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 ((CH3NH3)(3)Sb2I9, 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 Elliotts 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 [Sb2I9](3-) anion. Exciton dissociation at the interfaces with mesoporous TiO2 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.