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Generation of Coherent Optical Phonon in Methylammonium Lead Iodide Thin Films Chengbin Fei, Julio S Sarmiento, and He Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03529 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018
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The Journal of Physical Chemistry
Generation of Coherent Optical Phonon in Methylammonium Lead Iodide Thin Films
Chengbin Fei†, Julio S. Sarmiento†, He Wang†* †
Physics Department, University of Miami, Coral Gables, 33146, FL, USA
ABSTRACT Long-lived hot carrier in organic-inorganic hybrid perovskite materials is possibly assisted by the large polaron and hot phonon bottleneck. Phonon plays a significant role in the properties of hybrid perovskite. Coherent phonon refers to the in-phase vibration of crystal lattice. Here, a temperature-dependent, fluence-dependent, and polarizationdependent transient absorption spectroscopy is utilized to study the coherent phonon in methylammonium lead iodide. The coherent optical phonon is observed due to the existence of Pb-I-Pb angular distortion at 24-33 cm-1 in both tetragonal and orthorhombic phases. The oscillation from the excited state absorption is also observed, suggesting the couple between coherent phonon and photo-excited state. The oscillation of coherent phonon attenuates with increasing environmental temperature. The dependence of coherent phonon on the polarization of the pulse and the quantitative analysis indicates the generation mechanism is impulsive stimulated Raman scattering and transient depletion field screening.
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1. INTRODUCTION Organic-inorganic hybrid perovskite materials have attracted extensive attention due to the rapid increase in photovoltaic efficiency. The photovoltaic efficiency has increased to 22.7% in the past few years.1 This rapid increase in efficiency originates from the formation of highly crystalline perovskite film and superb optoelectronic properties of perovskite materials.2 The crystalline perovskite thin film is typically formed by thermal evaporation and spin coating with some treatment such as antisolvent deposition.3 Methylammonium lead iodide (CH3NH3PbI3) is the most commonly studied hybrid perovskite material. The superb optoelectronic properties include tunable bandgap for strong light absorption,4 small exciton binding energy at room temperature for easy exciton dissociation,5,6 decent electron and hole mobilities for efficient charge transport,7 and limited electron-hole recombination.8,9
Both crystal and liquid characteristics are proposed and observed for hybrid perovskite by Zhu and coworkers.10 By measuring the mobility at various temperatures, the band-like charge transport is observed.11 By comparing the organic-inorganic hybrid perovskite with inorganic perovskite via optical Kerr effect and 2D-infrared spectroscopy, liquidlike reorientation of the organic cation is observed.12,13 Large polaron is formed by the coupling between coherent transport charge and the reorientation of organic cation.10,14,15 These large polarons lead to Coulomb screening, which can protect the cooling of hot carrier at the low excitation intensities.10 Long-lived hot carrier in hybrid perovskite
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The Journal of Physical Chemistry
compared with inorganic perovskite has been generally accepted in the community,16-21 although the rapid carrier thermalization via carrier-carrier scattering was observed by 2D electronic spectroscopy with better temporal resolution.22 Theoretically, the successful collection of hot carrier by the selective electrode before cooling to the bottom of conduction band could potentially break the Shockley-Queisser limit (~33.1%) for solar cell application. Besides the protection via large polaron, long-lived hot carrier might also stem from the hot phonon bottleneck at high excitation densities. The hot phonon bottleneck refers to the optical phonon cannot be cooled by the acoustic phonon and the optical phonon coupled with the carrier assists the long-lived hot carrier.17,18 Regardless of the mechanism of large polaron or hot phonon bottleneck, phonon plays a significant role in organic-inorganic hybrid perovskite.
Coherent phonon, which refers to in-phase vibration of crystal lattice, has recently been observed in CH3NH3PbI3 by transient absorption spectroscopy (at 77 K)23 and 2D electronic spectroscopy (at room temperature).24 The coherent phonon has been attributed to the angular distortion of Pb-I-Pb with optical character.23-26 This vibrational mode has been observed by Raman and THz spectra.25-27 Although coherent optical phonon is only recently detected in hybrid perovskite, coherent optical phonon has been widely observed in inorganic semiconductors. The generation of coherent phonon typically includes two mechanisms, impulsive stimulated Raman scattering (ISRS) and transient depletion field screening (TDFS).28 ISRS refers to the coherent phonon is excited by the ultrafast laser pulse with a distribution of photon energy, whose difference equals to the phonon energy
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for the material with Raman active phonon mode. Another mechanism TDFS refers to the sudden screening and change of the electric field by the photo-excited carriers. The positive and negative ions move in opposite directions, resulting in a collective oscillation. This mechanism, in need of cations and anions, typically exists in a polar compound. ISRS mechanism is dependent on the polarization of optical excitation, while TDFS is polarization independent.28
Organic-inorganic hybrid perovskite materials are semiconductors that consist of cations and anions. It is not clear whether the coherent phonon is created by ISRS or TDFS. In this work, the effects of polarization, optical excitation density, and temperature effect on coherent phonon are studied. The coherent phonon is strongly temperature dependent and occurs in both tetragonal and orthorhombic phases. Some of the coherent phonons couple with photo-excited carriers and excitons. Besides, the coherent phonon is strongly dependent on the polarization of laser pulse. The quantitative analysis to separate angledependent component and angle-independent component suggests the mechanism of generating coherent optical phonon is a combination of ISRS and TDFS.
2. EXPERIMENTAL SECTION 2.1 Fabrication and Characterization of CH3NH3PbI3 film. 80 mg methylammonium iodide CH3NH3I (Yingkou You Xuan Trade Co., Ltd) and 231 mg lead iodide PbI2 (Yingkou You Xuan Trade Co., Ltd) were dissolved in 0.5 mL anhydrous dimethylformamide (Sigma Aldrich) in the nitrogen-filled glovebox (O2
