Rotor-phonon coupling in perovskite CH3NH3PbI3: the origin of exceptional transport properties

TL;DR

This study uncovers how rotor-phonon interactions in CH3NH3PbI3 influence its transport properties, revealing atomic dynamics that explain its exceptional photovoltaic performance and thermal conductivity.

Contribution

It provides a comprehensive atomic dynamic picture of CH3NH3PbI3 using neutron scattering, highlighting rotor-phonon coupling as a key factor in its transport properties.

Findings

Proton rotational modes activate around 80 K, reducing phonon lifetimes.

Phase transition at ~165 K involves C-N axis rotation.

Reduced phonon lifetimes correlate with charge mobility and thermal conductivity.

Abstract

Atomic dynamics takes a fundamental part in numbers of physical properties of solids like high-Tc superconductivity, semiconducting transports, and thermoelectricity. Perovskite CH3NH3PbI3 exhibits outstanding photovoltaic performances, but the exact physical scenario has not been established yet, due to the inadequate understanding of the atomic dynamics and exceptional transport properties. We present a complete atomic dynamic picture consisting of phonons, rotational modes of protons and molecular vibrational modes, which is constructed by carrying out high-resolution time-of-flight inelastic neutron scattering measurements in a wide energy window ranging from 0.0036 to 54 meV on a large single crystal sample. A three-fold rotational mode of protons activated around 80 K reduces the lifetimes of acoustic and optical phonons down to about 4.5 ps and below 1 ps at 150 K, respectively. The orthorhombic to tetragonal phase transition takes place with a slower four-fold rotational mode of the C-N axis concomitantly setting in at ~ 165 K, above which the optical phonons are too broadened to be distinguished whereas the acoustic ones are still robust. The significantly reduced lifetimes of optical phonons are linked to the smaller mobility of charge carriers while the ultralow lattice thermal conductivity is attributed to nanoscale mean free paths of acoustic phonons. These microscopic insights provide a solid standing point, on which perovskite solar cells can be understood more accurately and their performances are perhaps further optimized. The revealed rotor-phonon coupling opens up an emergent opportunity to create unprecedented functionalities of materials.