Exciton and Coherent Phonon Dynamics in the Metal-Deficient Defect Perovskite (CH₃NH₃)₃Sb₂I₉

The ultrafast charge carrier dynamics of the metal-deficient lead-free perovskite derivative methylammonium antimony iodide ((CH 3 NH 3 ) 3 Sb 2 I 9, MASbI) is studied using femtosecond UV–vis–NIR transient absorption spectroscopy. The transient and steady-state absorption spectra show strong excitonic features. This is confirmed by an analysis of the near-band-gap absorption using Elliott’s theory and its reformulation by Tanguy with subsequent modeling of the free-carrier and exciton contributions using the Saha equation. Time scales of the different carrier scattering processes are identified. The kinetics in the UV–vis range is largely independent of pump laser fluence suggesting that the dynamics involves localized excitons, with recombination time constants of 190 ps and >5 ns. Damped oscillations in the kinetics indicate coherent phonon dynamics and strong electron-phonon coupling. Fourier transformation of the time-domain data provides the steady-state Raman spectrum of MASbI which is dominated by vibrations of the [Sb 2 I 9 ] 3– anion. Exciton dissociation at the interfaces with mesoporous TiO 2 and a triarylamine-based hole transport material is found to be negligible. Our experiments suggest that it will be challenging to fabricate efficient photovoltaic devices from such lead-free antimony- and also bismuth-based zero-dimensional perovskite derivatives, because nonradiative exciton recombination largely governs their carrier dynamics.