Direct evidence of weakly dispersed and strongly anharmonic optical phonons in hybrid perovskites

A C Ferreira,J Even,S Paofai,A Létoublon,B Hehlen,S Raymond,S Cordier,B Rufflé,J Ollivier,C Katan,P Bourges

doi:10.1038/s42005-020-0313-7

Abstract

Hybrid organolead perovskites (HOP) have started to establish themselves in the field of photovoltaics, mainly due to their great optoelectronic properties and steadily improving solar cell efficiency. Study of the lattice dynamics is key in understanding the electron-phonon interactions at play, responsible for such properties. Here, we investigate, via neutron and Raman spectroscopies, the optical phonon spectrum of four different HOP single crystals: MAPbBr3, FAPbBr3, MAPbI3, and α-FAPbI3. Low temperature spectra reveal weakly dispersive optical phonons, at energies as low as 2-5 meV, which seem to be the origin of the limit of the charge carriers mobilities in these materials. The temperature dependence of our neutron spectra shows as well a significant anharmonic behaviour, resulting in optical phonon overdamping at temperatures as low as 80 K, questionning the validity of the quasi-particle picture for the low energy optical modes at room temperature where the solar cells operate.

Highlights

Hybrid organolead perovskites (HOP) have started to establish themselves in the field of photovoltaics, mainly due to their great optoelectronic properties and steadily improving solar cell efficiency
It typically goes from the high-temperature cubic phase (Pm3m), passing by a tetragonal phase (I4/mcm for MAPbI3 and MAPbBr3 and P4/mbm for FAPbBr326) down to the lowtemperature orthorhombic phase (Pnma for all compounds except for FAPbI3 where a trigonal phase P3m1 has been reported at low temperature)[27,28,29,30,31]
Only a broad inelastic contribution was observed around 12 meV in MAPB at RT and a tentative attribution of the various phonon modes inside this bundle was only possible through the numerical fitting of the experimental spectrum by a superposition of damped harmonic oscillators (DHOs)[18]

Summary

Introduction

Hybrid organolead perovskites (HOP) have started to establish themselves in the field of photovoltaics, mainly due to their great optoelectronic properties and steadily improving solar cell efficiency. Several of the properties responsible for the outstanding performance of hybrid perovskites are connected with electron–phonon interactions, which have been under intense debate[13,14,15,16,17] Besides governing their emission line broadening, phonon scattering is among the factors setting a fundamental intrinsic limit to the mobility of charge carriers in these materials. They have been well documented to undergo a series of crystallographic transitions which differ slightly depending on the organic cation or the halide atom It typically goes from the high-temperature cubic phase (Pm3m), passing by a tetragonal phase (I4/mcm for MAPbI3 and MAPbBr3 and P4/mbm for FAPbBr326) down to the lowtemperature orthorhombic phase (Pnma for all compounds except for FAPbI3 where a trigonal phase P3m1 has been reported at low temperature)[27,28,29,30,31]. By inducing more complex octahedra tiltings in the perovskite lattice, organic cations may indirectly affect their electronic structure

Methods

Results

Conclusion