Do Chalcogenide Double Perovskites Work as Solar Cell Absorbers: A

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Do Chalcogenide Double Perovskites Work as Solar Cell Absorbers: A First-Principles Study Qingde Sun, Hangyan Chen, and Wan-Jian Yin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04320 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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Chemistry of Materials

Do Chalcogenide Double Perovskites Work as Solar Cell Absorbers: A First-Principles Study Qingde Sun1,2, Hangyan Chen3, Wan-Jian Yin*,1,2 1College

of Energy, Soochow Institute for Energy and Materials InnovationS (SIEMIS), Soochow University, Suzhou 215006, China 2Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006, China 3School of Physical Science and Technology, Soochow University, Suzhou 215006, China ABSTRACT: Organic-inorganic hybrid perovskite solar cells have recently been developed at an unprecedented rate as an emerging solar cell technology, with its certified power conversion efficiency (PCE) (23.3%) surpassing conventional thinfilm contenders. However, the poor long-term stability and toxicity of Pb pose major setbacks to its commercialization. Theoretical calculations and experimental trail-and-error processes have recently aimed to find alternative perovskites, including inorganic halide perovskites (CsPbI3, CsPbIBr2, etc.), inorganic halide double perovskites (Cs2AgBiBr6, etc.), and chalcogenide single perovskites (BaZrS3, etc). However, their material properties are inferior to hybrid perovskite in terms of cell performance and material toxicity. Here, a class of lead-free chalcogenide double perovskites A2M(III)M(V)X6 [A = Ca2+, Sr2+, Ba2+; M(III) = Bi3+ or Sb3+; M(V) = V5+, Nb5+, Ta5+; X = S2-, Se2-] are comprehensively investigated with respect to its stability and electronic and optical properties. First-principles calculations on bandgaps, effective masses, optical absorption, and ideal power conversion efficiencies led to the selection of nine stable double chalcogenide perovskites that exhibit superior optoelectronic properties, i.e., quasi-direct bandgaps, balanced electron and hole effective masses, and strong optical absorption owing to the strong antibonding character both at the valence band maximum (VBM) and conduction band minimum (CBM). Unfortunately, thermodynamic stability calculations on massive decomposition pathways show negative decomposition energies ranging from 0 (-0.37) to -66 eV/atom, indicating the difficulty for a thin-film phase. The most likely compound is Ba2BiNbS6, with its decomposition energies (0 and -22 eV/atom for P21/n and R-3 phases, respectively) within the computational errors, which may be further stabilized by the confinement effect in nanocrystal form.

INTRODUCTION Recent extensive research Organic-inorganic hybrid halide perovskites (OIHP), CH3NH3PbX3 (X = Cl, Br, I), have revolutionized the solar cell field. The record power conversion efficiency (PCE) on the lab scale has been updated to 23.3%1 since 2009 (3.8%)2, which is on par with that of commercialized thin-film solar cells such as CdTe (22.1%) and CuInxGa1-xSe2 (22.9%). Nevertheless, the longterm stability and toxicity of Pb remain the main hurdles to industrial deployment. Extensive efforts have recently been made to develop and search for alternative candidates, aiming to overcoming these two issues. Replacing the organic CH3NH3+ with inorganic Cs+ offers a choice since inorganic cesium lead halide perovskites (ILHP) CsPbX3 (X = Cl, Br, I) have improved thermal stability with the absence of volatile organic components. CsPbBr3 shows good stability but a relatively wide bandgap (2.3− 2.4 eV),3, 4 while CsPbI3 has a more suitable bandgap (1.73 eV) but suffers from long-term phase instability5, 6, although recently You et al.7 achieved a PCE of 15.7 % via solvent-controlled growth of the precursor film in a dry environment, and Zhao et al.8 obtained a champion efficiency of 17.06 % by phenyltrimethylammonium bromide (PTABr) posttreatment. CsPbI2Br demonstrated a compromise between

the bandgap and phase stability, and the devices achieved reasonable efficiencies (13.3%,9 13.47%,10 and 14.45%11). However, disparity in the PCE still exists between ILHP and OHIP (23.3%) owing to the wide bandgaps of CsPbI3 (1.73 eV) and CsPbI2Br (1.91 eV), which are beyond the optimal range (1.0− 1.6 eV) for efficient single-junction solar cell absorbers according to the Shockley-Queisser limit.12 Moreover, the toxicity of Pb remains an intrinsic problem for ILHP. Inorganic cesium halide double perovskites Cs2M(I)M(III)X6 (X= Cl, Br, I) have eliminated the toxicity of Pb by splitting divalent Pb(II) of ILHP CsPbX3 (X = Cl, Br, I) into monovalent M(I) and trivalent M(III). Cs2AgBiBr6,13, 14 Cs2AgBiCl6,13,15 and Cs2AgSbCl616, 17, as three representatives of Cs2AgM(III)X6 (X = Cl, Br, I; M(III) = Sb, Bi) double perovskites, were synthesized and found to exhibit indirect bandgaps owing to the chemical mismatch of Ag d and Sb/Bi s orbitals at the band edges,18, 19 which are not ideal absorbers of thin-film solar cells. By substituting Sb/Bi for In, Cs2AgInX6 (X = Cl, Br) was prepared and showed direct bandgaps.16, 20 Unfortunately, Cs2AgInX6 (X = Cl, Br) exhibits inferior optical absorption owing to parity-forbidden transitions.21 Zhao et al. proposed Cs2InBiCl6 and Cs2InSbCl622 as promising absorbers with the advantage of direct bandgaps and strong transitions between band edges, but the oxidation for In+ → In3+ may drive them to decompose spontaneously.23 Meng et al. and Zhang et al.

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conducted systematical electronic and optical analyses on the class of inorganic cesium halide double perovskites Cs2M(I)M(III)X6 (X = Cl, Br, I) and concluded that although it directly mutated from OIHP, this class showed distinctly different electronic and optical properties and thus less promise as solar cell absorbers. To overcome the intrinsic instability of halides, chalcogenide perovskites were recently proposed as potential solar cell absorbers because the Coulomb interaction in chalcogenides is expected to be four times larger than the halides for purely ionic systems. Sun et al. theoretically studied the possibility of a class of perovskites ABX3 (A = Ca, Sr, Ba; B = Ti, Zr, Hf; and X =S, Se) as solar cell absorbers.24 Among them, BaZrS3 was proposed as a promising candidate and has been experimentally synthesized, showing a direct bandgap of 1.73– 1.85 eV.25–27 Ju et al.28 investigated another 18 types of chalcogenide perovskites ABX3 (A = Ca, Sr, Ba; B = Ge, Sn, Te; X = S, Se) and proposed that the proper mixing of SrSnS3 and SrSnSe3 can tune the bandgap for optimal sunlight absorption. Unfortunately, no chalcogenide perovskite solar cells have been reported. On the other hand, neither BaZrS3 nor SrSn(S,Se)3 carry lone-pair s orbitals, which is believed to be a crucial factor in the superior performance of OIHP. In this paper, by the chemical mutation of B-site cations based on BaZrS3, i.e., Zr (IV) → M(III) + M(V), where M(III) = Sb3+or Bi3+ and M(V) = V5+, Nb5+, Ta5+, one can introduce lone-pair s orbitals on a B site and derive a class of chalcogenide double perovskites A2M(III)M(V)X6 (X=S/Se). This is presented in Figure 1, where A = Ca, Sr, Ba. Consequently, the two crucial facts responsible for the superior performance of OIHP, i.e., lone-pair s elements and perovskite (-derived) structures, are retained in the class of chalcogenide double perovskites. Our study further shows that this class of compounds adopts an electronic structure similar to that OIHP, namely, (i) VBM is mainly composed of antibonding states between M(III) s and anion p, and CBM is a mixture of M(III) p - anion p and M(V) d - anion p, resulting in symmetric holes and electron masses; (ii) they may exhibit quasi-direct bandgaps, with indirect bandgaps just a little lower (0.0− 0.2 eV) than the direct bandgaps, which facilitates photo-excited electron-hole separation; and (iii) optical absorption is stronger than that of OIHP, ascribed to their higher joint density of states in the p-to-d transition.

METHODOLOGY First-principles calculations were performed using DFT as implemented in the VASP code.29, 30 The electron and core interactions were included using the frozen-core projected augmented wave approach.31 The generalized gradient approximation of PBE32 was used for the exchange correlation functional. The structures were relaxed for the 36 kinds of sulfide and selenide double perovskites A2M(III)M(V)X6 until the total energies converged to 10−4 eV with the kinetic energy cutoff for plane-wave basis functions set to 400 eV. K-point meshes with grid spacings of 2π × 0.02 Å−1 or smaller were used for Brillouin zone integration. To better evaluate the bandgaps, a hybrid Heyd-ScuseriaErnzerhot (HSE)42 and state-of-art GW calculation34, 35 was performed. Taking BaZrS3 as a reference to correct the HSE

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bandgap by varying α with the experimental bandgap, α = 0.25 is fairly close to the experimental value (Figure S3). Thus, an HSE functional with a standard 25% Hartree Fock exchange (i.e., PBE0) is used for the PBE selected double perovskites. Previous GW calculations based on the VASP code report that GW0, i.e., iterating only G but keeping W fixed, shows good agreement with the experimental results. Therefore, GW0 calculations are adopted for the selected PBE0 double perovskites. The parameters with kinetic energy cutoffs of 350 eV, 3 × 3 × 3 k-point meshes, and 3072 bands were chosen for GW0 bandgap calculations based on a convergence test of oxide double perovskites (Ca2BiVO6, Ba2BiVO6, and Ba2BiTaO6).36 The spectroscopic limited maximum efficiency (SLME),37, 38 based on the improved Shockley-Queisser model, is calculated by using a homemade code that considers intrinsic material properties such as the bandgap, optical absorption spectrum, and bandgap type (direct vs. indirect). As we know, PCE is influenced by the open-circuit voltage (Voc), the Voc subtract the bandgap difference between the indirect and direct gaps (ΔEg) based on Voc calculated via the direct optical transition, as all 36 kinds of chalcogenide compounds have indirect bandgaps. A simulation is performed under the standard AM1.5G solar spectrum at room temperature.

Figure 1. Schematic crystal structures of (a) cubic 

perovskite ( Pm 3m symmetry), (b) distorted perovskite (Pnma symmetry), (c) distorted perovskite (R-3c 

symmetry), (d) double perovskite ( Fm 3m symmetry), (e) distorted double perovskite (P21/n symmetry), and (f) distorted double perovskite (R-3 symmetry). The right panel (g) is the chemical mutation of 36 kinds of chalcogenide (S, Se) double perovskites from BaZrS3.

RESULTS AND DISSCUSSION A screening process based on the bandgap decomposition energy (to binary compounds) allows us to identify nine chalcogenide double perovskites (Sr2SbTaS6, Ba2SbTaS6, Sr2BiNbS6, Ba2BiNbS6, Sr2BiTaS6, and Ba2BiTaS6 in the P21/n phase; Ba2SbTaS6, Ba2BiNbS6, and Ba2BiTaSe6 in the R-3 phase) (Table 1) as possible candidates. These have superior optical absorption and high spectroscopic limited maximum efficiency (SLME) comparable to that of CH3NH3PbI3. Unfortunately, extensive studies on decomposition pathways show that the minimum decomposition energies of those compounds are negative,


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Chemistry of Materials indicating a potential problem of the compounds’ formability. Notably, some values are within the computational errors by DFT-PBE. The most stable one is Ba2BiNbS6, which has a decomposition energy close to zero (0 and -22 meV/atom for the P21/n and R-3 phases, respectively). Considering that the calculated decomposition energy of CH3NH3PbI3 ranges from -18 to 5 meV/atom (CH3NH3 is considered as one atom),39, 40 an experimental synthesis for Ba2BiNbS6 is needed. The ideal perovskite ABX3 has a cubic structure with a 

high symmetry of Pm 3m containing one type of octahedra, as shown in Figure 1(a). Cubic double perovskite A2B1B2X6 

belongs to the Fm 3m space group and can be viewed as ABX3 by replacing the B site with B1 and B2, resulting in two types of octahedra alternating in a rock-salt face-centered-cubic structure, as shown in Figure 1(d). Geometrically, for an ideal cubic perovskite, the A−X bond length equals the √2* B−X bond length, which is roughly assumed to be the sum of two ionic radii. In reality, ionic radii rarely match perfectly, and the cation and anion size mismatch is often described by the Goldschmidt tolerance factor t  (rA  rX ) / 2(rB  rX ) , where rA, rB, and rX are the ionic radii of ions A, B, and X, respectively. For double perovskite A2B1B2X6, an average of the B-cation radii was often used for rB. In general, materials with a tolerance factor of 0.9− 1.0 have an ideal cubic structure, and a tolerance factor of 0.71− 0.9 results in a distorted perovskite structure with tilted octahedra. Nonperovskite structures are often formed when the tolerance factor is much higher (>1) or lower ( 0] marked in red dash box; (c) and



(f): GW0-calculated (GW0-Eg) gaps of selected perovskites of (b) and (e) respectively, the criteria for materials screening of stable solar compounds, i.e., 1.0 eV < GW0-Eg < 1.7 eV and ΔHD > 0, is shaded in light yellow color. The decomposition energies (ΔHD) as calculated by firstprinciples density functional theory (DFT) are able to quantitatively reflect the thermodynamic stability of materials. For example, the calculated ΔHD values of CH3NH3PbI3 are close to 0 (5 meV/atom by Agiorgousis et al.,39 -12 meV/atom by Zhang et al.,44 and -18 meV/atom by Ganose et al.40), indicating the possible intrinsic thermodynamic instability of CH3NH3PbI3. The most common pathway is taken into consideration in previous literature for halides, i.e., decomposition into binary compounds, where ΔHD is defined as H D ( A2 M (Ⅲ) M (Ⅴ) X 6 )  2 E ( AX ) 

and the Γ-Y high symmetry line, respectively, and its ΔEg is 0.26 eV, as shown in Figure 3(b). Ba2SbTaS6 (R-3) exhibits an indirect bandgap with CBM and VBM located at the L and T points, respectively, and its ΔEg is 0.08 eV, as shown in Figure 3(c). The partial density of states shows the antibonding coupling of Sb lone-pair 5s with S 3p at VBM, resembling the case of CH3NH3PbI3, where VBM is the antibonding state of the Pb lone-pair s and I p orbital.

1 E ( M (Ⅲ ) 2 X 3 ) 2

1  E ( M (Ⅴ) 2 X 5 )  E ( A2 M (Ⅲ) M (Ⅴ) X 6 ) 2

i.e., the energy difference between A2M(III)M(V)X6 (X=S/Se) and corresponding decomposed binary compounds. Positive values of ΔHD mean that A2M(III)M(V)X6 (X=S/Se) will not decompose into its respective binary compounds spontaneously. Other pathways involving ternary compounds as secondary phases will be considered last, owing to the complication and high computational cost when considering complete pathways for all 36 candidates and 2 phases. On the other hand, the bandgap is crucial for evaluating a new material for potential solar cell applications. It is well known that DFT based on LDA/GGA calculations underestimates the bandgaps of crystalline materials. Although HSE06 and GW0 are state-of-the-art methods for relatively accurate calculations of bandgaps, the computational cost is highly demanding for all 36 kinds of chalcogenide double perovskites studied here. Therefore, we first calculated the bandgaps at the PBE level of all 36 kinds of double perovskites in the P21/n and R-3 phases, as shown in Figure 2(a) and 2(d), respectively, and Table S2. Then, we calculated the bandgap at the HSE06 level for compounds with PBE-Eg larger than 0, as shown in Figure 2(b) and 2(e). Finally, the bandgap values were refined by GW0 calculations for compounds with PBE0-Eg within 0.75− 1.55 eV, as shown in Figure 2(c) and 2(f). Based on the criteria of decomposition energy (ΔHD > 0) and GW0-Eg (1.0– 1.7 eV), six chalcogenide double perovskites (Sr2SbTaS6, Ba2SbTaS6, Sr2BiNbS6, Ba2BiNbS6, Sr2BiTaS6, and Ba2BiTaS6) with P21/n phase, and three chalcogenide double perovskites (Ba2SbTaS6, Ba2BiNbS6, and Ba2BiTaSe6) with R-3 phase, were selected as potential candidates for efficient single-junction solar cell absorbers. To further understand the electronic and optical properties of this class of chalcogenide double perovskites A2M(III)M(V)X6, band structure calculations were performed at the PBE levels, as shown in Figure S4 (Figure S5) for sulfide (selenide) with P21/n phase and Figure S6 (Figure S7) for sulfide (selenide) with R-3 phase. Ba2SbTaS6 was chosen as a representative to show the difference between single perovskite BaZrS3 and double chalcogenide perovskites with a P21/n phase and R-3 phase. BaZrS3 has a direct bandgap with both CBM and VBM located at the Γ point, as shown in Figure 3(a). Ba2SbTaS6 (P21/n) exhibits an indirect bandgap with CBM/VBM located at the U point

Figure 3. The PBE-calculated electronic band structures and partial density of states of (a) BaZrS3, (b) Ba2SbTaS6 (P21/n), and (c) Ba2SbTaS6 (R-3). The bandgap is corrected to GW0-gap using scissor operator. For BaZrS3, the VBM is mainly contributed from the localized anion p orbital, which is similar to conventional semiconductors such as GaAs and CuInSe2. This leads to a high hole effective mass (0.58 along Γ-Y and 0.78 along Γ-Z). In CH3NH3PbI3, a lone-pair s contribution at the antibonding VBM results in a low hole effective mass. This is responsible for ambipolar conductivity. A similar electronic structure is observed in Ba2SbTaS6, in particular with the dispersive upper valence band, which is derived from strong s-p coupling and leads to a low hole effective mass. On the other hand, Ba2SbTaS6 exhibits a much smaller electron effective mass than BaZrS3 by introducing a delocalized p orbital of Sb at CBM. The calculated carrier effective masses of nine selected double perovskites are 0.2– 0.3 me, as listed in Table 1. An indirect bandgap is often considered a deficiency in an optimal solar cell since the optical absorption at the band edge is extremely weak. On the other hand, an indirect bandgap with a small ΔEg can facilitate electron-hole separation to avoid fast recombination, which is beneficial for cell performance. The ΔEg of chalcogenide double perovskites is around 0.2− 0.5 eV (Table S2), which is not as large as that of halide double perovskites Cs2AgBiX6 (X = Cl, Br, I) (0.45– 0.71 eV).19 Notably, P21/n phases (ΔEg ~ 0.2– 0.5 eV) generally have larger ΔEg than R-3 phases (ΔEg ~ 0.0– 0.2 eV) (Table S2). In this aspect, the R-3 phase is more suitable for PV applications than the P21/n phase. Optical absorption is another crucial property for PV materials. For clarity, we show the calculated optical absorption of Ba2SbTaS6 (P21/n), Ba2SbTaS6 (R-3), Ba2BiNbS6 (P21/n), and Ba2BiNbS6 (R-3) in Figure 4(a) in comparison with CH3NH3PbI3 and GaAs. (The results of the other five compounds are shown in Figure S8). There are very sharp absorption edges and strong absorption strengths in the visible light region where the optical


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Chemistry of Materials absorption strength of the double perovskites is much stronger than that of GaAs, and even stronger than that of CH3NH3PbI3. Strong absorption ensures the harvesting of enough sunlight with extremely thin layers, thus shortening the diffusion length of the photoexcited carriers. The modified SLME for an indirect bandgap absorber (see Methodology Section) indicates that Ba2SbTaS6 (R-3) can achieve PCE of 20% when the layer thickness is just 300 nm, surpassing that of CH3NH3PbI3 [Figure 4(b)]. The SLME of the R-3 phases performed better than the corresponding P21/n phases, which can be explained by the small ΔEg of R3 phase. In CH3NH3PbI3, strong optical absorption is derived from direct p-p transitions and intra-atomic Pb s-p transitions. For sulfide double perovskite A2M(III)M(V)S6, the optical absorption at the band edge is mainly from S pto-M(V) d orbitals and intra-atomic M(III) s-p transitions.

Therefore. we calculated the decomposition energies of 9 selected double perovskites in 11 representative pathways, as shown in Figure 5. These 11 pathways were selected to include all possible decomposition pathways with secondary phases that can be found in the ICSD, as shown in Table S3 (see part 1 in the Supporting Information for details).

Figure 5. The heat map for decomposition energies (ΔHD) of nine compounds along eleven decomposition pathways. The pathways are determined by searching all related secondary phases in Inorganic Crystal Structure Database (ICSD). Cross indicates that that particular pathway does not apply due to the absence of corresponding secondary phases in ICSD.

Figure 4. (a) Optical absorption and (b) spectroscopic limited maximum efficiency (SLME) of Ba2SbTaS6 (P21/n), Ba2SbTaS6 (R-3), Ba2BiNbS6 (P21/n), and Ba2BiNbS6 (R-3), CH3NH3PbI3 and GaAs are taken for comparison.

The results show that although ΔHD is positive at common pathway ① ,the values are reduced by considering additional pathways. For example, ΔHD of Ba2SbTaS6 (P21/n) and Ba2BiTaS6 (P21/n) in pathway ④ becomes negative (Ba2SbTaS6: -49 meV/atom, Ba2BiTaS6: -40 meV/atom). Unfortunately, all nine compounds have a negative minimum ΔHD, as listed in Table 1, indicating these compounds may not be easy to form in experiments. However, previous calculations show that DFT-PBE may underestimate the thermodynamic stability of double perovskite. For example, the calculated minimum ΔHD for Cs2NaBiI6 (-25 meV/atom) is negative by the decomposition

For quaternary compounds, it is not enough to consider the decomposition pathways to binary compounds. For instance, ΔHD of Cs2AgBiI6 is positive when considering pathways to binary compounds. However, it becomes negative when ternary compounds such as Cs3Bi2I9 are 1 1 Cs NaBiI  CsI  NaI  Cs Bi I involved.19,45 This explains why there has not been a 2 2 pathway of , but they do exist in successful synthesis of thin-film Cs2AgBiI6. On the other experiments.46 Meanwhile, the confinement effect may hand, oxidation-reduction (redox) decomposition further stabilize perovskite nanocrystals such as Cs2AgBiI6, pathways are crucial when determining the stability of although the corresponding thin films are difficult to form.47 compounds that contain ions with uncommon valance charges such as Cs2InBiCl6 and Cs2InSbCl6 with In1+ ions.22,23 Table 1. Some key parameters, including direct/indirect bandgap [PBE-Eg (GW0-Eg)], carrier effective mass, decomposition energies to binary compounds, the lowest decomposition energy, SLME at 300 nm and 2 μm, for nine chalcogenide double perovskite, in comparison to GaAs and CH3NH3PbI3. 2

Perovskites

i E（ g eV）

E（ g eV）

6

3

2 9

Electron(Hole) mass

ΔHD binary (minimum)

300 nm (2 μm)

SLME

Sr2SbTaS6 (P21/n)

0.33 (1.02)

0.24

0.22 (0.49)

35 (-60)

22.28% (26.04%)

Sr2BiNbS6 (P21/n)

0.35 (1.38)

0.38

0.24 (0.32)

20 (-47)

17.39% (20.67%)

Sr2BiTaS6 (P21/n)

0.61 (1.46)

0.40

0.27 (0.35)

41 (-55)

16.31% (19.30%)

Ba2SbTaS6 (P21/n)

0.48 (1.26)

0.26

0.20 (0.66)

80 (-49)

20.77% (25.26%)

Ba2BiNbS6 (P21/n)

0.45 (1.51)

0.43

0.25 (0.62)

68 (0)

14.92% (17.77%)

Ba2BiTaS6 (P21/n)

0.71 (1.56)

0.44

0.30 (0.62)

89 (-40)

14.32% (17.04%)

Ba2SbTaS6 (R-3)

0.59 (1.21)

0.08

0.17 (0.17)

62 (-66)

24.04% (30.84%)



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Ba2BiNbS6 (R-3)

0.70 (1.69)

0.13

0.20 (1.19)

46 (-23)

20.75% (25.31%)

Ba2BiTaSe6 (R-3)

0.49 (1.11)

0.07

0.15 (>3.0)

7 (-84)

27.63% (30.81%)

GaAs

1.42 (exp)

0.00

0.07 (0.34)

353

12.95% (27.69%)

CH3NH3PbI3

1.50 (exp)

0.00

0.35 (0.31)

CONCLUSIONS

[-18–

5]39, 40

20.51% (28.97%)

AUTHOR INFORMATION

In summary, we have employed chemical mutation methods for property engineering of sulfide and selenide double perovskites to design lead-free stable perovskites for photovoltaic applications. By mutating transition metal element Zr(IV) ions to lone-pair s elements [M(III) = Bi3+ or Sb3+] and group VB elements [M(V) = V5+, Nb5+,Ta5+], a class of chalcogenide double perovskites A2M(III)M(V)X6 [A = Ca, Sr, Ba; X=S, Se] were formed. Based on systematical investigation, nine compounds have been selected with their key parameters related with solar cell applications (bandgaps, effective mass, decomposition energies, SLME) summarized in Table 1. Although those class of chalcogenide double perovskites show indirect bandgap, the differences between indirect and direct gaps for some of those perovskites are not large (< 0.2 eV), in particular to R3 phase, therefore can be viewed as quasi-direct bandgap. Meanwhile, VBM is main composed of antibonding state between M(III) ns and anion X 3p/4p, and CBM is is mixing of M(III) np- anion X 3p/4p and M(V) md- X 3p/4p, resulting in nearly-symmetric hole and electron masses. However, the formability and stability may be main issues, since they are unfortunately thermodynamically unstable (ΔHD < 0) under DFT-PBE calculation by considering full decomposition pathways. Considering the underestimate of ΔHD by DFT-PBE, those compound may still have chance to form, in particular for Ba2BiNbS6, with ΔHD close to zero. Selection of proper precursors with negative formation enthalpies to targeted compounds might be another route for synthesizing those compounds. Furthermore, size effect may further stabilize those compounds in nanocrystal form. Therefore, the experimental synthesis are called for.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Part 1: Eleven pathways and the details of the decomposition pathways of double perovskites with different formulas. Part 2: The data for the ionic radii and tolerance factors (t) of the 36 kinds of double perovskites (Table S1), the difference between the indirect and direct gaps (ΔEg) in the P21/n and R-3 phases (Table S2), the formula and space group of secondary phases (Table S3), and the value of bandgaps (Table S4). Crystal structures of four distorted double perovskite phases (Figure S1), the total energy difference between the cubic and distorted perovskite phases (Figure S2), the HSE bandgaps of BaZrS3 as a function of α (Figure S3), band structure of 36 kinds of double perovskites in the P21/n and R-3 phases (Figure S4– S7), and the optical absorption and SLME of Sr2SbTaS6 (P21/n), Sr2BiNbS6 (P21/n), Sr2BiTaS6 (P21/n), Ba2BiTaS6 (P21/n), and Ba2BiTaSe6 (R-3) (Figure S8).

Corresponding Author * E-mail: [email protected] ORCID

Qingde Sun: 0000-0002-6216-4604 Wan-Jian Yin: 0000-0003-0932-2789 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors acknowledge the funding support from National Key Research and Development Program of China under grant No. 2016YFB0700700, National Natural Science Foundation of China (under Grant No. 51602211, No. 11674237), Natural Science Foundation of Jiangsu Province of China (under Grant No. BK20160299) and Suzhou Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, China. The work was carried out at National Supercomputer Center in Tianjin, Lvliang and Guangzhou, China and the calculations were performed on TianHe-1(A) and TianHe-II.

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Do Chalcogenide Double Perovskites Work as Solar Cell Absorbers: A

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