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Behavior of Methylammonium Dipoles in MAPbX3 (X = Br andI)

机译:甲基铵偶极子在MAPbX3中的行为(X = Br和一世)

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摘要

Dielectric constants of MAPbX3 (X = Br, I) in the 1 kHz–1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature-independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX3 in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI3 that establish these dipoles as randomly oriented with a rotational relaxation time scale of ∼7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photoexcitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs–1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation.
机译:与CsPbBr3几乎不依赖温度的介电常数相比,在1 kHz–1 MHz范围内MAPbX3的介电常数(X = Br,I)在室温附近表现出强烈的温度依赖性。四方相中MAPbX3的这种强烈的温度依赖性归因于MA + 偶极子在探测时间尺度内自由旋转。这种解释得到MAPbI3上从头算分子动力学模拟的支持,该模拟将这些偶极子建立为随机定向,在300 K时旋转弛豫时间标度为约7 ps。此外,我们探索了这些偶极子在光激发过程中产生瞬态极化的有趣可能性。使用光激发泵和二次谐波产生效率作为探针,对延迟时间跨度为100 fs–1.8 ns会产生重大影响。在任何延迟时间都没有二次谐波信号排除了在光激发下任何瞬态铁电态的可能性。

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