The speed of sound and elastic properties of single crystals of inorganic and hybrid lead-free iodide perovskites obtained by femtosecond transient optical reflectivity

TL;DR

This study measures the speed of sound in lead-free iodide perovskites using femtosecond transient optical reflectivity to determine their elastic properties, revealing lower stiffness in hybrid materials relevant for optoelectronic device design.

Contribution

It introduces a femtosecond optical method to measure elastic constants in lead-free perovskites, providing new insights into their mechanical properties.

Findings

Hybrid perovskites have lower elastic stiffness.

Speed of sound measurements enable elastic constant extraction.

Elastic properties influence device stability and performance.

Abstract

Optoelectronic devices operate under continuous thermal stress that may influence both long-term stability and optical properties of the active material. It is therefore useful to know the elastic properties of the active materials. In general, the elastic constants of a material may be deduced by the measurement of the speed of sound in that material. In this work, we report on the measurements of the speed of sound in three lead-free halide perovskites, namely the inorganic Cs3Bi2I9 and the hybrid MA3Bi2I9 and MA3Sb2I9, by means of femtosecond transient optical reflectivity that shows oscillations of the signal intensity that are related to the strain field caused by the photoexcitation of electron-hole pairs. The values of the speed of sound thus obtained have allowed us to extract the elastic constant, c11, along the propagation direction of the light. The c11 values indicate a lower stiffness of the hybrid materials, an important aspect when designing optoelectronic devices.