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Air-Stable Direct Bandgap Perovskite Semiconductors: AllInorganic Tin-Based Heteroleptic Halides AxSnClyIz (A=Cs, Rb) Jiangwei Li, Constantinos C. Stoumpos, Giancarlo G. Trimarchi, In Chung, Lingling Mao, Michelle Chen, Michael R. Wasielewski, Liduo Wang, and Mercouri G. Kanatzidis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02232 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018
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Chemistry of Materials
Air-Stable Direct Bandgap Perovskite Semiconductors: All-Inorganic Tin-Based Heteroleptic Halides AxSnClyIz (A=Cs, Rb) Jiangwei Li,†,‡ Constantinos C. Stoumpos,‡ Giancarlo G. Trimarchi,‡ In Chung,‡ Lingling Mao,‡ Michelle Chen,‡ Michael R. Wasielewski, ‡ Liduo Wang,*,† and Mercouri G. Kanatzidis*,‡ †
Department of Chemistry, Tsinghua University, Beijing 100084, China
‡
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
ABSTRACT: Semiconducting halide perovskites are a class of materials with exciting photoelectronic properties. Compared to the widely studied hybrid organic-inorganic perovskites, the all-inorganic derivatives are less well understood even as they promise high inherent stability. Currently, such materials are limited due to the fact that there is a very narrow choice of inorganic cations that can stabilize the desirable perovskite structure. Herein we report on the synthesis and characterization of novel all-inorganic tin-based perovskites and perovskitoids that can be stabilized by the heteroleptic coordination of chloride and iodide anions, Cs2SnCl2I2 (1) and Cs2.38Rb1.62Sn3Cl8I2 (2), consist of two-dimensional (2D) layers of [SnCl4I2]4- octahedra with different connectivity modes. Compound 1 is an n=1 Ruddlesden-Popper type perovskite adopting the tetragonal archetype structure (I4/mmm space group; a = 5.5905(3) Å, c = 18.8982(13) Å), while compound 2 crystallizes as an orthorhombic modification (Cmcm space group; a = 5.6730(11) Å, b = 25.973(5) Å, c = 16.587(3) Å) with corrugated layers. The crystal chemistry changes drastically when Cs+ is replaced by the smaller Rb+ cation which leads to the isolation of the low dimensional compounds Rb3SnCl3I2 (3a), Rb3SnCl2.33I2.67 (3b) and Rb7Sn4.25Cl12I3.5 (4), thus illustrating the importance of the A-cation size in the formation of perovskites. The 2D perovskites show wide band gaps and relatively large resistivities, associated with their chemical stability against the oxidation of Sn2+. The chemical stability is coupled with remarkable electronic properties that derive from the perovskite structure. DFT calculations suggest that both compounds are direct band gap semiconductors with large bandwidths, consistently with the experimentally determined band gaps of Eg=2.62 eV and 2.81 eV for 1 and 2, respectively. The combination of stability and favorable electronic structure in heteroleptic-halide perovskites presents a new direction towards the realization of functional devices made exclusively from inorganic perovskites.
Introduction Halide perovskites have been intensively studied because of their diverse structures and extensive applications, such as energy storage, field-effect transistors (FET), radiation detection, light emitting devices (LED) and, most impressively, photovoltaics (PV).1-9 In particular, perovskite solar cells, with organicinorganic lead halide perovskites as light absorbers, have achieved a power conversion efficiency of over 22% recently.10 This fascinating class of perovskites adopt a generic formula ABX3 for the three-dimensional (3D) structure, where typically A = CH3NH3+, HC(NH2)2+, Cs+, B = Pb2+, Sn2+, Ge2+ and X = Cl-, Br-, I-. By slicing the 3D perovskites down with long spacer cations along a specific crystallographic plane ((100), (110) or (111)), twodimensional (2D) perovskites with greater synthetic versatility are formed.11 The Ruddlesden-Popper (RP) phases belong to the most common (100)-oriented type, with the general formula A’2An−1BnX3n+1 (A’ = spacer cation and n = layer thickness). It was reported that the n = 4 member of RP phase (C4H9NH3)2(CH3NH3)3Pb4I13 as light absorber demonstrated a 12.5% efficiency in solar cells.12 Meanwhile, the incorporation of long organic cations remarkably improves the device stability, showing the potential of PV application with better environmental endurance.13 Compared with the organic-inorganic halide perovskites, the all-inorganic homologues have higher intrinsic stability especially under thermal stress.14,15 Unfortunately, the exploration of allinorganic halide perovskites is restricted by the single choice of Cs+ as A-site cation due to the size limitation. Even so, diverse applications have been demonstrated with limited compositions. For example, CsSnBr3-xIx as light absorbers were reported to produce ~3% efficiency.15-17 CsSnI3 was involved in all-solid-state solar cells as a p-type semiconductor.18,19 CsPbBr3 was proposed to be a promising material for high-energy radiation detection.20 CsPbI3, though suffers from phase transition at room temperature, demonstrated an ~13% efficiency when doped with Bi3+ for better structural stabilization.21 Therefore, the development of all-
inorganic halide perovskites is believed to further expand the versatile applications of halide perovskite materials. Tin-based halide perovskites as environmentally friendly alternatives have attracted great attention recently, based on which decent efficiencies of solar cells have been progressively reported, but the intrinsic chemical instability of divalent tin compounds is always hindering their further development.19,22,23 Concerning that, halide mixing is widely used to effectively tune the physical properties of halide perovskites. For instance, the mixing of Cl/Br and Br/I halides were reported to broaden the emitting wavelength of perovskite materials from visible to near infrared regions, and introducing Br in triiodide perovskite absorbers was demonstrated to markedly improve the chemical and thermal stability of perovskite solar cells.7,24-26 However, unlike the solid solutions formed with neighboring halogens, the incorporation of nonadjacent Cl in triiodide perovskites, as claimed for the enhanced efficiency “CH3NH3PbI3-xClx” perovskite solar cells or the phase stable “CsPbI3-xClx” colloidal nanocrystals, cannot be achieved because of the large size mismatch between Cl and I ions (rI/rCl=1.234).2729 Thus, it is important to learn more about the structural evolution of the halide perovskites when the coexistence of Cl and I is synthetically imposed, as it can be achieved by following the solidstate chemistry approach. In this work, we report two new 2D perovskites in the AxSnClyIz (A=Cs, Rb) system, prepared using high temperature solid state synthesis. The first one, Cs2SnCl2I2 (1), is an n = 1 RP perovskite with ordered halide atoms, while the second one, Cs2.38Rb1.62Sn3Cl8I2 (2), which also exhibits a 2D structures with ordered anions, has a corrugated layer structure. Two types of [SnCl2I4]4- octahedra with different connectivity modes exist in these two structures, confirming the potential of Cl as a moderator for structural engineering. The crystallographically ordered positions of Cl and I rules out the possibility of forming Cl/I solid solution in halide perovskite structures. In addition, the A-cation size is demonstrated to play an important role as a structure determining factor in the all-inorganic 2D perovskites, similar to the trend observed in the regular 3D perovskites. By substituting Cs+
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with smaller Rb+ in compound 1, a 0D molecular salt Rb3SnCl3I2 (3a) can be obtained, which is composed of [SnCl3I2]3- anions and Rb+ cations. On the other hand, the structure of compound 2 can only be retained in Cs-rich composition, as more Rb+ will deform the structure into the low dimensional compound Rb7Sn4.25Cl12I3.5 (4), with isolated [SnI6]4- and [SnCl3]44- complex anions. DFT calculations clearly indicate that Cs2SnCl2I2 is a promising candidate for optoelectronics since it possesses a direct band gap with dispersive bands, similar to those observed in hybrid inorganicorganic 2D halide perovskites.
Experimental Section Synthesis. SnCl2 (99.99%), SnI2 (99.99%), CsCl (99.9%) and CsI (99.9%) were purchased from Sigma-Aldrich and used as received. Cs2SnCl2I2 (1): Stoichiometric mixtures of CsI (519.6 mg, 2 mmol) and SnCl2 (189.6 mg, 1 mmol) or CsCl (336.8 mg, 2 mmol) and SnI2 (372.5 mg, 1 mmol) were loaded in a 9 mm Pyrex tube in a N2 glovebox. The tube was then evacuated to 10-3 mbar and flame-sealed with liquid N2 protection. The sealed tube was subsequently transferred to a tube furnace, heated to 400 ºC over a period of 6 h, soaked at 400 ºC for 24 h, and then cooled down to room temperature over a period of 24 h. The completion of reaction gave an apparently black ingot, and the grinding exposes a yellow colored powder as the main product. The 2CsCl+SnI2 reaction gives purer samples without CsI impurity, which always forms in the 2CsI+SnCl2 reaction, suggesting that the quality of the starting materials plays an important role in maintaining the stoichiometry. Crystals suitable for X-ray diffraction were grown by a slow cooling process which produces highly crystalline samples. Cs2.38Rb1.62Sn3Cl8I2 (2): Stoichiometric mixtures of CsCl (336.8 mg, 2 mmol), RbCl (241.8 mg, 2 mmol) and SnI2 (372.5 mg, 1 mmol) or were loaded in a 9 mm Pyrex tube in a N2 glovebox. After evacuated the tube at 10-3 mbar and flame-sealed, the tube was subsequently transferred to a tube furnace, heated to 750 ºC over a period of 8 h, soaked at 750 ºC for 24 h, and then cooled down to room temperature over a period of 24 h. The final ingot was pale yellow. All the other Cs4-xRbxSn3Cl8I2 series of compound were prepared with similar procedure except for the stoichiometry of Cs and Rb halide salts. For x=4, an entirely different product is isolated, Rb7Sn4.25Cl12I3.5 (4) with orange color. Normally impurities are involved in the Rb-rich cases, but they seem to be amorphous and cannot be observed in PXRD patterns. Rb3SnCl3I2 (3a). Stoichiometric mixtures of RbI (424.8 mg, 2 mmol) and SnCl2 (189.6 mg, 1 mmol) were loaded in a 9 mm Pyrex tube in a N2 glovebox. After evacuated the tube at 10-3 mbar and flame-sealed, the tube was subsequently transferred to a tube furnace, heated to 750 ºC over a period of 8 h, soaked at 750 ºC for 24 h, and then cooled down to room temperature over a period of 24 h. The final ingot was orange with RbI and RbCl impurities. Rb3SnCl2.33I2.67 (3b). Stoichiometric mixtures RbI (424.8 mg, 2 mmol) and SnCl2 (189.6 mg, 1 mmol) were loaded in a 9 mm Pyrex tube in a N2 glovebox. After evacuated the tube at 10-3 mbar and flame-sealed, the sealed tube was subsequently transferred to a tube furnace, heated to 750 ºC over a period of 8 h, soaked at 750 ºC for 24 h, and then cooled down to 400 ºC followed by a quick air-quenching process. The final ingot was yellow with orange Rb3SnCl3I2 as a second phase. Single Crystal X-ray Diffraction. Single crystals of appropriate size were selected with the protection of Paratone oil for X-ray diffraction experiments. Frames were collected using either a STOE IPDS 2 or IPDS 2T diffractometer with graphite-
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monochromatized Mo Kα radiation (λ = 0.71073 Å), operating at 50 kV and 40 mA under N2 flow. Integration and numerical absorption corrections were performed using the STOE X-AREA program suite. All structures were solved by direct methods and refined by full-matrix least-squares on F2 using the OLEX2 or JANA2006 program package.30,31 Powder X-ray Diffraction. Powder XRD analysis was performed using a Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter. Optical Spectroscopy. Optical diffuse-reflectance measurements were performed at room temperature using a Shimadzu UV-3600 UV−vis-NIR spectrometer from 200 to 1500 nm. BaSO4 was used as the reflectance reference. The reflectance versus wavelength data were used to estimate the band gap of the material by converting reflectance to absorption data according to the Kubelka-Munk equation: F(R) = (1 − R)2/2R, where R is the reflectance.32 Steady State Photoluminescence. Steady-state photoluminescence (PL) spectra were acquired using a Horiba Nanolog fluorimeter. Cs2SnCl2I2 was excited at 375 nm whereas Cs2.38Rb1.62Sn3Cl8I2 was excited at 380 nm with a long-pass filter at 385 nm. The measurements were performed on powdered samples. Electrical Resistivity Measurements. Electrical resistivity of Cs2SnCl2I2 single crystal with a dimension of 0.5×0.4 mm2 (plane)×0.1 mm (thickness) was recorded using a Keithley 2400 source meter with a standard two-probe contact geometry. A colloidal graphite paste was used as an electrical contact between the 100 µm Cu wires and the single-crystal sample. For Cs2Rb2Sn3Cl8I2, the fine ground powders were put inside a 12.7 mm diameter graphite die and densified by spark plasma sintering (SPS, SPS-211LX, Fuji Electronic Industrial Co., Ltd.) at 180 ºC, respectively, for 5 min under an axial compressive stress of 40 MPa in vacuum. Highly dense disk-shaped pellets with ~10 mm thickness were obtained and then cut into specific dimension, 2.17×2.17 mm2 (electrodes)×4.95 mm thickness for Cs2Rb2Sn3Cl8I2. I-V data were collected using a Keithley 6517b source meter with a standard two-probe contact geometry. A bias voltage from 10 to -10 V was applied with 1 V step and 5 s delay time. The data were averaged over 5 points. Scanning Electron Microscopy (SEM). SEM images and Energy Dispersive X-ray (EDX) analyses were obtained using a Hitachi S3400N-II scanning electron microscope equipped with an Oxford Instruments INCAx-act SDD EDS detector. The accelerating voltage for image capture was set to 5 kV, and for EDX operation was15 kV. Differential Thermal Analysis (DTA). DTA measurements were performed on a Shimadzu DTA-50 thermogravimetric analyzer in aluminum boats using α-Al2O3 as reference. Ground materials (∼30 mg) were flame-sealed in a silica ampule evacuated to 10−3 mbar. Samples were heated and cooled at a speed of 10 °C/min with a soaking time of 10 min when reaching the high or low temperature limits. Band Structure Calculations. The electronic structure calculations were performed using density functional theory as implemented in the VASP ab initio package.33,34 We optimized the symmetry unconstrained lattice parameters and cell-internal atom coordinates by local minimization of the total energy starting from the experimental lattice vectors and atom positions. The structure optimization was performed applying the PBEsol DFT exchange and correlation functional.35 The electronic band dispersions were calculated for the fully relaxed structures including the spin-orbit coupling (SOC). The set of special k-points and the k-point paths in the Brillouin Zone used to obtain the band structure are those
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Chemistry of Materials
defined by Setyawan and Curtarolo.36 The analysis of the data from the ab initio calculations was performed via Python scripts in which we used the Atomic Simulation Environment (ASE) and the Python Materials Genomics (pymatgen) materials analysis libraries.37,38 The effective masses were calculated using the set of python scripts available at the emc repository.39
Results and Discussion Synthesis and Crystal Structural Analysis. The solid-state synthesis of Cs2SnCl2I2 (1) always gave a visually black ingot, though the ground powders were basically yellow and XRD analysis showed a pure phase with an occasionally observed CsI impurity. It was observed with microscopy that the black impurities distributed inside the yellow polycrystalline product form a lamellar structure or dispersive dots due to the cooling from the CsSnCl3/CsSnI3 eutectic melting at ~ 230 ºC.40 By screening the black impurity using single-crystal diffraction, we confirmed that this phase was CsSnI3. Accordingly, a slow cooling process was designed to produce high quality crystalline Cs2SnCl2I2 free of CsSnI3 inclusions. Yellow transparent single-crystal could be obtained from the polycrystalline ingot. Similarly to CsSnI3, Cs2SnCl2I2 is readily soluble in polar solvents like dimethyl formamide and dimethylsulfoxide, which allows for an analogous processability towards functional devices.18,19 Single-crystal Xray diffraction analysis showed that 1 adopts the K2NiF4-type structure, the RP perovskite prototype, crystallizing in the tetragonal space group of I4/mmm (No. 139) with the unit cell parameters of a = b = 5.5905(3) Å, c = 18.8982(13) Å (Figure 1).41 Detailed crystallographic data is listed in Table 1. Compound 1 consists of 2D [SnCl2I2]n2n- layers, with Cs+ ions occupying the interlayer voids. The Sn-centered [SnCl4I2]4- unit is an elongated octahedron, with four Cl ions located in the equatorial plane and two I ions at the polar positions. The bond length of Sn-I and Sn-Cl bond in 1 is 3.154 and 2.795 Å, respectively, comparable to that of the related 3D structures (310.29 pm Sn-I bond in CsSnI3 and 280.5 pm Sn-Cl bond in high temperature 3D phase CsSnCl3).19,42 The preference of the Cl atoms to occupy the linearly coordinated bridging sites in the structure lying on the plane of the [SnI2Cl2]2- layers and consequently the iodide atoms to occupy the out of plane terminal positions make good chemical sense. The linear M-Cl-M bridge requires partially hybridized s and p orbitals (sp2) on Cl ions. As the halide gets heavier, as in the iodide, the energy difference between s and p valence orbitals get smaller and the ability for mixing and full sp3 hybridization is greater and when it happens it results in bend M-I-M bonds. This is analogous to almost linear Al-O-Si bridges strongly favored in aluminosilicate structures, while such linkages are very rare for the heavier chalcogenides. Therefore, in a competitive situation as in Cs2SnI2Cl2 structure, the linearly bridging chloride will be the lowest energy configuration. In this context, it is interesting to point out that the almost linear M-I-M bridges in the hybrid Pb and Sn iodide perovskites are an exception when one considers the plethora of such iodides which adopt bent M-I-M bridges.
Figure 1. Crystal structure of Cs2SnCl2I2 (compound 1) from (a) the side view and (b) top-down view (without Cs, the neighboring planes are depicted in different color), (c) the [SnCl4I2] octahedron unit, and (d) the image of a single crystal under microscopy (scale bar: 500 µm).
Figure 2. (a) Crystal structure of Cs2.38Rb1.62Sn3Cl8I2 (compound 2), the Cs+ and Rb+ cations are depicted with mixed occupancy. (b) Side view of the irregular 2D plane, with the blue and green colors showing two [SnCl4I2] octahedron units. (c) Detailed structure of the connectivity [Sn3Cl8I2]4- units outlined in (b). When we adjusted the Cl/I ratio to 4:1, a new compound was obtained with the formula of Cs4Sn3Cl8I2. However, according to the XRD results and optical absorption data (Figure S2), Cs2SnCl2I2 was inevitably generated as a second phase. We found that when Rb+ was incorporated, this new series of compounds could be obtained as pure phases, which is due to the destabilization of Cs2SnCl2I2 when Rb+ is included as we discuss below. We solved and refined the crystal structure using the Cs:Rb = 1:1 sample with the nominal stoichiometry of Cs2Rb2Sn3Cl8I2, and the actual chemical formula was determined to be Cs2.38Rb1.62Sn3Cl8I2 (2) from structure refinement, which is also consistent with Energy Dispersive X-ray Spectroscopy (EDS) analysis (Figure S5). Compound 2 has an irregular 2D structure, crystallizing in the orthorhombic space group Cmcm (No. 63) with the unit cell parameters of a = 5.6730(11) Å, b = 25.973(5) Å and c = 16.587(3) Å. The 2D framework is composed of two [SnCl4I2]4- units with
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varied bond lengths, depicted in green and blue color in Figure 2. The compound technically is not a perovskite but a perovskitoid (i.e. a halide structure with octahedra sharing edges or faces) as defined previously.43 Different from the [SnCl4I2]4- unit in compound 1, the octahedra in 2 are distorted, having two adjacent Ion one side of the equatorial plane, allowing the Sn 5s2 lone pair to be expressed in between them. While the Sn1 octahedron is corner-connected in the structure, the Sn2 octahedron has a connectivity of I-I edge sharing to form a dimer structure. In the caxis, the Sn2 dimers construct a zigzag chain by sharing Cl- ions, and Sn1 octahedron connect the neighboring Sn2 dimers through I- ions. Then the infinite [Sn3Cl8I2]n4n- 2D plane is formed by connecting the polar position Cl- ions of all Sn1 and Sn2 octahedra along the a-axis. Rb+ and Cs+ cations reside at the interspatial sites between the layers showing no special preference for the two independent sites, giving a refined cation ratio of Cs:Rb=2.38:1.62. Detailed information of atomic coordinates is listed in Table S2.
Figure 5. (a) Crystal structure of Rb7Sn4.25Cl12I3.5 (compound 4) depicted in simplified models. The purple and blue octahedron are isolated [SnI6]4- and [IRb6]5+, gray and green squares represent two different type of [Sn4Cl8] rings (α- and β-, respectively). (b) Side-view of the three-dimensional tin chloride network. (c) and (d) show the connectivity of α- and β-[Sn4Cl8] rings by sharing the out-of-plane Sn-Cl bonds.
Figure 3. (a) Crystal structure of Rb3SnCl3I2 (compound 3a), (b) the top-down view along b axis, and (c) the [SnCl3I2]3- anion unit.
Figure 4. (a) Crystal structure of Rb3SnCl2.33I2.67 (compound 3b), (b) the top-view along c-axis, and (c) the 6H hexagonal perovskite polytype structure in 3b constructed by Rb-coordinated octahedra. The isolated [SnI6]4- octahedra are depicted in orange, [SnCl3]pyramids in yellow, [IRb6] and [ClRb6] units in the 6H framework in purple and green, respectively.
The results of Rb+ incorporation strongly support that the cation size plays an important role in the structure of 2D alkali tin-based halide perovskites. When Rb+ was used instead of Cs+ in the Cs2SnCl2I2 case with a synthetic reaction between SnI2 and 2 RbCl, no low angle peak (2θ
