Infrared Spectroscopic Study of Vibrational Modes in

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Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites Tobias Glaser, Christian Müller, Michael Sendner, Christian Krekeler, Octavi Escala Semonin, Trevor D Hull, Omer Yaffe, Jonathan S. Owen, Wolfgang Kowalsky, Annemarie Pucci, and Robert Lovrincic J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01309 • Publication Date (Web): 09 Jul 2015 Downloaded from http://pubs.acs.org on July 13, 2015

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The Journal of Physical Chemistry Letters

Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites Tobias Glaser1,2#, Christian Müller1,2,3# , Michael Sendner1,2, Christian Krekeler1,3, Octavi E. Semonin4, Trevor D. Hull4, Omer Yaffe4, Jonathan S. Owen4, Wolfgang Kowalsky1,2,3, Annemarie Pucci1,2,5*, and Robert Lovrinčić1,3* 1

InnovationLab GmbH, Heidelberg, Germany

2

Kirchhoff-Institute for Physics, Heidelberg University, Germany

3

Institute for High-Frequency Technology, Braunschweig Technical University, Germany

4

Department of Chemistry, Columbia University, New York, United States

5

Center for Advanced Materials, Heidelberg University, Germany

AUTHOR INFORMATION # These authors contributed equally to this work. Corresponding Author *E-mail: [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The organic cation and its interplay with the inorganic lattice underlie the exceptional optoelectronic properties of organo-metallic halide perovskites. Herein we report high quality infrared

spectroscopic

measurements

of

methylammonium

lead

halide

perovskite

(CH3NH3Pb(I/Br/Cl)3) films and single crystals at room temperature, from which the dielectric function in the investigated spectral range is derived. Comparison with electronic structure calculations in vacuum of the free methylammonium cation allows for a detailed peak assignment. We analyze the shifts of the vibrational peak positions between the different halides, and infer the extent of interaction between organic moiety and the surrounding inorganic cage. The positions of the NH3+ stretching vibrations point to significant hydrogen bonding between the methylammonium and the halides for all three perovskites.

TOC GRAPHICS

KEYWORDS hybrid lead halide perovskites, infrared spectroscopy, photovoltaics


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Introduction Hybrid organic-inorganic perovskites, with the general chemical formula ABX3 (A being the organic cation, B=Pb or Sn, and X the halide), exhibit a rich structural and electronic behavior which is strongly influenced by the complex interactions between the organic and inorganic subunits.1 Hybrid halide perovskites2 (X=Cl, Br, I) were studied in-depth already in the 1990s for their promising opto-electronic properties.3–7 Over the last few years the material science community has renewed its interest in the iodide8–11 and bromide12 varieties of the methylammonium (A=CH3NH3+, MA) lead halide perovskites due to their exceptional qualities in photovoltaic and light emitting13 applications. While the organic cation does not directly participate in the formation of electronic transport levels14, it influences the lattice constants and thereby indirectly the band gap.15 Furthermore, even though the MA is known to rotate rapidly at room temperature16,17, ferroelectric effects related to oriented MA domains under applied bias have been suggested.15,18,19 The dynamic interplay between MA cation and the inorganic Pb-X cage is currently under scrutiny,20,21 with several attempts to measure Raman active bands of CH3NH3PbX3 recently reported22–25. Since the organic–inorganic interplay is governed by hydrogen bonding interactions between the NH3+ group of the MA cation and the electronegative halide atoms20,26, detailed knowledge of the hydrogen bonding is necessary for a full understanding of the exceptional electronic properties of the material. Infrared (IR) vibrational spectroscopy is a powerful tool for the study of hydrogen bonding. Onoda-Yamamuro et al. published the mid-IR spectra of CH3NH3Pb(I/Br/Cl)3 in a pioneering work that focused on temperature induced phase transitions.27 However, IR spectra measured on pressed pellets as used by Onoda-Yamamuro are known to suffer sometimes from artifacts, especially in the case of halide containing samples due to hydrogen-bond interactions with the pellet materials KBr


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and KCl.28 We therefore measured the IR spectra of CH3NH3Pb(I/Br/Cl)3 films at room temperature and, for the first time, determined the full dielectric function in the range of 7007000 cm-1 with high accuracy. Thin film results are affirmed by measurements on single crystals. Peak positions differ by up to more than 100 cm-1 compared to Ref. 27. We infer conclusions on the strength of hydrogen bonding between the amine group and the halides in the perovskite structure (N+-H---X) from comparisons with the IR spectra of the corresponding methylammonium halides (CH3NH3(I/Br/Cl)) and 2nd order Møller Plesset perturbation theory29,30 (MP2) calculations for the CH3NH3+ cation in vacuum. Our results suggest that hydrogen bonding is medium-strong at room temperature, and, surprisingly, does not differ significantly between the three halide perovskites. Results Thin films of CH3NH3PbI3 and CH3NH3PbCl3 were fabricated by co-evaporation11 of the corresponding precursor materials in a high vacuum chamber onto Si wafers (see Methods for details). CH3NH3PbBr3 films were made by first evaporating PbBr2 on Si and subsequent transformation to CH3NH3PbBr3 via vapor assisted processing31 in an N2 filled glove box. Film quality was examined with UV/VIS absorption, X-ray diffraction measurements, and atomic force microscopy (see Methods and Supporting Information for details). All IR measurements were performed in either vacuum (thin films) or in dry air (single crystals). Figure 1 shows the relative IR transmittance spectra (full lines) of an approximately 500 nm thick CH3NH3PbI3 film at 0° and 70° angles of incidence, together with the result of an optical model of the 0° measurement (dotted curve). The curved baseline of the 0° measurement is related to thin film interferences and thus essentially depends on the film thickness as well as on


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the dielectric background of the MAPbI3 film. To model the experimental data, a dielectric function = + ∑

p,

,

i

(1)

was fitted (see Methods for details) to the measured spectrum. The sum over i Lorentz oscillators accounts for vibrational excitations with resonance frequency , oscillator strength p , and damping . We find an of 5.0, corresponding to a refractive index of n = 2.24, which is in excellent agreement with recently reported data32 obtained via ellipsometry in the near IR. The isotropic dielectric function was used to simulate the spectrum at 70° angle of incidence (dashed black curve in Fig. 1) and is reproducing very well the experimental spectrum at this angle. This indicates the optically isotropic behavior of the MA ions in the polycrystalline MAPbI3 film, despite the strong preferred orientation along the (110) direction in MAPbI3 layers prepared with PbCl2 as precursor11. Charge carrier densities and relaxation rates can in principle be determined from accurate IR spectra,33 as previously reported for the high carrier density material MASnI3.6 Here, we do not need a Drude-type term to describe the optical response of the sample, and can therefore only set an upper limit for the free charge carrier density ne in our MAPbI3 films. Only if the plasma frequency was above 600 cm-1, a response in the used spectral range would be detectable. With an effective mass34 of 0.1me, we can set the upper limit to e < p e ⁄ = 4 × 10 ! cm$.


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Figure 1. Relative transmittance of a 498 nm thick CH3NH3PbI3 film (deposited onto a Si wafer) measured at normal incidence (top) using non-polarized light and at an angle of incidence of 70° with respect to the surface normal (bottom) using p-polarized light. The black dotted curve shows a fit to the experimental spectrum taken at normal incidence. The black dashed curve shows a simulated spectrum at 70° using the isotropic dielectric function obtained from the fit at normal incidence. The main vibrational lines in the spectra of MAPbI3 can be assigned to fundamental modes of the MA cation and anharmonic combinations thereof.35 We performed analogous measurements and data analysis for all three types of MAPbX3 films (see supporting information for details and the complete tabulated values). Figure 2 a) shows the imaginary parts of the obtained dielectric functions (see SI for tabulated values of real and imaginary parts in the range of 700-7000 cm-1). All NH3 related peaks are stronger than the corresponding CH3 vibrations, a consequence of the positive charge that is located on the ammonium group and enhances the change in dipole moments related to the NH3 vibrations. Table 1 summarizes the peak positions including peak


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assignments. The peak assignments for the fundamental modes were supported by a comparison to the MP2-calculated spectrum (see Methods) of the MA cation, see Fig. 2 b). The weak features in the range of 2300-2900 cm-1 have not been reported previously for MAPbX327 and are not accounted for in the calculation. Peaks in this range are present in MAX salts and have been assigned to anharmonically coupled modes of the MA cation.35 We assume the same interpretation is valid here.

Figure 2. a) Imaginary part of the dielectric function %& of the methylammonium lead halide perovskites CH3NH3Pb(I/Br/Cl)3 obtained by fitting the corresponding relative transmission spectra at normal incidence (see Fig. 1 and Fig. S1 in SI). b) MP2-calculated IR absorption of a single methylammonium-cation in vacuum divided by the wavenumber ν. The inset shows the calculated intensity on a logarithmic scale for a better visibility of the weak CH3 stretch vibrations. The vertical lines indicate the mode assignments of the fundamental modes, see also Table 1.


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In the experimental spectra, a shift to smaller frequencies for almost all peaks in the series Cl – Br – I is apparent. We explain this effect with the increasing polarizability in the Cl – Br – I series (∞ MAPbCl = 4.0, ∞ MAPbBr = 4.7, ∞ MAPbI = 5.0). For a higher polarizabilty of the material, the local field decreases, and thereby also the resonance frequency of the vibrational excitations. This effect is known as Lorentz-Lorenz shift, and can be derived from the ClausiusMossotti relation.36 In other words, the peak shifts to smaller frequencies for the Cl – Br – I series can be intuitively understood as interaction of the MA cation with the surrounding Pb-X cage as effective medium. Importantly, this shift to smaller frequencies is also present for the two N-H stretch vibrations around 3150 cm-1. We note that this is in contrast to the results in Ref. 27, and speculate that the discrepancy might be related to interactions of the perovskite with the pellet materials28 KBr and KCl used in Ref. 27. It is known that the positions of the N-H stretch bands are very sensitive to the strength of hydrogen bonding in MA salts (N+-H---X).35,37,38 Specifically, hydrogen bonding decreases (N-H band positions increase) in the Cl – Br – I series for MAX salts due to decreasing electronegativity of the halide.35 The fact that the N-H stretch band positions in the perovskites follow the opposite trend due to the increasing polarizabilty of the Pb-X cage, allows us to conclude that N+-H---X hydrogen bonding does not differ significantly between the three halide perovskites. This finding is particularly surprising when we additionally consider the increasing lattice constants in the Cl – Br – I series, which intuitively should further decrease the hydrogen bonding. Moreover, the absolute position of the N-H bands (~3150 cm-1) point to medium-strong hydrogen bonding, as the stretching vibrations of free NH3+ are experimentally found37 above 3200 cm-1, and above 3400 cm-1 in the MP2 calculation (see Table 1).


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Figure 3. Relative reflectance spectra of CH3NH3PbI3 (top, red), CH3NH3PbBr3 (middle, green), and CH3NH3PbCl3 (bottom, blue) single crystals measured with non-polarized light and a gold mirror as reference (constant offsets were applied for baseline correction). The black dashed and dotted curves show simulated spectra using Fresnel equations and the dielectric function of the corresponding thin films displayed in Fig. 2. The dotted vertical lines mark the positions of three prominent peaks in CH3NH3PbI3. We performed additional IR micro-spectroscopy measurements on single crystals to verify that the thin film results are not governed by defect or impurity related effects. Figure 3 shows reflection spectra of MAPb(I/Br/Cl)3 single crystals (see Methods for details) together with simulated reflection spectra that were calculated from the Fresnel formula for the perovskite/air interface using the corresponding dielectric functions obtained from transmission measurements.


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All fundamental modes and, furthermore, the weak features in the range of 2300-2900 cm-1 are reproduced, supporting our assumption that these previously not reported peaks are indeed anharmonically coupled combinations of the fundamental modes. We find very good agreement in all peak positions and amplitudes for all three halide perovskites, which corroborates our conclusions on the hydrogen bonding in these materials. Table 1 Measured resonance frequencies of vibrational modes and peak assignments for the three halide perovskites CH3NH3Pb(I/Br/Cl)3 and, for comparison, those calculated for the free MA+ cation. The numbering νx of the peaks corresponds to the one given in Ref. 38. Peak assignments for coupled modes are suggestions only. All frequencies are given in [cm-1]. Mode frequencies are identical for thin films and single crystals. MP2 calculation of + CH3NH3 1032 982 1409 1597 1620 1691 1809

3164 3263 3446 3519

CH3NH3PbI3

CH3NH3PbBr3

CH3NH3PbCl3

Peak assignment

911 960 1250 1385 1423 1469 1577 1842

917 969 1252 too weak 1427 1477 1585 1852 2156 2392 2501 2724 2836 2937 2966 3148 3194

923 978 1257 too weak 1430 1485 1591 1870 2167 2405 2508 2737 2844 2950 2971 3156 3201

CH3 - NH3 rock, ν12 C-N stretch, ν5 + CH3 - NH3 rock, ν11 sym. CH3 bend, ν4 asym. CH3 bend, ν10 + sym. NH3 bend, ν3 + asym. NH3 bend, ν9 ν5 + ν12 ν11 + ν12 ν5 + ν10 2 × ν11 ν3 + ν11 ν9 + ν11 sym. CH3 stretch, ν2 asym. CH3 stretch, ν8 + sym. NH3 stretch, ν1 + asym. NH3 stretch, ν7

2380 2489 2712 2823 2916 2958 3132 3179

+

Conclusion In summary, we provide here high quality data for the mid-IR vibrational spectra of CH3NH3Pb(I/Br/Cl)3. Our thin film measurements are unambiguously confirmed by single


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crystal measurements. The results correct previous reports27 with deviations of more than 100 cm-1 for some modes. Significant hydrogen bonding is found for all perovskites irrespective of the halide. This new set of data will be crucial for comparisons with advanced theoretical calculations of the dynamic disorder in methylammonium lead halide perovskites. Furthermore, the detailed knowledge of the peak positions for the different perovskites provided here will be the basis for characterizations of mixed halide perovskites. Such investigations are currently ongoing in our laboratories. EXPERIMENTAL METHODS Thin film preparation. SiO2 substrates were cut and cleaned by sonication in acetone and isopropyl alcohol followed by drying in a N2 stream. The films were grown using 2 different solvent free methods, namely co-evaporation and a vapor assisted approach. MAPbI3 and MAPbCl3 films were fabricated using thermal co-evaporation. Therefore we used as received PbCl2 (Sigma-Aldrich) and MAI, respectively MACl, synthesized as reported elsewhere.39 The vacuum chamber was pumped down to 10-6 mbar. The MA source was then slowly heated up until the chamber pressure rose to 1–2x10-4 mbar (~120°C). Afterwards the PbCl2 source was heated up to reach an evaporation rate of 6 nm/min (~300°C). The as fabricated perovskite films were then transferred to a nitrogen filled glovebox or to atmosphere. Ex-situ X-ray diffraction measurements on co-evaporated films showed only peaks corresponding to the perovskite structure and no traces of PbCl2. MAPbBr3 films were fabricated using the vapor assisted approach. Therefore PbBr2 was thermally evaporated as received from Sigma-Aldrich. We evaporated a PbBr2 film at a pressure of 10-6 mbar and transferred the substrates to a nitrogen filled glovebox. The substrates were placed on Petri dishes surrounded by MABr. The Petri


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dishes were then put onto a hot-plate at a temperature of 140°C and kept there for several hours to allow the perovskite films to form. Single crystal preparation. Single crystals of CH3NH3PbI3 were grown by diffusion of isopropanol vapor into a 0.5M solution of 1:1 lead iodide (99.999%, STREM) and methylammonium iodide (synthesized following Ref. 39) in concentrated hydroiodic acid (57%, Sigma-Aldrich). Black rhombic dodecahedral crystals with dimensions approaching 1 mm grew overnight. Orange of isopropanol

cubic

vapor

CH3NH3PbBr3 crystals

into

a

1M

solution

were of

grown

1:1

similarly,

lead bromide

by (98%,

diffusion Sigma-

Aldrich) and methylammonium bromide (synthesized following Ref. 36) in dimethylformamide. Small colorless cubic CH3NH3PbCl3 crystals were also grown similarly, by diffusion of npropanol

vapor

into

a

0.15M

solution

of

1:1

lead chloride (98%,

Sigma-

Aldrich) and methylamine hydrochloride (Sigma-Aldrich) in concentrated hydrochloric acid (37%, Sigma-Aldrich). IR spectroscopy of thin films. The as formed perovskite films were transferred to a Bruker Vertex 80v Fourier transform IR (FTIR) spectrometer in nitrogen atmosphere or ambient air. Spectra were measured with the samples in vacuum (~2 mbar) using a DLaTGS detector and non-polarized light. The spectrum at 70° angle of incidence was taken using p-polarized light. Spectral resolution was set to 4 cm-1 and all spectra are referenced to the spectrum of the bare Sisubstrate with natural oxide layer taken at the corresponding angle of incidence. IR spectroscopy of single crystals. The spectra of the CH3NH3Pb(I/Br/Cl)3 single crystals were measured in an IR microscope (Bruker Hyperion 1000) purged with dry air (dew point -73°C). The microscope is coupled to an FTIR spectrometer (Bruker Tensor 27). The IR light was detected with a liquid nitrogen cooled mercury-cadmium-telluride detector. The spectra were


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measured in reflection with a Schwarzschild objective in an angular range of 10° up to 30° incidence and a measurement spot with 100 µm diameter. All spectra were acquired with a resolution of 4 cm-1 and 500 scans with a gold mirror as reference. Optical modeling of thin film IR spectra. The experimental thin film spectra were modeled using the commercial software package Scout.40 The optical model consisted of a 1 mm thick Si substrate with the perovskite layer on top. Both materials were described by their dielectric function. For Si the dielectric function consisted of a dielectric background ∞ Si = 11.69 and Brendel oscillators to account for phonon absorptions, while the dielectric function of the perovskite film consisted of a dielectric background and Lorentz oscillators as given in formula (1). Dielectric background and position, oscillator strength and damping of the Lorentz oscillators were all fitted to the experimental spectrum at 0° of the corresponding perovskite layer. Optical modeling of single crystal IR spectra. Modeling was done using the commercial J.A. Woollam WVASE-32 software package with an angle of incidence of 20° and without backside reflections in case of CH3NH3PbI3 and CH3NH3PbBr3 and with backside reflection for the smaller CH3NH3PbCl3 single crystal. The modeling bases on the Fresnel equations which depend only on the dielectric function of the perovskite single crystal for a certain angle of incidence 8. If the initial medium is air (air = 1, the reflection coefficients for the air/perovskite interface are: :p =

; ?@; >AB ? ; ?C@; >AB ?

, :s =

?@; >AB ?

.

?@; >AB ?

MP2 calculation of vibrational spectrum. Second order Møller Plesset perturbation theory (MP2) is used in combination with a def2-TZVP basis set within the TURBOMOLE v6.5 package.41 The structure was optimized and then the harmonic frequencies were calculated.


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ASSOCIATED CONTENT Supporting Information. IR spectra of MAPbBr3 and MAPbCl3 films, AFM measurements, visible absorption spectra, photographs of single crystals, tabulated values for the dielectric functions. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank Tasja Schwenke, Bernd Epding, and Marcel Plogmayer for invaluable experimental support and Artem Bakulin for helpful discussions. REFERENCES (1)

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(4)

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(8)

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Infrared Spectroscopic Study of Vibrational Modes in

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