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Entropy-Suppressed Ferroelectricity in Hybrid Lead-Iodide Perovskites Alessio Filippetti, Pietro Delugas, Maria Ilenia Saba, and Alessandro Mattoni J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02117 • Publication Date (Web): 20 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015
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The Journal of Physical Chemistry Letters
Entropy-Suppressed Ferroelectricity in Hybrid LeadIodide Perovskites Alessio Filippetti1,*, Pietro Delugas1,2, Ilenia Saba1, and Alessandro Mattoni1 1
Istituto Officina dei Materiali, CNR-IOM SLACS Cagliari, Cittadella Universitaria, Monserrato
09042-I (CA), Italy. 2
Istituto Italiano di Tecnologia - IIT, Via Morego 30, 16163 - Genova, Italy.
ABSTRACT. The actual nature of the electric polarization in hybrid lead-iodide perovskites is unveiled on the basis of ab-initio and model results. A finite, albeit small electric polarization of few µC/cm2 is found to be pervasive in this system, due to the polar-uncompensated alignment of methylammonium dimers, at least for temperature lower than the activation energy of dimer rotations; however, the presence of a large number of structural local minima corresponding to differently oriented polarization directions counteracts the stabilization of an ordered ferroelectric phase at the macroscale. According to our estimate, only for temperature lower than 40-50 K a clear ferroelectric behavior is displayed. At higher temperature the polarization is progressively suppressed and the ferroelectric ordering hindered by the large configurational entropy, giving rise to a super-paraelectric-like behavior at the macroscale.
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A remarkable emphasis was recently devoted to the presence of electric polarization in CH3NH3PbI3 (MAPI) perovskite, the undisputed star of last few year research on photovoltaic materials.1-6 Polarization has been invoked to explain some peculiar features such as hysteretic effects in the current-voltage curves, 7-10 the large dielectric response under illumination,11,12 and the extremely slow photoconductive response,13-17 which however can also be imputed to different causes, such as ion migration and photoinduced trapping defects. Also, ferroelectric behavior is sometime claimed as possible key to explain excellent photoconversion properties,1823
with the rationale that macroscopic electric fields resulting from incomplete depolarization
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screening24,25 or charged ferroelectric domains26 could eventually enhance the space separation of the photoinduced electron-hole plasma. Experimental evidence of electric polarization (P) or ferroelectric ordering in MAPI is highly controversial. Electric polarization is indicated by a number of piezoresponce force microscopy/atomic force microscopy (PFM/AFM) experiments:27-30 a polar response to the AFM tip is visible in the dark, and enhanced under illumination, with a remnant polarization at zero field. However, it is not clear whether P is actually persistent or it just fades away very slowly, as pointed out in a recent piezoresponse measurement30 which rules out the presence of steady P at room temperature. This conclusion is coherent with most of theoretical31-33 and experimental works (NMR,34 IR-spectroscopy and calorimetry,35,36 and neutron diffraction37,38) on the dynamical properties of MAPI which attest that the molecule rotation is fully activated at room temperature, with relaxation times in the range 1-5 ps (in ref. 37 a longer relaxation time of 12 ps at room-T is reported). Indeed, since P descends primarily by the relative orientation of the C-N dimers, in order to retain a well defined P the dimers should be steady, i.e. P can only survives at temperatures lower than the thermal activation of dimer rotations. A shown by finite-temperature molecular dynamics simulation,33 as temperature decreases, the fast reorientational dynamics of the cations is quenched at T
