Behavior of Methylammonium Dipoles in MAPbX₃ (X = Br and I)

Abstract

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.

Dielectric and ferroic properties of metal halide perovskites

“…110 The measurement of similar responses for both MAPbI 3 and CsPbI 3 by Ref. [119] supports this claim. Pb 2+ is highly polarisable due to its lone pair (6s 2 ) electrons, and is therefore expected to dominate the ionic dielectric response.…”

“…Govinda et al argue that the 1/T dependency of the dielectric response, observed above 160K in Figure 2(a), implies rotational disorder in the ab plane. 119,120 The established dynamic disorder of molecular orientation and inorganic octahedral titling evidences the absence of true ferroelectricity in MAPbI 3 .…”

“…120 In order to account for the dielectric contribution from interfacial polarization (to be introduced in Section III.D), the authors introduce additional Maxwell-Wagner terms to the Debye relaxation model with which they perform their spectral fitting. 119,138 The neglect of these effects can explain the larger value for ion (23 The Cochran-Cowley expression is a generalization of Eq. 13 that accounts for systems with more than two atoms in the unit cell (MAPbI 3 has 48 at room temperature).…”

Competition between Polar and Antiferrodistortive Modes and Correlated Dynamics of the Methylammonium Molecules in MAPbI3 from Anelastic and Dielectric Measurements

“…8 of SI and Refs. 3,22,23), leaves no doubt that the transition at this temperature involves only octahedral tilting. The steplike stiffening, instead of softening, below T OT is a less conventional behavior.…”

Light Generation in Lead Halide Perovskite Nanocrystals: LEDs, Color Converters, Lasers, and Other Applications

“…Changing the halogen species from chlorine to iodine drastically shifts the light emitted from the resulting LHPs from near‐ultraviolet to near‐infrared (Figure d) . Exploiting alloyed halide species at various ratios to occupy site‐X allows the flexible tuning of the bandgap of the resulting LHPs, resulting in continuously adjustable emissive spectra across the entire visible range (Figure d) . By comparison, the A‐cation plays a minor role in determining the energy level configuration of LHPs.…”