  Self-assembled hybrid perovskite quantum wells have attracted attention due
to their tunable emission properties, ease of fabrication and device
integration. However, the dynamics of excitons in these materials, especially
how they couple to phonons remains an open question. Here, we investigate two
widely used materials, namely butylammonium lead iodide
$(CH_3(CH_2)3NH_3)2PbI_4$ and hexylammonium lead iodide
$(CH_3(CH_2)5NH_3)2PbI_4$, both of which exhibit broad photoluminescence tails
at room temperature. We performed femtosecond vibrational spectroscopy to
obtain a real-time picture of the exciton phonon interaction and directly
identified the vibrational modes that couple to excitons. We show that the
choice of the organic cation controls which vibrational modes the exciton
couples to. In butylammonium lead iodide, excitons dominantly couple to a 100
cm-1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with
phonons with frequencies of 88 cm-1 and 137 cm-1. Using the determined optical
phonon energies, we analyzed PL broadening mechanisms. At low temperatures
(<100 K), the broadening is due to acoustic phonon scattering, whereas at high
temperatures, LO phonon-exciton coupling is the dominant mechanism. Our results
help explain the broad photoluminescence lineshapes observed in hybrid
perovskite quantum wells and provide insights into the mechanism of
exciton-phonon coupling in these materials.
