Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic–Inorganic Hybrid Perovskites

Two thin-film 2D organic–inorganic hybrid perovskites, i.e., 2-phenylethylammonium lead iodide (PEPI) and 4-phenyl-1-butylammonium lead iodide (PBPI) were synthesized and investigated by steady-state absorption, temperature-dependent photoluminescence, and temperature-dependent ultrafast transient absorption spectroscopy. PBPI has a longer organic chain (via introducing extra ethyl groups) than PEPI, thus its inorganic skeleton can be distorted bringing on structural disorder. The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics. In addition, the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy, suggesting its correlation with inorganic skeleton rather than organic chain. Moreover, the photoexcited coherent phonons were observed in both PEPI and PBPI, pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton. This work provides valuable insights into the optical properties, excitonic behaviors and dynamics, as well as coherent phonon effects in 2D hybrid perovskites.