Microscopic origin of the ultra-low thermal conductivity in hybrid perovskites

Description

Hybrid perovskites such as the archetypal methylammonium lead iodide (MAPbI3) are an important class of optoelectronic materials. Over the last decade, the efficiency of hybrid-perovskite photovoltaics has developed to compete with the best alternative materials, due to the unique combination of optimal electronic structures, large optical-absorption coefficients and high tolerance to defects.

A key feature of MAPbI3 is the tight coupling between its optoelectronic properties and structural (lattice) dynamics. Rotations and deformations of the PbI3- octahedral framework lead to a sequence of thermal phase transitions, the nature of which results in large deviations from the average crystallographic structure.[1] At the same time, correlated motion of the dipolar MA+ cations produces polarisation domains that help separate photogenerated charge carriers.[2,3] First-principles modelling has further demonstrated strong coupling between the cage and cation dynamics,[4] which suppresses the thermal conductivity and limits both non-radiative recombination and hot-carrier cooling.[5]

While introducing loosely-bound "rattler" ions to scatter phonons is an established strategy for reducing thermal conductivity (e.g. as in filled Skutterudites), the effect of replacing an ion with a molecule is less well understood.

We have combined high-precision neutron-scattering measurements with first-principles modelling to perform an in-depth analysis of the thermal transport in MAPbI3 to investigate how the interplay between the cage and molecule impacts the thermal transport.[6]

In contrast to binary semiconductors such as GaAs and CdTe, heat conduction via acoustic phonons is heavily suppressed in MAPbI3, with picosecond lifetimes and mean-free paths on the order of nanometres. Analysis of the contributions of individual phonon modes to the macroscopic thermal conductivity, within the framework of the phonon Boltzmann-transport equation, shows that the short lifetimes result from: (1) a high density of energy-conserving scattering channels, enabled by the additional degrees of freedom of the organic cation; and (2) strong phonon-phonon interactions. A more detailed study of the individual three-phonon scattering processes that limit the lifetimes further shows that low-frequency optic modes where the motion of the cage couples to the translations and rotations of the molecule serve as scattering "hotspots", providing a direct link between the organic molecule and the ultra-low thermal conductivity.

This study enables a microscopic understanding of thermal transport in MAPbI3 that, beyond implications for carrier dynamics, provides new insight into how hybrid inorganic/organic materials might be used to optimise thermal transport e.g. for thermoelectric applications. It also highlights the detailed information available from lattice-dynamics modelling, which will play a valuable role in understanding and optimising heat conduction in other material systems.

1. A. N. Beecher et al., ACS Energy Lett. 1 (4), 880-887 (2016), DOI: 10.1021/acsenergylett.6b00381
2. J. M. Frost et al., Nano Lett. 14 (5), 2584-2590 (2014), DOI: 10.1021/nl500390f
3. A. M. A. Leguy et al., Nature Comm. 6, 7125 (2015), DOI: 10.1038/ncomms8124
4. F. Brivio et al., Phys. Rev. B 92, 144308 (2015), DOI: 10.1103/PhysRevB.92.144308
5. L. D. Whalley et al., Phys. Rev. B 94, 220301(R) (2016), DOI: 10.1103/PhysRevB.94.220301
6. A. Gold-Parker et al., PNAS 115 (47), 11905 (2018), DOI: 10.1073/pnas.1812227115

Period	8 Jul 2019
Event title	14th International Conference on Materials Chemistry
Event type	Conference
Location	Birmingham, United KingdomShow on map
Degree of Recognition	International