Spectral signatures of exciton-polarons in two-dimensional hybrid lead-halide perovskites

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Thouin, Felix
Silva, Carlos
Mourigal, Martin
Trebino, Rick
First, Phillip N.
Brédas, Jean-Luc
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This thesis investigates the optical properties of two-dimensional hybrid organic inorganic perovskites (2D-HOIPs). These materials, consisting of stacked layers of metal-halide octahedras separated by long organic cations, feature a previously unexplained excitonic fine-structure in their low-temperature absorption and emission spectra. This thesis work shows that features within this fine-structure correspond to exciton-polarons of distinct polaronic nature. The first evidence to support this interpretation was provided by successfully reproducing the low-temperature absorption spectrum using a modified Wannier formalism supposing distinct excitonic states with contrasting couplings to the lattice. This hypothesis is then unambiguously confirmed by high-resolution resonant impulsive coherent stimulated Raman spectroscopy which revealed in detail the exact polaronic nature of each excitons within this fine structure. Having shown their distinct polaronic nature, we then show that it drives the relaxation dynamics of photoexcited species down this excitonic manifold. This highlights the importance of the proper consideration of these polaronic effects when optimizing these materials for optoelectronics. We also show, using two-dimensional coherent spectroscopy, that these effects protect the excitons from many-body interactions with other quasiparticles hinting at the importance of polaronic effects in polaritonic microcavities based on these materials. Finally, we also observe stable biexcitons in these materials up to room temperature using high-power two-dimensional coherent spectroscopy showing the importance of many-body interactions in these materials. Our conclusions are of broad fundamental importance to this class of materials for they suggests an avenue to unify the physics governing spectrally broad and narrow emitting 2D-HOIPs. Moreover, due to the high degree of tunability of their crystalline structure, 2D-HOIPs offer an ideal system on which to test models that account for both strong electron-hole and strong electron-phonon interactions. This is of fundamental importance to all two-dimensional polar semiconductors.
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