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Researchers have debated whether methylammonium lead iodide (MAPbI 3 ), with a perovskite crystal structure, is ferroelectric and therefore contributes to the current−voltage hysteresis commonly observed in hybrid perovskite solar cells (PSCs). We thoroughly investigated temperature-dependent polarization, dielectric, and impedance spectroscopies, and we found no evidence of ferroelectric effect in a MAPbI 3 thin film at normal operating conditions. Therefore, the effect does not contribute to the hysteresis in PSCs, whereas the large component of ionic migration observed may play a critical role. Our temperature-based polarization and dielectric studies find that MAPbI 3 exhibits different electrical behaviors below and above ca. 45 °C, suggesting a phase transition around this temperature. In particular, we report the activation energies of ionic migration for the two phases and temperaturedependent permittivity of MAPbI 3 . This study contributes to the understanding of the material properties and device performance of hybrid perovskites.

High-frequency supercapacitors are being studied with the aim to replace the bulky electrolytic capacitors for current ripple filtering and other functions used in power systems. Here, 3D edge-oriented graphene (EOG) was grown encircling carbon nanofiber (CNF) framework to form a highly conductive electrode with a large surface area. Such EOG/CNF electrodes were tested in aqueous and organic electrolytes for high-frequency supercapacitor development. For the aqueous and the organic cell, the characteristic frequency at − 45° phase angle was found to be as high as 22 and 8.5 kHz, respectively. At 120 Hz, the electrode capacitance density was 0.37 and 0.16 mF cm−2 for the two cells. In particular, the 3 V high-frequency organic cell was successfully tested as filtering capacitor used in AC/DC converter, suggesting the promising potential of this technology for compact power supply design and other applications. Electronic supplementary materialThe online version of this article (doi:10.1007/s40820-017-0162-4) contains supplementary material, which is available to authorized users.

Practical hybrid perovskite solar cells (PSCs) must endure temperatures above the tetragonal-cubic structural phase transition of methylammonium lead iodide (MAPbI ). However, the ionic and optical properties of MAPbI in such a temperature range, and particularly, dramatic changes in these properties resulting from a structural phase transition, are not well studied. Herein, we report a striking contrast at approximately 45 °C in the ionic/electrical properties of MAPbI owing to a change of the ion activation energy from 0.7 to 0.5 eV, whereas the optical properties exhibit no particular transition except for the steady increase of the bandgap with temperature. These observations can be explained by the "continuous" nature of perovskite phase transition. We speculate that the critical temperature at which the ionic/electrical properties change, although related to crystal symmetry variation, is not necessarily the same temperature as when tetragonal-cubic structural phase transition occurs.

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