Tunable Band Gap and Long Carrier Recombination Lifetime of Stable

Article Cite This: Chem. Mater. 2018, 30, 1556−1565

pubs.acs.org/cm

Tunable Band Gap and Long Carrier Recombination Lifetime of Stable Mixed CH3NH3PbxSn1−xBr3 Single Crystals

Dianxing Ju,† Yangyang Dang,† Zonglong Zhu,‡ Hongbin Liu,§ Chu-Chen Chueh,∥ Xiaosong Li,§ Lei Wang,† Xiaobo Hu,† Alex K.-Y. Jen,*,‡,§,⊥ and Xutang Tao*,† †

State Key Laboratory of Crystal Materials, Shandong University No. 27 Shanda South Road, Jinan, 250100, P.R. China Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States § Department of Chemistry, University of Washington, Seattle, Washington 98195, United States ∥ Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan ⊥ Department of Biology and Chemistry, City University of Hong Kong, Kowloon, Hong Kong ‡

S Supporting Information *

ABSTRACT: The mixed metal Pb/Sn halide perovskites have drawn significant attentions in perovskite photovoltaics due to their broad absorption spectra and tunable band gaps. To obtain a deeper understanding of these materials properties, single crystals are regarded as the best platform among various building blocks for fundamental study. Here, we report the mixed-metal MAPbxSn1−xBr3 (MA = CH3NH3) perovskite single crystals grown by top seeded solution growth (TSSG) method. Systematical characterizations were applied to investigating their structures and optoelectronic properties. These single crystals kept higher stability even exposed to air over one month than that of MASnBr3. The outstanding electrical properties, such as lower trap-state density and higher carrier mobility, were investigated by space charge-limited current (SCLC) and the Hall Effect measurements. More importantly, these perovskite single crystals exhibited much narrower optical band gap (1.77 eV) and longer carrier lifetime (∼2 μs) than those of MAPbBr3 and MASnBr3, which showed a greatly potential application in tandem solar cells based on hybrid organic−inorganic perovskites with the optimal bandgap of 1.70−1.85 eV.

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INTRODUCTION The recent development of solution process-able organic− inorganic hybrid perovskite materials has drawn both the academic and industrial attention in the optoelectronic fields based on their low-cost, lightweight, and transformative photovoltaic technique.1,2 Owing to their superior semiconducting properties, such as high absorption coefficient, long charge carrier diffusion length, and low density of deeplevel defect, the power conversion efficiency (PCE) of hybrid perovskites has reached up to 22.1% since its first debuted in 2009.2−4 In general, the formula of metal halide perovskites has been described as AMX3, where A+ is an organic cation or cesium cation (Cs+) or rubidium cation (Rb+), M2+ is a metal cation, and X− is a halide anion, respectively.2,5,6 In principle, its optical band gap and optoelectronic properties can be tailored by the selection of M+ and X- ions, while the variable cations on the A+ sites play a critical role on its lattice torsion and associated phase transformation.7,8 It is worthwhile to note that mixed halide anions (Br−I) compounds with tunable bandgaps often present photoinduced phase segregation caused by halide ion migration.9,10 Nevertheless, the researchers have found mixed Pb/Sn, can effectively mitigate the I/Br phase segregation and afford stable © 2018 American Chemical Society

and efficient PVSCs with tunable bandgaps ranging from 1.17 to 1.73 eV with a M-site alloy.11,12 Recently, benefiting from the flexibility in tunable bandgap, high-efficiency (over 20%) perovskite-perovskite FA0.75Cs0.25Pb0.5Sn0.5I3 absorber with bandgap of 1.2 eV and FA0.83Cs0.17Pb(I0.5Br0.5)3 layers with bandgap of 1.8 eV, as well as silicon-perovskite tandem solar cells have also been achieved based on similar small bandgap Pb/Sn alloys perovskite layer as the bottom cell.13 Nowadays, the progress of Pb/Sn based PVSCs has been still hindered by the chemical instability of Sn2+ element.14−16 Furthermore, the fundamental properties of this Pb/Sn based perovskite materials have not been fully understood yet. To further explore the fundamental properties of these materials, large-size high quality hybrid perovskite single crystals can be served as an ideal scaffold due to the fewer grain boundaries and surface defects. The better understandings of their structure, optoelectronic properties and stability can enable us to optimize their derived photoelectric devices. Early attention is mainly focused on the single-crystal growth of MAPbI3 and MAPbBr3 (MA = CH3NH3) using a top-seeded Received: October 31, 2017 Revised: February 3, 2018 Published: February 8, 2018 1556

DOI: 10.1021/acs.chemmater.7b04565 Chem. Mater. 2018, 30, 1556−1565

Article

Chemistry of Materials

Figure 1. MAPbxSn1−xBr3 single crystals obtained by tuning the Pb/Sn molar ratios. (a)MAPbBr3, (b) MASnBr3, (c) MAPb0.74Sn0.26Br3, (d) MAPb0.68Sn0.32Br3, (e) MAPb0.39Sn0.61Br3. (f) Experimental and calculated powder X-ray diffraction patterns for kinds of MAPbxSn1−xBr3 powder. The powder XRD patterns of the samples were consistent with the calculated XRD patterns of these single crystals. (g) The variation in cubic a lattice parameters with a change in Sn contents. (h−i) Ball-and-stick diagrams of crystal structures and their ((Pb/Sn)Br6] octahedral structure units in the MAPbxSn1−xBr3 single crystals. The C and N elements represent the disordered CH3NH3 groups; hydrogen atoms bonded to the C or N atoms were omitted for clarity.

instance, Bakr and co-workers reported the growth of MAPbX3 perovskite single crystals using an inverse temperature crystallization method.19 Later Lian et al. demonstrated the large single crystals of perovskite CH3NH3PbI3(Cl) via rapidly cooling chlorine-containing solutions.20 Recently, growth of mixed organic cations A+ or halide anions X− single crystals has also been reported.21 However, there have been few reports

solution method with a temperature gradient or antisolvent vapor assisted crystallization approach.17,18 Our research group reported the growth of bulk high-quality single crystal MAPbI3 by temperature-lowering method for the first time.17 After that, several approaches to growing single crystals of MAPbI3 and MAPbBr3 via temperature lowering method or inverse temperature crystallization method were also reported. For 1557


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the Kubelka−Munk function: α/S = (1 − R)2/(2R),26 where α is the absorption coefficient, S is the scattering coefficient, and R is the reflectance. PL and TRPL Measurements. Photoluminescence measurements were carried out by a laser of 405 nm with a photomultiplier (PMTHS1-CR131) and DSP lock-in amplifier (SRS 830). The time-resolved photoluminescence measurement was carried by FLS920 all functional fluorescence spectrometer (Edinburgh). The output laser wavelength was set to be 405 and 377.8 nm. The single crystals with dimensions of about 5 × 5 × 1 mm3 were used to get the TRPL spectra. Thermogravimetric Analysis, Thermal Expansion and Specific Heat Measurements. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out using a TGA/DSC/1600HT analyzer (METTLER TOLEDO Instruments). The MAPbxSn1−xBr3 samples were placed in a platinum crucible, and heated at a rate of 10 °C min−1 from room temperature to 800 °C under flowing nitrogen gas. Thermal expansion measurements of MAPbxSn1−xBr3 in the temperature range from 303 to 417 K were made on a polished sample with dimensions of about 5 × 5 × 3 mm3 with a PerkinElmer thermal dilatometer (Diamond TMA). Specific heat measurements of MAPbxSn1−xBr3 about 2 × 2 × 1 mm3 samples were made under a nitrogen atmosphere from 293 to 453 K using a PerkinElmer Diamond DSC analyzer. SCLC and Hall Effect Measurements. All current−voltage measurements were carried out by Keithley 2400 semiconductor parameter analyzer. The dielectric constant was measured by an Agilent 4294A impedance network analyzer, combined with the equation of ε = Cpt/Aε0. Where Cp, t, ε, ε0 and A are the capacitance, the thickness of single crystals, relative dielectric constant and the area of single crystals. The kinds of MAPbxSn1−xBr3 single crystals with the dimensions of about 4 × 4 × 0.5 and 4 × 3 × 0.35 mm3 were also used for the measurements of Hall Effect. All of them were deposited with carbon conduction paste as the electrodes. The resistivity and the carrier concentration measurements were performed at room temperature on a 4-probe sample holder placed between the plates of an electromagnet on Ecopia HMS-5000 instrument. The magnetic field intensity was at 0.5 T.

pertaining to mixed Pb/Sn hybrid perovskite single crystals due to the oxidation of Sn2+ to Sn4+ easily.22 Here, the mixed MAPbxSn1−xBr3 single crystals with large macroscopic dimensions (16 × 14 × 10 mm) have been grown with top seeded solution growth (TSSG) method in ambient atmosphere by our research group for the first time. The characterization techniques including powder and single crystal X-ray diffraction, UV−vis spectra, and thermogravimetric analysis, etc., were used to elucidate the intrinsic structure and optoelectronic properties of the synthesized single crystals in detail. Interestingly, mixed MAPbxSn1−xBr3 single crystals exhibited high stability when exposed to air. More importantly, mixed MAPbxSn1−xBr3 single crystals demonstrated much narrower optical band gap and longer carrier lifetime. These behaviors revealed the intrinsically optical and electric properties of the mixed Pb/Sn hybrid perovskite materials, which provide the guidance for further application of their optoelectronic devices.

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EXPERMENTAL METHODS

Materials. Analytical-grade reactants of SnO, PbO, Methylamine hydrochloride (CH5N·HCl), H3PO2 solution and HBr solution were purchased (Sinopharm Co. Ltd.) and used without further purification. Crystal growth process of MAPbxSn1−xBr3 is as follows: The raw materials including methylamine hydrochloride (CH5N·HCl), SnO and PbO were dissolved in a mixed solution of HBr/H3PO2 at 75 °C under an ambient atmosphere and then stirred to form a bright yellow solution, which were saturated at about 68 °C. Afterward, the black, shiny large size MAPbxSn1−xBr3 single crystals were grown over one month by top seeded solution growth method with the nonlinear decrease of solution temperature from 68 to 40 °C, such as 68−65 °C with the decrease temperature rate of 60 h/°C and then 65−60 °C with the decrease temperature rate of 48 h/°C. Seed crystals were obtained by spontaneous crystallization. By tuning the molar ratio of Pb and Sn elements (such as Pb/Sn = 2/1, 1/1, and 1/3), kinds of mixed Pb−Sn perovskites single crystals were successfully obtained. X-ray Diffraction. X-ray powder diffraction (XRD) patterns of polycrystalline powder were carried out on a Bruker-AXS D8 Advance X-ray diffractometer with CuKα1 radiation (λ = 1.54186 Å) in the range of 10−90° (2θ) with a step size of 0.004°. Single crystals’ structures were determined by Bruker SMART APEX-II diffractometer equipped with a CCD detector (graphite-monochromatized Mo−Kα radiation, λ= 0.71073 Å) at 296 K. Data integration and cell refinement were performed using the APEX2 software.23 The structure was analyzed by direct methods and refined using the SHELXTL 97 software package.24 All non-hydrogen atoms of the structure were refined with anisotropic thermal parameters, and the refinements converged for Fo2 > 2θIJFo2. All the calculations were performed using SHELXTL crystallographic software package.24 Symmetry analysis on the model using PLATON revealed that no obvious space group change was needed. In the refinement, the commands EDAP and EXYZ were used to restrain some of the related bond lengths and bond angles.25 X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS measurements of the MAPbxSn1−xBr3 samples with about 2 × 3 × 1 mm3 newly synthesized and exposure to air one month were performed on ESCALAB 250 (Thermo Fisher Scientific) instrument under vacuum atmosphere (1.7 × 10−10 mbar). Moreover, the same XPS measurements were also carried out before and after the humidity treatment of the newly synthesized single crystals. UV−vis-NIR Diffuse Reflectance Spectra Measurements. UV−vis-NIR diffuse reflectance spectra were carried out using a Shimadzu UV 2550 spectrophotometer equipped with an integrating sphere over the spectral range 200−900 nm. The series of MAPbxSn1−xBr3 single crystals were dried and ground into powders. A BaSO4 plate was used as the standard (100% reflectance). The absorption spectra were calculated from the reflectance spectrum using

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RESULTS AND DISCUSSION Figure 1a-e show the kinds of bulk MAPbxSn1−xBr3 single crystals with a black, shiny surfaces.17,27 Compared with the MAPbBr3 (Figure 1a) and MASnBr3 (Figure 1b), the color of MAPbxSn1−xBr3 single crystals (Figure 1c-1e) deepened obviously. This can also be observed from their powder and micrometer size single crystals, as shown in Supporting Information (SI) Figure S1−S2, which the color of MAPbxSn1−xBr3 microcrystals deepened with the increasing of Sn2+ ions. Powder X-ray diffraction patterns of these crystals confirmed the pure perovskite phase of MAPbxSn1−xBr3 single crystals. All the XRD diffraction patterns match well with the calculated XRD patterns, as shown in Figure 1f. With the increase of the Sn2+, a small shift in the XRD peaks toward the larger angle side was exhibited obviously, which demonstrated the decrease of lattice parameters due to the smaller ionic radius of Sn2+ than that of Pb2+, shown in SI Figure S3.28 This matches well with the variation of cubic lattice parameters for MAPbxSn1−xBr3 in Figure 1g. With the molar ratio of Pb and Sn elements of 3:1 and1:4, we got another two single crystals with their lattice parameters and compositions determined by the single crystal X-ray diffraction analysis. All the parameters followed the Vegard’s law with a progressive reduction of the lattice parameter by increasing the amount of Sn2+ ion.28,29 The crystal structures indicate that all the MAPbxSn1−xBr3 crystals belong to the cubic perovskite phase with a Pm-3m space group at room temperature, which are the same with MAPbBr3 and MASnBr3.30 The related single-crystal parameters are presented 1558


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Chemistry of Materials Table 1. Crystal Data and Structure Refinement for Series of MAPbxSn1−xBr3 Single Crystals empirical formula −1

formula weight/g·mol temperature/K wavelength/Å crystal color crystal system space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 crystal size (mm3) Z density/g·cm−3 μ(mm−1) F (000) theta range (data collection) limiting indices

reflections collected/unique GOF on F2 absorption correction data/restraints/parameters Robs, wRobs (I > 2σ (I)] Rall, wRall (all data)

MAPb0.74Sn0.26Br3a

MAPb0.68Sn0.32Br3b

455.98 293

450.67 293 0.71073 fuchsia cubic Pm-3m 5.922(16) 5.922(16) 5.922(16) 90 90 90 207.64 0.20 × 0.16 × 0.14 1 3.44 29.126 190 3.44 to 28.69 −7 ≤ h ≤ 8 −7 ≤ h ≤ 8 −8 ≤ l ≤ 8 2403/81 (R(int) = 0.0334] 1.00 semiempirical from equivalents 81/0/6 0.0213, 0.0301 0.0213, 0.0301

fuchsia cubic Pm-3m 5.943(3) 5.943(3) 5.943(3) 90 90 90 209.86 0.18 × 0.15 × 0.11 1 3.47 29.882 186 3.43 to 28.09 −7 ≤ h ≤ 8, −7 ≤ k ≤ 8, −7 ≤ l ≤ 8 2383/79 (R(int) = 0.0456] 1.00 79/0/6 0.0248, 0.0296 0.0248, 0.0296

MAPb0.39Sn0.61Br3c 424.93 293 black cubic Pm-3m 5.908(7) 5.908(7) 5.908(7) 90 90 90 206.25 0.13 × 0.12 × 0.11 1 3.373 24.285 174.5 3.45 to 28.27 −7 ≤ h ≤ 7, −7 ≤ k ≤ 7, −7 ≤ l ≤ 7 2452/78 (R(int) = 0.0329] 1.00 78/0/6 0.0188, 0.0234 0.0188, 0.0234

Figure 2. UV−vis diffuse reflectance spectroscopy plots for MAPbxSn1−xBr3: (a) MAPbBr3, (b) MAPb0.74Sn0.26Br3, (c) MAPb0.68Sn0.32Br3, (d) MAPb0.39Sn0.61Br3, (e) MASnBr3, (f) Experimental band gap evolution of MAPbxSn1−xBr3 solid solution as a function of composition x.

in Table 1 and Table S1−S2. Ball-and-stick diagrams of the crystal structures are shown in Figure 1 (h-i). C and N atoms in the C/N structural unit present the disorder phenomenon so that the distribution of C and N atoms is random around the C−N bond. In order to further demonstrate the composition of the MAPb x Sn 1 − x Br 3 , as an example, the EDX of

MAPb0.74Sn0.26Br3 was shown in SI Figure S4, after the friction of its surface. The single crystals exhibit the element of Pb and Sn with a moral ratio of 0.77:0.23, which is close to the analysis of crystal structure MAPb0.74Sn0.26Br3, and demonstrate their composition. 1559


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Figure 3. (a) The band structure (VBM and CBM) of MAPb0.74Sn0.26Br3, MAPb0.68Sn0.32Br3, and MAPb0.39Sn0.61Br3. (b) The density states of MAPb0.74Sn0.26Br3, MAPb0.68Sn0.32Br3, and MAPb0.39Sn0.61Br3 in the near VBM region. The zero-reference is aligned at the VBM. An incremental trend can be observed in the tin 5s state contribution when the tin concentration increases.

Figure 4. Typical PL spectra for the samples, (a) MAPb0.74Sn0.26Br3, (b) MAPb0.68Sn0.32Br3, (c) MAPb0.39Sn0.61Br3 excited by the laser of 405 nm. Time-resolved photoluminescence spectra for MAPbxSn1−xBr3 powders (d) MAPb0.74Sn0.26Br3, (e) MAPb0.68Sn0.32Br3, (f) MAPb0.39Sn0.61Br3. Timeresolved photoluminescence spectra for MAPbxSn1−xBr3 single crystals (g) MAPb0.74Sn0.26Br3, (h) MAPb0.68Sn0.32Br3, (i) MAPb0.39Sn0.61Br3. All the TRPL data were fitted by biexponential decay functions, excited by the laser of 377.8 nm.

bandgap (2.18 eV-1.77 eV), which is much narrower than both of the MASnBr3 (2.02) and MAPbBr3 (2.18 eV). Similar phenomenon has also been observed in MAPbxSn1−xI3 by Kanatzidis et al.31−33 Combined experiment with the firstprinciples electronic structure calculations, the antagonistic effect of band gap narrowing caused by strong spin−orbit coupling (SOC) and band gap enlargement caused by a x-

The optical band gaps (Eg) of the MAPbxSn1−xBr3 single crystals further demonstrate the color deepening with the increase of Sn2+ ions, determined by UV−vis diffuse reflectance spectroscopy, as shown in Figure 2. Notably, by stoichiometrically dissolving the SnO and PbO with methylamine hydrochloride into the mixed solutions of HBr and H3PO2, kinds of MAPbxSn1−xBr3 single crystals were obtained with a tunable 1560


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Chemistry of Materials dependent structural distortion of the perovskite network were proposed. They found that SOC caused a band gap linearly decrease with increases of x until a phase transition ca. x = 0.5. After that, a phase transition from P4 mm to I4 cm was exhibited when the x > 0.5, which led to a lattice distortion and further induced a band gap increase associated with subtle geometric changes. Such difference in band gap trend before and after the phase transition explains the substantial change of band gap behavior observed around x. Here, we also carried out the ab initio calculation, and revealed the same trend of the bandgap shrinkage when increase the concentration of tin in the lattice structure. The details of calculations can be found in the SI Figure S5. As shown in Figure 3a, the band gap shrinkage mainly comes from the lift of VBM when increase the tin concentration. This can be explained by the different magnitude of relativistic effect on lead and tin atoms. As shown in Figure 3b, when there are more lead atoms in the structure, the VBM is dominated by the iodine 6p states, due to the stabilization of the lead 6s states caused by the relativistic contraction effect. However, when more tin atoms are involved in the structure, the VBM is going to be contributed by both iodine 6p states and tin 5s states, since the relativistic contraction effect of tin is not as significant as that of lead. The exposure of the tin 5s states raises the VBM and thus causes a smaller band gap. This anomalous behavior is welcome in solar cell design, such as the tandem solar cells, because it can be utilized to extend the solar absorption and optimize the photogenerated current in halide perovskite solar cells.32 As we know, the grand challenge for tandem solar cells is to find a low cost and high-efficiency widebandgap top-cell. Based on simulations, > 30% PCE can be achieved if top-cell with bandgap of 1 0.70−1.85 eV is combined with Si bottom-cell.33 Beyond doubt, the tunable band gap of MAPbxSn1−xBr3 single crystals exhibit a potential application in the tandem solar cells with high performance. The gradual shift of the PL spectra is consistent with the variation of their UV−vis diffuse reflectance spectra without phase separation under the excitation of different laser, as shown in SI Figure S6 and Figure 4a-c, which exhibit an obvious red shift than that of pure MAPbBr3 and MASnBr3, as shown in SI Figure S7. To investigate the transport properties of perovskite single crystals, we estimated the carrier lifetime of fresh perovskite single crystals by using TRPL measurements. All the TRPL spectra of perovskite were fitted by biexponential decays (Figure. 4d−i, SI Figure S7c,d and SI Figure S8) and the corresponding lifetimes are exhibited in Table 2, including the fast and slow components. Generally, the fast component originates from the high trap density related to the crystal surface conditions, and slow component represents the carrier

transportation in bulk crystal due to the fewer defects which hindered the carrier recombination.34,35 Therefore, the slow component of the carrier lifetime always be used to characterize the bulk properties of the target single crystals. In general, the slow component lifetime is shorter in the powder compared with the single crystal, due to the more defects and surface states.36 This matches well with the carrier lifetime of MAPbxSn1−xBr3 single crystals. Here, kinds of hybrid perovskite single crystals with their carrier lifetime were summarized in Table 3. It can be found that MAPbxSn1−xBr3 single crystals Table 3. Carrier Lifetimes of MAPbxSn1−xPbBr3 Compared with That of Others Hybrid Perovskite Single Crystals materials MAPbI3 single crystal MAPbI3 single crystal MAPbBr3 single crystal FA0.1MA0.9PbI3 single crystal FAPbI3 single crystal FAPbI3 single crystal MA0.45FA0.55PbI3 single crystal MAPbBr3 powder MASnBr3 powder MAPb0.68Sn0.32Br3 powder MAPb0.68Sn0.32Br3 single crystal

τ1 (ns)

τ2 (ns)

references

22 ± 6 74 ± 5 122 32 131 124 11.17 322.42 386.74 348.66

567 (μs) 1032 ± 150 978 ± 22 1074.78 484 1714 2205 230.02 1916.49 2012.52 2098.09

20 34 34 35 39 40 40 our work

also exhibited a good carrier lifetime compared with other single crystals which has been widely used in the photovoltaic devices. Moreover, all the MAPbxSn1−xBr3 single crystals showed longer carrier lifetime than that of the MAPbBr3 (SI Figure S7), indicating the exceptional Sn2+ doping effect in the MAPbBr3 system. As the reported that hybrid perovskite materials used in the solar cells with a shorter photoluminescence lifetime are undesired which lead to the lower solar cell efficiency.35,37,38 The longer carrier lifetime of MAPbxSn1−xBr3 single crystals may provide a promising application in higher efficiency hybrid perovskite solar cell. The thermo-stability was determined by the thermogravimetric analysis. Figure 5 shows the TGA and DSC curves for different MAPbxSn1−xBr3 powders. The decomposition temperature of MASnBr3 is about 175 °C, whereas all the MAPbxSn1−xBr3 begins to lose weight above 190 °C, exhibiting

Table 2. Lifetimes Extracted from the PL Spectra of MAPbxSn1−xBr3 Powder and Single Crystals MAPb0.74Sn0.26Br3 powder τ1 (ns) τ2 (ns)

τ1 (ns) τ2 (ns)

MAPb0.68Sn0.32Br3 powder

MAPb0.39Sn0.61Br3 powder

333.16 (27.47%)

386.74 (28.94%)

335.55 (26.94%)

1753.10 (72.53%)

2012.52 (71.06%)

1838.63 (73.06%)

MAPb0.74Sn0.26Br3 single crystal 394.61 (27.58%)



2086.37 (72.42%)

2098.09 (75.74%)

1840.10 (78.78%)

Figure 5. TGA and DSC data for different MAPbxSn1−xBr3: (a) MASnBr3 , (b) MAPb 0.74Sn0.26 Br 3, (c) MAPb0.68 Sn 0.32 Br 3 , (d) MAPb0.39Sn0.61Br3.. 1561


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Chemistry of Materials a better thermal stability than MASnBr3. In order to determine their thermal decomposition products, we also investigated their residues by the PXRD patterns when they were annealed at 350 °C for 2 h under a flow of N2 gas. As an example, the residues of SnBr 2 , PbBr 2 and the undecomposed MAPb0.39 Sn 0.61 Br 3 were shown in SI Figure S9. The decomposition temperature of MABr was also provided in SI Figure S10, which is similar to the results, reported by Kanatzidis and co-workers.41 The reactions that occurred under a flow of N2 gas are given in eq 1: MAPbx Sn1 − xBr3(s) = MABr(g) + SnBr2(s) + PbBr2(s)

(1)

Then the MABr possibly in the form of the free amine and HBr, followed by a second eq 2: MABr(g) = CH3NH 2(g) + HBr(g)

(2)

Figure 6. Dark current−voltage curve of MAPbxSn1−xBr3 single crystals for space charge limited current analysis. (a) MAPb0.68Sn0.32Br3, (b) MAPb0.39Sn0.61Br3. Carrier concentration measurements of MAPbxSn1−xBr3 single crystals by Hall Effect measurements at room temperature. (c) MAPb0.68Sn0.32Br3, (d) MAPb0.39Sn0.61Br3. The results show that both the single crystals are n-type semiconductor.

The DSC at low temperatures ranging from 150 to 290 K was also provided to determine phase transition temperature, as shown in SI Figure S11. It can be observed clearly that the phase transition temperature of MASnBr3 is about 230 and 184 K, with the phase changing from Cubic (Pm-3m) into Orthorhombic (Pmc21) at 215 K. which is consistent with the previously reported.22 When the temperature was lower to 30 K, the MASnBr3 may exhibited the Triclinic crystal system with the P1 space group.42 However, there are no obvious phase changes for MAPbxSn1−xBr3 single crystals, as shown in SI Figure S11(b−d). The thermal expansion coefficients and the specific heat values of MAPbxSn1−xBr3 single crystals were also shown in SI Figure S12. SI Figure S12a shows the thermal expansions between 303 and 417 K. The thermal expansion coefficients of MAPbxSn1−xBr3, are calculated in the order of 10−5 with the values of 3.148 × 10−5 K−1, 2.718 × 10−5 K−1 and 2.906 × 10−5 K−1, respectively. The specific heat (Cp) values of MAPbxSn1−xBr3 single crystals were shown in SI Figure S12b. As a function of temperature, all the values increase almost linearly with the rising temperature from 293 to 453 K. The trap-state density ntrap and the carrier mobility μ (μ = μp ≈ μn, where μp and μn are the hole and electron mobility, respectively) of MAPbxSn1−xBr3 single crystals were also estimated according to the space charge-limited current (SCLC) model, as shown in Figure 6 (a, b).19,20,39 With the increasing bias voltage, ohmic and quadratic regions were clearly separated, where the kink point is the trap-filled limit voltage (VTFL), determined by the defect density,13 as follows (eq 3):

VTFL =

en tL2 2εε0

single crystals, applied voltage, and the thickness of the single crystal, respectively. The detailed estimated value of trap-state density ntrap and the carrier mobility μ were listed in the SI Table S3. This mobility values are of the same order of magnitude comparing to the result from SCLC measurement of MAPbBr3 single crystal.19 Here, kinds of hybrid perovskite single crystals, such as the reported MAPbBr3,19 MAPbI3,20 MA3Bi2I9,43 (NH4)3Sb2I9,44 and so on, with their trap-state density and the carrier mobility were summarized in SI Table S4. Hall Effect measurements were also carried out at room temperature, showing the n-type semiconductor behavior of the kinds MAPbxSn1−xBr3 single crystals. Both the samples of MAPb0.68Sn0.32Br3 and MAPb0.39Sn0.61Br3 possess the carrier concentrations of 1011 cm−3, as shown in Figure 6(c, d). The air stability is an important factor for the application of hybrid perovskite which has been attracted the world’s attention. Qi et al.45 has reported the instability of MASnBr3, which the Sn2+ ions were oxidized into Sn4+ after the air exposure of 60 min. Compared with MASnBr3 (Figure. 7a), MAPbxSn1−xBr3 single crystals exhibited good air stability even after exposure to the atmosphere for one month, which was determined by XRD and XPS. Figure 7(b) shows the powder XRD patterns of MAPbxSn1−xBr3 after exposure to the atmosphere for one month. Clearly, all the diffraction patterns match well with the initial XRD patterns, as shown in Figure 2(a). The XPS spectra of MAPb0.39Sn0.61Br3 single crystals were shown in Figure 7(c,d). The Sn 3d5/2 peak at binding energies from 490 to 484 eV shows the oxidation state of Sn (Figure 7d). Combined with the XPS of Sn4+ (SI Figure S13), the main binding energy at around 487 eV can be attributed to Sn2+.27,46,47 More importantly, the oxidation state of Sn remains unchanged before and after one month air exposure, which demonstrates the good stability of the MAPbxSn1−xBr3 single crystals. Moreover, the MAPb0.39Sn0.61Br3 single crystals also keep their relative stability even after exposure to high humidity atmosphere (70 RH%) for 24 h, shown in SI Figure S14.

(3)

Where e is elementary charge, nt is defect density, L is the thickness of MAPbxSn1−xBr3 single crystals (0.84 mm for MAPb0.68Sn0.32Br3, 0.82 mm for MAPb0.39Sn0.61Br3), ε0 is vacuum permittivity, and ε is relative dielectric constant. For instance, the defect density of both MAPb0.68Sn0.32Br3 and MAPb0.39Sn0.61Br3 were estimated in the order of 1011 cm−3. The carrier mobility of MAPb0.68Sn0.32Br3 and MAPb0.39Sn0.61Br3 was also estimated, according to the Mott−Gurney’s power law:39 JD =

9εε0μVb 2 8L3

(4)

Where JD, ε, ε0, μ, Vb, and L are the dark current, the relative dielectric constant, the vacuum permittivity, the mobility of 1562


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Chemistry of Materials *(A.K.-Y.J.) E-mail: [email protected]. ORCID

Zonglong Zhu: 0000-0002-8285-9665 Hongbin Liu: 0000-0001-9011-1182 Chu-Chen Chueh: 0000-0003-1203-4227 Xiaosong Li: 0000-0001-7341-6240 Alex K.-Y. Jen: 0000-0002-9219-7749 Xutang Tao: 0000-0001-5957-2271 Present Address #

(X.T.T.) State Key Laboratory of Crystal Materials & Institute of Crystal Materials, Shandong University, No. 27 Shanda South Road, Jinan, 250100, P. R. China. Author Contributions

All authors have given approval to the final version of the manuscript. Notes Figure 7. (a) X-ray diffraction patterns for MASnBr3 powder after air exposure for 3 days (20−36 RH%). (b) X-ray diffraction patterns for kinds of MAPbxSn1−xBr3 powders after exposure to air for one month (20−36 RH%). X-ray photoelectron spectra of MAPb0.39Sn0.61Br3 single crystals before and after exposure to air for one month. (c) XPS full survey spectrum, (d) Sn 3d spectrum.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 51321091, 51772170), National Key Research and Development Program of China (Grant No. 2016YFB1102201) and the Program of Introducing Talents of Disciplines to Universities in China (111 program no. b06015), the National Science Foundation (DMR1608279), ONR Perovskite Project with N00014-17-1-2260 and ONR OPV Project with N00014-17-1-2201, the Asian Office of Aerospace R&D (FA2386-15-1-4106), and the Department of Energy Sun Shot program (DE-EE0006710). Prof. Jen thanks the Boeing-Johnson Foundation for financial support. This work was facilitated though the use of advanced computational, storage, and networking infrastructure provided by the Hyak supercomputer system and funded by the STF at the University of Washington. Theoretical research is supported by the National Science Foundation (CHE1565520 and CHE-1464497 to X.L.). We greatly thank Jian Zhang, Xiufeng Cheng, Xiulan Duan, Tianyu Zhao, and Zhaozhen Cao for their help in structural refinements, thermal properties, XPS measurements and SCLC, Hall Effect and TRPL measurements.

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CONCLUSIONS In summary, we have successfully obtained a series of centimeter-size MAPbxSn1−xBr3 single crystals and demonstrated their structural and optoelectronic properties by XRD, UV−vis, thermogravimetric analysis, space charge-limited current (SCLC) and the Hall Effect measurements, etc. By carefully regulating the Sn2+ content, the optical band gaps of MAPbxSn1−xBr3 single crystals were greatly decreased and the carrier lifetime was greatly delayed compared with that of those MAPbBr3, which widely expanded their photovoltaic applications, especially for the tandem solar cells. Moreover, mixed Pb/Sn single crystals demonstrated the better stability than MASnBr3 even exposed to the ambient atmosphere over one month. These systematical investigations of Pb/Sn single crystals revealed in our work are beneficial for highlighting the role of mixed-metal cations in hybrid perovskite materials with a lower band gap and facilitating the future design of highly stable mixed-metal hybrid perovskite optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04565. XRD patterns of MAPbxSn1−xBr3 single crystals and their residues; The low temperature DSC curves of samples; The thermal expansions and specific heat capacities and stability; Electronic band structure of MAPbxSn1−xBr3 single crystals. X-ray diffraction and XPS patterns of the MAPb0.39Sn0.61Br3 single crystals (PDF) CCDC 1500653 (CIF, PDF) CCDC 1500655 (CIF, PDF) CCDC 1500656 (CIF, PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(X.T.T.) E-mail: [email protected]. 1563


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Tunable Band Gap and Long Carrier Recombination Lifetime of Stable

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