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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04565 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018
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Chemistry of Materials
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 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 organicinorganic perovskites with the optimal bandgap of 1.70 -1.85 eV.
INTRODUCTION The recent development of solution process-able organicinorganic hybrid perovskite materials has drawn both the academic and industrial attentions in the optoelectronic fields based on their low-cost, light-weight, 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 deep-level 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 photo-induced 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 and efficient PVSCs with tunable bandgaps ranging from 1.17 to 1.73 eV with a M-site alloy.11, 12 Recently, benefitting 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 topseeded 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 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,
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growth of mixed organic cations A+ or halide anions X- single crystals has also been reported.21 However, there have been few reports 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 Xray 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. 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 oC under an ambient atmosphere and then stirred to form a bright yellow solution, which were saturated at about 68 oC. Afterward, the black, shiny large size MAPbxSn1xBr3 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 oC with the decrease temperature rate of 60 h/oC and then 65-60 oC with the decrease temperature rate of 48 h/oC. 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 (graphitemonochromatized 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 nonhydrogen atoms of the structure were refined with anisotropic thermal parameters, and the refinements converged for Fo2 > 2σIJFo2. 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
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after the humidity treatment of the newly synthesized single crystals. UV-vis-NIR diffuse reflectance spectra measurements: UVvis-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 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 (PMTH-S1-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 nm 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 analyser (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 K 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 K to 453 K using a Perkin Elmer Diamond DSC analyser. SCLC and Hall Effect measurements: All current–voltage measurements were carried out by Keithley 2400 semiconductor parameter analyser. 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.
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 micro-meter size single crystals, as shown in Figure S1-S2 (see the Supporting Information), 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 towards the larger angle side was exhibited obviously, which demonstrated the decrease of lattice parameters due to the
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Chemistry of Materials
smaller ionic radius of Sn2+ than that of Pb2+, shown in 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 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 MAPbxSn1-xBr3, as an example, the EDX of MAPb0.74Sn0.26Br3 was shown in 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.
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 varia-
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tion 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. Table 1. Crystal data and structure refinement for series of MAPbxSn1-xBr3 single crystals. Empirical formula Formula weight/ g·mol
-1
Temperature/K
MAPb0.74Sn0.26Br3a
MAPb0.68Sn0.32Br3b
MAPb0.39Sn0.61Br3c
455.98
450.67
424.93
293
293
293
Wavelength/Å
0.71073
Crystal color
fuchsia
fuchsia
black
Crystal system
Cubic
Cubic
Cubic
Space group
Pm-3m
Pm-3m
Pm-3m
a/Å
5.943(3)
5.922(16)
5.908(7)
b/Å
5.943(3)
5.922(16)
5.908(7)
c/Å
5.943(3)
5.922(16)
5.908(7)
α/°
90
90
90
β/°
90
90
90
90
90
90
209.86
207.64
206.25
Crystal size (mm )
0.18 × 0.15 × 0.11
0.20 × 0.16 × 0.14
0.13×0.12 × 0.11
Z
1
1
1
3.47
3.44
3.373
µ(mm )
29.882
29.126
24.285
F (000)
186
190
174.5
Theta range (data collection )
3.43 to 28.09
3.44 to 28.69
3.45 to 28.27
-7
