The Raman Spectrum of the CH3NH3PbI3 Hybrid Perovskite: Interplay

Letter pubs.acs.org/JPCL

The Raman Spectrum of the CH3NH3PbI3 Hybrid Perovskite: Interplay of Theory and Experiment Claudio Quarti,*,† Giulia Grancini,‡ Edoardo Mosconi,† Paola Bruno,‡ James M. Ball,§ Michael M. Lee,§ Henry J. Snaith,§ Annamaria Petrozza,*,‡ and Filippo De Angelis*,† †

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, I-06123, Perugia, Italy Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, 20133, Milano, Italy § Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom ‡

S Supporting Information *

ABSTRACT: We report the low-frequency resonant Raman spectrum of methylammonium lead-iodide, a prototypical perovskite for solar cells applications, on mesoporous Al2O3. The measured spectrum assignment is assisted by DFT simulations of the Raman spectra of suitable periodic and model systems. The bands at 62 and 94 cm−1 are assigned respectively to the bending and to the stretching of the Pb−I bonds, and are thus diagnostic modes of the inorganic cage. We also assign the librations of the organic cations at 119 and 154 cm−1. The broad, unstructured 200−400 cm−1 features are assigned to the torsional mode of the methylammonium cations, which we propose as a marker of the orientational disorder of the material. Our study provides the basis to interpret the Raman spectra of organohalide perovskites, which may allow one to further understand the properties of this important class of materials in relation to their full exploitation in solar cells. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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related organohalide MAPbCl3 perovskite shows various bands between 40 and 321 cm−1, associated to the librations of the MA organic cations and a band at 483 cm−1, assigned to the MA torsional mode.27 The assignment of the MA torsional mode represents a very important issue in the characterization of hybrid organic−inorganic systems, with reported frequencies in the 281−487 cm−1 range.27,33−35 Such large spread of MA torsional frequencies is quite surprising. To the best of our knowledge, no experimental data are available for such mode in solution or in the gas phase, but a few theoretical calculations have set it at 267−324 cm−1.36,37 Hybrid perovskites are intrinsically complex materials, where the presence of various types of interactions and structural disorder15 may play an important role in the materials properties. In this work, the assignment of the Raman spectrum of MAPbI3 is supported by first-principles DFT calculations38,39 carried out on extended periodic systems and, for interpretative purposes, on simplified models. While this type of analyses have been applied to perovskites, especially in relation to their ferroelectric properties,40−47 to the best of our knowledge this is the first report on the Raman spectrum of the prototypical MAPbI3 perovskite, along with the first DFT simulation of the vibrational spectra of hybrid perovskites.

ybrid lead−halide perovskites are revolutionizing the photovoltaic landscape, having been claimed as “the next big thing in photovoltaics”.1 From their first application in 2009 by Kojima et al. as solar cells sensitizers,2 photovoltaic devices based on these materials showed a fast and continuous increase in their performance,2−14 with recent top efficiency exceeding 15%. Methylammonium lead-iodide, hereafter MAPbI3, represents a prototype system for photovoltaic applications.5,6,11,13 In spite of the wide interest in MAPbI3, the characterization of its hybrid structure and the understanding of its chemical-physical properties are still in an early stage. If, on the one hand, the study of the structure and of the properties of this material have been carried out by X-ray diffraction and UV−vis spectroscopy,15−18 along with electronic structure calculations,18−21 a comprehensive vibrational spectroscopy study of MAPbI3 is still missing. Here we report a combined experimental and theoretical Raman vibrational analysis of MAPbI3 in the low-frequency region, where the bands associated to the vibrations of the interacting inorganic/organic constituents are expected. To the best of our knowledge, the only vibrational investigation on MAPbI3 was carried out by IR spectroscopy in the 700−3500 cm−1 region,22 where only the internal modes of the MA organic cations are present. Several vibrational spectroscopy studies of inorganic23−26 and hybrid (layered) perovskites27−32 have been reported, which may provide useful guidelines for the MAPbI3 spectral assignment. The Raman spectrum of the © 2013 American Chemical Society

Received: November 28, 2013 Accepted: December 26, 2013 Published: December 26, 2013 279

dx.doi.org/10.1021/jz402589q | J. Phys. Chem. Lett. 2014, 5, 279−284

The Journal of Physical Chemistry Letters

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Figure 1. Left: Experimental resonant (a) and nonresonant calculated (tet-1, tet-2, ortho) Raman spectra of MAPbI3 (b−d). The tet-1, tet-2, and ortho optimized structures are shown on the right panel parallel to the ab plane. The periodic structures and Raman spectra were calculated by the Quantum Espresso program package.38

features, thus allowing us to establish some qualitatively interesting structure/property correlations. The experimental band falling at 62 cm−1, as well as that at 154 cm−1, find a good correspondence in the three theoretical spectra. The band at 94 cm−1 is well accounted in tet-2 and in ortho; in tet-1, many vibrational normal modes are found in the 80−100 cm−1 region, although the convoluted spectrum does not show a well-defined maximum. The band at 119 cm−1 instead is quite puzzling. This band does not find any parallel in tet-2 nor in ortho, while it can be possibly associated with the band at 141 cm−1 found in tet-1. Since the three models differ mainly in the orientation of the organic cations, the fact that the region around the experimental band at 119 cm−1 is differently described in the three simulated structures suggests that this is probably associated with the MA libration modes. The assignment of the Raman spectrum is reported in detail on the basis of the results obtained for the tet-1 structure. The assignments made for the tet-1 structure also hold for tet-2 and ortho, unless differently noted. The experimental band falling at 62 cm−1 can be assigned to the theoretical band predicted around 53 cm−1. The present band is mainly due to four normal modes falling at 50, 51, 56, and 60 cm−1, and the visual inspection of the eigenvectors associated to such modes (Figure 2a) demonstrates that they consist mainly in the bending of the I−Pb−I bonds and in the consequent libration of the cations due to the deformation of the inorganic cage. The assignment of the band at 62 cm−1 to the inorganic structure is confirmed by previous studies on related perovskites.28−31 Thus, the band measured for MAPbI3 at 62 cm−1 is a clear marker of the inorganic component. The slight underestimation of this vibrational frequency, which we calculated at 53 cm−1, is possibly due to the neglect of spin−orbit interactions,19,20 which are expected to lead to frequency increases.48,49 Inspection of the normal modes calculated in the region

The Raman spectrum measured under resonant conditions (excitation wavelength 532 nm) for a MAPbI3 film deposited on mesoporous Al2O3 is reported in Figure 1a (see Supporting Information for experimental and sample preparation details). The spectrum is quite similar to the spectra of related hybrid perovskites,28,30 and is characterized by a broad Raman signal in the 50−170 cm−1 region, with two well-defined maxima at 62 and 119 cm−1 and two less intense maxima at 94 and 154 cm−1. A broad and not resolved signal between 200 and 340 cm−1 is also found, plus an additional weak feature at 390 cm−1. In Figure 1b−d the experimental resonant Raman spectrum is compared to the nonresonant Raman spectra calculated in the harmonic approximation for two tetragonal structures (tet-1 and tet-2)19 and for one orthorhombic structure (ortho) composed by four MAPbI3 units (see models in the Supporting Information), representative of the room temperature and lowtemperature dominant phases.15,16 The two tetragonal structures differ for the initial orientation of the organic cations: tet-2 shows an ordered head-to-tail arrangement, while tet-1 shows a more disordered structure,19 inducing a different structural distortion, see Figure 1b−d. The lattice parameters for the investigated structures calculated at the LDA level used for the calculation of Raman intensities (see computational details in the Supporting Information) are in good agreement (within 1−2%) with experimental data15 (see Supporting Information). The tet-1 structure is 0.1 eV more stable than tet2, in agreement with our previous results.19 We stress here that while calculated vibrational frequencies can be directly compared to experimental values, the calculated Raman intensities are obtained in a nonresonant regime, thus in principle a different intensity distribution compared to resonant experimental data could be expected. Nevertheless, the calculated spectra reproduce quite well the experimental 280



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normal mode, neither Raman active or inactive, in the range 200−280 cm−1, though our results could be sensitive to both DFT functional and anharmonic effects, as we will explain below. Furthermore, since our calculations are performed in the nonresonant regime, we cannot exclude that second-order (two phonons) effects may contribute to the observed intensity.24−26 The torsional vibrational frequencies of the organic cations, Figure 2d, are the only modes predicted in the 200−400 cm−1 range. In particular, LDA places these modes at 295−315, 288− 301, and 391−399 cm−1 in tet-1, tet-2, and ortho, respectively. Notably, computing the structure and vibrational frequencies by PBE, a down-shift of the MA torsional modes to 271−309 cm−1 is predicted for tet-1. The large torsional frequencies difference predicted for the two tetragonal structures compared to those calculated for the orthorhombic structure may seem surprising at first glance, but it is actually not totally unexpected considering the large distribution of torsional frequencies reported in the literature for MA-based compounds.27,33−35 Furthermore, comparing the theoretical spectra of Figure 1, we observe that the Raman intensity predicted for the torsional modes increases with the structural disorder, in the order tet-1 > tet-2 > ortho. It is thus reasonable to wonder whether the experimental bands at 200−390 cm−1 can be associated to the torsional mode of the MA cations. To check this possibility and to explain the large differences in torsional frequencies found in the different considered structures, we have carried out further analyses on simplified models obtained by “decomposition” of the optimized crystal structures: (i) the single isolated MA cations; (ii) the four MA cations; and (iii) the four MA cations, plus the three nearest iodide anions, all at the crystal structure geometries (see Figure 3 and Table 1). The calculated vibrational frequency of the torsional mode of the isolated MA cation at its optimized structure is predicted at 309 cm−1 by LDA (278 cm−1 when optimized by the same method used for periodic simulations in a large supercell) in good agreement with previous results.36,37 This mode is predicted to be Raman inactive, consistent with the C3v symmetry of the isolated MA cation. The torsional frequency down-shifts to 302 cm−1 when calculated by PBE, paralleling the frequency shift observed for the perovskite models. Interestingly, anharmonic effects50 lead to further frequency down-shifts for the MA torsional mode, delivering a calculated frequency of 277 (298) cm−1 by PBE (LDA). Based on this data, it is not unreasonable to speculate that a combination of DFT functional and anharmonic effects could down-shift the calculated torsional modes for the tetragonal structures to ∼250 cm−1, as in the experimental Raman spectrum. The torsional mode frequency calculated in the harmonic approximation by LDA for the isolated MA cations at their crystal structures geometries are strongly blue-shifted, by more than 150 cm−1. In this case, the Raman intensities slightly differ from zero because of the partial loss of C3v symmetry. The observed shift is caused by the distortion of the molecule from the gas phase geometry, as suggested by a detailed structural analysis, see Table S1 and Figure S4, Supporting Information. From Table 1, it is interesting to notice that when the interactions between the MA cations and the three closest iodides are considered, the frequencies of the torsional modes shift to lower values. This frequency down-shift amounts to ∼100 cm−1 for tet-1 and tet-2, and to ∼60 cm−1 for the ortho structure and is originated by hydrogen bonding between the ammonium hydrogen and the iodide atoms. Alongside, the

Figure 2. Normal modes computed for the tet-1 structure associated to the most intense Raman bands of MAPbI3. (a) I−Pb−I bending mode at 60 cm−1 (Raman intensity ∼791 Å4/amu); (b) Pb−I stretching mode at 88 cm−1 (∼21.82 Å4/amu); (c) MA libration mode at 141 cm−1 (Raman intensity 789 Å4/amu); (d) MA torsional mode at 295 cm−1 (∼2042 Å4/amu).

around 94 cm−1 reveals a major component from librations of the organic cations and from Pb−I stretchings, Figure 2b. In particular, normal modes mainly corresponding to Pb−I stretching are predicted at 72, 73, 87, and 88 cm−1 and they are characterized by small or medium Raman intensity (15%, or less, of the intensity of the band at 60 cm−1). Thus, the 94 cm−1 band is reasonably associated with both the Pb−I stretching and to libration modes of the cations, and it mainly brings information on the inorganic component of the material. This is consistent with the assignment of bands in the 110−121 cm−1 range to the Pb−Cl stretching mode25,28 and with the assignment of a 100 cm−1 band of a putative KPbI3 perovskite to Pb−I stretching.26 As pointed out previously, the fact that the Raman spectra calculated on the different models are quite different in the 100−150 cm−1 region suggests that the band at 119 cm−1 is probably associated to the motion of the organic cations. The visual inspection of the eigenvectors associated to the modes calculated for tet-1 (Figure 2c) confirms this point, showing that the modes predicted above 90 cm−1 consist mainly in the librations of the organic cations. The band falling at 119 cm−1 can be tentatively associated to the band calculated at 141 cm−1. A similar result holds for the weak band measured at 154 cm−1, that can be safely associated with the band calculated at 156 cm−1, corresponding to the libration of the organic cations.27 The assignment of the broad and unresolved band ranging from 200 to 350 cm−1 and the additional weak feature at ∼390 cm−1 represent a very subtle point in the interpretation of the Raman spectrum of MAPbI3. In fact, the theoretical spectra calculated for the three investigated structures do not show any 281



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Raman intensity increases, going from ∼10 −3 to ∼20 Å4/amu for tet-1 and tet-2 and to ∼1 Å4/amu for the ortho structure. Notably, we find different hydrogen bonding patterns for the investigated structures (see Figure 3 and Table S2, Supporting Information). Tet-1 and tet-2 show two short (2.49−2.67 Å) and one long (2.82 Å) hydrogen bonds; the ortho structure shows instead three almost equivalent hydrogen bonds (∼2.5 Å), which preserve a C3v-like environment, leading to a decrease of the Raman intensity of the torsional modes. Our analysis shows that the frequency of the MA torsional modes in the crystal structures may be the result of two opposite effects, associated to the interactions between the cation and the inorganic cage. On the one hand, the deformation of the MA molecule shifts the torsional mode toward higher frequencies; on the other, the formation of specific hydrogen bonding interactions, typical of these compounds20,51 shifts this mode toward lower frequencies. Because of the known orientational disorder of the organic cations in MAPbI3,15,21 the different MA molecules are subject to different balances between molecular deformation and hydrogen bonding interactions and thus their torsional frequency is spread over a large range of values. It is also reasonable to expect that the torsional mode could be strongly influenced by all those parameters that affect the orientational order of the cations, like temperature and morphology. In particular, we expect that increasing the ordering of the cations, by, e.g., decreasing the temperature, a shift of the torsional mode to higher frequencies and an associated decrease in Raman intensity could be observed. In summary, we have reported the resonant Raman spectrum of the widely used MAPbI3 perovskite measured between 60 and 450 cm−1, assigning its main vibrational features with the help of electronic structure calculations. Important markers of the inorganic component have been found at 60 and at 94 cm−1, which are mainly associated to the I−Pb−I bending and Pb−I stretching. The region between 100 and 200 cm−1 is instead associated to librational motions of the MA cations. On the basis of electronic structure calculations, we have elucidated how the interactions between the organic cations and the inorganic counterpart may affect the vibrational frequency and the Raman intensity of the MA torsional mode in MAPbI3. The broad and unresolved band at 200−340 cm−1 is consequently assigned to the torsional mode of the MA cations, and we propose this mode as a possible marker of the orientational order of the organic cations in the material, and thus of the whole crystal. We believe the present study to provide the required assignments for the interpretation of the Raman spectra of organohalide perovskites, which may be extremely useful to understand in detail the properties of this class of materials in relation to their full exploitation in solar cells.

Figure 3. Reduced models used to investigate the torsional frequency of the MA cation in MAPbI3. Left the four isolated MA cations at their geometry in the crystal structures. Middle: the four isolated MA cations plus the three nearest neighbor iodide anions at the crystal structures geometries. Right: the periodic crystal models.

Table 1. Vibrational Frequencies (ω) and Raman Intensities Computed on the Isolated Cations with the Same Geometry They Have in the Crystals (Isolated MA), Including the Three Closest Iodides to the NH3+ Group (isolated MA +3 iodide) and on the Periodic Crystal Structuresa isolated MA ω (cm−1)

Raman int. (Å4/amu)

A A′ B B′

465 465 473 473

0.0050 0.0050 0.0011 0.0011

A A′ B B′

467 467 469 469

0.0003 0.0003 0.0003 0.0003

A A′ B B′

495 495 490 490

0.0003 0.0003 0.0044 0.0044

isolated MA + 3 iodide ω (cm−1)


Tet-1 344 344 327 327 Tet-2 367 367 355 355 Ortho 437 437 429 429

periodic structures ω (cm−1)


28.1350 28.1341 19.2620 19.2618

295 299 312 315

2042.4988 1542.5836 305.6091 128.0141

18.8574 18.9417 18.4804 18.4844

288 296 297 301

814.8004 1436.1303 15.4563 527.7518

1.5272 1.5271 1.5509 1.5509

391 392 398 399

3.3586 42.5448 3.2853 49.2765

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ASSOCIATED CONTENT

S Supporting Information *

Sample preparation, details on the Raman spectrum measurement and theoretical and computational details. Additional data and structural analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

a

The AB−A′B′ labels, defined in Figure 3, for the periodic structures refer to normal modes which are maximally localized on one of the MA molecules. Calculations on the isolated models are carried out by Gaussian 09.39

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 282



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*E-mail: fi[email protected].

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank FP7-NMP-2013 project 604032 “MESO” for financial support.

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The Raman Spectrum of the CH3NH3PbI3 Hybrid Perovskite: Interplay

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