Synthesis and Properties of a Lead-Free Hybrid Double Perovskite

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The Synthesis and Properties of a LeadFree Hybrid Double Perovskite: (CHNH)AgBiBr 3

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Fengxia Wei, Zeyu Deng, Shijing Sun, Fenghua Zhang, Donald M. Evans, Gregor Kieslich, Satoshi Tominaka, Michael A. Carpenter, Jie Zhang, Paul D. Bristowe, and Anthony K. Cheetham Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03944 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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Chemistry of Materials

The Synthesis and Properties of a Lead-Free Hybrid Double Perovskite: (CH3NH3)2AgBiBr6 Fengxia Wei,a,b Zeyu Deng,a Shijing Sun,a Fenghua Zhang,a Donald M. Evans,c Gregor Kieslich,a Satoshi Tominaka,d Michael A. Carpenter,c Jie Zhang,b Paul D. Bristowe,a* Anthony K. Cheethama* a

Department of Materials Science and Metallurgy, University of Cambridge, CB3 0FS, UK

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Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 2 Fusionopolis Way, Singapore. c

Department of Earth Science, University of Cambridge, CB2 2EQ, UK

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International Center for Materials Nanoarchitectonics, National Institute of Materials Science, Ibaraki 305-0044, Japan ABSTRACT: The discovery of lead-free hybrid double perovskites provides a viable approach in the search for stable and environmentally benign photovoltaic materials as alternatives to the lead-containing systems such as MAPbX3 (X = Cl, Br, I). Following our recent reports of (MA)2KBiCl6 and (MA)2TlBiBr6, we have now synthesized a hybrid double perovskite, (MA)2AgBiBr6, that has a low band gap of 2.02eV and is relatively stable and non-toxic. Its electronic structure and mechanical and optical properties are investigated with a combination of experimental studies and density functional theory calculations.

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INTRODUCTION

The organic-inorganic lead halide perovskites RPbX3 (R = CH3NH3+ (methylammonium, MA), NH2CHNH2+ (formamidinium, FA); X = Cl , Br and I ) have become very important materials for low cost photovoltaic applications in the last 5 years.1–5 The efficiency of single junction perovskite solar cells has soared during this period to 22.1% in 2016.6 Despite their outstanding solar cell performance, however, the toxicity of lead and the chemical instability of the lead halide perovskites are still the major drawbacks in relation to their commercialization. This has led to a search for alternative materials that are both stable and lead-free. One obvious option is to replace Pb by Sn and Ge from the same periodic group, but the chemical instability of Sn2+ and Ge2+ give rise to limitations for their further utilisation.7,8 Another environmentally benign cation, Bi3+, which is isoelectronic with Pb2+, shows great promise, but Bi3+-containing hybrid halides are normally low dimensional structures, such as (MA)3Bi2I9 and (NH4)3Bi2I9, giving rise to larger band gaps due to quantum confinement.9,10 In order to incorporate the trivalent Bi3+ into a hybrid 3-D perovskite architecture, a combination with a monovalent cation is necessary to form a double perovskite, e.g. (MA)2MIBiX6. Their inorganic analogues are well known as elpasolites,11,12 for instance A2MIMIIIX6 (A and MI = Li, Na, K, Rb, Cs, Tl, Ag, etc., MIII = Al, Bi, Fe, Ga, Ln, etc., and X = F, Cl, Br, CN). Very recently, Cs2AgBiX6 (X = Cl,

Br) was reported independently by three different groups;13–15 it crystalizes in cubic 3 symmetry and shows light absorption at the visible range of the spectrum. In the meantime, efforts to search for hybrid double perovskites have also been underway and the first hybrid double perovskites, (MA)2KBiCl6 with a bandgap (Eg) of 3.0eV16 and (MA)2TlBiBr6 with a direct bandgap of 2.16eV,17 have recently been reported. Being isoelectronic with MAPbBr3, (MA)2TlBiBr6 has a band structure that is very similar to the lead compound. However, despite its interesting electronic properties, the severe toxicity of Tl precludes (MA)2TlBiBr6 from being a practical alternative to the Pb analogue.

Figure 1. (a) Crystal structure of (MA)2AgBiBr6, obtained from single crystal X-ray diffraction. Purple and yellow octahedra represent BiBr6 and AgBr6, respectively. (b) The methylammonium cation was rotated by 5° clockwise around the c axis for better illustration. Partial occupancy is shown as different colours for one sphere.

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In the present work, we report a new, environmentally friendly hybrid double perovskite, (MA)2AgBiBr6, which has a smaller bandgap and better thermal stability than (MA)PbBr3. Optical spectroscopy, thermal analysis, resonant ultrasound spectroscopy (RUS) and nanoindentation are studied in combination with density function theory (DFT) calculations. 2.

EXPERIMENTAL SECTION

Various synthetic techniques can be used to obtain halide double perovskites, such as solvent evaporation,11,13 solution cooling,11,14 hydrothermal methods,16,17 solid state sintering12 and growth from a melt.18 Inorganic chlorides can be formed relatively easily, but the synthetic challenges increase significantly for the bromides and iodides. Although more than 20 inorganic, bromide-containing double perovskites have been synthesized,19 to the best of our knowledge, only 3 iodide compositions have been reported so far: Cs2NaLaI6, Cs2LiLaI6 and Cs2LiLuI6, which were obtained by growth from a melt.18,20 The synthesis of Cs2AgBiI6 has been attempted by melting the starting materials in a vacuum sealed quartz ampoule, but without success, and Xiao et. al. reported that the compound is thermodynamically unstable with respect to the starting materials, according to their DFT calculations.21 The synthesis of hybrid halide double perovskites has proved to be more difficult than that of their inorganic counterparts. The low decomposition and/or vaporization temperatures of the organic starting materials preclude techniques involving high temperatures, thus the synthetic options are quite limited. Of the two previously reported examples, single phase (MA)2KBiCl6 can be readily obtained from hydrothermal, solvent evaporation and solution cooling methods, but the formation of (MA)2TlBiBr6 is limited to hydrothermal methods and the product contains significant amounts of yellow (MA)3Bi2Br9. In the current study, crystals of (MA)2AgBiBr6 were synthesized by the hydrothermal method in a stainless steel Parr autoclave with 23ml Teflon liner, using 0.112g CH3NH3Br, 0.094g AgBr and 0.2243g BiBr3 (molar ratio 2:1:1) in 0.5ml HBr (48% in H2O) acid solution. In order to promote the formation of the target compound, additional 0.011g CH3NH3Br and 0.036g PbBr2 (molar ratio 1:1) were used to serve as a seed by forming MAPbBr3 in situ. The solution was heated at 433K for 3 days then slowly cooled to room temperature in 3 hours. Crystals were then filtered out and washed with ethanol. A small amount of yellow (MA)3Bi2Br9 is present as secondary product. The (MA)2AgBiBr6 crystals, red in colour, tend to crystalize with an octahedral morphology. The presence of a seed is crucial for the target compound formation, and its absence will yield yellow (MA)3Bi2Br9 crystals instead. We also performed unsuccessful attempts to obtain (MA)2AgBiI6, (FA)2AgBiI6 and (FA)2AgBiBr6, but these reactions yielded the layered R3Bi2X9 phases. 3.

RESULTS AND DISCUSSION

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Single crystal X-ray diffraction studies showed that (MA)2AgBiBr6 crystalizes in the cubic space group 3 with a lattice parameter a = 11.6370(1) Å. Alternating AgBr6 and BiBr6 octahedra form a 3D framework, resulting in a doubled cell compared to the normal hybrid perovskite (Figure 1). The Ag-Br bond length (2.952(2) Å) is slightly longer than that of Bi-Br (2.868(2) Å). The orientation of the methylammonium cation, which is disordered in 3 , was determined according to the shape of the electron density. In our final model, the C-N bonds align along the directions, with 6 possible orientations, each with probability of 1/6, thus resulting in an apparent octahedron on average.

Figure 2. Absorption spectrum (a) and Tauc plot (b) of (MA)2AgBiBr6. (c) DFT calculated band structure and its projected density of states (PDOS) including the effect of spin-orbit coupling. (d) Partial charge densities visualized

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in the CB and the VB at the X and L points. The following high symmetry k-points were used for the band structure calculation: Γ (0,0,0), X (0.5,0,0.5), W (0.5,0.25,0.75) and L (0.5,0.5,0.5). Within the inorganic framework the Ag, Bi and Br atoms are coloured silver, purple and brown respectively. The charge density levels vary between 0 (blue) and 0.005 e/Bohr3 (red). Chemical analysis was performed on single crystals of (MA)2AgBiBr6 by energy dispersive spectroscopy (EDS) in a scanning electron microscope (Nova NanoSEM 450) (Figure S2 and Table S1). The spectra were collected on an inner facet that was carefully cleaved from a large crystal. An average atomic ratio of Ag:Bi:Br = 1:1.05:5.3 was obtained, in reasonable agreement with the compound stoichiometry, and no obvious Pb peaks were present. However, the presence of Pb was observed from surfaces of uncleaved crystals that were directly collected from the autoclave without washing. The optical bandgap was estimated from both absorption and diffused reflectance spectra using UV-visible spectrometry. An absorption cutoff wavelength ~ 620nm was observed (Figure. 2a), and the Tauc plot from the reflectance spectrum (Figure. 2b), obtained by assuming an indirect bandgap, as indicated by our DFT calculations (below), resulted in an Eg of ~2.02 eV. This value is smaller than that of MAPbBr3, which was reported to be 2.2-2.3 eV,22 but is comparable with its inorganic analogue, Cs2AgBiBr6 (1.95-2.19 eV).13,14 This bandgap is considered to be narrow enough to exhibit semiconducting properties, but only ionic conductivity (4.8 x 108 Ω cm, details are available in SI) was observed by single-crystal conductivity measurements.23,24 This means that its electronic conductivity is probably far smaller than the ionic conductivity due to the low number of carriers and/or low mobility. Figure 2c shows the electronic band structure with the projected density of states (PDOS) of (MA)2AgBiBr6. The atomic structure used for the calculations is described in the SI and is based on the X-ray diffraction data. The band structure shows an indirect bandgap (X→L) of 1.25 eV and is similar to our previous DFT results which used a rhombohedral structural model (1.11 eV).17 However, we acknowledge that in both cases DFT (with spin-orbit coupling) will underestimate the bandgap significantly compared with experimental results. As shown by the PDOS, the valence band maximum (VBM) has contributions from Ag 4d, Bi 6s and Br 4p states, whereas the conduction band minimum (CBM) contains mostly Ag 5s, Bi 6p and Br 4p states. The charge densities of the VBM (X in VB) and the CBM (L in CB) are visualized in Figure 2d. In comparison with the corresponding lead compound, MAPbBr3, the Ag 4d states lead to an indirect band gap at the band edges for (MA)2AgBiBr6. As shown in Figure 2d, the X point in the VB is dominated by Bi 6s– Br 4p and Ag 4d–Br 4p antibonding states, whereas the L point in the VB consists mostly of Ag 4d–Br 4p antibond-

ing states with a small contribution from Bi 6p–Br 4p bonding states. For the CB, the X point consists of Bi 6p– Br 4p antibonding states plus Ag 4p–Br 4s bonding states, while at the L point has contributions from Bi 6p–Br 4p antibonding states. These specific orbital interactions result in the X and L points having the highest and lowest energies in the VB and CB respectively thus forming the indirect bandgap. Similar orbital interactions were found in our previous computational study on Pb-free hybrid double perovskites and used to explain and compare the nature of the bandgap in (MA)2AgBiI6 and (MA)2TlBiI6.17 The effect of the MA cations on the band edge is not significant (Figure S6 compares the band structure with that of the [AgBiBr6]2- inorganic framework in which the MA cations are replaced by background charges), since they do not contribute to the energy states at the band edges. However, substituting MA+ with Cs+ lowers the band gap by about 0.25 eV. This is to be expected since computationally the inorganic framework is held fixed (see SI) and the Cs ion has a smaller ionic radius than the MA ion.25 Experimentally, it is not clear whether Cs+ lowers the band gap since, as noted earlier, values of 1.95 eV14 and 2.19 eV13 have been measured for Cs2AgBiBr6 and these fall on either side of the value determined in this work for (MA)2AgBiBr6 (2.02 eV). The small difference in the computed band gaps of Cs2AgBiBr6 and (MA)2AgBiBr6 has little effect on the band dispersions near the band edges and as a consequence the effective masses of the two perovskites are very similar (Figure S6 and Table S3). Although some values near X and L are small implying high carrier mobility, the observed insulating nature of (MA)2AgBiBr6 probably indicates that the single crystal is an intrinsic semiconductor and can be conductive with optical excitation. Previous studies have shown that spin-orbit coupling (SOC) is crucial for obtaining the correct band dispersion and therefore we have compared the band structure of (MA)2AgBiBr6 with SOC to that without (Figure S6).26–28 It is seen that the reduction in band gap due to SOC is quite small (~0.18 eV) and significantly less than that found in MAPbI3 (1.06 eV).29 This is because the atomic number (Z) of Ag is much smaller than Pb and the SOC effect is strongly dependent on Z. In the double perovskite (MA)2KBiCl6, studied previously, the energy states near the band gap belong mostly to Bi and Cl and, unlike the Ag states in (MA)2AgBiBr6,, the K states are located deep in the VB. The effect of this is that a large reduction in band gap due to SOC is seen in this material as well.16 Despite the apparently small influence of SOC in (MA)2AgBiBr6, Figure S6 shows that it does change the shape of the conduction bands and therefore remains essential for determining the correct band structure of the material. Thermal stability determined by thermogravimetric analysis (TGA) showed the compound was stable until ~550K with heating rate of 10 K/min, where decomposition began (Figure 3a). (MA)2AgBiBr6 exhibits better thermal

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stability compared to MAPbBr3, which decomposes at ~490K at heating rate of 20 K/min;30 but it is less thermally stable than the Cs2AgBiBr6 (~700K).14 No obvious phase transition was detected from the corresponding differential scanning calorimetry (DSC) curve upon heating. Low temperature DSC was also conducted, and again no obvious phase transitions were detected (Figure S3). An approximately linear lattice expansion with a thermal expansion coefficient of ~4.410-5/K was obtained from 120K to 360K from variable temperature single crystal Xray diffraction (Figure 3b). The crystal colour actually changes from red to yellowish brown upon cooling (Figure S1).

Figure 3. (a) SDT (simultaneous DSC and TGA) measurements with TGA shown in black and DSC shown in green. (b) Lattice thermal expansion from 120K to 360K obtained from single crystal X-ray diffraction. (c) RUS spectra during cooling (blue) and heating (red); y axis is the amplitude in volts from the detecting transducer but the spectra are stacked in proportion to temperature at which they were collected and the axis labelled as temperature. Although both DSC and XRD indicate that there are no phase transitions on cooling from 300K to 120K, resonant ultrasound spectroscopy (RUS), suggests otherwise (Figure 3c). RUS is extremely sensitive to phase transitions through the influences of strain coupling on the elastic constants.31 Figure 3c shows segments of the spectra stacked in proportion to the temperature at which they were collected. Changes in peak shape and disappearance were observed at 185K and 158K, respectively. The elastic constants scale with the square of the frequencies, f, of

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individual mechanical resonances of the sample and the inverse mechanical quality factor, Q-1, obtained from the resonance peak widths, is a measure of acoustic loss. f2 decreases (i.e. elastic softening) from 300K to 185K, following by a slight increase down to 158K, and then an abrupt elastic hardening continued until 7K. The first discrete feature, at 185K, is a change in gradient of f2 and a small peak in Q-1 can be seen in Figure (S4). The second, at 158K, is another change in gradient and a steep increase in attenuation of the peaks. Sharp resonance peaks, i.e. with low attenuation, reappear below ~35 K. This evidence for one or two phase transitions contrasts with the DSC results which do not appear to show obvious anomalies in the heat capacity, suggesting perhaps that the changes in structure are only short range. Although the room temperature structure can be refined in a cubic cell with a ~ 11.637Å, weak indications of doubling along all three axes (a’ ~ 23.3 Å) can be observed throughout the temperature range on cooling. Therefore the temperature dependent behaviour is not fully understood at this point and further crystallographic studies on this system will be part of our work plan in the near future.

Figure 4. (a) Young’s modulus of (MA)2AgBiBr6 as a function of indentation depth. (b) Comparison of Young’s moduli of (MA)2AgBiBr6 with MAPbBr332 (on the left) and (MA)2TlBiBr617 (on the right). As shown in Figure 4, the elastic properties of (MA)2AgBiBr6 were measured by nanoindenation along the normals of the (110) and (111) facets in the cubic space group. The Young’s moduli (E) and hardnesses (H) (see Figure S5) were found to be E110 = 8.4 ± 1.7 GPa, H110 = 0.47 ± 0.09 GPa and E111 = 7.9 ± 1.4 GPa, H111 = 0.55 ± 0.11 GPa

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(Fig. 4b). The anisotropy observed here is attributed to the different orientations of the inorganic Ag-Br-Bi linkages. Comparing the mechanical properties of (MA)2AgBiBr6 with those of the analogous leadcontaining phase, MAPbBr3,32 the nanoindentation results on the {110} facets show that (MA)2AgBiBr6 is far more compliant than MAPbBr3; this is consistent with the expectation from Bennett et. al.33 that the replacement of two divalent metal cations (Pb in this case) by a monovalent one (Ag) and a trivalent one (Bi) should lead to a decrease in the stiffness of the material. In addition, comparing (MA)2AgBiBr6 with (MA)2TlBiBr6, the Ag phase shows a lower Young’s modulus than the Tlcontaining double perovskite along the same crystallographic orientation, probably due to the greater packing density in the thallium phase (Figure 4b; Table S2).17 4.

CONCLUSION

In conclusion, we have discovered a non-toxic, hybrid double perovskite, (MA)2AgBiBr6, which has an indirect band gap of ~2eV. The characteristic of the band gap is explained using density functional theory calculations and it is shown how the presence of Ag reduces the effect of spin-orbit coupling. The material is air and moisture stable, exhibits a higher decomposition temperature and smaller Young’s modulus than MAPbBr3. The discovery of (MA)2AgBiBr6 further demonstrates the importance of the double perovskite approach in the search for lead-free photovoltaic materials that exhibit good stability.

ASSOCIATED CONTENT Supporting Information. Additional information containing CIF, experimental details, and DFT calculations are in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT F. Wei is a holder of an A*STAR international fellowship granted by the Agency for Science, Technology and Research, Singapore. G. Kieslich and A.K. Cheetham thank the Ras al Khaimah Center for Advanced Materials for financial support. S. Sun, F. Zhang and Z. Deng would like to thank the Cambridge Overseas Trust and the China Scholarship Council. The calculations were performed at the Cambridge HPCS and the UK National Supercomputing Service, ARCHER. Access to the latter was obtained via the UKCP consortium and funded by EPSRC under Grant No. EP/K014560/1. RUS facilities were established in Cambridge through grants to MAC from the Natural Environment Research Council (NE/B505738/1, NE/F017081/1) and the Engi-

neering and (EP/I036079/1).

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TOC graphic The Synthesis and Properties of a Lead-Free Hybrid Double Perovskite: (CH3NH3)2AgBiBr6 Fengxia Wei, Zeyu Deng, Shijing Sun, Fenghua Zhang, Donald M. Evans, Gregor Kieslich, Satoshi Tominaka, Michael A. Carpenter, Paul D. Bristowe,* Anthony K. Cheetham*


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Synthesis and Properties of a Lead-Free Hybrid Double Perovskite

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