Composition-Dependent Light-Induced Dipole Moment Change in

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Composition-Dependent Light-Induced Dipole Moment Change in Organometal Halide Perovskites Xiaojing Wu, Hui Yu, Linkai Li, Feng Wang, Haihua Xu, and Ni Zhao J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Nov 2014 Downloaded from http://pubs.acs.org on November 25, 2014

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The Journal of Physical Chemistry

Composition-Dependent Light-Induced Dipole Moment Change in Organometal Halide Perovskites Xiaojing WU, 1 Hui Yu, 1 Linkai Li, 1 Feng Wang, 1 Haihua Xu,1,2 Ni Zhao*1,3 1

Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New

Territories, Hong Kong 2

Department of Biomedical Engineering, Medical School, Shenzhen University, Guangdong, PR

China 3

Shenzhen Research Institute, The Chinese University of Hong Kong

ABSTRACT: In this work we investigate the compositional dependence of electric dipole moment in AMX3 (A: organic; M: metal; X: halogen) perovskite structures using modulation electroabsorption (EA) spectroscopy. By sampling various device structures we show that the second harmonic EA spectra reflect the intrinsic dipolar property of perovskite films in a layered configuration. A quantitative analysis of the EA spectra of CH3NH3PbI3, NH2CHNH2PbI3 and CH3NH3Sn0.4Pb0.6I3 is provided to compare the impact of the organic and metal cations on the photoinduced response of dipole moment. Based on the EA results, we propose that the A and M cations could both largely affect the dielectric and dipolar properties of the perovskite materials, but through different mechanisms, such as ionic polarization, rotation of molecular dipoles and charge migration. These processes occur at different time scales and thus result in a frequency-dependent dipole response. KEYWORDS: electroabsorption, dielectric and dipolar property, perovskite solar cells, organic cation, metal cation

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INTRODUCTION Organometal halide perovskite materials have recently been studied extensively for photovoltaic (PV) applications due to their high power conversion efficiency (up to 19.3%)1 and compatibility with simple fabrication processes such as spin-coating or sequential deposition.2, 3 The materials can be described in a general form of AMX3, where ‘A’ is a monovalent organic cation and ‘M’ is a divalent metal cation that is coordinated to 6 ‘X’ halide anions to form an octahedron. In a perovskite solid, the MX6 octahedra are corner-connected to form a cage structure with the A cation located at the cage center (Figure 1). The electronic properties of the AMX3 perovskites can be tuned by modifying either the composition of the inorganic cage or the molecular structure of the organic cation. By replacing the A cation from methylammonium (MA) to formamidium (FA), the lattice structure changes from tetragonal to trigonal, accompanied by a small change in the unit cell dimensions;4 accordingly, the bandgap of the semiconductor changes from 1.57eV to 1.48eV.5 MASnxPb1-xI3 perovskite compound has also been demonstrated and exhibits a tunable bandgap from 1.17eV to 1.55eV according to the x value (0 ε01892345 across the frequency range tested. Figure 4(c) shows the calculated changes of dipole moment from the ground to exited states for the perovskite materials. While FAPbI3 shows the lowest increment (271%) of dipole moment change from 2000 Hz to 100 Hz, MAPbI3 and MASnPbI3 show an increment of 516% and 1063%, respectively.



15 5

∆ R/R (× 10 )

MAPbI3 FAPbI3

10

MASnPbI3

5 0

Dilectric Constant

(a)

2

10

1

(b)

10

3

10 ∆µ (D)

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2

10

(c) 2

10

3

10 Frequency (Hz)

Figure 4 (a) second harmonic EA peak, (b) dielectric constant and (c) the change of dipole moment ∆μ as a function of modulation frequency for MAPbI3, FAPbI3 and MASnPbI3 thin films. EIS experiments were carried out under the same illumination conditions (2 mW/cm2) as EA spectroscopy at room temperature. For EIS the dc and ac voltage was set to 0V and 50 mV, respectively. 3. Discussion The variation in the frequency dependence of the dielectric constant and dipole moment change is intriguing, and can be associated with the different ‘A’ and ‘M’ combinations in the three material systems. Firstly, MASnPbI3 perovskite shows a


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smaller dielectric constant than MAPbI3. This phenomenon can be qualitatively explained by the Clausius–Mossotti equation, which states that the dielectric constant ε: is given by ε: = 3V= + 8?@AB ⁄3V= − 4?@AB , where V= is the molar volume and @AB is the sum of ionic polarizability of individual ions.23 The molar volume can be related to the unit cell volume EFGHH through the equation V= = EFGHH IJ ⁄K , where NA is the Avogadro constant and Z is the number of formula units in the unit cell. From Table 1 we can see that MAPbI3 and MASnPbI3 have similar unit cell volume (990.0ÅM and 982.95ÅM, respectively);4 therefore, their V= should also be similar. The dielectric constant difference thus mainly originates from the different ionic polarizability of Pb2+ and Sn2+. Pb and Sn are both group IVA elements, but the larger diameter of Pb atoms allows higher degree of electron delocalization and therefore leads to greater ionic polarizability24 as compared to Sn. This polarizability difference may also account for the different extent of the dipole moment increase in MAPbI3 and MAPbSnI3 from the ground state to the excited state. Secondly, the dipole moment change is considerably reduced from MAPbI3 to FAPbI3. We have calculated the size of MA+ and FA+ by Marvin software and the results show that their minimum radius, maximum radius and volume are 2.33 Å, 2.67 Å, 42.58 Å3 and 2.63 Å, 3.14 Å, 46.17 Å3 respectively. Despite the similar size, the molecular dipole moment of MA+ (2.29 D) is much larger than that of FA+ (0.21 D);25 meanwhile, the energy barrier for rotation of MA+ and FA+ in the perovskite structure is predicted to be 0.3 and 13.9 kJ/mol, respectively, suggesting a greater freedom of rotation for MA+. The stronger molecular dipole together with rotational freedom may facilitate a higher degree of charge transfer upon light excitation in MAPbI3 and thus result in formation of large dipole moments. Finally, we note that the raise of the dielectric response at low frequency is most significant in MASnPbI3 among three perovskite systems. At the slow scale of ~ 100 Hz the dielectric property could be modified by space charge polarization resulted from long range charge migration, lattice distortion or dipolar domain alignment. Due to the different ionic radius and atomic orbital configuration of Pb and Sn, the conductivity and structural fluctuation may vary in their corresponding perovskite structures. Whether this could account for the different



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frequency-dependent response of the dielectric and dipolar properties requires further theoretical investigation. CONCLUSIONS In summary, we have applied electroabsorption (EA) spectroscopy to study three AMX3 perovskite systems with different ‘A’ and ‘M’ combinations (MAPbI3, FAPbI3 and MASnPbI3). By sampling various device structures we show that the second harmonic EA spectra reflect the intrinsic, rather than interfacial, properties of perovskite films. Based on the EA spectra we further estimate the change of dipole moment ( ∆µ) from the ground to excited states and find out that ∆µ01234 > 5

∆µ71234 > ∆µ0189234 . Our results suggest that the A and M cations could both 5

5

largely affect the dielectric and dipolar property of the perovskite materials, but through different mechanisms, such as ionic polarization, rotation of molecular dipoles and charge migration. These processes occur at different time scale and thus result in a frequency-dependent dipole response. Our study provides a quantitative analysis on the composition-dipole moment correlation in perovskite systems. The fundamental spectroscopic measurement, together with theoretical calculations, can help to improve the understanding on the remarkable electronic properties of the AMX3 perovskites and provide guidelines on the design of perovskite materials with new functionalities. Corresponding Author *Prof. Ni Zhao Add: Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong. Email: [email protected]. Phone: (+852)96427884.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS We gratefully acknowledge the fundings from Research Grants Council of Hong Kong (Grants No. CUHK419311 and T23-407/13-N), National Natural Science


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Foundation of China (Grants No. 61205036) and Shun Hing Institute of Advanced Engineering (Grant no. 8115041). Supporting Information Angle-dependent EA spectra and derivation of dipole moment; dielectric constant derivation process; XRD patterns of MAPbI3, FAPbI3 and MASnPbI3 perovskite films; SEM images of MAPbI3 and FAPbI3 perovskite films. This information is available free of charge via the Internet at http://pubs.acs.org REFERENCE [1] Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong. Z.; You. J.; Liu. Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542-546. [2] Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as A Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. [3] Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. [4] 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. [5] Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells. Energ. Environ. Sci. 2014, 7, 982-988. [6] Hao, F.; Stoumpos, C. C.; Chang, R. P.; Kanatzidis, M. G. Anomalous Band Gap Behavior in Mixed Sn and Pb Perovskites Enables Broadening of Absorption Spectrum in Solar Cells. J. Am. Chem. Soc. 2014, 136, 8094-8099. [7] Oh, D. H.; Sano, M.; Boxer, S. G. Electroabsorption (Stark Effect) Spectroscopy of Mono-and Biruthenium Charge-Transfer Complexes: Measurements of Changes in Dipole Moments and Other Electrooptic Properties. J. Am. Chem. Soc. 1991, 113, 6880-6890. [8] Bublitz, G. U.; Boxer, S. G; Stark Spectroscopy: Applications in Chemistry, Biology, and Materials Science. Annu. Rev. Phys. Chem. 1997, 48, 213-242. [9] Brunschwig, B. S.; Creutz, C.; Sutin, N. Electroabsorption Spectroscopy of Charge Transfer States of Transition Metal Complexes. Coord. Chem. Rev. 1998, 177, 61-79. [10] Vance, F. W.; Williams, R. D.; Hupp, J. T. Electroabsorption Spectroscopy of Molecular Inorganic Compounds. Int. Rev. Phys. Chem. 1998, 17, 307-329. [11] Chiang, H. C.; Iimori, T.; Onodera, T.; Oikawa, H.; Ohta, N. Gigantic Electric Dipole Moment of Organic Microcrystals Evaluated in Dispersion Liquid with Polarized Electroabsorption Spectra. J. Phys. Chem. C 2012, 116, 8230-8235.



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