Abstract
Researchers have debated whether methylammonium lead iodide (MAPbI3), 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 MAPbI3 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 MAPbI3 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 temperature-dependent permittivity of MAPbI3. This study contributes to the understanding of the material properties and device pe...
Highlights
Considering the commonly observed ferroelectric property of many inorganic perovskites in their tetragonal phase, it is intriguing to explore whether hybrid perovskites are ferroelectrics that contribute to J-V hysteresis.[12,15,16]
The temperature of a MAPbI3 layer in an encapsulated perovskite solar cells (PSCs) can reach more than 60 oC under typical terrestrial operating conditions, while the phase transition of MAPbI3 from a tetragonal to cubic structure occurs in this temperature range
Even though our macroscale studies could not exclude the possibility of ferroelectricity at nanoscale—and at low temperature—we conclude that MAPbI3 thin films used in PSCs exhibit no apparent ferroelectric property at the normal solar cell operating temperatures (10 oC−70 oC)
Summary
Considering the commonly observed ferroelectric property of many inorganic perovskites in their tetragonal phase, it is intriguing to explore whether hybrid perovskites are ferroelectrics that contribute to J-V hysteresis.[12,15,16] From the perspective of perovskites with tetragonal crystal structure, the polarization of MAPbI3 could be due to three components: (1) ionic polarization by the off-center shift of Pb in the PbI6 octahedral,[15] (2) ionic polarization by the displacement of the positive charge center (MA+) from the negative charge center (PbI3-), and (3) the orientation polarization of the MA+ dipole.[6]. Further experimental studies are still needed to determine unambiguously if MAPbI3 polycrystalline thin films have a ferroelectric property that could possibly contribute to the J-V hysteresis of PSCs. A ferroelectric material shows characteristic signatures in electric-field-dependent polarization studies and temperature-dependent dielectric spectroscopy.[21] For typical perovskitestructure-based ferroelectrics (e.g., perovskite oxide), in the phase transition from the hightemperature paraelectric cubic phase to the low-temperature ferroelectric tetragonal phase, the PE and the related C-E (C: capacitance) curves change correspondingly, giving an indication of a paraelectric to ferroelectric transition.
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