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Notwithstanding the success of lead-halide perovskites in emerging solar energy conversion technologies, many of the fundamental photophysical and charge transport phenomena in this material remain debated. Debated issues include the role of the organic and inorganic sublattices, and the coupling between them, on charge formation and transport. Here, we use several, primarily terahertz-based spectroscopies to elucidate electron-phonon coupling, electron-dipole coupling, phonon-band gap coupling and phonon-vibron coupling in this important class of materials. Electron-phonon coupling gives rise to polaron formation. We investigate polaron formation dynamics using the rise time of the terahertz photoconductivity, following femtosecond photo-generation of carriers, as a measure for the polaron formation rates. Polarons are formed on the timescale of 400 femtoseconds, independent of temperature or cation type. This timescale corresponds well with the timescale of the inverse of the LO phonon frequency.
Terahertz-pump, optical probe experiments reveal that strongly exciting the lattice phonons, the optical bandgap is modulated coherently because of excitation of the 1 THz phonon mode. These results explain the anomalous dependence of the optical bandgap on temperature.
Two-dimensional terahertz-infrared Raman spectroscopy reveals remarkably efficient coupling between the vibrations of the molecular cations and the phonon modes of the inorganic sublattice. This result reveals that even though there is no covalent link between the two sublattices, their mutual coupling a strong.
Together, our results illustrate the soft nature of hybrid organic-inorganic perovskites, resulting in strong coupling between different degrees of freedom in this fascinating material.
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