Letter pubs.acs.org/JPCL
Long-Lived Photoinduced Polarons in Organohalide Perovskites Tanja Ivanovska,†,⊥ Chiara Dionigi,† Edoardo Mosconi,*,‡,§ Filippo De Angelis,‡,§ Fabiola Liscio,∥ Vittorio Morandi,∥ and Giampiero Ruani*,† †
Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), via P. Gobetti 101, I-40129 Bologna, Italy ‡ Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO) CNR-ISTM, Via Elce di Sotto, I-06123, Perugia, Italy § D3-Computation, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy ∥ Consiglio Nazionale delle Ricerche (CNR), Istituto per la Microelettronica e Microsistemi (IMM), via P. Gobetti 101, I-40129 Bologna, Italy S Supporting Information *
ABSTRACT: The long diffusion length of charge carriers in the CH3NH3PbI3 perovskite is one of the most relevant properties for explaining the high photovoltaic efficiency of perovskite solar cells. As a possible mechanism for the large diffusion length of electrons and holes, several authors suggested a reduced coulomb attraction of the carriers due to the formation of polarons. Here we performed continuous wave far-infrared photoinduced absorption (PIA) experiments on CH3NH3PbI3; spectral changes are associated with local deformation of the lattice around the photogenerated long-lived charges, a typical signature of photoinduced polarons. Ab initio calculations show confinement of charge carriers at the interface between structural domains characterized by a different tilting of the PbI6 octahedra. The differential IR spectrum between unperturbed and perturbed simulated structures shows a close pattern to the experimental PIA. Positive and negative charges are confined in different varieties of the perovskites coherent with the low recombination and long diffusion length of photogenerated carriers.
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certain phonon modes with temperature in the prototype CH3NH3PbI3 (MAPI) perovskite also imply the presence of a sizable electron−phonon coupling.14 The interaction between photogenerated charge carriers and the crystal lattice can be effectively probed by performing infrared photoinduced absorption (PIA) measurements. By photoinducing charges in the material and measuring the IR absorption spectra, the coupling of the free carriers with the lattice can be detected.15 The photogeneration of electron− hole pairs could eventually distort the lattice around the photoinjected charges, which in turn might give rise to a modification of the electronic structure.16 PIA can thus deliver information on reversible photogenerated phase transformations and strong electron−phonon interactions, i.e., the characteristics of polarons. Here, we investigate the existence of polarons in MAPI by carrying out steady state PIA, which allows the detection of relatively long-lived photoexcited species, with lifetimes on the order of up to milliseconds. For related perovskite-like materials (e.g., cuprates,17,18 manganites,19 BaBiO3,20 and WO321), PIA measurements have shown the bleaching of normal modes and the appearance of infrared activated vibrational features (IRAVs) that imply the existence of
hanks to the extremely rapid improvement of the photovoltaic conversion efficiency observed in lead halide perovskite solar cells1,2 as well as the relatively simple processes for their fabrication,3 an ever increasing number of researchers are entering the race for further technological improvement. Despite such indisputable success, the physics behind the high performances of this class of materials is still an open issue. The charge carriers (both electron and holes) are photogenerated and carried within the same material confining the role of the other device components to select the charges at the interfaces.4,5 The information collected with different experimental techniques, data analysis and computer simulations regarding carrier generation and separation, mobility, diffusion lengths and recombination rates appears to be not always coherent and sometimes even contradictory.6−10 This is most likely related to the complex nature of the system composed by an inorganic lattice intercalated and organic ions with certain vibrational and rotational degrees of freedom within the cuboctahedral cavity.11 The imperative to understand the interaction between the constituents of the lead halide perovskite materials has recently taken a span, providing relevant results indicating electron−phonon coupling mechanisms as the foundation of the charge-transport mechanisms and of the exquisite optoelectronic properties. Steady state PL12 and Hall effect13 measurements have risen the possibility of polaron-mediated transport in lead-halide perovskites. Further evidence of strong anharmonicity and anomalous behavior of © XXXX American Chemical Society
Received: May 10, 2017 Accepted: June 16, 2017 Published: June 16, 2017 3081
DOI: 10.1021/acs.jpclett.7b01156 J. Phys. Chem. Lett. 2017, 8, 3081−3086
Letter
The Journal of Physical Chemistry Letters
The minimum observed around 53 cm−1 corresponds to a shoulder observed in the absorption spectrum at 53.8 cm−1. The bleaching at 73.5 cm−1 apparently does not correspond directly to any observed strong feature. Notably, the shape of the absorption peaks in this region is quite structured as if many peaks were grouped in the same spectral region. This is confirmed by DFT calculations that predict more than 10 IR active modes within the 70−90 cm−1 spectral range.14 Moreover, the minimum observed at 73.5 cm−1 is quite narrow and surrounded by two maxima, similar to a second derivative feature adding uncertainty to the precise determination of the bleaching energy. The observed PIA pattern is a clear indication of the presence of polaronic behavior.14,22 We also considered the possibility that photoinduced features are temperature artifacts due to a temperature increase of the sample due to laser irradiation, but we observed no common features between the temperature-dependent IR spectra and photoinduced spectral variations (see Figure S1 Supporting Information). Electron−phonon coupling in MAPI has been analyzed by Wright et al.12 monitoring the temperature dependence of the photoluminescence emission line width. In particular, the authors proposed a Fröhlich coupling with LO phonons at 11.5 meV (93 cm−1). Considering the values of the dielectric constants of MAPI reported by Ziang et al.,23 according to the Lydenne−Sachs−Tellers relationship,24 the TO counterpart of the LO phonon at 93 cm−1 should fall around 73 cm−1. This energy is close to the main feature of PIA spectrum. Nevertheless, we did not observe any direct absorption by large Fröhlich polarons. A Fröhlich polaron direct absorption should appear in the PIA spectrum as an asymmetric feature rising at the energy of the LO coupled phonon,25,26 while, apart from the relatively sharp IRAV modes and the bleachings, above 90 cm−1 the observed photoinduced spectrum is relatively flat. In any case, considering the noise signal, we cannot exclude that a weak polaron absorption ( 175 μm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (32) Karki, K. J.; Abdellah, M.; Zhang, W.; Pullerits, T. Different Emissive States in the Bulk and at the Surface of Methylammonium Lead Bromide Perovskite Revealed by Two-Photon Micro-Spectroscopy and Lifetime Measurements. APL Photonics 2016, 1, 046103. (33) Quarti, C.; Mosconi, E.; De Angelis, F. Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell Operation. Chem. Mater. 2014, 26, 6557−6569. (34) Weller, M. T.; Weber, O. J.; Henry, P. F.; Di Pumpo, A. M.; Hansen, T. C. Complete Structure and Cation Orientation in the Perovskite Photovoltaic Methylammonium Lead Iodide between 100 and 352 K. Chem. Commun. 2015, 51, 4180−4183. (35) Welch, E.; Scolfaro, L.; Zakhidov, A. Density Functional Theory + U Modeling of Polarons in Organohalide Lead Perovskites. AIP Adv. 2016, 6, 125037. (36) Santomauro, F. G.; Grilj, J.; Mewes, L.; Nedelcu, G.; Yakunin, S.; Rossi, T.; Capano, G.; Al Haddad, A.; Budarz, J.; Kinschel, D.; et al. Localized Holes and Delocalized Electrons in Photoexcited Inorganic Perovskites: Watching Each Atomic Actor by Picosecond X-Ray Absorption Spectroscopy. Struct. Dyn. 2017, 4, 044002. (37) Savenije, T. J.; Ponseca, C. S.; Kunneman, L.; Abdellah, M.; Zheng, K.; Tian, Y.; Zhu, Q.; Canton, S. E.; Scheblykin, I. G.; Pullerits, T.; et al. Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite. J. Phys. Chem. Lett. 2014, 5, 2189−2194. (38) Bischak, C. G.; Hetherington, C. L.; Wu, H.; Aloni, S.; Ogletree, D. F.; Limmer, D. T.; Ginsberg, N. S. Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites. Nano Lett. 2017, 17, 1028−1033. (39) Zhou, Y.; You, L.; Wang, S. W.; Ku, Z. L.; Fan, H. J.; Schmidt, D.; Rusydi, A.; Chang, L.; Wang, L.; Ren, P.; et al. Giant Photostriction in Organic-Inorganic Lead Halide Perovskites. Nat. Commun. 2016, 7, 11193. (40) Zheng, K. B.; Abdellah, M.; Zhu, Q. S.; Kong, Q. Y.; Jennings, G.; Kurtz, C. A.; Messing, M. E.; Niu, Y. R.; Gosztola, D. J.; Al-Marri, M. J.; et al. Direct Experimental Evidence for Photoinduced StrongCoupling Polarons in Organolead Halide Perovskite Nanoparticles. J. Phys. Chem. Lett. 2016, 7, 4535−4539. (41) Neukirch, A. J.; Nie, W.; Blancon, J.-C.; Appavoo, K.; Tsai, H.; Sfeir, M. Y.; Katan, C.; Pedesseau, L.; Even, J.; Crochet, J. J.; et al. Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials. Nano Lett. 2016, 16, 3809− 3816. (42) Zhu, H.; Trinh, M. T.; Wang, J.; Fu, Y.; Joshi, P. P.; Miyata, K.; Jin, S.; Zhu, X. Y. Organic Cations Might Not Be Essential to the Remarkable Properties of Band Edge Carriers in Lead Halide Perovskites. Adv. Mater. 2017, 29, 1603072. (43) Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584− 2590. (44) Zhu, H.; Miyata, K.; Fu, Y.; Wang, J.; Joshi, P. P.; Niesner, D.; Williams, K. W.; Jin, S.; Zhu, X.-Y. Screening in Crystalline Liquids Protects Energetic Carriers in Hybrid Perovskites. Science 2016, 353, 1409−1413.
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DOI: 10.1021/acs.jpclett.7b01156 J. Phys. Chem. Lett. 2017, 8, 3081−3086
