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

Abstract

The ultrafast charge carrier dynamics of the metaldeficient 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.

Formation of stable 2D methylammonium antimony iodide phase for lead-free perovskite-like solar cells*

“…Our results, displayed in table 1, show that compared to MAPbI 3 , the exciton binding energy values in MASI thin films are one order of magnitude higher [42]. However, our 2D-MASI films show significantly weaker exciton binding compared to the reported and our experimentally obtained values for the 0D-MASI films [43]. To understand the influence of the exciton binding energy on carrier dynamics, we employed a numerical method based on the Saha equation, described in detail in SI [42].…”

Interplay of Structure, Charge‐Carrier Localization and Dynamics in Copper‐Silver‐Bismuth‐Halide Semiconductors

“…Recent theoretical work on the double perovskites Cs 2 Ag(Bi/Sb) X 6 (X = Br − , Cl − ) by Biega et al [70] has suggested that swapping Bi 3+ for Sb 3+ leads to somewhat reduced exciton localization (without removing it entirely). [70] Separately, studies of BiI 3 , [35] (MA) 3 Bi 2 I 9 , [36] and (MA) 3 Sb 2 I 9 [57] by Scholz et al have found very strong electron-phonon couplings across all of these materials, confirming that the observed charge-carrier localization is linked intimately to the presence of bismuth or antimony, rather than the specifics of a particular crystal structure. This finding is in excellent agreement with our observation that structural and compositional variations across the solid solution line alter, but do not completely remove, charge-carrier localization.…”

“…Taken together, these works point toward two potential causes for charge-carrier localization in silver-bismuth materials: first, the easy structural distortion of bismuth-based systems and "soft" silver-halide bonds; second, the low electronic dimensionality that prevails in materials with alternating silver and bismuth structural units. Although similar charge-carrier localization effects have been observed across these and other materials, [7,30,32,36,37,57] an holistic understanding of the interplay between crystal structure, electronic structure, and optoelectronic properties in silver-bismuth materials, and how such effects may be tuned through stoichiometric engineering, has remained lacking thus far.…”

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

“…Many other studies also revealed the ultrafast behaviors of localized excitons, biexcitons, trions and multiple exciton generation in perovskites by transient techniques. He et al reported that the radiative recombination of the photogenerated species in solution‐processed methylammonium–lead–halide films was dominated by excitons weakly localized in band tail states, under the excitation level close to the working regime of solar cells, and such exciton localization effect was found to be general in several solution‐process perovskite films .…”