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

Activity: Talk or presentationOral presentation

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
Period8 Jul 2019
Event title14th International Conference on Materials Chemistry
Event typeConference
LocationBirmingham, United KingdomShow on map
Degree of RecognitionInternational