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

Sharada Govinda, Bhushan P. Kore, Menno Bokdam, Pratibha Mahale, Abhinav Kumar, Somnath Pal, Biswajit Bhattacharyya, Jonathan Lahnsteiner, Georg Kresse, Cesare Franchini, Anshu Pandey, D. D. Sarma*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

71 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)4113-4121
Number of pages9
JournalJournal of physical chemistry letters
Volume8
Issue number17
DOIs
Publication statusPublished - 7 Sep 2017
Externally publishedYes

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