Symmetry, Dynamics, and Defects in Methylammonium Lead Halide

Letter pubs.acs.org/JPCL

Symmetry, Dynamics, and Defects in Methylammonium Lead Halide Perovskites Wouter M. J. Franssen, Sverre G. D. van Es, Rıza Dervişoğlu,† Gilles A. de Wijs, and Arno P. M. Kentgens* Radboud University, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands S Supporting Information *

ABSTRACT: In order to better understand the structure and dynamics of methylammonium lead halide perovskites, we performed NMR, NQR, and DFT studies of CH3NH3PbI3 in the tetragonal and cubic phase. Our results indicate that the space group of the tetragonal phase is the nonpolar I4/mcm. The highly dynamic methylammonium moiety shows no indication of the occurrence of additional orientations of the C−N bond close to the c-axis at temperatures approaching the cubic phase. Crystal quality effects are shown to influence the 14N NMR and 127I NQR spectra, and the effects of high-temperature annealing on defects can be observed. A strong increase in T2 relaxation time of the 207Pb NMR signal on cooling is found, and is an indication of slow motions in the PbI6 octahedra at room temperature. These results aid in the understanding of the structure of methylammonium lead halides and enable further studies of defects in these materials.

in the tetragonal phase and its effect on the fluctuations in the octahedra;9,11,13 (ii) the space group in the tetragonal phase, which is debated to be either the polar I4/cm or the nonpolar I4/mcm;2,14−18 (iii) the appearance of defects and methods to measure the structural quality of methylammonium lead iodide.19−24 To investigate these areas, we have used solid state NMR spectroscopy, as it is sensitive to both mobilities over many different time scales, and local symmetries and distortions thereof. Previous NMR studies on methylammonium lead iodide in the late 80s and early 90s6,8,25,26 and recently by Baikie et al.27 have focused on the mobility of the organic group, showing the arresting of reorientation of the C−N axis in the orthorhombic phase, and increasing disorder in the tetragonal and cubic phase. Fast dynamics of reorientation motions of around 3 ps have been shown, together with interactions between the ammonium and the lead-iodide octahedra.27 In this Letter, we describe the results of solid state NMR experiments, and density functional theory (DFT) calculations used to interpret the resulting data. We demonstrate that 14N NMR spectra in the tetragonal phase show axial symmetry, pointing toward I4/mcm as the space group. A decrease in 14N quadrupolar coupling on raising the temperature in the tetragonal phase is shown to be caused by the change in c/aratio, and DFT calculations show that it is not necessary for the methylammonium group to visit additional orientations with the C−N bond close to the c-axis to allow for the sharp

I

n recent years there has been a resurgence in the interest for mixed organic/inorganic perovskites, initiated by the efficiency of methylammonium lead iodide (CH3NH3PbI3)based solar cells, reaching around 20% photo conversion efficiency.1 Apart from detailed studies on the production methods for making efficient cells, many authors have addressed the structural and electronic properties in order to understand the excellent performance in photovoltaic applications.2−5 Methylammonium lead iodide has an intriguing structure, with a mobile organic group that is rotating and reorientating in a cavity of corner sharing PbI6 octahedra. It exists in three different phases: orthorhombic below −110 °C (Pnma), tetragonal between −110 and 54 °C (I4cm or I4/ mcm), and cubic above 54 °C (Pm3̅m).2,6,7 In the orthorhombic phase, the PbI6 octahedra are distorted in numerous ways, as there are no reorientations of the methylammonium (MA) group. In the tetragonal phase, the MA is supposed to be 2D disordered in the ab-plane, leading to axial symmetry in the octahedra. The octahedra are rotated about the c-axis with the angle of rotation being temperature dependent, along with a relative elongation along the c-axis toward lower temperatures.2 In the cubic phase, the MA is 3D disordered and has cubic symmetry in the cavity and undistorted octahedra. Within the temperature range where the tetragonal phase exists, distortions in the octahedra are modest close to the tetragonal to cubic phase transition due to its second order nature,8 but increase on lowering the temperature toward the transition to the orthorhombic phase. While many structural characteristics of methylammonium lead iodide are known,9−12 there are still several areas which are not properly understood, among these are (i) reorientations and possible orientations close to the c-axis of the C−N bond © XXXX American Chemical Society

Received: October 31, 2016 Accepted: December 8, 2016 Published: December 8, 2016 61

DOI: 10.1021/acs.jpclett.6b02542 J. Phys. Chem. Lett. 2017, 8, 61−66

Letter

The Journal of Physical Chemistry Letters

Figure 1. DFT optimized structures of CH3NH3PbI3 in the tetragonal phase at 50 °C showing the four used C−N orientations.28

Figure 2. (A) Static 14N spectra of CH3NH3PbI3 at 20 (top) and −40 °C (bottom) measured using a solid echo at 850 MHz. Dashed traces are simulations using CQ = 59 (top) and 85 kHz (bottom) and η = 0.0 for both. (B) Temperature dependence of the experimental 14N CQ (full circles, measured using ∼9 kHz MAS at 400 MHz) overlaid with the scaled c/a ratio from Weller et al. (solid line).2 Diamonds represent DFT calculations showing the 14N CQ values of the individual MA (filled diamonds) and the average of all four positions (open diamonds).

decrease in CQ toward the tetragonal to cubic phase transition. Variable-temperature 207Pb NMR shows a strong increase in transversal relaxation time T2 at lower temperatures in the tetragonal phase that are attributed to low frequency motions in the PbI6 octahedra. Broadening of the 14N line in the cubic phase and the 127I nuclear quadrupolar resonance (NQR) line in the tetragonal phase is shown to be due to crystal strain, which can be reduced by annealing at moderate temperatures of around 100 °C. As the methylammonium reorientations in CH3NH3PbI3 are fast on the NMR time scale (several milliseconds),9 the average symmetry of the NMR interactions on the nitrogen site follows the symmetry of its average position as is included in the space group. In the tetragonal phase, this means there is a nonzero electric field gradient (EFG) for 14N, with either axial symmetry (η = 0) for I4/mcm, or no symmetry for I4c. Powder 14N NMR was used to distinguish between these two cases, as shown in Figure 2A. Both at room temperature and at −40 °C, the powder pattern shows axial symmetry, pointing toward I4/mcm as the most probable space group. Accidental axial symmetry is, however, possible in I4c, but is unlikely as the methylammonium lead iodide structure is heavily distorted at −40 °C compared to room temperature,2 and it is not probable that any local axial symmetry exists at both temperatures that is not included in the space group. Figure 2A furthermore shows an increase of CQ when lowering the temperature, and therefore an increase in the EFG along the c-axis. Weller et al.2 have shown the same effect for the lattice parameters, which are similar near the phase transition to cubic, but more different at lower temperatures. A more detailed temperature-dependent study of the 14N CQ is shown in Figure 2B, and is overlaid with the c/a-ratio as determined by Weller et al.2 The high correlation between the c/a-ratio and the 14N CQ is striking, and points at the lattice

deformation as the main cause for the EFG experienced by the nitrogen atom. While the C−N bond is close to the ab-plane at lower temperatures,2 and in the disordered cubic phase also exhibits orientations close to the c-axis (see Figure 2 of ref 2), it is debated whether these orientations also occur close to the tetragonal to cubic phase transition and provide a necessary contribution to explain the decrease in CQ.9,11 To examine this, we performed DFT calculations, starting from the four C−N orientations described by Weller et al.2 (see Figure 1). In the calculations, the temperature was simulated by using the c/aratio from Weller et al.2 (see Supporting Information (SI)). The CQ values of the MA group in a fixed lattice position are shown to be relatively constant over the temperature range at about 0.6 MHz (see Figure 2B), ca. 6 times higher than the highest experimental value, showing that dynamic averaging is crucial in understanding the 14N CQ in this material. Averaging over the four C−N orientations (thus modeling the effect of dynamic jumps between these orientations of the methylammonium cation) leads to CQ values close to the experimental values, following the same trend as a function of temperature. It is striking that the average calculated electric field gradient goes to zero at 50 °C, without the need for visiting additional out of plane orientations. When examining the C−N orientation as a function of temperature, hardly any rotation toward the c-axis is observed, with the angle of the C−N bond to the c-axis nearly constant at ∼60°. Nevertheless, the Vcc in the crystal frame goes through zero at 50 °C, being a special case, where, after averaging of the traceless EFG over the four orientations, no net electric field gradient remains. Close examination of the 14N signal in the cubic phase of CH3NH3PbI3 powder shows a spectrum where quadrupolar broadening still plays a role, despite the high symmetry point group (Oh), which should lead to an average EFG of zero. 62


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Figure 3. (A) 14N spectra of CH3NH3PbI3 at 85 °C with and without 1.1 kHz MAS (400 MHz). (B) 14N line widths in the cubic phase of a powder of CH3NH3PbI3 as a function of temperature upon heating and cooling at 400 MHz. The line width of a group of single crystals is also shown, along with the homogeneous line width due to T2 relaxation. (C) 127I NQR line widths measured at room temperature. Grind and regrind represent the same powder after one and two periods of mechanical grinding. Two annealing steps on the second powder are also shown, as well as the homogeneous line width.

Figure 4. (A) Variable temperature 207Pb spectra of CH3NH3PbI3 at 400 MHz using 10 kHz MAS. From top to bottom: 20, −30, −48, −68, −100 and −130 °C. The bottommost spectrum was measured without MAS. (B) Static 207Pb T2 of CH3NH3PbI3 as a function of temperature at 400 MHz. (C) 207Pb CSA as a function of temperature measured statically at 400 MHz. Overlayed is the scaled c/a data from Weller et al.2 The fit of the spectra in the tetragonal phase were performed using η = 0.

Figure 3A shows the spectrum at 85 °C with and without MAS, showing an unusual line shape, being neither a Lorentzian or Gaussian, nor a characteristic spin 1 powder pattern. In situ annealing of the powder shows a decrease of the line width as a function of temperature reaching a plateau at around 120 °C (Figure 3B). Cooling does not recover the broad line, and high quality crystals show hardly any broadening compared to the homogeneous line width shown in Figure 3B. These results indicate that the observed line broadening is caused by a change in microstructure induced by grinding of the material. This process induces a variety of defects and dislocations across the sample resulting in various local

distortions of the symmetry, inducing a distribution in (average) electric field gradients at the nitrogen sites. The fact that a significant reduction in line width and increase in crystal quality occurs at temperatures as low as ∼80 °C is a good indication of diffusion of ions taking place at relatively low temperatures. Assuming strain can not only be alleviated by rearrangement of methylammonium cations, the iodide anion, and possibly also the lead, must show some diffusion capability close to defects, in line with calculations and current−voltage sweep experiments.29 Similar line broadening effects can be studied in the tetragonal phase using 127I nuclear quadrupolar resonance 63


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The Journal of Physical Chemistry Letters (NQR) spectroscopy,8 which is unaffected by the angular dependence in the electric field gradient which is observed as a powder pattern in 14N NMR (see Figure 2A), and therefore makes distributions in the size of the orientation independent field gradient accessible as a line broadening effect. Figure 3C shows the 127I NQR line width of CH3NH3PbI3 powder after one and two steps of mechanical grinding and annealing. The axial iodine is more affected by the crystal quality, and for both sites the crystal quality is improved by high temperature treatments. Compared to the 14N spectra, higher temperatures are needed to have an appreciable effect on the 127I line width, showing that 127I NQR is more sensitive to these distortions. Only in spectra of single crystals is the line width close to the homogeneous line width. 127I NQR can therefore be used to study crystal quality at room temperature. Crystal quality effects can also be observed in CH3NH3PbCl3, which is in its cubic phase at room temperature and shows a similar 14N line shape as CH3NH3PbI3 in this phase.6 Single crystals give a line width of 0.8 kHz, while crushed powder leads to a significant broadening (maximum observed line width 7.3 kHz), and annealing of the same powder at 154 °C leads to a line width of 0.4 kHz (see SI). The fact that the annealed powder shows a narrower 14N line than the original single crystal is caused by the low growth temperature of ∼70 °C of the crystals, which would also benefit from an annealing treatment. When performing XRD on a different batch of material (14N line width 3.2 kHz, annealed 1.3 kHz), only a 6% reduction of average XRD line width was observed after annealing (see SI), showing that solid state NMR is more sensitive to these type of distortions than XRD. Apart from the mobilities and symmetries of the methylammonium group, there is a relative lack of understanding of the motions of the octahedra, and the influence of the organic group on them.13 Lead-207, being centered in the octahedra, is sensitive to both changes in the c/a-ratio via the chemical shift anisotropy (CSA), and mobilities via the transversal relaxation time (T2). Assuming I4/mcm, the lead environment has axial symmetry in the ab-plane (η = 0), and the δzz component of the chemical shift tensor along the c-axis. Figure 4A shows 207 Pb NMR spectra of CH3NH3PbI3 at several temperatures. At room temperature, the spectrum shows a single peak at 1450 ppm with a width of about 21 kHz (∼250 ppm at 400 MHz), as is also shown by Rosales et al. and Roiland et al.30,31 In contrast to the 14N case, the asymmetry of the atomic site is not visible in the NMR spectrum at room temperature due to the strong homogeneous broadening caused by a T2 lifetime of only 34 μs.30 This is further shown by the negligible effect of MAS (the Pb − I dipolar couplings are about 1 kHz) and no effect of 1H decoupling. At lower temperatures, the homogeneous broadening reduces, and spinning sidebands appear due to an increasing chemical shift anisotropy. T2 as a function of temperature is shown in Figure 4B and displays an interesting dependence on temperature, being stable on cooling until −60 °C, where a strong increase occurs. This lengthening of T2 on cooling is an unusual effect, as T2 values decrease on cooling following dipolar, CSA, or scalar coupling relaxation using the most common spectral densities.32 It is therefore evident that the observed increase upon cooling cannot exclusively result from an increase in correlation time and must therefore be due to slow motions at the NMR frequency (83.8 MHz), which enhance relaxation at higher temperatures. Cooling down the material changes these

motions, which leads to a reduction in the relaxation efficiency. Also, the temperature-dependent CSA can lead to variations in the relaxation efficiency. Figure 4C shows a plot of the 207Pb CSA as a function of temperature, overlaid with the scaled c/a-ratio from Weller et al.2 It is clear that the lack of CSA results in the high temperature regime inhibits the comparison with the c/a-ratio. However, the CSA goes to zero when approaching the tetragonal to cubic phase transition due to the symmetry in the cubic phase and the second order nature of the transition, and additional nonhomogeneous line broadening of the order of the expected CSA is found at room temperature, giving some merit to the comparison with the c/a-ratio. The transition to the orthorhombic phase leads to a 2-fold increase in CSA, displaying the decrease of the overall symmetry of the lead environment after the transition. In conclusion, we have shown that the methylammonium ion in methylammonium lead iodide is 2D disoriented in the tetragonal phase, even at temperatures close to the tetragonal to cubic phase transition. No additional reorientations of the MA moiety are necessary to explain the experimental behavior of the average electric field gradient at the nitrogen site as measured by 14N NMR, which is shown to follow the c/a-ratio of the unit cell. Crystal quality effects are shown to lead to significant broadenings in the 14N spectra in the cubic phase, and annealing leads to a reduction in the line width with a plateau reached at ∼120 °C. The same effects can be observed in the 127I NQR signal at room temperature in the tetragonal phase, and in the room temperature 14N signal of the closely related methylammonium lead chloride, where XRD does not show significant line broadening in this case. Evidence of low frequency motions in the PbI6 octahedra at room temperature is shown by the 207Pb transversal relaxation time T2, which lengthens upon cooling.

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EXPERIMENTAL AND COMPUTATIONAL METHODS Methylammonium lead iodide crystals were synthesized by precipitation using the method of Poglitsch and Weber.14 Powder was formed using solid state synthesis from methylammonium iodide and lead iodide (see SI). Methylammonium lead chloride was synthesized using the method of Maculan.33 Powder was prepared using manual grinding with mortar and pestle. The NMR spectra were recorded on Varian VNMRS systems operating at a magnetic field of 400 and 850 MHz. Probes used were Varian 3.2 mm T3 HXY and a Chemagnetics 3.2 mm APEX (400 MHz) and Varian 4.0 mm T3 HXY (850 MHz). The chemical shift was referenced using lead nitrate (−3494 ppm) for 207Pb and ammonium chloride (0 ppm) for 14N. T2 values were measured using the Hahn-echo experiment. NQR experiments were performed using a Bruker 300WB BL 4 mm probe. Electronic structure and electric field gradient34 (EFG) calculations were carried out with the Vienna ab initio simulation package (VASP)35−38 using the projector-augmented wave (PAW) method39,40 with Becke−Perdew− Ernzerhof (PBE) exchange-correlation functionals.41 We use a 2 × 2 × 2 super cell containing one MA+ ion and seven Cs+ ions at the 4b sites (I4/mcm) following the 180 K structure of Weller et al.2 In our model we assume that all Pb−I distances are identical. We assume a linear volume expansion (the Pb−I distance are temperature dependent) and use the c/a 64


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(6) Knop, O.; Wasylishen, R. E.; White, M. A.; Cameron, T. S.; Oort, M. J. M. V. Alkylammonium Lead Halides. Part 2. CH3NH3PbX3 (X = Cl, Br, I) Perovskites: Cuboctahedral Halide Cages With Isotropic Cation Reorientation. Can. J. Chem. 1990, 68, 412−422. (7) G, S.; Mahale, P.; Kore, B. P.; Mukherjee, S.; Pavan, M. S.; De, C.; Ghara, S.; Sundaresan, A.; Pandey, A.; Guru Row, T. N.; et al. Is CH3NH3PbI3 Polar? J. Phys. Chem. Lett. 2016, 7, 2412−2419. (8) Xu, Q.; Eguchi, T.; Nakayama, H.; Nakamura, N.; Kishita, M. Molecular Motions and Phase-Transitions in Solid CH3NH3PbCl3, CH3NH3PbBr3, CH3NH3PbI3, as Studied by NMR and NQR. Z. Naturforsch., A: Phys. Sci. 1991, 46, 240−246. (9) Bakulin, A. A.; Selig, O.; Bakker, H. J.; Rezus, Y. L.; Müller, C.; Glaser, T.; Lovrincic, R.; Sun, Z.; Chen, Z.; Walsh, A.; et al. Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites. J. Phys. Chem. Lett. 2015, 6, 3663−3669. (10) Lee, J.-H.; Bristowe, N. C.; Bristowe, P. D.; Cheetham, A. K. Role of Hydrogen-Bonding and Its Interplay With Octahedral Tilting in CH3NH3PbI3. Chem. Commun. 2015, 51, 6434−6437. (11) Leguy, A. M. A.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kochelmann, W.; Law, C.; Li, X.; Foglia, F.; Walsh, A.; O’Regan, B. C.; et al. The Dynamics of Methylammonium Ions in Hybrid OrganicInorganic Perovskite Solar Cells. Nat. Commun. 2015, 6, 7124. (12) Mattoni, A.; Filippetti, A.; Saba, M. I.; Delugas, P. Methylammonium Rotational Dynamics in Lead Halide Perovskite by Classical Molecular Dynamics: The Role of Temperature. J. Phys. Chem. C 2015, 119, 17421−17428. (13) Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goñi, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Lattice Dynamics and Vibrational Spectra of the Orthorhombic, Tetragonal, and Cubic Phases of Methylammonium Lead Iodide. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 144308. (14) Poglitsch, A.; Weber, D. Dynamic Disorder in Methylammoniumtrihalogenoplumbates (II) Observed by Millimeter-Wave Spectroscopy. J. Chem. Phys. 1987, 87, 6373−6378. (15) Kawamura, Y.; Mashiyama, H.; Hasebe, K. Structural Study on Cubic−Tetragonal Transition of CH3NH3PbI3. J. Phys. Soc. Jpn. 2002, 71, 1694−1697. (16) Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J. Synthesis and Crystal Chemistry of the Hybrid Perovskite (CH3NH3)PbI3 for Solid-State Sensitised Solar Cell Applications. J. Mater. Chem. A 2013, 1, 5628. (17) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites With Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038. (18) Dang, Y.; Liu, Y.; Sun, Y.; Yuan, D.; Liu, X.; Lu, W.; Liu, G.; Xia, H.; Tao, X. Bulk Crystal Growth of Hybrid Perovskite Material CH3NH3PbI3. CrystEngComm 2015, 17, 665−670. (19) Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defect Migration in Methylammonium Lead Iodide and its Role in Perovskite Solar Cell Operation. Energy Environ. Sci. 2015, 8, 2118−2127. (20) Kim, J.; Lee, S.-H.; Lee, J. H.; Hong, K.-H. The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite. J. Phys. Chem. Lett. 2014, 5, 1312−1317. (21) Delugas, P.; Caddeo, C.; Filippetti, A.; Mattoni, A. Thermally Activated Point Defect Diffusion in Methylammonium Lead Trihalide: Anisotropic and Ultrahigh Mobility of Iodine. J. Phys. Chem. Lett. 2016, 7, 2356−2361. (22) Yin, W.-J.; Shi, T.; Yan, Y. Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903. (23) Buin, A.; Comin, R.; Xu, J.; Ip, A. H.; Sargent, E. H. HalideDependent Electronic Structure of Organolead Perovskite Materials. Chem. Mater. 2015, 27, 4405−4412. (24) Buin, A.; Pietsch, P.; Xu, J.; Voznyy, O.; Ip, A. H.; Comin, R.; Sargent, E. H. Materials Processing Routes to Trap-Free Halide Perovskites. Nano Lett. 2014, 14, 6281−6286. (25) Furukawa, Y.; Nakamura, D. Cationic Dynamics in the Crystalline Phases of (CH3NH3)PbX3 (X: Cl, Br) as Studied by

ratio of ref 2 to construct a series of cells as a function of temperature. For each temperature, we only relax the MA+ coordinates and the coordinates of the 12 nearest neighbor iodide ions. To converge the symmetry averaged EFGs, the stopping criterion for relaxation had to be as small as ∼10−8 eV. More detail can be found in the SI.

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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.jpclett.6b02542. Additional experimental and computational details. 1H and 13C spectra and XRD of the prepared powders. 127I NQR spectra of CH3NH3PbI3. Point groups of all relevant nuclei. XRD line widths of annealed and nonannealed CH3NH3PbCl3 powders (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Arno P. M. Kentgens: 0000-0001-5893-4488 Present Address †

Max Planck Institute for Biophysical Chemistry, Department of NMR-based structural biology, Am Fassberg 11, 37077 Göttingen, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Netherlands Organization for Scientific Research (NWO) is acknowledged for their financial support of the programs in which this research is conducted: The Graduate School for Molecules and Materials (W.M.J.F), Stichting voor Fundamenteel Onderzoek der Materie (R.D.) and the VIDI research program 700.10.42 (R.D.). Moreover, NWO supports the solidstate NMR facility for advanced material science. This research used resources of the CFN at BNL under Contract No. DESC0012704 (RD). We thank Paul Tinnemans for the XRD measurements and Herma Cuppen, Bardo Bruijnaers, and René Janssen for fruitful discussions.

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REFERENCES

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Symmetry, Dynamics, and Defects in Methylammonium Lead Halide

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