Perovskite Chalcogenides with Optimal Bandgap and

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Perovskite Chalcogenides with Optimal Bandgap and Desired Optical Absorption for Photovoltaic Devices Ming-Gang Ju, Jun Dai, Liang Ma, and Xiao Cheng Zeng*

a general formula of ABX3, allow property engineering via A, B, or X site substitution, and the site substitution provides a synthesis strategy that has achieved great success in both fundamental physics and chemistry, as well as in a wide range applications. This adaptability of the perovskitestructure type takes advantage of meeting particular requirement for semiconductor with fourfold-coordinated tetrahedral network structure. Recently, several theoretical and experimental studies have shown predictions or successful synthesis of transition metal perovskite chalcogenides with promising properties.[14–18] Sun et al.[18] predicted that CaTiS3, BaZrS3, CaZrSe3, and CaHfSe3 with suitable bandgaps are candidates for single-junction solar cell. Mixed perovskite chalcogenides BaZr1−xTixS3 were proposed by Meng et al.[17] based on theoretical computations. Perera et al.[15] synthesized BaZrS3 and CaZrS3 with a perovskite structure via using high-temperature sulfurization of the oxide counterparts. They found that BaZr(OxS1–x)3 perovskites possess tunable bandgaps within a range of 1.73–2.87 eV. Most recently, α-SrZrS3 (1.53 eV), BaZrS3 (1.81 eV), and β-SrZrS3 (2.13 eV) were also synthesized for photovoltaic applications.[16] These advancements demonstrate that perovskite chalcogenides may have a great potential to make impacts in photovoltaics. In this communication, we present a comprehensive study of a series of perovskite chalcogenides, based on the first-principles computations. We predict that SrSnSe3 and SrSnS3 are promising solar-cell materials due to their suitable direct bandgaps within the optimal range of 0.9–1.6 eV, as well as very good optical absorption properties and good carrier mobility. Moreover, elemental mixing strategy is predicted to be capable of tuning the bandgaps of the perovskite chalcogenides to an appropriate value for specific application, such as for tandem photovoltaic device. Calculation of ratios of ionic radii can be utilized to qualitatively assess whether or not spheres of a particular size can be packed together into a particular structure. Such a simple calculation has been widely used in understanding and predicting stabilities of ionic solid-state structures, including the perovskite structures. For perovskite compounds with the chemical formula ABX3, the most commonly used and quite reliable geometric ratio is the Goldschmidt’s tolerance factor,[19,20] t, defined as t = (rA + rX ) /  2 (rB + rX ) , where rA and rB are the ionic radius of the A- and B-site cations, respectively, and rX is

Solar cells with organic-inorganic lead halide perovskites have achieved great success and their power conversion efficiency (PCE) has reached to 22.1%. To address the toxicology of lead element and some stability issues associated with the organic-inorganic lead halide perovskites, inorganic lead-free perovskites have gained more attentions from the photovoltaic research community. Herein, a series of chalcogenide perovskites are proposed as optical absorber materials for thin-film solar cells. SrSnSe3 and SrSnS3 are predicted to be direct bandgap semiconductors with the bandgap value being within the optimal range of 0.9–1.6 eV. Both SrSnSe3 and SrSnS3 not only exhibit good optical absorption properties and carrier mobility, but also possess flexible bandgaps that can be continuously tuned within the grange of 0.9–1.6 eV via the element-mixing strategy, thereby render both perovskites as promising candidates for photovoltaic applications.

Perovskites are a class of materials with a highly symmetric close packed structure, which have been extensively studied in decades due to versatility of their chemical and physical properties. Over the past few years, inorganic-organic halide perovskites have attracted unprecedented attention and represented a major breakthrough[1–11] after the initial report in 2009 by Kojima et al.[12] To data, polycrystalline thin-film perovskite photovoltaic devices have reached power conversion efficiencies exceeding 22.1%.[13] However, the involvement of a toxic element of lead and some instability issues are still concerns for large-scale commercial application of the perovskite photovoltaic devices. To address these concerns, inorganic lead-free perovskites with nontoxic elements and higher structural stability have been introduced into the realm of perovskite solar cell. Current semiconductor materials for solar cell mostly employ fourfold coordinated tetrahedral network structures, such as silicon, GaAs, CdTe, etc. It is well known that perovskites, with Dr. M.-G. Ju, Dr. J. Dai, Dr. L. Ma, Prof. X. C. Zeng Department of Chemistry University of Nebraska–Lincoln Lincoln, NE 68588, USA E-mail: [email protected] Prof. X. C. Zeng Collaborative Innovation Center of Chemistry for Energy Materials University of Science and Technology of China Hefei, Anhui 230026, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201700216.

DOI: 10.1002/aenm.201700216

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BaTiO3. For compounds ASnX3, their bandgaps decrease in the order of oxide > sulfide > selenide, corresponding to the aforementioned energy difference. For compounds with distorted perovskite structure, eight compounds exhibit bandgaps within the optimal range. Among the eight compounds identified above, four compounds are predicted to possess direct bandgaps, namely, SrSnS3 (1.56 eV), SrSnSe3 (1.00 eV), CaSnS3 (1.58 eV), and CaGeO3 (1.15 eV), which are more suitable for making thin-film solar cell. It is known that semiconducting materials with optimal direct bandgap would give higher performance than materials with optimal indirect bandgap for making thinfilm solar cell. For instance, typical silicon solar cells have to be hundreds of micro meters in thickness due to silicon’s indirect bandgap, whereas CdTe solar cell can be made with a very thin active layer (often 1, the corner-sharing octahedra network structure tends to be unfavorable whereas alterfour compounds, SrSnSe3 clearly exhibits the highest absorpnative structures, such as hexagonal structure or NH4CdCl3 tion in the visible-light range (from 1.59 to 3.26 eV) and the absorption intensity is comparable with that of MAPbI3. structure is more likely to form. Here, we first examine structural stabilities of all compounds considered in this study Moreover, SrSnSe3 shows modest absorption in infrared range through computing the empirical Goldschmidt’s tolerance facbecause SrSnSe3 has a smaller bandgap than other compounds, tors (see Figure S1, Supporting Information). Clearly, most t indicating that most output of Sun’s total irradiance spectrum factors of the compounds are between 0.71 and 0.9, suggesting can be absorbed by SrSnSe3. These desirable properties render that most compounds likely adopt the distorted perovskite SrSnSe3 with the distorted perovskite structure a very promstructure. As such, our focus will be mainly placed on the comising solar absorber material with potentially high efficiency. By pounds with either a perfect or a distorted perovskite structure. the contrary, CaGeO3 exhibits the lowest absorption. For SrSnS3 For a single p–n junction, the maximum theoretical effiand CaSnS3, both show similar absorption range due to their ciency of a solar cell is 33.7% based on an AM 1.5 solar specnearly the same bandgaps, and their absorption intensities are trum, a value known as the Shockley–Queisser limit.[21] The slightly lower than that of SrSnSe3. However, both compounds electronic bandgap of absorber material is one of key quantities still exhibit reasonably good absorption (the maximum absorpto assure high efficiency for a solar cell. Ideally, the bandgap tion coefficient ≈1.4 × 105 cm−1) in the visible-light range. value should be within the optimal range of 0.9–1.6 eV in order To gain better understanding of the most promising comto render theoretical efficiency >25%. Figure 1b shows the compound, SrSnSe3, we computed its electronic band structure and puted bandgaps of all 26 perovskite chalcogenides with either density of states (DOS) (see Figure 2b,c). We found that both perfect or distorted perovskite structure. For compounds with the conduction band minimum (CBM) and valance band maxa formula of ATeX3, most of the bandgaps are rather small and imum (VBM) are localized at Γ point. From the projected DOS (PDOS), VBM is contributed by Se 4p-orbital while CBM is some compounds are even metallic. None of the compounds mostly contributed by Se 4p-orbitals and Sn 5s-orbitals. These with the perfect perovskite structure gives a bandgap within features are also confirmed by distribution of charge densities the optimal range. For compounds ASnO3, their relatively large corresponding to the VBM and CBM (Figure S3, Supporting bandgaps (>2.5 eV) render them unsuitable as absorber mateInformation). The dispersion of the bottom conduction band is rials due to the large energy difference between Sn 5p-orbital stronger than the top valence band. These electronic-structure conduction band and O 2p-orbital valence band, similar to features are markedly different from those of CsSnI3, another typical transition-metal oxide perovskites, e.g., BaZrO3 and Adv. Energy Mater. 2017, 1700216

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Figure 2. a) Computed optical absorption spectra (based on the HSE06 functional) of several predicted materials with distorted perovskite structure, compared with optical absorption spectra of prototype Si and MAPbI3. The absorption coefficient of MAPbI3 is computed using the PBE functional without considering the spin-orbit coupling (SOC) effect. Note that the computed absorption spectrum of MAPbI3 is coincidentally in good agreement with the experimental spectrum due to cancellation of errors by using the PBE functional without considering SOC. b) Computed band structure of SrSnSe3 with a distorted perovskite structure; here Γ (0.0, 0.0, 0.0), X (0.5, 0.0, 0.0), Z (0.0, 0.0, 0.5), R (0.5, 0.5, 0.5), T (0.0, 0.5, 0.5), Y (0.0, 0.5, 0.0), and M (0.5, 0.5, 0.0) refer to the high-symmetry special points in the first Brillouin zone. c) Computed DOS and projected DOS of SrSnSe3 with distorted perovskite structure.

prototype Sn-based perovskite that has gained increasing attention in photovoltaic filed. Notably, the dispersion of the top valance band of CsSnI3 is stronger than the bottom conduction band, a feature of p-type semiconductor. The VBM of CsSnI3 is mostly contributed by Sn 5s-orbitals and I 5p-orbitals, while the CBM is contributed by Sn 5p-orbitals.[22,23] Note also that SrSnS3 exhibits a similar band structure and DOS as SrSnSe3, except for its larger bandgap (Figure S2, Supporting Information). Carrier mobility is also a key factor of solar-cell materials for achieving high performance of devices. To assess the carrier mobility, we computed the effective masses of SrSnSe3 and SrSnS3. Table 1 presents the effective mass tensors corresponding to the [100], [110] and [111] directions, respectively. It can be seen that both SrSnSe3 and SrSnS3 possess relatively small electron and hole effective masses, suggesting that both compounds possess good carrier transport properties. It is worthy of noting that electron and hole effective masses of SrSnSe3 are much smaller than SrSnS3 due to 4p-orbitals being much less localized than 3p-orbitals. These orbitals contribute to the VBM and CBM the most. Furthermore, to estimate the exciton effect, we calculated the exciton binding energies of SrSnSe3 and SrSnS3, which are 9.6 and 42.7 meV, respectively. Table 1. Computed effective masses of SrSnSe3 and SrSnS3 with distorted perovskite structure at the bandgap edges (in electron mass) by using PBE functional. Sample SrSnSe3

SrSnS3

[100]

[110]

[111]

mh*

−0.12

−0.19

−0.31

me*

0.18

0.19

0.24

mh*

−0.23

−0.33

−0.55

me*

0.43

0.48

0.65

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Both values are comparable to that of MAPbI3 (reported in the range of 19–50 meV[24]), suggesting that fast exciton dissociation can occur in both materials. To access likelihood of experimental realization of the two compounds, we examined dynamical and thermal stabilities of both compounds by calculating their phonon dispersion along the high-symmetry lines in the first Brillouin zone and by performing ab initio molecular dynamics (AIMD) simulations. There is no appreciable imaginary frequency in the phonon spectra of both compounds (see Figures S4a and S5a, Supporting Information), suggesting dynamic stabilities of both materials. Snapshots of SrSnSe3 and SrSnS3, taken at initial and end of 5-ps AIMD simulations (with temperature controlled at 300 K), are presented in Figures S4b,c and S5b,c (Supporting Information), respectively. It can be seen that the perovskite framework of the two compounds are well sustained in the final configuration with networks of corner-sharing octahedrons. In addition, AIMD simulations with temperature controlled at 500 and 1000 K are also carried out to examine thermal stabilities of SrSnSe3 and SrSnS3 (see Figure S6a–d, Supporting Information). It can be seen that both compounds show robust stabilities at 500 K after 5 ps simulation. At 1000 K, SrSnS3 still exhibits ordered configuration with networks of corner-sharing octahedrons after 5 ps simulation. However, SrSnSe3 shows slightly disordered configuration after 5 ps simulation, suggesting that this compound may lack thermal stability at the elevated temperature of 1000 K. Since the conduction band and valence band are primarily derived from the metal cation and chalcogenide anion, the electronic structures of ABX3 can be tuned by partially replacing either B or X ion with homologous elements to achieve suitable bandgap by design. The element-mixing strategies have been extensively investigated in the field of lead halide perovskite solar cells.[4,6,7,9,25,26] Here, we first consider partial

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Figure 3. a) Schematic crystal structure of mixed S and Se perovskite chalcogenides SrSnSxSe3–x. b) Computed bandgaps of a series of mixed compounds SrSnSxSe3–x with distorted perovskite structure, using the HSE06 functional. c) Computed optical absorption spectra of a series of compounds SrSnSxSe3–x with distorted perovskite structure, using the HSE06 functional, and compared with those of prototype Si and MAPbI3. d) Computed band structure of SrSnS2.5Se0.5 with distorted perovskite structure. e) Computed DOS and projected DOS of SrSnS2.5Se0.5 with distorted perovskite structure.

substitution of the anions in SrSnSe3 via replacing Se with S (see Figure 3a). A series of mixed S and Se compounds SrSnSxSe3–x are optimized. Their electronic properties are verified. Interestingly, computed bandgaps of the new mixing compounds do not decrease with increasing proportion of Se (Figure 3b). When the Se/S ratio exceeds 50%, bandgaps of the compounds are smaller than that of SrSnSe3, as in the case of MAPb1–xSnxI3.[27,28] Figure 3c shows the computed optical absorption spectrum of the mixed compounds together with the spectra of SrSnS3, SrSnSe3, Si, and MAPbI3 for comparison. It can be seen that the maximum absorption coefficient increases with the increase of Se/S ratio. When the Se/S ratio is