Anharmonic exciton-phonon coupling in metal-organic chalcogenides hybrid quantum wells

In stark contrast to inorganic quantum wells, hybrid quantum wells based on metal-organic semiconductors are characterized by relatively soft lattices. In the latter, excitonic states are deeply affected by coupling with optical phonons. A detailed understanding of the lattice role in exciton dynamics is therefore essential to improve the optoelectronic performance of these materials. Beyond 2D metal halide perovskites, layered metal-organic chalcogenides (MOCs) are an air-stable, underexplored material class hosting complex excitonic phenomena that could be exploited as photodetectors, light emitting devices and ultrafast photoswitches. Here, we elucidate the role of lattice phonons in the optical transitions at different temperatures in the prototypical MOC [AgSePh]∞. We detect coherent exciton-phonon coupling by pump-probe transient absorption spectroscopy, dominated by a Fröhlich interaction with optical phonons at 7 and 12 meV. Through a concerted use of ab initio calculations, linear and Raman spectroscopies, we reveal a distinct phonon anharmonicity clearly manifests in a non-trivial temperature-dependent Stokes shift, deeply impacting the excitonic photoluminescence. The temperature-dependent absorption and photoluminesce data further reveals a Huang-Rhys parameter of about 1.7 suggesting a strong exciton-phonon coupling in both optical transitions. Finally, angle-resolved Raman spectroscopy hints at a distinct anisotropy in the phonon coupling process. Our results indicate shared characters in the exciton-phonon interaction for MOCs and the 2D perovskites, either implying shared polar character of the inorganic sublattices and/or a more important role of the hybrid quantum well superlattice structure.