5 years ago

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

Behavior of Methylammonium Dipoles in MAPbX3 (X = Br and I)
Abhinav Kumar, Cesare Franchini, Jonathan Lahnsteiner, D. D. Sarma, Anshu Pandey, Sharada Govinda, Pratibha Mahale, Menno Bokdam, Somnath Pal, Bhushan P. Kore, Georg Kresse, Biswajit Bhattacharyya
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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpclett.7b01740

DOI: 10.1021/acs.jpclett.7b01740

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