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C2.07 - Composition-Dependent Electric Dipole Moment in Organometal Halide Perovskites 
April 7, 2015   4:00pm - 4:15pm

Organometal halide perovskite materials have been studied extensively for photovoltaic applications due to their high power conversion efficiency and compatibility with simple fabrication processes. Despite the theoretical studies and macroscopic electrical characterizations on the electrical and electro-optical properties of the perovskites, a microscopic picture that correlates the chemical composition with electric dipoles in the perovskite solids is still lacking. Herein, we investigate the compositional dependence of electric dipoles in AMX3 (A: organic; M: metal; X: halogen) perovskite structures using modulation electroabsorption (EA) spectroscopy, which measures the change in the reflection of light through a material upon application of a modulated electric field. By sampling various device structures we show that the second harmonic EA spectra reflect the intrinsic, rather than interfacial, properties of the perovskite films. 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. We further correlate the dielectric property with charge transport and charge trapping processes in the perovskites using charge modulation spectroscopy, electrochemical impedance spectroscopy and time-resolved THz experiments. Our 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.

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