction band that is derived mainly from the Cu 3d states.

Figure 3. Calculated absorption spectra of Cs3Sb2I9, Cs4SnSb2I12, and Cs4GeSb2I12, along with that reported from Ref. 22 for Cs4CuSb2Cl12 for the comparison. As shown above, the photovoltaic-relevant properties of Cs4SnSb2I12 and Cs4GeSb2I12 layered double perovskites are significantly improved, as compared to their parent phase Cs3Sb2I9. However, their bandgaps are still larger than the ideal bandgap for single-junction solar cells.


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Fortunately, the layer number of such layered double perovskites could be tuned by controlling the chemical composition to form a family of Cs3+nSnnSb2I9+3n layered double perovskites, providing the possibility to tune the photovoltaic-relevant properties. We performed calculations for Cs3+nSnnSb2I9+3n with n = 1, 3, 5, 7, 9 (the crystal structure with n = 3 is shown in Figure 1). As shown in Figure 4a, the bandgap of Cs3+nSnnSb2I9+3n decreases from 1.67 eV to 0.98 eV as n increases from 1 to 9, which is mainly attributed to the VBM upshift (Figure 4c). The n = 3 member (i.e., Cs6Sn3Sb2I18) has a bandgap of 1.31 eV, which is optimal for single-junction solar cells. The absorption curves (see Figure S5) near the absorption edge (low energy region) redshift with the increase of octahedral layers, consistent with gradually reduced bandgaps. The carrier effective masses decrease as layer number n increase (Figure 4b) and mh significantly reduces and approaches to me when n = 5. For n = 9, me and mh reach as low as 0.35 m0 and 0.37 m0 respectively, which is very close to these of MAPbI3 (me = 0.32 and mh = 0.36).28 The decrease of the hole (electron) effective masses originate from the enhanced orbital overlap between Sb and Sn/5s-I/5p (Sb/5p-Sn/5p) due to the increased [SnI6] octahedral layers.29 Therefore, we can see that inserting variable octahedral layers is an effective way to tune the electronic properties of perovskite absorber materials in the solar cells


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Figure 4. (a) Calculated bandgaps, (b) carrier effective masses, and (c) band structures of layered double perovskite Cs3+nSnnSb2I9+3n with n = 1, 3, 5, 7, 9. Note that the energy scales are aligned with respect to the Cs 5s states, and the zero energy is set to the VBM of Cs3Sb2I9. Finally, we would like to exam the thermodynamic stability of Cs4M(II)Sb2X12 (M = Sn, Ge; X = I, Br, Cl) against chemical decomposition. We have tested many possible decomposition pathways (see Table S4 in the Supporting Information) and found the following one shows the lowest decomposition enthalpies (△Hd) for Cs4M(II)Sb2X12. Cs4MSb2X12 → Cs3Sb2X9 + CsMX3

(1)

The decomposition enthalpies ( △ Hd) and decomposition pathways are shown in Table 2. Among the six compounds, Cs4SnSb2Br12 show a positive decomposition enthalpy (△Hd = 0.30 meV/atom), the | △ Hd| values of all the compounds are less than 5 meV/atom and within calculation error, indicating all these compounds may be synthesized. The other compounds show negative but very small decomposition enthalpies. Given the error of DFT calculation, all these hypothetical Cs4M(II)Sb2X12 compounds deserve experimental synthesis.


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Table 2. Decomposition enthalpies (

△

Hd) and associated decomposition pathways of

Cs4M(II)Sb2X12 (M = Sn, Ge; X = I, Br, Cl). Compounds Cs4SnSb2Br12 Cs4SnSb2I12 Cs4GeSb2I12 Cs4GeSb2Br12 Cs4SnSb2Cl12 Cs4GeSb2Cl12

△Hd (meV/atom)

0.3 -2.2 -2.4 -2.6 -1.2 -4.4

Decomposition Pathway CsSnBr3 + Cs3Sb2Br9 CsSnI3 + Cs3Sb2I9 CsGeI3 + Cs3Sb2I9 CsGeBr3 + Cs3Sb2Br9 CsSnCl3 + Cs3Sb2Cl9 CsGeCl3 + Cs3Sb2Cl9

In addition to the thermodynamic stability, we also checked the mechanical stability of these compounds by calculating the elastic stiffness constants Cij of Cs4MSb2X12. For each compound with monoclinic symmetry (C2/m space group), thirteen independent elastic constants were obtained, as shown in Table 3. According to the mechanical stability criteria30, we found that all compounds we discussed satisfy the mechanical stability criteria required for a monoclinic crystal (the mechanical stability criteria are shown as equations in the Supporting Information). Table 3. Elastic stiffness constants Cij of Cs4MSb2X12 (M = Sn, Ge; X = Cl, Br, I) in units of GPa. Cij (GPa) Cs4SnSb2Cl12 Cs4SnSb2Br12 Cs4SnSb2I12

C11 C22 C33 C44 C55 C66 C12 C13 C23 C15 17.63 17.83 20.47 5.40 6.02 4.67 7.85 9.27 9.26 -0.91 16.09 16.41 17.83 5.75 5.41 4.58 7.21 7.86 7.77 -0.41 14.00 14.73 16.08 4.79 5.12 4.04 6.22 6.70 6.74 -0.08

C25 0.40

C35 C46 -0.39 0.50

0.33 0.33

-0.18 0.52 0.09 0.05

Cs4GeSb2Cl12

21.00 20.79 22.25 6.18 6.44 5.87 8.93 9.82 9.16 -0.48

0.12

-0.47 0.32

Cs4GeSb2Br12

18.12 17.91 18.48 5.96 6.01 5.35 7.04 7.33 6.94 -0.19

-0.01

-0.32 0.15

Cs4GeSb2I12

16.13 16.67 16.18 5.20 5.27 5.07 6.32 6.21 6.19 0.07

-0.06

-0.10 0.06

From Table 3, we can observe two trends from the principle elastic constants C11, C22 and C33 of these compounds. One is that the stiffness constants of Cs4SnSb2X12 and Cs4GeSb2X12 (X = Cl, Br, I) become smaller from Cl to Br to I. Another one is that for the same halide, the stiffness constants of Ge-based compounds are larger than that of Sn-based compounds. The observations


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are consistent with the reports about elastic properties of prototype perovskites ABX3 (A = CH3NH3, HC(NH2)2; B = Pb, Sn, Ge; X = halide).31, 32 In conclusion, we have proposed a strategy to design layered halide double perovskite family of Cs3+nM(II)nSb2X9+3n with desired optoelectronic properties by inserting [MIII6] octahedral layers into Cs3Sb2X9, based on the principles of increased electronic dimensionality. The increased electronic dimensionality is achieved by combining the matched cation orbitals and the increased octahedral layer thickness. Our first-principles calculations show that the decreased carrier effective masses, enlarged dielectric constants, and reduced exciton binding energies can be obtained by inserting variable [SnI6] or [GeI6] octahedra into the [Sb2I9] bilayers. The absorption in the visible light region can also be significantly improved in this strategy. Moreover, our results show that adjusting the thickness of inserted octahedral layers is an effective approach to tune the bandgaps and carrier effective masses in a large range. Finally, the as-designed layered double perovskites Cs3+nM(II)nSb2X9+3n show small negative decomposition enthalpies, holding the possibility of being synthesized. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by the National Science Foundation of China (Grant No. 11572040), the Thousand Young Talents Program of China, and Graduate Technological Innovation Project of


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Beijing Institute of Technology. Theoretical calculations were performed using resources of National Supercomputer Centre in Guangzhou, which is supported by Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No.U1501501. Z. X, T. K and H. H acknowledge the funding from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through the Element Strategy Initiative to Form Core Research Center. Supporting Information Available: Computational details, Structural information, HSE06 calculated total and projected density of states, HSE06+SOC calculated band structure and total and projected density of states, antiferromagnetic configuration of Cs4CuSb2X12 (X = I, Cl) and band structure of Cs4CuSb2Cl12, schematic diagram of different energy levels of valence electrons for Sb, Ge, and Sn, detailed effective masses, dielectric constants, Born effective charges, decomposition enthalpies associated possible decomposition pathways for Cs4SnSb2Br12, the mechanical stability criteria are available in the Supporting Information. REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; and Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide Management in Formamidinium-Lead-Halide– Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376−1379. (3) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500963. (4) Xiao, Z.; Meng, W.; Wang, J.; Mitzi, D. B.; Yan, Y. Searching for Promising New Perovskite-Based Photovoltaic Absorbers: the Importance of Electronic Dimensionality. Mater. Horiz. 2017, 4, 206-216.


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Figure 1. Illustration of the general design principle for the layered halide double perovskites Cs3+nM(II)nSb2X9+3n (M = Sn, Ge) by inserting [MIII6] octahedral layers between [SbI6] layers in Cs3Sb2I9, left and right panels show the crystal structures of Cs3Sb2I9 and hypothetical Cs3+nM(II)nSb2X9+3n with n = 3, respectively. Cs and I atoms are shown in green and orange spheres, respectively; Sb and Sn(Ge) atoms coordination polyhedra are displayed in purple and cyan , respectively.

Figure 2. Band structures of (a) Cs3Sb2I9, (b) Cs4CuSb2I12, (c) Cs4SnSb2I12, and (d) Cs4GeSb2I12 from HSE06 calculations. Note that the energy scales are aligned with respect to the Cs 5s states, and the VBM of Cs3Sb2I9 is set to 0 eV. The band structures from HSE06+SOC are also shown in Figure S4 (Supporting Information) for comparison.


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Figure 3. Calculated absorption spectra of Cs3Sb2I9, Cs4SnSb2I12, and Cs4GeSb2I12, along with that reported from Ref. 22 for Cs4CuSb2Cl12 for the comparison.

Figure 4. (a) Calculated bandgaps, (b) carrier effective masses, and (c) band structures of layered double perovskite Cs3+nSnnSb2I9+3n with n = 1, 3, 5, 7, 9. Note that the energy scales are aligned with respect to the Cs 5s states, and the zero energy is set to the VBM of Cs3Sb2I9.


Layered Halide Double Perovskites Cs3+nM(II)nSb2X9+3n (M

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